Advanced In Situ Characterization Techniques for Direct Observation of Gas‐Involved Electrochemical Reactions

Gas‐involved electrochemical reactions provide feasible solutions to the worldwide energy crisis and environmental pollution. It has been recognized that various elements of the reaction system, including catalysts, intermediates, and products, will undergo real‐time variations during the reaction process, which are of significant meaning to the in‐depth understanding of reaction mechanisms, material structure, and active sites. As judicious tools for real‐time monitoring of the changes in these complex elements, in situ techniques have been exposed to the spotlight in recent years. This review aims to highlight significant progress of various advanced in situ characterization techniques, such as in situ X‐ray based technologies, in situ spectrum technologies, and in situ scanning probe technologies, that enhance our understanding of heterogeneous electrocatalytic carbon dioxide reduction reaction, nitrogen reduction reaction, and hydrogen evolution reaction. We provide a summary of recent advances in the development and applications of these in situ characterization techniques, from the working principle and detection modes to detailed applications in different reactions, along with key questions that need to be addressed. Finally, in view of the unique application and limitation of different in situ characterization techniques, we conclude by putting forward some insights and perspectives on the development direction and emerging combinations in the future.


Introduction
[3][4] Therefore, it is urgent to exploit renewable energy reasonably to construct a green and sustainable energy system.7][8][9][10] Back to the source, the core of these electrochemical energy conversion reactions is a series of gas-phase electrochemical reactions.Most of them occur at the cathode, including carbon dioxide reduction reaction (CO 2 RR), [11,12] nitrogen reduction reaction (NRR), [13,14] and hydrogen evolution reaction (HER), [15][16][17] whose fundamental principles will be briefly described in the next section.Among them, CO 2 RR and NRR are excellent energy conversion reactions with N 2 and CO 2 in the air as feedstock.At the same time, HER is a side reaction of the two reactions mentioned above, but it is also an excellent hydrogen production reaction in its own right.However, there are still various impediments to the practical application of these reactions.For instance, electrocatalysts for HER are usually derived from noble metals, which seems to be an expensive expense. [18]Meanwhile, CO 2 RR is a multi-electron transfer process, which has high energy transition states and requires corresponding catalysts for target products. [1,19]oreover, the N 2 barrier in NRR is too high to be activated by conventional catalysts, which need suitable catalyst design. [20]23] Various practical strategies have been put forward to overcome these obstacles and achieve specific results.However, most mechanism research is based on theoretical calculation without direct evidence to prove the reaction pathway and active catalytic site. [24,25]Conventional characterization methods are often exploited to explore the active phase of the catalyst.Nevertheless, these ex-situ characterizations can only monitor the state of the beginning or end, while they cannot detect the change of the catalyst along with some essential intermediates during the reaction process in real-time. [26,27]Therefore, it is vital to fully understand and develop in situ characterization techniques to investigate the electrocatalytic reaction, construct the structureperformance relationship, and design superior catalysts.In general terms, in situ characterization can be understood as the characterization of catalysts during their regular operation under actual reaction conditions to obtain characteristic variations of surface morphology, electronic valence, structural composition, and so on in real-time. [28]In recent years, a variety of characterization methods include X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), infrared spectroscopy (IR), Raman spectroscopy (Raman), mass spectrometry (MS), gas chromatography (GC) and microscopic techniques have made significant progress in the field of in situ measurement and have been applied in the detection of electrochemical catalytic reactions to varying degrees (Figure 1). [29][32] Thus, it is essential to combine several techniques to gain insight into electrocatalytic reaction principles.In a nutshell, rational and flexible application of these advanced in situ characterization techniques can explain electrochemical catalytic reactions from their origin and provide a shortcut to design more efficient catalysts to facilitate the practical application of energy conversion reactions. [33]n this review, we systematize several currently common in situ characterization techniques and briefly introduce their operating principles along with detection modes, which are helpful for optimizing the design and expanding their application.In addition, the specific applications of different in situ characterization techniques in the three mentioned electrochemical reduction reactions with hot spots, including CO 2 RR, NRR, and HER, are described and thus provide valuable insights for further understanding of the activity-function relationships and reaction mechanisms of electrochemical processes (Figure 2).Finally, we present an outlook on the future development of in situ characterization techniques, hoping that this review will better help researchers understand in situ characterization techniques and expand their application scope in electrochemical reduction reactions.

Carbon Dioxide Reduction Reaction
A large number of greenhouse gas emissions, mainly carbon dioxide (CO 2 ), have caused a series of environmental and climate problems such as global warming, which seriously threaten the survival and development of human beings. [34]To reduce greenhouse gas emissions and efficiently utilize CO 2 resources, several strategies for carbon capture and storage (CCS) and utilization (CCU) have been proposed, which are suitable for the goal of "carbon peak and carbon neutrality". [35,36]onsidering the principle of a sustainable energy system, carbon dioxide reduction reaction stands out from CCU strategies due to its mild reaction conditions, small plant footprint, mature reaction devices, and other advantages. [37]Since CO 2 is the highest valence oxide of carbon, electrons usually need to be transferred to the carbon to complete the oxidation process, thus obtaining higher energy products.Such a CO 2 RR process can also be recognized as CO 2 hydrogenation, [38] in which the electron and proton source is usually renewable water, and the overall reaction process is as follows: (1) In the CO 2 RR process, electric energy needs to be generated through low-carbon or zero-carbon energy, such as solar energy and wind energy, to ensure that there will be no subsequent generation of extra CO 2 in the whole reaction, which reduces the life cycle of CO 2 .Specifically, CO 2 RR can be carried out with the addition of catalysts, as the C=O bond energy is up to 806 kJ mol −1 .Under the application of high voltage, the C=O bond breaks, and electron transfer occurs to produce CO 2 À or other intermediates. [39]Unlike the NRR process to produce ammonia and hydrazine, CO 2 RR is a multielectron reaction with diversified products.Typically, two-electron, four-electron, six-electron, and eight reduction reactions can be realized through multiple electron transfers in an aqueous solution, thus obtaining C 1 products such as CO/HCOOH, formaldehyde, methanol, and methane, respectively. [37,40,41]Moreover, based on these essential reactions, more electron transfers such as 12-electron reactions can lead to higher value-added C 2 products such as ethylene and ethanol. [42] 2  annual production ranks first in all chemicals and plays a vital role in the global economy.Nowadays, the Haber-Bosch process is the primary mean of ammonia synthesis in industry, which employs Fe/Ru-based material as the catalyst and reacts under extremely high temperature (400-600 °C) and pressure (20-40 MPa) to achieve products. [58,59] Such a traditional way of ammonia production significantly consumes a large quantity of fossil fuels and emits many greenhouse gases, which causes irretrievable damage to the environment. [60]herefore, finding a clean and sustainable ammonia production process is necessary.3] 2N With N 2 as a nitrogen source and water as a proton source, electrochemical synthesis of ammonia can be achieved by applying a specific voltage in a three-electrode system under mild conditions.Due to the negative electron affinity (−1.8 eV), high bond breaking energy (410 kJ mol −1 ), and sizeable HOMO-LUMO energy gap (22.9 eV) of nitrogen molecules, the activation energy of ammonia synthesis reaction is up to 335 kJ mol −1 . [64,65]68][69] The reaction mechanism of NRR mainly includes dissociation and association mechanisms. [70]In the dissociation mechanism, N 2 molecules first carry out chemisorption on the catalyst's surface to weaken the chemical bonds between nitrogen atoms.Subsequently, the triple bond of the nitrogen molecule is broken before hydrogenation occurs, leaving two adsorbed nitrogen atoms on the catalyst surface.The adsorbed nitrogen atoms hydrogenated with the chemically adsorbed hydrogen atoms on the catalyst surface and gradually formed -NH, -NH 2 , and NH 3 .Finally, ammonia molecules are desorbed from the catalyst surface to form gaseous NH 3 . [71,72]herefore, the dissociation mechanism requires a high energy consumption due to the predestruction of the triple bond of N 2 molecules and requires high catalyst performance.Unlike the dissociation mechanism, N 2 molecules tend to adsorb on the surface physically and are subsequently activated by the electron injected into the highest orbital in the association mechanism. [73]Briefly, the triple bond of N 2 is broken by hydrogenation at one or both ends, reducing energy consumption and improving catalytic efficiency.In addition, the hydrogenation process in the association mechanism can be carried out through two processes, namely the alternating way and the distal way. [74]In an alternating way, the two N 2 atoms adsorbed on the catalyst surface can alternately hydrogenate and receive electrons.All the bonds between N 2 atoms are broken, and the first NH 3 molecule is successfully removed from the catalyst surface, followed by the release of the second ammonia molecule.While in a distal way, hydrogenation preferentially occurs on the nitrogen atom farthest from the catalyst surface, and the distal nitrogen atom does not directly interact with the catalyst.When the first ammonia molecule is released, the remaining nitrogen atom continues to hydrogenate to generate the second ammonia molecule and release it accordingly. [75]ased on the reaction mechanisms proposed above, the possible key factors that affect the NRR performance are as follows.The first is the adsorption and desorption capacity of nitrogen molecules on the catalyst. [76]In the ideal state, the adsorption rate of N 2 molecules is equal to the desorption rate to achieve adsorption equilibrium.However, the significant difference between the adsorption activation energy and the desorption activation energy is not conducive to the desorption of NH 3 molecules.The second is the hydrogenation of N 2 molecules on the catalyst's surface during the reaction process.Theoretically, the equilibrium potential required in the hydrogenation step is −3.2 V, which is far more negative than that of other steps.Therefore, the ratedetermining step (RDS) of the NRR process is mostly this step.According to previous reports, when the pH value is lower than 9.25, the ammonia in the solution is mainly in the form of ionic ammonium ammonium (NH 4 þ ), while when the pH value is ≥9.25, it mainly exists in the form of NH 3 .It seems that the lower the pH, the more favorable the medium is in terms of onset potential. [58,77]Finally, the selectivity of NRR competing with the hydrogen evolution reaction also plays a vital role in the reactions.There is a region between the N 2 -NH 3 line and the H 2 O-H 2 line where theoretically only NRR occurs, completely avoiding HER. [58]However, the theoretical voltage applied to electrocatalytic decomposing water is smaller than decomposing nitrogen and the actual HER onset potential may be positively offset.Thus, HER is preferentially generated in thermodynamics, which seriously inhibits NRR performance.Most of the transition metal catalysts, such as Fe, Mo, Ru and Rh, show superior activity in the volcanic diagram, which can be said to have a positive promoting effect on NRR reaction.However, their adsorption capacity of H atom is not weaker than that of N atom, leading to pronounced competing HER.Some other transition metals, such as Sc, Y, Ti and Zr, have stronger binding with N adsorption atoms than H adsorption atoms, which seems to be dominated by NRR, but there is still some deviation in practical application. [78]Nowadays, several outstanding strategies have been proposed from the above three perspectives to improve the performance of NRR. [79,80]However, the performance achieved so far still falls far short of the goals set by the US Department of Energy. [81]Due to the above problems, in situ characterization techniques combined with electrochemical experiments are developed to design superior catalysts and understand the reaction process.The study of reactions is no longer limited to a particular state but is moving toward real-time observation, as described in the following sections.

Hydrogen Evolution Reaction
[84] It is considered as a perfect carrier to support the future global energy structure.Hydrogen energy can be used directly as a fuel gas, promising to replace fossil fuels.In addition, more energy is stored through the integration of renewable electricity into the energy system, which is driven by hydrogen energy.However, over 95% of current hydrogen production is based on the conversion of fossil fuels, which produces a large amount of CO 2 , extensively damaging the ecological environment, and is not conducive to the development of sustainable energy systems. [85,86]herefore, the development of new hydrogen generation technology is significant and meaningful.Water electrolysis, which accounts for <4% of hydrogen production, seems to be an affordable and green way to produce hydrogen. [87]At present, most water electrolysis reactions exist in the form of side reactions in the Chlor-alkali industry. [88]However, with the decrease in power generation cost of sustainable energy technology, the hydrogen evolution reaction, the primary water electrolysis reaction, will be downstream and become the mainstream method of hydrogen production in the future. [89,90]s early as 1789, [91] water electrolysis was first proposed and can be decomposed into two continuous proton-electron transfer reactions, and there is no side reaction in the HER occurring at electrolytic cells. [92,93] þ þ 2 e À !H 2 (15)   Thus, the research around it is pretty extensive.According to the media with different pH values, the HER mechanism can be mainly described as the following equation. [94]n acidic media, In neutral or alkaline media, where * represents the active site on the catalyst surface and *H represents the proton adsorbed at the active site.No matter in acidic or neutral and alkaline media, the first step of the HER process is always the adsorption and reduction of proton at the active site on the catalyst surface, namely the Volmer step (Equations 16 and 19). [95]Subsequently, there are two possibilities for the following H 2 evolution step, one through a second proton/electron transfer step, namely the Heyrovsky step (Equations 17 and 20), and the other through the recombination of two adsorptive protons, namely the Tafel step (Equations 18 and 21). [96,97]he HER process can be carried out through either the Volmer-Heyrovsky or Volmer-Tafel pathways, which are strongly dependent on the catalyst's intrinsic chemical and electronic properties. [98]The RDS of the reaction can be briefly determined by evaluating the Tafel slope obtained from the HER polarization curve, which has been explained in detail by Conway and Tilak. [99]The reaction steps show that the chemisorption and desorption of H atoms on the catalyst surface are competitive processes.The adsorption free energy of hydrogen (ΔG *H ) is a crucial parameter of HER kinetics, which has been associated with the experimental value i 0 for different catalysts and surfaces in the form of a "volcano curve". [100,101]Therefore, an excellent catalyst for HER needs to meet the following requirements.First, a suitable catalyst needs to form a strong chemical bond with the adsorbed *H to facilitate the first proton-electron transfer step.Second, it is necessary to ensure that the generated H 2 can be easily desorbed from the catalyst surface.If the ΔG *H of the catalyst is too low, the Tafel or Heyrovsky steps will be limited, and H 2 will be difficult to release.On the contrary, if the ΔG *H is too large, the Volmer step will slow down, and the formation Energy Environ.Mater.2023, 6, e12552 of hydrogen intermediates is untoward.According to Sabatier's principle, the reaction rate of the whole HER is maximum when ΔG *H is almost zero.Therefore, the ideal HER catalyst should have moderate hydrogen intermediate bonding energy (ΔG *H ≈ 0), and the ability to bind H 2 atoms is neither weak nor strong. [102,103]Many non-noble metals and their compounds have been proven to have extremely high *H adsorption strength.Besides, previously reported work derived the theory ζ from the Butler-Volmer equation, where ζ is the Tafel slope.When ζ ≈ 29 mV dec −1 , Volmer-Tafel pathway will take place, while when ζ ≈ 38 mV dec −1 , Volmer-Heyrovsky will be the mechanism of HER reaction. [104,105]However, the catalyst change during the HER process is still difficult to detect.The active site, bond formation, and rupture processes are still unclear.Therefore, in situ characterization experiments are particularly important for further exploration of HER mechanism and optimization of catalyst design.The following chapters will introduce some recent in situ work related to HER and look into the future development of HER.

In Situ X-Ray Spectroscopy
X-ray is a kind of electromagnetic wave with a short wavelength and high energy.Different phenomena such as scattering and absorption can be observed when the X-ray interacts with materials.X-ray analysis refers to the analysis methods that take X-ray as the radiation source.According to different phenomena, it can be divided into the following three types: X-ray absorption spectrum, X-ray photoelectron spectroscopy, and X-ray diffraction spectrum.All of them are useful for the quantitative or qualitative analysis of materials, especially catalysts for electrochemical reduction reactions including CO 2 RR, NRR, and HER.With the advent of the third generation of the high-energy synchrotron, [106] ultra-high brightness high-energy X-rays have brought great advantages for in situ studies of catalytic reactions.The following part introduces the principle and application of three different in situ X-ray analysis methods in detail.

Fundamentals of In Situ X-Ray Absorption Spectroscopy
X-ray absorption spectroscopy (XAS) has been widely used in gasinvolved electrochemical reactions to investigate the coordination structure of electrode materials.Typically, the X-ray beams are emitted by the light source and interact with the sample, which will be partially absorbed.By measuring the change curve of X-ray absorption with photon energy, XAS can provide basic information in the local environment according to the X-ray absorption coefficient as a function of incident X-ray. [107]As a precise characterization with element specificity, XAS is sensitive to extremely low concentrations (10-100 particles mol −1 ).Therefore, it can be used to analyze crystalline and amorphous nanostructures. [108]Besides, the dynamic change in electronic structure can also be detected.Thanks to these factors, the reaction mechanisms in the electrochemical process could be meticulously investigated.
During the process of XAS measurements, the core electrons will be excited by photons, whose energy is commonly ranged between 50 and 100 keV.Generally speaking, X-ray absorption spectroscopy that uses X-rays of energy between 50 and 2000 eV is called soft XAS (sXAS) and is a robust characterization to explore the structural properties of all the materials in a catalytic environment. [109,110]niversally, sXAS covers the K-edge of the light elements such as carbon, nitrogen, and oxygen and the L-edge of 3d transition metals. [111]esides, it can uncover important surface structure information such as oxidation state, spin state, orbital hybridization, and adsorption species due to its high sensitivity to the solid material surface.As some electrochemical processes are two-phase reactions, they mainly occur on the surface of the solid catalysts.Therefore, in situ sXAS plays an indispensable role in studying the atomic-scale catalytic mechanism.Unlike the sXAS, an X-ray with energy up to 5000 eV is called a hard XAS. [112]ue to the penetrating X-rays, hard XAS is frequently utilized in operando research, especially for three-phase reactions with the presence of liquid, gas, and solid. [113]Specifically, hard XAS is divided into X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). [114]XANES can provide the absorption spectrum range immediately before and after the absorption edge.Through understanding the density of states characterized by XANES, the valence states and orbital hybridization due to coordination at the absorption edge and pre-edge could be revealed. [115]As for the EXAFS, it covers the energy region above the absorption edge, where quantitative structural information, such as coordination number and coordination shell distance, can be obtained. [116][119] Thus, in situ XAS is convenient for detecting the catalyst structure and active sites during electrochemical processes.

Applications for CO 2 RR
Since in situ XAS characterization is able to detect oxidized states and investigate element sites in the crystal lattice and atomically dispersed structures, it has been widely used in CO 2 RR to monitor the stability of catalysts and identify active catalytic sites.For Cu-based catalysts, which have been commonly utilized in the CO 2 RR process, many in situ XAS characterizations have been carried out to investigate the chemical state transitions on the catalyst surface.Sargent and co-workers carried out in situ XAS on the Cu catalyst during CO 2 RR based on the change of Cu L 3 -edge, which was typically detected to explore the real-time valence changes of Cu as the voltage was applied. [120]The results have confirmed that the peak at 931 eV became weaker when the applied voltage went from high potential to 0.28 V versus reversible hydrogen electrode (RHE), which meant the disappearance of Cu 2+ .At the same time, a new strong peak appeared at 934 eV, which could be attributed to the generation of Cu + , and the transformation was completed within 5 min.The observation of in situ XAS demonstrated the transition from Cu 2+ to Cu + in the CO 2 RR process.As the potential became more negative (−1.87 V vs RHE), Cu + transited to Cu 0 completely.These findings proved the existence of Cu + at a more negative reduction potential, which contributed to the stability of ethylene intermediates (*OCCOH), illuminating the correlation between the selectivity of C 2 products and oxidation state of Cu catalysts.On this basis, Kauffman's team further explored Cu oxide changes by in situ XAS. [121]The results showed that when the applied voltage was −0.2 V versus RHE, the reduction reaction of Cu oxide began.Once the voltage became more negative (−0.6 V vs RHE), almost no oxidation state existed, meaning the Cu oxide was completely reduced (Figure 3a).
As confirmed by Percec's team, [122] the presence of Cu + realized simpler activation of CO 2 molecular and C-C bond coupling, which Energy Environ.Mater.2023, 6, e12552 6 of 40 was conducive to the generation of C 2 products with high added value.Therefore, it is significant to monitor the change in Cu valence state and maintain the presence of Cu + during the CO 2 RR process.Chen et al. [123] successfully confirmed the existence of stable chemical states of half-Cu 0 -and-half-Cu + in an oxide-derived copper electrocatalyst during the CO 2 RR process by time-resolved in situ XAS characterization, which was conducive to the selective generation of asymmetric C 2 product-C 2 H 5 OH (Figure 3b).Similarly, by studying the effect of Cu 2 O coating on silver nanowires, Chang et al. [124] have proven that the chemical state of the active Cu center is determined by the oxidation of Cu 0 in an aqueous solution through in situ XAS characterization.In addition, the presence of trace oxidizing species in the electrolyte enables a dynamic equilibrium between Cu 0 and Cu + to be achieved under mild conditions, which facilitates the selectivity of hydrocarbon/ alcohol products (Figure 3c).Moreover, with in situ XAS being used to monitor the changes of Cu + species and valence states during the CO 2 RR process under neutral conditions by Rodriguez's group, titanic acid, for instance, which presents no CO 2 RR activity, was found making the oxidized Cu + species stay longer in the reaction process compared with ordinary Cu catalyst. [125]It provided great suggestions for the follow-up study of catalysts with better CO 2 RR product activity and selectivity (Figure 3d).
In addition to the common Cu-based catalysts, the structure and composition of bimetallic catalysts for CO 2 RR can also be investigated by in situ XAS.For instance, Cu and Zn species are in the highest valence state in the Cu 50 Zn 50 catalyst.When CO 2 RR occurred, the  [121] Copyright 2019, The Royal Society of Chemistry.b) Time-resolved EXAFS spectra without phase-correction of CuO x using Redox Shuttle and chronoamperometry.Reprinted with permission from ref. Lin et al. [123] Copyright 2020, Nature Publishing Group.c) In situ XANES and EXAFS of the Ag@CuO x -32 sample.Reprinted with permission from ref. Chang et al. [124] Copyright 2019, American Chemical Society.d) Representative in situ XANES spectra of 10 wt % CuO@NTO and corresponding linear combination fit determination of Cu species composition.Reprinted with permission from ref.
Lawrence et al. [125] Copyright 2022, Diamond Light Source Ltd.Published by American Chemical Society.e) Cu and Zn K-edge XANES spectra for Cu 50 Zn 50 nanoparticles at different times.Reprinted with permission from ref. Jeon et al. [126] Copyright 2019, American Chemical Society.
Energy Environ.Mater.2023, 6, e12552 reduction of Cu oxide species took only a few minutes, far more than several hours for Zn oxide species.Fourier transform EXAFS spectra of Cu K-edge performed by Cuenya et al. [126] showed that the interatomic distance of Cu-Zn (2.58 AE 0.01 Å) was between copper foil (2.58 AE 0.01 Å) and copper-zinc foil (2.62 Å), indicating that Cu and Zn atoms alloy during the CO 2 RR process (Figure 3e).Such a study provided a strong insight into the selective conversion from CH 4 to CO on Cu-Zn alloy, which is caused by the evolution of the Cu-Zn interaction.

Applications for NRR
In NRR, another small molecule reaction with various catalysts, in situ XAS also has a wide range of applications.The observation of the adsorption and desorption of different species during the NRR reaction is helpful in understanding the intrinsic characteristics of catalysts.The FT-EXAFS spectrum is the most direct observation method to monitor the chemical bond expansion or contraction caused by species adsorption. [127]As reported by Tan et al., [128] Mo 2 CT x nanosheets modified by Ru single atom exhibited excellent NRR performance.The standard Ru K-edge XANES and FT-EXAFS spectra were obtained by in situ XAS characterization, which explicitly revealed this excellent NRR performance with an ammonia yield rate of 40.57μg h −1 mg −1 and Faradaic efficiency of 25.77%.Compared with that of the Ar-saturated K 2 SO 4 solution, the Ru K-edge shifted to higher energy, and the radial distance of the Ru-C(O) peak shifted from 1.58 to 1.51 Å when the open-circuit voltage was applied in N 2 -saturated K 2 SO 4 solution.This phenomenon can be attributed to the coordination of the Ru atom and N. When the electrochemical N 2 reduction potential was applied to −0.3 V versus RHE, the Ru K-edge shifted to lower energy.The corresponding oxidation state decreased to +3.15, and the radial distance of the central peak moved to a longer direction of 1.56 Å, which suggested that the electron density redistribution led to the distortion of Ru atoms (Figure 4a).
In addition to detecting atomic structure changes of catalysts, in situ XAS can also reveal the origin of the intrinsic activity of catalysts by monitoring the state of the active center under operating conditions during the NRR process, which is helpful to further understand the reaction mechanism in NRR reactions.In the same work mentioned above, the XANES of Ru single-atom catalysts exhibited a shift toward lower energy during the NRR process, and the oxidation state of Ru increased from +3.27 to +3.56 (Figure 4b). [128]This phenomenon could be attributed to the delocalization of unpaired electrons in the Ru 3d orbital and the charge transfer of the Ru atom to the N 2p orbital, forming a series *N adsorption species.This electron back-donation effectively weakened the N≡N bond, making N 2 more easily to be activated.A similar phenomenon also appeared in another n-Pd 3 Bi catalyst reported by Wang et al. [129] As the working bias potential increased from −0.05 V to −0.1 V versus RHE, the intensity of the white line peak of Bi L 3 -edge XANES spectrum increased significantly, suggesting that during the NRR reaction, the interaction between N species and Bi atoms led to electron transfer from Bi atom to N 2p orbital (Figure 4c).This means that the Bi sites can fully adsorb N 2 molecules, reduce the energy barrier of *N 2 , and promote N 2 adsorption and activation.At  [128] b) The oxidation state of Ru and radial distance of the main peak for SA Ru-Mo 2 CT x .Reprinted with permission from ref. Peng et al. [128] Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) Normalized in situ Bi L 3 -edge XANES spectra for np-Pd 3 Bi.Reprinted with permission from ref. Wang et al. [129] Copyright 2021, Wiley-VCH GmbH.d) Time-dependent XAFS results of VN catalysts at −0.2 V versus RHE and pre-edge area at different potentials.Reprinted with permission from ref. Yang et al. [130] Copyright 2018, American Chemical Society.
Energy Environ.Mater.2023, 6, e12552 the same time, it can effectively accelerate electron transfer, which greatly facilitates the NRR performance.
For better NRR performance, the stability of the catalyst is also a vital factor to be considered in the comprehensive design of the catalyst.In situ XAS can be used to investigate the active site of the catalyst and the cause of inactivation.Xu's group designed a VN nanoparticle catalyst and achieved excellent cycling performance. [130]VN x O y has been proven to be the best active phase in the NRR process, and its composition was VN 0.7 O 0.45 , while VN almost had no activity on NRR.In the FT-EXAFS spectrum of the catalyst, there were two peaks at 1.6 and 2.4 Å, corresponding to V-N and V-V bonds in cubic VN, respectively (Figure 4d).However, in the NRR process, the intensity of the edge front peak decreased with time, indicating that the active phase VN 0.7 O 0.45 gradually transformed, and the more negative the potential was, the faster the transformation rate.VN 0.7 O 0.45 was converted 35.7% in 2 h at a potential of −0.1 V versus RHE and 57.8% in 1 h at a potential of −0.2 V versus RHE.Furthermore, almost all of VN 0.7 O 0.45 was converted to VN under a more negative potential within an hour.This indicates the transition from active phase VN 0.7 O 0.45 to inactive phase VN, clearly providing a strong insight into the reason for the inactivation of VN catalysts.

Applications for HER
As for the HER, an electrochemical catalytic reaction with a long history, its reaction mechanism and catalyst design have been quite mature, among which the in situ XAS characterization has played a significant role. [131,132]At present, Pt/C catalyst has been considered the most commonly utilized catalyst for HER with the best performance.A great deal of in situ XAS research has been carried out on Pt or Pt-based catalysts to explore their active sites and dynamic structures. [133,134]Chen's group repeated 100 LSV scans of Pt single-atom catalysts in a potential window of −0.1 to 0.7 V versus RHE and conducted in situ XAS characterization. [135]It was found that after LSV scanning, the white line intensity of Pt L 3 -edge decreased slightly, which meant that Pt occupied a higher 5d orbit compared with the initial situation, and the valence state of Pt species was lower.Besides, Pt atoms had a significant charge transfer to the d-band state during the HER process.As exhibited by FT-EXAFS spectra, there was a dominant peak representing Pt-N/C at 1.5 Å.However, an apparent Pt-Pt scattering peak with increased scanning times indicated that Pt changed from a single-atom form to a small cluster form during the HER process (Figure 5a).Such structural reconstruction was owing to the weakening of the bond between isolated Pt atoms and N atoms in the continuous HER process, which was considered to be the reason for the significant increase in the activity of Pt single-atom catalyst for HER.According to research reported by Li's group, the extended Pt-O bond has also been proved to be the active site in the HER process which was conducive to accelerating the coupling of protons and electrons and promoting the release of H 2 . [136]n addition to Pt catalysts, in situ XAS studies have shown that many other elements, such as Fe, Ni, and Ru, exhibit excellent HER activity.In situ XAS is commonly used to investigate the changes in the active structure of these catalysts during hydrogen evolution, which is conducive to the design and optimization of a new generation of electrocatalysts. [108]For instance, as reported by Wu et al., [137] compared with the K-space value of Ru obtained in the ex-situ conditions, when the voltage was applied to −0.06 V versus RHE, the K-space value of Ru tended to decrease due to the gradually weaker scattering power of the Ru-O signal, revealing that hydroxyl or water molecules were adsorbent on the surface of the Ru site (Figure 5b).This work demonstrated that Ni-Ru-P interface sites played an active role in the HER process and participated in the catalytic process.Moreover, the phosphide phase [138][139][140] has been confirmed as the active phase for HER.The original cobalt phosphide would be converted to hydroxide in an alkaline solution, which impeded the formation of the active substance.XANES spectra performed by Chen's group showed that the K-side EXAFS peaks of Co and Fe did not fluctuate under a wide range of negative voltages after the Fe introduction, indicating that Fe doping could stabilize the phosphating phase and prevent it from converting into Co(OH) 2 , which was conducive to the stabilization of HER activity of the catalyst (Figure 5c). [141]This finding implies that the appropriate catalytic mechanism is related to the phase transition and crystal transition rate.
Due to the particularity of HER, electrolyte also has a specific influence on the activity of catalyst.Therefore, it is crucial to study the stability of catalysts in an impassable electrolyte.Pattengale et al. [142] reported that Ni SACs had apparent transformation behavior in acidic and alkaline electrolytes during the HER process.In an acidic electrolyte, the Ni K-edge in XANES was transferred to the lower energy direction either at an increased voltage or at operating voltage (−0.76 V vs RHE).The atomic structure of Ni remained unchanged, but the valence state changed from +2 to +1.In an alkaline electrolyte, a significant second-shell scattering caused by the Ni-Ni bond could be observed when the catalyst entered the electrolyte.The first-shell peak shifted to smaller positions due to the formation of Ni-O-Ni bonds by the coordination of Ni and O (Figure 5d).In the working state, the Ni coordination number decreases, and thus metallic Ni clusters form.Based on this result, different active sites in acidic and alkaline electrolytes were proposed, laying a foundation for future research.

Summary
All in all, in situ XAS should be a powerful tool to monitor the electronic information.Thereinto, the XANES spectrum could obviously reflect the chemical valence states and electronic structure information of the measured elements.Besides, through fitting the data from the EXAFS spectrum, some equally important information including the true spatial distribution, the bonding conditions, and the coordination environment of the atoms could be obtained.With these reliable advantages, in situ XAS technology has been gradually matured and widely exploited in gas-involved electrochemical reduction reactions.Nevertheless, it is not a panacea.Owing to the high sensitivity of XAS and the powerful photon energy required for operation, it cannot be used for the overall characterization but only for the analysis of average information at the local area.Besides, rigorous operation conditions are required during the in situ XAS performance, which makes component analysis of complex systems relatively difficult.Therefore, not only in situ XAS should be developed, but also other in situ characterization techniques should be continuously coupled for in-depth analysis and comprehensive understanding of complex systems.

Fundamentals of In Situ X-Ray Photoelectron Spectroscopy
Another powerful surface analysis technology, in situ XPS, can measure catalyst surface composition, chemical states, and Energy Environ.Mater.2023, 6, e12552 electronic structures with high surface sensitivity. [143]The operation of in situ XPS is based on the photoelectric effect.X-rays hit the catalyst's surface, which excites electrons and produces photoelectrons that can be emitted into the vacuum.Elemental and chemical states on the catalyst surface can be detected by quantifying the binding energy of these electrons, which is related to the kinetic energy of the electrons emitted.However, the mean free path of these photoelectrons is concise, and thus the in situ XPS is used chiefly to detect properties at a few nanometers deep on the surface of the catalyst. [144]evertheless, as the X-ray penetration length is only 10 mm in the ambient atmosphere, the spectroscopic experiment usually takes place in an ultra-high vacuum (UHV) chamber, which dramatically limits in situ application in ambient conditions, especially with the presence of gases and liquids. [53]With the advent of the third generation of synchrotron light sources, two kinds of in situ measurement devices for XPS under ambient pressures (APXPS) have been developed. [145,146]The first is a differential pumped analyzer, and the second is a membrane-based device.The first device is often employed to investigate the electrochemical process at the solid-gas interface.This device requires the sample to be exposed to extremely high pressure and the hemispherical analyzer to maintain an extremely high vacuum during several differential pumping conditions.The photoelectrons will be collected and analyzed by a narrow cone placed  [135] Copyright 2021, Wiley-VCH GmbH.b) WT of the Ru K-edge of the EXAFS spectra of Ex situ and −0.06 V versus RHE for Ru Sas-Ni 2 P. Reprinted with permission from ref. Wu et al. [137] Copyright 2020, Elsevier Ltd.All rights reserved.c) Co and Fe K-edge X-ray absorption near edge structure for cobalt phosphide.Reprinted with permission from ref. Hung et al. [141] Copyright 2019, American Chemical Society.d) In situ XANES spectra and Fourier-transformed R-space spectra and fits for Ni@1T-MoS 2 .Reprinted with permission from ref. Pattengale et al. [142] Copyright 2020, Nature Publishing Group.
Energy Environ.Mater.2023, 6, e12552 extremely close to the surface of the sample.On the other hand, the second device is often used to detect electrochemical processes at the solid-liquid interface.The sample should be placed in an electrolytic cell with a membrane tightly covered, where the isolation film between the external high-pressure environment and the internal environment can support electron transfer, but not gas/liquid/solid molecular infiltration.Typically, a Si 3 N 4 film which is almost 10-15 nm thick is commonly used. [147]

Applications for CO 2 RR
Different from the focus of in situ XAS characterization, surfacesensitive (quasi) in situ XPS technology mainly aims to explore the valence changes of elements in chemical reactions and the electron transfer process at the active sites of catalysts.It has been widely used in electrochemical reduction, providing in-depth insights into the design of high-performance catalysts.Specifically, the most common use of Mistry et al. [148] Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.b) Quasi in situ Sn MNN Auger spectra of SnO x /AgO x and SnO x /Ag.Reprinted with permission from ref. Choi et al. [149] Copyright 2019, American Chemical Society.c) Quasi in situ Cu LMM XPS spectra of Ag-and Ptsupported Cu dendrites before and after 1 h of CO 2 RR.Reprinted with permission from ref. Scholten et al. [151] Copyright 2019, American Chemical Society.Eilert et al. [154] Copyright 2017, American Chemical Society.f) Quasi in situ Br 3p and I 3d XPS spectra of the Cu_Br and Cu_I measured before and after CO 2 RR.Reprinted with permission from ref. Gao et al. [155] Copyright 2019, The Authors.Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Energy Environ.Mater.2023, 6, e12552 in situ XPS is to detect state changes at a depth of about 5 nm on the surface of catalysts.Cuenya et al. [148] treated silver samples with O 2plasma and obtained a silver oxide catalyst with a binding energy of 367.5 eV.Quasi in situ XPS characterization was used to detect surface state changes.At the beginning of the CO 2 RR reaction, the oxide could still be detected on the surface, but it completely disappeared within 3 min, which is associated with the reduction of Ag oxides, indicating that Ag maintained as the active sites during the subsequent CO 2 RR process (Figure 6a).Based on this study, they further electrodeposited tin onto the O 2 plasma-treated silver surface and obtained SnO x /AgO x catalysts with high C 1 product selectivity. [149]Surprisingly, Quasi in situ XPS characterization showed a gradual decline in SnO x species over time.However, stable Sn δ+ /Sn species could be observed on the highly roughened surface of the catalyst either before the CO 2 RR process or after 20 h of the reaction, which helped enhance the activity and stable selectivity of CO/formate (HCOO − ) (Figure 6b).Such a quasi in situ XPS study revealed the influence of the composition or chemical state of the catalyst surface on the activity and selectivity of the CO 2 RR process.
Moreover, the chemical state transition of Cu oxide catalysts surface can also be investigated by quasi in situ XPS characterization.Typically, a complete reduction of Cu 2 O species occurs in the CO 2 RR process.The selectivity of copper oxide in the CO 2 RR process to ethylene and alcohol is related to its subsurface oxidation state as reported by Schmidt, [150] which is subsequently investigated with emphasis.For instance, Scholten et al. [151] realized the coexistence of Cu and Cu oxides within the copper dendrites grown on Ag and Pt foils through electrodeposition.After O 2 -plasma treatment, Cu dendrites were converted to CuO.However, when a potential of −0.9 V versus RHE was applied, the reduction of Cu x O species was observed in the quasi in situ XPS, proving that the Cu x O species lacked stability (Figure 6c).Gong's group introduced metallic Cu nanoparticles (NPs) on the Cu 2 O thin film to improve the selectivity of the catalyst. [152]A hydroxyl peak formed by adsorption of *H on O atoms was observed at 531.4 eV.With the increase in annealing temperature, the hydroxyl peak decreased gradually.However, the hydroxyl signal intensity of the Cu/ Cu 2 O sample decreased much more slowly than that of pure Cu 2 O, indicating a stronger *H binding energy and a weaker *CO binding energy, which is conducive to regulate the product distribution (Figure 6d).Although the copper oxide electrocatalyst is reduced in the CO 2 RR process, the residual oxygen still plays an important role and can be examined by in situ XPS. [153]Nilsson's group observed the presence of residual subsurface oxygen through in situ APXPS characterization. [154]The results showed a small peak at 531.7 eV observed in the spectroscopy, representing the residual oxygen, which was replaced by two components, CuCO 3 /Cu(OH) 2 and Cu 2 O after oxidation treatment, and the intensity of the peak increased significantly (Figure 6e).Combined with the quasi in situ electron energy-loss spectroscopy, the binding energy of CO was confirmed to be enhanced by the reduction of σ − repulsion by oxygen, promoting the formation of C-C bonds.Similar results were also confirmed by Cuenya's group. [155]After the CO 2 RR process, most Cu + and Cu 2+ species were reduced to metallic Cu, but a small amount of halides still existed on the surface of the Cu_CO 3 sample.However, considerable amounts of halides were observed on the Cu_I surface before and after the reaction, indicating the presence of residual subsurface oxygen and Cu + species in the halogen-derived Cu catalyst (Figure 6f).Due to the stability of the subsurface oxygen as well as Cu + species and adsorbed halides on the surface, a high C 2 selectivity of the Cu_I can be achieved.
In addition to conventional Cu-based catalysts, Kim et al. [156] successfully used in situ XPS to directly monitor the formation of dissociated *CO separated from chemisorbed CO 2 on Rh surfaces in a timeresolved manner.This discovery provided strong evidence for the rupture of *O-CO chemical bonds and formation of ordered intermediate structures on the atom-flat Rh (111) surface at room temperature, which is helpful for the rational design of catalysts to improve CO 2 RR in the wide application of frontier science of multiphase catalysis.

Applications for NRR
In situ XPS technology also has many applications in the field of NRR.Kelber et al. [157] designed a series of VN films deposited by plasma sputtering.When O 2 -plasma oxidized the film, a peak with non-lattice N 1 s characteristics was observed at 396.5 eV by in situ XPS characterization, representing V≡N, which is considered as an essential intermediate in the NRR process but not observed during thermal oxidation.When the film was treated with NH 3 plasma, the binding energy was close to 400.5 eV, and this peak represented the V-NH 2 sites.When O 2 + NH 3 plasma was used, both N 1 s characteristics representing V≡N and V-NH 2 could be observed in the in situ XPS.When annealed to less than 1000 K in UHV, the plasma-induced N 1 s spectrum was reversed.These results provided practical insights into important applications of plasma-modified catalyst surfaces (Figure 7a).In addition to the promoting effect of oxygen vacancy on NRR catalyst, cationic vacancy activation strategies such as Fe vacancy were proposed by Li et al. [158] and verified by in situ XPS characterization.As the NRR process progressed, a slightly negative shift in the Fe 2p XPS spectrum indicated that more low-valence Fe atoms were produced.Besides, the proportion of low coordination O atoms decreased, and the low-coordination-number Fe atoms closest to the Fe-O shell decreased slightly and accordingly.Moreover, the binding energy of the C element shifted to a lower value with the increase of time, indicating the electron-rich state of graphdiyne heterostructure (GDY) and the strong electron transferability of the electrocatalyst in the NRR process (Figure 7b).Such an anodic vacancy activation strategy provides a farsighted vision for designing novel and efficient electrocatalysts, which can be widely used in the field of electrocatalysis.
Moreover, a whole new work recently reported by Chu's group has delved into the excellent NRR performance and reaction mechanism of Wse 2-x in the water-in-salt electrolyte (WISE) by in situ XPS. [159]When the applied potential was increased from the open circuit potential to −0.5 V versus RHE, a shift in the W 4 f spectra of Wse 2-x toward higher binding energies was observed in WISE, while there was almost no change in dilute electrolytes (DEs) (Figure 7c).This result implied that WIES could promote the W-W-W active sites of the catalyst and back donate numerous electrons to the antibonding orbitals of the adsorbed nitrogen molecules, effectively facilitating the N 2 activation process.

Applications for HER
Just as functioning in the field of CO 2 RR and NRR, in situ XPS also plays a significant role in the HER process.As for the HER, amorphous Ni (OH) is more energetically favorable to HER than amorphous NiO.The Ni(OH) 2 crystal nucleus plays a vital role in improving the current density distribution.Thus, Xu et al. designed a crystal-amorphous Ni-Ni (OH) 2 core-shell assembly nanosheet (c-Ni@a-Ni(OH) 2 ) and observed a weak shoulder peak representing metallic Ni at 852.7 eV by Ar + Energy Environ.Mater.2023, 6, e12552 etching in situ XPS technology (Figure 8a).The results verified the coreshell structure of c-Ni@a-Ni(OH) 2 and accounted for highly efficient HER activity and durability in 1.0 M KOH. [160]Liu's group designed hydroxide-mediated Ni 4 Mo nanoparticles modified by FeO x and fixed on MoO 2 nanosheets (h-NiMoFe), which showed excellent alkaline HER performance at high current density. [161]In situ XPS showed that nickel existed in a metallic state and was modified by hydroxide during the HER process (Figure 8b).The intense interaction between Ni and Mo/Fe changed the local electronic structure of Ni, and there was more hydroxide on the Ni surface, which was beneficial to the HER process.Besides, in the work reported by Hayden et al., in situ spot XPS technique was used to study the proportions of two metal components in PdAu alloys. [162]Solid solution alloys were produced within the complete compositive range.In situ XPS shows that the surface composition  [158] Copyright 2021, The Authors.Advanced Science published by Wiley-VCH GmbH.c) In situ W 4f XPS spectra and corresponding contour maps of Wse 2-x in DEs and WISEs.Reprinted with permission from ref. Shen et al. [159] Copyright 2022, American Chemical Society.
Energy Environ.Mater.2023, 6, e12552 of these unannealed alloys is the same as the matrix, contrary to the observed and expected behavior of annealed alloys.Moreover, in situ XPS found that the reduction of Au concentration resulted in an increase in Au (4F 7/2 ) binding energy from the volume value, providing evidence for electronic interactions along with the compositional dependence of the HER Between alloy components in PdAu alloys (Figure 8c).

Summary
In summary, thanks to the in situ XPS technology, changes in valence states and coordination environments of specific elements can be accurately analyzed in real-time, which significantly contributes to the design of new effective electrocatalysts.However, the in situ XPS technology has very high environmental requirements and sometimes cannot meet the extremely high vacuum and experimental conditions simultaneously, which is a major obstacle to the expansion of its application.At the same time, due to the limited depth of detection, this surface-sensitive characterization technique cannot detect information inside bulk catalysts.Therefore, the future widespread application of in situ XPS is still full of challenges.

Fundamentals of In Situ X-Ray Diffraction
X-ray diffraction (XRD) analysis is also an essential method of X-ray characterizations in electrochemical reduction reactions, which is based on the Bragg equation: [163,164] 2dsinθ ¼ kλ (22)   where d is the crystal plane spacing, k is the reflection series, θ is a grazing angle, and λ is the wavelength of X-rays.The crystal is composed of a regular arrangement of atoms.When a monochromatic X-ray incident into the crystal, the distance between the atoms is in the same order of magnitude as the wavelength of the incident X-ray, and thus the X-rays scattered by different atoms interfere with each other will result in strong X-ray diffraction in some directions.
The orientation and intensity of diffraction lines in space are closely related to the crystal structure. [165]Therefore, XRD can quantitatively and qualitatively analyze the average grain size, crystallinity, strain, and crystal defects of catalysts in electrochemical reduction reactions.
For catalysts that function in electrochemical reduction reactions, the crystal structure constantly evolves as the reaction progresses. [166]herefore, in situ XRD technology was proposed to investigate the real-time change of crystal structure during the reaction processes. [167,168]According to the different X-ray diffraction light sources, in situ XRD mainly has two modes of operation: transmission mode and reflection mode.In transmission mode, The X-ray enters from one end of the electrolytic cell and acts on the surface of the catalyst, and the diffraction X-ray appears from the other side and is received by the detector to get the results.In this mode, a synchrotron radiation light source with good monochromaticity is generally utilized as the diffraction source to realize fast measurement time and accurate measurement results. [169]The reflection mode is that the X-ray enters from the electrolytic cell window and acts on the catalyst, and the diffraction X-ray comes out from the same window and is accepted by the detector.The advantages of this mode are that the device is simple and easy to operate, and the common X-ray diffractometer can be realized after modification. [170]

Applications for CO 2 RR
Generally speaking, the type and position of atoms will determine the diffraction intensity of in situ XRD, and the distribution of unit cells determines its diffraction distribution.The in situ XRD patterns obtained from real-time measurements can reveal the changes in catalysts during the reaction process and further analyze the stability and phase transformation of catalysts.As early as 2017, in situ XRD was used in CO 2 RR to detect the transformation of catalyst composition during the reaction by Sheng et al. [171] DFT calculation showed that the generation of β-phase Pd hydride (PdH) significantly reduced the binding energy of adsorbed *CO and *H, which can be regarded as reaction descriptors to adjust the relative adsorption intensity of CO and H, regulating the selectivity of CO and H 2 in CO 2 RR process simultaneously.To confirm the above result, in situ XRD characterization was carried out during the LSV  [160] Copyright 2020, The Royal Society of Chemistry.b) Quasi in situ Ni 3s XPS spectrum of h-NiMoFe and relative percentages of surface Ni species of Ni, NiMo, and h-NiMoFe.Reprinted with permission from ref. Luo et al. [161] Copyright 2021, The Royal Society of Chemistry.c) Au (4f 7/2 ) (black diamonds) and Pd (3d 3/ test on the Pd/C catalyst.The peak representing face-centered cubic shifted to small angles due to lattice expansion in PdH, which proved that β-PdH was generated in the CO 2 RR process (Figure 9a).Therefore, combined with in situ XRD, the reaction mechanism of promoting CO 2 RR to produce high purity syngas through Pd catalyst was revealed.Similarly, the catalytic performance of Pd loaded on transition metal nitride (TMC) for CO 2 RR was also well illustrated.In situ XRD patterns performed by Chen's group showed that the Pd peak of Pd/TaC moved to lower angles during the CO 2 RR process (Figure 9b). [172]In contrast, the peak representing TaC did not have a peak shift phenomenon, indicating the formation of PdH and the great stability of TaC support.Compared with conventional PdH/C, TaC as the substrate could promote the formation of *HOCO on the carrier PdH in the CO 2 RR process by adjusting the adsorption of *HOCO, thus promoting the CO 2 RR kinetics and activity.

Applications for NRR
Likewise, in the NRR process, in situ XRD also plays an essential role in real-time monitoring of the phase transition of catalysts and investigating specific reaction paths.Zou and his co-workers reported on a method to accelerate nitrogen fixation through gold electrocatalyzing reduction of lithium. [173]In situ XRD was carried out to explore the cycling process of lithium intermediates in the NRR process.The decrease in the concentration of Li + and the appearance of Li 3 N peaks could be observed from the in situ XRD patterns of Au/CP.However, the peak of Li 3 N was not observed in the in situ XRD spectrum of CP, which meant that Au could better promote the crystallization and product of lithium metal (Figure 10a).With the addition of the proton source, the lithium metal and Li 3 N signals weakened rapidly, which indicated the formation of ammonia.Meanwhile, Li 3 N and lithium metal were transformed into Li + again.Therefore, in situ XRD proved that the lithium-ion reduction step was the crucial step of the reaction.Moreover, it also provided vital evidence to prove that Au, as a model electrocatalyst for non-aqueous NRR, dramatically improves the adsorption energy of Li and electron transfer efficiency.
In addition, in situ XRD can also be used to detect the structure variations of catalysts during the NRR process.As previously reported by our group, the crystallinity change of COF was investigated by in situ XRD.Obvious characteristic peaks could be detected in the pristine electrode, while when applied with a negative potential, all signals weakened gradually and completely disappeared at about 12 min. [174]However, there was no change in the peaks of COF when tested under Ar atmosphere, clearly demonstrating that COF tended to transform from crystalline to amorphous phase during the NRR process.Such a structural transformation that occurred in N 2 atmosphere enables COF to exhibit a superior NRR performance.

Applications for HER
In situ XRD also has many applications in the HER process, which is often used to explore the stability of electrocatalysts in the synthesis or reaction process.The HER activity of the core-shell, hollow-structured iridium-nickel nitride nanoparticles has been demonstrated to be Pt/C equivalent The formation of this core-shell structure during annealing under NH 3 was well described by synchronous accelerated in situ time-resolved XRD characterization as reported by Adzic et al. [175] The three prominent peaks became more intense as the temperature rose.The appearance and enhancement of a significant new peak were observed between Ir (220) and Ni ( 220) reflections (Figure 11a).With the increase in annealing temperature and the change of atmosphere, the peak position shifted significantly to a higher angle, indicating the introduction of nitrogen and confirming the formation of the Ni 4 N core.The formation of such a Ni 4 N core can increase the HER activity of Ir shell to a level comparable to Pt/C.Similarly, Zhou and co-workers exploited in situ high-temperature XRD techniques to investigate phase transitions during the reduction of NiMoO 4 microsphere which act as a rapid Tafel-step-decided electrocatalyst toward HER. [176]Diffraction peaks representing (312), (132), and (152) crystal planes of NiMoO 4 were observed at 33.6°, 42.6°, and 59.1°, respectively, when the sintering temperature was lower than 375 °C.Once the temperature rose above 375 °C, new diffraction peaks appeared at 42.6°and 65.8°, indicating that the (033) and (352) crystal planes of MoNi preferred to grow, among which the (352) plane evolved Sheng et al. [171] Copyright 2017, The Royal Society of Chemistry.b) In situ XRD patterns of Pd/TaC during CO 2 RR test.Reprinted with permission from ref. Wang et al. [172] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
gradually from the (152) plane of NiMoO 4 .When the temperature was further increased to 600 °C, a new diffraction peak representing the (231) crystal plane of MoNi appeared at 36.8°(Figure 11b).These results showed that the (312) phase of NiMoO 4 gradually transformed into the (033) crystal plane of MoNi with increasing temperature, forming a crystal (231) plane at higher annealing temperature, and also proved that MoNi was a stable thermal phase.Such NiMoO 4 microspheres possessed excellent HER activity in 1.0 M KOH aqueous solution, with an initial potential of −7 mV versus RHE and a Tafel slope of 36.6 mV dec −1 .Moreover, as a most popular electrocatalyst, highentropy alloy (HEA), was also investigated through in situ XRD technology as reported by Zhu et al. [177] They synthesized single-phase FeCoNiXRu solid solution HEA nanoparticles (X = Cr, Mn, and Cu) in carbon nanofibers (FeCoNiXRu/CNFs) through a polymer nanofiber reactor strategy.In situ XRD results indicated the transformation from a multiphase to a single-phase HEA and revealed a thermodynamically driven phase transition.The as-formed electrocatalysts achieved low overpotentials of 71 mV to drive a current density of 100 mA cm −2 for the HER, which only required only 1.65 V to realize overall water splitting with a remarkable stability.

Summary
Unlike the in situ XAS and XPS, which focuses on valence and electronic structure, in situ XRD pays more attention to detecting catalyst phase transition and crystal structure changes during reaction processes.This focus is conducive to exploring the generation and phase transformation of intermediates in the reaction process and further understanding the specific path in the reaction process.In addition, some self-properties of catalysts can also be evaluated by in situ XRD.However, it is worth noting that in situ XRD has low spatial resolution and can only be used to analyze crystal samples.Thus, using in situ XRD alone cannot obtain all helpful information.

Discussion
In general, in situ X-ray spectroscopy should be practical and widespread in situ technology.A great deal of useful information can be obtained from different applications of X-rays.In situ XAS could detect vital electronic information including electronic structure, chemical valence states, the true spatial distribution, the bonding conditions, and the coordination environment of the atoms, which are of great significance to understand the intrinsic properties of electrocatalysts.However, due to the limitations of various harsh conditions, in situ XAS cannot achieve ideal results in the face of complex reaction systems.In situ XPS, another application of X-ray could determine surface compositions, oxidation states, and electronic structures with high surface sensitivity, which is typically employed to identify the local chemical environment and electronic structure.However, the penetration length of X-rays in ambient atmosphere is too short to investigate the internal changes of bulk electrocatalysts.The high-vacuum degree also inhibits the atmospheric pressure used, especially in the presence of gases and liquids.As for in situ XRD, it typically focuses on detecting catalyst phase transition and crystal structure changes during reaction processes, which should be an outstanding tool for monitoring the synthetic process of catalysts.Nevertheless, its spatial resolution is extremely low and can only be used to analyze samples with structure.Copyright 2020, Wiley-VCH GmbH.b) In situ XRD contour plots of the transformation of COF from crystalline to amorphous phase.Reprinted with permission from ref. Liu et al. [174] Copyright 2019, Nature Publishing Group.

In Situ Vibrational Spectrum Spectroscopy
[180] It can be divided into infrared and Raman spectra and is a non-destructive, non-contact, and non-vacuum analysis technology. [181,182]The mechanism complexity and product diversity of electrochemical reduction reactions such as CO 2 RR, NRR, and HER require an accurate and practical characterization, and thus in situ vibration spectroscopy technology comes into being.The principles and applications of these two in situ vibration spectroscopy techniques are introduced in detail in the following chapters.

Fundamentals of In Situ Infrared Spectroscopy
For the infrared spectrum, when the infrared beam irradiates the sample, the molecule will selectively absorb some infrared beam whose wavelength frequency is the same as the vibration frequency or rotation frequency of groups inside the molecule, and the vibration energy level and rotation energy level of the molecule will have corresponding transitions. [183]In essence, infrared spectroscopy is an analytical method to determine the molecular structure of substances and identify compounds based on the relative vibration and molecular rotation of atoms in molecules.Through exciting the vibrations and rotations of bonds in fragmented molecules, functional groups, and radicals, the dipole moment changes can be detected in the infrared range (143 00-14 320 cm −1 ). [184]Compared with the other in situ technologies, in situ FTIR spectrum focuses more on the detection of catalytic reaction intermediates and has the advantages of high sensitivity and fast characterization, further revealing details about the catalytic mechanism. [185,186][189] It is worth noting that water and ionic liquids have prominent infrared absorption peaks, which can easily interfere with the final test results in liquid systems.Therefore, to shorten the infrared path length and reduce the interference of external factors, IRAS and ATR are more suitable for in situ infrared characterizations.There is an infrared-transparent window in IRAS mode, and the working electrode is placed 1-10 μm away from the window.The infrared beam is measured by touching the electrode surface through this transparent window.Although the electrolyte is still present inside the electrode, the distance from the infrared ray to the electrode surface is extremely short.It just attenuates a little, which minimizes the interference of the electrolyte on the test.Likewise,  [175] Copyright 2014, The Royal Society of Chemistry.b) The image plot, the corresponding diffraction patterns, and selected 2θ region plot of the in situ HT-XRD results.Reprinted with permission from ref. Yang et al. [176] Copyright 2019, Elsevier B.V. All rights reserved.
Energy Environ.Mater.2023, 6, e12552 electrodes are typically prepared as thin films in ATR mode and deposited on a transparent prism.In this case, the incidence angle of the infrared beam is greater than the critical angle, and the infrared ray is fully reflected.The infrared ray passes through the 1-2 μm electrolyte by evanescent wave, acting on the molecular groups adsorbed on the catalyst surface. [190]ince the absorption intensity is proportional to the amplitude of the electric field and the dipole moment of the vibration, a surfaceenhanced infrared absorption spectrum (SEIRAS) was designed to optimize the in situ infrared spectra. [186]The technique functions by depositing thin layers of metal such as gold, silver, and copper on the crystal plane to increase oscillating electrical resonance and thus enhance the infrared absorption signal. [191]ATR-SEIRAS, the advanced infrared technology obtained by combining ATR and SEIRAS, has been proven to have extremely high time resolution and sensitivity.Besides, this technique can effectively reduce the attenuation of infrared rays and interference from electrolytes and is widely used in the electrochemical reduction reactions. [183,192]

Applications for CO 2 RR
In electrochemical reduction reactions, many pieces of research are based on in situ infrared spectroscopy.The changes in reaction products and intermediates are detected by in situ infrared spectroscopy, and then the selectivity of catalysts to different products and specific reaction mechanisms can be investigated.Dunwell and his team reported a work in which they studied the CO 2 RR process on the Au surface using the SEIRAS technique, specifically exploring the role of bicarbonate in the overall reaction and suggesting that the rapid equilibrium of bicarbonate species and dissolved CO 2 is beneficial for the CO 2 RR process. [193]heng et al. [194] improved the infrared spectroscopy by using a subtractively normalized interfacial FTIR spectroscopy (SNIFTIRS) method to more intuitively trace the intermediates, such as *COO − and *COOH, resulting from the CO 2 RR on the surface of gold catalysts and electrolytes.It could be observed by in situ infrared spectroscopy that a small water peak was located at 1645 cm −1 , but its intensity hardly changed at low potential ranges, suggesting that the hydrogen evolution potential of Au is high.When the potential was applied to −0.5 V versus RHE, two peaks representing asymmetric and symmetric stretching of *COO − on gold nanoparticles appeared at 1560 and 1410 cm −1, respectively (Figure 12a).When the potential was applied to −0.6 V versus RHE, the water peak became positive, indicating that the consumption rate of water exceeded the formation rate of water, and hydrogen evolution reaction began to dominate.Moreover, the H 2 O peak of the solvent at high electrode potential overlaps the *COO − peak generated by CO 2 RR in the infrared spectrum and is difficult to distinguish.However, the peak position of D 2 O (1200 cm −1 ) was different from that of H 2 O (1645 cm −1 ) (Figure 12b).Thus, the in situ infrared spectrum test was also carried out in the electrolyte configured with D 2 O, and the information obtained was consistent with the above information.Based on in situ FTIR and DFT calculations, at the potential of −0.5 V versus RHE, the CO 2 RR process on the gold surface tends to produce formate through the formation of stable *COO − intermediates rather than *OCHO intermediates.Besides, when the potential is higher, as shown in the infrared spectrum, the peak intensity representing *COO − increases significantly, while the peak intensity representing formic acid almost remains unchanged, which means that more *COO − is directly converted to CO along with a small amount of formate under this condition.This study provided direct evidence for the reduction mechanism of CO 2 on gold nanoparticles through in situ IR characterization, which is beneficial to the design and optimization of subsequent catalysts.
As a further complement, Yang et al. [195] analyzed the reaction products and intermediates of CO 2 RR on the surfaces of Au, Pt, and Cu by in situ SEIRAS.They explored the affinity of these three catalysts for carbon and oxygen bond adsorbents.On the surface of Pt, CO ad obtained from COOH ad was very stable and eventually poisoned the catalyst, which means the Pt could not promote the CO 2 RR process and mainly contributed to the hydrogen evolution reaction.For gold, due to the weak Au-CO interaction, CO ad exited the surface as CO gas; thus, the formation of CO and formate could be stably observed in the in situ SEIRAs spectrum, accompanied by hydrogen generated by the hydrogen evolution reaction.Still, no other C 1 or C 2 products appear.When it comes to Cu, the configuration of the initial adsorbent, such as carbon bonds (HCO ad ) and oxygen bonds (CO 3, ad ), determined the final product of CO 2 RR.The presence of O-bound CO 3, ad , OCH 3 , ad , and OC 2 H 5, ad , as well as C-bound (H)CO ad in the in situ SEIRAS spectra confirmed the formation of CO, formate, methane, and ethylene on the surface of the Cu catalyst (Figure 12c).These in situ infrared results indicate that the selectivity of CO 2 RR can be optimized in terms of metal-carbon and metal-oxygen affinity, and that the final product is determined by the configuration of the initial intermediate, which may be different for each catalyst.Similarly, Zhang's group studied C-C coupling in the reduction of deoxy carbon to ethylene and ethanol. [196]hen F-modified Cu catalysts were used for CO 2 RR, infrared peaks were observed at 2117, 1972, and 1920 cm −1 , indicating the presence of CO intermediates on the Cu surface (Figure 12d).In addition, CHO species, which are key intermediates for C-C coupling, were observed when the potential was applied to −0.4 V. providing an experimental basis for the hydrogen-assisted C-C coupling mechanism of C 2+ products generated by CO 2 RR electrocatalysis on halogen modified copper catalyst.Through in situ infrared spectroscopy, various products and corresponding intermediate groups in the CO 2 RR process were well monitored, revealing the necessary conditions for the generation of various necessary intermediate groups and providing ideas for the control of product selectivity.

Applications for NRR
Different from CO 2 RR, NRR normally produces single product, but it still exists in two ways: the dissociative pathway and the associative pathway, which can be understood in depth by in situ IR characterization.The presence of various groups in the reaction process detected by in situ infrared spectroscopy can assist in determining the specific reaction pathway of NRR.For example, in the previous work of Sun et al., [197] in situ infrared spectroscopy was used to detect the intermediates generated in the NRR process using carbon shells coated with oxygen-doped molybdenum carbide nanoparticles as catalysts.Three weak bands at 3230, 1505, and 1330 cm −1 were observed in in situ IR spectra, caused by N-H stretching, H-N-H bending, and -NH 2 wagging, respectively, indicating that the N intermediate adsorbed on the surface of the catalyst was hydrogenated.In addition, N-N stretching bands were observed at 1120 cm −1 , meaning that the N≡N triple bond was broken, and -N 2 H y (1 ≤ y ≤ 4) species were formed on the catalyst surface (Figure 13a).These shreds of evidence demonstrated that the two Energy Environ.Mater.2023, 6, e12552 nitrogen atoms remained attached to each other on the catalyst's surface during hydrogenation, which explains that the reaction follows the associative pathway.Similarly, Yuan [198] and Wang's Team [199] also made full use of in situ infrared spectroscopy in their study to detect the intermediates of NRR reaction on phosphorus co-doped porous carbon and N-doped porous carbon with secondary heteroatoms, respectively.The NRR process was carried out mainly according to the dissociation path (Figure 13b).
Moreover, the competition between HER and NRR can be understood by observing the changes of some key groups in the in situ infrared spectra, and the advantages and the reaction performance of NRR can be further qualitatively investigated.As previously reported by our group, [200] in situ FTIR was employed to detect reaction intermediates at different potentials in the N 2 atmosphere.A small peak representing N-H stretching at almost 3700 cm −1 was observed in the in situ FTIR spectra of boron-doped carbon paper (BCP), and the change of this peak can be analyzed to explore the activity of NRR.As the potential gradually became negative, the intensity of the small peak gradually increased.Notably, the small peak disappeared with the potential applied to −0.5 V versus RHE, indicating that the HER process occupies the dominant position.However, as for the boron-doped carbon paper located with an exfoliated COF layer (BCP@ECOF), the N-H peak detected in the infrared spectrum was stronger.It did not disappear with negative potential, indicating that ECOF has a good inhibition effect on HER reaction and can significantly promote NRR activity (Figure 13c).

Applications for HER
As the most basic and important electrochemical reduction reaction, HER has no complex intermediates and products.However, as introduced in chapter 2.3, there are two possible hydrogen production pathways: Volmer-Tafel and Volmer-Heyrovsky.Overpotential deposited hydrogen (H opd , also known as terminal H) and underpotential deposited hydrogen (H upd ) are key intermediates of HER, which have been controversial for a long time. [201]Therefore, in situ infrared spectroscopy is often used to detect the formation of the two intermediates and further understand the dynamic electrocatalytic reaction process from the molecular level.As early as 1988, an absorption band was observed at 2090 cm −1 in operando SEIRAS reported by Bewick and Nichols, which was the first time H opd was observed during the HER process on the Pt electrode. [202]Subsequently, in 2007, Osawa et al. [203] also reported the existence of H opd in the HER process using the ATR-SERIAS device and further established a quantitative relationship between H opd band strength and HER dynamics.When 0.1 V versus RHE was applied to the polycrystalline Pt electrode, the characteristic vibration mode of the H opd was observed at 2080-2095 cm −1 , which was consistent with the previous report.The more negative the potential was, the stronger the band was.When the potential was reduced to −60 mV versus RHE, the bandwidth reached the maximum.Moreover, SEIRAS is 28 times more sensitive to p-polarization than IRAS and 8.5 times more sensitive to unpolarized radiation than IRAS (Figure 14a).Thanks to this advantage of high sensitivity, band intensity data can be quantitatively compared with HER dynamics (Tafel diagram).Analysis showed that the H opd was the intermediate of HER reaction, and HER was generated by the Volmer-Tafel mechanism, in which the Tafel step is the RDS.Thus, in-situ ATR-SEIRAS were used to deeply explore the hydrogen evolution reaction on the surface of the polycrystalline platinum electrode, which made it possible to quantitatively compare the band strength data with HER kinetics (Tafel diagram), and laid a solid foundation for subsequent research.
In addition to water electrolysis, with the development of new, cheap, and efficient HER catalysts, the HER processes in new nonaqueous solvents such as acetonitrile can also be investigated by in situ infrared spectroscopy.Early in situ infrared data obtained by Juan's group showed that chemisorption of hydrogen, acetonitrile, and cyanide occurred only in the presence of water at the interface. [204]Koper's group specifically investigated the HER process of platinum electrodes in an acetonitrile-based electrolyte using in situ SNIFTIRS. [205]A positive band appeared in the O-H stretching zone (3300-3750 cm −1 ) and O-H bending zone (tail observed at 1633 cm −1 ), indicating that water was being consumed from the electrode surface.A negative band appeared in the Cl-O stretch region around 1130 cm −1 , indicating that perchlorate anions were attracted or migrated to the electrode surface (Figure 14b).This result proved that the reversibility of the HER process in acetonitrile depends to some extent on the solvation degree of each component in the electrocatalytic reaction, especially in the presence of water.Based on this study, they further studied the HER process of the polymetallic microelectrode in acetonitrile with or without water in detail by in situ infrared spectroscopy. [206]The results showed that water left the interface with more negative potentials in the absence of a proton source (Figure 14c).While in the presence of a proton source, water had a preferential solvation effect on protons, promoting the HER process on the Au electrode.In addition, the Tafel diagram illustrated that the RDS of polycrystalline gold is the same as that of polycrystalline platinum, that is, the process of first electron transfer reaction generating adsorbed hydrogen.
In addition to the analysis of reaction steps, in situ infrared spectroscopy can also be used to detect catalyst properties during the HER process.For instance, the active site of the MoS 2 single-atom-layer catalyst for HER was well revealed by in situ infrared spectroscopy reported by Wei et al. [207] Under open-circuit voltage, no significant absorption band was observed in the vibration frequency range of 2500-2600 cm −1 for single-layer Co-MoS 2 .However, an absorption band representing S-H intermediate was observed at 2550 cm −1 when a potential of −0.05 V versus RHE was applied, which gradually strengthened in intensity with the increase of potential (Figure 14d).While for the original MoS 2 , no S-H vibration absorption band was observed in the reaction process, which proved that the active catalytic center was the S atom in the base plane of single-layer Co-MoS 2 , rather than the Mo atom or doped Co atom.Besides, for a Ru-N 4 -P catalyst with a triadic coordination structure, an obvious peak at 2400 cm −1 was observed by Song's group from in situ synchrotron radiation infrared spectrum at a potential of −0.1 V versus RHE, and the peak intensity did not change with time (Figure 14e). [208]This finding meant that H atoms were adsorbed on P atoms of catalysts during the HER process and continuously formed P-H ads intermediates.Such research proved that the nonmetallic P atom was promoted as a new nonmetallic reaction center, which accelerated the dissociation of water and the adsorption of H, thus enhancing HER dynamics.

Summary
In conclusion, the development of in situ infrared spectroscopy makes real-time detection of reaction intermediates a reality, which is of great significance for optimizing reaction conditions, understanding reaction mechanisms, and comparing reaction performance.However, the original in situ infrared spectra are extremely sensitive to water molecules and have a low temporal resolution.Thus, the peaks of relevant  [197] Copyright 2019, American Chemical Society.b) In situ-FTIR spectra of the NRR on the N, P co-doped hierarchical porous carbon (NPC) electrode and associative mechanism of the NRR process on NPC.Reprinted with permission from ref. Song et al. [199] Copyright 2019, The Royal Society of Chemistry.c) In situ FTIR spectra of BCP and ECOF@BCP under different applied potentials with nitrogen as the feed gas.Reprinted with permission from ref. Liu et al. [200] Copyright 2021, Nature Publishing Group.
intermediates obtained in the aqueous system are very weak and have a low signal-to-noise ratio. [209]In addition, the existence time of intermediates is short, which means it is difficult to detect the detailed adsorbent composition in a small region.Therefore, it is necessary to combine with other in situ techniques such as in situ Raman spectroscopy to perform secondary verification of the results.

Fundamentals of In Situ Electrochemical Raman Spectroscopy
Raman spectroscopy (RS) is a widely used analyte technique that can accurately characterize the vibrational, rotational, and low-frequency  [203] Copyright 2007, Elsevier Ltd.All rights reserved.b) Potential-dependent SNIFTIR spectra for a polycrystalline platinum electrode in acetonitrile under different atmospheres.Reprinted with permission from ref. Ledezma-Yanez et al. [205] Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) Potential-dependent infrared absorbance spectra for a polycrystalline gold electrode with or without 10 mM of HClO 4 as proton donor and corresponding Tafel plots measured for gold and platinum microelectrodes.Reprinted with permission from ref. Ledezma-Yanez et al. [206] Copyright 2016, Elsevier B.V. All rights reserved.d) In-situ SR-micro-FTIR spectra of monolayer Co-MoS 2 during the HER process.Reprinted with permission from ref. Duan et al. [207] Copyright 2021, Wiley-VCH GmbH.e) The synchrotron radiation infrared spectrometer, in situ cell, and in situ synchrotron radiation infrared spectra of Ru-NPC at different test times.Reprinted with permission from ref. Wu et al. [208] Copyright 2020, Exclusive Licensee Science and Technology Review Publishing House.modes of analytes. [210,211]Through this characterization, the catalyst states, the reaction intermediates, and products during the gas-involved reactions can be detected non-destructively.214] Electrochemical in situ RS is performed based on the Raman scattering, an inelastic scattering phenomenon in which the frequency of incident light changes significantly, bringing about wavelength shifts.In general, RS results from the superposition of vibrational energy or rotational energy and photon energy when the incident photons collide with molecules.It can transfer the molecular energy spectrum in the infrared region to the visible region for observation and thus can be recognized as complementary to infrared spectroscopy. [215]It is worth noting that, unlike the absorption-based IR spectroscopy, water would not cause strong Raman scattering, and scattering-based RS can be applied to aqueous environmental characterization.
[218] Raman spectrometer consists of a laser source, acquisition system, spectroscopic system, and detection system.The light source generally utilizes laser with concentrated energy and high-power density; the collection system is composed of a lens group; the optical splitting system adopts grating or notch filter combined with grating to filter Rayleigh scattering and stray light, and the detection system uses photomultiplier tube detector, semiconductor array detector or multi-channel electric charge coupling device. [219]In situ electrochemical Raman cells are generally constructed by working electrodes, auxiliary electrodes, reference electrodes, and ventilation devices.
SERS, which is operated as an enhanced technique, is designed based on the SERS enhancement effects.In short, the local electric field amplified by the resonant excitation of surface plasmon and the non-resonant lightning rod effect in the enhanced source concentration will lead to SERS enhancement effects. [220]Thus, the electromagnetic intensity of the surface will be significantly increased.Raman signals can be considerably improved in this approach, which meets the requirements of detecting surface absorbents and reaction species in gas-involved electrochemical reactions.As some of the most common catalysts for electrochemical reduction reactions, the roughly nanostructure surface of gold, silver, and copper would produce strong surface enhancement under laser excitation, so they are the research object of the SERS spectrum with a wide range similar to the SEIRAS.SHINERS was designed to reduce the interference of some external factors on the sample characteristic signal. [221]This improved technology is functioned by adjusting the laser source and can make SERS suitable for various sample surfaces.In addition, the combination of Raman spectroscopy and scanning probe microscopy, known as TERS, can provide actual nanoscale spatial resolution and high sensitivity for Raman analysis, which provides an effective way for in situ characterization of nano-scale catalysts. [222]

Applications for CO 2 RR
In situ Raman spectroscopy combined with electrochemical measurement develops rapidly and is expected to be a powerful tool for studying the properties of catalysts in electrochemical reduction reactions.Similar to in situ IR spectrum, it can detect the reaction on the catalyst surface, monitor the adsorbed intermediates and final products on the surface, and then determine the active site and reaction mechanism.
One application is detecting intermediates such as adsorbed carbonates, CO, OH, and C species on the Cu electrode surface during the CO 2 RR process and investigating the dependence of these intermediates on potential.According to in situ SERS reported by Moradzaman, the adsorption of monodentate carbonate was observed mainly at ~1067 cm −1 when the potential was between 0.2 and −0.2 V versus RHE, and the reduction reaction of Cu(I) oxide took place. [223]Cu-CO vibration was observed at 360 cm −1 under the potential range of −0.4 to −1.0 V versus RHE, suggesting the generation of CO intermediates.In addition, the production of Cu-OH and Cu-C intermediates at 525 and 500 cm −1, respectively, was first detected by in situ SERS when the potential was around −1.0 V versus RHE.When the applied potential was higher than −1.0 V versus RHE, the CO intermediate produced on the surface gradually disappeared while Cu-OH and Cu-C still persisted (Figure 15a).It is worth noting that this process was completely reversible.This report proved that CO was a key intermediate of hydrocarbons and laid the foundation for understanding the role of Cu-OH or Cu-C species in CO 2 RR.Besides, Tong et al. [224] also studied the adsorption engineering of intermediates for highly selective heterogeneous Cu-based catalysts by in situ Raman spectroscopy.When the potential was applied to −0.5 V versus RHE, the HCO 3 − band and *COOH band were observed at 1064 and 1640 cm −1, respectively (Figure 15b).This finding suggested that HCO 3 À was first chemically adsorbed on the electrode surface and then reduced to the *COOH intermediate.This intermediate could then be reduced to adsorbed *CO species, which promoted the formation of gaseous CO.Furthermore, by comparing the peak intensity of *COOH in in situ Raman spectra of different metal-doped Cu-based catalysts, it was also proved that the modification of metal nanoparticles could control the adsorption of *COOH intermediates, thus optimizing the CO 2 RR process.Moreover, in situ Raman spectroscopy was also exploited by Zhu's group to probe the formation of possible intermediate on atomically dispersed CuN 4 and NiN 4 bimetal sites supported by electrospun carbon nanofibers (CuNi-DSA/CNFs) during the CO 2 RR process.With potentials increasing, a new peak attributed to the symmetric stretching of v s CO 2 − was observed at 1060 cm −1 , and the intensity gradually increased, indicating the formation and aggregation of *COOH intermediates.Thus, the in situ Raman results verified that CuNi-DSA can facilitate the formation of *COOH intermediates during CO 2 reduction to CO. [225] Besides, in addition to the detection of C 1 products, in situ Raman spectroscopy is also used to detect the changes of intermediates in the reactions that produce C 2 products such as ethane.For example, Qiao and co-workers demonstrated the presence of Obound ethoxy (*OCH 2 CH 3 ) in the C 2 path of the CO 2 RR processes by clearly observing symmetric stretching bands of -CH 2 and -CH 3 in the vicinity of 2900 and 2925 cm −1 using in situ Raman spectroscopy (Figure 15c). [226]It was found that the optimal charge state of the Cu active site could make *OCH 2 CH 3 bound to oxygen stably, which was favorable for hydrocarbonization to produce ethane but not for hydrogenation to produce ethanol or ethylene.Not only being widely used to monitor surface changes of Cu in the CO 2 RR process, but in situ Raman spectroscopy is also applied to some other potential catalysts.As early as 1995, the CO 2 RR process on the surface of the Ag electrode had been investigated by SERS. [227]Subsequently, Gewirth's team promoted CO 2 RR by adding 3, 5-diamino-1,2, 4-triazole (DAT) to the Ag electrode and Energy Environ.Mater.2023, 6, e12552 studied the promoting mechanism of DAT through in situ SERS. [228]In the absence of DAT, two Raman bands were located at 1880 and 1945 cm −1 , which represented the adsorption of *CO.In the presence of DAT, the original band at 1880 cm −1 disappeared.Two new bands appeared at 2049 and 2099 cm −1 , respectively, indicating the physical adsorption or weak coordination of *CO on the surface of DAT (Figure 15d).Therefore, the efficiency of the AG-DAT electrode was greatly improved because DAT can reduce the coordination between CO and the surface.The product was more easily desorbed from the electrode surface.Similarly, in situ Raman spectroscopy has also been used to investigate the dynamic chemical state of CO 2 RR over Zn electrocatalysts.As reported by Chen et al., [229] in situ Raman spectroscopy revealed that the peak intensity representing the A 1 (LO) mode of ZnO at 566 cm −1 decreased with the increase of cathode potential (Figure 15e).This phenomenon can be attributed to the electrochemical reduction of zinc oxide to metallic zinc.Combined with in situ XAS spectroscopy, the results showed that zinc electrocatalyst has better selectivity for CO at lower cathode potential and high selectivity for formate due to the conversion between zinc and zinc oxide.The results explained the early misunderstanding between zinc electrocatalysts and CO production and thus revealed the  [226] Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.d) In situ SERS spectra during cathodic and anodic polarization without and with DAT.Reprinted with permission from ref. Schmitt et al. [228] Copyright 2014, American Chemical Society.e) The Raman peak intensity and in situ Raman mapping of O-Zn-400 (15 × 15 μm 2 ) based on the peak intensity at 566 cm −1 .Reprinted with permission from ref. Chen et al. [229] Copyright 2020, The Royal Society of Chemistry.
important influence of the dynamic chemical state on the selective CO 2 RR process over zinc electrocatalysts by in situ technique for the first time.

Applications for NRR
Similar to in situ IR spectroscopy, in situ Raman spectroscopy can also be used to detect intermediates in the NRR process.Du and his team synthesized a B-enriched VC nanocrystalline based on carbon nanofibers. [230]During the NRR process, an obvious peak representing -NH intermediates could be found located at 1525 cm −1 in in situ Raman spectrum, and the peak intensity gradually increased with the reaction, indicating that a strong NRR occurred (Figure 16a).Another application is to use in situ Raman spectroscopy to detect the formation and rupture of chemical bonds on the surface of catalysts and thus explore how to optimize the NRR process from the mechanism perspective.Wang et al. [231] employed in situ  [230] Copyright 2021, Wiley-VCH GmbH.b) In-situ Raman spectra of O-CoP/CNT at −0.5 V versus RHE from 0 to 1 h.Reprinted with permission from ref. Meng et al. [231] Copyright 2021, Elsevier B.V. All rights reserved.c) In situ electrochemical Raman spectra of np-B 13 C 2 at various applied potentials in N 2 -saturated electrolyte.Reprinted with permission from ref. Lan et al. [232] Copyright 2021, Wiley-VCH GmbH.d) In situ Raman spectra and corresponding contour plots of np-PdH 0.43 and np-Pd at various potentials in N 2 -saturated electrolyte.Reprinted with permission from ref. Xu et al. [233] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ Raman spectra with the N 2 -saturated and Ar-saturated 10 M LiCl as the electrolyte.Reprinted with permission from ref. Wang et al. [234] Copyright 2021, Nature Publishing Group.
Raman spectroscopy to detect the reaction intermediates on the surface of O-CoP/CNT during the NRR.In the beginning, the presence of the C-H bond at 720 cm −1 could be observed, which meant that H + occupied the surface.As the reaction progressed, the peak gradually weakened until it disappeared, accompanied by the emergence of a new peak located at 215 cm −1 , representing the Co-N bond.The N 2 molecule was adsorbed on Co species.The adsorbed part of H + preferentially combined with a large number of nitrogen molecules to form ammonia, which inhibited the effect of HER.Combined with DFT, it was proved that the O-group could effectively reduce the potential barrier of the RDS (*N 2 → *N-N *H) and effectively promote the NRR (Figure 16b).Moreover, Tan et al. [232] found obvious Raman peaks at 674, 1612, and 2900 cm −1 through in situ Raman spectra measured in the NRR reaction of the nanoporous B 13 C 2 .These peaks represented the stretching of the N-H bond in NH 2 -NH 2 , NH 3 , and N-H, respectively, which were associated with the NRR and proved that the introduction of carbon was conducive to the N 2 chemical adsorption (Figure 16c).
Likewise, Chen et al. [233] observed that in in situ Raman spectrum, NRR reaction intermediates can be produced both on the surface of nanoporous PdH 0.43 (np-PdH 0.43 ) and nanoporous Pd (np-PdH), while the signal of np-PdH 0.43 was stronger.In addition, ammonia formation was detected on the surface of np-PdH 0.43 at 50 mV versus RHE.However, the formation of ammonia on the np-Pd surface was detected at almost −100 mV versus RHE, indicating that hydrogen dopants in palladium hydrides can reduce the activation barrier of N 2 and promote the NRR kinetics (Figure 16d).The previous work of our group also detected two obvious peaks at 1152 and 1526 cm −1 in the N 2 -saturated environment by in situ Raman spectroscopy, representing -NH 2 and -NH, respectively. [234]Moreover, the intensity gradually increased as the NRR process proceeded.While the same peaks were not observed in the Ar-saturated environment.These results provided strong evidence for the occurrence of NRR (Figure 16e).

Applications for HER
A great deal of work has been put into designing efficient HER catalysts to improve water electrolysis efficiency.However, the reasons behind the high HER activity of these catalysts remain unclear.On the premise of such research, in situ Raman spectroscopy can be an excellent characterization for further exploring how catalysts function in the HER process.Although Pt-based catalyst has been proven to possess extremely high HER activity, the understanding of its HER mechanism is still mysterious.With nickel-platinum alloy nanoparticles working as a catalyst, Sun's group studied the dynamic role of water structure at the electrode-electrolyte interface in the HER process using in situ Raman spectroscopy. [235]In the 2030-2100 cm −1 band region, a representative terminal H (H opd ) vibrational mode was observed, which was independently coordinated on the surface of an apical metal atom.Its peak intensity increased as the potential became negative, which was consistent with the increase of HER reaction current, indicating that H opd was the intermediate in the HER process (Figure 17a).By comparing in situ Raman spectra of acidic and alkaline electrolytes, it was found that the adsorbed H in the base was not transferred by hydrogen bonding but discharged from water, suggesting that the pH dependence of the interfacial water structure of Pt-based alloys was mainly due to the difference in reactants and charge transfer rates.This study elucidates the dynamic control of the water structure at the electrode/electrolyte interface during HER process and provides a choice for the design of novel efficient electrocatalysts.
Oxygen vacancies have also been confirmed to promote the desorption of hydrogen on the surface of Pt catalyst.As reported by Lu et al., [236] the peak representing the E g (1) vibration mode of anatase TiO 2 was observed at 144 cm −1 , which was related to the symmetric tension vibration of O-Ti-O and was very sensitive to the change of TiO 2 surface structure.When the potential of −0.1 V versus RHE was applied, the blue shift of Raman peak in V O -rich Pt/TiO 2 became more obvious with the progress of the HER process.This blue shift phenomenon was believed to be caused by hydrogen spillover on Pt/NCs and TiO 2 carriers.The further comparison of the contrast samples without the influence of pH also proved that the blue shift of V O -rich Pt/TiO 2 was more obvious than that of V O -deficient Pt/TiO 2 , indicating that more *H atoms were transferred from Pt NCs to V O -rich vacancy TiO 2 carrier, and thus further enhancing HER activity (Figure 17b).In addition, Pt/WO 3 has been shown to have excellent HER activity close to that of commercial Pt/C catalysts by Wang's group. [237]The promoting effect of WO 3 was further studied by in situ Raman spectroscopy.The Raman characteristic peak of WO 3 disappeared when the potential was swept from 0.1 to 0.05 V versus RHE and reappeared when the potential was swept back.This was consistent with the phenomenon of the catalyst surface color changing from yellow to green and then to yellow.Such a result proved that Pt clusters were the real active sites of HER in Pt/WO 3 catalyst and that the electrochemical H + insertion/out process leads to the reversible in situ transformation of WO 3 to H x WO 3 , which can promote the rapid transfer of electrons and hydrogen.
Moreover, molybdenum sulfide (MoS x ) materials are also considered as promising electrocatalysts due to their robustness and excellent catalytic performance. [238,239]In recent years, great progress has been made in studying the detailed mechanism of the MoS xcatalyzed HER process by in situ Raman measurement.Yeo's team found a Raman peak related to H 2 evolution at 2530 cm −1 by in situ Raman spectroscopy, which could be attributed to the S-H stretching vibration of MoS x -H moieties. [240]However, the stretching vibration of Mo-H (or Mo-D) was not observed, so the Mo center was excluded as the catalytic site for HER.The S atom in amorphous MoSx was proved to be the active catalytic site for H 2 generation through a series of experimental measurements and quantum chemical simulation (Figure 17c).Based on this research, the effect of the relationship between Mo and S atoms on the HER process is further studied by Nakamura and Li. [241]In situ Raman spectra showed that the band of S-S terminal species appeared with the synchronous change of Mo-Mo, Mo 3 -μ 3 S, and Mo-S vibration band frequencies.This implied a synergistic relationship between the formation of terminal S-S species and the weakening of the Mo-Mo bond.Thus, the metal center and sulfur ligand became a whole with each other to promote efficient HER at neutral pH.In addition to the study of the intrinsic activity of MoS x , the composition-property relationship of MoS x -based electrocatalyst and the structural changes induced by HER could also be well explored by in situ Raman spectroscopy.In a previous work reported by Gao's group, in situ Raman spectroscopy was used to explore the structural evolution of MO 2 C-MoO x /CC during HER under acidic conditions. [242]Raman peaks representing MoO 3 species were observed at 240, 821, and 995 cm −1 .These peaks disappeared completely after 5 min, and a series of new peaks  17d).Ni oxides and their derivatives have also been reported as efficient HER catalysts.Nickel/nickel oxide (Ni/NiO) nanosheet heterostructure is a cheap and effective catalyst for the hydrogen evolution in alkaline electrolytes reported by Sunde et al. [243] In situ Raman spectroscopy showed that the catalyst initially maintained β-Ni(OH) 2 morphology during the resonance process.Copyright 2020, Wiley-VCH GmbH.b) In situ Raman spectra of V O -rich Pt/TiO 2 at −0.1 V versus RHE with different electrolysis durations, magnified view of the E g (1) mode, and in situ Raman spectra of V O -rich Pt/TiO 2 , V O -deficient Pt/TiO 2 , and TiO 2 at a potential of −0.2 V versus RHE.Reprinted with permission from ref. Wei et al. [236] Copyright 2021, Wiley-VCH GmbH.c) In situ Raman spectra recorded of MoS x -CE during the cathodic and anodic half sweep.Reprinted with permission from ref. Deng et al. [240] Copyright 2016, American Chemical Society.d) In situ Raman spectra of Mo 2 C-MoO x /CC with chronoamperometry curve at η = 200 mV in 0.1 M HClO 4 .Reprinted with permission from ref.He et al. [242] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ Raman-chronoamperometry at different time intervals for Ni/NiO and in situ Raman after 10 000 s of different Ni catalysts.Reprinted with permission from ref. Faid et al. [243] Copyright 2020, Elsevier.f) In situ Raman spectra of SANi-I at OCP and constant potentials (vs RHE).Reprinted with permission from ref. Zhao et al. [245] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Zhang et al. [244] reported a highly efficient NiMo alloys catalyst for HER.In situ Raman spectroscopy indicated that Mo was oxidized and dissolved as MoO 4 2À during the reaction.The dissolved MoO 4 2À re-adsorb on the electrode surface and polymerize into Mo 2 O 7 2À , which was more conducive to improving the HER activity of single metal Ni.In addition, a single atom nickel iodide (SANi-I) electrocatalyst with atomically dispersed nonmetallic iodine atoms has also been proved by Qiao's group to exhibit very high HER activity. [245]In situ Raman spectroscopy was employed to investigate the origin of this high catalytic activity.Compared with the catalysts without doping I, a wide peak was observed at 2460 cm −1 caused by the vibration of H atoms adsorbed on the I site (I-H ads ) when the applied voltage ranges from −0.015 to −0.065 V versus RHE, which meant that the evolution of I-H ads intermediates played a key role in promoting HER (Figure 17f).

Summary
In summary, in situ Raman spectroscopy is widely used in gas-involved reactions, which can analyze the intermediates generated during the reaction based on chemical structure and provide valuable information for exploring the reaction mechanism.Nevertheless, since it mainly relies on intrinsic vibrational, rotational, and other modes in molecules under the inelastic scattering of monochromatic light, in situ Raman spectroscopy can only be limited to monitoring electrocatalysts with chemical functional groups that can scatter light at different frequencies.

Discussion
To sum up, in situ FTIR is typically employed to detect dipole moment changes during gas-involved electrochemical reactions, further provide information including the generation of intermediates and products regarding bonds in fragmented molecules, functional groups, and radicals.Under this premise, in situ Raman can be recognized as a complementary technique to in situ FTIR.However, Raman or FTIR alone cannot provide a holistic view of a complex catalytic system.For example, as water does not induce strong Raman scattering, Raman has advantages over FTIR when it comes to the representation of the water environment.Thus, the combination of Raman and FTIR should offer more multidimensional information about the underlying mechanisms of gas-involved electrochemical reactions.Despite the coupling between the above two in situ vibrational spectrum spectroscopy, other in situ characterizations including in situ XAS and in situ XPS can be combined with Raman and FTIR to obtain extended insights into the surface evolution and interfacial processes occurring during electrochemical gas-involved electrochemical reactions.It is essential to continuously develop in situ vibrational spectrum spectroscopy for further understanding of the structurefunction relationships and underlying mechanisms of electrochemical energy conversion reactions.

Other In Situ Technology
In addition to in situ characterization techniques mentioned above, several other techniques such as in situ mass spectrometry and in situ gas chromatography play important roles in detecting the kinetic evolution of CO 2 RR, NRR, and HER.Besides, in situ microscopy also has a relatively mature application.

Fundamentals of In Situ Mass Spectrometry
Merely characterizing the catalyst changes cannot help understand the catalytic reaction dynamics.Thus, it is vital to immediately characterize the changes in catalytic intermediates or products under working conditions.The MS technique affords a promising meaning to monitor the catalytic products in real-time. [246]Unlike in situ IR or Raman spectrometry, by continuously separating and collecting products by pervaporation, MS can accurately and efficiently obtain the information on catalytic intermediates and products in electrochemical reduction reactions.In contrast to gas or liquid chromatography, which typically takes a few minutes to flow a sample through the column, mass spectrometry can analyze the mass charge ratio of the sample in only a second.Based on this principle, by recording the sectional ion current densities of analytes, the formation rates of gaseous and/or volatile reaction products can be semi-quantified in real-time. [247]Therefore, online electrochemical mass spectrometry (OLEMS) is widely employed in in situ research to monitor the changes of reaction products and the local surrounding of three-phase interfaces in gas-involved reactions.
OLEMS analytical technique involves the utilization of a mass spectrometer in conjunction with an electrolytic cell to detect the gaseous product composition in real-time by placing a sampling needle near the electrode surface.Although OLEMS can detect the reaction intermediates and products, it cannot efficiently collect analytes and is extremely sensitive to the distance between the sampling tips and the electrode surface.Meanwhile, OLEMS could not detect the liquid species of gasinvolved reactions in real-time. [248]Thus, differential electrochemical mass spectrometry (DEMS) was developed to optimize this defect.DEMS are mainly composed of three parts: a three-electrode cell, a porous Teflon membrane that allows gas products to pass through, and a vacuum system that can be connected to the mass spectrum.The components ionized by electrons are separated and analyzed by a four-stage mass spectrometer according to different mass to charge ratios.This modified in situ mass spectrometry technique has a notable short response time and offers a pathway to monitor gaseous-phase and liquid-phase products or intermediates in electrochemical reduction reactions. [249]Thus, it is frequently employed to help investigate the selectivity of catalysts in a time-resolved manner.

Applications for CO 2 RR
As an effective method to analyze the immediate products of electrochemical reactions, in situ mass spectrometry has been widely used in the CO 2 RR.Grote and co-workers reported a finding that Cu selectivity could be improved by mixing Co. [250] Through in situ mass spectrometry, they detected the formation of various C 1 and C 2 products during CO 2 RR, which was confirmed to be adjusted by the ratio of introduced Co.For Cu-Co thin film alloy catalyst, when the Co atom ratio was between 5% and 15%, ethane would be generated and precipitated preferentially than methane, significantly increasing the proportion of C 2 products.However, when the proportion of Co was further increased, the hydrogen evolution reaction would gradually strengthen and dominate.Such research Energy Environ.Mater.2023, 6, e12552 proposed a superior method that employs a scanning flow cell coupled to an online electrochemical mass spectrometry (SFC-OLEMS) to quickly and easily screen initial catalyst composition (Figure 18a).Besides, complex intermediates in the CO 2 RR process could be well identified by in situ mass spectrometry.Back in 2012, Koper's group analyzed two possible pathways and intermediates in the CO 2 RR to produce ethylene by DEMS. [251]As an essential intermediate in the process of CO 2 RR to methane and ethylene, CO has been observed in Cu (111) and Cu (100).Significantly, by generating an adsorbed dimer of CO, *CO was selectively reduced to ethylene on Cu (100) at a relatively low overpotential, which was proved by DEMS results (Figure 18b).

Applications for NRR
In addition to the CO 2 RR, in situ mass spectrometry also plays a vital role in the NRR.Recently, the possible NRR reaction pathways on Bi catalysts were well revealed by DEMS as reported by Qiao's group. [252]he signal generated at 15 points represents NH 3 or N 2 H 4 , but there were no mass-to-charge ratio (m/z) signals at 27, 31, and 33 points, and no characteristic signal of N 2 H 4 was detected.Therefore, the apparent m/z signal at 15 points could be proved to be caused by NH 3 .DEMS results showed that the m/z signal at 17 changed with the change of potential.Although it may be caused by evaporated water, the characteristic signal of water did not vary with electric potential.Therefore, the m/z signal at 17 could be used to track the NRR process.These findings proposed a possible pathway that absorbed nitrogen molecules are reduced to *N 2 H 2 by a two-electron transfer process and then desorb and decompose in the electrolyte to generate NH 3 .(Figure 19a).

Applications for HER
The most extensive application of in situ mass spectrometry in the field of HER is to establish the HER dynamics.Garcia's team reported such work, using DEMS to accurately measure the activity of composites in NaOH 0.1 M media. [253]The onset potential and Tafel slope of different composite materials were obtained by DEMS, which proved that the Heyrovski step was the rate-determining step for all TMCs and TMCs-OPy materials.The onset HER overpotential of Mo 3 P catalysts was also confirmed to be as low as 21 mV versus RHE by DEMS as Asadi's group described. [254]Meanwhile, DEMS indicated that the number of electrons transferred to generate per H 2 in the HER process was 1.99 e − , which confirmed that all the current generated by Mo 3 P nanoparticles at 200 mV versus RHE was used to generate H 2 , meaning the high selectivity of Mo 3 P (Figure 19b).Besides, Ticianelli et al. [255] found that all TM-Mo 2 C catalysts exhibited excellent HER activity by DEMS, with onset potential lower than −0.06 V versus RHE and mass activity ranging from 29 to 50 mA mg −1 , suggesting that they are promising electrocatalysts for non-noble metal catalytic reactions.In addition to determining the kinetics of the reaction, similar to the application in NRR, in situ mass spectrometry can also be used to investigate the effect of catalyst transformation on the reaction.Fiechter's group studied the structural transformation of sputtering amorphous MoS x during the HER process. [256]DEMS was used to detect gas components precipitated on electrodes.When the potential swept from +0.2 to −0.3 V versus RHE, the H 2 signal curve peaks could be observed at 4350, 5380, 6000, and 8790 s due to the release and accumulation of H 2 on the electrode surface.In the first scanning process, the signal of H 2 S + is strong, and then the signal began to weaken gradually, accompanied by the increase of the H 2 + signal.The H 2 S + peak almost wholly Schouten et al. [251] Copyright 2012, American Chemical Society.
Energy Environ.Mater.2023, 6, e12552 disappeared at the end of the measurement, which indicated that the HER process was accompanied by the loss of sulfur in the form of H 2 S (Figure 19c).

Summary
In summary, in situ MS is helpful for monitoring the catalytic reaction process and provides real-time qualitative analysis of desired catalytic intermediates and products.However, for reactions with complex and diverse products, such as CO 2 RR, it is difficult to accurately identify and quantitatively analyze a single product by the intermittent detection method of in situ mass spectrometry.

Fundamentals of In Situ Gas Chromatography
In situ gas chromatography is also a practical separation and analysis technology often used in electrocatalysis.GC refers to chromatography using gas as the mobile phase.Because of the different stationary phases used, gas chromatography can be divided into gas-solid chromatography with solid adsorbent as the stationary phase and gas-liquid chromatography with a fixed liquid-coated monomer as the stationary phase.Due to the rapid transfer of the sample through the gas phase, the sample components can reach an instant equilibrium between the mobile phase and the stationary phase.In addition, there are many substances that can be selected as stationary phase, and thus GC is a separation and analysis method with high analysis speed and high separation efficiency.In recent years, the utilization of susceptive and selective detectors makes it has the advantages of high sensitivity and wide application.
Typically, in situ GC consists of the following systems: gas path system, sampling system, separation system, temperature control system, detection recording system, and electrochemical system.After a period of electrochemical reaction, the product to be analyzed is fed into the vaporization chamber for vaporization and then carried into the column by carrier gas (mobile phase).More concentrated components in the carrier gas flow out of the column first, followed by the transfer of more concentrated components in the stationary phase.As the components leave the column, they enter the detector immediately.The detector can convert the sample component into an electrical signal, and the size of the electrical signal is proportional to the amount or concentration of the component being measured.When these signals are amplified and recorded, corresponding gas chromatograms can be obtained.

Applications
At present, in situ GC has been used in the field of HER to analyze the amount of hydrogen produced quantitatively.As previously reported by Sawangphruk's group, due to the degassing of electrolytes, the presence of H 2 could be observed in chromatograms taken at 360, 720, 1440, 2160, and 2880 s under the condition of extremely high O 2 concentration (Figure 20a). [257]Under this premise, the quantification  [254] Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) In situ DEMS result of RT sputtered MoS x with glassy-carbon CE.Reprinted with permission from ref. Xi et al. [256] Copyright 2019, American Chemical Society.
Energy Environ.Mater.2023, 6, e12552 of the produced H 2 could be integrated into the following formula: where A is the area of H 2 peak in the chromatogram (cm 2 ), A s is the area of standard H 2 peak in the chromatogram (cm 2 ), C s is the concentration of normal H 2 (%), P a is atmospheric pressure (kPa), V s is the volume of injection (mL), and P c is the pressure of carrier gas (kPa).In addition, the hydrogen evolution rate and Faradaic efficiency can be further calculated by the following formula: where φ is the flow rate of carrier gas (mL s −1 ), 22 400 (mL mol −1 ) is the conversion factor for quantitative calculation of volume gas, S is the area of electrode surface (cm 2 ), N is the theoretical mole of H 2 , F is Faraday constant (96 485 C mol −1 ), N is the number of electrons transferred in the Faraday process, and Q is the total charge of the whole reaction.Moreover, in situ gas chromatography can calibrate the degree of adverse reaction HER in the NRR process.Such measurements can provide further certification of the NRR performance (Figure 20b). [258]

Summary
To conclude, in situ GC can be used for qualitative or quantitative analysis of the composition or yield of simple gas products.However, the technology is not entirely comprehensive.In the direct qualitative analysis of components, it is necessary to compare the known material or known data with the corresponding chromatographic peak.Besides, in quantitative analysis, pure samples of known substances are often needed to correct the output signal after detection.Therefore, when it comes to the multi-product and complex electrochemical reduction reaction such as CO 2 RR, it needs to be combined with other in situ characterization such as in situ mass spectrometry to precisely determine the changes of intermediates and products.

In Situ Microscopy
In situ microscopy technology also has a mature application in the field of electrochemical reduction reactions, especially for CO 2 RR.It can be divided into three types: in situ optical microscopy, in situ electron microscopy, and in situ scanning probe microscopy. [28]

In Situ Optical Microscopy
In situ optical microscopy can be considered the most basic in situ microscopy technique, commonly used to observe the changes in electrode materials during cycling.In recent years, with the development of electrocatalysis, in situ optical microscopy has been gradually popularized in the field of electrochemical reduction.According to Kanoufi's group, a recent work revolves around in situ optical microscopy. [259]isual images of the indium tin oxide/electrolyte interface were recorded during CV at 0.  21a).In addition, previous work regarding NRR reported by our group successfully applied in situ optical microscopy for captive bubble experiments. [260]During the NRR process, the process of N 2 bubble adsorption and consumption on the electrode surface was observed in detail.Different from the normal electrode, the N 2 bubbles gradually shrink until they were completely consumed within 10 min on the  [257] Copyright 2019, Elsevier Ltd.All rights reserved.b) H 2 selectivity of PNG and GNG measured at different potentials by in situ gas chromatography.Reprinted with permission from ref.

In Situ Electron Microscopy
In situ electron microscopy mainly refers to in situ transmission electron microscopy (TEM), which seems to be a traditional imaging characterization technique.By combining ultramodern in situ techniques, TEM can provide real-time data related to changes in catalyst morphology or composition at the atomic scale during the reaction process.A successful example is that Li's group observed a phenomenon that the Bi-MOF on N-doped carbon network was successfully reduced to Bi nanoparticles (NPs) in the CO 2 RR process through in situ TEM, and atomization happened with the assistance of NH 3 released by dicyandiamide decomposition (Figure 22a). [261]Moreover, the dissolution/redeposition of Cu nanocatalyst during the CO 2 RR process was also well revealed by Buonsanti's group. [262]When the open circuit potential was applied, oxidation and dissolution coincided.Under this condition, Cu 2 O and dissolved Cu ions coexisted in the electrolyte.When a negative potential was applied, the dissolved copper ions were reduced to solid copper.With the help of surface Cu 2 O, the reconstruction process was completed, and larger secondary copper nanoparticles were formed.The mechanism of the catalyst phase transition has been investigated in detail by time-resolved TEM at the nanoscale (Figure 22b).Additionally, 2D CuO nanosheets have been confirmed to transform into nanoscale fragments under time and potential changes.These nanoscale fragments then agglomerated to form Cu dendrites, generating undercoordinated Cu sites that facilitate the generation of C 2+ products.The relationship between operating conditions, morphology, and properties was explained successfully by in situ TEM as reported by Strasser et al. (Figure 22c). [263]However, there are still many limitations of in situ TEM currently, even in the field of CO 2 RR.Moreover, it is still difficult to be applied for the NRR or HER.First, the high electron dose can destroy the catalyst in a short time, so obtaining the appropriate electron dose is tedious, and the sample preparation is relatively strict, especially for the high-resolution mode.Second, electrodes placed inside the device of TEM can cause uneven distribution of electric fields, forming hot spots.In a microreaction system with only a small amount of electrolyte, the gas produced by the reaction can significantly affect the local reaction environment, which will result in the loss of resolution and contrast. [264]Thus, it is a challenge to obtain a clear and reliable image.It is believed that more efforts are needed to promote the development of in situ TEM for its widespread application in all electrochemical reduction reactions.

In Situ Scan Probe Microscopy
In situ scanning probe microscopy is another tool that can reveal surface morphology and sub-nanometer characteristics.In this technique, light or electrons are replaced by various probes for information retrieval.As the basic in situ scanning probe microscopy, in situ scanning electrochemical microscopy (EC-SECM) mainly detects the local current changes on the surface of the catalyst by using an ultrafine electrode (UME) or nanoelectrode (NE) to obtain the transformation of chemical species in the reaction process.As early as 2014, Phani's group explored the morphology transformation and reaction mechanism of Ciocci et al. [259] Copyright 2021, Elsevier Ltd.All rights reserved.b) In situ optical images of the dynamic consumption process of N 2 bubbles on common WE and NME@WE, schematic illustration of the common process of electrochemical ammonia synthesis, schematic of strategies to improve efficiency in NRR by NME coating, and the illustration of device structure for in situ optical microscopy visualization experiments.Reprinted with permission from ref. Shen et al. [260] Copyright 2022, Wiley-VCH GmbH.
Energy Environ.Mater.2023, 6, e12552 Au catalyst in the CO 2 RR process by combining SECM and electrochemical experiments. [265]The Pt 10 μm UME served as a probe for all the SECM experiments and the substrates were Au, Pd, Ag, and thin films of Hg and Bi deposited on a glassy carbon surface.Substrate generation-tip collection mode (SG-TC) was chosen to collect the available information.With the Pt 10 μm UME kept and maintained at a constant distance of ≈20 μm from the substrate, positive feedback mode for noble metals (Au, Ag and Pd) and negative feedback mode (Bi and Hg) of SECM were carried out in ferrocenemethanol (1 mM) + 0.2 M KCl solutions.Bicarbonate reduction was followed at the substrate that is kept at a potential (called substrate potential [E S ]) to obtain the voltammetric patterns for the oxidation of substrategenerated species, which could help investigate bicarbonate and CO 2 reduction at different pH values.EC-SECM results showed that CO intermediate was mainly derived from CO 2 RR and HCOO − from bicarbonate ion in low pH electrolyte when a negative potential was applied (Figure 23a).Likewise, the time-resolved quantitative detection of products was also proved by Kim's group, who used EC-SECM to investigate the CO 2 RR performance on the surface of Au nanoparticles and demonstrated an extremely high CO selectivity at a low overpotential without the interference of HER. [266]Recently, through employing the EC-SECM, Kanan found that the grain-boundary surface terminations of Au electrode were more active than the grain surface, providing a strategy for further development of the grain-boundary effects of heterogeneous catalysis. [267]Besides, due to the time-resolved detection capability of in situ SECM, the tip generation/substrate collection mode was used for the first time to detect unstable CO 2 À radical in the CO 2 RR process by Bard et al. [268] The dimerization velocity and halflife were evaluated by experiments and theoretical calculations to be 6.0 × 10 8 M −1 s −1 and 10 ns (Figure 23b).In addition to its application in the field of CO 2 RR, there is also research in NRR using EC-SECM characterization.Yu's group found that Ti 3+ had twice the N 2 adsorption rate of pure TiO 2 by in situ SECM. [269]This finding not only experimentally verified that Ti 3+ species are the most likely active sites for HER and NRR but also provided an accurate measure of the kinetic rates of active sites at which these two competing reactions occur (Figure 23c).As for HER, EC-SECM has been used to measure hydrogen adsorption on polycrystalline Pt electrodes quantitatively as reported by Bard's group. [270]Through comparing with electrochemical low potential deposition (UPD) of hydrogen on Pt, such quantitative method for H ads could be verified (Figure 23d).However, this noncontact technique has significant limitations.Due to the influence of mass transfer of the product from the surface to the tip of the probe, its spatial resolution fluctuates.Therefore, this technique is highly dependent on probe size and the distance between the probe and samples and needs to be further developed for widespread application.
As for other in situ scanning probe microscopes, in situ electrochemical scanning tunneling microscopy (EC-STM) and in situ atomic force microscopy (AFM) can investigate the structural state of the catalyst surface in the reaction environment.Among them, in situ EC-STM  [261] Copyright 2019, American Chemical Society.b) Sequence of in situ TEM images at given times during the experiment and corresponding magnified images.Reprinted with permission from ref.
Vavra et al. [262] Copyright 2020, Wiley-VCH GmbH.c) Schematic overview of the experimentally observed evolution of the CuO NS morphology probed by in situ TEM.Reprinted with permission from ref. Wang et al. [263] Copyright 2021, Nature Publishing Group.
Energy Environ.Mater.2023, 6, e12552 mainly relies on the quantum tunneling effect of the tiny gap between the probe tip and the plane surface.Especially, it is appropriate for exploring the surface state of catalysts at the molecular or atomic level under atmospheric pressures.Through the assistance of EC-STM, Soriaga's group found that Cu (100) on air-oxidated surfaces can undergo cathode regeneration under typical CO 2 RR conditions.In addition, polycrystalline Cu is converted to Cu (111) after 30 min when a potential of −0.9 V versus RHE is applied. [271]Another work by the same group found that Cu (111) was further converted to Cu (100) after an additional 30 min (Figure 24a). [272]In a recent work reported by Lingenfelder's group, EC-STM also revealed the dynamics of the morphologic evolution of polycrystalline Cu (p-Cu) and graphenecovered polycrystalline Cu (g-Cu) surfaces. [273]Even in halogen-free electrolytes, Cu surfaces underwent drastic reconfiguration after long periods of exposure to a negative potential, evolving from polycrystalline Cu to nanocuboids.The addition of a single graphene layer to the copper catalyst could effectively achieve protection (Figure 24b).In addition to its application in the field of CO 2 RR, Horch's group explored the formation process of Au (111)-Cd alloy in an H 2 SO 4 electrolyte by EC-STM and obtained an overpotential increase of 130 mV for hydrogen evolution reaction compared with pure Au, which was proved to be favorable to those electrocatalytic reactions competing with HER. [274]Such an EC-STM technique, which can directly detect the catalyst surface, is beneficial to the construction of the "structurecomposition-reaction relationship" of multiphase electrocatalysts and opens the way for the design of electrocatalysts in the future.
With the assistance of in situ AFM, surface profiles and potentials can be monitored by detecting the tip-sample interaction formed by van der Waals forces.As reported by Cuenya et al., [275] The change of Cu nanoparticles during the CO 2 RR process was carefully observed by in situ AFM.Within 1 min of the reaction, the corners of the Cu nanocube gradually smoothed out.Three hours later, a 10% drop in the size of the copper nanoparticles was detected.This morphologic change resulted in a sharp decrease in EF and increased selectivity for C 1 products (Figure 24c).Moreover, Simon et al. [276] studied the morphology change and surface reconstruction of Cu (100) electrocatalyst during the CO 2 RR process by in situ AFM.Epitaxial Cu 2 O (111)/Cu (100) phase was formed on the electrode surface under open-circuit voltage.At a potential of −0.5 V versus RHE, the surface of the electrode exhibits circular, smooth island and terrace structures.When a more negative potential was applied (−0.1 V vs RHE), the surface became rectangular stepped, corresponding to p (2 × 2) reconstruction induced by specific adsorption and (1 × 1) Cu surfaces, respectively.This work proved that there was a complex interrelationship between the morphology, structure, defect density, applied potential and electrolyte of Cu catalyst in CO 2 RR process (Figure 24d).In situ AFM with excellent atomic resolution has been widely used in the field of CO 2 RR.However, there are no precedents for applications in NRR and HER due to their simple reaction pathways and products.

Conclusions and Perspectives
In situ characterization techniques have played a significant role in investigating electrochemical energy conversion reactions.With the assistance of real-time detection of catalysts, reaction intermediates, and catalytic products, valuable information such as the adapted operating conditions and the underlying reaction mechanism can be compiled in  [268] Copyright 2017, American Chemical Society.c) Illustration of SI-SECM for determining and the reaction rate constants of H + and N 2 adsorption on Ti 3+ site in TiO 2 NTs and s-TiO 2 NTs.Reprinted with permission from ref. Li et al. [269] Copyright 2020, Wiley-VCH GmbH.d) Formation of H ads by electrochemical UPD of H on Pt and depiction of surface interrogation results.Reprinted with permission from ref. Rodríguez-López et al. [270] Copyright 2010, American Chemical Society.
detail, which is of excellent significance meaning for the targeted design of catalysts with high selectivity and efficient catalytic systems for the practical application of these electrochemical energy conversion reactions.In this review, we have briefly introduced three cathodic electrochemical reactions that are currently in the limelight and presented the operating principles, advantages, limitations, and related research advances of the current common in situ characterization techniques such as X-ray energy spectroscopy, vibrational spectroscopy, mass spectrometry, chromatography, and microscopy.It was taken for granted that a deeper understanding of in situ characterization techniques would help understand electrochemical reduction reactions, and this is indeed the case.Nonetheless, due to the diverse nature of electrochemical reactions, the complexity of the multiple electron transfer steps, and the wide variety of active sites of different catalysts, numerous potential catalytic mechanisms remain unrecognized.Besides, the quantitative understanding and dynamic synergistic investigation of interfacial multi-component reactions are still a blind spot, and the kinetic parameters of each component in the interfacial evolution process are difficult to be accurately obtained, which seriously restricts the in-depth understanding of the scientific and rational optimization of the critical physicochemical properties of the reactions.Therefore, future in situ characterization techniques are still some distance away  [272] Copyright 2014, American Chemical Society.b) Schemes of EC-STM and series of in situ EC-STM images of a g-Cu surface during CO 2 RR in 0.1 M CO 2 -saturated KHCO 3 .Reprinted with permission from ref. Phan et al. [273] Copyright 2021, The Authors.Published by American Chemical Society.c) In situ AFM images of Cu cubes electrodeposited on highly oriented pyrolytic graphite (HOPG) acquired in air and a CO 2 -saturated 0.1 M KHCO 3 aqueous solution.Reprinted with permission from ref. Grosse et al. [275] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.d) In situ EC-AFM images of electropolished Cu (100) in CO 2 -saturated 0.1 M KHCO 3 recorded at −0.5, −1.0, −1.1 V versus RHE.Reprinted with permission from ref. Simon et al. [276] Copyright 2020, The Authors.Angewandte Chemie International Edition published by Wiley-VCH GmbH.
Energy Environ.Mater.2023, 6, e12552 from being applied to thoroughly investigate electrochemical reduction reactions, accompanied by challenges and opportunities.This review presents some suggestions and perspectives on the future refinement of in situ characterization techniques.

Improving the Accuracy of In Situ Characterization Techniques from the Perspective of Temporal Resolution
The current optimization of in situ characterization techniques mainly aims at improving spatial resolution and energy resolution.For instance, the previously reported in situ fluorescence detection X-ray absorption spectroscopy with small incidence angles (HERFD-XAS) with high energy resolution according to Chen et al. [277] breaks the shackles of conventional X-ray analysis techniques and broadens the application of X-ray absorption spectroscopy K-edge leap signals in catalysis.Moreover, in situ tip-enhanced Raman spectroscopy, a combination of scanning probe microscopy and Raman spectroscopy, has been designed and demonstrated to improve Raman spectral intensity and spatial resolution, which was conducive to reveal potential active sites and verified induced molecular reorientation and nanoscale redox by electrochemistry. [278]However, it is equally important to optimize the temporal resolution of in situ characterization techniques, which can provide more precise insight into dynamic pathways.Still, until now, there has been little research focusing on improving from this perspective.Professor Bert M. Weckhuysen and his co-workers optimized in situ Raman characterization by designing a time-resolved surface-enhanced in situ Raman spectroscopy (TR-SERS). [279]It has a sub-second temporal resolution (about 0.7 s) compared to normal in situ Raman spectroscopy that requires several minutes to acquire data.Through this novel improvement, the production of CO intermediates and dimerization reactions at high potentials can be observed in the spectroscopy, which is hardly achieved by the in situ Raman spectra with low temporal resolution.Recently, sub-second time-resolution in situ XAS and XRD techniques have also been successfully used to reveal the surprising complexity of catalysts exposed to dynamic reaction conditions. [280]Furthermore, the up-to-date free-electron laser (FEL) light sources have a unique temporal system and can be used for in situ microscopy or in situ spectroscopy with temporal resolution up to the femtosecond level (1 fs =10 −15 s), allowing ultrafast time-resolved material properties as well as molecular dynamics studies. [281]Most of the FEL facilities in the world are currently located in Asia, Europe, and the United States.Notably, the largest xFEL situated in Hamburg, Germany, has a high-energy electron beam of 8.5-17.5 GeV and can emit up to 27 000 pulses per second.These unique advantages of FEL can be applied to ultrafast pump-probe spectroscopy to study ultrafast electron dynamics.Several pioneering in situ studies have been carried out, such as ultrafast in situ XAS has been used to monitor the subfemtosecond motion of valence electrons, offering promise for application to explore electrocatalytic reactions. [282]By organically combining femtosecond spectroscopy with existing in situ characterization techniques, the time resolution limitation will no longer exist.With the aid of the time characteristics of femtosecond pulses, the primary proton-coupled electron transfer reactions forming transition states can be tracked and thus the ultra-fast kinetic process of catalytic reaction can be studied, capturing the kinetic characteristics of each elemental reaction dynamically and quantitatively.Moreover, the dynamic evolution process of interface reaction can be fully revealed, and the dynamic coordination mechanism of multicomponent interfacial reactions can be discussed at the molecular level.Thus, it is reasonable to believe that improving the temporal resolution of in situ characterization techniques will be the main development direction in the next phase.

Coupling of Diverse In Situ Characterization Techniques for more Comprehensive Insights into Electrochemical Reaction Mechanisms
Electrochemical energy conversion reactions are generally complex and diverse, especially those involving multiple electron transfer steps such as CO 2 RR, which have numerous reaction intermediates and pathways.Due to the different applications of different characterization techniques, it may be too one-sided to elucidate the possible intermediates or paths in the reaction process only through the aspect of a single in situ characterization technique, which may lead to inappropriate misunderstandings.Nowadays, there is a tendency to comprehensively combine different in situ characterization techniques to interpret the reaction mechanism exhaustively.For instance, the structure and morphology evolution of P-substituted CoSe 2 electrocatalysts during the HER and OER processes were investigated by blending in situ TEM, in situ Raman, and in situ XAS techniques according to Zhu et al. [31] In situ liquid phase TEM is responsible for observing the significant structural and morphological changes of catalysts during the reaction process, which cannot be obtained by in situ Raman or in situ XAS.Likewise, in situ Raman cannot provide the same information about the electronic structure and chemical states as in situ XAS but is a powerful characterization technique for detecting the surface state along with numerous intermediates.Besides, Zhu's group developed s-PdNi nanoparticles as electrocatalysts and in situ XRD was performed to investigate crystal structure and phase evolution during the synthesis process with different temperatures, revealing thermodynamically induced strain relaxation in s-PdNi NPs. [283]In situ spectroscopic investigations including in situ Raman and in situ ATR-FTIR were further carried out to demonstrate that strain relaxation optimized the formation energies of *COOH and *CO intermediates on s-PdNi alloy surfaces, which effectively improved CO 2 RR activity and selectivity.Therefore, the establishment of a network of complementary in situ characterization techniques by sorting out their respective applications, which can help to systematically track the dynamic structure of catalysts, identify the real active sites and precisely investigate the underlying reaction mechanisms, is an inevitable direction for the future development of in situ characterization techniques.

Smart Combination of In Situ Characterization Techniques with Theoretical Calculation to Complementarily Study Electrochemical Systems
Although in situ monitoring of the reaction process has reached an extremely detailed level, in situ characterization alone is not sufficient for a thorough understanding of a complex reaction.For electrochemical conversion reactions, the reaction environment (pH, concentration, and composition of electrolyte and cationic action), reaction intermediates (type, configuration, and selectivity to products), and catalysts (morphological and structural transformation) are all factors that need Energy Environ.Mater.2023, 6, e12552 in-depth investigation.In situ characterization alone cannot ensure that all key intermediates are detected for some multi-step reduction reactions.Moreover, some demanding in situ reactions, such as the detection of the CO 2 RR dynamic evolution under extreme conditions (high temperature and pressure), are challenging to achieve with the current experimental conditions.In recent years, with the booming development of theoretical calculation, various computational methods such as density functional theory (DFT), finite element simulation (FES), molecular simulation (MS), and hydrodynamic simulation have emerged.Through machine learning, which controls calculation conditions and optimizes reaction parameters, these computational simulation methods can effectively simulate the whole electrochemical reaction process and predict the results for some experiments where realistic conditions are difficult or impossible to meet.For instance, Yu's group employed in situ DRIFTS to demonstrate that the formate pathway would be the most suitable reaction scheme on both Ni and NiFe alloy catalysts, while a moderate addition of Fe facilitated the activation of CO 2 via hydrogenation to *HCOO.Meanwhile, the DFT results revealed that the overall energy barrier for CH 4 was lower on the alloy surface and also proved that CO 2 activation via hydrogenation to *HCOO was more preferred than its direct dissociation on both Ni and NiFe alloy surfaces, which was consistent with and complement the in situ DRIFTS results. [284]Besides, as reported by Wang et al., [285] in situ Ranman, in situ ATR-SEIRAS spectrum, and DFT technology were combined to demonstrate the synergy mechanism for NRR caused by engineering ordered vacancies and atomic arrangement over the intermetallic PdM/CNT (M = Pb, Sn, In) nanocatalysts.In situ Raman results indicated the occurrence of N 2 chemisorption and activation on OVs-induced unsaturated Pb sites of the intermetallic Pd 3 Pb in the NRR process.The in situ ATR-SEIRAS was further carried out to investigate the proton supply.Meanwhile, DFT results revealed the whole path diagram of electrocatalytic NRR, and the introduction of the ordered atomic arrangement and vacancies greatly reduced the energy barrier.With the combination of in situ technologies and theoretical calculations, the ordered atomic arrangement was proved to drive nitrogen adsorption and promote water cleavage to supply protons, facilitating the NRR performance.As the theoretical calculation and in situ characterization technology complement each other, they can jointly help to design superior catalysts, optimize material structures, and predict the possible reaction pathways or reaction intermediates.Combining in situ characterization techniques with theoretical calculations and applying them to the investigation of electrochemical reduction reactions will be a crucial research direction in the future, which is expected to play a critical role in vastly improving research efficiency.

)
Tao Qian received his Ph.D. degree from Nanjing University in 2014.During 2015-2020, he was an associate professor at the College of Energy, Soochow University.In May 2021, he became a full professor in the College of Chemistry and Chemical Engineering, Nantong University.His research interests focus on the functional materials in energy storage and conversion systems.Chenglin Yan is a Professor and Dean of the College of Energy at Soochow University in Suzhou, China.He received his Ph.D. from Dalian University of Technology in 2008.In 2011, he became a staff scientist and a group leader at the Institute for Integrative Nanoscience at the Leibniz Institute in Dresden (Germany).His primary research interests focus on electrochemical energy storage.Energy Environ.Mater.2023, 6, e12552 2 of 40

Figure 2 .
Figure 2. Schematic of in situ devices and their unique application to catalysts, intermediates, and products involved in gas-involved electrochemical reduction reactions.The green dash lines represent the electron based in situ characterization technologies, the blue dash lines represent the X-ray based in situ characterization technologies, and the red dash lines represent the spectrum in situ characterization technologies.Copyright 2019, American Chemical Society, Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Copyright 2022, Diamond Light Source Ltd.Published by American Chemical Society, Copyright 2021, The Authors.Advanced Science published by Wiley-VCH GmbH, Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Copyright 2019, American Chemical Society.

Figure 3 .
Figure 3.In situ XAS for CO 2 RR investigation.a) Potential-dependent k 2 -weighted R-space EXAFS for Cu oxide.Reprinted with permission from ref. Nguyen-Phan et al.[121]Copyright 2019, The Royal Society of Chemistry.b) Time-resolved EXAFS spectra without phase-correction of CuO x using Redox Shuttle and chronoamperometry.Reprinted with permission from ref. Lin et al.[123]Copyright 2020, Nature Publishing Group.c) In situ XANES and EXAFS of the Ag@CuO x -32 sample.Reprinted with permission from ref. Chang et al.[124]Copyright 2019, American Chemical Society.d) Representative in situ XANES spectra of 10 wt % CuO@NTO and corresponding linear combination fit determination of Cu species composition.Reprinted with permission from ref.Lawrence et al.[125]Copyright 2022, Diamond Light Source Ltd.Published by American Chemical Society.e) Cu and Zn K-edge XANES spectra for Cu 50 Zn 50 nanoparticles at different times.Reprinted with permission from ref. Jeon et al.[126]Copyright 2019, American Chemical Society.

Figure 4 .
Figure 4.In situ XAS for NRR investigation.a) Normalized in situ Ru K-edge XANES spectra and corresponding FT-EXAFS spectra for SA Ru-Mo 2 CT x .[128]b) The oxidation state of Ru and radial distance of the main peak for SA Ru-Mo 2 CT x .Reprinted with permission from ref. Peng et al.[128]Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) Normalized in situ Bi L 3 -edge XANES spectra for np-Pd 3 Bi.Reprinted with permission from ref. Wang et al.[129]Copyright 2021, Wiley-VCH GmbH.d) Time-dependent XAFS results of VN catalysts at −0.2 V versus RHE and pre-edge area at different potentials.Reprinted with permission from ref. Yang et al.[130]Copyright 2018, American Chemical Society.

Figure 5 .
Figure 5.In situ XAS for HER investigation.a) Normalized XANES at the Pt L 3 -edge and corresponding FT-EXAFS fitting in R-space for Pt SA /N-C.Reprinted with permission from ref. Wang et al.[135]Copyright 2021, Wiley-VCH GmbH.b) WT of the Ru K-edge of the EXAFS spectra of Ex situ and −0.06 V versus RHE for Ru Sas-Ni 2 P. Reprinted with permission from ref. Wu et al.[137]Copyright 2020, Elsevier Ltd.All rights reserved.c) Co and Fe K-edge X-ray absorption near edge structure for cobalt phosphide.Reprinted with permission from ref. Hung et al.[141]Copyright 2019, American Chemical Society.d) In situ XANES spectra and Fourier-transformed R-space spectra and fits for Ni@1T-MoS 2 .Reprinted with permission from ref. Pattengale et al.[142]Copyright 2020, Nature Publishing Group.

Figure 6 .
Figure 6.In situ XPS for CO 2 RR investigation.a) Quasi in situ Ag MNN Auger spectra of O 2 plasma-treated Ag catalysts.Reprinted with permission from ref.Mistry et al.[148]Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.b) Quasi in situ Sn MNN Auger spectra of SnO x /AgO x and SnO x /Ag.Reprinted with permission from ref. Choi et al.[149]Copyright 2019, American Chemical Society.c) Quasi in situ Cu LMM XPS spectra of Ag-and Ptsupported Cu dendrites before and after 1 h of CO 2 RR.Reprinted with permission from ref. Scholten et al.[151]Copyright 2019, American Chemical Society.d) Quasi in situ O 1 s spectra of Cu 2 O and Cu/Cu 2 O under different annealing temperature.Reprinted with permission from ref. Chang et al.[152]Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ O 1 s APXPS spectra of different modified Cu foils.Reprinted with permission from ref.Eilert et al.[154]Copyright 2017, American Chemical Society.f) Quasi in situ Br 3p and I 3d XPS spectra of the Cu_Br and Cu_I measured before and after CO 2 RR.Reprinted with permission from ref. Gao et al.[155]Copyright 2019, The Authors.Published by Wiley-VCH Verlag GmbH & Co. KGaA.
Figure 6.In situ XPS for CO 2 RR investigation.a) Quasi in situ Ag MNN Auger spectra of O 2 plasma-treated Ag catalysts.Reprinted with permission from ref.Mistry et al.[148]Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.b) Quasi in situ Sn MNN Auger spectra of SnO x /AgO x and SnO x /Ag.Reprinted with permission from ref. Choi et al.[149]Copyright 2019, American Chemical Society.c) Quasi in situ Cu LMM XPS spectra of Ag-and Ptsupported Cu dendrites before and after 1 h of CO 2 RR.Reprinted with permission from ref. Scholten et al.[151]Copyright 2019, American Chemical Society.d) Quasi in situ O 1 s spectra of Cu 2 O and Cu/Cu 2 O under different annealing temperature.Reprinted with permission from ref. Chang et al.[152]Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ O 1 s APXPS spectra of different modified Cu foils.Reprinted with permission from ref.Eilert et al.[154]Copyright 2017, American Chemical Society.f) Quasi in situ Br 3p and I 3d XPS spectra of the Cu_Br and Cu_I measured before and after CO 2 RR.Reprinted with permission from ref. Gao et al.[155]Copyright 2019, The Authors.Published by Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 7 .
Figure 7.In situ XPS for NRR investigation.a) In situ XPS data of N 1 s of VO 0.4 N 0.6 obtained before plasma treatment, after O 2 , NH 3 , or O 2 + NH 3 plasma treatment, and after annealing under different conditions.Reprinted with permission from ref. Osonkie et al. [157] Copyright 2020, AIP Publishing.b) In situ Fe 2p XPS, O 1 s XPS of IVR-FO/GDY, and p-FO and C 1 s XPS spectrum of IVR-FO/GDY.Reprinted with permission from ref. Fang et al.[158]Copyright 2021, The Authors.Advanced Science published by Wiley-VCH GmbH.c) In situ W 4f XPS spectra and corresponding contour maps of Wse 2-x in DEs and WISEs.Reprinted with permission from ref. Shen et al.[159]Copyright 2022, American Chemical Society.

Figure 8 .
Figure 8.In situ XPS for HER investigation.a) Ar + etching in situ Ni 2p XPS spectra of the surface and etched surface of c-Ni@aNi(OH) 2 .Reprinted with permission from ref. Hu et al.[160]Copyright 2020, The Royal Society of Chemistry.b) Quasi in situ Ni 3s XPS spectrum of h-NiMoFe and relative percentages of surface Ni species of Ni, NiMo, and h-NiMoFe.Reprinted with permission from ref. Luo et al.[161]Copyright 2021, The Royal Society of Chemistry.c) Au (4f 7/2 ) (black diamonds) and Pd (3d3/

Figure 9 .
Figure 9.In situ XRD for CO 2 RR investigation.a) Contour map of in situ XRD patterns of Pd/C catalyst under LSV and theoretical XRD patterns of Pd and PdH.Reprinted with permission from ref.Sheng et al.[171]Copyright 2017, The Royal Society of Chemistry.b) In situ XRD patterns of Pd/TaC during CO 2 RR test.Reprinted with permission from ref. Wang et al.[172]Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 10 .
Figure 10.In situ XRD for NRR investigation.a) Contour maps of in situ XRD patterns of Au/CP and CP under different conditions.Reprinted with permission from ref. Gao et al.[173]Copyright 2020, Wiley-VCH GmbH.b) In situ XRD contour plots of the transformation of COF from crystalline to amorphous phase.Reprinted with permission from ref. Liu et al.[174]Copyright 2019, Nature Publishing Group.

Figure 11 .
Figure 11.In situ XRD for HER investigation.a) In situ synchrotron XRD patterns of the IrNi 3 nanoparticles during reduction in N 2 and NH 3 flow at increasing temperatures and comparison of in situ synchrotron XRD patterns for IrNi 3 nanoparticles under different conditions.Reprinted with permission from ref. Kuttiyiel et al.[175]Copyright 2014, The Royal Society of Chemistry.b) The image plot, the corresponding diffraction patterns, and selected 2θ region plot of the in situ HT-XRD results.Reprinted with permission from ref. Yang et al.[176]Copyright 2019, Elsevier B.V. All rights reserved.

Figure 13 .
Figure 13.In situ infrared spectroscopy for NRR investigation.a) In situ FTIR spectra of NRR recorded on O-MoC@NC-800 in a N 2 -saturated electrolyte.Reprinted with permission from ref. Qu et al.[197]Copyright 2019, American Chemical Society.b) In situ-FTIR spectra of the NRR on the N, P co-doped hierarchical porous carbon (NPC) electrode and associative mechanism of the NRR process on NPC.Reprinted with permission from ref. Song et al.[199]Copyright 2019, The Royal Society of Chemistry.c) In situ FTIR spectra of BCP and ECOF@BCP under different applied potentials with nitrogen as the feed gas.Reprinted with permission from ref. Liu et al.[200]Copyright 2021, Nature Publishing Group.

Figure 14 .
Figure 14.In situ infrared spectroscopy for HER investigation.a) In situ SEIRA spectra of the Pt electrode surface observed in 0.5 M H 2 SO 4 at the sample potentials.Reprinted with permission from ref. Kunimatsu et al.[203]Copyright 2007, Elsevier Ltd.All rights reserved.b) Potential-dependent SNIFTIR spectra for a polycrystalline platinum electrode in acetonitrile under different atmospheres.Reprinted with permission from ref. Ledezma-Yanez et al.[205]Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) Potential-dependent infrared absorbance spectra for a polycrystalline gold electrode with or without 10 mM of HClO 4 as proton donor and corresponding Tafel plots measured for gold and platinum microelectrodes.Reprinted with permission from ref. Ledezma-Yanez et al.[206]Copyright 2016, Elsevier B.V. All rights reserved.d) In-situ SR-micro-FTIR spectra of monolayer Co-MoS 2 during the HER process.Reprinted with permission from ref. Duan et al.[207]Copyright 2021, Wiley-VCH GmbH.e) The synchrotron radiation infrared spectrometer, in situ cell, and in situ synchrotron radiation infrared spectra of Ru-NPC at different test times.Reprinted with permission from ref. Wu et al.[208]Copyright 2020, Exclusive Licensee Science and Technology Review Publishing House.

Figure 15 .
Figure 15.In situ Raman spectroscopy for CO 2 RR investigation.a) In situ SERS of a Cu surface in CO 2 saturated 0.1 M KHCO 3 and development of the bands at 502 and 524 cm −1 .Reprinted with permission from ref. Moradzaman et al. [223] Copyright 2021, The Authors.ChemElectroChem published by Wiley-VCH GmbH.b) In situ Raman spectra of Cu 2 O/CuO@Ni and in situ Raman spectra of Cu 2 O/CuO, Cu 2 O/CuO@Co, Cu 2 O/CuO@Fe, and Cu 2 O/CuO@Ni.Reprinted with permission from ref. Yang et al. [224] Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) The −CH x stretching region between 2700 and 3000 cm −1 at −0.8 V versus RHE based on in situ Raman monitoring and possible ethoxy intermediate between ethane and ethanol in C 2 reaction pathway.Reprinted with permission from ref. Vasileff et al.[226]Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.d) In situ SERS spectra during cathodic and anodic polarization without and with DAT.Reprinted with permission from ref. Schmitt et al.[228]Copyright 2014, American Chemical Society.e) The Raman peak intensity and in situ Raman mapping of O-Zn-400 (15 × 15 μm 2 ) based on the peak intensity at 566 cm −1 .Reprinted with permission from ref. Chen et al.[229]Copyright 2020, The Royal Society of Chemistry.

Figure 16 .
Figure 16.In situ Raman spectroscopy for NRR investigation.a) Time-resolved in situ Raman spectra of B 11 -VC/CNFs collected at −0.6 V versus RHE.Reprinted with permission from ref. Wen et al.[230]Copyright 2021, Wiley-VCH GmbH.b) In-situ Raman spectra of O-CoP/CNT at −0.5 V versus RHE from 0 to 1 h.Reprinted with permission from ref. Meng et al.[231]Copyright 2021, Elsevier B.V. All rights reserved.c) In situ electrochemical Raman spectra of np-B 13 C 2 at various applied potentials in N 2 -saturated electrolyte.Reprinted with permission from ref. Lan et al.[232]Copyright 2021, Wiley-VCH GmbH.d) In situ Raman spectra and corresponding contour plots of np-PdH 0.43 and np-Pd at various potentials in N 2 -saturated electrolyte.Reprinted with permission from ref. Xu et al.[233]Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ Raman spectra with the N 2 -saturated and Ar-saturated 10 M LiCl as the electrolyte.Reprinted with permission from ref. Wang et al.[234]Copyright 2021, Nature Publishing Group.
Energy Environ.Mater.2023, 6, e12552 appeared at 192, 340, 458, and 495 cm −1 , representing Mo-O 1 mode.This phenomenon indicated the electrochemical generation of MoO 2 , revealing the structural transformation of the catalyst from Mo 2 C-Mo(VI)O x to Mo 2 C-Mo(IV)O x during the HER process (Figure

Figure 17 .
Figure 17.In situ Raman spectroscopy for HER investigation.a) In situ Raman spectra at the Acid-PtNi 1.5 surface in 0.1 M HClO 4 and 0.1 M NaOH.Reprinted with permission from ref. Fan Shen et al. [235] Copyright 2020, Wiley-VCH GmbH.b) In situ Raman spectra of V O -rich Pt/TiO 2 at −0.1 V versus RHE with different electrolysis durations, magnified view of the E g (1) mode, and in situ Raman spectra of V O -rich Pt/TiO 2 , V O -deficient Pt/TiO 2 , and TiO 2 at a potential of −0.2 V versus RHE.Reprinted with permission from ref. Wei et al. [236] Copyright 2021, Wiley-VCH GmbH.c) In situ Raman spectra recorded of MoS x -CE during the cathodic and anodic half sweep.Reprinted with permission from ref. Deng et al. [240] Copyright 2016, American Chemical Society.d) In situ Raman spectra of Mo 2 C-MoO x /CC with chronoamperometry curve at η = 200 mV in 0.1 M HClO 4 .Reprinted with permission from ref.He et al. [242] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.e) In situ Raman-chronoamperometry at different time intervals for Ni/NiO and in situ Raman after 10 000 s of different Ni catalysts.Reprinted with permission from ref. Faid et al.[243]Copyright 2020, Elsevier.f) In situ Raman spectra of SANi-I at OCP and constant potentials (vs RHE).Reprinted with permission from ref. Zhao et al.[245]Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 18 .
Figure 18.In situ mass spectroscopy for CO 2 RR investigation.a) In situ mass spectrometer signals for hydrogen, methane, and ethylene in dependence of potential and composition.Reprinted with permission from ref. Grote et al. [250] Copyright 2016, Elsevier Inc.All rights reserved.b) Cyclic voltammograms and MS signals for CO reductions on the Cu (111) and Cu (100) facets in PBS and NaOH solutions.Reprinted with permission from ref. Schouten et al.[251]Copyright 2012, American Chemical Society.

Figure 19 .
Figure 19.In situ mass spectroscopy for NRR and HER investigation.a) Mass spectra obtained during NRR tests with Bi NPs at −0.7 V versus RHE in N 2saturated 0.10 M Na 2 SO 4 and ion current responses of the m/z signal at 17 at different reaction potentials, conditions, and reaction time.Reprinted with permission from ref. Yao et al. [252] Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.b) In situ DEMS results for determining HER onset potential, CV plot of Mo 3 P at a scan rate of 50 mV s −1 , and the number of electrons per mole of evolved hydrogen at an overpotential of 200 mV.Reprinted with permission from ref. Kondori et al.[254]Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.c) In situ DEMS result of RT sputtered MoS x with glassy-carbon CE.Reprinted with permission from ref. Xi et al.[256]Copyright 2019, American Chemical Society.

Figure 20 .
Figure 20.In situ gas chromatography application in electrochemical cathode reaction.a) The chronoamperometry curve of GO@Ni electrode at −0.5 V versus SCE with in situ GC measurement of hydrogen gas evolution.Reprinted with permission from ref. Sarawutanukul et al.[257]Copyright 2019, Elsevier Ltd.All rights reserved.b) H 2 selectivity of PNG and GNG measured at different potentials by in situ gas chromatography.Reprinted with permission from ref. Wang et al.[258]Copyright 2020, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 21 .
Figure 21.In situ optical microscopy application in electrochemical cathode reaction.a) Examples of sequential optical images recorded at different electrode potentials and reconstructed image of different regions on the ITO h electrode.Reprinted with permission from ref. Ciocci et al.[259]Copyright 2021, Elsevier Ltd.All rights reserved.b) In situ optical images of the dynamic consumption process of N 2 bubbles on common WE and NME@WE, schematic illustration of the common process of electrochemical ammonia synthesis, schematic of strategies to improve efficiency in NRR by NME coating, and the illustration of device structure for in situ optical microscopy visualization experiments.Reprinted with permission from ref. Shen et al.[260]Copyright 2022, Wiley-VCH GmbH.

Figure 22 .
Figure 22.In situ electron microscopy application in electrochemical cathode reaction.a) Scheme of the transformation from Bi-MOF to single Bi atoms and corresponding representative in situ TEM images of Bi-MOF.Reprinted with permission from ref. Zhang et al.[261]Copyright 2019, American Chemical Society.b) Sequence of in situ TEM images at given times during the experiment and corresponding magnified images.Reprinted with permission from ref.Vavra et al.[262]Copyright 2020, Wiley-VCH GmbH.c) Schematic overview of the experimentally observed evolution of the CuO NS morphology probed by in situ TEM.Reprinted with permission from ref. Wang et al.[263]Copyright 2021, Nature Publishing Group.

Figure 23 .
Figure 23.Applications of in situ SECM in electrochemical cathode reaction.a) Schematic of the SG-TC mode for bicarbonate reduction-formate oxidation.Reprinted with permission from ref. Sreekanth et al. [265] Copyright 2014, The Royal Society of Chemistry.b) Schematic depiction of the collection of the CO 2 -radical in TG/SC mode of SECM, CVs of 20 mM CO 2 reduction (top) acquired at a SECM tip, and corresponding i s of CO 2 -radical.Reprinted with permission from ref. Kai et al.[268]Copyright 2017, American Chemical Society.c) Illustration of SI-SECM for determining and the reaction rate constants of H + and N 2 adsorption on Ti 3+ site in TiO 2 NTs and s-TiO 2 NTs.Reprinted with permission from ref. Li et al.[269]Copyright 2020, Wiley-VCH GmbH.d) Formation of H ads by electrochemical UPD of H on Pt and depiction of surface interrogation results.Reprinted with permission from ref. Rodríguez-López et al.[270]Copyright 2010, American Chemical Society.

Figure 24 .
Figure 24.Applications of in situ STM and AFM in electrochemical cathode reaction.a) Low-resolution (200 nm × 200 nm) and zoomed-in (2 nm × 2 nm) in situ EC-STM images of Cu(pc).Reprinted with permission from ref. Kim et al.[272]Copyright 2014, American Chemical Society.b) Schemes of EC-STM and series of in situ EC-STM images of a g-Cu surface during CO 2 RR in 0.1 M CO 2 -saturated KHCO 3 .Reprinted with permission from ref. Phan et al.[273]Copyright 2021, The Authors.Published by American Chemical Society.c) In situ AFM images of Cu cubes electrodeposited on highly oriented pyrolytic graphite (HOPG) acquired in air and a CO 2 -saturated 0.1 M KHCO 3 aqueous solution.Reprinted with permission from ref. Grosse et al.[275]Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.d) In situ EC-AFM images of electropolished Cu (100) in CO 2 -saturated 0.1 M KHCO 3 recorded at −0.5, −1.0, −1.1 V versus RHE.Reprinted with permission from ref. Simon et al.[276]Copyright 2020, The Authors.Angewandte Chemie International Edition published by Wiley-VCH GmbH.