Deciphering the Local Environment of Electrocatalytic Metal Sites with X‐ray Absorption Fine Structure

Electrocatalysis plays a pivotal role in energy conversion and holds significant promise for the development of new energy sources. Understanding the intricate atomic‐level interplay between active sites and electrocatalytic activity is essential for comprehending catalytic behavior and advancing high‐performance catalysts. In this review, an overview of the recent advances in X‐ray absorption fine structure (XAFS) is provided for deciphering the local environment of electrocatalytic metal sites. The application of XAFS in disclosing the electronic interaction and coordination environment of metal sites is summarized, offering insights into the correlation between ligand arrangement and catalytic activity. Special attention is given to advanced XAFS techniques for exploring active species, including determining the actual active sites, monitoring the local structure transformation, and deciphering metal sites–catalysis relationship. The limitations of traditional XAFS have naturally prompted the development of high‐spatiotemporal‐resolution and high‐energy‐resolution techniques as well as in‐situ/operando methods to better understand the entire catalytic process. This review is anticipated to provide a comprehensive understanding of the capabilities of XAFS in exploring the fine structure of electrocatalysts, with implications for the design of advanced catalytic materials.

metal sites during catalytic reactions, and elucidates the structure-catalysis relationship.The review also outlines the main challenges and opportunities for XAFS in exploring electrocatalytic sites.The recent advances and associated challenges discussed in this article will deepen researchers' understanding of this field and thus help guide the future design of highperformance electrocatalysts.

Probing Local Environment of Metal Sites
XAFS is a technique used to measure the absorption coefficient of X-rays, providing detailed information about the local atomic structure and electronic properties.When the energy of the incident X-ray is lower than the binding energy of an elemental orbital, the electron will not be excited to the highest unoccupied state or the vacuum state.However, when the X-ray energy is sufficient, core electrons can be excited to an unoccupied state with strong X-ray absorption, leading to the generation of intense resonance peaks in the spectrum.Additionally, as the X-ray energy increases further, interference effects between emitted and backscattered waves will produce oscillations in the extended XAFS spectrum region.XAFS spectrum typically consists of two components: X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS).XANES is very sensitive to the oxidation state and electronic structure of the measured element, making it an effective strategy for investigating the electronic interactions between metal sites and coordination ligands.EXAFS can selectively determine structural information such as coordination number, bond length, and atomic disorder based on the short-range interactions.Electrocatalytic metal sites are typically found in the form of nanoparticles to maximize the exposure of the surface area to reactive molecules.Single-atom metal sites, clusters, defects as well as well-defined surfaces are regarded as active centers for electrocatalysis. [7,23][26] In this section, we primarily delineate the applications of XAFS in probing the local environment of metal sites, including the electronic interaction and coordination environment.

Electronic Interaction
The oxidation state of the metal site and its electron interaction with the surrounding ligands significantly influence the stability and reaction kinetics of electrocatalyst. [27,28]This part will focus on the use of XANES to investigate the electronic interaction between metal sites and neighboring atoms or molecules, providing insights into the charge transfer for electrocatalytic metal sites.By creating topologies or defects on the supports to provide different coordination environments for metal atoms, the electronic structure of electrocatalytic sites can be precisely controlled. [29,30]For instance, by selectively anchoring Ir metal sites onto the three-fold hollow sites (Ir 1 /T O -CoOOH) and oxygen vacancies (Ir 1 /V O -CoOOH) on defective CoOOH surface, the oxygen evolution catalytic performance can be modulated. [31]s shown in Figure 2a, the slightly higher white line intensity of Ir 1 /V O -CoOOH in Ir L 3 -edge XANES spectra suggests that the Ir(OH) 6 2À anions are more likely to bond to electron-accepting oxygen vacancy sites, where the deficiency of oxygen gives rise to localized positive charge to attract anions.In this context, the Ir(OH) 6 2À sites are deprotonated under the oxidative potential, leading to the Ir valence state of Ir 1 /V O -CoOOH higher than þ4 due to loss of electrons.Additionally, an electron transfer from CoOOH to Ir sites due to metal-support interaction can also be observed in Co L-edge due to the energy shift of Co L 2 and L 3 peaks of Ir 1 /V O -CoOOH (Figure 2b).Thus, the strong electronic interaction between Ir sites and coordination ligands modifies the electronic structure of the active center for stronger electronic affinity to intermediates.
34] Exploring electron interactions between metal centers and coordination atoms using XANES will help us understand the evolution mechanism of catalytic active species.Taking Ni-based electrocatalysts as an example, the reconstructed NiOOH obtained from different nickel-based precatalysts including NiS 2 , NiSe 2 , Ni 5 P 4 (denoted as NiS 2 -Ni(OH) 2 , NiSe 2 -Ni(OH) 2 , Ni 5 P 4 -Ni(OH) 2 , respectively) exhibit different oxygen evolution activities. [35]As shown in Figure 2c

Coordination Environment
XANES is caused by multiple scattering of excited photoelectrons to surrounding atoms.In addition to revealing electronic interaction, XANES can also be utilized to a certain extent to reveal the spatial geometry of the atomic environment. [36]Taking Ga-Sn alloy as an example, the solid-liquid phase transition of Ga-Sn alloys can induce an instant and radical transformation of the coordination environment for metallic Sn sites. [37]The Sn K-edge XNAES of liquid Ga-Sn exhibits an apparent shift of the absorption edge toward a lower absorption energy by 1.3 eV after solidliquid phase transition, indicating a significant change of Sn local environment.By utilizing a Sn 1 Ga 12 icosahedral cluster with a Sn atom in the center and twelve Ga atoms on the vertices as the input structure model, the fingerprints of experimental XANES spectrum of the Sn K-edge are well reproduced in the simulated spectrum.The transition of active components from phase-segregated Sn clusters to dispersed Sn single sites during melting leads to a substantial increase in the selectivity of CO 2 reduction to formate.The local environment of the metal site also greatly influences the electrolytic water-splitting performance.For example, Zang et al. found that the coordination environment of Ni-N x site can affect the performance of hydrogen evolution reaction in alkaline medium. [38]As shown in Figure 3a EXAFS is one of the most effective methods for studying the atomic local environment, which can provide intuitive coordination information through Fourier-transformed (FT)-EXAFS. [39]or example, EXAFS can be utilized to establish correlations between catalyst surface and local structure to investigate the adsorption mechanism of metal sites.Li et al. find that stacking faults can collectively enhance the adsorption energy and transform the non-active Ag into a highly active catalyst. [40]In Figure 3b, FT-EXAFS illustrates that laser-generated Ag nanoparticles (L-Ag NPs) shows 3.5% longer Ag-Ag bond length of than that of monocrystal (S-Ag) and twin crystal (T-Ag) Ag NPs.The Reproduced with permission. [31]Copyright 2022, Springer Nature.c) Normalized Ni K-edge XANES spectra of X-Ni(OH) 2 (X = NiS 2 , NiSe 2 , Ni 5 P 4 ) with Ni(OH) 2 as the benchmark.d) The correlation between the NiO 6 octahedron distortion extent and the corresponding electron transfer ability for X-NiOOH (X = NiS 2 , NiSe 2 , and Ni 5 P 4 ).Reproduced with permission. [35]Copyright 2023, Royal Society of Chemistry.
average coordination number value can be determined from the fitting of Ag-Ag shell.L-Ag has the lowest coordination number of %8.37 compared with other samples (Figure 3c), suggesting the stacking faults and surface steps of L-Ag jointly contribute to its ultralow coordination number.Ag sites with low coordination number have weaker Gibbs free energy of hydrogen adsorption, resulting in a high hydrogen evolution activity.[43] In this regard, the analysis of M-N x catalysts with different coordination configurations using EXAFS can aid in the development of highperformance M-N x sites with well-defined structures.For instance, in the case of Fe-N 4 sites, [44] the ammonia assisted pyrolysis can convert pyridine nitrogen to pyrrole nitrogen, forming Fe-N 4 site of pyridine nitrogen coordination (Figure 3d).In Figure 3e of the N K-edge X-ray absorption spectra (XAS), the FeN 4 catalyst synthesized from ammonia pyrolysis (HP-FeN 4 ) exhibits a pyrrole-coordinated Fe structure.Fe K-edge FT-EXAFS also reveals that Fe-Fe interaction was not observed in both pyridine-type Fe-N 4 (FeN 4 ) and HP-FeN 4 catalysts.The main peaks at %1.55 Å can be ascribed to the Fe-N coordination shell, confirming the existence of atomically dispersed FeN 4 sites on the graphene framework.Combined with catalytic performance tests, it can be found that pyrrole-type Fe-N 4 sites exhibit better oxygen adsorption energy and ultra-high four-electron reaction selectivity, which behaves significantly enhanced intrinsic activity compared with other Fe-N 4 sites.
In summary, XAFS provides a valuable characterization method for studying the local environment of the electrocatalytic active site, which is crucial for exploring the key factors affecting catalytic activity.

Advanced XAFS Techniques in Exploring Active Species
The development of XAFS has also facilitated the use of advanced characterization techniques of high-spatiotemporal resolution or high-energy resolution, such as HERFD-XAS, GI-XAFS, Q-XAFS, and various in-situ/operando methods, to delve deeper into the potential reaction mechanisms of active species.For example, HERFD-XAS provides significantly higher energy resolution (%1 eV) compared to conventional solid-state fluorescence detection (50-200 eV), allowing for the detection of fine features in XANES pre-edge regions. [14,45]GI-XAFS, with a near-surface probing depth of nanometers, can be employed to investigate the surface active species during catalysis. [15,46]Q-XAFS can offer time-resolved XAFS spectra, serving as a powerful tool for studying electrocatalytic reaction kinetics. [16]Moreover, in-situ/operando methods based on advanced XAFS also provides a valuable non-destructive tool for detecting the active sites and  [38] Copyright 2021, Wiley-VCH GmbH.b) Ag K-edge FT-EXAFS spectra for L-Ag, S-Ag, and T-Ag catalysts with standard Ag foil as reference.c) The average coordination numbers of L-Ag, S-Ag, and T-Ag by EXAFS fitting.Reproduced with permission. [40]Copyright 2019, Springer Nature.d) Preparation process of high-purity pyrrole-type Fe-N 4 structure.e) N K-edge XAS spectra of HP-FeN 4 and FeN 4 .Reproduced with permission. [44]Copyright 2020, Royal Society of Chemistry.
structural changes of electrocatalysts under reaction conditions.For electrocatalytic reactions such as CO 2 reduction, water splitting and fuel cell, the real-time XAFS measurements are usually carried out on a custom-made glass or metal electrochemical cells combined with a computer-controlled electrochemical analyzer. [12]The catalyst is loaded onto a self-supporting electrode or a carbon paper support as the working electrode. [10]XAFS spectra are collected in fluorescence or transmission mode.By recording the spectra of catalysts at different potentials and saturated gas molecules, the corresponding XANES and EXAFS information can reflect the structural evolution of catalysts during catalysis.In this section, we primarily focus on the applications of advanced XAFS techniques in exploring active species, including determining the actual active sites, monitoring the local structure transformation and deciphering metal sites-catalysis relationship.

Determining the Actual Active Sites
Until now, the identification of actual catalytic active sites has been a topic of wide attention. [47,48][51] Figure 4a demonstrates the application of operando HERFD-XAS to elucidate the active sites of Cu nanoparticles (Cu NPs). [52]The HERFD-XANES region reveals that Cu NPs exhibits the same pre-edge energy as bulk Cu 2 O, indicating full oxidation of the NPs.However, Cu NPs displayed edge features similar to those of Cu foil during CO 2 reduction.Upon post-electrolysis air exposure, the NPs completely re-oxidized to Cu 2 O. Quantitative valence analysis using HERFD-XANES shows the metallic Cu fraction increasing from 0 to 100% over 1 h. Figure 4b [52] Copyright 2023, Springer Nature.d) GI-XANES, e) GI-EXAFS of Ir L 3 -edge, and f ) O K-edge XAS of SrIrO 3 under different reaction time.g) Proposed crystalline-to-amorphous transformation pathway in SrIrO 3 .Reproduced with permission. [55]Copyright 2021, American Association for the Advancement of Science.
highlighting the importance and potential of operando methods in contributing to the rational design of future nanoscale electrocatalysts.
Moreover, electrocatalyst often undergoes structural evolution of amorphous layer forming from the crystalline surface during catalytic process. [53,54]Taking SrIrO 3 film as an example, the structural evolution of the amorphous layer during potential cycling can be observed by GI-XAFS. [55]As depicted in Figure 4d, the Ir L 3 -edge GI-XANES indicates a decrease and subsequent increase in the white-line intensity, suggesting the reduction-oxidation process of surface Ir atoms.The reduction of Ir starts with lattice oxygen activation at high oxidation potential, resulting in lattice oxygen loss and increased oxygen vacancy concentration, transforming the surface SrIrO 3 layer into low-coordination amorphous Ir 3þ oxide.This transformation is further confirmed by GI-EXAFS, which shows a decrease in the Ir-O coordination number from 6.0 to 4.5 (Figure 4e).The presence of oxygen vacancies accelerates the leaching of Sr 2þ , destabilizing the defect-rich structure and causing Ir to reoxidize, forming a disordered mixture of Ir 3þ /Ir 4þ octahedral clusters (Figure 4g).Additionally, the change in the π and σ transition peak of O K-edge in Figure 4f suggests the transformation of the Ir octahedral active site network with gradually increasing disorder.This amorphization process turns SrIrO 3 into a highly disordered Ir octahedral network with an Ir square-planar motif, demonstrating the signature of active moiety.

Monitoring the Local Structure Transformation
The rational design and optimization of targeted catalysts with desirable performance depend on comprehensive insights into the nature of the active structure of catalysts under real conditions.[58] For instance, Cheng et al. utilized operando XAFS to disclose the local structure transformation of Ni sites in Ni-based metal-organic framework (Br-Ni-MOF) catalyst. [59]As shown in Figure 5a, a red shift of the first and second dominant peaks at 1.10 V suggests the initial reconstruction of Ni sites into in Br-Ni-MOF.When the potential increases to 1.35 V, the second major peak at about 2.85 Å undergoes a split into two peaks.Notably, at higher potentials of 1.45 and 1.55 V, the first and second dominant peaks show a blue shift, indicating the formation of final active structure of Ni centers toward oxygen evolution.Additionally, XANES analyses of Br-Ni-MOF show an average metal valence state of Ni 3.3þ at 1.55 V, indicating the in-situ formed γ-NiOOH analog.As shown in Figure 5b, when the applied potential is turned back to 1.10 V from 1.55 V, this in-situ formed γ-NiOOH structure can be quickly converted into β-Ni(OH) 2 analog.The potentialdependent metastable γ-NiOOH sites are confirmed to be the real highly active structure for the Br-Ni-MOF system based on the oxygen evolution test.
Another type of catalyst system with homogeneous active sites is the single-atom catalyst.Understanding the dynamic evolution of single-atom sites at the solid-liquid interfaces under working conditions is essential for fundamental understanding of electrocatalytic mechanisms and optimal design of advanced electrocatalysts. [60,61]Through operando XAFS, Su et al. identify a general evolution of single-atom Ni sites at the atomic level into a nearfree atom state during oxygen reduction. [62]In Figure 5c, the Reproduced with permission. [59]Copyright 2021, American Association for the Advancement of Science.c) The first-shell fitting of EXAFS spectra under 0.95, 0.85 and 0.70 V versus RHE working conditions for Ni-N 4 single-atom catalyst.The insets show the corresponding geometric configurations.Reproduced with permission. [62]Copyright 2020, American Chemical Society.d) In situ XANES e) FT-EXAFS spectra for NiS at various operation conditions compared to Ni-NC single-atom catalyst.Reproduced with permission. [66]Copyright 2022, Wiley-VCH GmbH.
single dominant peak at 1.3 Å in Ni K-edge FT-EXAFS of Ni-N 4 single-atom catalyst has greatly changed, with %20% damping of the peak intensity and a slightly positive shift of 0.05 Å for the peak centers as the applied potential decreased from 0.95 to 0.70 V, suggesting distinct variations in the local coordination environments of Ni single sites.Quantitative EXAFS fitting showed that the coordination number of Ni-N gradually decreases with the applied potential decreasing, followed by the appearance of Ni-O coordination.At 0.70 V, the Ni atoms are released from the N-C substrate to form a metastable, near-free, isolated Ni (2Àδ)þ -N 2 active site with a much less coordinated and near-neutral atomic state structure, which facilitates surface adsorption of oxygen molecules, thus achieving efficient oxygen reduction.
[65] For example, in the case of NiS, with the increase of CO 2 reduction current density, the shape of XANES peak of NiS gradually becomes similar to that of the Ni-NC single-atom catalyst such as the decreasing white line peak intensity and energy shift of absorption edge (Figure 5d). [66]In Figure 5e

Deciphering Metal Sites-Catalysis Relationship
Due to its minimal sample type and preparation requirements, XAFS greatly facilitates in-situ/operando monitoring of samples and has been proven to be a powerful tool for studying the reaction mechanism and establishing the structure-catalysis relationship of electrocatalytic metal sites. [67,68]For example, the controversy over the correlation between reaction product selectivity and metal active species has hindered the design and application of high-performance CO 2 reduction electrocatalysts, whereas operando Q-XAFS can reveal the complexity and the relationship between structure and selectivity under dynamic reaction conditions. [69,70]By using operando Q-XAFS and a pulsed reaction protocol, Timoshenko et al. have dynamically perturbed the Cu 2 O nanocube derived catalyst to decouple the effect of coexisting copper species on the product distribution. [70]n Figure 6a,b, the periodic reversible changes of Cu 0 /Cu 1þ / Cu 2þ in the catalyst structure and composition under pulsed CO 2 reduction with anodic pulse Δt a = 10 s and anodic pulse potential E a = 0.6 V are evidenced by second-resolution Q-XAFS.Linear combination analysis and EXAFS fitting reveals that that the Cu 1þ fraction increases and Cu-O bond formation during the anodic pulse within a few seconds, while an opposite trend is observed in the Cu 0 concentration, and the Cu 2þ concentration varies much less.This implies that only the near-surface layers of the catalyst are reoxidized during the anodic pulse, while the metallic core structure is maintained.As a result, a kinetic model can be proposed to predict the time dependencies of the concentrations of Cu 0 , Cu 1þ , and Cu 2þ species at arbitrary pulse to identify the relationship between catalyst surface compositions and distinct catalytic properties (Figure 6c).The enhanced ethanol production can be attributed to an optimal balance between oxidized and reduced copper species on the catalyst surface, and the presence of a distorted copper oxide phase.
[73] Taking the Ni-Fe-N-C catalyst as an example, molecule-like bimetallic Ni-Fe active sites can be synthesized from the single-atom Ni-N-C precursors via in-situ electrochemical transformation. [71]As depicted in Figure 6d, the HERFD-XANES Ni K-edge spectra of the Ni-N-C precatalyst exhibits two regions, a lower energy region at %8335 eV and a higher energy region at around 8340 eV.These two regions correspond to the dipole 1s!4p transition caused by 3d-4p mixing and a metal to ligand electronic transition of hybridization between 2p(N/O) and 4p(Ni) states, respectively. [74]Upon immersion in the electrolyte, the widening and decrease in the pre-edge peak at the low-energy region indicate an increase in coordination number due to solvation caused by OH À or H 2 O around Ni. Following cyclic voltammetry cycles, the symmetry of Ni transformed to a highly symmetrical octahedral due to the desorption of nitrogen and attaching of oxygen, leading to a further decrease in the pre-edge peak area.The decreased intensity of metal to ligand electronic transition at higher energy region also suggests the change of the coordination environment of Ni sites.The atomic ratio of Fe to Ni during electrochemical transformation can also be examined by HERFD-XANES.In Figure 6e, the number of Fe ions notably increased upon activation and reached a steady state of about 25% relative to Ni after 3 h catalysis.Moreover, the 1s !3d transition at about 7115 eV Fe K-edge HERFD-XANES can be fit to two Gaussian contributions, E g and T 2g configurations, respectively.The energy split of the two configurations was about 1.3 eV, similar to that of Fe(NO 3 ) 3 , suggesting an asymmetric structure of the Ni-Fe units with symmetrical Oh of six coordinated H 2 O ligands.The average oxidation state of Fe ion was higher than þ3, suggesting the formation of substantial Fe 4þ species during OER.Consequently, the incorporation of Fe can facilitate the formation of O-O bond due to the easier Fe 3þ -OH to Fe 4þ =O oxidation process compared with Ni sites, resulting in nearly 1000-fold increases of turnover frequencies.By bridging advanced XAFS techniques and operando characterization methods, an attractive platform is provided for the study of the structure-catalysis relationship of electrocatalytic metal sites.

Future Outlook
XAFS technique is a powerful method for investigating the local environment of electrocatalytic metal sites.It enables the clear elucidation of the relationship between metal sites and catalysis in electrocatalysts by identifying the metal active centers and monitoring the dynamic reaction process in real time.This is crucial for gaining a comprehensive understanding of the catalytic mechanism and for the rational design of efficient catalytic systems.Nonetheless, there are still some challenges and opportunities for future research in this field.

High-Spatiotemporal-Resolution Exploration of Metal Sites
Although the techniques and applications for characterizing electrocatalytic metal sites using XAFS have seen remarkable progress, improving temporal and spatial resolution remains a primary objective for researchers in this field.On one hand, the pursuit of high spatial resolution has spurred the development of modern X-ray sources, monochromators, optics, and detectors.Consequently, synchrotron radiation-based XAFS has achieved a spatial resolution of less than 10 nm. [16,75]Notably, Ajayi et al. have demonstrated that by linking synchrotron X-rays with a quantum tunneling process, it is possible to use X-rays to characterize the elemental and chemical state of a single atom. [76]s depicted in Figure 7a,b, the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique can detect the X-ray-excited currents generated from an iron atom coordinated to organic ligands, enabling a X-ray absorption spectroscopy of just one atom.On the other hand, the development of X-ray monochromators has facilitated the rapid scaling XAFS experiments with a temporal resolution below 10 ms, which holds great potential catalytic kinetics studies. [77]In the future, the utilizing of high-spatiotemporal-resolution XAFS to separate the real active site signal and investigate their structure evolution from the ensemble average signal will be a focal point of future research on electrocatalytic metal sites.

ML-Assisted Heterogeneity and Complex Processes Analysis
While XAFS is a leading technique for understanding the local environment of electrocatalytic metal sites, its ensemble averaging nature presents significant limitations.The coexistence of metal ions in different chemical states and environments, such as active and inactive phases under electrocatalytic conditions, poses challenges for the analysis of the active species' local environment.[80] ML methods, such as principal component analysis, K-means  [70] Copyright 2022, Springer Nature.d) Operando Ni K-edge pre-edge and e) Fe K-edge HERFD-XANES of Ni-Fe-N-C at different activation times.Reproduced with permission. [71]Copyright 2021, Springer Nature.
[83] For instance, Timoshenko et al. have utilized operando Q-XAFS coupled with unsupervised and supervised ML methods to unveil the real active species and active states of spinel-like Co x Fe 3Àx O 4 . [82]As shown in Figure 7c, d, they employed principal component analysis to discern subtle changes in XANES spectra and applied an artificial neural network to interpret the EXAFS spectra.This enabled them to track the evolution of tetrahedral and octahedral coordinated Co-O x sites, and to elucidate the chemical changes and several phase transitions that occurred on the Co x Fe 3Àx O 4 catalyst.Overall, the ML-assisted XAFS analysis method holds great potential for establishing a bidirectional mapping between catalyst structure and spectrum, and is poised to be a powerful tool for future studies of the local environment of metal sites.

Industrial Catalysis Applications
The rational design of electrocatalysts should not only aim to understand the structure-catalysis relationship at a fundamental level, but also to meet the practical requirements of industrial applications.In this regard, there is a need to develop advanced XAFS methods and operando devices that are suitable for industrial catalytic systems.86] Currently, XAFS characterization techniques for electrocatalysis predominantly focus on in-situ measurements in three-electrode systems.Few studies have reported operando monitoring of electrocatalytic active sites in flow battery systems or membrane electrode assemblies, which are suitable for industrial applications.Industrial electrocatalysis often involves a more complex reaction environment, larger reaction currents or voltages, and extreme working conditions. [87,88]This necessitates a deep consideration of whether the catalytic mechanism established under laboratory conditions is equivalent to that of industrial conditions.To transition from evaluation models to industrial applications, it is imperative to place greater emphasis on the application and understanding of XAFS in electrocatalytic reaction devices with high industrial potential, such as flow cells and fuel cell systems.This will be critical for addressing the unique challenges and requirements of industrial electrocatalysis and for advancing the practical implementation of XAFS in this field.

Integration XAFS with Other Characterization Techniques
In addition to local environments of electrocatalytic metal sites, there are dynamic changes in the spatial distribution of metal species, long-range ordered structures, reaction intermediates, and other structural parameters during the electrocatalytic process, all of which are important considerations.Therefore, the Reproduced with permission. [76]Copyright 2023, Springer Nature.c) Evolution of partial radial distribution functions for tetrahedrally and octahedrally coordinated Co-O x sites in Co 2.25 Fe 0.75 O 4 during oxygen evolution.d) Schematic depiction of the structure and composition-dependent transformations of the Co x Fe 3Àx O 4 electrocatalyst after activation and during oxygen evolution.Reproduced with permission. [82]Copyright 2023, American Chemical Society.
integration of XAFS with X-ray diffraction, electron microscopy, Raman/infrared spectroscopy, and other in-situ/operando characterization techniques to explore the reaction process from a deeper and broader perspective based on the combined use of multiple technologies will be an inevitable trend in future catalyst research.For example, operando scanning transmission electron microscopy, Raman combined with XAFS have been used to identify the active species of Cu-based catalysts for CO 2 electrochemical reduction, providing deeper insights into morphology, reaction intermediates, and atomic local structure changes. [52,89]The development of combined techniques will provide the possibility for a more comprehensive and in-depth study of electrocatalysts.

Conclusion
In conclusion, this review highlights the recent advancements in elucidating the local environment of electrocatalytic metal sites through XAFS techniques.Utilizing XAFS measurements allows for a comprehensive understanding of the coordination environment and electronic interaction of metal sites, providing insights into the structural and functional correlations at the atomic level.Furthermore, we have outlined the progress made in advanced in-situ/operando XAFS for identifying actual active sites and monitoring the local environmental evolutions of metal sites.Although XAFS has significantly contributed to establishing the structure-catalysis relationship of metal sites, there are still some areas that require attention, particularly the development of advanced high-spatiotemporal-resolution XAFS techniques, machine learning (ML)-assisted analysis methods, and their integration with industrial applications.It is our hope that these advancements will yield more exciting results in the field of electrocatalysis, further enhancing the capabilities of XAFS techniques and contributing to the development of efficient and sustainable electrocatalytic systems.
, NiS 2 -Ni(OH) 2 exhibited the lowest white line intensity, followed by NiSe 2 -Ni(OH) 2 , Ni 5 P 4 -Ni(OH) 2 , and Ni(OH) 2 , respectively.The white line related to the electron jumping from the 1s to the 4p orbital can reflect the geometrical distortion of the NiO 6 octahedron.The extent of NiO 6 octahedron distortion is significantly increased in the order of Ni(OH) 2 , Ni 5 P 4 -Ni(OH) 2 , NiSe 2 -Ni(OH) 2 , and NiS 2 -Ni(OH) 2 due to the different electron interactions between Ni and coordination anions.The high-distortion NiO 6 octahedron can lead to the stronger broadening of the e g * band aroundthe Fermi level, which may enhance the electron transfer mobility to further improve the oxygen evolution reaction (OER) activities (Figure2d).

Figure 1 .
Figure 1.Schematic illustration of XAFS in deciphering the local environment of electrocatalytic metal sites.
, the Ni K-edge XANES of Ni-SA/NC catalyst can be best reproduced by a mixed coordination structure composed of Ni-N 4 and Ni-N 3 -O 2 , with a calculated Ni-N/O coordination number of 4.3, suggesting the adsorption of one oxygen molecule in the Ni-N 3 structure.By employing density functional theory calculations, they find that Ni-N 3 is capable of stabilizing hydrogen adsorbates, whereas Ni-N 4 interacts with the adsorbate too weakly.These results confirm that the Ni-N 3 coordination accounts for hydrogen evolution catalytic activity.

Figure 3 .
Figure 3. Probing coordination environment of metal sites.a) Experimental Ni K-edge XANES curve of Ni-SA/NC and calculated curves of the proposed Ni-N 4 and Ni-N 3 -O 2 coordination.Inset: experimental EXAFS curve of Ni-SA/NC and fitting curve of Ni-N 4 and Ni-N 3 -O 2 mixed structure.Reproduced with permission.[38]Copyright 2021, Wiley-VCH GmbH.b) Ag K-edge FT-EXAFS spectra for L-Ag, S-Ag, and T-Ag catalysts with standard Ag foil as reference.c) The average coordination numbers of L-Ag, S-Ag, and T-Ag by EXAFS fitting.Reproduced with permission.[40]Copyright 2019, Springer Nature.d) Preparation process of high-purity pyrrole-type Fe-N 4 structure.e) N K-edge XAS spectra of HP-FeN 4 and FeN 4 .Reproduced with permission.[44]Copyright 2020, Royal Society of Chemistry.
illustrates the scattering amplitude change of the Cu-Cu interactions in Cu K-edge FT-EXAFS spectra, indicating the electroreduction/ reoxidation life cycle of Cu NPs from Cu oxide to undercoordinated metallic Cu sites, aggregated Cu nanograins, and back to Cu oxide.By exploring the structural transformation of Cu NPs with different particle sizes, the fraction of active Cu nanograins is found to correlate with higher C 2þ selectivity (Figure 4c),

Figure 4 .
Figure 4. Determining the actual active sites.a) Operando HERFD-XANES spectra of 7 nm Cu NPs with bulk Cu and Cu 2 O as references.b) Operando HERFD-EXAFS of 7 nm Cu NPs under CO 2 RR conditions and following exposure to air.c) Structure-activity correlation of relative fraction of active Cu nanograins and C 2þ faradaic efficiency of Cu NPs ensembles with three different particle sizes.Reproduced with permission.[52]Copyright 2023, Springer Nature.d) GI-XANES, e) GI-EXAFS of Ir L 3 -edge, and f ) O K-edge XAS of SrIrO 3 under different reaction time.g) Proposed crystalline-to-amorphous transformation pathway in SrIrO 3 .Reproduced with permission.[55]Copyright 2021, American Association for the Advancement of Science.

Figure 5 .
Figure 5. Monitoring the local structure transformation.a) FT-EXAFS and b) XANES spectra of Ni K-edge for Br-Ni-MOF hollow prisms under various potentials.Reproduced with permission.[59]Copyright 2021, American Association for the Advancement of Science.c) The first-shell fitting of EXAFS spectra under 0.95, 0.85 and 0.70 V versus RHE working conditions for Ni-N 4 single-atom catalyst.The insets show the corresponding geometric configurations.Reproduced with permission.[62]Copyright 2020, American Chemical Society.d) In situ XANES e) FT-EXAFS spectra for NiS at various operation conditions compared to Ni-NC single-atom catalyst.Reproduced with permission.[66]Copyright 2022, Wiley-VCH GmbH.
, the FT-EXAFS spectra show that the introduced O species can induce a shortened bond length (Ni-O x S y ) compared to that of pristine NiS.As the applied current density increased, the bond length of the NiS electrode (Ni-O x-z S y ) became slightly longer owing to the deoxygenation by the cathodic potential.This modulated Ni-O x-z S y bond length of NiS is similar to that of Ni single-atom catalyst, suggesting the critical role of balancing between S and O species.These results demonstrate that the NiS electrode can mimics the electronic structure and catalytic activity of Ni-NC single-atom catalyst by having a broken D4h symmetry and modulated O xÀz S y ligand.

Figure 6 .
Figure 6.Deciphering metal sites-catalysis relationship.Changes in a) XANES b) FT-EXAFS under an anodic pulse of the operando Cu K-edge during CO 2 reduction, data corresponding to different times since the onset of the anodic pulse are shown in color bar.c) Schematic depiction of the catalyst structure and composition during a cathodic pulse.Reproduced with permission.[70]Copyright 2022, Springer Nature.d) Operando Ni K-edge pre-edge and e) Fe K-edge HERFD-XANES of Ni-Fe-N-C at different activation times.Reproduced with permission.[71]Copyright 2021, Springer Nature.

Figure 7 .
Figure 7. Future outlook.a) Schematic of SX-STM in the tunnelling regime.b) Simultaneously recorded XAS spectra of tip channels in the tunnelling regime.Reproduced with permission.[76]Copyright 2023, Springer Nature.c) Evolution of partial radial distribution functions for tetrahedrally and octahedrally coordinated Co-O x sites in Co 2.25 Fe 0.75 O 4 during oxygen evolution.d) Schematic depiction of the structure and composition-dependent transformations of the Co x Fe 3Àx O 4 electrocatalyst after activation and during oxygen evolution.Reproduced with permission.[82]Copyright 2023, American Chemical Society.