Tandem catalysis in electrocatalytic nitrate reduction: Unlocking efficiency and mechanism

The electrochemical nitrate reduction reaction (NO3RR) holds promise for ecofriendly nitrate removal. However, the challenge of achieving high selectivity and efficiency in electrocatalyst systems still significantly hampers the mechanism understanding and the large‐scale application. Tandem catalysts, comprising multiple catalytic components working synergistically, offer promising potential for improving the efficiency and selectivity of the NO3RR. This review highlights recent progress in designing tandem catalysts for electrochemical NO3RR, including the noble metal‐related system, transition metal electrocatalysts, and pulsed electrocatalysis strategies. Specifically, the optimization of active sites, interface engineering, synergistic effects between catalyst components, various in situ technologies, and theory simulations are discussed in detail. Challenges and opportunities in the development of tandem catalysts for scaling up electrochemical NO3RR are further discussed, such as stability, durability, and reaction mechanisms. By outlining possible solutions for future tandem catalyst design, this review aims to open avenues for efficient nitrate reduction and comprehensive insights into the mechanisms for energy sustainability and environmental safety.


| INTRODUCTION
[31] First, compared with traditional methods like ion exchange, reverse osmosis, and electrodialysis, electrocatalytic reduction technology offers the advantage of being applicable under a wide range of conditions, including controlled pH levels and various nitrate concentrations. [32,33]][36] Third, the selectivity of the electrocatalytic reduction process can be adjusted to produce either ammonium or nitrogen gas, offering flexibility and control over the desired outcome.[39] Fifth, through the conversion of nitrate into ammonium, the electrocatalysis system enables the recovery of valuable nitrogen resources, which can be utilized in agriculture or other applications, offering additional economic and environmental benefits. [40,41]As depicted in Figure 1, the potential applications of electrocatalytic reduction of nitrate and the corresponding reaction pathways are illustrated, providing a visual representation of the various uses and processes involved in this technology.
][44] Typically, in the nitrate reduction process, the formation of nitrite is considered the rate-determining step, while the subsequent hydrogenation steps are acknowledged as crucial for achieving desired selectivity. [19,45,46]These mechanistic pathways illustrate the sequential formation of different intermediates through either an indirect autocatalytic pathway or a direct reduction reaction mechanism, providing a comprehensive understanding of the stages involved in the electrocatalytic reduction of nitrate. [47]For high NO 3 − concentrations (>1 M), the reduction process usually takes place indirectly through intermediates such as NO + or NO 2 • in an acidic medium, rather than directly reducing NO 3 − . [48,49]On the contrary, in the direct electrocatalytic reduction mechanism, the process involves the adsorption of hydrogen and electron reduction pathway, where active hydrogen acts as the reducing agent. [26,50,51]Thus, the rational design of catalysts with active sites within a single unit for different reaction steps is crucial for achieving highly efficient and selective reduction of NO 3 − .][62][63] Through the rational design of multiple active sites, electrochemical NO 3 RR catalysis can synergistically facilitate various reaction steps, resulting in enhanced overall performance in nitrate reduction.[66][67][68] Accordingly, there is an urgent need for a detailed and systematic review based on the previous tandem catalysis research in electrochemical NO 3 RR.The existing reviews have largely focused on the advancements summarized in the electrocatalysts design for the reduction of NO 3 − to nitrogen gas or ammonium.In this review, we present a systematic summary of the recent advancements in the electrocatalytic reduction of nitrate using a relay mechanism, drawing inspiration from the benefits of tandem catalysis and the efficient generation of ammonium or nitrogen gas.This review encompasses the examination of the reaction mechanism, principles of tandem catalyst design, criteria for performance evaluation, and various in situ characterization methods employed in the tandem catalysis of electrochemical NO 3 RR.In addition, this review provides detailed discussions on electrocatalysts incorporating transition metals, noble metals, and important electrochemical strategies, such as pulse electrocatalysis.Furthermore, the principles and rules for designing associated tandem catalysts are also summarized.Finally, the discussion encompassed the understanding of large-scale implementation for electrochemical NO 3 RR, including the bottlenecks identified by fundamental research, challenges and perspectives along tandem catalysis, possible designs for electrochemical reactors, and the reduction mechanism of half-cells.

| MECHANISMS OF ELECTROCHEMICAL NO 3 RR
Electrochemical NO 3 RR in aqueous solutions involves a complex proton-coupled electron transfer reaction, with options for either a five-electron reduction process yielding N 2 or an eight-electron side reaction producing ammonia.The presence of diverse reaction intermediates, including ammonia, nitrite, hydrazine, hydroxylamine, nitric oxide, and nitrous oxide, with oxidation states ranging from −3 to +5, adds complexity to comprehending the reaction mechanism on the specific electrode surface.[71] In addition, practical conditions such as pH, electrolyte composition, temperature, and catalyst selectivity exert a significant influence on the final reaction products and detailed reaction pathways, as demonstrated by Equations ( 1) and ( 2). [72] In the typical indirect mechanism, the Vetter and Schmid pathways are involved with high nitrate concentrations and high acidic media, including electron transfer, intermediate step, and nitrous acid formation processes. [38]n most cases of nitrate reduction, which are typically conducted at low concentrations in neutral or alkaline environments, a direct mechanism is commonly associated. [73]Figure 2 illustrates the related processes of electron transfer reduction and atomic hydrogen reduction, which typically involve the steps of nitrate adsorption, nitrate-to-nitrite conversion, and nitrite reduction.Thus, the overall rate and efficiency of electrochemical NO 3 RR are significantly influenced by the nitrate concentration and electrocatalyst design.[76] The electron reduction pathways begin with the initial adsorption and reduction of nitrate into nitrite, followed by subsequent reductions that contribute to the formation of adsorbed NO (Equations 3-5).Subsequently, the generated NO (ads) can form in the solution and undergo dimer evolution into N 2 O, which ultimately undergoes transformation into nitrogen gas (Equations 6-8). [34]his reaction pathway typically occurs on electrocatalysts with a high nitrate adsorption capacity, such as the Cu electrode.
On the other hand, the atomic hydrogen pathway is also widely reported due to the simultaneous reduction of water through the Volmer process during electrochemical NO 3 RR (Equation 9). [77]The highly reactive active hydrogen can participate in the subsequently various reductions, for example, the reduction of NO 3 − into the intermediate of NO 2 − and NO (Equations 9-15). [34]Typically, in nitrogencontaining species, the N─H bond can react to yield NH 4 + ions, where active hydrogen donates an electron to the nitrogen atom, while two active hydrogen atoms (N ads ) can also potentially combine to form nitrogen gas (N 2 ). [78]owever, despite the high activation energy (0.75 eV) required for the N─N triple bond, it is not the preferred selectivity because the formation of the N─H bond, initiated by H ads , has a significantly lower energy barrier (0.1 eV) and is more favorable. [79]Hence, the multielectron transfer reactions involving numerous intermediates and various reaction steps emphasize the importance of designing specialized active sites with specific purposes.
In electrochemical NO 3 RR, tandem catalysts usually comprise "promoter" and "selector" sites, which synergistically enhance both the conversion of nitrate into nitrite and the subsequent reduction of nitrite.The key to controlling tandem catalysis lies in the coordinated transport of reactive intermediates among catalytic sites, highlighting the inadequacy of random physical mixtures of catalysts in mediating the desired reaction sequence.Hence, precise construction of the proximity and hierarchical arrangement of catalytic sites within the tandem catalyst is essential for regulating the transport of crucial intermediates.As shown in Figure 2, the presence of multistep electron/proton transfers and multiple intermediates in the reaction can lead to slow kinetics, low activity, and poor selectivity, but the use of specially designed tandem catalysts with decoupled steps can optimize the interaction between electrocatalysts and intermediates for improved performance.

| TANDEM CATALYST DESIGN STRATEGY
The tandem electrocatalyst design strategy involves the deliberate arrangement of multiple catalysts or active sites in a sequential manner, allowing for efficient and The illustration of nitrate reduction mechanism and the design of active sites for tandem catalyst.NO 3 RR, nitrate reduction reaction.
selective electrochemical reactions.Several factors that need to be considered for the design of tandem catalysts, including compatibility of catalysts, sequential reaction steps, stability, selectivity, and efficiency.First, the different sites should in close proximity to facilitate efficient transport of nitrogen-related intermediates within the context of electrochemical NO 3 RR.Second, the arranged catalysts sites could behave in a sequential manner to enable the step-by-step reaction.Third, each site or catalyst needs a hierarchical fashion to achieve the specific role of the interaction with different intermediates.Fourth, the individual could work synergistically to enhance the overall performance of the tandem system.Fifth, the resulted interfaces between the catalytic sites could optimize the charge transfer and activity.Lastly, the scalability and cost-effectiveness of the catalysts sites for potential large-scale applications should be considered.In the summary below, we provide a comprehensive overview of recent advancements in tandem catalysts for efficient electrochemical NO 3 RR, elucidating their detailed mechanisms and significance.

| Noble metal involved tandem catalysts
Conventionally, Cu-based electrodes were utilized as the host for tandem catalysts due to their exceptional capability in adsorbing nitrate and converting it into nitrite.For example, Wang et al. demonstrated that the CuPd/N-doped-C (CuPd/CN) electrocatalyst (Figure 3A) exhibited a metal-support interaction (EMSI), utilizing Pd atom sites separated by Cu atoms to enhance water molecule dissociation with boosted reduction of nitrate to ammonia. [80]The rationally designed catalyst demonstrated excellent nitrate reduction performance, with Cu atoms contributing to enhanced adsorption of nitrate ions, as observed in Figure 3B-D 3E-G). [81]The study demonstrated a NH 3 FE of 98.7% and a production rate of 555 μg h -1 cm -2 at −0.2 V versus reversible hydrogen electrode (RHE), along with negligible activity decay following a durability test.Electron paramagnetic resonance experiments using 5,5-dimethyl-1-pyrroline-N-oxide in KOH electrolyte at −0.2 V versus RHE under an argon atmosphere were conducted, confirming the production of *H through H 2 O activation in electrochemical NO 3 RR (Figure 3H).Additionally, density functional theory (DFT) calculations revealed  3I).
In a recent study by Han et al., a three-step relay mechanism involving spontaneous redox reaction, electrochemical reduction, and electrocatalytic reduction was proposed for the Ru 15 Co 85 catalyst. [83] h −1 with a corresponding FE of 97% ± 5%, as depicted in Figure 4B.In situ attenuated total reflection Fouriertransform infrared spectroscopy (ATR-FTIR) analysis provided insights into the vital contribution of Ru in the reduction of NO 2 − to NH 3 and the exceptional hydrogen supply facilitated by Co (Figure 4C).The presence of characteristic peaks at 3190, 3037, and 1160 cm −1 for *NH 2 , and at 1610 cm −1 for *NO, suggests the existence of active sites for the electrochemical NO 3 RR process.The possible reaction pathways, including the dissociative, distal-O associative, distal-N associative, and alternating-N associative pathways, can be indicated through in situ FTIR analysis.Furthermore, the proposed reaction mechanisms, as mentioned above, were supported by the electrochemical online differential electrochemical mass spectrometry (DEMS) tests, which confirmed the presence of m/z signals corresponding to NO (30), NH 3 (17), N 2 (28), HNO (31), and NH 2 OH (33).Gao et al. demonstrated the efficient synthesis of RuCu alloy catalysts supported on reduced graphene oxide (Ru x Cu x /rGO), showcasing high ammonia yield rate (0.38 mmol cm −2 h −1 ) and impressive ammonia FE (98%) at an ultralow potential of −0.05 V (vs.RHE), as illustrated in Figure 4D-F. [84]The remarkable activity of Ru 1 Cu 10 /rGO can be attributed to the synergistic effect between Ru and Cu sites, where Cu exhibits an efficient reduction of NO 3  5A). [85]Ag 30 Pd 4 exhibited excellent catalytic performance towards electrochemical NO 3 RR, showing NH 3 selectivity of ~88% and maximal NO 3 − removal rate of 92% at −0.6 V (Figure 5B,C).111), V-Cu nanosheets (111), and V Cu -Au 1 Cu SAAs (111) surfaces.Reproduced with permission. [81]Copyright 2022, Elsevier.DMPO, 5,5-dimethyl-1-pyrroline-N-oxide; EPR, electron paramagnetic resonance; LSV, linear sweep voltammetry; RHE, reversible hydrogen electrode; SAAs, single atom alloys.
In situ FTIR analysis reveals Ag site plays a more critical role in converting NO  Reproduced with permission. [83]Copyright 2023, Springer Nature.Reproduced with permission. [84]Copyright  5G). [86] The innovative triple reaction facilitated by the unique catalyst results in efficient conversion of Reproduced with permission. [86]Copyright 2023, Wiley-VCH.EDX, energy dispersive X-ray; FTIR, Fourier-transform infrared spectroscopy; LSV, linear sweep voltammetry; NMR, nuclear magnetic resonance; NO 3 RR, nitrate reduction reaction; RHE, reversible hydrogen electrode.
Except for above-mentioned cases, other noble metalrelated construction of tandem catalysts, for example, the Cu-Pd interfaces design with wide-pH-range ammonia electrocatalysis, [87] Ru@C 3 N 4 /Cu dural-active sites for enhanced activity, [88] in situ reconstructed Ru&Cu/Cu 2 O catalyst with high efficiency, [89] specially designed oxidederived silver catalyst, [90] dural sites of Pd-decorated MnO 2 , [91] bimetallic alloy with modulated atomic coordination environment, [92] and ultralow-content Pd incorporated Cu 2 O. [93] Noble metals possess notable advantages as catalysts in electrochemical NO 3 RR due to their high catalytic activity, selectivity, durability, and electron transfer capability.Particularly, in tandem catalyst system designs, the integration or incorporation of diverse active sites can significantly enhance the water dissociation step and finely tune the associated selectivity.For example, Pd-based catalysts exhibit high selectivity towards N 2 , whereas Ru demonstrates a propensity for ammonium production.Through meticulous design of the noble metal catalyst's composition and structure, it becomes feasible to direct the reduction pathway and minimize the formation of undesirable by-products.Additionally, the synergistic effects inherent in noble metals can confer higher stability and improved electron transfer kinetics, thereby making the tandem system a promising avenue for diverse sustainable applications linked with nitrate reduction.However, it is important to consider the cost implications and complications involved in catalyst synthesis when employing noble metals, as these factors may present challenges for largescale production and scalability.Moreover, noble metals can be susceptible to the detrimental effects of poisoning by certain species present in nitrate solutions encountered in real-world water treatment scenarios, such as sulfur compounds.Consequently, when compared with transition metals, noble metals emerge as the preferred choice for tandem catalyst design in the nitrate reduction process due to their elevated catalytic activity, enhanced selectivity, and improved electron transfer kinetics.Indeed, promoting active hydrogen provision and optimizing catalyst interfaces are key factors in achieving synergistic effects of noble metal sites in tandem electrocatalyst design for efficient electrochemical NO 3 RR.However, it is crucial to address the challenges related to stability and cost to ensure practical and scalable applications of these catalysts, which needs further research and development efforts.

| Transition metal contributed nitrate conversion
Using transition metals as tandem catalysts in the electrochemical NO 3 RR offers several advantages, including their variable oxidation states, diverse catalytic activities, thermal stability, and potential for synergistic effects in bimetallic systems.These properties make transition metals highly valuable for the rational design and optimization of catalytic systems aimed at efficient and sustainable nitrate reduction.For example, Fu et al. reported the construction of high-performance Cu nanosheets catalyst with tandem interaction of Cu(100) and Cu (111) facets, as shown in Figure 6A. [94] − in the electrochemical NO 3 RR process, leading to a gradual increase in NH 3 FE, consistent with previous tandem catalysis studies (Figure 6E).Shen et al. designed an efficient dendritic Cu 2 O/Cu tandem catalyst on Cu foam via the facile electrodeposition method. [95]In electrolytes containing 90 ppm of nitrate, an exceptional nitrate removal rate of 98.9% was achieved, accompanied by a relatively high ammonia yield rate of 11.2 mg h −1 cm −2 (Figure 6F,G).
Both in situ and ex situ data provide clear evidence of a synergistic effect where Cu sites play a crucial role in initiating the conversion of nitrate to nitrite, while Cu + sites serve as the active centers for the subsequent reduction of nitrite to ammonia (Figure 6H).Except for the pure Cu-based electrodes, Co was the mostly considered couple with Cu for tandem catalysts design in electrochemical NO 3 RR.For instance, Zhang et al. discovered that the tandem catalyst of Cu 2 O + Co 3 O 4 in electrochemical NO 3 RR significantly enhances the NH 3 production rate, showing a 2.7-fold increase compared with Co 3 O 4 and a 7.5-fold increase compared with Cu 2 O (Figure 7A-C). [96]The tandem catalysis proceeds by first generating NO  Reproduced with permission. [95]Copyright 2023, American Chemical Society.FE, Faradaic efficiency; FTIR, Fourier-transform infrared spectroscopy; LSV, linear sweep voltammetry; RHE, reversible hydrogen electrode.
exhibited a significant increase in the Raman signal of Co 3+ oxyhydroxide at 503 cm −1 , while the signal of Co (OH) 2 became weaker.In contrast, the CoO Reproduced with permission. [96]Copyright 2023, Wiley-VCH.(G) Schematic illustration of the prepared tandem catalyst.The in situ Raman measurements carried in 0.01 M KOH, 0.04 M K 2 SO 4 , and 0.1 M NO 3 − of CoO x (H) and modified CoO x on carbon paper (I).Reproduced with permission. [97]Copyright 2023, Wiley-VCH.FE, Faradaic efficiency; LSV, linear sweep voltammetry; RHE, reversible hydrogen electrode; SEM, scanning electron microscope.
WU ET AL.
| 255 effectiveness in stabilizing the Co-based phases during the electrochemical NO 3 RR reaction.Additionally, He et al. also developed tandem catalysts by transforming Cu─Co binary sulfides into Cu/CuO x and Co/CoP core-shell phases, achieving efficient electroreduction of nitrate to ammonia. [98]The unique tandem catalyst system shows exceptional performance, with a NO 3 − to NH 3 FE of 93.3%, a high NH 3 yield rate of 1.17 mmol cm −2 h −1 , and a half-cell energy efficiency of 36%, surpassing previous reports in nitrate-to-ammonia electroreduction (Figure 8A .Gong et al. also successfully synthesized Cudoped antiperovskite Co 4 N, which effectively enhanced the hydrogenation process in the electrochemical NO 3 RR reaction. [99]Cu dopants in Co 4 N enhanced nitrate adsorption, resulting in significantly higher electrochemical NO 3 RR activity compared with pure Co 4 N, with an optimal FE of 97% and an ammonia yield of 455.3 mmol h −1 cm −2 at −0.3 V versus RHE, as revealed by advanced spectroscopic techniques and density functional theory calculations (Figure 8G-I).
Other Co-based actives for enhanced active hydrogen provision were also reported.Fang et al. introduced a bio-inspired CuCo nanosheet catalyst for achieving efficient electrochemical NO 3 RR, where Co serves as the electron/proton donating center and Cu acts as the adsorption/association sites for NO x − (Figure 9A). [100]e bio-inspired CuCo nanosheet electrocatalyst demonstrated an impressive FE of 100% ± 1% at a high current density of 1035 mA cm −2 at −0.  9G,H). [101]The superior performance is due to the synergistic catalytic effect between [W─O] and CoP active sites, resulted from the inhibited release of hydrogen (H 2 ) with improved adsorption of H ads on Co.In situ Raman spectroscopy confirmed the presence of symmetric NO 3 − stretching at 1036 cm −1 , NO 2 − stretching at 1317 cm −1 , and NH 2 deformation at 1586 cm −1 , supporting the related electrochemical NO 3 RR process as predicted by DFT.
Fe, as the one of the most studied electrodes in electrochemical NO 3 RR, was also widely considered for the tandem sys design.For example, Zhang et   10A). [102]As a result, at a voltage of −0.6 V versus RHE, a maximum NH 3 FE of 96.8% was achieved, corresponding to an NH 3 yield of 25.5 mg h −1 (Figure 10B,C).Analysis of the characteristic peaks at 1350 cm −1 (NO  10G). [103]Significantly, a remarkable NH 3 FE of 97.1% was achieved, along with a corresponding NH 3 yield of 12.5 mg h −1 cm −2 , at a potential of −0.7 V versus the RHE (Figure 10G-I).Additionally, a series of in situ technologies strongly suggested that the induction of Lewis acid Fe─V pairs enhanced the generation of hydrogenation intermediates, resulting in improved kinetics of the overall electrochemical NO 3 RR process.
In the structural design of tandem catalysts for the two pathways of ammonium and nitrogen formation, the key points could include: (a) Interface engineering: designing the catalyst with a well-defined and optimized interface between different active sites or phases to facilitate efficient charge transfer and reaction kinetics; (b) Synergistic effects: ensuring a synergistic interaction between different active components in the catalyst to enhance catalytic activity and selectivity.This can be achieved by promoting electronic coupling or providing complementary active sites; (c) Stability and durability: keeping long-term stability Reproduced with permission. [100]Copyright 2022, Springer Nature.(G) EDS elemental mapping images of W─O─CoP.(H) LSV curves for the W─O─CoP@NF and CoP@NF recorded at a scan rate of 50 mV s −1 in 1 M KOH with and without 0.1 M NaNO 3 addition.(I) Electrochemical in situ Raman spectra of W─O─CoP@NF collected at different potentials during electrochemical NO 3 RR.Reproduced with permission. [101]Copyright 2023, Wiley-VCH.EDS, energy-dispersive X-ray spectroscopy; FTIR, Fourier-transform infrared spectroscopy; NO 3 RR, nitrate reduction reaction; RHE, reversible hydrogen electrode.
and durability of the catalyst by selecting appropriate support materials, optimizing surface compositions, and minimizing catalyst deactivation mechanisms such as catalyst poisoning or surface reconstruction; (d) Mass transport: designing catalyst structures that facilitate the diffusion of reactants, intermediates, and products, ensuring efficient mass transport within the catalyst system; (e) Active site exposure: maximizing the exposure of active sites by controlling the catalyst morphology, particle size, and surface area to enhance catalytic performance and utilization efficiency.These key points in structural design contribute to improving the overall catalytic performance and efficiency of tandem catalysts for ammonium and nitrogen formation pathways.

| Pulsed electrocatalysis for decoupled electrochemical NO 3 RR
Recently, pulsing strategy was also employed to address the reduction of nitrate with low NH 3 yield and FE, especially those hampered by the competing HER process and the massive accumulation of NO 2 − byproducts due to the reduction potential gap.For example, Yu et al. utilized a pulsed electrocatalysis strategy to carry out tandem catalysis, aiming to achieve efficient reduction of nitrate to ammonia. [119]In conventional electrochemical NO 3 RR, the Cu single-atom catalyst demonstrated two distinct reduction processes: NO 3 − to NO 2 − and subsequent conversion of Reproduced with permission. [103]Copyright 2023, Wiley-VCH.FE, Faradaic efficiency; FTIR, Fourier-transform infrared spectroscopy; LSV, linear sweep voltammetry; NO 3 RR, nitrate reduction reaction; RHE, reversible hydrogen electrode.potential method typically involves the accumulation and conversion of NO 2 − intermediates, leading to either the main product of NO 2 − at low overpotential or an intensive HER (hydrogen evolution reaction) side reaction at high overpotential (Figure 11C).Thus, a pulse electrosynthesis strategy was proposed, where applying a potential within a range of low and high overpotential for a specific duration of seconds can simultaneously achieve efficient NO 2 − formation and significant inhibition of the HER, as illustrated in Figure 11D.The simulation of the tandem accumulation and conversion process of nitrite intermediates clearly demonstrated the high efficiency of converting nitrite intermediates while effectively suppressing competition from the HER (Figure 11E).Bu et al. also reported a programmable pulsed electrolysis strategy that enables the formation of a Cu/Cu 2 O heterojunction, effectively overcoming the ratedetermining NO 3 − to NO 2 − step.Additionally, they utilized Ni alloying to further regulate the adsorption of hydrogen (Figure 11F). [120]Under optimal pulsed conditions, the implementation of this approach resulted in a significant enhancement in NH 3 formation, achieving a high FE of approximately 88% and a yield rate of 583.6 μmol cm −2 h −1 Snapshots of nitrite concentrations at different electrolysis times for the P3 protocol.Reproduced with permission. [119]Copyright 2023, American Chemical Society. . [121]As illustrated in Figure 12A-C, a pulsed potential approach involving periodic alternation of positive and negative potentials was employed to enhance the low-concentration nitrate reduction to ammonia.This approach resulted in a significantly improved performance compared with potentiostatic testing, with notable achievements in terms of FE (97.6%), yield rate (2.7 mmol −1 h −1 mg Ru −1 ), and conversion rate (96.4%).In situ Raman spectroscopy analysis showed that the characteristic peak of nitrate (~1050 cm −1 ) gradually diminished with increasing scanning times and nearly disappeared after 125 s of reduction.However, when the potential is switched to +0.6 V, the peak at ~1050 cm −1 reappears, indicating the accumulation of nitrate near the positively charged electrode (Figure 12D).The normalized peak intensity of nitrate exhibits a periodic variation trend, with a higher intensity at +0.6 V and a relatively lower level at −0.1 V, Reproduced with permission. [122]opyright 2023, Springer Nature.ATR-FTIR, attenuated total reflection Fourier-transform infrared spectroscopy; CNTs, carbon nanotubes; RHE, reversible hydrogen electrode.
when the applied potential is periodically switched between the two values (Figure 12E).The observation provided evidence of nitrate accumulation and suggests that the applied potential plays a role in controlling this process.In addition, under both potentiostatic and pulsed conditions, bands associated with ammonia production (~1460 cm −1 ) and (~1510 cm −1 ), as well as bridged NO (~1650 cm −1 ) and (~1740 cm −1 ) attributed to the bending vibration of NOH, are observed.However, the additional bands at ~1690 cm −1 , designated to the on-top adsorption of NO, are only observed under pulsed conditions (Figure 12F).Kim et al. also investigated the coupling of nitrate capture with ammonia production using bifunctional redox-electrodes at various working potentials (Figure 12G). [122]The combination of the nitrate-selective redox-electrosorbent (polyaniline) and electrocatalyst (cobalt oxide) enables efficient nitrate capture at anodic potential, followed by its electrochemical release at low overpotential, leading to the conversion into ammonia at lower overpotential (Figure 12H,I).The study demonstrated significant improvements in ammonium production rate, with a 24fold enhancement (108.1 μg h −1 cm −2 ), as well as a more than 10-fold increase in energy efficiency compared with direct electrocatalysis in a dilute stream.In tandem catalysis, the pulse strategy plays a crucial role in optimizing the design and performance of the tandem catalyst.The pulse strategy involves applying discrete and controlled voltage pulses to the electrode surface, which facilitates the coordinated action of multiple catalysts in a tandem configuration.The pulse strategy helps in several ways: (a) Enhancing reaction kinetics: by applying voltage pulses, the pulse strategy allows for the activation of catalysts at specific reaction steps, ensuring rapid and efficient electron transfer and improving the overall reaction kinetics.This can be particularly useful when different catalysts have different activation energies or reaction rates.(b) Reducing side reactions: the pulse strategy enables precise control over the reaction conditions, including potential, current, and duration of the pulses.By carefully tuning these parameters, it is possible to minimize unwanted side reactions and enhance the selectivity of the tandem catalysis process.(c) Facilitating intermediate transfer: in some tandem catalysis systems, the pulse strategy can be used to facilitate the transfer of intermediates between catalysts.By applying pulses at appropriate times, intermediates can be generated, stabilized, and efficiently shuttled between different active sites, allowing for sequential reactions to occur smoothly.(d) Optimizing catalyst performance: the pulse strategy provides a flexible and tunable approach for optimizing the performance of each catalyst in the tandem configuration.By adjusting the pulse parameters, such as frequency, amplitude, and duration, the activity, selectivity, and stability of the catalysts can be finely tuned, leading to improved overall performance.Efficient electroreduction of low-concentration nitrate (≤10 mmol) remains a significant challenge, primarily due to the restricted migration of nitrate ions near the working electrode and the presence of competing hydrogen evolution reaction.Therefore, developing an alternative strategy to overcome mass transfer limitations near the cathode surface and mitigate competitive reactions is of great significance for achieving efficient electroreduction of low-concentration nitrate into ammonia.Pulsed electrocatalysis presents a promising strategy for achieving stable and reliable active sites in catalytic systems.Using Cu-based tandem catalysts as a model, electro-pulse-driven strategy that combines peroxide generation and self-repair, resulting in a stable and highly active Cu/Cu 2 O heterogeneous interface was typically studied.This strategy effectively breaks the rate-limiting step of NO 3 − to NO 2 − conversion.However, the potential in situ reconstruction of the catalyst highlights the need for accurate identification and understanding of the active components involved in the reaction mechanism.Furthermore, to tackle the intricate proportional relationships in electrocatalysis, it is essential to integrate pulsed electrocatalysis with other strategies, such as electronic structure regulation and tandem catalysis, in a systematic and rational manner.
This combined approach has the potential to enhance catalytic performance and pave the way for more efficient and sustainable electrochemical processes.

| Others
Aside from their application in the NO 3 RR with ammonium production, tandem catalysts have also been employed in the nitrogen formation pathway.For example, Zhao et al. reported the successful electrochemical reduction of nitrate to nitrogen gas by leveraging the relay catalytic effects of neighboring Fe─Ni sites on MOFderived catalysts, demonstrating high efficiency in the process. [123]The electrocatalyst derived from metalorganic frameworks, containing earth-abundant bimetallic sites, exhibited remarkable performance in the quantitative (approximately 97.9% conversion) and selective (approximately 99.3%) transformation of nitrate into nitrogen gas (N 2 ) (Figure 13A

| CONCLUSION AND OUTLOOK
In conclusion, tandem electrocatalysts have shown promising results for nitrate reduction.The use of multiple catalytic sites in sequence allows for enhanced activity and selectivity, as each site can perform a specific function in the overall reaction.Tandem systems offer the advantage of utilizing different materials with complementary properties, leading to improved performance.However, there are still challenges that need to be addressed in the design of tandem electrocatalysts for nitrate reduction.One potential issue is the stability and durability of these catalysts under the harsh conditions of electrolysis.In situ reconstruction or degradation of the catalysts could occur, affecting their performance over time.Moreover, the complex stoichiometry and kinetics involved in nitrate reduction require a comprehensive approach.Below are the summarized electrochemical NO 3 RR performance for different tandem catalysts (Table 1).Besides, some design suggestions and challenges for tandem electrocatalysts with high-efficiency nitrate reduction are also provided.
(1) It is crucial to accurately identify the active components and understand the working mechanism of the catalysts.This will help in designing more stable and efficient catalysts by optimizing their composition and structure.Besides, the integration of tandem electrocatalysis with other strategies, such as electronic structure tuning and series catalysis, can result in a synergistic effect and further improve performance.
Exploring new materials with enhanced activity and stability also holds great potential in developing environmentally friendly technologies for nitrate removal.(2) Despite the significant progress made in electrochemical NO 3 RR, a comprehensive understanding of the Reproduced with permission. [123]Copyright 2022, Elsevier.DEMS, differential electrochemical mass spectrometry; EDS, energydispersive X-ray spectroscopy; IR, infrared; NO  of tandem electrocatalysts under harsh and practical electrolysis conditions.For example, shifting the focus towards practical applications holds more appeal from an environmental perspective, such as the use of actual wastewater from an environmental perspective.Additionally, flow cells equipped with gas diffusion electrodes are more reliable for scaling up the process instead of the normally employed H-type cell or single cell.Moreover, it is important to explore potential applications of nitrate reduction in real conditions for energy storage and value-added chemical production (C─N coupling and batteries).Additionally, the development of efficient collection systems for the resulted valuable chemicals from the nitrate reduction process is also an attractive avenue for future research.
Looking ahead, future research should focus on developing strategies to mitigate the stability issues of tandem electrocatalysts, exploring new materials with enhanced activity, and gaining a deeper understanding of the reaction mechanisms involved in nitrate reduction.By addressing these challenges, tandem electrocatalysts hold great potential for environmentally friendly nitrate removal technologies.

2 −
Figure 4A illustrates the spontaneous redox reaction of Co with NO 3 − resulting in the formation of Co(OH) 2 and NO , which effectively bypasses the rate-determining NO 3 − to NO 2 − step.The subsequent electrochemical and electrocatalytic processes involve the reduction of Co(OH) 2 and NO 2 − to Co and NH 3 , facilitated by the involvement of active hydrogen.At an onset potential of 0.4 V (vs.RHE), the Ru 15 Co 85 catalyst demonstrates a NH 3 production rate of 3.2 ± 0.17 mol g cat −1

3 −. 3 −
− and Ru demonstrates superior activity in converting NO 2 − to NH 3 .The analysis of charge density differences and charge transfer in Cu, Ru, and RuCu solid solution after NO 3 − and NO 2 − adsorption revealed favorable adsorption on the RuCu surface compared with Cu and Ru (Figure 4G).Additionally, the in situ Raman spectra captured during electrochemical NO 3 RR demonstrated that the adsorption of NO 3 − on Ru 1 Cu 10 is significantly stronger compared with Cu, as depicted in Figure 4H-I.Raman spectra are commonly utilized to investigate the electrochemical NO 3 RR process by analyzing the vibrational modes of aqueous NO 3 and the adsorbed NO It was observed that the Raman band at 1326 cm −1 , attributed to aqueous NO 2 − , appeared at 0.4 V and gradually decreased as the potential was further reduced to 0.1 V.The Raman signals confirmed the process of generation, desorption, adsorption, and conversion of NO 2 − into NH 3 through relay catalysis on the Ru 1 Cu alloy catalyst.Besides, the appearance of NH 3 − associated Raman bands at 1140 and 1479 cm −1 at 0 V on Cu, indicates the higher catalytic activity of Ru 1 Cu 10 with the 100 mV higher potential.Other noble metal, for example, Ag has also recently attracted great attention due to the excellent ability in NO 3 − to NO 2 − step.Qin et al. reported an atomically precise [Ag 30 Pd 4 (C 6 H 9 ) 26 ](BPh 4 ) 2 (Ag 30 Pd 4 ) nanocluster as a model catalyst towards the electrochemical NO reduction reaction (eNO 3 − RR) to elucidate the different role of the Ag and Pd site and unveil the comprehensive catalytic mechanism (Figure

F
I G U R E 3 (A) The illustration of the conversion from nitrate to ammonia over the CuPd/CN.(B) LSV curves of CuPd/CN in 0.5 M K 2 SO 4 with and without 200 ppm of NH 3 .(C) NH 3 selectivity of different samples.(D) NH 3 yield rate of sample CuPd/CN.Reproduced with permission. [82]Copyright 2023, Wiley-VCH.(E) The possible mechanism for NO 3 RR on the V Cu -Au 1 Cu SAAs surface.(F) The colored intensity map of V Cu -Au 1 Cu SAAs.Red arrows highlight Au atoms, and the white circle areas indicate the vacancy of Cu atoms.(G) LSV curves tested in KNO 3 /KOH electrolyte.(H) EPR signals of DMPO spin adducts by Cu nanosheets, V-Cu nanosheets, and V Cu -Au 1 Cu SAAs at −0.2 V versus RHE.(I) Free energy diagrams of H 2 O dissociation on Cu nanosheets (

F
I G U R E 4 (A) Illustration of the three-step relay mechanism during electrochemical NO 3 RR for the RuCo catalyst.(B) LSV curves for the electrochemical NO 3 RR over Ru x Co y sample.(C) Isotope-labeling electrochemical in situ ATR-FTIR spectra of the Ru 15 Co 85 .
(D) Aberration-corrected TEM dark field and bright field (built-in) images of Ru 1 Cu 10 /rGO.(E) Mass-normalized r(NH 3 ) and FE of Ru x Cu y /rGO catalysts with different Ru/Cu ratios.(F) Mass-normalized r(NH 3 ) and FE of Ru 1 Cu 10 /rGO in designed electrolyte at −0.02 V versus RHE with 80% current-resistance (iR) compensation with different NO 3 − concentrations.(G) Charge density differences and charge transfer of Cu, Ru, and RuCu solid solution after adsorption of NO 3 − and NO 2 − , respectively.In situ Raman spectra of (H) Ru 1 Cu 10 and (I) Cu during electrochemical NO 3 RR at different potentials (V vs. RHE).
NO 3 − to NH 3 , achieving a high FE of 94.3% and a remarkable NH 3 yield rate of 253.7 μmol h −1 cm −2 in 1 M KOH and 0.1 M KNO 3 solution at -0.25 V (vs.RHE) as shown in Figure 5H.In situ FTIR spectrum indicates Ag phases preferentially reduce NO 3 − to NO 2 − , followed by conversion of NO 2 − intermediates to NO on Co 3 O 4 phases.Specially, CoOOH phases enhanced the hydrogenation of NO, facilitating its reduction to NH 2 OH and subsequently to NH 3 by providing additional H atoms during the electrochemical NO 3 RR reaction.F I G U R E 5 (A) The tandem electrocatalysis of nitrate in Ag 30 Pd 4 .(B) LSV curves of Ag 30 Pd 4 in 1 M NaOH with NO 3 − and Ar.(C) 1 H NMR spectrum of the product from the electrochemical reaction using 14 NO 3 − and 15 NO 3 − as the N source, respectively.The electrochemical in situ FTIR potential-dependent spectra on the (D-F) Ag 30 Pd 4 , Pd x (C 6 H 9 ) y , and Ag 15 electrode, respectively.Reproduced with permission. [85]Copyright 2023, American Chemical Society.(G) EDX elemental mapping images of i-Ag/Co 3 O 4 .(H) LSV curves of the as-synthesized catalysts in 1 M KOH electrolyte with and without KNO 3 .(I) Electrochemical in situ FTIR spectra of i-Ag/Co 3 O 4 .
NH 3 partial current density of 665 mA cm −2 and yield rate of 1.41 mmol h −1 cm −2 in the flow at −0.59 V (vs.RHE) were achieved (Figure 6B,C).To gain further insights into the surface structure of these Cu catalysts during electrochemical NO 3 RR, the postreaction electrode was subjected to electro-sorption of hydroxide (OH ads ) in Arpurged 1 M KOH, allowing for a more comprehensive investigation.Distinct OH ads peaks were observed on the CV curves at potentials around 0.35 V (Cu(100)), 0.4 V (Cu(110)), and 0.45 V (Cu(111)), indicating OH − adsorption on specific Cu facets, with Cu(111)/Cu(100) ratios of 1.26, 0.08, and 0.13 for Cu nanosheets, Cu-25, and CuO-600-3 electrodes after electrochemical NO 3 RR, as shown in Figure 6D.Both CuO nanosheets and Cu-25 catalysts showed high FE for NH 3 production from NO 2 − , with a substantial generation of NO 2

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I G U R E 6 (A) Illustration for the boosted tandem catalysis over Cu nanosheets.(B) NH 3 yield rate and NH 3 partial current density over Cu nanosheets.(C) Total current density and FEs of various products over Cu nanosheets.(D) Fitted OH − adsorption peaks of Cu nanosheets, Cu-25 and CuO-600-3.(E) FEs of NH 3 over Cu nanosheets and Cu-25.Reproduced with permission. [94]Copyright 2023, Wiley-VCH.(F) LSV curves of D-Cu 2 O/Cu/CF and Cu foam in a 0.1 mol L −1 Na 2 SO 4 solution with or without 90 ppm NaNO 3 .(G) FEs, conversions, selectivity, and NH 3 yield rates of D-Cu 2 O/Cu/CF and Cu foam.(H) In situ infrared (FTIR) spectra of D-Cu 2 O/Cu/CF during the nitrate reduction at varied potentials (1 min).
x after modification showed significantly weaker signals of CoO(OH) and a sharp attenuation in signals of Co (OH) 2 , indicating an accelerated reduction of Co-based phases.Furthermore, the modified CoO x -VRP catalyst demonstrated a sustained oxidized state (V 2+ ) at −0.1 V (vs.RHE) due to the efficient redox cycling of the polymer-bound viologen units, highlighting the VRP's F I G U R E 7 (A) SEM image of the synthesized Cu 2 O + Co 3 O 4 on the carbon paper.(B) LSV plots in 0.1 mol L −1 NO 3 − and 0.1 mol L −1 NaOH over Cu 2 O, Co 3 O 4 , and Cu 2 O + Co 3 O 4 .(C) Yield rate, FE, and selectivity comparisons of Cu 2 O, Co 3 O 4 , and Cu 2 O + Co 3 O 4 .In situ Raman electrochemistry of Cu 2 O (D), Co 3 O 4 (E), Cu 2 O + Co 3 O 4 (F).

3 − to NO 2 −
-C).In situ Raman spectra demonstrated that the inner Cu/CuO x phases displayed a preference for catalyzing the reduction of NO 3 − to NO 2 − , which is subsequently rapidly reduced to NH 3 at the adjacent Co/CoO shell, as illustrated in Figure 8D-F.Cu-based electrodes displayed Cu─O and Cu─OH modes, indicating the catalytic nature of Cu/CuO x phases.In Co-based catalysts, Raman signals associated with CoOOH and Co 3 O 4 increased at high potentials due to oxidative NO 2 species, while Co 3 + based phases and Co(OH) 2 weakened at low overpotentials, suggesting fast reduction of NO 3 − to NO 2 − with in situ F I G U R E 8 (A) LSVs at a scan rate of 5 mV s −1 and (B) Faradaic efficiencies on CuSP, CoSP, and CuCoSP in 0.01 M NO 3 − 0.1 M KOH (pH 13).(C) The LSV-derived potentials at a current density of −1 mA cm −2 and the calculated reaction constants for NO 3 − and NO 2 − reduction on different samples.(D-F) In situ Raman spectra during the electrochemical NO 3 RR process on CuSP, CoSP, and CuCoSP, respectively.Reproduced with permission. [98]Copyright 2022, Springer Nature.(G) Elemental mapping information of the Co 3 CuN sample.(H) LSV curves of Co 4 N and Co 3 CuN in 0.5 M KOH (dash line) and 0.5 M KOH with 2000 ppm NO 3 − (solid line).(I) NH 3 Faradaic efficiency and NH 3 yield of Co 4 N and Co 3 CuN.Reproduced with permission. [99]Copyright 2023, Wiley-VCH.LSV, linear sweep voltammetry; NO 3 RR, nitrate reduction reaction; RHE, reversible hydrogen electrode.formation of metallic Co.The conversion of NO 2 − to NH 3 is predominantly governed by a Co 2 + -dominated CoO x phase.The CuCo-based catalyst efficiently reduces NO instead of forming oxidative NO 2 species on the Cu-based phases, while the active Co 2 + -based phases of CuCoSP are stabilized by the Cu/CuO x phases, creating a tandem system for low-overpotential cascade reduction of NO 3 −

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I G U R E 9 (A) EDS mapping analysis of Cu 50 Co 50 .(B) j-E curve (80% iR corrected) over Cu 50 Co 50 , pure Cu, and pure Co modified Ni foams (catalysts loading was 5 mg cm −2 ) in 1 M KOH solution containing 100 mM KNO 3 (solid lines) or in the absence of KNO 3 (dotted line).(C) Faradaic efficiencies of NH 3 − and NO 2 − during electrochemical NO 3 RR.Electrochemical thin-layer in situ FTIR spectra of electrochemical NO 3 RR on Cu 50 Co 50 (D), Cu (E), and Co (F).

NO 2 −
to NH 4 + at cathodic potentials of −0.5 V and −0.8 V (vs.RHE), respectively (Figure 11A,B).The constant F I G U R E 10 (A) Illustration of the tandem catalysis on FeB 2 .(B) LSV curves of FeB 2 in 1 M KOH with and without 0.1 M NO 3 − .(C) NH 3 yield rates and NH 3 -FE of FeB 2 at various potentials.Potential-dependent operando FTIR spectra during electrochemical NO 3 RR of (D) FeB 2 without SCN − , (E) with SCN − , and (F) Fe 2 O 3 .Reproduced with permission. [102]Copyright 2023, Wiley-VCH.(G) Elemental mapping images of Fe─V 2 O 5 sample.(H) LSV curves of Fe─V 2 O 5 in 1 M KOH with and without 0.1 M NO 3 − .(I) NH 3 yield and FE NH 3 of Fe─V 2 O 5 at various potentials.

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I G U R E 11 (A) LSVs of Cu SAGs in 0.1 M PBS solution (pH = 7.0) with and without 20 mM nitrate or nitrite.(B) FE and YR of H 2 , NO 2 − , and NO 3 − production over Cu SAGs, Cu foil, and PPy-C at −0.8 V. Schematic of reaction pathways with CE (C) and PE (D).(E) (F) Alternated potential program for the pulsed electrolysis.(G) The NH 3 yield rate and FE of Cu and Cu 50 Ni 50 at different E c .(H) The NH 3 yield rate and FE of Cu─Ni alloy at E c = −1.0V. Reproduced with permission. [120]Copyright 2023, Wiley-VCH.FE, Faradaic efficiency; LSV, linear sweep voltammetry; PBS, phosphate-buffered saline; RHE, reversible hydrogen electrode; SAGs, singleatom-modified gels; YR, yielding rate.(Figure 11G,H).Significantly, the incorporation of Ni/Ni (OH) 2 sites in the rationally designed tandem catalyst significantly enhanced hydrogen adsorption coverage, resulting in a more balanced rate of NO 3 − to NO 2 − and NO 2 − to NH 3 reactions during electrochemical NO 3 RR.Huang et al. presented a study showcasing the effectiveness of a pulsed potential approach in achieving high-performance electrochemical NO 3 RR.They observed that the periodic application of anodic potential resulted in the optimization of adsorption configuration for the key *NO intermediate and increased the local concentration of NO 3 −

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I G U R E 12 (A) Schematic illustration of potentiostatic and pulsed potential tests.(B) Ammonia Faradaic efficiency (FE) and yield rate, (C) ammonia partial current density and energy efficiency.(D) In situ Raman spectra of RuIn 3 /C under −0.1 V for 125 s and then +0.6 V in 0.1 M KOH + 10 mM NO 3 − .(E) In situ Raman spectra of RuIn 3 /C under −0.1 V and then +0.6 V in 0.1 M KOH + 10 mM NO 3 − solution with 3 cycles.(F) Time-dependent in situ ATR-FTIR spectra of pulsed potential (E c = −0.1 V, E a = +0.6V, t c = 4 s, t a = 0.5 s).Reproduced with permission. [121]Copyright 2023, Springer Nature.(G) Schematic illustration of two scenarios for treating a dilute nitrate stream (corn/soy tile drainage sample collected from the University of Illinois Energy Farm). (H) FE and ammonia yield rate of PANI-Co 3 O 4 /CNT electrodes in the different scenarios.(I) Comparison of the energy consumptions (kWh kg −1 -N) in different scenarios.

2 −, 2 −.
-C).During four consecutive electrocatalytic cycles, online DEMS analysis detected m/z values of 46 (NO 2 ), 30 (NO), 44 (N 2 O), and 28 (N 2 ) within the applied potential range of 0 to −1.6 V versus saturated calomel electrode.Through the online DEMS analysis, the reaction pathways of nitrate electroreduction were elucidated, revealing the sequential transformation of NO 3 − to NO NO, N 2 O, and eventually to N 2 (Figure 13D).The in situ IR measurements, performed with varying reaction times and applied potentials, provided valuable information regarding the nitrate electroreduction mechanism.It was observed that NO 3 − is initially reduced to NO 2 − by Fe, accompanied by the oxidation of Fe to Fe 3+ .Subsequently, active Ni[H] species are formed on the surface of Ni sites, allowing for the reduction of Fe 3+ back to Fe, thereby enabling the continual formation of NO Additionally, it was observed that the excess Ni[H] species have the ability to reduce NO 2 − to form Ni[N 2 O], which can be subsequently converted directly into N 2 (Figure 13E,F).

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I G U R E 13 (A) TEM image and corresponding EDS mapping information of Fe 3 Ni─N─C.(B) Faradaic efficiency under different applied cathode potentials.(C) NO 3 − removal rate and N 2 selectivity of N─C, Ni─N─C, Fe─N─C, and Fe 3 Ni─N─C.(D) DEMS measurements of Fe 3 Ni─N─C during electrochemical NO 3 RR.In situ IR spectra of Fe 3 Ni─N─C as a function of (E) reaction time and (F) the applied potential.
nitrate reduction.This includes cyclic voltammetry, chronoamperometry, and impedance spectroscopy.Through these techniques, the catalytic activity, reaction kinetics, and charge transfer processes can be monitored and analyzed to provide insights into the relay catalytic mechanism.(b) In situ spectroscopy: in situ spectroscopic techniques, such as infrared spectroscopy (IR), Raman spectroscopy, or X-ray absorption spectroscopy, can be used to probe the surface intermediates and reaction products during nitrate reduction.By monitoring the changes in the vibrational or absorption spectra, valuable information about the reaction pathways and the involvement of different catalysts in the relay mechanism can be obtained.(c) Operando characterization: operando characterization techniques allow for the real-time monitoring of catalyst behavior under working conditions.For example, operando X-ray diffraction or X-ray photoelectron spectroscopy can provide insights into the structural changes and oxidation states of the catalyst during nitrate reduction.This can help identify any dynamic structural transformations or active site changes that occur during the relay catalytic process.(d) Isotopelabeling experiments: Isotope-labeling experiments, such as using isotopically labeled nitrate or water molecules, can provide information about the involvement of specific catalysts in different reaction steps.By tracking the fate and distribution of isotopes during the reaction, it is possible to determine the extent of participation of each catalyst in the relay catalytic mechanism.(3)Challenges remain in terms of stability and durability values of 0.21 eV for Cu nanosheets, 0.13 eV for V-Cu nanosheets, and 0.11 eV for V Cu -Au 1 Cu single atom alloys (SAAs), suggesting that V Cu -Au 1 Cu SAAs exhibited superior H 2 O adsorption and enhanced capacity in capturing H 2 O molecules (Figure 2 − from Cu 2 O, which is then transferred to the nearby Co 3 O 4 surface for subsequent conversion into NH 3 .Furthermore, the tandem mechanism was confirmed by utilizing identical location transmission electron microscopy to reveal the structural and phase evolution of individual Cu 2 O and Co 3 O 4 nanocubes during the electrochemical NO 3 RR process.In situ Raman characterization also suggested the phase evolution of CuO to metallic Cu and Co 3 O 4 to x underwent a significant structural reconstruction due to the formation of hexagonal Co(OH) 2 nanosheets and aggregated CuO x nanoparticles.The in situ formation and accumulation of NO 2 on electrocatalysts at low applied overpotentials can lead to oxidation of most transition metals, thereby presenting a significant 2 on the CuCoO x -VRP sample, indicating an accelerated reduction of Cobased phases (Figure 7H,I).Specifically, CoO x was chosen as the model catalyst due to its more prominent Raman signals associated with phase transformation compared with CuCoO x .Over time, the CoO x catalyst al. illustrated the high feasibility of MBenes as tandem catalysts for electrochemical NO 3 RR, utilizing B sites for NO 3 − activation and Fe sites for H 2 O dissociation to DFT) is crucial for a more comprehensive understanding of the reaction mechanism and establishing the relationships between catalyst structure, activity, and selectivity.Therefore, the integration of in situ characterizations, experimental investigations, and theoretical simulations can greatly contribute to elucidating the detailed reaction mechanisms and designing improved catalysts for nitrate reduction.Here are some possible tests that can be employed to investigate and validate the relay catalytic mechanism in the context of electrocatalytic nitrate reduction: (a) Electrochemical techniques: various electrochemical techniques can be employed to investigate the behavior of tandem catalysts during T A B L E 1 The performance comparison of different tandem catalysts.