Cobalt-enabled tandem catalysis on copper-based electrocatalysts for nitrate-to-ammonia conversion

Xin Wang; Ling Zhang; Tianyi Zhu; Yue Chen; Mengyao Han; Shaowei Zhang; Wen Lei; Haijun Zhang

doi:10.63823/20260401

Composite Functional Materials ›› DOI: 10.63823/20260401

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Cobalt-enabled tandem catalysis on copper-based electrocatalysts for nitrate-to-ammonia conversion

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Ammonia plays an essential role in agriculture, chemical industry, and emerging energy systems, whereas the conventional Haber-Bosch process suffers from high energy consumption and large CO₂ emissions. Meanwhile, nitrate pollution caused by excessive fertilizer use and industrial wastewater discharge poses serious threats to ecosystems and human health. Electrocatalytic nitrate reduction to ammonia (NO₃RR) offers a promising strategy for simultaneously removing nitrate contaminants and producing value-added NH₃ under ambient conditions. Recently, CuCo-based catalysts have attracted increasing attention due to their tunable electronic structures, abundant active sites, and pronounced bimetallic synergy. In this review, the fundamental reaction mechanisms of NO₃RR are first introduced, followed by a systematic summary of recent advances in CuCo-based catalyst design, including heteroatom doping, heterostructure and interface engineering, MOF- and coordination-derived catalysts, atomically dispersed catalysts, in-situ reconstructed catalysts, and electrode architecture/mass-transfer engineering. Finally, the remaining challenges and future opportunities are discussed, aiming to provide guidance for the rational design of efficient, stable, and practical CuCo-based NO₃RR electrocatalysts.

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Nitrate reduction reaction / Electrocatalytic ammonia synthesis / CuCo-based catalysts / Bimetallic synergy / Interface engineering

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Xin Wang, Ling Zhang, Tianyi Zhu, Yue Chen, Mengyao Han, Shaowei Zhang, Wen Lei, Haijun Zhang. Cobalt-enabled tandem catalysis on copper-based electrocatalysts for nitrate-to-ammonia conversion. Composite Functional Materials DOI:10.63823/20260401

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1. Introduction

Ammonia (NH₃) is a fundamental chemical in modern agriculture, chemical manufacturing, and emerging energy systems [1 -4]. It is also regarded as a promising carbon-free energy carrier and fuel. Currently, industrial NH₃ production mainly relies on the Haber-Bosch process [5 -7], which requires harsh operating conditions and fossil-fuel-derived hydrogen, leading to high energy consumption and substantial CO₂ emissions. Therefore, developing mild, low-carbon, and sustainable ammonia synthesis technologies is of great significance for future energy and environmental sustainability.

In recent years, electrochemical ammonia synthesis has attracted considerable attention because it can be driven by renewable electricity under ambient conditions [8,9]. As shown in Fig. 1a, bibliometric analysis has identified ‘copper catalyst’, ‘ammonia synthesis’ and ‘nitrate reduction’ as the main research focal points in the development of the electrochemical nitrate reduction reaction. The past decade has seen rapid developments in electrocatalytic nitrate reduction technology, with a corresponding surge in the number of research papers published on the subject (Fig. 1b). The main electrochemical routes include the nitrogen reduction reaction (NRR) [10 -12], nitrate reduction reaction (NO₃RR), and NOx reduction reaction (NORR) [13 -15]. However, NRR is severely limited by the low solubility of N₂ in aqueous electrolytes [16,17], the high dissociation energy of the N≡N triple bond [18], and the competing hydrogen evolution reaction (HER), usually resulting in low NH₃ yield rates and Faradaic efficiencies. In contrast, NO₃⁻ possesses high water solubility and a much lower N-O bond energy than the N≡N bond, making NO₃RR kinetically more favorable [19,20]. Moreover, nitrate is widely present in agricultural runoff, industrial wastewater, and contaminated groundwater. Thus, NO₃RR provides a promising strategy for simultaneously producing NH₃ and remediating nitrate pollution.

At present, nitrate-to-ammonia conversion technologies can be generally classified into biological reduction, photocatalytic reduction, and electrocatalytic reduction (Fig. 1c-e). Biological and photocatalytic routes show advantages in environmental compatibility and relatively low cost, but they are usually limited by slow reaction rates and low product selectivity. Recent advances in photocatalysis have demonstrated that heterojunction construction [21,22], defect engineering [23], interfacial charge-transfer regulation [24,25], and hierarchical structural design [26,27] can effectively improve charge separation and surface reaction kinetics, providing useful general principles for catalytic material design. In comparison, electrocatalytic NO₃RR has been widely explored for ammonia production because of its high selectivity, controllability, adaptability, and mild operating conditions.

Nevertheless, NO₃RR is a complex eight-electron multistep reduction process involving coupled proton/electron transfer, and its overall reaction depends on the electrolyte pH [28]:

(1)

$\mathrm{N}{\mathrm{O}}_{3}^{-}+9{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}=\mathrm{N}{\mathrm{H}}_{3}+3{\mathrm{H}}_{2}\mathrm{O}$

(2)

$\mathrm{N}{\mathrm{O}}_{3}^{-}+6{\mathrm{H}}_{2}\mathrm{O}+8{\mathrm{e}}^{-}=\mathrm{N}{\mathrm{H}}_{3}+9\mathrm{O}\mathrm{H}$

The reaction pathway involves multiple intermediates, such as *NO₃, *NO₂, *NO, *NOH, *NH₂OH, and *NH₂ [29,30]. During this process, nitrite accumulation, byproduct formation such as N₂ and N₂O, and HER competition can significantly reduce NH₃ selectivity and energy efficiency [31]. Therefore, the development of highly active, selective, and stable electrocatalysts is crucial for efficient nitrate-to-ammonia conversion.

Among various NO₃RR electrocatalysts, Cu-based materials are considered among the most promising non-noble-metal catalysts owing to their good electrical conductivity [14,32], strong capability for NO₃⁻ adsorption/activation, and relatively weak HER activity. Compared with noble-metal catalysts, Cu is earth-abundant and cost-effective, making it more suitable for practical applications [33]. However, monometallic Cu catalysts often suffer from sluggish kinetics in the further reduction of NO₂⁻ and hydrogenation of nitrogen-containing intermediates, which may lead to nitrite accumulation and unsatisfactory NH₃ selectivity [34 -36]. Therefore, Cu sites alone are usually insufficient to simultaneously achieve efficient NO₃⁻ activation and deep hydrogenation.

Constructing CuCo binary catalysts offers an effective strategy to overcome these limitations [37,38]. Fig. 1f exhibits the chronological evolution of representative CuCo-based catalysts, highlighting the continuous diversification of material systems and electrocatalytic applications. In general, Cu sites mainly promote NO₃⁻ adsorption and initial reduction, whereas Co sites can regulate active hydrogen (*H) supply and facilitate the subsequent hydrogenation of NO₂⁻/*NO intermediates. Compared with pure Cu catalysts, the uniqueness of CuCo catalysts can be mainly reflected in following three aspects: 1) the introduction of Co can modulate the electronic structure of Cu and shift the Cu d-band center upward, thereby strengthening the interaction between Cu sites and key nitrogen-containing intermediates (Fig. 1g). 2) Cu-Co electronic coupling can lower the deoxygenation barrier of the NO₃⁻-to-NO₂⁻ step, promoting nitrate activation and initial reduction (Fig. 1h). And 3) the Cu-Co dual-site configuration can reduce the energy barrier of the rate-determining step, thus breaking the kinetic limitation of deep hydrogenation toward NH₃ (Fig. 1i). Through electronic coupling, interfacial synergy, and dual-site functional division between Cu and Co, the NO₃RR pathway can be optimized, while nitrite accumulation and HER competition can be suppressed. As a result, NH₃ yield rate, Faradaic efficiency, and catalytic stability can be improved. Recently, diverse material design strategies have been developed for CuCo-based catalysts, including heteroatom doping, heterostructure and interface engineering, MOF- and coordination-derived structures, atomically dispersed sites, in-situ reconstruction, and three-dimensional self-supported electrode architectures. Several recent reviews have addressed different aspects of electrocatalytic NO₃RR. Saafie et al. [39] focused on Cu- and Co-based catalyst engineering together with paired electrolysis, Li et al. [40] emphasized Cu-based electrocatalysts from catalyst design to practical applications, Wu et al. [41] discussed bimetallic active-site catalysts and their synergistic mechanisms, while Zhou et al. [38] summarized alloy catalysts and multisite cooperation. In contrast, the present review specifically focuses on Co-enabled tandem catalysis in Cu-based systems, with particular emphasis on Cu-Co functional division, active-hydrogen regulation, dynamic interfacial reconstruction, and electrode-scale mass-transfer enhancement.

This review focuses on recent advances in CuCo-based electrocatalysts for nitrate reduction to ammonia. First, the fundamental reaction pathways and key mechanistic features of NO₃RR are introduced. Then, CuCo-based catalyst design strategies and their structure-performance relationships are systematically summarized, with emphasis on Cu-Co bimetallic synergy, interfacial electronic modulation, hydrogen spillover, in-situ reconstruction, and mass-transfer enhancement. Finally, the remaining challenges and future perspectives are discussed, particularly regarding low-concentration nitrate wastewater treatment, high-current-density ammonia production, and practical reactor applications.

2. Mechanistic understanding of NO₃RR

NO₃RR is a complex eight-electron/nine-proton transfer process involving multiple adsorbed intermediates and competing reaction pathways [69]. Therefore, understanding how high-valence nitrate species are progressively reduced to NH₃ is essential for the rational design of efficient electrocatalysts.

In general, nitrate electroreduction can proceed through direct reduction and indirect autocatalytic mechanisms [70]. The indirect autocatalytic mechanism, including the Vetter and Schmid pathways (Fig. 2), usually occurs in highly acidic electrolytes with high nitrate concentrations [71,72]. In contrast, the direct reduction mechanism dominates in most neutral and alkaline NO₃RR systems.

The direct reduction mechanism dominates in most commonly studied NO₃RR systems, especially in neutral or alkaline electrolytes with NO₃⁻ concentrations below 1 mol·L^-1. The direct pathway can be further divided into electron-transfer-mediated reduction and active-hydrogen-mediated reduction. Typically, NO₃⁻ is first adsorbed on the catalyst surface to form *NO₃, followed by consecutive proton-coupled electron transfer steps to generate key intermediates such as *NO₂ and *NO. Since NO₃⁻ adsorption and initial N-O bond activation strongly influences the subsequent reaction barriers, this step is usually considered a critical kinetic process in NO₃RR.

During the subsequent reduction process, *NO can be converted to NH₃ through different pathways. In one possible route, *NO undergoes successive hydrogenation to form *NOH, *NHOH, and *NH₂OH intermediates, followed by deoxygenation and hydrogenation of *NH₂OH to *NH₂, and finally NH₃ formation. In another route, *NO is first hydrogenated to *NOH, which then undergoes dehydroxylation to form *N, followed by stepwise hydrogenation to *NH, *NH₂, and NH₃. The competition between these pathways is closely associated with the adsorption strengths of key intermediates, including *NO, *NOH, *N, and *NH₂OH, on the catalyst surface [73].

In the active-hydrogen-mediated pathway, water molecules are reduced on the cathode surface to generate adsorbed active hydrogen species (*H). These *H species subsequently participate in the deoxygenation and hydrogenation of *NO₂, *NO, *N, *NH, and *NH₃ intermediates, thereby promoting the conversion of NO₃⁻ to NH₃. Theoretical studies suggest that N-N coupling to form N₂ is generally less favorable than N-H bond formation toward NH₃. For instance, the migration barrier of *N on the catalyst surface is much higher than that of *H, making the coupling between *N and mobile *H more kinetically favorable than the recombination of two *N species. Therefore, an appropriate supply of *H is beneficial for enhancing N-H bond formation and improving NH₃ selectivity [74]. However, the coverage of *H must be precisely regulated. Insufficient *H supply limits the subsequent hydrogenation steps, whereas excessive *H accumulation promotes H₂ formation through *H dimerization.

Because NO₃RR and the HER have similar thermodynamic potentials, HER is usually an unavoidable competing reaction during NO₃RR. At more negative potentials, HER can occupy active sites and consume electrons and protons, leading to decreased NH₃ Faradaic efficiency. Under neutral and alkaline conditions, HER generally uses water as the proton source and proceeds through the Volmer, Heyrovsky, and Tafel steps:

(3)

${\mathrm{H}}_{2}\mathrm{O}+{\mathit{e}}^{-}=\mathrm{*}\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}\mathrm{ }\mathrm{ }\left(\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{m}\mathrm{e}\mathrm{r}\right)$

*H+H₂O+e⁻=H₂+OH⁻ (Heyrovsky) (4)

*H+*H=H₂ (Tafel) (5)

Therefore, the design of efficient NO₃RR catalysts should not simply aim to completely suppress HER. Instead, it is more important to balance sufficient *H supply with the suppression of excessive *H dimerization [75]. In other words, an ideal catalyst should promote water activation and active hydrogen generation while directing *H preferentially toward the hydrogenation of NOx intermediates rather than H₂ evolution.

Based on this mechanistic understanding, CuCo bimetallic catalysts provide a promising platform for regulating the NO₃RR pathway [76]. In many reported CuCo systems, Cu-related sites are proposed to favor NO₃⁻ adsorption and initial reduction, whereas Co-related sites may facilitate water activation, *H generation, and the subsequent hydrogenation of nitrogen-containing intermediates. However, this functional division is not universal, as the catalytic roles of Cu and Co strongly depend on their oxidation states, local coordination environments, interfacial structures, and reaction conditions. In particular, Cu⁰, Cu⁺, Cu²⁺, and dynamically reconstructed Cu/CuO_x interfacial species can exhibit distinct adsorption and activation behaviors toward NO₃⁻ and key intermediates, while Co species with different valence states may differently regulate water dissociation, *H availability, and intermediate hydrogenation. Importantly, Co sites may also directly participate in NO₃⁻ and NO₂⁻ adsorption and reduction rather than serving exclusively as hydrogenation centers, while Cu sites can contribute to subsequent reduction steps beyond the initial activation of NO₃⁻. In addition, Cu-Co interfacial sites may exhibit electronic and adsorption properties distinct from those of isolated Cu or Co sites, thereby creating alternative reaction pathways. By constructing Cu-Co dual active sites [77], heterointerfaces, or atomically adjacent sites, these complementary functions can be coupled to improve the kinetic matching between initial NO₃⁻ activation and subsequent deep hydrogenation. Therefore, Cu-Co synergy should be regarded as a dynamic and state-dependent interaction rather than a fixed “Cu for nitrate activation and Co for hydrogenation” model. The dominant roles of Cu, Co, and Cu-Co interfacial sites may vary with the applied potential, electrolyte composition, local microenvironment, and operando reconstruction, and should therefore be determined on a case-by-case basis using appropriate mechanistic evidence.

3. Material design strategies for CuCo-based electrocatalysts toward NO₃RR

In general, bimetallic catalysts offer greater structural and electronic tunability than monometallic catalysts [78,79]. On the one hand, bimetallic systems can provide multiple active sites and enable functional division among different reaction steps. On the other hand, electronic interactions and interfacial synergy between different metal species can optimize the adsorption energies of key intermediates, thereby improving catalytic activity and product selectivity. For NO₃RR, Cu sites are generally favorable for NO₃⁻ adsorption, N-O bond activation, and the initial conversion of NO₃⁻ to NO₂⁻, whereas Co sites can promote water dissociation, regulate *H supply, and accelerate the subsequent hydrogenation of NO₂⁻/*NO intermediates. Therefore, the rational construction of Cu-Co dual active sites are regarded as an effective strategy for enhancing nitrate-to-ammonia conversion [80,81]. It should be emphasized that Cu-Co synergy does not necessarily follow a universal functional division. Depending on catalyst structure and operating conditions, the enhancement may originate from four distinct scenarios: electronic promotion without direct intermediate transfer, adjacent-site tandem relay of nitrogen-containing intermediates, Co-mediated H generation and hydrogen spillover, or electrochemical reconstruction into a new CuCo-containing active phase. These mechanisms should therefore be distinguished according to the available experimental evidence rather than inferred solely from improved catalytic performance.

In recent years, various material design strategies have been developed for CuCo-based catalysts, including heteroatom doping [82], heterostructure and interface engineering [83], MOF- and coordination-derived structures, atomically dispersed sites [84], in-situ reconstruction, and three-dimensional electrode architecture/mass-transfer engineering. These strategies regulate NO₃RR at multiple levels, including electronic structure, local coordination environment, interfacial interaction, real active phase, and macroscopic mass transport, thereby improving NH₃ Faradaic efficiency, yield rate, and long-term stability. In this section, the main design strategies of CuCo-based NO₃RR electrocatalysts are systematically summarized, with emphasis on their structural features, catalytic performance, and Cu-Co synergistic mechanisms. Representative electrochemical performances of CuCo-based catalysts are summarized in Table 1.

3.1. Heteroatom-doped CuCo-based catalysts

Heteroatom doping is an effective strategy for regulating the electronic structure of CuCo-based catalysts and optimizing the NO₃RR pathway. The introduction of nonmetallic elements [97,98], such as B, P, S, and N, can induce charge redistribution around metal centers and modify the local coordination environments of Cu and Co sites, thereby tuning the adsorption strengths of key intermediates, including NO₃⁻, NO₂⁻, *NO, and *H. For CuCo binary systems, heteroatom doping not only enhances the intrinsic activity of individual metal sites but also improves the synergistic interaction between Cu and Co, enabling better kinetic matching between NO₃⁻ activation and subsequent hydrogenation steps.

For example, Liu et al. [99] reported a B-doped Cu₂O/Cu composite catalyst, denoted as B-Cu₂O/Cu/CP, to investigate the effect of nonmetal doping on Cu active sites for NO₃RR. Although this system is not a typical CuCo binary catalyst, it provides useful insight into dopant-induced regulation of Cu centers. In 0.1 M KOH containing 1000 ppm NO₃⁻, B-Cu₂O/Cu/CP achieved an NH₃ Faradaic efficiency of 92.74% and an NH₃ yield rate of 2.44 mg·h^-1·cm^-2 at −0.5 V vs. RHE. The NO₂⁻ Faradaic efficiency was only 1.52%, much lower than that of undoped Cu₂O/Cu/CP, indicating that B doping promoted further NO₂⁻ reduction and suppressed nitrite accumulation. Structural characterizations and theoretical calculations revealed that B doping induced lattice distortion and Cu-B coordination, leading to the electronic reconstruction of Cu sites (Fig. 3a). This work suggests that nonmetal doping can optimize NO₃RR intermediate conversion by modulating the electronic structure of Cu sites, providing guidance for dopant engineering in CuCo-based catalysts.

More directly, Yan et al. [52] introduced P doping into a CuCo binary system and constructed a biphasic nanoflower-like P-Cu/Co(OH)₂ catalyst composed of P-doped Cu clusters and P-doped Co(OH)₂ nanosheets. The incorporation of P induced lattice contraction in both Cu and Co(OH)₂ phases, suggesting that P doping effectively modified the local coordination environments and electronic structures of Cu and Co sites. In 1 M KOH containing 0.1 M KNO₃, P-Cu/Co(OH)₂ delivered an NH₃ yield rate of 42.63 mg·h^-1·cm^-2 and a Faradaic efficiency of 97.04% at −0.4 V vs. RHE, significantly outperforming the undoped Cu/Co(OH)₂ catalyst (Fig. 3b). The enhanced activity can be mainly attributed to the synergistic modulation of Cu-Co dual sites by P doping. Specifically, P-doped Cu clusters facilitate NO₃⁻ adsorption and initial reduction, whereas P-doped Co(OH)₂ nanosheets promote the subsequent hydrogenation of NO₂⁻/*NO intermediates, thereby improving the kinetic matching of the tandem catalytic process (Fig. 3c). Furthermore, the catalyst was assembled into a Zn-NO₃⁻ battery, delivering a peak power density of 13.78 mW·cm^-2 and maintaining an NH₃ Faradaic efficiency of 90.11% at 60 mA·cm^-2, demonstrating the potential of heteroatom-doped CuCo catalysts for both nitrate valorization and energy conversion.

P doping has also been extended to CuCo alloy systems. Wang et al. [85] prepared P-doped CuCo alloy catalysts on Ni foam by electrodeposition, among which P-Cu₅₀Co₅₀ delivered an NH₃ yield rate of 45.5 mg·h^-1·cm^-2 and a Faradaic efficiency of 96.7% at −0.5 V vs. RHE in 1.0 M KOH containing 0.1 M KNO₃. The low NO₂⁻ Faradaic efficiency indicated that P-Cu₅₀Co₅₀ effectively suppressed nitrite accumulation. Spectroscopic and radical-trapping results suggested that P-metal coordination promoted water dissociation and *H generation, while multivalent Cu and Co species optimized intermediate adsorption and hydrogenation. This work further confirms that coupling P doping with CuCo alloying can efficiently balance NO₃⁻ activation and *H-mediated hydrogenation.

Overall, heteroatom doping can optimize the adsorption and transformation of NO₃RR intermediates by inducing lattice distortion, forming metal-heteroatom coordination bonds, regulating *H supply, and modulating the local electronic structure of CuCo-based catalysts. In CuCo binary systems, rational heteroatom doping can enhance the NO₃⁻ activation capability of Cu sites while improving the hydrogenation ability of Co sites toward NO₂⁻ and *NO intermediates. As a result, nitrite accumulation and the competing hydrogen evolution reaction can be effectively suppressed, ultimately leading to improved NH₃ Faradaic efficiency, yield rate, and selectivity.

3.2. CuCo heterostructures and interface engineering

Constructing CuCo heterostructures is an important materials-design strategy for improving NO₃RR performance [100,101]. Compared with single Cu or Co components, CuCo heterointerfaces can regulate the adsorption and transformation of key intermediates, including NO₃⁻, NO₂⁻, NO and H, through interfacial electronic coupling [102], local charge redistribution, and dual-site cooperation. In general, Cu sites are more favorable for NO₃⁻ adsorption and initial reduction, whereas Co-related sites can promote the subsequent hydrogenation of NO₂⁻ intermediates or regulate the supply of active hydrogen species. Therefore, the rational construction of Cu-Co, Cu/CuO_x-Co/CoO [103,104], Cu-LDH, CuCo oxide, and CuCo alloy interfaces is beneficial for establishing efficient tandem catalytic pathways, thereby suppressing nitrite accumulation and the competing hydrogen evolution reaction.

He et al. [86] constructed a core-shell Cu/CuO_x@Co/CoO catalyst through an electrochemically driven phase-separation strategy (Fig. 4a). In 0.1 M KOH containing 0.1 M NO₃⁻, this catalyst achieved an NH₃ Faradaic efficiency of 93.3 ± 2.1% at −0.175 V vs. RHE. When the NO₃⁻ concentration was increased to 0.1 M, the NH₃ yield rate reached 19.9 mg·h^-1·cm^-2. Kinetic analysis, in-situ Raman spectroscopy, and scanning electrochemical microscopy revealed that the inner Cu/CuO_x phase preferentially catalyzed the reduction of NO₃⁻ to NO₂⁻, while the generated NO₂⁻ subsequently diffused to the outer Co/CoO phase and was rapidly reduced to NH₃. This spatially decoupled dual-site structure enables functional division between initial NO₃⁻ activation and deep NO₂⁻ reduction, highlighting the advantage of CuCo heterointerfaces in tandem NO₃RR catalysis.

Similarly, Liu et al. [53] constructed an amorphous CoO/Cu₂O heterojunction, denoted as a-CoO/Cu₂O, to enhance active hydrogen supply at the Cu-Co oxide interface. In 1 M KOH containing 50 mM KNO₃, a-CoO/Cu₂O achieved an NH₃ Faradaic efficiency of 95.72% and an NH₃ yield rate of 16.4 mg·h^-1·mg_cat^-1 at −0.4 V vs. RHE, outperforming crystalline CoO/CuO and pristine Cu₂O. EPR and in-situ ATR-SEIRAS results revealed that amorphous CoO promoted water dissociation to generate abundant *H, accelerated NO₂⁻ consumption, and facilitated the hydrogenation of intermediates to NH₃. This work highlights the importance of amorphous Cu-Co oxide interfaces in regulating *H supply and improving NO₃RR hydrogenation kinetics.

Layered double hydroxides (LDHs), featuring tunable metal compositions, abundant active sites, and open interlayer channels, have also been explored as CuCo-based NO₃RR catalysts [105]. Wang et al. [87] reported a CuCoAl LDH catalyst, which delivered an NH₃ Faradaic efficiency of 99.5% and an NH₃ yield rate of 0.22 mol·h^-1·g^-1 at −0.8 V vs. RHE (Fig. 4b-c). In this system, the Cu component facilitated NO₃RR at a lower overpotential, while the Co component suppressed nitrite byproduct formation and improved NH₃ selectivity. The Al component mainly contributed to stabilizing the LDH structure, thereby maintaining highly active interfaces and favorable reaction stability. These results indicate that regulating the coordination environments of Cu and Co within multimetallic LDH frameworks can effectively enhance NO₃RR selectivity and efficiency. Similarly, Fan et al. [54] constructed an oxygen-vacancy-rich Cu-doped CoFe LDH catalyst, in which Cu sites promoted NO₃⁻-to-NO₂⁻ conversion, Co sites facilitated NO₂⁻ hydrogenation, and oxygen vacancies enhanced *H supply while suppressing HER, thereby enabling efficient NO₃RR even at ultralow nitrate concentration.

To further clarify the tandem catalytic differences between Cu and iron-group metals, Yan et al. [55] constructed Fe/Cu, Co/Cu, and Ni/Cu binary catalysts (Fig. 4d-i). The results showed that Cu sites efficiently promoted the conversion of NO₃⁻ to NO₂⁻, but they were insufficient for further NO₂⁻ capture and conversion. Fe sites also favored the NO₃⁻-to-NO₂⁻ step and therefore aggravated nitrite accumulation. Ni sites exhibited strong NO₂⁻ reduction ability at low overpotentials, but their pronounced HER activity limited ammonia production at higher current densities. In contrast, Co sites showed balanced *H activation capability and excellent NO₂⁻-to-NH₃ conversion activity, enabling an ordered tandem pathway with Cu sites. Consequently, the Co/Cu catalyst achieved an NH₃ yield rate of 48.44 mg·h^-1·cm^-2 and a Faradaic efficiency of 96.46% at −0.4 V vs. RHE. This comparative study demonstrates that Co is a particularly suitable secondary metal component for constructing Cu-based tandem NO₃RR catalysts.

Beyond metal/metal tandem interfaces, Cu oxide/Co-based hydroxide interfaces can further promote NO₃RR by integrating interfacial electron transfer with hydrogen spillover. Zheng et al. [57] developed a CoNi-LDH@Cu₂O heterostructure (Fig. 5a-b), which achieved an NH₃ Faradaic efficiency of 97.8% at −0.3 V vs. RHE and an NH₃ yield rate of 75.2 mg·h^-1·cm^-2 at an industrially relevant current density of approximately 1 A·cm^-2. In-situ EPR, DEMS, XPS, and DFT calculations indicated that CoNi-LDH promoted water dissociation and enhanced the availability of active hydrogen species near the Cu₂O interface. Although these results support Co-mediated H generation and interfacial hydrogen-assisted cooperation, they do not by themselves directly prove the migration of H from CoNi-LDH to Cu₂O. Therefore, the proposed hydrogen-spillover pathway should be interpreted cautiously and in conjunction with complementary kinetic, isotopic, and theoretical evidence. Meanwhile, interfacial electron transfer from CoNi-LDH to Cu₂O lowered the reduction barrier of *NO₃ adsorbed on Cu sites. The synergy between hydrogen spillover and interfacial electron transfer is therefore critical for achieving efficient NO₃RR at high current densities.

Similarly, Liu et al. [56] stabilized Cuδ⁺ species in CuO_x nanorods through Ni and Co co-doping, obtaining a Ni,Co-CuO_x catalyst (Fig. 5c-e). In 1.0 M KOH containing 0.1 M KNO₃, the catalyst delivered an NH₃ yield rate of 130.64 mg·h^-1·cm^-2 at −0.5 V vs. RHE and an NH₃ Faradaic efficiency of 98.53% at −0.3 V vs. RHE. In contrast to undoped CuO_x, which was completely reduced to metallic Cu during electrolysis, Ni,Co-CuO_x maintained a Cu/Cu₂O heterostructure through Ni/Co-O-Cu linkages and preserved a higher Cuδ⁺/Cu⁰ ratio. This result suggests that interfacial coordination bonds can not only tune the electronic structure of Cu sites but also stabilize reconstruction-prone Cu species under NO₃RR conditions, thereby improving both catalytic activity and durability.

For more complex multicomponent interfaces, Zuo et al. [58] prepared Cu nanocluster-anchored CoMoO₄ nanosheets. During NO₃RR, Cu induced the accelerated reconstruction of CoMoO₄, forming a CoMoO₄/Co(OH)₂/Cu ternary active heterostructure (Fig. 5f). The catalyst exhibited an NH₃ Faradaic efficiency of 98.5% and an NH₃ yield rate of 20.1 mg·h^-1·cm^-2 at −0.2 V vs. RHE, significantly outperforming pristine CoMoO₄. Mechanistic studies indicated that Cu nanoclusters accelerated the NO₃⁻-to-NO₂⁻ conversion and suppressed *H dimerization, Co(OH)₂ promoted water dissociation and regulated intermediate adsorption, and CoMoO₄ further facilitated NO₂⁻-to-NH₃ conversion. This system highlights the advantages of multicomponent interfacial cooperation in simultaneously regulating NO₃⁻ activation, *H supply, and deep NO₂⁻ reduction.

CuCo alloy interfaces can also achieve efficient NO₃RR by balancing the surface coverage of *H and NOx intermediates. Fang et al. [59] reported a Cu₅₀Co₅₀ alloy nanosheet catalyst, which achieved an NH₃ Faradaic efficiency of 100 ± 1%, a current density of 1035 mA·cm^-2, and an NH₃ yield rate of 81.8 mg·h^-1·cm^-2 at −0.2 V vs. RHE. Compared with pure Cu and pure Co, the CuCo alloy balanced the surface coverage of NO₃⁻/NOx and *H through electronic redistribution. This compensated for the insufficient *H supply on pure Cu and avoided excessive *H occupation on pure Co, thereby suppressing HER competition. This biomimetic bifunctional-site design provides an important reference for achieving highly selective NO₃RR at ampere-level current densities.

CuCo oxide heterostructures also show significant advantages. Li et al. [60] prepared a series of CuCo₂O₄ catalysts by regulating the Cu/Co ratio. The Cu-rich 4- CuCo₂O₄ catalyst achieved an NH₃ Faradaic efficiency of 100% and an NH₃ yield rate of 24.58 mg·h^-1·mg_cat^-1 at −0.70 V vs. RHE in a neutral electrolyte. Post-reaction structural analysis showed that part of CuO was reduced to metallic Cu during electrolysis, forming a Cu@CuCo₂O₄ real active phase. The synergy between Cu and CuCo₂O₄ significantly accelerated the initial NO₃⁻-to-NO₂⁻ conversion (Fig. 5g), as evidenced by the nearly identical NH₃ yield rates when NO₃⁻ and NO₂⁻ were used as substrates. This indicates that the catalyst effectively overcame the kinetic bottleneck of nitrate-to-nitrite conversion. Wu et al. [61] further constructed a Cu-doped Co₃O₄ nanoneedle array self-supported electrode by using Cu foam as both the substrate and Cu source. The resulting Cu-Co₃O₄/CF catalyst reached an industrial-level current density of approximately 1000 mA·cm^-2 at −0.5 V vs. RHE, with an NH₃ yield rate of 58.4 mg·h^-1·cm^-2 and a Faradaic efficiency of 98.3%, demonstrating that Cu-doped Co₃O₄ interfaces can maintain excellent NO₃RR activity under high-current-density operation.

Overall, the essence of CuCo heterostructure and interface engineering lies in constructing Cu and Co active sites with clear functional division. Cu-related sites mainly promote NO₃⁻ adsorption and initial reduction, whereas Co-related sites facilitate NO₂⁻ capture, *H regulation, and subsequent hydrogenation. Through interfacial electron transfer, hydrogen spillover, lattice distortion, coordination modulation and in-situ reconstruction, the energy barriers of key reaction steps can be effectively reduced, while nitrite accumulation and HER competition are suppressed. Consequently, CuCo heterostructures provide a powerful platform for simultaneously improving NH₃ yield rate, Faradaic efficiency and long-term stability.

3.3. MOF- and coordination-derived CuCo catalysts

MOF- and coordination-framework-based materials, featuring well-defined metal nodes, ordered pore structures, and tunable coordination environments, provide an attractive platform for constructing CuCo dual-metal active sites. Compared with conventional inorganic supports, MOFs can not only improve the dispersion of metal sites but also regulate NO₃⁻ mass transport, intermediate retention, and the local reaction microenvironment through pore confinement [106]. In particular, conductive MOFs (cMOFs), which combine ordered channels with enhanced charge transport capability, offer a promising solution to the intrinsically poor conductivity of conventional MOFs. Through post-synthetic metal exchange, pore-confined loading, coordination reconstruction, or pyrolysis-derived transformation, Cu and Co sites can be integrated into framework structures as atomic sites, nanoclusters, or heterointerfaces, thereby enabling cooperative regulation of NO₃⁻ adsorption, NO₂⁻ conversion and subsequent hydrogenation. However, post-synthetic metal modification may also damage the parent framework, leading to pore collapse, structural degradation, or active-site instability. Therefore, the rational design of CuCo-MOF catalysts requires a careful balance among metal-site incorporation, framework stability, and electron-transfer efficiency.

Liu et al. [62] constructed bimetallic Cu_xCo_yHHTP (hexahydroxytriphenylene hydrate) conductive MOFs by partially replacing Cu sites in CuHHTP with Co (Fig. 6a-b). The resulting material retained the ordered porous structure and good conductivity of CuHHTP, while introducing Co sites to modulate the local electronic environment of Cu (Fig. 6c-d). Benefiting from the Cu-Co synergy, Cu_xCo_yHHTP showed superior NO₃RR performance compared with monometallic CuHHTP. Among different compositions, Cu₁Co₁HHTP with an n(Co):n(Cu) ratio of 1:1 exhibited the best electrocatalytic activity toward NH₃ synthesis. The improved performance can be attributed to two main factors. On the one hand, the conductive HHTP framework facilitates rapid charge transfer and increases the contact probability between NO₃⁻/reaction intermediates and active sites within the porous channels. On the other hand, Co substitution tunes the electronic structure of Cu sites, resulting in more favorable adsorption and conversion of key NO₃RR intermediates. This work demonstrates that metal-node regulation in conductive MOFs is an effective strategy for constructing high-performance CuCo dual-metal catalysts for NO₃RR.

In addition to direct metal-node modulation, pore confinement provides another effective route to constructing Cu-Co tandem catalytic sites. Sun et al. [63] used cage-structured MOF-818(Cu) as a host to confine Co nanoclusters within the MOF cages, obtaining MOF-818(Cu)-Co (Fig. 6e). This catalyst exhibited excellent NO₃RR performance in a neutral electrolyte, achieving an NH₃ Faradaic efficiency of 99.5% at −1.5 V vs. Ag/AgCl and maintaining an NH₃ Faradaic efficiency above 90% over a wide potential window from −1.3 to −1.8 V. Compared with pristine MOF-818(Cu) and Co nanoparticles, MOF-818(Cu)-Co delivered a much higher mass-normalized NH₃ yield rate. The enhanced performance originates from the pore-confinement effect of the MOF cages and the tandem cooperation between Cu and Co sites. The Cu sites in MOF-818(Cu) favor NO₃⁻ adsorption and initial activation, whereas the confined Co nanoclusters promote the subsequent hydrogenation of nitrogen-containing intermediates. Meanwhile, the MOF cages restrict Co aggregation and enrich local NO₃⁻/NO₂⁻ species, thereby facilitating the continuous reduction process. This study highlights that MOF confinement can stabilize metal nanoclusters and construct spatially adjacent Cu-Co bifunctional sites for efficient nitrate-to-ammonia conversion under neutral conditions.

MOF-derived carbon composites provide another route to integrating CuCo alloy sites with conductive porous supports. Cang et al. [88] prepared a ZIF-derived CuCo-NC catalyst, in which CuCo alloy nanoparticles were embedded in a nitrogen-doped carbon framework. Benefiting from the high surface area of the porous carbon matrix and the synergistic CuCo alloy sites, CuCo-NC achieved an NH₃ Faradaic efficiency of 97% and an NH₃ yield rate of 7.17 mg·h^-1·cm^-2 at −0.3 V vs. RHE in 1 M KOH containing 0.1 M KNO₃. It also showed efficient nitrite-to-ammonia conversion. This work demonstrates that ZIF-derived N-doped carbon frameworks can simultaneously improve metal dispersion, electron transport, and Cu-Co cooperative catalysis, thereby expanding the design space of MOF-derived CuCo catalysts.

Related COF-shell-modified and MOF-derived catalysts, although not always typical CuCo binary systems, can also provide useful guidance for microenvironment engineering in CuCo framework catalysts. Tahir et al. [107] constructed pyridine- or imidazole-functionalized COF shells on Cu₂O nanocubes. Among them, Cu₂O@Py-COF exhibited higher NO₃RR activity than Cu₂O@Im-COF and pristine Cu₂O in neutral electrolyte (Fig. 6f-g), indicating that functionalized organic framework shells can regulate NO₂⁻ enrichment, intermediate transport, and interfacial stability through molecular-level channels and coordination environments. Similarly, Dong et al. [64] prepared a Ce-Cu/MoO₂@C catalyst using Ce-doped NENU-5 MOF as a precursor. In this system, Ce sites promoted water dissociation to generate active hydrogen species (*H), which were supplied to neighboring Cu sites through hydrogen spillover, thereby accelerating the hydrogenation of nitrogen-containing intermediates (Fig. 6h). Although these two systems are not typical CuCo catalysts, they demonstrate the importance of organic-framework-shell modulation and heterometal-assisted hydrogen regulation, which may guide the future design of CuCo-MOF or CuCo-COF-derived catalysts.

Overall, the advantages of MOF- and coordination-derived CuCo catalysts lie in their ability to regulate NO₃RR at three levels: metal nodes, pore confinement, and local microenvironment. By rationally introducing Co sites into Cu-based frameworks, cooperative bimetallic centers can be constructed, where Cu sites mainly promote NO₃⁻ adsorption and initial reduction, while Co sites facilitate NO₂⁻ capture, *H supply, or subsequent hydrogenation. Meanwhile, ordered pore channels can improve NO₃⁻ mass transport, stabilize metal nanoclusters, and suppress active-site aggregation. Future efforts should focus on developing highly conductive and structurally robust MOF/COF-derived catalysts with well-defined Cu-Co coordination relationships, which will be crucial for further improving the NO₃RR performance of CuCo-based catalysts.

3.4. Atomically dispersed CuCo catalysts

Atomically dispersed catalysts have attracted increasing attention in NO₃RR because of their high metal utilization efficiency, well-defined coordination structures, and tunable electronic environments [108 -110]. Compared with conventional nanoparticles or bulk catalysts, single-atom and dual-atom sites can maximize the exposure of metal centers, while strong metal-coordination interactions help stabilize isolated active sites, thereby improving catalytic activity, selectivity and durability. For CuCo-based catalysts, atomically dispersed structures not only regulate the individual electronic states of Cu and Co, but also create electronic coupling and functional cooperation between adjacent Cu-Co sites, enabling more efficient NO₃⁻ adsorption, NO₂⁻ conversion, and subsequent hydrogenation.

Wei et al. [65] reported a nitrogen-doped carbon-supported Co-Cu dual-atom catalyst, denoted as CoCu-NC DAC. In this catalyst, atomically dispersed Co and Cu were anchored in the carbon matrix as Co-N₄ and Cu-N₄ coordination structures, with a Co-Cu atomic distance of approximately 2.65 Å. The dual-atom catalyst exhibited excellent NO₃RR performance in a neutral electrolyte, delivering an NH₃ Faradaic efficiency of 95.3% and an NH₃ yield rate of 2.41 mg·h^-1·cm^-2 at −0.6 V vs. RHE, outperforming monometallic Co-NC and Cu-NC single-atom catalysts. DFT calculations indicated that CoCu-NC showed stronger NO₃⁻ adsorption than Co-NC and Cu-NC, while significantly lowering the energy barrier of the key hydrogenation step (Fig. 7a-b). In particular, the adjacent Co site could provide adsorbed hydrogen for Cu-bound nitrogen intermediates, forming a dual-atom cooperative hydrogenation pathway. This work demonstrates that atomically adjacent Cu-Co dual sites can simultaneously optimize electronic structure and *H supply, providing an effective strategy for enhancing NO₃RR kinetics.

Besides true dual-atom sites, the coupling of single atoms with metal nanoclusters can also create bifunctional catalytic structures. Yi et al. [66] constructed a self-supported carbon nanofiber electrode co-modified with Cu nanoclusters and Co single atoms, denoted as CuCo-CNF. In this structure, Co mainly existed as Co-N₄ single-atom sites, while Cu was dispersed as nanoclusters within the carbon nanofiber framework (Fig. 7c-f). In a neutral electrolyte, CuCo-CNF achieved an NH₃ yield rate of 11.3 mg·h^-1·cm^-2 and a Faradaic efficiency of 95.84% at −0.7 V vs. RHE. Theoretical calculations showed that Cu nanoclusters exhibited much stronger NO₃⁻ adsorption than Cu single atoms, and their d-band center was closer to the Fermi level, which favored electron transfer and intermediate activation. Meanwhile, Co single-atom sites could regulate the local electronic environment and participate in subsequent reaction steps. This study suggests that integrating Cu nanoclusters and Co single atoms into a conductive self-supported framework can simultaneously provide high active-site density, rapid charge transport, and good mechanical stability.

Xue et al. [67] further utilized the confinement effect of mesoporous carbon spheres to construct a CuCo/MCS catalyst containing Cu nanoparticles and atomically dispersed Co sites. The catalyst exhibited high applicability to low-concentration nitrate remediation. In a neutral electrolyte, it achieved almost complete NO₃⁻-N removal within 150 min at −0.60 V vs. RHE, with an NH₃ selectivity of 91.2%, a NO₂⁻-N residue below 0.02 mg/L, and a Faradaic efficiency of 92.1%. DFT calculations revealed that the Cu sites underwent significant charge-density variation after NO₃⁻ adsorption, while Co single-atom sites supplied electron density to neighboring Cu nanoparticles, thereby strengthening the chemisorption of *NO₃ (Fig. 7g-h). By confining Cu nanoparticles and Co single atoms in nanoscale proximity, this system mimics the spatial cooperation between reaction channels and active centers in natural nitrite reductase, providing a useful strategy for low-energy electrochemical treatment of nitrate-contaminated water.

More recently, Su et al. [89] developed an atomically paired Cu-Co dual-site catalyst on an interpenetrating polymer network (Fig. 7i), denoted as IPN-CuCo, for industrial-current-density NO₃RR. In a two-electrode flow cell, IPN-CuCo achieved an NH₃ Faradaic efficiency of 99% and an NH₃ yield rate of 272.6 mg·h^-1·cm^-2 at −0.40 V vs. RHE, corresponding to an NH₃ partial current density as high as 4.7 A·cm^-2. Moreover, the catalyst operated continuously for 200 h at 3.5 A·cm^-2 while maintaining an NH₃ Faradaic efficiency of 97%-100%. This work indicates that polymer-network-regulated atomically adjacent Cu-Co sites can not only enhance the intrinsic activity and selectivity of NO₃RR (Fig. 7j), but also meet the practical requirements of high current density, long-term operation, and continuous product collection.

It should be noted that some non-CuCo single-atom systems also provide important references for understanding the role of atomically dispersed sites in NO₃RR. For example, Chen et al. [111] reported a Cu₁/ZnO single-atom catalyst, in which isolated Cu atoms enhanced NO₃⁻ adsorption and *H retention while suppressing HER competition (Fig. 7k-l), thereby lowering the energy barrier of the *NO → *NHO step. Although this system is not a CuCo-based catalyst, the revealed mechanism of isolated Cu-site regulation provides useful guidance for designing Cu sites in atomically dispersed CuCo catalysts.

Overall, the key advantage of atomically dispersed CuCo catalysts lies in the construction of spatially adjacent and electronically coupled Cu-Co active sites. Cu sites generally facilitate NO₃⁻ adsorption and initial N-O bond activation, whereas Co sites can promote *H supply, NO₂⁻ capture, and subsequent hydrogenation. Through dual-atom pairing, single-atom/nanocluster coupling, mesoporous confinement, and polymer-network anchoring, the utilization efficiency and cooperation degree of Cu-Co sites can be significantly enhanced, leading to lower reaction barriers and suppressed nitrite accumulation and HER competition. Future studies should focus on the precise structural identification of atomic Cu-Co sites, stabilization of high-loading isolated sites, and dynamic evolution under realistic NO₃RR conditions.

**3.5. In-situ reconstructed CuCo catalysts**

Electrochemical in-situ reconstruction has become an important strategy for identifying and generating catalytically relevant active interfaces in CuCo-based catalysts. Under NO₃RR conditions, Cu-based oxides, Co-based oxides, phosphides, sulfides, and MOF precursors often undergo phase transformation, valence-state evolution, or surface reconstruction. It should be emphasized that such reconstruction is not unique to CuCo systems. In particular, the partial reduction or structural evolution of Cu oxides under cathodic potentials is a common electrochemical phenomenon and may occur even in the absence of Co. Therefore, the occurrence of reconstruction itself should not be regarded as evidence of Cu-Co synergy or as an indication that the reconstructed phase is intrinsically more active. For instance, CuO/Cu₂O can be partially reduced to Cu⁰/Cu⁺ interfaces, Co₃O₄/CoO may transform into CoOOH, Co(OH)₂, or low-valence Co species, while phosphides and sulfides can form oxidized or hydroxylated surface layers. MOF structures may also partially collapse and generate metal/oxide/carbon composite active phases. Therefore, the initial catalyst structure is not necessarily identical to the real active phase. For CuCo catalysts, a more meaningful distinction should be made between unavoidable potential-induced reconstruction and genuinely beneficial Co-regulated reconstruction. The latter requires evidence that Co alters the reconstruction pathway, stabilizes specific Cu/Co oxidation states or interfacial configurations, or generates reconstructed sites with measurably improved NO₃RR kinetics or selectivity. By appropriately controlling such reconstruction, CuCo catalysts with metal-oxide interfaces, oxygen vacancies, and multivalent metal centers can be generated under reaction conditions, thereby potentially regulating NO₃⁻ adsorption, NO₂⁻ conversion, *H supply and HER suppression.

Zhu et al. [68] reported a Co-Cu₂O nanorod catalyst that underwent dynamic reconstruction during NO₃RR (Fig. 8a). In an environmentally relevant nitrate electrolyte, the catalyst achieved an NH₃ yield rate of 9.35 mg·h^-1·cm^-2 and a Faradaic efficiency of 94.3% at −0.7 V vs. RHE, outperforming Co-free Cu₂O nanorods. In-situ Raman spectroscopy, XANES, and EXAFS revealed that Co-Cu₂O evolved into a metastable CoO/Co-Cu₂O/Cu heterointerface during electrolysis, where part of Cu⁺ was reduced to Cu⁰ and Co species existed as CoO/Co. This reconstructed interface created a clear functional division: Cu sites preferentially promoted the conversion of NO₃⁻ to NO₂⁻, whereas Co-related sites facilitated water dissociation and supplied *H for subsequent hydrogenation. DFT calculations further showed that the d-band center of CoO/Co-Cu₂O/Cu shifted upward compared with that of Cu₂O/Cu, strengthening the adsorption of NO₃RR intermediates and lowering the energy barrier of key reaction steps. This work demonstrates that Co-induced dynamic reconstruction can convert Cu₂O precursors into more active Cu-Co oxide/metal composite interfaces for tandem nitrate-to-ammonia conversion.

Mei et al. [90] further constructed an oxygen-vacancy-rich Ov-Co(OH)₂/Cu heterostructure through in-situ electrochemical reconstruction. The reconstructed catalyst delivered an NH₃ yield rate of 167.8 mg·h^-1·cm^-2 and a Faradaic efficiency of 97.7% at −0.4 V vs. RHE, corresponding to a current density above 2 A·cm^-2. Even after continuous operation at 2 A·cm^-2 for 25 h, it maintained high NH₃ production activity and Faradaic efficiency, indicating excellent tolerance to industrial-level current density. Mechanistic studies revealed that the oxygen vacancies generated during reconstruction significantly accelerated the water-dissociation kinetics of Co(OH)₂ (Fig. 8b), enhanced the supply of *H, and optimized the adsorption of key intermediates such as *NOOH. Compared with pristine Co(OH)₂, Ov-Co(OH)₂/Cu greatly reduced the energy barrier of the rate-determining *NO₂ → *NOOH step, thereby accelerating deep hydrogenation of nitrite intermediates. This study highlights that introducing stable oxygen vacancies and Co(OH)₂/Cu interfaces through in-situ reconstruction is an effective route toward ampere-level NO₃RR.

Zhao et al. [91] reported a carbon-supported CuCo tandem catalyst, denoted as CuCo-CNO, which also underwent in-situ reconstruction under NO₃RR conditions. In a neutral electrolyte, CuCo-CNO achieved an NH₃ Faradaic efficiency of 96.5% and an NH₃ yield rate of 2.9 mg·h^-1·cm^-2 at −0.8 V vs. RHE, outperforming the corresponding Cu-CNO and Co-CNO catalysts. In-situ Raman spectroscopy revealed that the initial metallic Cu-Co sites were reconstructed into Cu₂O and CoOOH during electrolysis, which served as the real active phases. DFT calculations further showed that CuCo-CNO strengthened NO₃⁻ adsorption, lowered the energy barrier of the key *NO → *NH step, and increased the HER barrier. This work indicates that carbon-supported CuCo precursors can in-situ generate oxide/oxyhydroxide active phases, thereby improving nitrate activation, hydrogenation kinetics, and NH₃ selectivity.

Beyond oxide and carbon-supported metallic precursors, conductive MOF precursors can also be electrochemically reconstructed into highly stable CuCo composite active interfaces. Luo et al. [92] used CoCuHHTP (2,3,6,7,10,11-hexahydroxytriphenylene) as a precursor and constructed CoCu₂O nanoclusters supported on partially reduced CoCuHHTP through in-situ electrochemical reduction, denoted as CoCu₂O@CoCuHHTP. In alkaline nitrate electrolyte, this catalyst achieved an NH₃ Faradaic efficiency of 97.9% and an NH₃ yield rate of 20.4 mg·h^-1·cm^-2 at −0.6 V vs. RHE, and exhibited excellent long-term stability in both H-type and flow-cell systems. Structural characterization showed that CoCuHHTP did not completely collapse after electrochemical reduction. Instead, part of the conductive organic framework was retained, exposing abundant uncoordinated hydroxyl groups and forming a synergistic interface with in-situ generated CoCu₂O nanoclusters. This hydrogen-bond-rich metal-organic framework/metal oxide composite structure is beneficial for stabilizing reaction intermediates (Fig. 8c-d), promoting proton/electron transfer, and improving long-term NO₃RR durability. This work suggests that controlling the reconstruction degree of MOF precursors can preserve the advantages of framework structures while generating real active metal oxide interfaces.

In addition, although the nanoporous Co₂P/CoOOH system reported by Sun et al. [112] is not a CuCo catalyst, it provides useful insight into the reconstruction behavior of non-oxide precursors during NO₃RR. During electrolysis, Co₂P/CoOOH transformed into a real active Co₂P/Co₃O₄ heterostructure, which promoted water dissociation, optimized *NO intermediate adsorption, and lowered the energy barrier of the *NO → *NOH step. This result indicates that phosphide/oxyhydroxide precursors often undergo complex redox reconstruction under NO₃RR conditions (Fig. 8e), and the real active phase should be identified by operando Raman, ATR-SEIRAS, XAS and other techniques. This mechanism may also provide guidance for designing reconstructed CuCo phosphide or sulfide catalysts.

Overall, in-situ reconstruction in CuCo catalysts should not be regarded as intrinsically beneficial, because part of the structural evolution may simply originate from unavoidable potential-induced reduction, oxidation-state changes, or surface reorganization under NO₃RR conditions. The key mechanistic question is whether Co specifically modifies the reconstruction pathway or stabilizes reconstructed Cu-Co states that exhibit superior catalytic behavior relative to appropriate Cu-only, Co-only, or unreconstructed references. Beneficial reconstruction may involve the stabilization of multivalent Cu⁺/Cu⁰, Co²⁺/Co³⁺, and Co(OH)₂/CoO species, as well as the construction of oxygen vacancies, metal-oxide interfaces, and MOF/oxide composite structures, which can regulate NO₃⁻ adsorption, deep NO₂⁻ reduction, and *H supply can be simultaneously enhanced. Future studies should place greater emphasis on operando and in-situ characterizations to clarify the valence evolution, coordination changes, and interfacial reconstruction of Cu and Co during NO₃RR, rather than attributing catalytic activity only to the initial catalyst structure.

3.6. Electrode architecture and mass-transfer engineering

In addition to intrinsic active-site regulation, electrode architecture and mass-transfer behavior are also crucial for determining the NO₃RR performance of CuCo-based catalysts. Conventional powder catalysts are usually immobilized on current collectors with polymer binders, which may block active sites, increase interfacial resistance, and reduce mechanical stability. In contrast, self-supported CuCo electrodes enable the direct integration of catalytic structures with conductive substrates, thereby improving electron transport and structural robustness. More importantly, three-dimensional porous architectures can enhance NO₃⁻ mass transport, facilitate bubble release, and alleviate local concentration polarization. Therefore, the rational design of Cu foam-supported electrodes, nanowire arrays, aerogel networks, nanoporous alloys, and ordered microchannel electrodes is highly important for achieving high-current-density and scalable nitrate-to-ammonia conversion.

Zhang et al. [93] reported a branched CuCo nanowire self-supported electrode grown on Cu foam, denoted as CuCo NW/Cu foam, in which Cu(111) and Co(111) facets were simultaneously exposed to construct a dual-facet tandem catalytic system. In a static H-type cell, this electrode achieved an NH₃ Faradaic efficiency of 91.3% and an NH₃ yield rate of 21.8 mg·h^-1·cm^-2 at −0.3 V vs. RHE. More importantly, in a flow reactor, the catalyst maintained a Faradaic efficiency of 90.3% at 1.0 A·cm^-2 and operated stably for 200 h at 100 and 200 mA·cm^-2. When the nitrate concentration was increased to 0.3 M, the NH₃ yield rate reached 10.41 mmol·h^-1·cm^-2 at 3.0 A·cm^-2. The excellent performance can be mainly attributed to the branched nanowire architecture, which provides abundant exposed active sites and fast mass-transfer channels, while the Cu foam substrate ensures efficient electron transport and mechanical support. Meanwhile, the cooperation between Cu(111) and Co(111) facets facilitates the coupling of initial NO₃⁻ reduction and subsequent hydrogenation steps. Notably, the authors further constructed a scaled-up prototype reactor with an active electrode area of 20.0 cm², which delivered a total NH₃ production rate of 1474.1 mg·h^-1 and a Faradaic efficiency of 91.26% at a total current of 20.0 A, demonstrating the feasibility of self-supported CuCo electrodes for scalable ammonia production.

Jiang et al. [94] further extended CuCo electrode design to three-dimensional aerogel networks (Fig. 9a). The Cu₅₀Co₅₀ aerogel possessed an interconnected porous skeleton, which provided abundant accessible active sites and rapid nitrate-transport pathways. In 1 M KOH containing 1000 ppm NO₃⁻-N, it achieved an NH₃ yield rate of 56.2 mg·h^-1·cm^-2 with nearly 100% Faradaic efficiency at −0.2 V vs. RHE, and also showed good adaptability under low nitrate concentration and flow-cell operation. Kinetic analysis indicated that Cu sites mainly promoted NO₃⁻-to-NO₂⁻ conversion, whereas Co sites facilitated NO₂⁻ hydrogenation. The superior performance over physically mixed Cu and Co aerogels highlights the importance of continuous Cu-Co relay sites within a connected three-dimensional framework.

Biomass-derived porous carbon supports also provide a sustainable platform for constructing CuCo catalytic interfaces with improved mass transport and wastewater adaptability. Eziz et al. [95] prepared a CuCo alloy supported on walnut-green-husk-derived porous carbon, denoted as CuCo/BC (Fig. 9b-d). The porous carbon framework promoted metal dispersion and nitrate accessibility, while the CuCo alloy enabled electronic interaction between Cu and Co sites. In 1 M KOH containing 0.1 M KNO₃, CuCo/BC delivered an NH₃ yield rate of 20.9 mg·h^-1·mg_cat^-1 and a Faradaic efficiency of 83.80%. It also showed good tolerance to interfering ions such as Cl^-, SO₄^2-, and HCO₃^-. In-situ FTIR and DFT calculations indicated that Co-to-Cu electron transfer enhanced NO₃⁻ adsorption, promoted the *NO-to-*NOH hydrogenation step, and improved water dissociation while suppressing HER. This work highlights the potential of low-cost biomass-derived porous carbon supports for developing sustainable CuCo catalysts toward practical nitrate wastewater conversion.

Zhou et al. [96] prepared a nanoporous CuCo alloy with a bimodal pore structure through a dealloying strategy. The material contained large pores of approximately 110 nm and small pores of approximately 15 nm, which simultaneously provided fast ion-transport channels and abundant electrochemically active interfaces (Fig. 9e-g). In 1 M KOH containing 200 ppm NO₃⁻-N, np-CuCo delivered an NH₃ yield rate of 4.7 mg·h^-1·cm^-2 and a Faradaic efficiency of 85.3% at −0.23 V vs. RHE, outperforming monometallic np-Cu and np-Co. When the NO₃⁻-N concentration was increased to 1400 ppm, the NH₃ yield rate further increased to 14.3 mg·h^-1·cm^-2 with a Faradaic efficiency of 91.5%. The enhanced performance originates not only from the bimetallic synergy between Cu and Co, but also from the high surface area, continuous conductive network, and improved mass transport enabled by the nanoporous architecture. The large pores facilitate rapid nitrate diffusion into the electrode interior, whereas the small pores provide abundant active sites and confined spaces for reaction intermediates, thereby promoting continuous NO₃RR.

Ordered microchannel electrodes also provide a promising route for enhancing mass transport and achieving high-current-density NO₃RR. Ren et al. [113] fabricated a three-dimensional CuNi self-supported electrode with ordered 200 μm microchannels and interconnected porous frameworks by selective laser melting (Fig. 9h-j). Although this system is not a CuCo catalyst, it clearly demonstrates the advantages of 3D-printed electrode architectures in strengthening mass transfer and constructing stable self-supported catalytic interfaces. Compared with commercial CuNi foam, the 3D-printed CuNi-P electrode exhibited much higher NH₃ Faradaic efficiency and yield rate, indicating that ordered microchannels can effectively promote nitrate transport, electrolyte penetration, and product release. This strategy provides an important reference for the future design of three-dimensional CuCo self-supported electrodes, flow electrolyzers, and industrial NO₃RR reactors.

Overall, electrode architecture and mass-transfer engineering are key to bridging the gap between laboratory-scale CuCo-based NO₃RR studies and practical applications. Self-supported electrodes can reduce interfacial resistance and avoid binder-induced blockage of active sites. Nanowire arrays aerogel networks, biomass-derived porous carbon supports, and nanoporous structures can increase active surface area and improve mass-transfer efficiency. Flow reactors and scaled-up electrolyzers further enable high-current-density operation, long-term durability and continuous ammonia production. Future CuCo catalyst design should integrate three-dimensional conductive frameworks, ordered pore architectures, hydrophilic/hydrophobic interface regulation, and flow-electrolysis systems to meet the requirements of low-concentration nitrate wastewater treatment and scalable electrosynthesis of ammonia.

4. Advanced characterization techniques for understanding CuCo-based NO₃RR catalysts

Although substantial progress has been achieved in developing CuCo-based electrocatalysts for NO₃RR, identifying the real active sites and elucidating the synergistic roles of Cu and Co under working conditions remain challenging. This is mainly because CuCo catalysts usually undergo dynamic structural, electronic, and interfacial evolution during electrochemical operation, and the initial catalyst structure may not necessarily represent the actual active phase. Therefore, advanced operando characterization techniques combined with theoretical calculations are essential for establishing the relationships among catalyst structure, reaction intermediates, and catalytic performance. In this review, greater evidentiary weight is given to mechanistic interpretations supported by operando/in-situ characterization and isotope or kinetic evidence than to those inferred solely from ex-situ characterization or theoretical calculations. Rather than treating these methods simply as characterization tools, this section discusses them in relation to four key mechanistic questions for CuCo-catalyzed NO₃RR: (1) how the oxidation states and coordination environments of Cu and Co evolve under reaction conditions; (2) whether the initial catalyst or a reconstructed phase represents the real active state; (3) whether active hydrogen is merely generated locally or actually transferred between neighboring sites; and (4) whether the spatial coupling of nitrate activation and subsequent hydrogenation can be directly resolved.

Various experimental and computational approaches, including electrochemical impedance spectroscopy (EIS), operando Raman spectroscopy (Fig. 10a-f), differential electrochemical mass spectrometry (DEMS) (Fig. 10g), Fourier-transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), operando synchrotron-based X-ray absorption fine structure (XAFS), isotope labeling experiments, and DFT calculations (Fig. 10h), have been employed to investigate nitrate activation, intermediate evolution, *H generation, electron transfer, and reaction pathways during NO₃RR. Among these techniques, operando characterization methods provide direct insights into the dynamic reaction process and are particularly valuable for understanding Cu-Co synergistic catalysis.

Li et al. [114] developed a multimodal operando imaging platform to directly visualize the spatial-temporal evolution of NH₃ production (Fig. 10i-k), NO₂⁻ accumulation, and H₂ evolution during NO₃RR. By integrating operando fluorescence imaging, polarization imaging, and reflective absorption imaging, the authors simultaneously monitored product formation, hydrogen evolution, and intermediate distribution on Fe, Co, Ni, and Cu foam electrodes. The results revealed that Cu foam efficiently promoted nitrate activation but suffered from severe NO₂⁻ accumulation due to insufficient hydrogenation capability, whereas Ni foam exhibited excessive HER activity. In contrast, Co foam achieved a balance between NO₂⁻ conversion and hydrogen supply, leading to superior NH₃ production performance. Although this study was performed on individual metal foams rather than a CuCo bimetallic catalyst, it provides an important methodological basis for spatially distinguishing nitrate activation, intermediate accumulation, and competing H₂ evolution. Extending such multimodal imaging to CuCo interfaces could directly test whether different reaction steps are spatially partitioned between Cu-rich, Co-rich, and interfacial regions rather than inferred solely from bulk catalytic performance.

Beyond product and intermediate mapping, spatially resolved characterization can also clarify how electrode architecture modifies the local reactant environment surrounding active sites. Li et al. [115] constructed an operando electrochemical confocal laser scanning microscopy (CLSM) platform using RhB-labeled nitrate (RhB-NO₃⁻) as a fluorescent probe to visualize nitrate transport within a hierarchical hollow mult-shell CuO catalyst. The three-dimensional imaging results demonstrated that NO₃⁻ could be initially enriched on the positively charged surface and subsequently migrate into internal cavities under applied potential, confirming the enhanced ion transport and confinement effect of the hierarchical structure. Combined with operando XAS, Raman, FTIR, and DEMS analyses, this work further revealed the dynamic CuO reconstruction and NO₃RR pathway. Although this system does not contain Co, the approach is transferable to porous CuCo electrodes, where spatially resolved visualization of nitrate concentration gradients could help distinguish intrinsic Cu-Co catalytic cooperation from apparent activity enhancement caused by local mass-transfer or reactant-enrichment effects.

In addition to mass-transfer regulation, understanding the generation and utilization of active hydrogen is critical for controlling nitrate hydrogenation and suppressing HER. Wu et al. [116] integrated operando Raman spectroscopy, NMR (Fig. 10l), EPR, scanning vibrating electrode technique (SVET), finite element simulation (Fig. 10m), and DFT calculations to investigate the role of active hydrogen and localized enhanced electric fields (LEEFs) in NO₃RR. Operando Raman spectroscopy identified the sequential transformation of nitrate-derived intermediates, while EPR and isotope experiments confirmed that generated *H species were preferentially consumed for nitrate hydrogenation rather than H₂ evolution. Furthermore, SVET measurements combined with simulations revealed that tip-induced LEEFs on Cu nanostructures promoted water activation and accelerated *H generation. DFT calculations further demonstrated that moderate *H coverage decreased the hydrogenation barrier of NO intermediates, whereas excessive *H accumulation favored HER. This work provides a comprehensive understanding of the coupling between electric-field regulation, hydrogen supply, and reaction kinetics.

Accordingly, mechanistic assignments supported by operando/in-situ spectroscopy, isotope experiments, site-specific perturbation, or matched kinetic controls should be regarded as stronger evidence than those inferred mainly from ex-situ characterization, performance enhancement, or DFT calculations alone. Methodological validation is equally important for reliable interpretation of CuCo-catalyzed NO₃RR. NH₃ signals should be verified by appropriate blank and control experiments to exclude background contamination, while ¹⁵NO₃⁻ isotope labeling can provide stronger evidence that the detected NH₃ originates from nitrate reduction. In addition, matched NO₃RR and NO₂RR measurements are necessary to distinguish genuinely enhanced nitrate activation from intrinsically high nitrite-reduction activity. For CuCo catalysts, prolonged electrolysis may also induce Cu/Co dissolution, migration, and redeposition, thereby altering the apparent active-site structure. Therefore, mechanistic assignment of Cu, Co, or Cu-Co interfacial sites should combine catalytic controls with operando and post-reaction structural analyses rather than relying solely on the initial catalyst structure. Collectively, these advanced characterization strategies provide important insights into the fundamental processes governing CuCo-based NO₃RR, including nitrate adsorption, intermediate conversion, hydrogen transfer, and competing HER. For CuCo tandem catalysts, the combination of operando spectroscopy, spatially resolved imaging, and theoretical modeling enables a more accurate understanding of the functional roles of Cu sites, Co sites, and Cu-Co interfaces, thereby providing guidance for rational catalyst design.

Future characterization approaches should focus on improving temporal and spatial resolution to capture dynamic structural evolution at the atomic scale. Emerging techniques such as operando transmission electron microscopy, ultrafast spectroscopy, and advanced synchrotron-based methods may provide deeper insights into transient active species and reaction pathways. Of particular importance for CuCo systems is the simultaneous correlation of local Cu/Co oxidation states, interfacial structure, adsorbed intermediates, and spatial product distribution under the same operating conditions. Meanwhile, integrating multiple characterization techniques with theoretical calculations, kinetic analysis, and machine-learning-assisted modeling will facilitate the establishment of quantitative structure-activity relationships. Moreover, developing operando methods compatible with realistic wastewater environments will be essential for bridging fundamental mechanistic studies and practical NO₃RR applications.

5. Conclusions and perspectives

NO₃RR provides an attractive and sustainable route for simultaneous nitrate pollution remediation and green ammonia synthesis. In recent years, CuCo-based catalysts have attracted increasing attention owing to their low cost, tunable composition, structural diversity, and pronounced bimetallic synergistic effects. In general, Cu sites are favorable for NO₃⁻ adsorption, activation, and initial reduction to NO₂⁻, whereas Co sites can regulate *H supply, promote the subsequent hydrogenation of NO₂⁻/*NO intermediates, and suppress the competing hydrogen evolution reaction to some extent. Therefore, rationally designed Cu-Co dual active sites can effectively overcome the limitations of monometallic Cu or Co catalysts, such as nitrite accumulation, sluggish hydrogenation kinetics, and unstable product selectivity.

This review summarizes recent progress in CuCo-based NO₃RR catalysts from the perspective of materials design, including heteroatom doping, heterostructure and interface engineering, MOF- and coordination-framework-derived catalysts, atomically dispersed catalysts, in-situ reconstructed catalysts, and electrode architecture/mass-transfer engineering. Heteroatom doping can induce lattice distortion and electronic redistribution, thereby optimizing the adsorption of key intermediates. CuCo heterostructures enable tandem catalysis through interfacial electronic coupling and functional division between Cu and Co sites. MOF/COF- and carbon-confined structures are beneficial for stabilizing CuCo sites and regulating the local reaction microenvironment. Single-atom, dual-atom, and subnanocluster catalysts provide atomic-level platforms for constructing well-defined adjacent Cu-Co sites. In-situ reconstruction strategies can generate real active interfaces during NO₃RR, such as Cu⁺/Cu⁰, Co(OH)₂/Cu, CoO/Co-Cu₂O/Cu, and MOF/oxide composite structures. Meanwhile, self-supported three-dimensional electrodes and flow reactors further promote CuCo catalysts toward high-current-density operation and scalable applications.

Despite these advances, several key challenges remain. First, many CuCo catalysts undergo significant valence evolution and structural reconstruction during NO₃RR, meaning that the initial structure is not necessarily the real active phase. Therefore, operando Raman, FTIR, XAS, XPS, isotopic labeling experiments, and other advanced techniques are required to clarify the real roles of Cu and Co sites under working conditions. Second, the Cu-Co synergistic mechanism still needs to be further understood, especially the specific functions of Cu sites, Co sites, and interfacial sites in NO₃⁻ adsorption, NO₂⁻ conversion, *NO hydrogenation and *H supply. Third, most current studies are still performed in high-concentration nitrate solutions and ideal electrolytes, whereas real wastewater usually contains low nitrate concentrations and various interfering species, such as Cl^-, SO₄^2-, HCO₃^-, organic pollutants, and heavy-metal ions. These complex conditions impose higher requirements on catalyst selectivity, stability, and poisoning resistance. In addition, the commonly used H-type cell is insufficient for evaluating practical performance, and more attention should be paid to flow cells, membrane-electrode reactors, in-situ product separation, and ammonia recovery. For practical CuCo systems, long-term operation should also be accompanied by quantitative analysis of Cu/Co leaching to distinguish apparent electrochemical stability from true structural durability. In addition, process-level evaluation should consider not only NH₃ Faradaic efficiency and production rate, but also NH₃/NH₄⁺ recovery efficiency and the overall energy consumption associated with electrolysis and downstream separation.

Future development of CuCo-based NO₃RR catalysts should focus on the following aspects (Fig. 11). First, well-defined Cu-Co dual active sites should be constructed with controllable Cu/Co coordination environments and spatial proximity, so as to clarify whether electronic promotion, intermediate relay, or hydrogen-assisted cooperation dominates under specific reaction conditions. Second, controllably reconstructable CuCo precursors should be developed to generate stable and highly active real interfaces under reaction conditions. Third, MOF, COF, and carbon-confined structures should be used not only to improve site dispersion and stability, but also to regulate the local proximity between Cu and Co sites and the transport of NOx intermediates between them. Fourth, high-density single-atom, dual-atom, and subnanocluster CuCo sites should be designed to establish clearer relationships among Cu-Co distance, electronic coupling, and tandem reaction kinetics. Fifth, self-supported, three-dimensional, and porous CuCo electrodes should be developed to preserve effective Cu-Co cooperation under high-current-density operation while improving nitrate transport and local microenvironment regulation. Sixth, catalytic performance and structural durability should be further evaluated under low nitrate concentrations, complex ionic environments, and real wastewater conditions, with particular attention to catalyst poisoning and Cu/Co leaching. Seventh, operando characterization, theoretical calculations, and kinetic analysis should be integrated to establish quantitative structure-activity relationships among CuCo catalyst structures, real active sites, and NO₃RR performance.

In summary, CuCo-based catalysts provide an important materials platform for efficient, selective, and sustainable nitrate-to-ammonia conversion. Through multiscale design of atomic active sites, interfacial electronic structures, dynamic reconstruction behavior, and macroscopic electrode architectures, energy-efficient, highly selective, durable, and scalable NO₃RR systems are expected to be realized in the future.

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Abstract

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Keywords

Cite this article

1. Introduction

2. Mechanistic understanding of NO₃RR

3. Material design strategies for CuCo-based electrocatalysts toward NO₃RR

3.1. Heteroatom-doped CuCo-based catalysts

3.2. CuCo heterostructures and interface engineering

3.3. MOF- and coordination-derived CuCo catalysts

3.4. Atomically dispersed CuCo catalysts

**3.5. In-situ reconstructed CuCo catalysts**

3.6. Electrode architecture and mass-transfer engineering

4. Advanced characterization techniques for understanding CuCo-based NO₃RR catalysts

5. Conclusions and perspectives

References

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Just Accepted

Abstract

Graphical abstract

Keywords

Cite this article

1. Introduction

2. Mechanistic understanding of NO3RR

3. Material design strategies for CuCo-based electrocatalysts toward NO3RR

3.1. Heteroatom-doped CuCo-based catalysts

3.2. CuCo heterostructures and interface engineering

3.3. MOF- and coordination-derived CuCo catalysts

3.4. Atomically dispersed CuCo catalysts

3.5. In-situ reconstructed CuCo catalysts

3.6. Electrode architecture and mass-transfer engineering

4. Advanced characterization techniques for understanding CuCo-based NO3RR catalysts

5. Conclusions and perspectives

References

RIGHTS & PERMISSIONS

Just Accepted

2. Mechanistic understanding of NO₃RR

3. Material design strategies for CuCo-based electrocatalysts toward NO₃RR

**3.5. In-situ reconstructed CuCo catalysts**

4. Advanced characterization techniques for understanding CuCo-based NO₃RR catalysts