Nitrate (NO₃⁻) pollution in industrial and agricultural wastewater poses significant environmental and health risks. Conventional biological NO₃⁻ removal methods are energy-intensive. Electrocatalytic reduction of NO₃⁻ to ammonia (NH₃) offers a sustainable alternative, as NH₃ is a crucial industrial chemical and a potential carbon-free hydrogen carrier. The Haber-Bosch process, currently used for industrial NH₃ synthesis, is energy-intensive and environmentally unfriendly. Electrocatalytic NO₃⁻ reduction powered by renewable energy is therefore gaining significant attention as a sustainable complementary approach. This approach offers the possibility of fulfilling ammonia demand while addressing water pollution concerns. This study is motivated by the highly efficient enzymatic nitrate reduction observed in nature. Copper-type nitrite reductases (Cu-NIRs) are particularly noteworthy for their role in nitrogen fixation. These enzymes possess two types of copper active centers: T1Cu (electron donating) and T2Cu (catalytic). The T2Cu center binds NO₂⁻, and electron transfer from T1Cu facilitates its reduction to NH₃. This bio-inspired mechanism guides the design of efficient electrocatalysts. Previous research has highlighted the importance of moderate hydrogen adsorption energy for selective NH₃ production and suppressing the competing hydrogen evolution reaction (HER). Recent studies on CuCo binary metal sulfides have suggested a tandem mechanism where NO₂⁻ intermediates are formed on Cu and then split on Co sites, although the role of hydrogen and the electronic structure's influence on intermediate adsorption require further investigation. This research aims to develop a high-performance electrocatalyst for NH₃ production, mimicking the Cu-NIR’s bifunctional nature.

The existing literature extensively explores various electrocatalysts for nitrate reduction to ammonia. Studies have demonstrated the effectiveness of different materials, including copper-based catalysts, and single-atom catalysts featuring metals like rhodium, iron, and cobalt. These catalysts exhibit varying degrees of activity and selectivity. The importance of hydrogen adsorption and its interplay with the reduction process have been emphasized. Furthermore, understanding the electronic structure of the catalysts and its effect on intermediate adsorption is a crucial aspect of catalyst design. Previous research has also focused on tandem mechanisms involving multiple catalytic sites to improve efficiency. However, studies with tandem mechanisms involving Cu and Co often lack clarity on hydrogen's role during the nitrate reduction reaction and its influence on the overall performance. Understanding the influence of hydrogen coverage on the catalytic surface is crucial in designing more efficient catalysts. This work builds upon the existing knowledge to develop a high-performance electrocatalyst, focusing on mimicking the natural mechanism found in nitrite reductase enzymes for more efficient and selective ammonia production.

CuCo bimetallic nanosheet electrocatalysts were synthesized using a one-step electrodeposition method onto Ni foam substrates. The Ni foam acts as a conductive support. The Cu/Co molar ratio was controlled during the electrodeposition. The resulting materials were characterized using various techniques. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM) were used for structural and morphological analysis. Inductively coupled plasma optical emission spectrometry (ICP-OES) determined the elemental composition. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) probed the electronic properties and oxidation states of Cu and Co. Electrochemical activity was evaluated using linear sweep voltammetry (LSV), rotating disk electrode (RDE) measurements, and chronoamperometry. The number of electrons transferred during the nitrate reduction process was determined using Koutecký-Levich (K-L) plots. Ammonia and nitrite were quantified using the Nessler's reagent method and ion chromatography, respectively. Gas products were analyzed using gas chromatography (GC) and online electrochemical mass spectrometry (OEMS). Electrochemical in situ Fourier transform infrared spectroscopy (FTIR) and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) were employed to study the reaction mechanism. Density functional theory (DFT) calculations further elucidated the reaction pathways and the role of the CuCo bimetallic structure at the atomic level. The influence of various parameters, such as the Cu/Co ratio, applied potential, and nitrate concentration, on the catalytic performance was systematically investigated. The stability of the catalyst was assessed through long-term chronoamperometry and subsequent material characterization.

The Cu₅₀Co₅₀ nanosheet electrocatalyst exhibited superior performance for electrochemical nitrate reduction to ammonia. It achieved an ampere-level current density of 1035 mA cm⁻² at −0.2 V vs. RHE with 100 ± 1% Faradaic efficiency for NH₃ generation. The NH₃ production rate reached 4.8 mmol cm⁻² h⁻¹ (960 mmol gcat⁻¹ h⁻¹). A wide potential window (−0.1 to −0.4 V) maintained >90% Faradaic efficiency. In situ FTIR and SHINERS revealed the formation of intermediates including NO₂, NH₂OH, and HNO during the reaction. DFT calculations indicated that the CuCo alloy lowered the energy barrier for initial NO₃⁻ adsorption and facilitated electron transfer, while the Co sites enhanced hydrogenation of intermediate species. The optimal Cu/Co ratio (50:50) resulted in a balanced coverage of *H and *NO₃ species on the catalyst surface, leading to high activity and selectivity. The catalyst demonstrated excellent stability, maintaining high NH₃ production rates and Faradaic efficiency over multiple cycles. Batch electrolysis with an initial nitrate concentration of 100 mM (∼6200 ppm) achieved 99.5% nitrate removal with 96% Faradaic efficiency within 10 hours, resulting in a final nitrate concentration below the World Health Organization drinking water limit.

The results demonstrate that the CuCo bimetallic nanosheet catalyst effectively mimics the bifunctional nature of Cu-NIR enzymes, achieving high efficiency and selectivity in electrochemical nitrate reduction to ammonia. The synergy between Cu and Co is crucial for both the kinetics and selectivity of the reaction. Cu sites facilitate NO₃⁻ adsorption and subsequent reduction steps, while Co sites provide protons and electrons for hydrogenation of intermediates, thus preventing the accumulation of nitrite. The balanced surface coverage of *H and *NO₃ species is critical for achieving high Faradaic efficiency. The high activity and stability of the catalyst make it a promising candidate for practical applications in wastewater treatment and sustainable ammonia production. This study advances the understanding of bio-inspired catalyst design and provides valuable insights into the development of efficient electrocatalysts for challenging multi-electron transfer reactions.

This work demonstrates a novel CuCo bimetallic nanosheet electrocatalyst for highly efficient and selective electrochemical nitrate reduction to ammonia. The catalyst exhibits exceptional performance, achieving an ampere-level current density and near-perfect Faradaic efficiency. The synergistic effect between Cu and Co, mimicking the bifunctional nature of Cu-NIRs, is key to its high activity and selectivity. This study provides a promising pathway towards sustainable ammonia production and wastewater treatment. Future work could explore further optimization of the catalyst structure and composition, as well as scale-up studies for industrial applications.

While the catalyst demonstrated excellent performance in laboratory settings, further research is necessary to optimize the catalyst for industrial-scale applications. Long-term stability under more realistic conditions should be investigated thoroughly. The effects of potential impurities or variations in wastewater composition on the catalytic performance warrant further study. The scale-up process needs optimization to maintain high efficiency and selectivity during large-scale production. Though the final nitrate concentration after the electrolysis is below the drinking water standards, further ammonia recovery steps would be necessary to fully implement the process.