Enhanced nitrate reduction to ammonia using Cu-Ni catalyst: Synergistic mechanisms and reaction pathways

Accelerated industrialization combined with over-applied nitrogen fertilizers results in serious nitrate pollution in surface and ground water, disrupting the balance of the global nitrogen cycle. Electrochemical nitrate reduction (eNO₃RR) emerges as an attractive strategy to simultaneously enable nitrate removal and decentralized ammonia fabrication, restoring the globally perturbed nitrogen cycle. However, complex deoxygenation-hydrogenation processes and sluggish proton-electron transfer kinetics significantly hinder practical application of eNO₃RR. In this study, we developed carbon-coated Cu-Ni bimetallic catalysts derived from metal-organic frameworks (MOFs) to facilitate eNO₃RR. The unique structural features of catalyst promote enhanced synergy between Cu and Ni, effectively addressing critical challenges in nitrate reduction. Comprehensive structural and electrochemical analysis demonstrate that electrochemical nitrate-to-nitrite conversion mainly takes place on active Cu sites, the introduction of Ni could efficiently accelerate the generation of aquatic active hydrogen, promoting the hydrogenation of oxynitrides during eNO₃RR. In addition, Ni introduction could push up the d-band center of the catalyst, thus enhancing the adsorption and activation of nitrate and the corresponding intermediates. Detailed reaction pathways for nitrate-to-ammonia conversion are illuminated by rotating disk electrode (RDE), in-situ Fourier-transform infrared spectroscopy, in-situ Raman spectrum and electrochemical impedance spectroscopy (EIS). Benefiting from the synergistic effect of Cu and Ni, optimum catalyst exhibited excellent nitrate reduction performance. This work provides a new idea for elucidating the underlying eNO₃RR reaction mechanisms and contributes a promising strategy for designing efficient bimetallic electrocatalysts.