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 (eNORR) 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 eNORR. In this study, we developed carbon-coated Cu-Ni bimetallic catalysts derived from metal-organic frameworks (MOFs) to facilitate eNORR. 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 eNORR. 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 eNORR reaction mechanisms and contributes a promising strategy for designing efficient bimetallic electrocatalysts.