Efficient Electrochemical Nitrate Reduction to Ammonia Driven by a Few Nanometer-Confined Built-In Electric Field

Abstract

Converting nitrate (NO₃^–) to ammonia (NH₃) through the electrochemical reduction method offers an appealing approach for wastewater treatment and facilitates nitrogen cycling in nature. However, this electrolytic method involves a series of proton-coupled electron transfer processes and comes with severe competing reactions. Consequently, there is a significant demand for catalysts exhibiting good catalytic activities and selectivities. Here, a series of copper–cobalt binary sulfide nanosheets with varying Cu/Co compositions were prepared to investigate the synergy effects between the components copper sulfide and cobalt sulfide on their catalytic performance. As a result, a volcano-like correlation between the Cu/Co ratio and electrocatalytic performance was built. The optimal catalyst Cu_xS–Co_0.5 exhibited a maximum Faradaic efficiency (FE) of ∼95.6% for ammonia at −1.4 V vs Ag/AgCl. The highest ammonia yield rate of 5.36 mg/h·cm² was achieved at −1.6 V vs Ag/AgCl, which was 6.5- and 3.8-fold relative to those of pure Cu_xS and CoS₂, respectively. By combining spectroscopy characterizations with theoretical calculations, we revealed that catalyst Cu_xS–Co_0.5 with a built-in electric field confined to a few nanometers played a critical role in enhancing electron transfer and creating more active sites. Besides, its improved water dissociation capability was essential for the hydrogenation of reduction intermediates, collectively contributing to the enhanced catalytic performance.