Potential Dependence of Ammonia Selectivity of Electrochemical Nitrate Reduction on Copper Oxide
Rong Yang, Huan Li, Jun Long, Huijuan Jing, Xiaoyan Fu, Jianping Xiao
Abstract
Nitrate electrochemical reduction (eNO3RR) has been considered as an alternative strategy for decentralized ammonia production, while there are two critical issues regarding high overpotential and poor selectivity because of nitrite production at low overpotentials. Therefore, it is significant to understand the reaction mechanism of ammonia and nitrite selectivity. In addition, it was found that hydrogen evolution reaction (HER) can become notable and drastically affects the Faradaic efficiency. In this work, taking copper oxide as an example, we have combined density functional theory (DFT) calculations and microkinetic modeling (MKM) to understand the products selectivity and Faradaic efficiency of eNO3RR to NH3 at varying potentials. Our models are consistent qualitatively with the experimental results, describing well the competition of ammonia, nitrite, and hydrogen. As the charge transfer coefficient (β) for *NO2 protonation (*NO2 to HNO2(g)) is smaller than that of *NO hydrogenation (*NO to *NHO), ammonia production becomes more favorable gradually. As the larger charge transfer coefficient (β) for HER compared to *NO hydrogenation (*NO to *NHO), ammonia production becomes less preferable at high overpotentials. The present mechanistic insights can provide design principles of eNO3RR activity and selectivity over ammonia, nitrite, and hydrogen.