- Electrocatalytic recycling of waste nitrate (\(NO_{3}\)\(^-\)) to valuable ammonia (\(NH_3\)) at ambient conditions is a green and appealing alternative to the Haber−Bosch process. However, the reaction requires multi-step electron and proton transfer, making it a grand challenge to drive high-rate \(NH_3\) synthesis in an energy-efficient way. Herein, we present a design concept of tandem catalysts, which involves coupling intermediate phases of different transition metals, existing at low applied overpotentials, as cooperative active sites that enable cascade \(NO_{3}\)\(^-\)-to-\(NH_3\) conversion, in turn avoiding the generally encountered scaling relations. We implement the concept by electrochemical transformation of Cu−Co binary sulfides into potential-dependent core−shell \(Cu/CuO_x\) and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in−situ Raman spectra reveal that the inner \(Cu/CuO_x\) phases preferentially catalyze \(NO_{3}\)\(^-\) reduction to \(NO_{2}\)\(^-\), which is rapidly reduced to \(NH_3\) at the nearby Co/CoO shell. This unique tandem catalyst system leads to a \(NO_{3}\)\(^-\)-to-\(NH_3\) Faradaic efficiency of 93.3 \(\pm\) 2.1% in a wide range of \(NO_{3}\)\(^-\) concentrations at pH 13, a high \(NH_3\) yield rate of 1.17 mmol \(cm^{−2} h^{−1}\) in 0.1 M \(NO_{3}\)\(^-\) at −0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.
