Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia.

Abstract

Electrocatalytic recycling of waste nitrate (NO₃^-) to valuable ammonia (NH₃) 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 NH₃ 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 NO₃^--to-NH₃ 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 NO₃^- reduction to NO₂^-, which is rapidly reduced to NH₃ at the nearby Co/CoO shell. This unique tandem catalyst system leads to a NO₃^--to-NH₃ Faradaic efficiency of 93.3 ± 2.1% in a wide range of NO₃^- concentrations at pH 13, a high NH₃ yield rate of 1.17 mmol cm^-2 h^-1 in 0.1 M NO₃^- at -0.175 V vs. RHE, and a half-cell energy efficiency of ~36%, surpassing most previous reports.