Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia
Wenhui He, Jian Zhang, Stefan Dieckhöfer, Swapnil Varhade, Ann Cathrin Brix, Anna Lielpētere, Sabine Seisel, João R. C. Junqueira, Wolfgang Schuhmann
Abstract
Abstract 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 ± 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.