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

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/CuOx and Co/CoO phases. Electrochemical evaluation, kinetic studies, and in-situ Raman spectra reveal that the inner Cu/CuOx 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 similar to 36%, surpassing most previous reports.

Automated summary

In this study, a design concept of tandem catalysts was proposed to achieve cascade conversion of waste nitrate to valuable ammonia using different transition metal phases. By transforming Cu-Co binary sulfides into Cu/CuOx and Co/CoO phases, efficient NH3 synthesis was achieved. The tandem catalyst system exhibited high Faradaic efficiency and NH3 yield rate in the presence of waste nitrate, and surpassed previous studies in terms of energy efficiency.