Metal–organic framework derived low-crystallinity cobalt–nitrogen–carbon electrocatalysts for nitrate reduction to ammonia

Abstract

Electrocatalytic ammonia synthesis using nitrate as the nitrogen source is a sustainable strategy for ammonia synthesis. Although there have been reports on composites of transition metals and carbon–nitrogen, the correlation between nitrate reduction to ammonia (NRA) activity and the status of metal nanoparticles has been overlooked. Herein, we synthesize a series of cobalt–nitrogen–carbon (Co–N–C) electrocatalysts to systematically investigate the impact of nanoparticle states on the NRA reaction. The low-crystallinity Co–N–C-500 presents the highest ammonia yield rate of 1.14 mg h⁻¹ cm⁻² in neutral electrolytes. The Faraday efficiency (FE) remains stable at 81% after a duration of 73 hours. Well-dispersed and smaller-sized Co nanoparticles (14.87 nm) resulted in more reactive active sites. The coordination-unsaturated Co facilitates the critical step of the conversion of NO₃⁻ and NO₂⁻. The deoxidation and hydrogenation processes in the NRA reaction are confirmed based on the reaction intermediates detected by in situ ATR-FTIR. The performance of a Zn–NO₃⁻ battery using the Co–N–C-500 cathode is also relatively superior. This investigation of the structure–activity relationship based on catalysts offers a novel perspective for designing highly efficient NRA electrocatalysts.