Hollow cobalt phosphate microspheres for sustainable electrochemical ammonia production through rechargeable Zn–N2 batteries

Abstract

Electrochemical nitrogen reduction to produce ammonia beyond overall two-electrode electrolysis with less energy input is necessary but challenging. Herein, hollow cobalt phosphate microspheres (CoPi/HSNPC) derived from corresponding cobalt phosphonates are employed as an efficient electrocatalyst toward both nitrogen reduction reaction (NRR) and oxygen evolution reaction (OER) under alkaline conditions. Benefiting from the sufficient active sites and the enhanced proton-conducting pathways, the CoPi/HSNPC exhibits a high NH₃ yield rate of 16.48 μg h⁻¹ mg_cat.⁻¹ with a faradaic efficiency of 4.46% at −0.2 V vs. the reversible hydrogen electrode, associated with excellent stability for cycling operation. The Co²⁺ sites of cobalt phosphate microspheres are revealed as the active component for the NRR through the associative pathway, and the phosphate groups are also beneficial for the enhanced NRR activity. The CoPi/HSNPC also exhibits extraordinary OER activity with an overpotential of 341 mV to deliver 10 mA cm⁻² of anodic current density and a negligible current decay during 30 h. When employed as the cathode catalyst of a rechargeable Zn–N₂ battery, a large NH₃ yield rate of 11.62 μg h⁻¹ mg_cat.⁻¹ and faradaic efficiency of 24.42% with decreased charge–discharge potential gaps as well as an impressive power output including a peak power density of 0.31 mW cm⁻² and an energy density of 138.6 mW h g_zn⁻¹ are achieved, providing an attractive perspective for practical applications. Moreover, the assembled Zn–N₂ battery can maintain robust cycling operation with a stable NH₃ yield rate driven by the sustainable electricity input from solar energy.