Microwave-assisted fabrication of a self-supported graphene-based high-entropy alloy electrode for efficient and stable electrocatalytic nitrate reduction to ammonia†

Abstract

Direct electrochemical nitrate reduction to ammonia (NRA) synthesis is an efficient and environmentally friendly production technology. However, the development of highly selective electrocatalysts is still a challenge due to the nine-proton and eight-electron reduction reaction. High-entropy alloys (HEAs) contain a wide range of elements and have adjustable properties, giving them excellent application potential in multi-step reactions. In this work, we skillfully use the local high temperature and excellent thermal conductivity generated at the reduced graphene oxide (rGO) defect in a microwave process to achieve a rapid quenching process in 10 seconds. This approach overcomes element immiscibility and results in a self-supported, single-phase, non-precious metal and uniform FeCoNiCuSn alloy electrode. The HEAs reach a remarkable NH₃ yield of 883.7 ± 11.2 μg h⁻¹ cm⁻², maximum faradaic efficiency (FE) of 94.5 ± 1.4%, and highest NH₃ selectivity of 90.4 ± 2.7%. Experimental and theoretical calculations reveal that the presence of multiple adjacent elements in HEAs triggers a synergistic catalytic effect, while the excellent mass and charge transfer properties of rGO jointly encourage the performance of the electrochemical NRA. In particular, NO₃⁻ favors vertical adsorption at Fe–Fe sites, and the desorption of NH₃ is identified as the rate-determining step (RDS) with an extremely small ΔG value of 0.7 eV.