One-step synthesized Nb2O5−y-decorated spinel-type (Ni,V,Mn)3O4−x nanoflowers for boosting electrocatalytic reduction of nitrogen into ammonia

Abstract

Two renewable methods for reducing greenhouse gas emissions associated with ammonia (NH₃) production are the renewable H₂-combined Haber–Bosch process and the electrochemical nitrogen reduction reaction (eNRR). However, progress in eNRR has fallen short of expectations. Here, we demonstrated a well-controlled electrochemically viable and clean NH₃ production with spinel-type (V,Ni,Mn)₃O_4−x/Nb₂O_5−y nanoflowers, successfully synthesized via a facile one-pot hydrothermal method on nickel foam, aimed at boosting the eNRR performance, after surveying many Ni–V–Mn–Nb quaternary, ternary, and binary oxide systems for their electrochemical and structural properties. The best NH₃ yield from a quaternary system exhibited the most promising results, achieving 3.19 mg h⁻¹ cm⁻² (1.28 mg h⁻¹ mg_cat.⁻¹) at a potential of −0.5 V vs. RHE, with a remarkable faradaic efficiency (FE) of 22.7%, which is superb among the current state of the art methods. Besides, the electrocatalyst performance, evaluated in terms of energy conversion, showed a cell energy efficiency of 37.66%. An NN bond-weakening micro-mechanism of electron transport for the nonstoichiometric Ni–V–Mn–Nb oxide system is proposed to elucidate this exceptional activity. Furthermore, we suggest a reaction micro-mechanism encompassing electron transport and N₂ trapping, contributing to the observed excellent activity in NH₃ production.