Engineering a Stable High Entropy Infused Spinel Oxide for Electrocatalytic Conversion of Nitrate to Ammonia

Abstract A sustainable strategy to mitigate nitrate (NO ₃ ⁻ ) contamination in water involves its electrochemical reduction to ammonia (NH ₃ ), a valuable green fuel. However, nitrate reduction reaction (NO ₃ RR) is hindered by sluggish kinetics and poor selectivity due to the competing hydrogen evolution reaction (HER). Herein a strategic synthesis of a spinel high‐entropy oxide (HEAO) heterostructure is reported as an efficient electrocatalyst for NO ₃ RR. The HEAO exhibits a Tafel slope of 275 mV dec ⁻¹ , a Faradaic efficiency of 84%, and an impressive NH ₃ production rate of 314 µmol h ⁻¹ cm ⁻² at −0.6 V versus RHE. Mott–Schottky analysis reveals a high donor density and a low flat‐band potential, which contribute to the enhanced reaction kinetics. Hydrophobicity studies demonstrate that the water‐repellent nature of the HEAO suppresses the HER, favoring NO ₃ RR selectivity. Finally, an energy conversion device based on this catalytic system is proposed, which delivers a promising open‐circuit potential of 0.61 V and a peak power density of 2.3 mW cm ⁻² at 27 mA cm ⁻² . A comprehensive density functional theory (DFT) analysis reveals that the Fe‐Mn bridge site in HEAO possesses optimal adsorption energy for NO ₃ ⁻ ions and its capability to selectively reduce NO ₃ ⁻ toward NH ₃ .