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 .