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

A sustainable strategy to mitigate nitrate (NO₃ ^-) contamination in water involves its electrochemical reduction to ammonia (NH₃), a valuable green fuel. However, nitrate reduction reaction (NO₃RR) 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 NO₃RR. The HEAO exhibits a Tafel slope of 275 mV dec^-1, a Faradaic efficiency of 84%, and an impressive NH₃ 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 NO₃RR 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 NO₃ ^- ions and its capability to selectively reduce NO₃ ^- toward NH₃.