Sulfate-Terminated High-Entropy Oxyhydroxide Porous Nanocubes for Efficient Nitrate-to-Ammonia Conversion.

Abstract

Electrochemically coupling the nitrate reduction reaction (NO₃RR) with the oxygen evolution reaction (OER) enables simultaneous pollution mitigation and efficient ammonia synthesis. However, slow kinetics in both reactions, particularly water dissociation and NO₃^- hydrogenation, limit Faradaic efficiency (FE), yield rate, and energy consumption. Designing catalysts that overcome these dual kinetic barriers is challenging. High-entropy materials (HEMs) offer promise due to compositional diversity and lattice distortion effects, but precise synthesis is difficult. This work employs porous high-entropy sulfide nanocubes (NiCoFeCuMn-S) as precatalysts. These electrochemically transform into sulfate-terminated oxyhydroxides (NiCoFeCuMnOOH-SO₄^2-), which serve as the active species. The resulting catalyst delivers exceptional bifunctional performance in alkaline electrolyte: an ultralow OER overpotential (216 mV @ 10 mA cm^-2), high NH₃ FE (94.5%), and yield rate (21.8 mg h^-1 mg_cat^-1). <i>In situ</i> spectroscopy shows that multimetallic synergy enables efficient OER mechanisms. Density functional theory reveals that coordinated sulfate lowers the water dissociation barrier, facilitating proton transfer and accelerating NH₃ synthesis. This work presents a promising design strategy for efficient bifunctional high-entropy electrocatalysts.