Spin-State Engineering of Co 3+ in ZnCo 2 O 4 for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrocatalytic nitrate reduction to ammonia (NO 3 RR) represents a promising strategy for nitrate remediation and sustainable ammonia synthesis, yet its efficiency remains strongly limited by sluggish reaction kinetics. Herein, spin-state engineering is demonstrated as an effective approach to enhance NO 3 RR by regulating the spin-state of octahedral Co 3+ sites in ZnCo 2 O 4 spinel catalysts. A series of ZnCo 2 O 4 samples were synthesized via a sol–gel method followed by calcination at different temperatures, enabling precise modulation of the Co 3+ spin state while eliminating the influence of Co 2+ through Zn 2+ substitution. Electrochemical tests show that ZCO-500 (calcined at 500 °C) delivers an optimal NO 3 RR performance, achieving a high Faradaic efficiency (FE) of 92.2 ± 0.5% and an ammonia yield rate of 8.63 ± 0.08 mg h –1 cm –2 at −0.4 V vs RHE. A comprehensive combination of in situ attenuated total reflectance Fourier-transform infrared spectroscopy with density functional theory calculations elucidates the mechanistic origin of the spin-state effect, demonstrating that high-spin Co 3+ lowers the energy barrier for the rate-determining step (*NO protonation to *NOH) from 1.02 to 0.75 eV. This work highlights spin-state engineering as a general design principle for developing efficient electrocatalysts for nitrate-to-ammonia conversion.