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

The electrocatalytic nitrate reduction to ammonia (NO3RR) 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 NO3RR by regulating the spin-state of octahedral Co3+ sites in ZnCo2O4 spinel catalysts. A series of ZnCo2O4 samples were synthesized via a sol–gel method followed by calcination at different temperatures, enabling precise modulation of the Co3+ spin state while eliminating the influence of Co2+ through Zn2+ substitution. Electrochemical tests show that ZCO-500 (calcined at 500 °C) delivers an optimal NO3RR 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 Co3+ 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.