Optimal Ni⁰/Ni2 + modulates H* coverage to accelerate FeNi-C electro-catalyzing nitrate reduction

Electrocatalytic nitrate reduction to ammonia (NO₃RR) offers a promising route for wastewater treatment and nitrogen resource recovery. In this work, a FeNi alloy-loaded carbon material (FeNi-C) was evaluated about NO 3 RR performance and the role of different active hydrogen (H*) was illustrated. FeNi(1:1)-C obtained at 700 °C exhibits a 96.6% NO 3 ⁻ -N removal and 99.6% NH₄ ⁺ -N selectivity at −1.5 V and it maintains excellent performance over 10 cycles without metal leaching. Furthermore, high-concentration nitrate can be converted to ammonium, recovered as struvite. Mechanism analysis indicate that a high surface coverage of adsorbed atomic hydrogen (H* ads ) is more crucial for enhancing the selectivity of nitrate-to-ammonia conversion compared with the free H*（H* free ）in the electrolyte. Tafel slope combined with verification experiments reveal that an optimal Ni 0 /Ni 2+ (≈2) facilitates water electrolysis to enables FeNi(1:1)-C forming a high surface coverage of H* ads . Thereby, FeNi(1:1)-C maintains an abundant H* ads -covered reductive-microenvironment surface, promoting fast conversion between H* ads and the adjacent nitrogen-containing intermediates and suppressing the competing hydrogen evolution reaction (HER). Furthermore, this reductive-microenvironment surface also helps protect the FeNi alloy from oxidation, endowing its superior stability. This work highlights the superior role of H* ads over H*（H* free ）in the electrolyte.