Atomically Dispersed Cobalt on Ionic Carbon Nitrides for Selective and Efficient Nitrate Electroreduction to Ammonia

Direct electrochemical conversion of nitrate to ammonia (NH₃) represents a sustainable route for NH₃ production while simultaneously mitigating nitrate pollution. Carbon nitrides (CNs) have emerged as promising supports for transition-metal single-atom catalysts due to their high nitrogen content and abundant coordination sites. However, conventional CNs generally suffer from poor electrical conductivity and difficulty in stabilizing high densities of atomically dispersed metal centers, which limits catalytic efficiency and selectivity in the nitrate reduction reaction. Herein, we overcome these limitations by constructing cobalt poly(heptazine imides) (CoPHI), an ionic carbon nitride in which Co²⁺ species are coordinated to negatively charged imide-bridging nitrogen atoms. This coordination environment enables a high density of isolated Co active sites (1.092 wt.%) while enhancing charge transport through the PHI framework. As a result, CoPHI achieves a Faradaic efficiency of 93.5% and an NH₃ yield rate of 46.1 mg·h^-1·mg_cat. ^-1 at -0.8 V versus RHE, outperforming conventional Co─N─C and Co─C₃N₄ systems. Combined experimental and theoretical studies show that CoPHI promotes strong nitrate adsorption, facilitates water dissociation to supply protons, and stabilizes key intermediates, collectively enabling efficient and selective nitrate-to-ammonia conversion.