S- and N-Enriched Cyclophosphazene- and Triazine-Based Inorganic–Organic Hybrid Nanoporous Electrocatalyst for Nitrate Upcycling to Ammonia

Heteroatoms-enriched (S = 46 wt % and N = 24 wt %) cyclophosphazene- and triazine-moiety-based hybrid nanoporous material (CTHM-2) is reported as an efficient metal-free catalyst for the electrochemical nitrate (NO3–) upcycling to ammonia (NH3). The electrocatalyst exhibits a high NH3 yield rate of 43 μg h–1 mgcat–1 (2.6 mmol h–1 gcat–1) and a faradaic efficiency (FE) of 42%, at a low potential of −0.8 V vs RHE at the mass loading of 1.6 mg. A fast kinetics marked by a low Tafel slope of 48 mV dec–1 in the CTHM-2 electrocatalyst is attributed to the Hads-mediated pathway, validated using TBA as a H* scavenger. Density functional theory simulations validate the mechanism with NO* as a key intermediate, confirming the reaction pathway for NO3RR. The sulfur enrichment in the framework enhances charge delocalization, which stabilizes Hads species to facilitate NO3– and intermediate adsorption. CTHM-2 demonstrates consistent NO3RR activity up to 5 cycles and sustained 17 h of continuous operation under varied experimental conditions.