Strong d–p orbital hybridization in cobalt porphyrin cages promotes electrochemical nitrate reduction to ammonia

The electrocatalytic reduction of nitrate (NO₃RR) to ammonia presents a viable solution for addressing nitrate pollution and offers an environmentally-friendly, energy-efficient alternative for industrial ammonia synthesis. However, the absence of efficient electrocatalysts impedes its industrial application. In this study, we constructed a porphyrin organic cage (PB-2) through the covalent-bonded self-assembly. Subsequently, metalized porphyrin organic cages, PB-M (M = Co, Ni, Cu), were synthesized <i>via</i> post-modification of PB-2. These PB-M catalysts were utilized to elucidate the reaction pathway and intrinsic structure-performance relationship of the NO₃RR. Experimental results indicate that PB-Co exhibits the highest activity and ammonia selectivity (FE_NH3 = 95.8 ± 1.06%, NH₃ yield rate = 995.5 ± 28.4 µmol h^-1 mg_cat ^-1). Theoretical calculations reveal that the d-p orbital hybridization between the Co 3d orbital in PB-Co and the NO₃ ^- 2p orbital is the strongest one. PB-Co possesses a high d-band center of -0.97 eV and high adsorption energies for NO₃ ^- and H₂O, promoting charge transfer and the production of active hydrogen, thereby reducing the activation energy barrier of NO₃ ^-. This research illuminates the intrinsic structure-activity relationship of metalized PB-M for the NO₃RR, potentially providing valuable insights for the design of efficient electrocatalysts.