Polyoxometalate-based complex@graphene composite electrodes for efficient nitrate reduction to ammonia.

To replace the energy-intensive and polluting traditional ammonia synthesis process, in this study, we designed two polyoxometalate (POM)-based nickel/cobalt metal-organic composite catalysts, namely (HNCP)2[Ni(H2O)4]2[NiMo12(HPO4)4(PO4)4O30] (Ni-P4Mo6) and (HNCP)2[Co(H2O)4]2[CoMo12(HPO4)4(PO4)4O30] (Co-P4Mo6). These catalysts utilized {P4Mo6} as the structural unit, nickel/cobalt as the metal node, and π-conjugated organic ligands as the linkers, and they were loaded onto graphene oxide (GO) to enhance the conductivity and reaction contact area. Experimental results showed that in acidic electrolytes, Ni-P4Mo6/GO achieved an ammonia yield of 2.62 mg h-1 mg-1cat. at -0.6 V (vs. RHE) with a faradaic efficiency (FE) of 83.9% at -0.5 V, outperforming Co-P4Mo6/GO (1.63 mg h-1 mg-1cat. at -0.7 V; FE 85.3% at -0.3 V). Under neutral conditions, both the catalysts exhibited significantly improved performances (Ni-P4Mo6/GO: 11.6 mg h-1 mg-1cat. yield, 88.4% FE; Co-P4Mo6/GO: 11.1 mg h-1 mg-1cat. yield, 78.5% FE), surpassing most comparable catalysts. This work provides a novel strategy for developing efficient electrocatalysts for nitrate reduction to ammonia (e-NO3RR).