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

Abstract

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⁻¹_cat. 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⁻¹_cat. 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⁻¹_cat. yield, 88.4% FE; Co-P₄Mo₆/GO: 11.1 mg h⁻¹ mg⁻¹_cat. 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).