Graphene quantum dot engineered nickel-cobalt phosphide as highly efficient bifunctional catalyst for overall water splitting

Graphene quantum dot (GQD) is the most recent addition to the nanocarbon materials family which promises a wide spectrum of novel applications. On the other hand, bimetallic phosphides are emerging for their unique potentials for electrocatalysis. Herein, we have demonstrated the fabrication of heterostructured nanosheet arrays of ternary nickel-cobalt phosphide (NiCo₂P₂) and GQD hybrid (NCP/G NSs) and the use as bifunctional catalysts for overall water splitting in alkaline medium. NCP/G NSs exhibit excellent electrocatalytic activity towards hydrogen evolution reaction (reaching 100 mA cm⁻² at an extremely low overpotential of 119 mV), superior to any other non-noble metal catalyst. Furthermore, an electrolyzer equipped with two identical NCP/G NS electrodes at an exceptionally small amount of catalyst loading (0.31 mg cm⁻²) is able to achieve efficient overall water splitting (10 mA cm⁻² at 1.61 V) with high stability. The careful comparison with NiCo₂P₂ nanowires (NCP NWs) synthesized under the same conditions without GQDs (in terms of electrocatalytic performance, atomic and electronic structures, and electrochemical properties) reveals the mechanistic roles of GQDs in morphology control and performance enhancement. In addition, the performance comparison with ternary nickel-cobalt oxide (NiCo₂O₄) and GQD hybrid (NCO/G NSs) suggests the advantage of bimetallic phosphides over oxide counterparts.