Graphite encapsulated high active nickel-molybdenum/nickel oxide porous nanosheet composites as enhanced oxygen evolution reaction electrocatalysts

Electrocatalytic water splitting for hydrogen production is one of the effective solutions to alleviate energy crisis and reduce carbon emission. The large overpotential and the high sluggish reaction kinetics of the oxygen evolution reaction (OER) at the anode seriously restrain the overall efficiency of water splitting. Nickel-based materials are widely studied in alkaline electrolytes attributed to the low cost and good stability. However, the insufficient exposure of active sites and inferior actual catalytic activity hinder the further enhancement of its OER activity and stability. In the present work, a morphological and microstructural synergistic regulation strategy is developed to prepare NiMo/NiO porous nanosheet composites (PNC) by using Mo element doped Ni metal-organic frameworks (MOFs) as precursors. The nanocrystals of NiMo alloy and NiO are subsequently encapsulated by ultrathin graphite layers with abundant Ni^{3 +} oxides and oxygen vacancy formed on the surface of NiO. Electrochemical studies have revealed that NiMo-2 PNC exhibits excellent oxygen evolution reaction (OER) electrocatalytic activity. The corresponding overpotentials of NiMo-2 PNC are only 292 mV and 311 mV at 10 and 20 mA cm⁻², respectively, along with a lower Tafel slope and good stability. The remarkable catalytic performance is primarily attributed to the synergistic effects of the specific morphology, Mo element, Ni³⁺ and oxygen vacancy defects, which can increase the number of exposed effective active sites, regulate the electronic structure of Ni metal and optimize the adsorption energy of NiO for the intermediates of OER, leading to the improved catalytic activity and electron transport kinetics.