Efficient homogeneous electrochemical water oxidation catalysed by macrocyclic nickel complex with redox non-innocent pyridine coordination structure

Abstract

Efficient electrocatalysts are anticipated to mitigate the high overpotential and slow kinetics of water splitting, which is a feasible way to produce hydrogen as an environmentally friendly renewable fuel. However, the development of low-cost catalysts with high activity and stability is still challenging. Herein, Ni complex [Ni(Me3pyclen)(CH3CN)2](ClO4)2 (1) with macrocyclic pyridine-triamine ligand is developed as efficient molecular catalyst for electrocatalytic water oxidation, which occurs with onset overpotential of only 520 mV and high Faradaic efficiency of 93% under neutral condition. The single-site catalytic mechanism involving proton-coupled electron transfer (PCET) processes of both Ni center and ligand is proposed based on electrochemical test results. Furthermore, comparative studies on catalytic behaviors of 1 and the derivative all-amine coordinated Ni complex [Ni(12-TMC)(OAc)]PF6 (2) illustrate that the stability of 1 is dependent on the hybridization form of the coordinated nitrogen atom, which avoiding the decompsotion of 2 into nickel-hydroxides during oxygen evolution. Therefore, the pyridine-triamine ligand shows its superiority in constructing homogeneous electrochemical water oxidation system than the all-amine based ligand.