Highly enhanced electrocatalytic OER activity of water-coordinated copper complexes: effect of lattice water and bridging ligand

Abstract

The use of metal–organic compounds as electrocatalysts for water splitting reactions has gained increased attention; however, a fundamental understanding of the structural requirement for effective catalytic activity is still limited. Herein, we synthesized water-coordinated mono and bimetallic copper complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O, and CuMorph-H₂O) with varied intermetallic spacing (pyrazine/4,4′-bipyridine) and explored the structure-dependent oxygen evolution reaction (OER) activity in alkaline medium. Single crystal structural studies revealed water-coordinated monometallic complexes (CuMorph-H₂O) and bimetallic complexes (CuPz-H₂O·H₂O, CuPz-H₂O, CuBipy-H₂O·H₂O). Further, CuPz-H₂O·H₂O and CuBipy-H₂O·H₂O contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-H₂O·H₂O) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm⁻². Bimetallic copper complex (CuPz-H₂O) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-H₂O·H₂O) exhibited relatively lower OER activity. CuPz-H₂O and CuBipy-H₂O·H₂O required an overpotential of 236 and 256 mA cm⁻², respectively. Monometallic CuMorph-H₂O showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-H₂O·H₂O facilitated faster charge transfer kinetics at the electrode surface and supported the enhanced OER activity. The chronoamperometric studies indicated good stability of the catalyst. Overall, the present structure-electrocatalytic activity studies of copper complexes might provide structural insight for designing new efficient electrocatalysts based on metal coordination compounds.