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

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-HO·HO, CuPz-HO, CuBipy-HO·HO, and CuMorph-HO) 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-HO) and bimetallic complexes (CuPz-HO·HO, CuPz-HO, CuBipy-HO·HO). Further, CuPz-HO·HO and CuBipy-HO·HO contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-HO·HO) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm. Bimetallic copper complex (CuPz-HO) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-HO·HO) exhibited relatively lower OER activity. CuPz-HO and CuBipy-HO·HO required an overpotential of 236 and 256 mA cm, respectively. Monometallic CuMorph-HO showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-HO·HO 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.