Integrating Homogeneous and Heterogeneous Catalytic Systems for Synergistic Water Oxidation: Role of Geometrically Distinct Bispidine Metal Complexes

This study investigates the integration of two geometrically distinct nickel complexes with a heterogeneous catalytic counterpart to enhance the water oxidation performance. Bispidine-based Ni(II) complexes with a flexible backbone, NiL1Flex, and a rigid backbone, NiL2Rigid, act as homogeneous catalysts, whereas β-Ni(OH)2@NF nanoflowers serve as a heterogeneous catalyst counterpart. NiL2Rigid, featuring an octahedral geometry, exhibits superior electrocatalytic performance with an overpotential of 330 mV at 10 mA/cm2 and a Tafel slope of 97 mV/dec in 0.1 M phosphate buffer solution (pH 7), outperforming NiL1Flex with a distorted tetrahedral geometry. Single-crystal XRD reveals that the octahedral geometry with cis-positioned labile water sites of NiL2Rigid promotes O–O coupling for O2 generation, explaining its lower overpotential and enhanced charge transfer kinetics compared to the distorted tetrahedral NiL1Flex complex, which is devoid of such attached cis-labile water molecules. The enhanced activity of NiL2Rigid is attributed to its minimized potential difference across the redox couple (Ni2+/Ni3+), aiding favorable synchronization among homogeneous and heterogeneous catalytic systems. Mullikan spin density calculations reveal that the rigid complex NiL2Rigid has a highly localized spin density (∼80%) on the nickel center, whereas the spin density population mainly lies between the metal and oxygen bonds in NiL1Flex reducing the redox activity of nickel in a flexible backbone. This research work establishes a model system that integrates homogeneous and heterogeneous catalysis for efficient water oxidation, leveraging the advantages of both systems while addressing their limitations through their integration.