Concomitant polymorphs: an alternative to modulate the oxygen evolution performance of mononuclear nickel complexes

Abstract

The availability of polymorphs of metallic complexes provides an opportunity to reveal the relationship between crystal packing and catalytic activity. Herein, we immobilize two stable concomitant polymorphs (green NiL₂-G and red NiL₂-R) of mononuclear nickel NiL₂ on electrodes as heterogeneous oxygen evolution reaction catalysts. Density functional theory calculations reveal that the formation of L₂Ni^IV=O from L₂Ni^III-OH is the rate-determining step, whereas R-L₂Ni^III-OH is more stable than G-L₂Ni^III-OH, and forming L₂Ni^IV=O from the former has a much lower barrier than from the latter. Consistently, NiL₂-R exhibits a lower overpotential (339 mV) than NiL₂-G (466 mV) at 10 mA cm⁻².