Spectroelectrochemistry of water oxidation kinetics in molecular versus heterogeneous oxide iridium electrocatalysts

23 February 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectro-chemical analyzes are employed to make a direct comparison of water oxidation reaction kinetics between a molecu-lar catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(µ-O)]2+ (IrMolecular, pyalc = 2-(2’pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogenous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the for-mation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with poten-tial-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogenous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinc-tions between molecular and heterogenous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.


Water oxidation
molecular catalysis
iridium oxide
iridium dimer

Supplementary materials

Supporting Information
Supporting Figures S1-11 and Table S1


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