Improving the kinetics of oxygen electrocatalysis is key to increasing the efficiency of hydrogen production from renewable sources, production of carbon-neutral fuels such as ethylene and rechargeable metal-air batteries¹. Metal oxides exhibit state-of-the-art activity, but fundamental atomic-level insights into the reaction mechanism are often unknown. Particularly, various differences between materials, including the differences in active surface area, chemical state of the metal cations, fractional coverage of oxidized species and the range of ordered structure, renders it difficult to identify the origin of the differing activity. Thus far, ambiguities in measuring the number of sites participating in the reaction have prevented the accurate measurement of intrinsic catalytic activity, or turnover frequency. Furthermore, while recent studies have indicated that the density of oxidized species can enhance reaction rates on metal oxides surfaces via cooperative effects between adjacent adsorbates^2-4, extending these mechanistic implications to a range of oxides remains a challenge.

In this talk, I will present developments in time-resolved optical spectroscopy to identify the density of different oxidized species as a function of potential and establish how this controls the reaction kinetics. These results will be combined with (i) X-ray absorption spectroscopy to measure the oxidation state and coordination of the active site (ii) time of flight secondary ion mass spectrometry to measure the depth of redox active states⁵ and (iii) on chip electrochemical mass spectrometry to measure the degree of lattice oxygen participation. As an example, a range of iridium-based catalysts – namely monolayers of molecular iridium dimers⁶, amorphous⁷ and crystalline oxides will be compared for the water oxidation reaction. For all the catalysts investigated, three redox transitions can be observed, and the physical origin of these redox processes can be assigned using density functional theory studies. Although similar oxidized species are found to accumulate at water oxidation potentials, the correlation between the density of oxidized species and water oxidation kinetics is very different. On molecular catalysts, there is limited interaction between isolated iridium centres, and thus the intrinsic activity per oxidized site is invariant with potential⁵. On the contrary, for heterogeneous oxide catalysts, a high degree of cooperative effects results in faster kinetics with increasing accumulation of oxidized species on the surface. The potential for accumulation for oxidized species and the degree of interaction of these oxidized species will be compared for the amorphous and crystalline oxides. Therefore, through this work, I will highlight the power of operando time-resolved spectroscopy in unravelling the critical role of oxidized species in facilitating water oxidation kinetics.

The author would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (bp-ICAM), which made this research possible.

References:

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