Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.198
Publication date: 7th February 2022
Iridium oxide is the state-of-the-art electrocatalysts for water oxidation in polymer electrolyte membrane (PEM) electrolysers for green hydrogen production. Hydrous, amorphous iridium oxides (IrOx) show far higher activity than the rutile crystalline iridium oxides (IrO2), but the reason why they are more active is still under intense debate. Various differences between IrOx and IrO2, including the differences in active surface area, chemical state of Ir, fractional coverage of -OH groups and the range of ordered structure, renders it difficult to identify the origin of the differing activity.[1], [2] 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, for amorphous iridium oxides and rutile iridium oxide.
In this talk, I will present the results from a range of different spectroscopic techniques and electrochemical measurements to identify the redox species present in IrOx and IrO2 and establish how these control the reaction kinetics. The techniques include (i) time-resolved UV-vis absorption spectroscopy (ii) on chip electrochemical mass spectrometry (iii) time of flight secondary ion mass spectrometry.
We have identified the same short-lived and long-lived oxidised species for both IrOx and IrO2 at potentials from 1.4 V to 1.55 V. We have correlated these species to the O2 evolution kinetics. We attribute the lower activity of rutile IrO2 to a lower concentration of the species responsible for driving water oxidation. The electrochemical mass spectrometry allows us to separate capacitive currents from O2 evolution on the two catalysts. Combining TOF-SIMs with a paraffin wax cover method,[3] the depth of proton penetrating into the bulk catalysts was also revealed.
By comparing the concentration of oxidised species to reaction rates, we can directly measure turnover frequency (TOF) for IrOx and IrO2 as a function of applied potential. In summary, this study quantitively measured the intrinsic activity of amorphous and crystalline iridium oxides without any assumption of surface area and therefore shines insights into the origin of activity difference between amorphous and rutile iridium oxides.
The authors would like to acknowledge the funding support from BP through the BP International Centre for Advanced Materials (bp-ICAM), and Imperial College-Chinese Scholarship Council Studentship, which made this research possible.
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