Oxygen Evolution Reaction on Ir-Oxide Based Materials Studied By Modulation Excitation X-Ray Absorption Spectroscopy
Authors: Diklić Nataša , Clark Adam Hugh , Herranz Juan , Diercks Justus Sebastian , Aegerter Dino , Beard Alexandra , Schmidt Thomas J. 
Published: July 2022

The price of electricity produced from renewable sources has sharply sunk in the last decade, making these a cheaper choice than newly built thermal power plants in almost all parts of the world [1]. The missing link towards the full decarbonisation of the energy sector is a crossover vector that can account for the intrinsic intermittency of these renewable energy sources. Hydrogen seems to be the best candidate for this, particularly if produced using water electrolysis powered by excess energy from renewable sources. In particular, the load capability and high power density of polymer electrolyte water electrolysis (PEWE) renders this technology an excellent match to these renewables’ sporadic nature [2]. However, the need for scarce Ir to catalyze the sluggish oxygen evolution reaction (OER) taking place in PEWE anodes hinders the large scale application of this technology [3]. Therefore, a better understanding of the OER mechanism on Ir-based electrocatalysts is needed to guide catalyst design and decrease the efficiency losses related to this reaction.
Over the last decades, the OER mechanism on Ir-based materials has been extensively investigated through operando/in-situ X-ray absorption as well as X-ray photoelectron spectroscopy (XAS, XPS) [4-7]. Despite enormous efforts, there is still a lack of consensus regarding the oxidation state of Ir under OER conditions and the nature of the OER-active sites. Therefore, in this work we employed operando modulation excitation XAS to study the OER mechanism on several calcined and uncalcined Ir-oxide catalysts. By combining this modulation excitation approach with phase sensitive analysis, we enhanced the sensitivity of XAS towards oxidation state changes that are otherwise undiscernible on low surface area materials, such as calcined IrO2. To derive more precise information from these phase resolved spectra, standard materials with Ir oxidation states ≥ +5 were also characterized. The comparison of the demodulated, operando spectra of a given material with the corresponding difference between its low-potential spectrum and the standard compounds’ spectra indicates that all oxides reach a maximum Ir oxidation state of +5 under OER conditions. In addition, multivariate curve resolution (MCR) analysis was employed to extract kinetic information from these period averaged spectra. This analysis unveiled that the oxidation of the surface iridium proceeds at the same rate as its reduction in the case of highly OER-active materials, while reduction is slower than oxidation for the less OER-active samples.
In summary, this contribution presents novel insights on the Ir oxidation under potential control, and in doing so also features the capabilities of modulation excitation XAS and MCR to elucidate kinetics of electrochemical reactions.

References

[1] IRENA (2021), Renewable Power Generation Costs in 2020, International Renewable Energy Agency
[2] M. Carmo et al., International Journal of Hydrogen Energy, 38 (2013) 4901-4934.
[3] M. Bernt et al., J Phys Chem Lett, 9 (2018) 3154-3160.
[4] V. A. Saveleva et al., J Phys Chem Lett, 9 (2018) 3154-3160.
[5] V. Pfeifer et al., Chem Sci, 7 (2016) 6791-6795.
[6] D. F. Abbott et al., Chemistry of Materials, 28 (2016) 6591-6604.
[7] A. Minguzzi et al., ACS Catalysis, 5 (2015) 5104-5115.