Tuning the Electronic Structure of Iridium-Based OER Electrocatalysts by Particle Proximity and Metal-Support Interactions Probed by Operando XAS
Authors: Klein Jochen , Kramer Nils , Blaseio Sonja , Guo Qiang , Ioselli Giulia , Oezaslan Mehtap 
Published: December 2024

Iridium (Ir) is a widely used anode electrocatalyst material for the oxygen evolution reaction (OER) in PEM water electrolysis.[1] However, high loadings of very costly and scarce Ir are currently needed to achieve sufficient performance and durability, which hinder the broad implementation of large-scale PEM water electrolyzers. To overcome these challenges, size-controlled Ir nanoparticles (NPs)[2] are typically used with different loadings in combination with robust support materials such as titanium dioxide (TiO2)[3] or antimony doped tin oxide (ATO)[4]. The variation of the metal loading and support material allows tailoring the metal-support interactions (MSI) and particle proximity effect based on the interparticle distance. Although the particle proximity effect of other catalyst material systems, e.g. Au NPs supported on TiO2
[5], is well-established, the influence of different support materials and loadings (interparticle distance) on the OER activity and durability for Ir NPs catalysts are not fully understood to date.

This study focuses on the metal-support interactions (MSI) and particle proximity effect for Ir NPs deposited on three different support materials: antimony doped tin oxide (ATO), titanium oxide (TiO2) and graphitized carbon (C) at different metal loadings between 20 – 60 wt.%. First, surfactant-free Ir NPs with controlled particle size were obtained by a colloidal route in methanol.[2] Afterwards, the Ir NPs were homogeneously distributed on the support materials with similar loadings and particle sizes and characterized by TEM and XPS. Thin-film rotating disc electrode (TF-RDE) technique was employed to determine the OER activity in 0.05°M H2SO4 electrolyte solution. The ex-situ XAS and XPS data show significant differences in oxidation behavior of Ir depending on the loading and support material. Interestingly, the OER mass activity at 60 wt.% Ir loading increases in the order: TiO2 (36 ± 4 A/gIr) < C (54 ± 5 A/gIr) < ATO (92 ± 8 A/gIr) measured at 1.5 VRHE-iR and 25 °C. Although the NPs on the support materials are aged similar, the iridium on TiO2 is more strongly oxidized than on ATO. In addition to the effect of the support material, the proportion of oxidized iridium increases with decreasing loading. However, no changes in the chemical state of Ir as a function of the loading for TiO2 support material used are observed.

Furthermore, operando XAS investigations on the as-prepared Ir catalysts were preformed to correlate the OER activity and stability with the loading and support material. The operando Ir LIII edge XAS experiments were performed at the P64 beamline at DESY and the ID24 beamline at the ESRF using a home-made spectro-electrochemical flow cell consisting of a three-electrode configuration. Cyclic voltammetry (CV) measurements were performed using two different protocols. The first protocol included a fixed lower vertex potential and starting from 0.06 VRHE, with increasing upper vertex potentials of 0.8, 1.2, 1.4, or 1.6 VRHE. The second protocol used a fixed upper vertex potential of 1.6 VRHE as the starting point, and a variable lower vertex potential of 1.4, 1.2, or 0.8 VRHE, respectively. All CV profiles were conducted at a scan rate of 2 mV/s in 0.05°M H2SO4. Based on the Quick-XANES data, the Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) was used to uncover the electrochemical reversibility behavior of iridium oxidation state as a function of metal loading and support material. Very interestingly, when the iridium loading is very high, a loss of the reversibility of the chemical state of Ir species on carbon and ATO are observed during the CV scan up to 1.6 VRHE, but not on TiO2 support material. In contrast, the chemical state of the Ir species was found to be highly reversible for low-loaded catalyst materials irrespective of the support material.

In summary, our operando XAS investigations provide fundamental insights into the dynamic changes in electronic structure of Ir NPs depending on the support material and interparticle distance. Knowledge about the complex metal-support interactions and particle proximity effect can help to design more efficient and robust OER catalyst materials for PEM water electrolysis.

Literature:

[1] P. Mazúr, Int. J. Hydrog. Energ., 2012, 37, 12081-12088, DOI: 10.1016/j.ijhydene.2012.05.129

[2] F. Bizotto, Catal. Sci & Tech., 2019, 9, 6345-6356, DOI: 10.1039/C9CY01728C

[3] E. Oakton, ACS Catal., 2017, 7, 2346–2352, DOI: 10.1021/acscatal.6b03246

4] H. Tada, J. Phys. Chem. C., 2022, 126, 32, 13539–13547, DOI: 10.1021/acs.jpcc.2c03648

[5] B. Hayden, Acc. Chem. Res., 2013, 46, 1858-1866, DOI: 10.1021/ar400001n