Influence of Support Physicochemical Properties on the Oxygen Evolution Reaction Performance of ITO-Supported IrO_x Nanoparticles

The widespread implementation of proton exchange membrane water electrolyzers (PEMWEs) is greatly hindered by the availability and high costs associated with key catalyst and electrode components. The anodic oxygen evolution reaction (OER) is known to be kinetically challenging and therefore requires substantially high loadings of Ir-based catalysts (∼2 mg cm^–2). A promising strategy to lower the amount of iridium is to enhance the electrocatalytically active surface area, by means of dispersing finely divided nanoparticles of iridium/iridium oxide over cheaper, electronically conductive, stable oxide support materials. In this work, we use a metal–organic chemical deposition (MOCD) technique to deposit IrO_x (x = 0–2) nanoparticles on tin-doped indium oxide (ITO) supports with different physicochemical properties such as specific surface area and Sn dopant concentration. The MOCD technique has been proven to deliver catalysts with well-dispersed nanoparticles and narrow particle size distributions. This approach, in combination with the chosen ITO supports, allows for decoupling of particle size and Ir loading effects from other catalyst–support interactions that are contributing to catalyst activity, stability, and nature of deposited nanoparticles. In this way, we were able to investigate the effect of IrO_x nanoparticle coverage effects on ITO and found that high nanoparticle coverage, i.e., low interparticle distances of the IrO_x nanoparticles supported on low surface area ITO, increased the stability of the support. The specific surface area of ITO supports was found to correlate with the nature of surface Sn and In species, which in turn influenced the nature of deposited IrO_x nanoparticles for enhanced OER activity. The most stable OER catalyst had highly dispersed, small (2.4 ± 0.7 nm), predominantly metallic Ir nanoparticles with a high mass-specific OER activity of 207 ± 34 A g_Ir^–1 at 1.525 V vs RHE. This was obtained by the interplay of the Ir phase, optimized for improved activity and low interparticle distance for catalyst stability.