Doped tin oxide supports for the oxygen evolution reaction

Abstract

The utilisation of hydrogen as a clean and renewable energy carrier in transport applications and as a chemical feedstock, is a promising strategy to limit fossil fuel emissions and mitigate climate change. Water electrolysis, especially proton exchange membrane water electrolysis (PEMWE), efficiently produces clean, high purity hydrogen, with virtually no carbon dioxide (CO2) emissions when coupled with primary renewable energy sources such as solar and wind. The oxygen evolution reaction (OER) occurring at the PEMWE anode, under highly oxidative conditions, is kinetically challenging and requires large quantities of noble metal oxide electrocatalysts for feasible operations. Iridium oxide (IrO2) is seen as the most suitable OER electrocatalyst due to its high activity and corrosion resistance. However, the high cost and scarcity of iridium limits its widespread application as an OER electrocatalyst materials in PEMWEs. As such, ways of reducing the Ir content includes the use of suitable support materials to improve the utilisation and operational lifetime of the metal. In this thesis, the use of tin-doped indium oxide, commonly referred to as indium tin oxide (ITO), as a support material for Ir-based OER electrocatalysts was explored. Various phases of Ir-based nanoparticles, ranging from metallic or oxidic (generally referred to as IrOx: where x ranges from 0–2) were deposited on ITO by an in-house developed metal-organic chemical deposition (MOCD) technique. The physicochemical properties of the supports and the supported electrocatalyst counterparts were determined by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), high resolution-scanning transmission electron microscopy (HR-STEM) and X-ray photoelectron spectroscopy (XPS). The OER performance in terms of activity and stability were investigated using the rotating disk electrode (RDE) technique in acidic electrolyte. The results showed that the support physicochemical properties influenced the nature, activity, and stability of the MOCD IrOx/ITO electrocatalysts. The electrocatalysts that were prepared on a low BET surface area ITO support had higher coverage of IrOx nanoparticles over the support, higher surface Sn2+/Sn4+ and Ir4+/Ir3+ with lower surface In2O3/In(OH)3 component ratios than those utilised on high BET surface area ITO support. The best performing electrocatalyst, which was active and the most stable, had uniformly distributed, small (2.4 ± 0.7 nm) size, predominately Ir metal nanoparticles with a good mass specific OER activity of 207 ± 34 A gIr-1 at 1.525 V vs. reversible hydrogen electrode (RHE).