Influence of Crystallinity, Particle Size, and Microstructure on the Oxygen Evolution Activity of IrO2

Abstract

The development of cost-effective polymer electrolyte membrane (PEM) electrolyzers is key to the implementation of a hydrogen-based infrastructure. It cannot be denied that there is an ever-increasing demand to shift away from the use of nuclear power and the traditional burning of fossil fuels as primary energy systems. The only foreseeable long-term solution has been increasingly focused on the implementation of clean, renewable power supplies, such as wind farms and solar power stations. In this regard, the electrochemical conversion of water to hydrogen is expected to play a key role in the development of scalable energy storage that is required for such intermittent power supplies(1). The cathodic generation of hydrogen from water splitting is known to be extremely facile on Pt-based catalysts(2). The simultaneous oxygen evolution reaction (OER) occurring at the anode, however, is limited by sluggish kinetics and requires a considerable overpotential to achieve modest current densities. Moreover, the harsh acidic environment and high anodic operating potentials limits the choice of stable electrocatalyst materials to those of the noble metal oxides. Reduction of the noble metal loading at the anode and enhancing catalyst stability for OER in PEM electrolyzers remains a challenge. Perhaps the most widely implemented approach for reducing the noble metal content is that which considers reducing the catalyst particle size(3, 4). A major issue that arises from this approach, however, is that it becomes increasingly difficult to establish structure-activity relationships for the OER due to the transformation processes, e.g. changes in microstructure and crystallinity, that commonly occur during the preparation of the materials. The research reported herein is focused on expanding the fundamental understanding of the influence of crystallinity, particle size, and microstructure on the electrochemical OER activity of nanocrystalline IrO2 with particular emphasis pointed towards formation of a hydrous surface layer. Chlorine−free iridium oxide nanoparticles are synthesized using the modified Adams fusion method(5), which is capable of producing spherical 1.7 ± 0.4 nm particles with a specific surface area of 150 m2/g using a low temperature synthesis (350 °C). Increasing the synthesis temperature to 600 °C results in the formation of larger, rod−shaped particles mostly terminated by highly ordered non−defective (110) surfaces. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and electrochemical studies indicate the presence of a hydrous surface layer, i.e. Ir(O)OH, that leads to an enhanced OER activity. We report that it is possible to create a larger hydrous layer on smaller nanoparticles and thus increase the specific OER activity.