Towards Low Iridium Loading in Proton-Exchange Membrane Water Electrolysis: Scalable Synthesis of IrOx@TiO2 Core-Shell Particles

Abstract

Proton-exchange membrane water electrolysis (PEMWE) is a promising technology for producing hydrogen from renewable energy sources. However, the high demand for noble metals hinders the widespread adoption of this technology. Especially the anodic catalyst layer is usually manufactured with high loadings of the scarce element iridium, typically ~2 mgIr·cm-2. As an active and stable noble-metal-free alternative to iridium catalysts has yet to be developed, the most viable strategy for the near future development of PEMWE is to reduce iridium loadings as much as possible. Lowering the iridium loading to values below 0.5 mgIr·cm-2, however, often leads to inhomogeneous catalyst layers resulting in disconnected catalyst islands.1, 2 In two related contributions, we report a novel anodic catalyst for PEMWE that can overcome this limitation by forming homogeneous layers even at lower loadings, thus exhibiting high full cell performance. In this first contribution, we synthesized catalysts with less than 50 wt% iridium content to obtain low iridium loadings while maintaining electrical conductivity. The catalyst is based on the IrO2@TiO2 core-shell microparticles introduced in our previous work,3 which we optimized towards their structural and electrochemical properties. The structural properties of these novel catalysts are analyzed by scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Furthermore, the catalytic activity is evaluated by rotating disk electrode (RDE) measurements, and the electrochemical stability is assessed in a scanning flow cell setup combined with an inductively coupled plasma mass spectrometer (SFC-ICP-MS). Our catalysts outperform state-of-the-art materials in terms of mass activity in RDE measurements, while a comparable dissolution stability is achieved, as evidenced by SFC-ICP-MS measurements. In our second contribution, we show a high performance of this novel catalyst in catalyst-coated membrane (CCM) configuration with iridium loadings below the above mentioned threshold of 0.5 mgIr·cm-2. This proves that our novel IrOx@TiO2 particles with low iridium contents enable a significant reduction of iridium for the anode in PEMWE.