Strategies Towards Low-Ir-Catalysts for PEM Electrolysis: From Support Material and Morphology to Depositing Iridium Oxide

The transition from fossil fuel based and CO₂-intensive energy production to a future sustainable energy-based economy requires solutions for efficient energy conversion and storage for later re-electrification of green energy such as solar or wind power. One possibility is to convert energy from sustainable sources into chemical energy in the form of hydrogen by electrolysis of water. To meet this challenge, Proton Exchange Membrane (PEM) electrolysis is a powerful state-of-the-art technology that uses scarce iridium-based catalysts to promote the oxygen evolution reaction (OER). To improve the sustainability and economics of the PEM process, it is necessary to minimize the use of rare and precious iridium to reduce the overall density of iridium-based catalysts for the OER.<br/>Here we present different strategies for the synthesis of low overall iridium OER catalysts. In addition to the choice of support material and support morphology, we also consider the coating strategy to produce highly active and durable catalysts.<br/>The coating of corrosion-resistant high surface area metal oxide supports of various morphologies with an ultrathin layer of amorphous iridium hydroxide by a simple wet chemical synthesis at low temperature and ambient pressure or by a hydrothermal approach leads to the formation of interconnected amorphous hydrous iridium oxide particles. In a subsequent step, the amorphous phase is oxidized at elevated temperatures using an Adams fusion salt melt. This process allows for a controllable phase transformation and crystallization to form a layer of interconnected partially crystalline IrO_x nanoparticles of ≈1.5 - 2 nm domain size on the surface of the supporting metal oxides, thus providing an electrical percolation path in the catalyst layer.<br/>The appropriate choice of support material in terms of crystal lattice parameters can modulate the crystal growth of the iridium oxide in terms of epitaxy and significantly increase the stability due to tight anchoring to the support. The final core-shell nanostructures comprise highly active iridium oxide with total Ir weight fractions as low as 7 at% on SnO₂ nanoparticles, compared to a state-of-the-art benchmark catalyst with 56 at% Ir supported on TiO₂ nanoparticles. Transmission electron microscopy (TEM) imaging was used to elucidate the impact of different core-shell designs, with a particular focus on iridium coating and crystallinity. Rotating disc electrode (RDE) measurements were also carried out to assess the electrochemical properties of each catalyst.