Hollow Iridium-Based Catalysts for the Oxygen Evolution Reaction in Proton Exchange Membrane Water Electrolyzers

Abstract

The performance of Proton Exchange Membrane Electrolyzers (PEMWE) is mostly impeded by the efficiency of the anode catalysts allowing for the Oxygen Evolution Reaction (OER) to occur. New catalytic materials should possess at least as high performance and stability as state of the art materials, since the price of electrolysis is mostly impacted by the price of electricity and higher overpotentials would render the catalysts not viable for application in PEMWE. The use of noble metal at the anode for the OER is at the current stage essential due to the highly oxidative and strongly acidic environment. Nevertheless, due to cost and resources considerations, it is necessary to reduce the noble metal content. In addition, according to present industrial requirements, PEMWE cells should be able to work at very high current density and under high pressure. Water and gases management are also a major challenge in PEMWE catalyst layers, and unlike in fuel cells or PEMWE cathodes the use of a cheap carbon based porous support for electronic conduction at the anode is highly undesirable due to highly anodic potential. We have developed a cheap, clean and easily scalable synthesis method to produce highly porous iridium-based materials. The materials are synthesized in aqueous solutions in presence of a porogen. The resulting material is further calcinated to remove the porogen and initiate crystallization of the inorganic phase. By adjusting reactant concentrations and calcination conditions, metallic iridium, iridium oxide or mixed oxides can be obtained. This last point is of high importance since in addition to reducing noble metals content compared to current technologies, the atomic neighborhood of dissimilar metals is expected to modify the monometallic behavior and potentially gives rise to new desired characteristics, i.e.to decrease the OER overpotential and/or increase the material stability. Structural characteristics and chemical composition of the materials have been determined using XRD, XPS and XRF. The influence of the thermal treatment has a strong impact on the crystallite size and the composition of the particle surface. SEM-FEG and BET characterizations have shown that the type of the porogen used for material synthesis directly impacts its morphology and porosity. To assess the catalytic performances of these hierarchically porous materials, a screening of the electrocatalytic activity of a series of prepared materials has been performed using a three-electrodes electrochemical setup. The optimized material shows lower overpotential than commercial Ir or IrO2 nanoparticles as shown on the figure thereafter. Finally, selected materials have been used to prepare anode catalyst layers and have been tested in a complete electrolysis device. With a loading of 2 mg cm-2 in Iridium, a potential of 1.65 V at 1 A cm-2 and of 1.85 V at 2 A cm-2 is obtained. The produced specific morphologies are very well adapted to the formation of porous percolating networks, which promote gases and water transport throughout the catalyst layer. Figure 1