title: Electrocatalyst Advancements for Acidic Oxygen Evolution Reaction: From Synthesis to Reliable Performance Evaluation
Creators: name: Bornet, Aline
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: Storage of renewable energy surplus is of paramount importance for tackling the ongoing climate change. Proton exchange membrane water electrolysers (PEMWEs), which split water into H2, the energy carrier, and O2 by electrical means, are valuable technologies for both storing renewable energy as well as providing an important base chemical. While the catalyst at the cathodic side, where H2 evolves, is well-established, the materials that catalyse the anode reaction still require optimisation to overcome the sluggish kinetics of the oxygen evolution reaction (OER). Therefore, this thesis focuses on the acidic OER, investigating model and high-surface-area Ir-based catalysts using both fundamental and more applied approaches. In the first part of the thesis, a commercially available IrO2 catalyst served as a benchmark to deepen the fundamental understanding of the commonly used galvanostatic stability test in an aqueous model system (AMS). This constant current stability protocol was systematically investigated using three distinct backing electrode materials (glassy carbon (GC), Au and Ti), along with two different electrochemical testing platforms, namely a standard rotating disc electrode (RDE) and a 180° inverted RDE. In addition to buoyancy, periodic oxygen reduction reaction on top of galvanostatic screening was conducted to ensure the elimination of O2 gas bubbles obstructing active sites. This way, it was demonstrated that in contrast to claims in literature, electrogenerated gas bubbles have a negligible effect on the observed abrupt potential rise. Combined characterisation methods such as identical location scanning electron microscopy and energy dispersive X-ray spectroscopy, Raman spectroscopy and gas detection, provided evidence that instead, the degradation of the backing electrode material is the main factor responsible for the discrepancies between AMS and membrane electrode assemblies observed in the literature. By utilising half-covered stationary GC electrodes in an H-type cell, the mechanism underlying the substrate passivation was elucidated, providing a conclusive description of the typically observed potential transient. In the second part of the thesis, the focus was directed on developing nanoparticulate Ir-based catalysts to optimise the utilisation of iridium given its scarcity on Earth. Metallic Ir and Ir0.4Ru0.6 nanoparticles (NPs) were synthesised via a reproducible and straightforward colloidal route, avoiding the use of any surfactant. These NPs were subsequently immobilised onto commercially available carbon black and antimony-doped tin oxide (ATO) supports. The performance evaluation of these four catalysts was conducted using a gas diffusion electrode (GDE) setup at temperatures of 30, 40 and 60 °C. This test bed enables conducting experiments under conditions resembling those of a PEMWE, such as the usage of an electrolyte membrane, high loadings, and elevated temperatures. The catalysts were first activated (i.e., the NPs were oxidised) and subsequently tested and compared through 5-minute galvanostatic steps. Surprisingly, despite the tendency of carbon to corrode, carbon-immobilised NPs exhibited superior performance than ATO-immobilised counterparts. This phenomenon was particularly pronounced at 60 °C. This unexpected trend was attributed to the instability of the ATO support caused by antimony leaching. The GDE setup was further employed to investigate the influence of the substrate on the performance of (Ir0.7Ru0.3)0.96Ni0.04 NPs immobilised onto a developed ATO support. The catalyst was synthesised in a solvothermal flow reactor, wherein the NPs are directly formed onto the support material. Two substrates were compared: one derived from fuel cell approaches, namely a carbon gas diffusion layer (GDL), and the one prevalent in PEMWE, namely a titanium porous transport layer (PTL). Our investigations revealed that while GDL is more appropriate than PTL for studying catalyst activity due to its simple and cost-effective electrode preparation, it is unsuitable for stability studies. Indeed, the GDL cannot withstand the harsh oxidative conditions and degrades rapidly. Conversely, despite requiring more elaborated electrode preparation methods, PTLs are more resistant and must therefore be the preferred choice for stability measurements.