title: Determination of oxygen reduction reaction (ORR) catalyst activity in gas diffusion electrode half-cells
Creators: name: Schmitt, Nicolai
Publisher: name: UNSPECIFIED
Year: 2023

description: In order to face one of the biggest challenges of our time, climate change and its consequences, along with the limited availability of fossil fuels, the transition towards renewable energies is inevitable. Following the current global political strategies, a key player in energy transition will be hydrogen. Hydrogen can directly be used as an emission free-fuel or as a long-term energy storage. Key technologies for hydrogen as energy storage solution therefore are water electrolysis, which allows production of green hydrogen by the use of renewable electricity, and fuel cells, which can reconvert the chemical energy stored in hydrogen to produce electricity when required. Along the different types of fuel cell technologies, especially polymer electrolyte membrane fuel cells (PEMFC) are promising, as they exhibit high power density and electrical efficiency, and allow quick start-up and shut-down due to their low operating temperature. For a wide-spread commercialization of the PEMFC technique, one hurdle is the sluggish kinetics at the cathodic catalyst layer along with the required high overpotentials to drive the oxygen reduction reaction (ORR). Therefore, development of improved ORR catalyst is a main focus of ongoing PEMFC research. In this regard, a major issue is the limited transferability of ORR catalyst activity data collected in lab-scale rotating disk electrode (RDE) testing to real membrane electrode assemblies (MEA). In order to overcome this limitation, this work focuses on the introduction of a novel technique for ORR catalyst evaluation, namely the gas diffusion electrode (GDE) half-cell approach. Thereby, the mass transport limitations observed in RDE testing with the catalyst coated on a bulky electrode surface and immersed in liquid electrolyte is circumvented by using a porous gas diffusion media as electrode material, allowing to directly distribute the reactant gas to the catalyst surface. Requirements to the GDE approach are to enable the study of realistic catalyst layers in fuel cell relevant current and potential regimes, while keeping advantages of the RDE technique, such as simplicity, fastness and good reproducibility. In the present work, therefore a setup using a commercially available half-cell is established. In the first step best practice advices are developed for electrode preparation and measurement of the electrochemical performance. Also pitfalls in GDE evaluation, such as electrolyte heating and falsified iR correction are identified and solutions to avoid these are presented. In the next step, the GDE setup is further developed to avoid limitations in the maximum current density that can be reached, thus allowing to study the full current range of real MEAs. Therefore, different measures are proposed and the effect of those on the maximum achievable current density is investigated individually. Lastly, the established GDE half-cell approach is compared to real MEA testing with the use of two model catalysts analyzed by using both techniques. Thereby, it can be shown that the differing interphase of the catalyst in GDE testing (catalyst in contact with liquid acidic electrolyte) compared to the MEA (catalyst in contact with solid ionomer membrane) can result in different trends observed with both techniques. In case of differences in catalytic activity being linked to oxygen mass transport, GDE evaluation could very well give trends for catalytic activity in a MEA and is superior compared to RDE testing in this regard. However, trends in catalytic activity being linked to proton transport could less be described within the GDE half-cell. In sum, the setup presented in this work combines advantages of the RDE technique such as simplicity, fastness, comparability and reproducibility of the results, as well as minimum material consumption along with the possibility to test realistic catalyst layers in the full potential and current range of real PEMFCs. On one hand the presented results show that GDE testing with the catalyst layer in contact with liquid electrolyte gives reliable insides in oxygen mass transport properties of realistic catalyst layers at fuel cell relevant potentials and current densities. On the other hand, due to the different catalyst environment in GDE testing in the configuration used in this work, resulting in partial flooding of the catalyst layer by the electrolyte, no full and reliable description of all transport phenomena in real fuel cells (e.g. dry operating conditions, proton accessibility and the complex interaction with the ionomer) is possible. Therefore, future research will have to figure out, whether solid ionomer membranes, utilized in real MEAs, can be introduced in GDE half-cells between catalyst layer and liquid electrolyte, while keeping the advantages of GDE testing such as technical simplicity and faster evaluation compared to MEA testing. Based on these findings, this work helps to guide future application of the GDE technique in PEMFC catalyst research.