Mario Kircher

In the context of the energy transition, the fuel cell represents a key technology. On the way to higher performance and efficiency, the development and testing of catalysts for the oxygen reduction reaction (ORR) is crucial [1]. Over the past years, the gas diffusion electrode (GDE) half-cell setup has been introduced as a bridging tool between RDE half-cell testing and MEA single cell testing for catalyst characterization [2,3]. Compared to the MEA testing, the GDE proved to be pronounced faster and cheaper, while still reaching relevant current densities (3 A cm^-2) other than RDE setups. In recent studies the potential and reliability of this technique has been proven. It was shown, that the GDE can correctly predict trends of catalytic activity in the MEA until the mass transport limitation in the MEA appears. Furthermore, the mass transport influence of ionic liquid modifications of the catalyst layer could also be deduced in the GDE half-cell, without the tedious ink development needed for MEA preparation [4]. Proton transport is not limited in the GDE half-cell, if the catalyst layer is directly immersed within the liquid electrolyte. Depending on if the intrinsic activity of the catalyst or the proton resistance wants to be studied this is an advantage or disadvantage. This opens the question, whether proton mass transport resistance can also be studied in the GDE setup, when separating the catalyst layer from the liquid electrolyte through a membrane. Adding a membrane will change the mass transport of the system by two aspects: First, the protons must be transported through the membrane to reach the catalyst layer and second, the formed water during the reaction can no longer be flushed away from the liquid electrolyte but must leave through the gas diffusion layer (GDL). Developing such a manufacturing procedure and measurement protocol to gain valid, reproducible, and reliable results is scope of this study. In the first step the membrane (Nafion™ NR211) was spray coated via an ultra-sonic spray coater with a commercial Pt/C catalyst (HiSPEC®3000, 20 wt% Pt). Afterwards the catalyst coated membrane (CCM) was hot-pressed to the GDL (Toray™ TP-090-T5). Different pressing times and pressures were examined. The resulting electrode was characterized using a commercial GDE half-cell (Flexcell® PTFE, Gaskatel GmbH) and the measurement protocol according to Schmitt et al. [5]. On the way towards the final manufacturing procedure many challenges have been overcome. The biggest was how to deal with membrane swelling and detachment when measuring at high current densities where large quantities of water are produced. To cope this issue a GDL without a microporous layer was chosen to support the water transport. Furthermore, the application of a membrane in a GDE half-cell brings up - yet unsolved – challenges, when using impedance spectroscopy for iR-correction. With our final manufacturing procedure and measurement protocol additional mass transport resistances and limitations can be captured in the ohmic region and at high current densities in the GDE half-cell-setup. In summary we showed a working proof of concept for the membrane application in GDE half-cell setups, which give comparable results to actual MEA measurements and remaining the advantages of GDE testing. Literature: [1] Banhamet al. ACS Energy Lett. 2017, 2, 629. [2] Ehelebe et al. ACS Energy Lett. 2022, 7, 816. [3] Schmitt et al. Journal of Power Sources 2022, 539, 231530. [4] Schmitt et al. Energy Adv. 2023, 2, 854. [5] Schmitt et al. Electrochemistry Communications 2022, 141, 107362.

The shelf-life of catalyst ink for fabricating polymer electrolyte fuel cells (PEFCs) is relevant for large-scale manufacturing with unforeseen production stops. In this study, the storage effects on the physicochemical characteristics of catalyst ink (Pt/C, Nafion, 2-propanol, water) and subsequently manufactured catalyst layers are investigated. Sedimentation analysis showed that catalyst particles are not fully stabilized by charge interaction induced by Nafion. Acetone was found to be an oxidation product, even in freshly prepared ink with platinum catalyzing the reaction. Rotating disk electrode analysis revealed that the electrochemically active surface area is, overall, minimally increased by storage, and the selectivity towards water formation (4-electron pathway) is unharmed within the first 48 h of storage. MEAs prepared from stored ink reach almost the same current density level after conditioning via potential cycling. The open-circuit voltage (OCV) increases due to increased catalyst availability. Scanning electron microscopy and mercury intrusion porosimetry showed that with increasing acetone content, the pore structure becomes finer, with a higher specific surface area. Electrochemical impedance spectroscopy revealed that this results in a more hindered mass transfer but lowered charge transfer resistance. The MEA with the highest OCV and power output and the lowest overall cell resistance was fabricated from catalyst ink stored for a duration of four weeks.