Influence of Lowering the Pt Loading in Gas Diffusion Electrodes for HT PEMFC Application

Kurzfassung

Fuel cells, as environmentally friendly power generation devices, have been recognized as a powerful tool to reduce the need of fossil fuels. High temperature proton exchange membrane fuel cells (HT PEMFC) using green methanol as hydrogen source are promising devices to contribute the increasing demand of renewable energy storage and conversion devices. They are attractive for stationary and mobile applications. In comparison to low temperature (LT) FCs, HT PEMFCs show a higher tolerance towards catalyst poisoning by CO. Due to phosphate adsorption, a high amount of noble metal (e.g. Pt) is needed at the cathode to perform sufficiently. To promote the commercialization of FCs, the lowering of the amount of Pt is necessary. The investigation of the catalyst performance under realistic conditions is one major aim to improve the FC performance, as activity and long-term stability. Recently, half-cell setups were proven to determine the activity of gas diffusion electrodes (GDEs), containing the catalyst of interest. Applying realistic PEMFC loadings and current densities, conclusions for the realization of the whole FC setup are possible [1]. LT PEMFC applications were investigated in several studies [1-4], whereas only Hu et al. characterized catalyst activities under HT PEMFC conditions, using a GDE with relatively low loadings of Pt [5]. The presented work focusses on the influence of decreased Pt loadings in relation to the state-of-the-art GDEs with loadings in the order of 0.85 mgPt cm-2 and the investigation of the catalytic performance. Ultrasonic spray coating is used for the precipitation of the catalyst layers (CL) containing a commercial PtNi/C catalyst onto a gas diffusion layer (GDL), resulting in a GDE. Real conditions of a HT PEMFC were represented within the commercial GDE half-cell setup by the usage of concentrated H3PO4 at a temperature of 140 °C. In a first step, the testing procedure and the binder content of the CL were optimized. Afterwards, Pt loading was varied for the preparation of GDEs. The influence of the Pt loading towards the oxygen reduction reaction (ORR) and possible limitations regarding the CL were investigated. The obtained results showed that the self-prepared GDEs with comparable Pt loadings but also with lower Pt contents surpassed the state-of-the-art GDE, which contained the same PtNi/C catalyst. Mass transport features as a function of the catalyst loading is investigated by the comparison of oxygen and air as cathode gases. In the final step, the self-prepared GDEs were transferred into MEAs using polybenzimidazole (PBI) membranes doped with phosphoric acid to achieve the required proton conductivity. The investigations observed in single-cell measurements yield in further knowledge of transferring GDE half-cell results to MEAs and single-cell tests. µ-computed tomography (CT) is used to determine the catalyst layer thicknesses and homogeneity [6]. [1] M. Inaba et al., Energy & Environmental Science, 11 (2018) 988-994. [2] K. Ehelebe et al., Journal of The Electrochemical Society, 166 (2019) F1259-F1268. [3] B.A. Pinaud et al., Journal of The Electrochemical Society, 164 (2017) F321-F327. [4] P. Mardle et al., ACS Applied Materials & Interfaces, 12 (2020) 42832-42841. [5] Y. Hu et al., Journal of Power Sources, 375 (2018) 77-81. [6] H. Schmies et al., Journal of Power Sources, 529 (2022), 231276-231286.