The extremely promising activities of advanced fuel cell catalyst materials achieved in an ideal rotating disk electrode (RDE) environment can frequently not be transferred to technologically relevant membrane electrode assemblies (MEAs). This can mainly be ascribed to a non-optimal catalyst layer composition, which significantly affects the management of educts and products on the catalyst layer surface for the oxygen reduction reaction (ORR). Therefore, the composition of the catalyst layer has enormous potential to significantly improve the performance of these systems in MEA. However, it has to be optimized for each individual catalyst system. Currently, MEA experiments have to be employed for catalyst layer optimization.^[1] However, such investigations are time consuming, expensive (large quantities of catalyst and extended test equipment needed) and do not allow independent investigation of either cathode or anode catalyst layer. In order to accelerate catalyst layer optimization screening, the gas diffusion electrode (GDE) half-cell setup for investigating realistic catalyst layers was recently proposed.^[2]

In a Pt loading study, it was previously shown that similar performance trends compared to MEA experiments can be achieved.^[2] In the present work, we introduce advanced electrochemical characterization methods, such as Oxygen Transport Resistance^{[3, 4]} and CO-Displacement^[5], which have been developed for MEA technique, to the GDE method. By using a commercial Pt on Vulcan catalyst system, we investigate the impact of Nafion loading on the electrochemical performance. The results show, that high ionomer loadings lead to severe O₂ mass transport limitations, whereas for small loadings, lower ionomer coverage are measured. Both result in a significant performance loss at high current densities. Therefore, an intermediate ionomer loading which forms a thin layer of ionomer leads to an optimal performance for the Vulcan carbon support.

This work demonstrates that advanced electrochemical methods can also be applied to GDE setups to shed light on the optimal composition of the triple phase interface of catalyst layers. This is an innovative step for the future to efficiently optimise catalyst systems and to gain fundamental insight into the understanding of catalyst layers.

Literature

[1] K. Ehelebe, D. Escalera-López, S. Cherevko, Current Opinion in Electrochemistry 2021, 29, 100832.

[2] K. Ehelebe, D. Seeberger, M. T. Y. Paul, S. Thiele, K. J. J. Mayrhofer, S. Cherevko, Journal of The Electrochemical Society 2019, 166, F1259-F1268.

[3] D. R. Baker, C. Wieser, K. C. Neyerlin, M. W. Murphy, ECS Transactions 2006, 3, 989-999.

[4] D. R. Baker, D. A. Caulk, K. C. Neyerlin, M. W. Murphy, Journal of The Electrochemical Society 2009, 156, B991.

[5] T. R. Garrick, T. E. Moylan, V. Yarlagadda, A. Kongkanand, Journal of The Electrochemical Society 2016, 164, F60-F64.