(Invited) Comparison of Hydrogen Pump and Electrochemical Impedance Spectroscopy Methods for Proton Transport Resistance Measurements in Catalyst Layers

Optimizing ionomer content in polymer electrolyte fuel cells (PEFCs) catalyst layers is critical to achieve sufficient coverage of electrocatalyst and provide low-resistance proton transport pathways. This has resulted in a need for advanced diagnostic techniques to understand delineation of proton transport resistance from the other contact, Ohmic and electric resistances within the catalyst layer. Electrochemical impedance spectroscopy (EIS) method for H2/O2 or H2/N2 experiments along with the transmission lime models has been used for measuring proton transport resistance within the catalyst layers [1,2]. This diagnostic is easy to implement to catalyst layers during PEFC operation and the perturbation signal can be overlaid over cell applied currents or potentials. One of the challenges of the method is data interpretation, as 45o region on Nyquist plot in many cases is not well-defined, what introduces a source of error. An alternative approach is to use hydrogen pump (HP) method, where pseudo catalyst layer (carbon and ionomer layer without electrocatalyst) is electrically insulated between two membranes within the PEFC configuration and hydrogen is used on the anode and cathode [3]. With this method hydrogen polarization curves are extracted and slopes of the curves are proportional to the proton transport resistance within the pseudo catalyst layer. Iden et al. [3] used high frequency resistance to correct the measurements for membrane and contact resistances. They have shown that it is possible to extract tortuosity values of the ionomer within the pseudo catalyst layer, when ionomer content is estimated for each RH. The challenge of this diagnostic technique is that Pt cannot be used within the layers as it can short the current. Currently, the two techniques have not been compared and the comparison can provide better understanding of the EIS method, which is easier to conduct and implement. In this study we use two methods to measure proton transport resistance in pseudo catalyst layers. The experiments are performed on pseudo catalyst layers containing 3M 825 ionomer and Vulcan carbon black of varying ionomer to carbon (I/C) ratios. The I/C ratios used in this study are 0.6, 1.0 and 1.4. The thickness of the pseudo catalyst layers varied for the HP experiment by sandwiching 2, 4 and 6 layers between the two membranes. Varying thickness of the layers is an alternative method to obtain contact and membrane resistances that should be subtracted from the overall measured resistance. During HP experiment both DC and EIS data was collected. For the EIS method pseudo catalyst layers were cathodes of conventional configuration. H2/N2 method was used to extract the length of the 45o segment on the Nyquist plot, which corresponds to proton transport resistance divided by three. With this method proton transport resistance within the catalyst layer dependence on applied potential can be extracted too. We show that by dividing effective conductivity obtained with the HP method and that from the EIS and adjusting for ionomer content within the pseudo catalyst layers we can obtain values of ionomer tortuosity within the catalyst layers. References: M Eikerling, A.A Kornyshev, Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, Journal of Electroanalytical Chemistry, Volume 475, Issue 2, 1999, Pp 107-123 R. Makharia, M. F. Mathias, and D. R. Baker, Measurement of Catalyst Layer Electrolyte Resistance in PEFCs Using Electrochemical Impedance Spectroscopy J. Electrochem. Soc. 2005 152(5): A970-A977 H. Iden, A. Ohma, K. Shinohara, Analysis of Proton Transport in Pseudo Catalyst Layers J. Electrochem. Soc. 2009 156(9): B1078-B1084