Investigation of a Method of Evaluating Proton Transport Resistance in PEFC Catalyst Layers

Abstract

Introduction In polymer electrolyte fuel cells (PEFCs), the catalyst layers, consisting of electrocatalysts and ionomers, greatly affect current-voltage (IV) performance. In other words, not only catalytic activity and electronic conductivity, but also proton conductivity become important to achieve higher performance in PEFCs. Even though the proton conductivity within the catalyst layer is one of the most important factors, there is no method to directly measure the proton conductivity in catalyst layers. Therefore, the objective of this study is to establish a method to directly measure the proton resistance and to quantitatively evaluate the proton conductivity within the catalyst layer. Experimental A schematic diagram for the proton conduction measurement used in this study is shown in Fig. 1. A commercial cell holder commonly used for PEFCs was utilized in our measurement system. As show in Fig. 1, Pt/C gas diffusion electrodes and water-repellent GDLs were used for both the anode and cathode sides. The sample to be analyzed was sandwiched between two Nafion® membranes in order to block electrons. In detail, the 1 cm2 gas diffusion electrode was prepared by spray printing 46.5% Pt/KB (TEC10E50E) on each Nafion®212 film. Then, for one of the two Nafion® membranes, the sample to be analyzed was also spray-printed on the reverse side. Finally, two Nafion® membranes were hot pressed first and assembled with two GDLs as described in Fig. 1. With this setup, ohmic resistance was measured using a current interrupt method, in which direct current (DC) was applied to the sample but electrons were blocked. Since it is important to select a reaction with a small overvoltage for the DC reaction, we have used hydrogen oxidation/reduction reaction in this study and flowed hydrogen on the anode side and nitrogen on the cathode side. Results and Discussion Before the actual sample measurement, a Nafion®212 was first sandwiched instead of the sample to be analyzed and tested. When the H2 generated at the cathode side was quickly removed along with the N2 flow, a gradient of hydrogen concentration was produced. The resulting concentration cell generated the additional voltage. Also, unlike a fuel cell reaction, since water was not produced during the reaction, the Nafion® membrane dried up as the reaction proceeded, leading to increase in resistance. Therefore, in order to avoid formation of a concentration cell, the flow rate of gases was minimized and also N2 gas was stopped at the cathode right before the measurement. Additionally, the water-repellent GDLs normally used in PEFCs were replaced with GDLs without a water-repellent treatment. Using the optimized measurement system described in the above, two samples, a catalyst layer containing Pt/Vulcan, and a Vulcan carbon support layer without Pt, were tested. As a result, ohmic resistance of the Pt/Vulcan layer was very small like 0.011 Ω. On the other hand, the Vulcan catalyst layer showed 0.095 Ω. The reason for this is most likely because the presence of Pt in the layer brings up some other chemical reactions, causing the electron transfer within the sample layers, and the electron movement cannot be completely blocked.Consequently, by directly evaluating the Pt-free carbon support layer, deriving the proton conductivity in the catalyst layer becomes the best way. Figure 1