Direct Measurement of Proton Conductivity in the Catalyst Layer of PEFC

Abstract

I ntroduction Since the commercialization of FCVs, PEFCs have attracted attention. For further improvement of the efficiency, raising the operating temperature is focused as one possible solution. We have been studying higher temperature and low humidity operation, and have found that decrease in ionomer’s proton conductivity enhances degradation of the cathode and there is a correlation between the proton conductivity of ionomers in the cathode and PEFC’s durability [1, 2]. Even though the proton conductivity within the cathode is important, such proton conductivity has not been directly measured. Therefore, in this study, we investigated a method of directly measuring proton conductivity in the cathode layer. Experimental By referring to a common method of making MEAs, electrocatalyst layers were prepared by spraying the slurry containing 46%Pt/KB (TEC10E50E) and three different ionomers (Nafion, AquivionD72, and AquivionD83) on the Nafion membrane (Nafion 212). The I/C was kept 0.72 and 0.76 for Nafion and Aquivion, respectively. Then, another Nafion membrane was placed further on the sprayed electrocatalyst layer. The size of electrocatalyst layer and Nafion membranes were 30 mm×10 mm and 35 mm×14 mm, respectively, as shown in Fig. 1(a). Here, the catalyst layer was smaller than the membrane for the catalyst layer not to contact with Pt electrode of the holder shown in Fig. 1(b) even after hot-pressing. Two carbon papers adhered by carbon paste were put between the membrane and Pt electrode. With this holder, impedance was measured in the nitrogen or hydrogen atmosphere by applying DC or AC (10 mV, 0.1 mA, 32 MHz~10 Hz). Measurements were performed under 80oC-RH100% and 105oC-RH57% as a common operating condition and a low humidity condition, respectively. Results and discussion The proton conductivity was calculated based on the impedance. Measuring impedance in nitrogen atmosphere with AC, the movement of electrons within the electrocatalyst layer cannot be excluded completely. If Nafion ionomer within the electrocatalyst layer forms a condensed thin film, the thickness will be 1.25 μm based on the mass and density of Nafion ionomer, resulting in 0.0025 Ω. However, in reality, Nafion ionomer spreads out through the catalyst layer and results in higher resistance. In this study, the obtained resistance for the catalyst layer with Nafion ionomer was 0.031 Ω, which was more than ten times larger. Larger resistance is reasonable at this moment, but again there is high possibility of including electron conductivity within the catalyst layer. Therefore, in the next step, we tried to apply DC. Since the reaction with low overvoltage was desired for this measurements, DC was applied under hydrogen atmosphere where the reaction is reversible and its overvoltage is small. We’ll report the results obtained by applying DC under the hydrogen atmosphere.