Load Cycle Durability of Pt/Nb-SnO2 in Polymer Electrolyte Fuel Cells Under Potentiostatic/Galvanostatic Operation

Abstract

Polymer electrolyte fuel cells (PEFCs) used in fuel cell vehicles (FCVs) are still faced with the challenge of increasing the stack durability. It is essential to resolve the degradation of the cathode catalyst layer due to carbon corrosion, dissolution and particle growth of Pt, and decomposition of membranes under various types of FCV operating conditions such as startup/shutdown, load cycling, and idling. In particular, it is difficult to suppress the degradation of Pt, because the speed of the FCV is controlled by the driver, leading to load cycling, which is known to decrease the durability of carbon black-supported Pt cathode catalysts. In particular, the degradation of Pt is accelerated at high relative humidity and long holding times at open circuit voltage (OCV) due to Pt oxide growth [1]. The dissolved Pt2+ ions are likely to diffuse into the ionomer and membrane during load cycling due to the limited ability of the carbon black support to trap these ions. In contrast, Pt on an Nb-SnO2 support might be expected to be slower to diffuse into the ionomer and membrane due to strong metal-substrate interactions [2]. In fact, we reported that the durability of a Pt/Nb-SnO2 cathode catalyst was superior to that of a Pt/GCB cathode catalyst during potentiostatic simulated load cycling [3]. In this research, we evaluated the load cycle durability for membrane electrode assemblies using a Pt/Nb-SnO2 cathode catalyst in PEFCs under galvanostatic and potentiostatic cycling operation. The Pt loading amount for both the Pt/Nb-SnO2 cathode catalyst and the Pt/CB anode catalyst was 0.1 mg cm-2. The cell performances were evaluated under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. Figure 1a shows the protocol for the durability evaluation involving OCV and load holding times at a low current density of 0.33 A cm-2 (sample A1), at a high current density of 0.55 A cm-2 (sample A2), and, in the case of Fig. 1b, the load cycles between the upper cell voltage at 0.94 V and the lower cell voltage at 0.6 V were operated by a potentiostat under H2/N2 (sample A3). A1, A2 and A3 conditions were performed with H2; utilizations of the reactant gases were H2 (70%)/air (40%), H2 (70%)/O2 (20%) and H2 (100 mL min-1)/N2 (100 mL min-1) at 80 °C and 80% RH in Fig. 2. These durability evaluations can be characterized as follows: A1, A2, with the OCV/load holding times of 60 s/3 s; and A3, with upper/lower cell voltage holding times of 60 s/3 s; the sweep rate of 165 mV s-1 from the lower to upper cell voltage, simulates the durability test of A1. Figures 3 and 4 show the cell performances and mass activities under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. After the durability evaluation, the cell performances for both A1 (H2/air condition) and A2 (H2/O2 condition) were nearly the same as their initial performances, but that for A3 (H2/N2 condition) decreased. The mass activity change decreased with the higher O2 partial pressure in the cathode after the durability evaluation. The mass activity of A2 (H2/O2 condition) was the highest of all conditions after the durability evaluation, in spite of the upper cell voltage being the highest. The behavior was in contrast to the case of a carbon-supported Pt catalyst (not shown here). These results suggest that the durability of the Pt/Nb-SnO2 cathode catalyst could correlate with the thickness of the electron depletion layer between the surface of the Nb-SnO2 and Pt. We reported that the electron depletion layer was induced by adsorption of oxygen species on the surface of SnO2 [4]. Therefore, we consider that the degradation from Pt dissolution in the electrochemical oxidation reaction (Pt → Pt2+ + 2e-) could be suppressed by increasing the thickness of the electron depletion layer due to an increase in the amount of adsorbed oxygen species as a function of O2 partial pressure. We will discuss in detail the degradation mechanisms of the Pt/Nb-SnO2 cathode catalyst in comparison with Pt/GCB during the load cycle durability evaluation. C. Takei, K. Kakinuma, K. Kawashima, K. Tashiro, M. Watanabe, and M. Uchida, J. Power Sources, 324, 729-737 (2016).Y. Chino, K. Taniguchi, Y. Senoo, K. Kakinuma, M. Hara, M. Watanabe, and M. Uchida, J. Electrochem. Soc., 162, F736-F743 (2015).R. Kobayashi, K. Kakinuma, A. Iiyama, and M. Uchida, 232nd ECS Meeting, 1388 (2017).Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv., 4, 32180-32188 (2014). Figure 1