Mitigation of Cathode Catalyst Degradation during Air/Air Startup Cycling Via the Atmospheric Resistive Switching Mechanism of a Hydrogen Anode with a Pt Catalyst Supported on Ta-Doped TiO2

One of the most important issues for polymer electrolyte fuel cells (PEFCs) is the severe degradation of cathode catalysts during startup and shutdown. The degradation is brought about by the reverse current mechanism [1]. In the present study, we propose a new strategy for reducing the reverse current using unique characteristics of an “atmospheric resistive switching mechanism” (ARSM) of the metal oxide semiconductor support, such that the electrical resistivity changes depending on gas atmosphere (Fig. 1) [2].

       10 atom% Ta-doped TiO₂ nanoparticles with a fused-aggregate structure were synthesized by a flame oxide-forming method [3]. The powder obtained was heat-treated at 850 °C in a 4% H₂ atmosphere for 2 h to form Ti_0.9Ta_0.1O_2–δ having a surface area of 14.7 m² g⁻¹. Pt nanoparticles were deposited on the Ti_0.9Ta_0.1O_2–δ by the colloidal method. The powder obtained was heat-treated at 900 °C in a 4% H₂ atmosphere for 2 h to produce 19.8 wt% Pt on the Ti_0.9Ta_0.1O_2–δ support catalysts (Pt/Ti_0.9Ta_0.1O_2–δ). We fabricated two membrane-electrode assemblies (MEAs), one using the Pt/Ti_0.9Ta_0.1O_2–δ and the other a graphitized carbon black-supported platinum (Pt/GCB) (TEC10EA50E, Tanaka Kikinzoku Kogyo K. K. (TKK)) as the anode catalysts. Both of the cathode catalysts were Pt/GCB.

       The electrochemically active surface area (ECSA) values of Pt/Ti_0.9Ta_0.1O_2–δ and Pt/GCB were 30.7 m² g_Pt⁻¹ and 49.3 m² g_Pt⁻¹, respectively. The resistance of the Pt/Ti_0.9Ta_0.1O_2–δ anode cell was showed approximately one order of magnitude greater resistance in air than in hydrogen, which could be explained by the ARSM of Pt/Ti_0.9Ta_0.1O_2–δ. The overpotentials of the hydrogen oxidation reaction (HOR) of both anodes were compared by the H₂ pump method [4]. The HOR overpotential for the Pt/Ti_0.9Ta_0.1O_2–δ anode was negligible, at the same level as that for the Pt/GCB anode, up to at least 1.5 A cm⁻².

       The MEAs were subjected to repeated anode gas switching from air to H₂, thus simulating air/air (for anode/cathode) startup cycling. Figure 2 shows the H₂/air polarization curves measured at 65 °C with 100% relative humidity. Initially, the Pt/Ti_0.9Ta_0.1O_2–δ anode cell showed slightly lower performance than the Pt/GCB anode cell due to the greater loss of oxygen mass transport in the cathode, which was probably caused by more accumulation of generated water than the latter cell, because back-diffusion of the generated water from the cathode toward the anode must be small due to the limited capability for water expulsion via the hydrophilic anode.. On the other hand, the performance of the Pt/Ti_0.9Ta_0.1O_2–δ anode cell after 1000 cycles exceeded that of the Pt/GCB anode cell. The percentages of retention of the ECSA of cathode at 1000 cycles were 64.7% for the Pt/Ti_0.9Ta_0.1O_2–δ anode cell and 42.4% for the Pt/GCB anode cell. The retention percentages of mass activity at 0.9 V were 51.4% for the Pt/Ti_0.9Ta_0.1O_2–δ anode cell and 39.1% for the Pt/GCB anode cell. Therefore, the low performance degradation of the Pt/Ti_0.9Ta_0.1O_2–δ anode cell at low current density could be attributed to the low ECSA degradation. At high current density (> 0.4 A cm⁻²), the Pt/Ti_0.9Ta_0.1O_2–δ anode cell showed distinctively higher performance than the Pt/GCB anode cell after 1000 cycles. The thickness of the cathode catalyst layer in the tested MEA with Pt/Ti_0.9Ta_0.1O_2–δ anode was nearly the same as that of the pristine MEA. On the other hand, thinning of the cathode catalyst layer of approximately 40% was observed in the tested MEA with the Pt/GCB anode (Fig. 2 insets). Such a difference indicates that the carbon corrosion of the cathode catalyst during air/air startup cycling was significantly suppressed by the use of the Pt/Ti_0.9Ta_0.1O_2–δ anode. The decrease in the degradations is attributed to a reduction of the reverse current due to a low oxygen reduction reaction rate at the Pt/Ti_0.9Ta_0.1O_2–δ anode, which showed a high resistivity in air atmosphere. These results demonstrate the effectiveness of the ARSM in mitigating the catalyst degradation and the oxygen mass-transport overpotential in the cathode during air/air startup cycling [5].

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2. Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, M. Watanabe, RSC Adv. 4 (2014) 32180.

3. K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta 56 (2011) 2881.

4. K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B631.

5. H. Shintani, Y. Kojima, K. Kakinuma, M. Watanabe, M. Uchida, submitted in J. Power Sources.