Tomoyuki Tada

Effect of Particle Size of Platinum and Platinum-Cobalt Catalysts on Stability Against Load Cycling

Tanaka Kikinzoku Kogyo KK, Technical Centre, 2–73 Shinmachi, Hiratsuka, Kanagawa 254-0076, Japan; *E-mail: k-matsutani@ml.tanaka.co.jp To investigate the effect of load cycling, platinum (Pt) and platinum-cobalt (PtCo) fuel cell catalysts with different particle sizes were prepared and evaluated for their durability against load cycling. The particle size of the Pt and PtCo catalysts was controlled by changing the catalyst loading and by applying heat treatment. Pt catalysts with particle sizes of 2–3 nm and 4–5 nm and PtCo catalysts with sizes of 3–4 nm, 4–5 nm and 7–8 nm were obtained. A potential sweep from 0.65 V to 1.05 V was applied to the cathode of membrane electrode assemblies (MEAs) prepared with these catalysts,and the degradation of their mass activity and cell voltage were evaluated. As a result of this investigation, it was found that Pt catalysts with particle sizes of 4–5 nm and PtCo catalysts of particle sizes 7–8 nm showed better stability against potential sweep, with the Pt catalysts of sizes 4–5 nm showing the best stability of all the catalysts tested.

Effect of the Kind of the Cathode Catalyst on the Cell Performance under Low and High Relative Humidity

Both the lack and the excess of water in the cathode catalyst layer of polymer electrolyte fuel cell (PEFC) causes the loss of cell performance. The lack of water in the cathode catalyst layer causes the decrease of the proton conductivity of binder ionomer [1, 2], catalyst utilization [3], and so on. On the other hand, the excess water produced in cathode catalyst layer inhibits the gas transport [4]. Liu et al. had reported the effect of RH of the cell voltage at 0.1 A cm including the ratio of ionomer binder to carbon, in the case of the Pt / Vulcan catalyst [1], and concluded that the dominant cell voltage loss under lower RH was due to the decrease of protonic conductivity of ionomer and not due to the kinetic loss of oxygen reduction reaction. However, the effect of the kind of the catalyst under low and high relative humidity (RH) was not examined in detail up to date. Here we report the effect of the kind of the cathode catalyst on the cell performance under low and high RH, including the different carbon support, the heat treatment of catalyst, and the ionomer / carbon ratio in the cathode catalyst layer. Three kinds of carbon were used as cathode catalyst support. Vulcan (Surface Area (SA)= c.a. 250 m g), Graphitized Vulcan (SA= c.a. 85 m g), and HSAC (SA= c.a. 800 m g). The weight ratio of platinum to catalyst was around 50% in all catalysts. Nafion NR212 and DE521CS were used as membrane and binder ionomer, respectively. Cell voltage was examined under these conditions; anode and cathode gas was H2 and air, respectively. Pt loading was 0.4 mgPt cm. Cell temperature was 80C. The actual RH of the cell was difficult to be measured; therefore, the temperature of humidifier was instead used to indicate RH. First, the effect of the carbon support with the different surface area on the cell performance under low and high RH was examined. Figure 1 shows the cell voltages under various RH at the current density of 0.8 A cm. As the decrease of RH, the cell voltage was also decreased in all cases. However, no significant difference was observed among three carbon supports though their carbon surface area was from 85 m g to 800 m g. Heat-treatment process on catalyst was frequently used to enhance the stability, for example, against potential cycling. To examine the effect of the heat-treatment process on the cell performance under low or high RH was important. Figure 2 shows the effect of the heat treatment of the Pt / HSAC catalyst on the cell voltages under various RH. In the case of no heat treated catalyst, the cell voltages at 0.2 A cm under the lowest RH condition (the temperature of humidifier was 60C) were around 30 mV lower than the cell voltage under the highest RH condition (the temperature of humidifier was 90C), respectively. It indicated that the loss of the cell voltage under low RH independent of the catalyst (for example, due to the protonic resistance of membrane or binder ionomer) was less than 30 mV in this operating condition. However, in the case of the catalyst annealed at 900C, the loss of the cell voltage under low RH was around 70 mV and higher than that of no heat treated catalyst, though the kind of membrane and the kind and the amount of binder ionomer were identical. It was indicated that the catalyst itself affected the cell voltage loss under low RH. In addition, at the current density of 0.8 A cm, the cell voltage losses from the highest RH condition to the lowest RH condition in the case of no heat treated catalyst and catalyst annealed at 900C were around 100 mV and 280 mV, respectively. Wash treatment by acid recovered the catalytic activity, but not completely. To explain the difference of the behavior under lower RH among these catalysts, the surface condition was examined. The quantitative analysis of acidic organic groups in the surface of catalysts was progressing. The optimization of the amount of ionomer in the electrode was also important to obtain the better cell performance. The correlations of the ionomer / carbon ratio under lower and higher RH among the different kind of catalysts were also in progress.