Pt/C and PtCo/C electrocatalysts are well known as highly active cathode catalysts for PEFCs (1). In the previous work, in order to exceed activity of the existing catalysts, we investigated oxygen reduction reaction (ORR) activity of PtCoX (X = Mn, Ni, Fe, Cr, Ti, Cu, Zn, Mo, V, Sn, In, Pd, Ir, Ru, Au) / high surface area carbon (HSAC, Surface area of carbon = ca. 800 m²/g) ternary alloy catalyst (2). PtCoMn / HSAC showed the highest ORR activity in all of the catalysts evaluated. The ORR activity of PtCoMn / HSAC was two times higher than that of Pt / HSAC measured in MEA. In addition, we evaluated the stability of PtCoMn / HSAC in MEA by voltage cycling test. As a result of this test, it was found that the stability of PtCoMn / HSAC was as low as the existing Pt and PtCo catalysts. To understand the mechanism of decreasing ORR activity of PtCoMn / HSAC, we investigated MEA which was used in voltage cycling test by SEM / EDX and EPMA line analysis. From results of the investigation, it was found that not only Pt but also Co and Mn in the PtCoMn catalyst dissolved during the test. It was considered that the stability of PtCoMn / HSAC would be improved by suppressing the dissolution of the metal in the PtCoMn / HSAC. In this study, in order to improve the stability of the existing PtCoMn catalysts, we tried to prepare protective layer on the catalyst. In addition, we evaluated the stability of the catalyst.

Fig. 1 shows mass activities and electrochemical surface area (ECSA) of the existing PtCoMn and Improved-PtCoMn catalyst with protective layer before and after the voltage cycling test. In addition, the result of the existing Pt catalyst shows as reference. As a result of this test, ORR activity and ECSA of the existing PtCoMn catalyst decreased gradually in each test. On the other hand, ORR activity and ECSA of the Improved-PtCoMn catalyst was higher than the existing catalyst. These results suggest that protective layer on the PtCoMn catalyst has good effective for improving catalyst stability against the voltage cycling test.

To understand the PtCoMn dissolution phenomenon of PtCoMn catalyst, we investigated the MEA before and after the voltage cycling test by TEM. Fig. 2 shows TEM images of cross-section of the MEA before and after the test. In the case of the existing PtCoMn catalyst, a lot of huge metal particles were observed after the test. These results suggest that Pt in the PtCoMn catalyst dissolved and turn to be Pt ions, and they were re-deposited as Pt metal particles during the test　(3). In contrast, in the case of the Improved-PtCoMn catalyst, the huge metal particles were less compare to the existing catalyst. Fig. 3 shows average particle size of the existing PtCoMn catalyst and Improved-PtCoMn catalyst before and after the voltage cycling test. In the case of the existing catalyst, the average particle size was significantly increased. On the other hand, in the case of the Improved-catalyst, the growth of the catalyst particle was suppressed. These results suggest that PtCoMn dissolution was prevented by the protective layer. Therefore, the stability of the catalyst was improved.

1. K. Matsutani, K. Hayakawa, T. Tada, Precious Metal Review, 54(4), 223-232 (2010).

2. M. Ishida, K. Matsutani, ECS Trans., 64(3), 107-112 (2014).

3. M. Ishida, K. Matsutani, ECS Trans., 69(17), 651-656 (2015).