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).