Investigation of Stability Reduction in PtCoMn Ternary Alloy Catalyst for PEFCs

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. Therefore, we focused on the development of PtCoMn / HSAC catalyst. In this study, we evaluated the stability of PtCoMn / HSAC in MEA by voltage cycling test (650 mV-1050 mV). In addition, the MEA were investigated by SEM / EDX and EPMA line analysis.

To evaluate the stability of PtCoMn / HSAC, we utilized the voltage cycling test. Fig. 1 shows mass activities and electrochemical surface area (ECSA) of Pt, PtCo and PtCoMn catalysts before and after the voltage cycling test. As a result of this test, ORR activity and ECSA of PtCoMn catalyst decreased gradually in each test. It was found that the stability of PtCoMn / HSAC was not comparable to existing Pt and PtCo catalysts.

To understand the mechanism of reducing ORR activity of PtCoMn / HSAC, we investigated MEA which was used in voltage cycling test by SEM / EDX. For comparison, MEA before testing was investigated in the same way. Fig. 2 shows SEM image of cross -section of the MEA which was used in the test. A lot of particles were found at the interface of the membrane and the cathode catalyst. On the other hand, there were no particles in the fresh MEA.

To investigate the particles which are composed of any elements, we utilized EDX mapping analysis. Fig. 3 shows SEM / EDX mapping analysis of cross -section of the MEA which was used in the test. As a result of analysis the particles, Pt element was detected. On the other hand, Co and Mn elements were not found in the particle. It became clear that the particles are composed of Pt. These results suggest that Pt in the PtCoMn catalyst dissolved and turn to be Pt ions, and then they were re-deposited as Pt metal particles at the interface of the membrane and the cathode catalyst during the voltage cycling test.

To understand the Pt dissolution phenomenon of PtCoMn / HSAC, we utilized cross-section of MEA by EPMA line analysis. Fig. 4 shows EPMA line analysis of cross -section of the MEA which was used to the test. As a result of analysis, Co and Mn elements were detected in the Nafion membrane. On the other hand, these transition metal elements did not observed in the fresh MEA. The intensity of Pt, Co and Mn elements decreased gradually in the interface of the membrane and PtCoMn cathode catalyst, and the intensity of Pt element increased significantly at the interface of the membrane and the cathode catalyst with the generation of particles. These results suggest that not only Pt but also Co and Mn in the PtCoMn catalyst dissolved during the voltage cycling test. And these results indicate that Pt, Co and Mn in the cathode catalyst dissolved from the near of the interface between the membrane and PtCoMn cathode catalyst. From the above results, it is considered that the stability of PtCoMn / HSAC would be improved by suppressing the dissolution of the metal in the PtCoMn / HSAC.

[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).