Development of Pt-Co-Mn Ternary Alloy Catalyst for PEFCs

Pt/C and PtCo/C electrocatalysts are well known as a highly active cathode catalyst for PEFCs [1]. In order to exceed activity of the existing catalysts, we investigated PtCoX ternary alloy catalyst. In this study, 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 catalysts were synthesized and evaluated by oxygen reduction reaction (ORR) activity. In addition, these catalysts were characterized by XRD, TEM-EDX and cyclic voltammetry.

Fig. 1 shows mass activities of various PtCoX alloy catalysts. The order of ORR activities of PtCoX catalysts were as follows; Mn > (PtCo), Cr, Ti, Ni, In > Pd, Zn, Fe > Mo, Ir, Ru > (Pt), V > Sn > Cu > Au. 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.

To understand the mechanism of expressing activity of PtCoMn / HSAC catalyst, we picked up three samples of three different states from preparation process and evaluated the ORR activities. The preparation method for PtCoMn / HSAC alloy catalysts was as follows. First, Co(II) and Mn(II) ions were reduced and precipitated onto the platinum particles on HSAC in aqueous solution by using reduction agent (referred as (i) Reduction in Fig.2).  Second, this raw material was heat-treated at high temperature in the reduction atmosphere in order to make alloy particles (referred as (ii) Heat treatment in Fig.2). It was well known that Pt and transition metals (such as Co, Ni etc.) in the cathode catalyst dissolve under the severe conditions (such as low pH, highly positive potential and high temperature) during the fuel cell operation. The dissolved metal species cause decrease in the conductivity of protons in proton exchange membrane such as Nafion membrane. Therefore, Pt-alloy catalysts were generally treated in acid solution to leach the excess transition metals from the surface of the alloy particles in advance of the operation [2, 3]. Hence, the PtCoMn / HSAC catalyst was treated in acid solution, too (referred as (iii) Acid treatment in Fig.2).

Fig. 2 shows the ORR activities of three catalysts. In the case of (i), ORR activity was quite low. However, (ii) showed higher ORR activity than that of (i). These result indicated that it was necessary for PtCoMn / HSAC to be carried out the heat treatment in order to express the ORR activity. In case of (iii), the ORR activity was almost same as (ii). This result shows that acid-leaching treatment for the PtCoMn alloys catalyst didn’t effect on ORR activity of initial performance.

In order to make clear the mechanism of expressing activity, we investigated the crystal structures of PtCoMn / HSAC with and without heat treatment by XRD. Fig. 3 shows XRD patterns of PtCoMn / HSAC catalysts with and without the heat-treatment. In case of (i), only the profile of pure Pt was observed in the XRD pattern. However, XRD profile showed the existence of Pt₃Mn and Pt₃Co in the pattern of (ii) and (iii). The formation of these alloys and the expressing ORR activity correlated to each other. These results suggest that the Pt₃Mn and Pt₃Co alloys are active species for ORR in case of PtCoMn / HSAC ternary alloy catalyst. 

To evaluate the stability of PtCoMn / HSAC, we utilized the voltage cycling test. As a result of this test, it was found that the stability of PtCoMn / HSAC was not comparable to existing PtCo alloy catalyst. Hence, improvement of the stability of PtCoMn / HSAC ternary alloy catalyst could be next target.

[1] K. Matsutani, K. Hayakawa, T. Tada, Precious Metal Review, 54, (4), 223-232 (2010). [2] S. Takenaka, N. Suzuki, H. Miyamoto, E. Tanabe,  J. Catalysis, 279, 381-388 (2011). [3] N. M. Marckovic, V. R. Stamenkoviv, J. Am. Chem. Soc. 133, 14396-14403 (2011)