Activity and stability of Pt_xCr/C catalyst for oxygen reduction reaction: Effect of the Pt:Cr ratio and heat treatment atmosphere

A series of Pt_xCr/C catalysts (where x is the Pt atomic number, equal to 1, 2, 3, 4 and 5) was prepared by impregnation-seeding technique for the oxygen reduction reaction (ORR). The effect of the Pt:Cr ratio (1:1 to 5:1) and the heat-treatment atmosphere (H₂ or N₂) on the ORR activity and stability were explored in an acid solution and in a PEM fuel cell under H₂/O₂ and H₂/air environments. The preliminary results demonstrated that increasing the Pt:Cr ratio increased the electrochemical surface area (ESA) of the catalyst, but decreased the degree of alloying. The heat treatment of the Pt_xCr/C catalyst under H₂ or N₂ environments induced an increased degree of alloying, but decreased the ESA as well as the formation of different catalyst morphologies, which consequently affected the ORR activity and stability of the Pt_xCr/C catalyst. In the absence of heat treatment, the Pt₃Cr/C catalyst provided higher ORR activity and stability than Pt/C and other Pt_xCr/C catalyst. In the presence of heat treatment, the Pt₃Cr/C(N₂) catalyst exhibited the highest ORR activity in both acid electrolyte and in PEM fuel cell at 60 °C and ambient pressure. It also provided the highest stability in the acid solution compared with the other Pt_xCr/C catalysts.