The use of fossil fuels has been increasing environmental pollutants, resulting in rapid climate change and environmental pollution. As one of the eco-friendly energy resources that can replace fossil fuels to overcome the environmental problems, fuel cell technology has gained a lot of attention. Among the fuel cell technologies, polymer electrolyte membrane fuel cell (PEMFC) has high power density and low operating temperature, but there are limitations in commercial applications due to its high cost of PEMFC catalysts which remains a major issue in the fuel cell research. In particular, platinum (Pt) used as electrodes in fuel cells shows the highest efficiency for oxygen reduction reaction (ORR), but has the limitation of the noble metal for economic feasibility.
In order to overcome the limitation, research has been steadily conducted to replace the Pt catalyst with alternative catalysts. Among them, Pt-based binary alloys have received attention as a promising alternative catalyst for ORR due to the high efficiency and durability. However, some factors such as complex ORR mechanisms and unclear properties of alloys have made it hard to develop new catalysts.
In this study, the ORR mechanism was analyzed by using density function theory (DFT) on the Pt-based binary catalysts of Pt3M and PtM (M = Co, Ni, Mn, Ir) with a Pt skin layer. The ORR efficiencies of the catalysts were then evaluated by applying electrode potentials and solvation effect that mimic the electrochemical environment of the fuel cell. As results, the efficiency of ORR was evaluated in terms of ORR onset potential (in Volt) and enhanced in the order of PtNi (0.88 V) > Pt3Co (0.81 V) > PtCo (0.81 V) > Pt3Ni (0.81 V) > PtIr (0.80 V) > Pt3Ir (0.73 V) > Pt3Mn (0.62 V) > Pt (0.60 V) > PtMn (0.54 V). Also, from the ORR mechanism analysis, it was demonstrated that a weaker oxygen adsorption strength of approximately 0.5 eV of a new catalyst than that of the pure Pt surface would result in an enhanced ORR efficiency of fuel cell. This can be used as a powerful descriptor that can readily evaluate the ORR efficiencies of catalysts. In addition, the durability of the Pt-based binary alloys was evaluated by calculating the segregation energies of M atoms from the Pt3M and PtM catalysts, demonstrating that the durability was enhanced in the order of Pt-Ni > Pt-Mn > Pt-Co > Pt-Ir > Pt.
In conclusion, the Pt-Ni alloy system (Pt3Ni and PtNi) was found to be the best catalyst among the platinum ? based binary alloys investigated in terms of both ORR efficiency and durability. Furthermore, the developed activity prediction descriptor based on the simple calculation of the oxygen adsorption strength may be applied to screening potential candidates of Pt ? based catalysts by efficiently evaluating the ORR efficiencies of a number of new catalyst materials. The fundamental mechanism analyses of the current study would provide useful insight on the development of Pt-based active cathode catalysts of fuel cells.