Polymer electrolyte membrane fuel cell (PEMFC) is a promising energy conversion system to replace the conventional power generators operating on fossil fuels. In the PEMFC, there are two main reactions; oxygen reduction reaction (ORR) at the cathode and hydrogen oxidation reaction (HOR) at the anode. Theoretical equilibrium voltage of PEMFC is 1.23 V, but actual PEMFCs produce lower cell voltages mostly due to slow kinetics of ORR. In order to increase oxygen reduction reaction rate, Pt catalyst is used in PEMFC. Moreover, in fuel cell electric vehicle (FCEV), durability of Pt/C catalyst is a crucial issue to be overcome for commercialization. In FCEV, high potentials (> 1.4 V) are present at the cathode upon startup and shutdown cycles. When exposed to such high potentials, carbon support oxidizes to carbon dioxide. Corrosion of carbon support is one of the main reasons for degradation of Pt/C catalysts. As a result, platinum nanoparticles become detached from the carbon and the cell performance degrades.

To prevent corrosion of carbon and degradation of cathode, a variety of metal oxides have been explored. SnO₂ and TiO₂ are promising candidates to replace carbon support because of their stability under the acidic and high potential condition. However, although those metal oxides have been doped with other metals in order to improve their electrical conductivity, such as Nb-doped TiO₂^[1], In_2-xSn_xO₂^[2], metal oxides have not yet fulfilled the requirements as a catalyst support in terms of electrical conductivity, stability and cell performance. On the other hand, HOR-selective catalysts can be a solution to prevent the cathode carbon corrosion upon startup/shutdown. To achieve HOR-selectivity, an organic molecule, e.g., calix[4]arene, has been applied to Pt surface.^[3] However, HOR-selectivity and stability of the molecule-treated Pt have not yet been demonstrated through single cell tests.

Oxygen evolution reaction (OER)-functional ORR catalysts are also expected to prevent the cathode carbon corrosion effectively. When high voltage is present at the cathode, OER-functional catalyst can oxidize water instead of carbon support. In this study, we developed PtIr/C catalysts for ORR & OER bifunctional catalysts to prevent carbon corrosion at cathode. We synthesized PtIr/C catalyst with various ratio of Pt and Ir using polyol method at 160 ^oC. Synthesized PtIr nanoparticles had uniform distribution on carbon black with average particle size of 4 nm. With increasing of Pt to Ir ratio in the alloy catalysts, PtIr/C showed higher ORR and lower OER catalytic activity. With an optimal composition, PtIr/C exhibited slightly lower initial ORR activity than Pt/C and superior stability during high-voltage cycling up to 1.5 V with half-cell and single cell configuration. Interestingly, PtIr/C catalyst showed higher mass activity and better stability than simple mixture of Pt/C and Ir/C during the high-voltage cycling.

References

[1] Kim et al. Scientific Reports, 7, 44411 (2017)

[2] Schmies et al. Adv. Energy Mater., 8, 1701663 (2018)

[3] Genorio et al. Nat. Mater., 9, 998-1003 (2010)