Surface Modifications of Pt Cathode Catalysts in Polymer Electrolyte Fuel Cells with Site-Selective or Nonselective Overlayers

For the wider commercialization of polymer electrolyte fuel cells (PEFCs), it is essential to improve the long-term durability of cathode catalysts, which are exposed to harsh conditions of potential cycles at low pH and high temperature (> 60°C) during the operation. Improving the durability without sacrificing the i-V performance is, however, a big challenge because those two properties have often a trade-off relation. Surface modifications of Pt catalysts by foreign materials are a promising method for improving both the activity and durability, and as the materials, ionic liquids [1-3] and a carbon thin layer formed by the heat-treatment (700°C) of dopamine [4] have been reported. The roles of the modifiers in the improvements, however, have not been clarified, and are studied in the present study. The role of ionic liquid (IL) was examined by using a stepped Pt single crystal. Figure 1 shows the cyclic voltammograms (CV) of Pt (443) in an aqueous electrolyte of 0.1 M HClO4 with and without the modification by the IL of [C4C1im][NTf2]. Only the Pt(110) step peaks for underpotential hydrogen deposition (Hupd) disappeared by the modification while the Pt(111)-terrace plateaus remained, and thus the IL molecules are judged to be selectively adsorbed on the step sites. This behavior is reminiscent of the results with the modification by a solution of Au complex, which exhibited selective depositions of Au atoms on step sites and improvements in both the activity and stability of Pt surface [5]. Although the improvements in the activity and stability by the IL has not been confirmed for the Pt single crystal electrode, where the IL molecules face the bulk electrolyte and can easily dissolve away, the CV results can explain the reported beneficial effects of ILs on high-surface-area Pt catalysts [1-3], where IL molecules can remain in micro pores of the catalysts for a long term. Analyses with surface-enhanced infrared absorption spectroscopy (SEIRAS) are underway to clarify the adsorption state of the IL molecules on Pt surface. The effect of the dopamine modification followed by the heat-treatment (DM-HT) was examined using a carbon-supported single-metal Pt nanoparticles (Pt/C) catalyst in the present study whereas a Pt-alloy catalyst (PtFe/C) was used in the previous study [4]. The analysis with the single-metal Pt/C enables us to evaluate the effect of the carbon thin layer separately from that of the formation of ordered intermetallic PtFe phase through the heat-treatment. Figure 2a shows the effect of the DM-HT on the activity of Pt/C. The results show the increase in the ORR activity accompanied by the suppression of sulfonate adsorptions on the Pt surfaces with the increase in the amount of dopamine added to the cathode catalyst, indicating the blockages of detrimental sulfonate adsorptions from ionomer by the carbon layers. The DM-HT Pt/C also exhibits a larger electrochemical surface area (ECSA) throughout potential cycles and thus a higher durability than the pristine Pt/C and a heat-treated Pt/C without the dopamine modification (HT-Pt/C) as shown in Fig.2b. The effect of the carbon layer on the O2 transport property near the catalyst surface (power density of the cell) will also be discussed in the meeting. References [1] Snyder et al., Nat. Mater., 9, 904 (2010) [2] Zhang et al., Angew. Chem. Int. Ed., 55, 2257 (2016) [3] Huang et al., J. Electrochem. Soc., 164, F1448 (2017) [4] Chung et al., J. Am. Chem. Soc., 137, 15478 (2015) [5] Kodama et al., J. Am. Chem. Soc., 138, 4194 (2016) Figure 1