Development of Metal Oxide-Supported Metal and Ordered Intermetallic Nanoparticles to Enhance the Oxygen Reduction Reaction in PEMFC

Abstract

Recently, there has been an increased interest in polymer electrolyte membrane fuel cells (PEMFCs). PEMFCs have energy conversion efficiencies as high as 80%. However, the durability of PEMFCs has been recently recognized as one of the most important issues to be addressed before the commercialization of PEMFCs. Pt surface area loss due to carbon corrosion and Pt nanoparticle (NP) dissolution from the carbon support and/or aggregation on the carbon support are considered one of the major problems to address. Carbon black (CB) is normally used as a support material for the Pt catalyst to maximize the mass activity of the catalyst. However, the oxidized sites of the CB accelerate the degradation of the support material for Pt NPs. The CB will oxidize at edge sites because polar functional groups can form at those sites. The polar functional groups will be further oxidized and finally corroded away. Recently, carbon nanotubes (CNTs) have been proposed as promising support materials for Pt NPs because the CNT supports exhibit high conductivity and mass transport capability as well as high chemical stability. However, there are no novel binding sites for adsorbing Pt ions on the CNT surfaces, although the surfaces of CNT are composed of non-oxidized graphitic carbon. Usually, functional groups are generated on the external walls to make the binding sites for Pt source using harsh oxidative treatments, such as refluxing in HNO3 or H2SO4-HNO3. However, the treated CNT surfaces will suffer serious carbon corrosion under conditions such as low pH, high potential, high humidity, and high temperature (~80 °C). A new methodology for the deposition of catalyst NPs on CNT surfaces should be developed that satisfies both requirements for high NP dispersion and chemical stability. Recently, nanocrystalline TiO2 as a catalyst support has received increasing attention due to its inherent stability in the electrochemical environment, its commercial availability, and its enhancement of electrocatalytic activity due to its corrosion resistance and the synergistic effect between NPs and TiO2. We considered the preparation of PtPb/TiO2 on cup-stacked carbon nanotube (CSCNT) to enhance the oxygen reduction reaction (ORR). CSCNTs are tubular carbon nanostructures with a stacked cup arrangement of graphene layers. Therefore, the edges of the graphene layers are densely exposed on the surface of the CSCNT. The graphene edges are used as scaffolds to thoroughly coat the CSCNT surface with TiO2 layers. The PtPb ordered intermetallic NP electrocatalyst was selectively deposited on the TiO2 layers. The CSCNT functions as an electron conducting path. Recently, we reported that PtPb/TiO2 showed enhancement of electrocatalytic activity for the ORR [1]. The nature of the support, the composition of catalytic sites, the sites’ interaction with the support, and the electronic structure of the catalytic sites all most likely influenced the observed electrochemical behavior. In this study, PtPb NPs were chemically deposited on small, thin TiO2 layers that were prepared on CSCNTs. The unique catalytic property of PtPb ordered intermetallic NPs and the strong interaction between PtPb NPs and TiO2 layers successfully achieved the enhancement of ORR in acidic aqueous solutions. The improved ORR performance of PtPb NPs/TiO2/CSCNT is not an effect of the CSCNT support material. Rather, the higher performance is due to the interaction between PtPb NPs and TiO2. The CSCNTs can provide a large amount of structure to PtPb NPs on TiO2 on the CSCNT surface. [1] T. Gunji, G. Saravanan, T. Tanabe, T. Tsuda, M. Miyauchi, G. Kobayashi, H. Abe, F. Matsumoto, Catalysis Science and Technology, 2014, 4, 1436.