Oxide-Supported PEFC Electrocatalysts

Abstract

Even after the start of the commercialization of fuel cell vehicles from December 2014, further improvement of PEFC electrocatalysts is still desired for the next-generation PEFCs. While the Pt-based electrocatalyst supported on carbon black is the state-of-the-art material, further development using alternative electrocatalyst support materials are helpful especially to improve the durability of electrocatalysts for e.g. FCV applications. Among various degradation mechanisms, start-stop-cycle up to a high potential can cause carbon corrosion related degradation [1-3], one of the major degradation mechanisms of PEFCs. Various alternative carbon-based electrocatalyst support materials have been proposed so far, including graphitized carbon black [4], meso-porous carbon [5], graphene [6], and carbon nanofibers [7]. This paper gives an overview on the materials design principles to tailor oxide-supported PEFC electrocatalysts. In general, as thermochemically calculated and shown in Fig. 1 [8], oxides e.g. SnO2 could be alternative support materials to the conventional carbon black [9-13]. Thermochemical stability is essential with which only several oxides could be stable in the Nafion-containing strongly-acidic PEFC environment. It has been revealed that doped SnO2 is a promising electrocatalyst support material with the 60,000-cycle durability between 1.0 and 1.5 VRHE[13]. In addition, specific surface area has to be larger to prevent agglomeration of Pt-based catalyst nanoparticles, while an increase in oxide surface area leads to an increase in the ohmic resistance as electrons involved in the oxygen reduction reaction (ORR) should be transported across more grain boundaries. Lower electronic conductivity could be one of the major reasons for poor ORR activities. In order to improve catalytic and electrochemical performance of such oxide-supported PEFC electrocatalysts, one possible solution is to tailor highly-conductive pathways within the electrocatalyst layers. Figure 2 shows the I-V characteristics of MEAs using Pt/Nb-doped SnO2electrocatalysts to which highly-conductive vapor-grown carbon nanofibers (VGCF) have been mixed, as shown in Figure 3. Carbon nanofibers can act as the fast transport pathways for electrons involved in the ORR processes at the PEFC cathodes. Figure 2 clearly demonstrate that I-V characteristics of MEAs using the oxide-supported electrocatalysts approach those using the standard Pt/C electrocatalyst. Further improvement may be made by using such conductive fillers as the framework of the electrocatalysts. Such conductive-pathway-integrated electrocatalysts exhibit satisfactory electrochemical performance in case the Pt catalyst particles could be impregnated selectively on the oxide surfaces, rather than on the carbon surfaces on which Pt catalysts accelerate carbon corrosion. The cell performance depends strongly on the microstructure of the electrocatalyst layers, and thus the critical preparation conditions including the catalyst-Nafion ratio and the catalyst-filler ratio. The importance of the 3-dimentional optimization of the electrocatalyst layer will also be discussed based on microstructural characterizations of the cells. The materials design principles and technological issues in applying such oxide support materials to PEFC electrocatalyst layers are summarized and discussed. Acknowledgement: Financial support by the Grant-in-Aid for Scientific Research (S) (No. 23226015), JSPS Japan, is gratefully acknowledged.