Electrocatalyst Support Durability

While commercialization of PEFCs has been started in residential application and is planned in automotive application, detailed understanding of degradation phenomena is essential. A large part of PEFC degradation, especially in automotive applications, is known to be related to carbon support degradation, mainly caused by carbon corrosion and related phenomena. During the fuel cell start-stop cycles, the potential of the cathode can reach up to 1.5 V (versus reversible hydrogen electrode, RHE). Fluctuation of cell voltage up to such higher potentials can cause oxidation-induced carbon support corrosion especially for cathode electrocatalysts [1-3].

 In this tutorial talk, after summarizing typical degradation mechanisms with the state-of-the-art carbon black support, various efforts to develop alternative electrocatalyst support materials are reviewed. Table 1 compiles possible electrocatalyst support materials considered recently. There are three possible strategic directions in developing alternative durable supports as shown in Fig. 1: (1) graphitized carbon black, (2) alternative carbon support, and (3) alternative non-carbon support.

 (1)     Graphitized carbon black 

While carbon is thermochemical unstable under the PEFC cathode condition, carbon corrosion can be kinetically slow for graphitized carbon black [4]. As graphite is relatively stable, so that higher temperature heat-treatment to increase the degree of graphitization is a useful procedure to ensure longer-term durability.

 (2)     Alternative carbon support

Various nanostructured carbon-based materials, such as mesoporous carbon [5] and graphene [6] may be applicable to design nanostructure of electrocatalysts. Generally speaking, graphene-like surface is stable against carbon corrosion, but Pt particles on the graphene surface tend to agglomerate each other [7]. More defective surfaces are suitable to stabilize Pt particles but such surface defects could be a site where carbon corrosion may start.

 (3)     Alternative non-carbon support such as oxides

As alternatives to the conventional carbon black electrocatalyst support, conductive oxides are mainly considered as carbon-free catalyst support materials [8-10]. Thermochemical calculations have revealed, as shown in Fig. 2 [8], that several oxides could be stable under the PEFC cathode conditions.

 As an example, Figure 3 shows the I-V curves before and after such start-stop cycle tests for an MEA with the cathode electrocatalyst layer consisting of the Pt/Sn_0.98Nb_0.02O₂ mixed with 5wt % vapor grown carbon fibers (VGCF-H) as the electron-conductive filler. Start-stop cycle degradation was almost negligible up to 60,000 cycles. This result confirms that Nb-doped SnO₂ can be an alternative electrocatalyst support material for e.g. fuel cell vehicles (FCVs) which are suffered from the voltage cycling up to a high potential. Materials design principles for PEFC electrocatalyst supports will be discussed.

Acknowledgement

Financial support by the Grant-in-Aid for Scientific Research (S) (No. 23226015), JSPS Japan, is gratefully acknowledged.

References

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