Sn02 Aerogels: Towards Performant and Stable PEFC Catalyst Supports
Carbon blacks supported Pt, currently widely used as electrocatalysts in Polymer Electrolyte Fuel Cells (PEFC) are thermochemically unstable in PEFC operating conditions. This is especially true at the cathode side where, on top of relatively elevated temperature (80°C) and acidic conditions, both the potential and the relative humidity may be high. The resulting carbon oxidation is partially responsible for the PEFC performance decrease observed over time. Hence, long term durability still needs to be improved in order to consider PEFC as credible alternatives to conventional power sources for automotive, stationary or portable applications.
Tin dioxide has already been studied as an alternative catalyst support to replace carbon (1, 2). We will show how SnO2 aerogels may even be more promising candidates. Due to their remarkable nanotexturation, aerogels have indeed already proven their ability to efficiently support catalyst for PEMFC application (3).
Our SnO2 aerogels were obtained from supercritical CO2 drying of gels synthetized following an acid catalyzed sol-gel route, starting from metal alkoxides precursors. Nb or Sb doping were realized adding the corresponding dopant alkoxide to the sol. The nanoscale morphology can be controlled during the sol-gel synthesis route and is retained after drying.
The effect of doping with Nb5+ or Sb5+ on the structure and the morphology of the material was investigated by XRD, SEM and nitrogen sorption. Our materials showed reasonable specific surface areas (80-90 m²/g). The bimodal narrow pore size distribution, centered on around 25 and 45 nm after calcination at 600 °C, is particularly well adapted to the foreseen application. Such morphology (see Figure 1) was directed to i) favor both good dispersion and high activity of the catalyst (Pt) and ii) efficiently manage the water produced at the cathode during the oxygen reduction reaction (ORR). All Sb doped samples exhibited an impressive improvement of electronic conductivity, reaching 1 S/cm for 5 at.% Sb. This is representing a 5 orders of magnitude increase compared to pure SnO2. This level of conductivity is almost equivalent to that of carbon Vulcan XC-72R measured in the same conditions (4 S/cm).
These properties make SnO2: Sb a very promising material as catalyst support for PEFC cathode. Our latest results, including platinum deposition and electrochemical activity, will be discussed with respect to the foreseen application.
This work is funded by the FP7 NANOCAT European program (SP1-JTI-FCH.2012.1.5).
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