Porous carbon supports prepared by ultrasonic spray pyrolysis for direct methanol fuel cell electrodes

Abstract

Carbon powders composed of porous micrometer-sized spheres were synthesized from simple organic salt precursors using ultrasonic spray pyrolysis (USP). These materials were tested as catalyst supports for a direct methanol fuel cell (DMFC) catalyst and as pore formers in a membrane electrode assembly (MEA). The effect of these materials on unit cell performance was evaluated and compared to traditional Vulcan XC-72 carbon nanoparticle powder. USP provides a simple and facile way to prepare porous carbons with various morphologies and pore sizes. In exploring these new morphologies of carbon, two types of micrometer-sized spherical porous carbons were tested as pore-forming additives for both the anode catalyst (i.e., PtRu/C for methanol oxidation) and the cathode catalyst (i.e., Pt/C for O-2 reduction). The anode catalyst mixture of PtRu/Vulcan and PtRu/PC-I (weight ratio 2:1, 33 wt % PC-I) showed the highest performance improvement and is attributed to the synergic effect of two carbons (PC-I and Vulcan XC-72); the mixture improves the effective mass transport of reactant methanol and products while maintaining a reasonably high conductivity. For the reduction of O-2 at the cathode, the addition of relatively small amounts of carbon microspheres (PC-II) significantly improved the performance of the cathode when they were added to the E-TEK commercial Pt/C catalyst. Even at a relatively slow airflow rate, the maximum power density of the MEA with 1.25 wt % PC-II was maintained without a significant decrease. O-2 is, of course, much less viscous than methanol, and consistent with that, the amount of pore loading required is much less for the cathode than for the anode (i.e., 1.25 wt % vs 33 wt %). These results demonstrate that the inclusion of carbon microspheres is an effective way to facilitate the mass transport of air and methanol, emphasizing the importance of pore structure at both the cathode and anode in the development of efficient self-breathing direct methanol fuel cells.