Mesoporous Platinum Electrodes for Solid Oxide Fuel Cells
Description:... Solid oxide fuel cells (SOFCs) have long offered promise of renewable and highly efficient direct chemical to electrical energy conversion, though their high cost and operating temperatures have limited commercial adoption and applications. Recently, research efforts have focused on reducing the operating temperature by thinning the fuel cell membrane, thereby eliminating the requirement of high-temperature (800∘C to 1000∘C) compatible ceramic electrodes, and allowing the use of more traditional metallic catalysts that were previously limited to lower temperature fuel cell types due to materials compatibility constraints. However, because these operating temperatures (400∘C to 600∘C) remain much higher than those found in low temperature fuel cells, the morphological stability of these metallic catalytic electrodes at these intermediate temperatures is a critical factor impacting fuel cell performance. Our first study investigates the morphological stability of mesoporous platinum thin films deposited via nanosphere lithography on yttria-stabilized zirconia (YSZ). Carboxylate-modified polystyrene nanospheres were spin-coated onto YSZ substrates, diameter-reduced via reactive ion etching, and used as a shadow mask for e-beam platinum deposition. The spheres were then removed by sonication, leaving behind a hexagonal close-packed array of holes in the Pt film. Hole diameters were measured pre- and post-annealing in 5% forming gas and pure oxygen environments, mimicking the anode and cathode environments, respectively, of a fuel cell. Annealing temperatures and times were varied, and the hole evolution model by Srolovitz and Safran was used to extract an activation energy for platinum surface diffusion under each annealing environment. Calculated activation energies were 1.16 eV and 1.22 eV for forming gas and oxygen environments, respectively, with a slightly smaller diffusion prefactor for the latter environment. Our second study evaluates the performance of these mesoporous platinum films as SOFC electrodes. We compare the mesoporous platinum films to standard nanoporous platinum electrodes, using simple SOFC devices with 100 μm thick polycrystalline YSZ as the electrolytes. Four electrode configurations were investigated (noted as cathode-anode): nano-nano, meso-nano, nano-meso, and meso-meso. SEM images of the electrodes before and after testing are compared. We also evaluated the electrical performance of the electrodes by measuring current output as a function of time at constant voltage, current as a function of voltage, and impedance spectra pre- and post-constant voltage testing. The data suggest that, for these devices and operating conditions, oxygen reduction kinetics and cathode triple phase boundary lengths are the main factors limiting performance. Devices with nanoporous cathodes exhibited higher absolute current density compared to devices with mesoporous cathodes. The nanoporous cathode devices also exhibited decreasing current at constant voltage over time, whereas the mesoporous cathode devices reached a stable current after approximately 5 h. SEM images show that the nanoporous electrodes underwent significant morphological change during testing, as compared to the mesoporous electrodes and impedance spectra quantitatively show that the nanoporous cathode devices exhibit greater resistance increase after testing than the mesoporous cathode devices.
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