An important issue that has limited the potential of Solid Oxide Fuel Cells (SOFCs) for portable applications is its high operating temperatures (800-1000 ºC). Lowering the operating temperature of SOFCs to 400-600 ºC enable a wider material selection, reduced degradation and increased lifetime. On the other hand, low-temperature operation poses serious challenges to the electrode performance. Effective catalysts, redox stable electrodes with improved microstructures are the prime requisite for the development of efficient SOFC anodes. The performance of Nb-doped SrT iO3 (STN) ceramic anodes with various loadings of Ni-CGO by infiltration technology is illustrated. The Knudsen and bulk gas phase diffusion in the porous electrode and its influence on the gas diffusion impedance have been studied in detail. The new contributions of these studies includes combined electrocatalysts materials (Ni, Pt, Pd, Ru and CGO (Gd-doped CeO2), which could significantly enhance the electrode performances. The best electrode performance is obtained using ternary catalyst i.e., anode polarization resistance of 0.1 and 0.3 Ohm cm2 at 600 and 500ºC, respectively for the combination of Pt, Ni and a CGO. At 400ºC, the polarization resistance was 2 Ohm cm2, these values were determined in moisturized hydrogen. STN based ceramic anodes have been prepared with the novel concept of a palladium functional layer at the interface of electrode/electrolyte. Pd nanoparticles formed at the interface have a significant effect in reducing the polarization losses compared to PdCGO infiltrated anodes without modifications. Similarly, the interfacial modification of the electrode/electrolyte with a ceramic functional layer (e.g. CGO) infiltrated with Pd-CGO electrocatalysts, yielded rather a low polarization resistance of 1.5 Ohm cm2 at 400ºC. The potential of using WO3 ceramic as an alternative anode materials has been explored. The relatively high electrode polarization resistance obtained, 11 Ohm cm2 at 600 ºC, proved the inadequate catalytic activity of this system for hydrogen oxidation. At the end of this thesis, an investigation on the effect of application of cathodic polarization on Ni-YSZ anodes is described.
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Høgh, Jens Valdemar Thorvald, Bonanos, Nikolaos
Department of Energy Conversion and Storage, Technical University of Denmark, 2012