Anodes for solid oxide fuel cell (SOFC) systems operating in multiple fuel environments: Effects of microstructure and composition

This work examined several cermet materials as possible anode materials for solid oxide fuel cell (SOFC) systems operating in multiple fuel environments. In particular, these materials consisted of two phase M-YSZ (M = Ni, Cu or Co, YSZ = yttria stabilized zirconia) cermets synthesized via the gel-precipitation method. Relations between synthesis conditions and cermet structure/performance dominated the first section of this work. SOFC anode performance was correlated back to original synthesis conditions, whereby dM/dYSZ was controlled via the production of a crystalline intermediate.;Electrochemical characterization of Ni- and Co-YSZ anodes in H2 and H2S/H2 fuels was performed and each arc observed in the impedance curves was ascribed to a specific process. The high- and mid-frequency arcs were related to transport and charge transfer processes respectively. Non-traditional relationships between inverse Tafel (Lefat) slopes and temperature were observed. This resulted in a change in charge transfer coefficients with respect to temperature. By evaluating this dependence, a charge transfer coefficient of 1.5 for hydrogen oxidation on Ni-, Co- and NiS-anodes was determined. CoS-anodes resulted in a charge transfer coefficient of 0.2.;NiS- and CoS-anodes were stable in sour gas environments and selective towards the electrochemical oxidation of H2 over H2S as suggested by open circuit voltage and mass spectrometry.;Cell performances of and CoS-anodes in sour gas environments were stable over a 6 day period and showed no signs of deterioration. However, H 2S was required in the fuel stream to maintain optimal performance of these anodes. Once H2S was removed, the CoS-anodes reverted to Co-YSZ. For CoS-based anodes, optimum cell performance was maintained at a H2S content >1v/v%.;Investigations of bimetallic NiCu- and NiCo-anodes were also performed. The addition of Cu to Ni-based anodes resulted in larger metal particle sizes and decreased anode activity for hydrogen oxidation. Small additions of Co to Ni-based anodes showed a marked improvement in hydrogen oxidation activity. The bimetallic anodes were also tested in H2S/CH4 mixtures and showed good activity for all anodes tested over a 15 hour period.