Correlating Electrochemical Performance with In Situ Optical Spectroscopy in Solid Oxide Fuel Cells
Eigenbrodt, Bryan Christopher
Walker, Robert A
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Solid oxide fuel cells are versatile electrochemical devices that can operate with a wide variety of carbon containing fuels. However, SOFCs operate at high temperatures (650ªC) making in situ characterization of material properties and reaction mechanisms, associated with electrochemical fuel oxidation, difficult to perform. Experiments described employed in situ vibrational spectroscopy and standard electrochemical methods to characterize the properties of SOFC electrolytes and anodes at temperatures up to 715ªC. Collectively these methods address long standing questions about fuel oxidation and degradation mechanisms. One investigation coupled in situ Raman with surface specific EIS measurements to show that yttria stabilized zirconia electrolytes form a slightly conductive, surface reduced layer under anodic conditions commonly encountered during SOFC operation. Polarizing the SOFC further depleted oxide ion concentrations close to the three phase boundary. Additional ex situ XAS measurements confirmed that only zirconium, in the crystal structure, plays a role in the formation of conductive surface states in reduced YSZ. A second project combined electrochemical measurements with kinetically resolved in situ spectroscopic data to compare the performance and oxidation mechanisms of SOFCs operating with methanol and methane. Methanol exhibited higher percentages in utilization and conversion at the anode as compared to methane. Results emphasized that this increase in reactivity enabled the methanol fuel to pyrolyze into carbon deposits on the anode at a faster rate and significant amounts leading to greater degradation in electrochemical performance than compared to methane. The final project investigated the internal oxidative and steam reforming of methane and ethanol using in situ Raman, real time electrochemical, and FTIR exhaust measurements. EIS data show that direct methane and ethanol lead to anode degradation that correlates directly with the appearance of aggressive carbon deposits on the anode surface. Ex situ FTIR measurements revealed that methane doesn¡&hibar;t undergo pyrolysis but when reformers were introduced into the fuel, a mixture of H2, CO, CO2, and H2O was created. These same measurements show a decrease in acetylene when H2O was introduced into ethanol. In situ Raman measurements showed that carbon formation could be completely suppressed in the presence of these reformers, especially at high cell overpotentials.