ELECTROCHEMICAL OXIDATION KINETICS OF HYDROGEN AND HIGHER HYDROCARBON FUELS ON SOLID OXIDE FUEL CELLS.
Eichhorn, Bryan W
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Solid oxide fuel cells (SOFCs) are promising electrochemical energy converting devices due to their ability to use not only hydrogen but also hydrocarbons as a fuel. Although conventional SOFCs with Ni/YSZ anodes and hydrocarbon fuels form carbon deposits that inhibit SOFC performance, an enhancement in performance is observed for the Cu/CeO2/YSZ anode with carbon deposits and H2 fuel. Structural and compositional analyses of these carbon deposits show that graphitic carbon forms on the Cu/CeO2/YSZ. The reason of the 2 to 3 fold enhancement in performance is due to increase in anode conductivity by graphitic carbon deposits. An important problem for fuel oxidation kinetics on SOFC anodes is determining the rate limiting step(s) for fuel oxidation. To assess the effects of YSZ surface chemistry on oxidation processes, porous and dense Au anodes on YSZ electrolytes were prepared to study H2 oxidation. Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS) were used to identify critical processes in the Au/YSZ anodes as a function of Au geometry. The results show that the surface diffusion on the SOFC anodes and electrolytes is believed most likely to be the rate limiting step. To address the contribution of reduced YSZ on SOFC anode performance, porous Au anodes with different geometrical porous YSZ layers were fabricated. Studies of porous YSZ layers on Au anodes demonstrate that these layers block active sites on Au anodes for dissociative adsorption of hydrogen but help charge transfer reaction of adsorbed species on anode. Other than regular hydrogen as a fuel, isotopically-labeled D2 fuel were used to differentiate effects of both gas phase and surface diffusion on Au anode performance. An observed ~25 % decrease in current and power densities with D2 relative to H2 is attributed to lower surface diffusion of adsorbed D2 fuel species relative to H2 fuel species. Finally, modeling studies for these systems are used to understand more fully the mechanisms of H2 oxidation on SOFC anodes. The interpretations of experimental results are confirmed by using the model that manipulates the effect of various fuel partial pressures on the diffusion parameters of anode surface species. The model developed is able to describe qualitatively the isotope effect on the gas and surface diffusion coefficients by the mass affect. The implementation of the surface diffusion parameters of the water species into this model is critical to manipulate the effect of the fuel partial pressure values on diffusion processes.