An Investigation of Solid Oxide Fuel Cell Chemistry: A Spectroscopic Approach
Walker, Robert A
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Solid oxide fuel cells (SOFCs) represent an alternative power source that utilizes hydrogen and/or hydrocarbon fuels at significantly higher efficiencies than combustion systems; however, molecularly specific fuel oxidation mechanisms in SOFCs are not well understood. This research uses ex-situ techniques to quantify the physicochemical properties of SOFC materials and novel optical methods to identify chemical intermediates present in-situ. Ex-situ experiments include quantifying gas-phase exhaust composition using Fourier transform infrared spectroscopy and characterizing material properties of SOFC components with Raman and X-ray photoelectron (XPS) spectroscopies. The exhaust studies show that fuel oxidation efficiencies depend on anode materials and fuels used. Nickel is a more active fuel oxidation catalyst than ceria; however, poor exhaust carbon balance indicates that nickel also promotes carbon deposition. Studies of YSZ indicate that the surface is chemically altered by reducing environments. This observation can complicate our understanding of SOFC chemistry if the reduced surface participates in fuel oxidation chemistry. In-situ Raman spectroscopy is combined with traditional electrochemical methods to characterize SOFC anodes at 715 °C. Raman spectra of the anode indicate the presence of NiO, graphite, and Ni-COO as fuel oxidation intermediates depending on operating conditions. Graphite formed with carbon-containing fuels is tracked by Raman spectroscopy on a minute timescale, allowing the study of deposit formation and oxidation. CO, CH4, C2H4, C3H6, and C4H10 have been used to correlate fuel molecule structure to the deposit formed. Higher-weight fuels lead to carbon deposits with smaller domain sizes. Singular intermediate-weight fuels (C2 and C3) lead to permanent degradation in cell performance, while extended use of intermediate-weight fuel mixtures and C4H10 result in performance recovery. These studies present the first identification of intermediates on working SOFC anode surfaces, providing new insight to the fuel oxidation mechanism. Species identified in these studies will enable development of more accurate models of SOFC chemistry.