(5ba) Catalytic Phenomena Limiting Solid Oxide Fuel Cell Performance

Solid oxide fuel cells (SOFCs) show promise for generating clean power from a variety of fuels. The major roadblocks against their implementation are a large cathodic resistance, which causes insufficient power densities and high fabrication costs, and anodic deactivation caused by carbon-based fuels such as coal and biomass-derived gases and their sulfur impurities. The large cathodic resistance is caused by slow oxygen activation kinetics and oxide ion transport of the current manganite-based cathode. Thus, the development of catalytically active materials suitable for use as electrodes is needed to help SOFCs realize their wide-scale application.

Recent research has shown that cathode behavior becomes co-limited by oxygen reduction kinetics and ionic transport for transition metal perovskite materials. The present work studies the kinetics of the oxygen reduction reaction, oxygen mobility, oxygen-surface interactions, and the surface chemistry of doped-lanthanum ferrites between 500 and 700°C. Results show trends with strontium dopant levels on the lanthanum site, transition metal dopant levels on the iron site, temperature, and oxygen partial pressure in the surrounding environment.

Oxygen reduction kinetic measurements are made through oxygen equilibration experiments during simultaneous thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses, cyclic voltammetry (CV), and isotopic oxygen exchange studies. The kinetic results are supported by a thorough surface characterization by using methanol as a probe molecule and X-ray photoelectron spectroscopy (XPS). The nature and quantity of active surface sites and their surface chemistry are determined by pulsed methanol chemisorption and methanol temperature-programmed desorption, respectively. The majority of sites are basic in nature, but redox sites also exist. Further insight is provided through techniques such as X-ray diffraction (XRD), vibrational spectroscopy, and temperature-programmed techniques.

The current research also demonstrates perovskite materials that are stable in highly reducing conditions and possess catalytic activity for the oxidation of hydrogen and carbon monoxide in the desired temperature range. The effect of the increased oxidation activity and lattice oxygen mobility on the steam requirements and the influence of hydrogen sulfide concentration on the oxidation activity are discussed. Characterization is performed by XRD, XPS, and vibrational spectroscopy to complement the anodic oxidation results.