Steven C. DeCaluwe

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Experimental Characterization of Thin-film Ceria Solid Oxide Fuel Cell Anodes

This study presents an experimental effort to elucidate the role of CeO2-x in composite SOFC anodes, by characterizing electrochemical oxidation of small fuel molecules (H2, CO) on thin-film ceria anodes. Thin ceria films were sputter-deposited onto thick YSZ-electrolyte-supported button cells with highsurface-area, porous LSM/YSZ cathodes. The mixed ionicelectronic conductivity of ceria provides multiple reaction pathways for electrochemical oxidation and electrochemical characterization of the ceria films over a range of gas compositions at temperatures from 650 to 850 °C provide a basis for elucidating the importance of competing mechanisms for fuel oxidation on the ceria. Analysis of the experiment results provides a basis for formulating a micro-kinetic mechanism for the oxidation of small fuel species on ceria anodes which can be implemented in SOFC modeling efforts for optimizing ceria-based composite anodes.

In Situ XPS for Evaluating Ceria Oxidation States in SOFC Anodes

Ceria is being considered as an electrocatalyst component for fuelflexible SOFC anodes. With Ce 3+ and Ce 4+ oxidation states, ceria can function as a mixed ionic-electronic conductor, transporting both O 2- ions and electrons. It remains unclear what role these oxidation states play during SOFC operation. This study addresses these issues using ambient-pressure XPS in a single-chamber cell to characterize thin-film CeO2-x electrodes on a YSZ electrolyte during electrochemical oxidation of H2 and reduction of H2O at 620 °C. Voltage-current curves for the ceria film electrodes reveal sharp changes in resistance as a function of applied voltage from positive to negative bias. XPS measurements during electrochemical characterization show the extent of near-surface Ce 4+ reduction to Ce 3+ with increasing cell voltage. Increased oxidation to Ce 4+ at large negative voltages correlates with a resistance drop due to H2 oxidation activity. These findings provide insight into how CeO2-x influences electrochemical fuel oxidation or H2O reduction.

The Predicted Effects of Anode Microstructure on SOFC Overpotentials

A one-dimensional SOFC model with button-cell geometry has been developed to explore the impact of electrode microstructure on overpotentials. The model couples porous media transport with charge transport and detailed electrochemical kinetics. Exploration of microstructural parameters over a range of values shows that model results are most sensitive to variations in tortuosity, porosity, pore radius, and length of three-phase boundary. In order to explore the relation of these parameters to one another and the coupling between microstructure and performance in experimental results, modeling efforts focus on humidified H2 fuel with Ni/YSZ anodes and LSM/YSZ cathodes. To match experimental results from Zhao & Virkar (1), it is shown that simulations must include correlation between porosity, tortuosity, and the thickness of the electrochemically active region in the anode. These results indicate the importance of models that allow electrochemical reactions to occur throughout the depth of the electrode.

Integration of catalytic combustion and heat recovery with meso-scale solid oxide fuel cell system

To facilitate high-power density operation of a meso-scale solid oxide fuel cell (SOFC) system, fuel processing and anode exhaust catalytic combustor with waste heat recovery are critical components. An integrated modeling study of a catalytic combustor with a solid oxide fuel cell and a catalytic partial oxidation (CPOx) reactor indicates critical aspects of the butane-fueled system design in order to ensure stable operation of the SOFC as well as the combustor and CPOx reactor. The modeled system consists of: 1) a Rh-coated ceramic foam catalytic partial oxidation reactor, 2) a SOFC with a Ni/YSZ structural anode, a dense YSZ electrolyte, and a LSM/YSZ cathode layer, and 3) a Pt-coated anode exhaust combustor with waste heat recovery. Model results for a system designed to produce < 30 W electric power from n-butane show how the design of the inlet-air cooled catalytic combustor can maximize combustion efficiency of the anode exhaust and heat recovery to the system inlet air flow. The model also shows the need to minimize heat loss in the air flow passages in order to maintain stable SOFC operation at 700 °C or higher. There is a strong sensitivity of the system operation to the SOFC operating voltage as well as the overall air to fuel ratio, and these sensitivities place important bounds on the range of operating conditions.