Christopher D. Johnson

Validation and Application of a CFD-Based Model for Solid Oxide Fuel Cells and Stacks

The National Energy Technology Laboratory (NETL) has developed a solid oxide fuel cell (SOFC) model based on commercial computational fluid dynamics (CFD) software. This new tool is being used to support the US DOE Solid State Energy Conversion Alliance Fuel Cell Program, which will require advanced fuel cell designs in order to meet the program goal of reaching

Dynamics of Solid Oxide Fuel Cell Operation

400/kW for small (∼5kW) systems. The NETL model combines a special SOFC electrochemical model with an electrical potential field model in the finite-volume commercial CFD code from Fluent Incorporated (Lebanon NH). Mass and energy sources and sinks resulting from the electrochemical reactions and electrical current flow are coupled to the fluid flow, chemical species transport, heat transfer, porous media flow, and gas phase chemistry capabilities available in the base CFD model. The NETL SOFC model has also been recently extended to model SOFC stacks with cells connected in electrical series. The model is able to predict detailed, spatially resolved current flow through the electrolyte and through all conducting media in three-dimensional SOFC cells and cell stacks. In conjunction with the SOFC model development program, NETL has an experimental facility in place to generate data for validation of the SOFC model. The experimental program includes collaboration with the University of Utah, a supplier of test specimens and preliminary cell performance data. Well-characterized SOFC test specimens are being tested in the NETL fuel cell test stands for single cell and short-stack arrangements. Anode-supported cells with controlled electrode microstructures, electrode thickness, and electrolyte thickness are being tested. Operating data from the test stands includes cell and stack polarization curves, temperature data, and chemical composition of reactant streams. Using NETL and University of Utah data, an extensive validation program is now underway for the NETL SOFC model. The model is being tested using a simple button-cell configuration. A parametric study of varying operating conditions, cell geometries and cell properties is being performed. Good agreement between predicted and measured cell performance has been observed and is presented. The model has also been applied to planar single cell and cell stack configurations to help in the design of NETL experimental test facilities.Copyright

Thermal Gradient Induced Current Recirculation on Load Change in Solid Oxide Fuel Cells

The dynamics of solid oxide fuel cell operation (SOFC) have been considered previously, but mainly through the use of one-dimensional codes applied to co-flow fuel cell systems. In this paper a cross-flow geometry is considered. The details of the model are provided, and the model is compared with some initial experimental data. For parameters typical of SOFC operation, a variety of transient cases are investigated, including representative load increase and decrease and system shutdown. Of particular note are results showing cases having reverse current over significant portions of the cell, starting from the moment of load perturbation up to the point where equilibrated conditions again provide positive current. Consideration is given as to when such reverse current conditions might most significantly impact the reliability of the cell.Copyright

Effect of Pressure on Catalyst Activity and Carbon Deposition During CO2 Reforming of Methane over Noble-Metal Catalysts

This paper considers recent model results examining the transient performance of three common solid oxide fuel cell (SOFC) geometries (cross-flow, co-flow, and counter-flow) during load reduction events. Of particular note for large load decrease (e.g., shutdown) is the occurrence of reverse current over significant portions of the cell, starting from the moment of load loss up to the point where equilibrated conditions again provide positive current. This behavior results from the temperature gradients that exist in an SOFC stack. Also reported are test results from an experiment employing two separate button cells coupled together electrically (anode-to-anode and cathode-to-cathode) which are used to confirm the model predictions. The test results confirm the predictions of the model in that temperature gradients are a driver for current circulation within a cell. Also reported are test results of a button cell operated under reverse current to help begin to identify what effects such operation may have on fuel cell performance and durability.Copyright

Impact of Temperature and Porosity on Sc 2 O 3 -CeO 2 -ZrO 2 Intermediate Temperature Solid Oxide Fuel Cells Performance

The reforming of methane with CO2 was studied over 1wt% Rh/alumina, Pt/ZrO2, and Ce-promoted Pt/ZrO2 catalysts at 800°C and pressures of 1, 8, and 14 bar. It was found that high pressure resulted in greater carbon formation, lower methane and CO2 conversions, as well as a lower H2/CO ratio. Temperature-programmed oxidation (TPO), of the catalysts after reaction, shows several CO2 peaks for the Ce-promoted catalyst, indicating several sources or types of carbon and/or several locations on the catalyst are involved with carbon deposition. The change in the temperature and intensity of the TPO peaks with pressure indicates that more stable carbon is deposited at high pressure. Thermodynamic calculations for the endothermic reaction of CH4 with CO2, CH4 decomposition, and CO disproportionation were also performed. The results of these calculations are consistent with CO disproportionation being a larger contributor to carbon deposition at high pressure.

The performance of solid oxide fuel cells with Mn–Co electroplated interconnect as cathode current collector

Two commercial 10mol% Sc2O3 – 1mol% CeO2 –ZrO2 powders manufactured by Praxair Surface Technologies, Specialty Ceramics, USA and DKKK, Japan were compared for use as an electrolyte material for Intermediate Temperature SOFCs. Single element SOFCs were developed using ScCeZrO2 dense ceramics as an electrolyte, composite 50 wt% La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) + 50 wt% Gd2O3 + CeO2 (GDC) porous ceramics as a cathode, and Ni-ScCeZrO2 cermet as an anode. It is found that the ScCeZrO2 DKKK powder densifies more easily than that of Praxair powder. It is also found that the optimal sintering temperature for 50wt% LSFC – 50wt% GDC cathode powders is 1100°C to 1200°C, but that sintering begins as low as 800°C. The optimal sintering temperature for the 50 wt% DKKK ScCeZrO2 – 50 wt% NiO anode powder is between 1050°C and 1100°C. The electrolyte tapes have been produced by the tape casting followed by sintering at 1500°C for 2 hours. Then, the anode and cathode has been screen printed on both sides of the electrolyte layer. Completed button cells containing the screen printed anode and cathode were sintered in air at 1100°C for 2 hours. Button cells were tested at 800°C using a 97% H2 and 3% H2O on the anode side and air on the cathode side. The first preliminary results showed the cell performance of only 22 mW/cm maximum power density achieved at 0.7 V. This poor performance may be due to the sintering of the electrodes during testing and the formation of poorly conducting secondary phases at one of the electrode/electrolyte interfaces. The performance of the cell is modeled using one-dimensional model, which showed that most of the losses are due to diffusion and contact resistance.