Effect of Bi-Layer Interconnector Design on the Current Density of Solid Oxide Fuel Cells

The concentration gradient of fuel and oxidant gas is great in the plane normal to the solid oxide fuel cells (SOFC) three-phase-boundary (TPB) layer, especially in the porous electrode. We present a novel interconnector design, termed bilayer interconnector, for SOFC. It can distribute the fuel and air gas in the plane normal to the SOFC TPB layer. In this paper, we develop a 3D model to study the current density of the SOFC with conventional and novel bi-layer interconnectors. The numerical results show that the novel SOFC design Rib1 can slightly enhance the mass transfer in the porous anode and current density. The novel SOFC design Rib2 can improve the current density significantly under low electrical conductivity of interconnector.

Numerical Studies of Heat Transport for Polymer Electrolyte Fuel Cell Stack in Sub-Freezing Environment

Two cases of heat transfer processes for a general polymer electrolyte fuel cell (PEFC) stack in a sub-freezing environment are studied in this paper: cooling-down and heating-up. We investigate the time consumption problem for both of these two cases in order to find the way to normally restart fuel cell stack without regard to electrochemical reaction. We consider the action of heat transfer in lieu of generated chemical energy to PEFC in sub-freezing environment by means of heat insulator. In the numerical simulation, we define a combined finite element/upwind finite volume discretization to approximate the heat transport equation for different cases of heat transport process, and obtain the stable and reasonable numerical solutions. These results correspondingly provide explicit ways to preserve heat in PEFC stack in the sub-freezing environment.

Low-Temperature Prepared Multi-Elements Doped CeO2 Electrolyte

CeO2 materials doped with the di- or tri-valent metals possess high oxide ionic conductivity at low temperature for potential electrolyte use in intermediate temperature solid oxide fuel cell (SOFC). However, multi-elements doped CeO2-based electrolyte, (La1-x-ySrxBay)0.175Ce0.825O2-δ (LSBC) in this work, with pure phase is difficultly synthesized at low calcination temperature. High sintering temperature, e.g. > 1500°C, is also needed in conventional mixed oxide method. In this work, nanoparticles less than 50nm of LSBC can be prepared by solution-evaporation method at constant temperature. Pure fluorite crystal structure can be obtained lower than 700°C. The optimal mole ratio of LSBC/citric acid in prepared solution is 1/2 to achieve homogeneous composition and pure phase of LSBC. Small grain size of about 1μm average is observed for 1300°C-microwave sintered LSBC by solution-evaporation method. The ionic conductivity of 1400°C-conventional sintered and 1300°C-microwave sintered LSBC prepared by solution-evaporation method is about 0.006 S/cm at 600°C but less than 0.004 S/cm at 600°C even for 1500°C-conventional sintered LSBC prepared by mixed oxide method.

Analysis of Pressure Drop and Water Flooding of Two-Phase Flow Along Channels for a PEM Fuel Cell System

The flow in gas flow channels of an operating polymer electrolyte membrane (PEM) fuel cell has a two-phase characteristic that includes air, water vapor and liquid water and significantly affects the water flooding, pressure distribution along the channels, and subsequently the performance of the cell and system. Presence of liquid water in channels prevents transport of the reactants to the catalysts and increases the pressure difference between the inlet and outlet of channels, which leads to high parasitic power of pumps used in air and fuel supply systems. We propose a model that enables prediction of pressure drop and liquid water distribution along channels and analysis of water flooding in an operating fuel cell. The model was developed based on a gas-liquid two-phase separated flow that considers the variations of gas pressure, mass flow rate, relative humidity, viscosity, void fraction, and density along the channels on both sides. Effects of operating parameters that include stoichoimetric ratio, relative humidity, and inlet pressure on the pressure drop and water flooding along the channels were analyzed.

System Integration Issues for a Direct Sodium Borohydride Fuel Cell (DNBFC)

A fuel cell for air independent mobile applications using Direct Sodium Borohydride/Hydrogen Peroxide fuels in a low temperature PEM configuration is under development [1, 2]. As part of the development of this unique all liquid fuel cell, we have been studying methods for system integration, including methods for water management, stacking issues involving fluid conductivity control, and the design of a composite catalysis-diffusion layers. [3, 4] The goal is to find optimal conditions (minimum activation, ohmic and transport losses plus maximum run time per fuel loading) in this unique all liquid fuel cell. In contrast to conventional H2/O2 cells, the high electron and ion conductivity of the aqueous solution based fuels introduces special design considerations. For example, in stack design, the path length of flow channels connecting cells must be lengthened to increase the electric resistance which would otherwise introduce serious electrical shorting. With the catalyst coated throughout the diffusion layer, increasing ion conductivity from reaction sites to the PEM region also becomes a key design consideration, involving the porosity and entanglement of catalyst materials. Water management in this type of cell involves unique issues beyond humidification of the PEM which is automatically wetted by the liquid fuels. Here the issue is recirculation of product water from the cathode side back to the borohydride side to prevent reaction product NaBO2 from exceeding its solubility limit. These system integration issues are studied by a coordinated experimental approach which will be described in the presentation.

Microstructure, Sinterability and Electrochemical Properties of a Sm-Sr-(Co,Fe,Ni)-O Cathode System for Solid Oxide Fuel Cells

Samarium (Sm) is a rare earth material that shows promise for use in cathodes of intermediate temperature-operating solid oxide fuel cells (IT-SOFCs). Perovskite-structured oxide containing Sm has very attractive electrocatalytic properties, and spinel-structured oxide generally exhibits low thermal expansion, indicating its suitability for application as a SOFC cathode. In this paper, the characteristics of the various Sm-based oxide materials (Sm-Sr-(Co,Fe,Ni)-O) deposited on Sm0.2Ce0.8O1.9 (SDC) electrolyte pellets were investigated in terms of their microstructure, sinterability and electrochemical properties. The relationship between the composition and the sintering temperature was studied and discussed. Results show that the substitution of iron (Fe) and nickel (Ni) in Co-sites affects the sinterability, adhesion to the electrolyte and electrochemical activity, such that the different sintering temperatures for these compositions should be considered. The microstructure and sinterability of the cathodes were investigated using a scanning electron microscope (SEM). Area specific resistance (ASR) values for all cathode compositions were measured using AC electrochemical impedance spectroscopy (EIS).

A Finite Volume SOFC Model for Coal-Based Integrated Gasification Fuel Cell System Analysis

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for energy production. Most IGFC system analyses performed to date have used non-dimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work, a one-dimensional finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Results are presented for SOFC operation on humidified hydrogen and methane-containing syngas, under co-flow and counter-flow configurations; detailed internal profiles of species mole fractions, temperature, current density and electrochemical performance are obtained. The effects of performance, fuel composition and flow configuration on SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed.

In-Situ Estimation of Faradaic Efficiency for a Direct Methanol Fuel Cell Stack by a Carbon Dioxide Saturated Solution Method

A direct methanol fuel cell (DMFC) system consisting of 40 single cells was assembled to study the influence of the transport phenomena at the anode and stack faradaic efficiencies by a CO2 saturated solution method. This method corrected the common experimental error in measuring methanol crossover caused by the simultaneous CO2 permeation from the anode to cathode. Both anode and stack faradaic efficiencies were estimated using this method. An equivalent “carbon-flow current” has been defined and a relationship between the transport phenomena and efficiencies was developed. Also the effect of methanol concentration, methanol flow rate and air flow rate on stack efficiency was studied. The results show that lower methanol flow rate, lower methanol concentration and higher air flow rate are all helpful in decreasing the methanol crossover and increase the stack faradaic efficiency.

A Phenomenological Model for Degradation of Solid Oxide Fuel Cell Anodes Due to Impurities in Coal Syngas

A new phenomenological one-dimensional model is formulated to simulate the typical degradation patterns observed in Solid Oxide Fuel Cell (SOFC) anodes due to coal syngas contaminants such as arsenic (As) and phosphorous (P). The model includes ordinary gas phase diffusion including Knudsen diffusion and surface diffusion within the anode and the adsorption reactions on the surface of the Ni-YSZ based anode. Model parameters such as reaction rate constants for the adsorption reactions are calibrated to match the degradation rates reported in the literature. Preliminary results from implementation of the model demonstrated that the deposition of the impurity on the Ni catalyst starts near the fuel channel/anode interface and slowly moves toward the active anode/electrolyte interface which is in good agreement with the experimental data. Parametric studies performed at different impurity concentrations, operating temperatures and current densities showed that the coverage rate increases with increasing temperature, impurity concentration and current density, as expected.

Extracting Model Parameters and Paradigms From Neutron Imaging of Dead-Ended Anode Operation

In a PEMFC, feeding dry hydrogen into a dead-ended anode (DEA), reduces the overall system cost, weight and volume due to reduced need for a hydrogen-grade humidification and recirculation subsystems, but requires purging to remove the accumulated water and inert gas. Although the DEA method of operation might be undesirable due to its associated high spatial variability it provides a unique perspective on the evolution of the water accumulation in the anode. Sections of the channel nearest the inlets are significantly drier than those nearest the outlet as shown in the neutron imaging of a 53 cm2 PEMFC. This method allows in-situ visualization of distinct patterns, including water front propagation along the channels. In this paper we utilize neutron imaging of the liquid water distributions and a previously developed PDE model of liquid water flow in the GDL to (a) identify a range of numerical values for the immobile saturation limit, (b) propose a gravity-driven liquid flow in the channels, and (c) derive the two-phase GDL boundary conditions associated with the presence of liquid water in the channel.