Stationary FEM Model for Performance Evaluation of Planar Solid Oxide Fuel Cells Connected by Metal Interconnectors

Abstract

A modeling framework is presented based on a stationary finite element method (FEM) model. The model geometry is a twodimensional repeat unit representing all individual layers of an anode supported cell sandwiched between metallic interconnect (MIC) structures. The model is capable of analyzing performance limiting factors for planar solid oxide fuel cell (SOFC) stacks. These factors arise from material composition, microstructure, layer thickness, or MIC flowfield design. Herein, setup and validation of the modeling framework are presented and discussed in detail. Charge-transfer chemistry is modeled with detailed Butler-Volmer kinetics. Ion and electron conduction is modeled with Ohm's law and porous-media gas species transport is represented by the Dusty-Gas model. All physical parameters in the model equations were determined by our own measurements, conducted on anode supported cells and components thereof. The stationary finite element model was validated against measured current/voltage characteristics for a temperature range of 621 to 871 • C, fuel humidification from 5.5% to 60%, and current densities up to a maximum of 2 A/cm 2 . This more than exceeds the standard operating range for multi-kW SOFC stack units. The applicability of the model is demonstrated using the electrical and dimensional data of anode supported cells and the MIC design similar to stationary 5 and 10 kW stacks (from Forschungszentrum Jülich). It is shown that the MIC flow field design induces gas transport limitations and electronic current constrictions in the cell components. The simulation results clearly indicate the cathode layer thickness as the most sensitive factor in limiting stack performance.Although the performance of planar solid oxide fuel cells (SOFC) at the single cell layer has been continuously improved over the past decade, 1 there are vast optimization potentials at stack-level, where the area specific power density decreases by 50%. 2 Stacking single cells sandwiched between metal interconnects (MICs) is a practical way to increase the voltage of SOFC systems. The MIC structure, however, contributes massively to immediate power losses and the long term degradation of cells. The applied MIC material, a ferritic high-temperature steel, emits chromium species that poison the SOFC cathode, thereby inducing a strong degradation of the stack performance in the long-term. 3-6 Independent thereof, the ab initio power loss compared to single cells is inherently determined by the interconnector design. 6 Besides providing current collection, MICs also act as gas flow manifold, providing the porous electrodes with gaseous reactants. Common MIC designs accordingly feature contact ribs and gas channels in a parallel setup. Underneath the gas channel, current constriction arises from non-ideal electronic pathways in the porous electrode structures. Meanwhile, non-ideal gas supply beneath the contact rib induces gas transport limitations. As of the moment of writing, there is no overall understanding of how these system-imma...

Effects of changes in solid oxide fuel cell electrode thickness on ohmic and concentration polarizations

“…Obviously, for a typical ASC, the oxygen concentration beyond the x>1.4 mm zone is very close to zero. This phenomenon has also been encountered in paper [18,30], which is to a great degree caused by the high current and thin cathode thickness. So, numerically, the electrochemical reaction can't occur in this zone due to the lack of the oxygen.…”

“…An oxidant starvation zone was found under the rib cover zone due to the very thin cathode, which implied a dramatic slowdown in the performance. Similarly, the produced electric current density under the rib dropped from 1.6 A cm -2 to almost zero [18].…”

“…As the open circuit voltage of a single cell is only around 1 V, many single cells are assembled into a stack through interconnectors in the form of series, parallel or seriesparallel connection to meet the high potential output demands [17][18][19][20]. Many grooves are milled on the interconnector to act as the gas channels to supply the reactant gases and exhaust the products.…”

“…However, based on two facts, CFL can be degraded into a boundary condition at the interface of cathode and electrolyte. First, CFL is generally thinner than CCCL and the actual electrochemical reaction depth is even shallower [18], second the CCCL thickness is the main factor that influence the gas and charge transfer, as paper [30] revealed, more electronic current flows through CCCL. So, the cathodes appear in Fig.…”

Numerical simulations of non-stationary distributions of electrochemical potentials in SOFC

“…Also, some studies addressed thermal-stress analyses with detailed three-dimensional models of a cell or a cell stack of specific SOFCs to investigate their mechanical stability and reliability under operational temperature environments (Lin et al , 2007; Lin et al , 2009; Nakajo et al , 2012; Greco et al , 2014). The failure mechanisms of SOFCs can also be explored by numerical analyses to evaluate stresses induced by both thermal and reduction strains, in consideration of the coupling behavior with electrochemical transport and gas diffusion phenomena (Nakajo et al , 2009a; Nakajo et al , 2009b; Geisler et al , 2014). Moreover, the characterization of the material behavior coupled with the diffusions of electrochemical defects is within the scope of some numerical studies (Swaminathan et al , 2007a; Swaminathan et al , 2007b).…”