A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting solid oxide fuel cell (SOFC) cathodes that considers the complex coupling of gas diffusion, surface exchange, ionic bulk-diffusion and electrolyte conductivity is presented. By using the finite element method, this model enables the time-dependent and space-resolved simulation of the physicochemical processes in a porous cathode microstructure. The developed model is used for a detailed analysis of the formation of a 'Gerischer-type' impedance. It is detected that the low-frequency part is dominated by the surface exchange reaction, whereas the typical 45 • ramp of the Gerischer impedance is related to the ionic diffusion in the bulk. The capability of the time-dependent 3D impedance model is evaluated versus a well-established homogenized analytical model. For homogeneous 3D microstructures both models calculate impedance curves which are in excellent agreement. Further impedance simulations with microstructures containing features of high-performance SOFC cathodes clearly show that model separates and quantifies the contribution of the gas diffusion in a porous cathode layer. At an oxygen partial pressure of 0.21 atm the gas diffusion accounts for only 2% of the total polarization resistance, whereas a depletion of oxygen to 0.01 atm significantly increases this value to 38%.Intermediate temperature solid oxide fuel cells (SOFCs) own high potential and flexibility for an efficient conversion of fuels (from pure hydrogen to higher hydrocarbons) to electric power. The performance of SOFC single cells with a thin-film electrolyte of 1 μm is dominated by the polarization losses of both electrodes. Thus, mixed ionic-electronic conducting (MIEC) cathode materials such as La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF) are indispensable at operating temperatures below 750 • C. 1,2 Due to the high oxygen ion conductivity of LSCF, the cathode performance is determined by the oxygen ion transport in the porous solid phase and the exchange kinetics of oxygen between the gas phase and the LSCF surface. Extending this electrochemically active region results in a lower value of the area specific resistance of the cathode (ASRcat), which is the mostly reported performance index. However, the coupling of transport process and surface reaction process in a single phase demands for a properly adapted chemical composition and microstructure.Electrochemical impedance spectroscopy (EIS) has become a state-of-the-art method for the characterization of SOFC electrodes. Recent developments have introduced the combined approach of EIS measurements and the distribution of relaxation times (DRT) method, 3 leading to a physically motivated equivalent circuit model for a complete SOFC single cell. 4 The DRT method offers a higher resolution of the individual loss mechanisms due to their specific kinetics, and enables a more detailed investigation on operating conditions. The MIEC cathode is modeled by a Gerischer-type impedance, 5 and detailed investigation...
A three-dimensional finite element method (FEM) model that enables the performance simulation of mixed ionic-electronic conducting (MIEC) oxygen transport membranes (OTM) has been developed. In order to evaluate the influence of a porous functional layer on the membrane performance a numerical geometry generator was implemented that allows to create arbitrary porous microstructures. The 3D OTM model includes the spatially coupled physicochemical processes i) gas diffusion in the porous functional layer, ii) oxygen exchange at the feed-side between gas phase and MIEC material, iii) oxygen ion diffusion across the membrane, iv) oxygen excorporation at the permeate-side. The performed simulation carried out for the state-of-the-art MIEC composition La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF) was validated with the help of oxygen permeation measurements carried out on an asymmetric LSCF thin-film OTM in the temperature range of 750.. . 1000 • C. The simulation results identified a surface exchange dominated regime for membrane thicknesses below 50 μm. While the application of a porous functional layer on the feed side could only increase the permeation flux by around 26%, the model demonstrates the significant improvement by a factor of two (for the given conditions) that can be achieved with a functional layer on the permeate side in case of a 20 μm thin-film membrane.
The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resistance of the electrodes. Electrochemical impedance spectroscopy (EIS) enables a deconvolution of individual electrochemical processes. In case of mixed ionic-electronic conducting (MIEC-) cathodes the impedance spectra result from the coupling of gas diffusion, surface exchange and bulk diffusion of oxygen ions. In this paper we present a three-dimensional (3D) finite element method (FEM) model which allows the transient simulation of the underlying processes in a porous cathode structure. The developed model is validated with a well established homogenized 1D model by comparing the area specific resistance and the corresponding impedance spectra. In case of a homogeneous 3D microstructure the FEM simulation results show an excellent agreement with the homogenized 1D model. Furthermore, the 3D FEM model is applied for impedance simulations of a technical MIEC cathode which microstructure was reconstructed from FIB tomography.
A three-dimensional (3D) finite element method (FEM) model that enables the performance simulation of mixed ionic-electronic conducting (MIEC) oxygen transport membranes (OTM) has been developed. In order to evaluate the influence of a porous functional layer on the membrane performance a numerical geometry generator was implemented that allows to create arbitrary porous microstructures. The 3D OTM model includes the spatially coupled physicochemical processes i) gas diffusion in the porous functional layer, ii) oxygen exchange at the feed-side between the gas phase and the MIEC material, iii) oxygen ion diffusion across the membrane, iv) oxygen excorporation at the permeate-side. The performed simulation carried out for La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) identified a surface exchange dominated regime for membrane thicknesses below 50 µm. While the application of a porous functional layer on the feed-side could only increase the permeation flux by around 26 %, the 3D OTM model demonstrates the significant improvement that can be achieved with a functional layer on the permeate-side of the membrane.
In this contribution a three-dimensional (3D) finite element method (FEM) model that enables the performance simulation of solid oxide fuel cell (SOFC) Ni/YSZ cermet anodes is presented. The 3D FEM anode model enables the spatially resolved simulation of the electrochemical processes at the triple phase boundary (TPB) and the intrinsic coupling with the ionic, electronic and gaseous transport in the complex three-phase microstructure. Experimental data obtained from measurements with patterned model anodes are used for a detailed description of the electro-oxidation kinetics at the TPB. This numerical approach is combined with 3D reconstructions gained by focused ion beam (FIB) tomography used as the model geometry. Hence, all critical microstructure parameters are intrinsically considered in the 3D anode model. Simulations for a reconstructed anode functional layer were conducted for a temperature range between 450°C and 950°C. The simulated performance showed a good agreement with experimental data of technical Ni/YSZ cermet anodes.
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