Multi-Physics Simulation of SOFC Button Cell with Multi-Step Charge Transfer Model in Composite LSM/YSZ Cathode

YSZ ( yttria-stabilized zirconia)-modified LSM composite cathodes were fabricated by formation of YSZ film on triple phase boundary (TPB) of LSM/YSZ/gas. The YSZ coating film greatly enlarged electrochemical reaction sites from the increase of additional TPB. The composite cathode was formed on thin YSZ electrolyte (about 30 Um thickness) supported on an anode and then I-V characterization and AC impedance analyses were performed at temperature between . As results of the impedance analysis on the cell at with humidified hydrogen as the fuel and air as the oxidant, R1 around the frequency of 1000 Hz represents the anode Polarization. R2 around the frequency of 100Hz indicates the cathode polarization, and R3 below the frequency of 10 Hz is the resistance of gas phase diffusion through the anode. The cell with the composite cathode produced power density of at air and oxygen atmosphere, respectively. The I-V curve could be divided into two parts showing distinctive behavior. At low current density region (part I) the performance decreased steeply and at high current density region (part II) the performance decreased gradually. At the part I the performance decrease was especially resulted from the large cathode polarization, while at the part H the performance decrease related to the electrolyte polarization.

Solid oxide fuel cell (SOFC) is well known as power and heat generation device which converts chemical energy directly from fuel into electricity. SOFC operate at high temperature becomes obstacle for SOFC which reducing ionic conductivity material of current electrolyte, reduce lifetime of cell components, high fabrication cost, limited durability and performance issues. This introduce to environment pollution and decrease the SOFC lifetime. The fabrication of durability and stability composite cathode are comprised from mixing of perovskite La0.6Sr0.4CO0.2Fe0.8 (LSCF) powders with nanoscale ionically conducting ceramic electrolyte materials, SDC-carbonate (SDCc) was overcome this problems. Powder preparation and composite cathode fabrication must consider which as main factors in the development of durability and stability of LSCF-SDCc composite cathode. Powders must in nanoscale to enhance the conductivity and decrease the interfacial polarization resistance and the composite cathode should in nanoporous morphology for achieve high power density over than 500 h and remarkable durability. Calcination also plays in important role and its operations will effects to the SOFC durability and performance. The necessary to prolong the lifetime and increase the SOFC performance has lead to development of durability and stability of SOFC. This paper reviews the durability and stability of the composite cathode and focus on the challenges in material technology.

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

The electrochemical reduction of CO2 was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO2 electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO2 into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H2, CO and hydrocarbons and use these chemicals when necessary. Yet it is a challenging task to realize an efficient reduction of CO2 by SOEC due to the non-polar nature of CO2 fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO2 reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO2 electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO2through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc. Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO2 by SOEC and to obtain more understandings of the mechanism of CO2 electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O3 (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O2 (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO2 electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO2 electrolysis was characterized in various CO2-CO mixtures and applied potentials in 900-750oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs. In this paper, impedance behaviour of the electrochemical reduction reaction of CO2 from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO2-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO2electrochemical reduction. By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO2 electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO2-CO 50-50 mixture.

Understanding the electrochemical behaviour of LSM-based SOFC cathodes. Part I — Experimental and electrochemical

Evaluation of electrochemical reaction mechanisms of La0.6Sr0.4CoO3-δ-Gd0.1Ce0.9O2-δ composite cathodes by 3D numerical simulation

Solid oxide fuel cells (SOFCs) have drawn remarkable attention in recent years due to their distinctive advantages of high efficiency, fuel variety and low environment pollution. Along the way to designing of new cathode materials for SOFCs, an understanding of the mechanism of oxygen reduction reaction (ORR) plays a key role, especially the interaction between O2 molecule and surface of cathode. The results of the first principles calculations of the atomic and electronic structure of a layered perovskite LaSrCo0.5Feo.5O4-δ used as a cathode material for solid oxide fuel cells (SOFCs) are presented and discussed. The oxygen vacancy formation energies is found to be smaller than LaSrCoO4 and slightly lower than the other perovskite, e.g. (La, Sr)MnO3, which widely used as a cathode because of its excellent catalytic activity. From a detailed analysis of difference electron densities, the presence of oxygen vacancies can induce local charge redistribution. Furthermore, a strong hybridization between Co-Oad and Fe-Oad for molecular adsorption was obtained by analysis of density of states. Our results show how the mixed B-site ions produce the excellent adsorption and conduction performance of LaSrCo0.5Feo.5O4-δ and fully support its potential application in next-generation SOFCs.

Solid oxide fuel cell (SOFC) technology requires further maturations to become commercialized broadly. Many existing problems are expected to be solved by intermediate temperature SOFC (IT-SOFC). On the other hand, electrochemical resistances of cathode materials increase rapidly at lower temperature. So it is necessary to understand the underlying mechanisms of oxygen reduction reaction and develop new materials. Although many experimental and theoretical researches for this purpose are in progress, traditional trial-and-error materials search must be inefficient and take too long time. Considering the above situation, we explore critical factors that can explain the oxygen exchange rate of perovskite oxide cathode in SOFCs. It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing reported experimental data on 18 materials in figure 1. Materials which have high oxygen exchange rate can be explained by the multiplication of bulk ionic conductivity and electronic conductivity while others explained by the ionic conductivity. Fig. 1 Relationship between oxygen exchange rate and bulk electronic conductivity, ionic conductivity, and the model function with both conductivities. Figure 1

The functional layer of a flat‐tubular solid oxide fuel cell (SOFC) is examined using a three‐dimensional microscale electrode model. SOFC electrodes essentially include two types of layers: a structural layer and a functional layer. The structural layers, which are the anode support layer and the cathode current collector layer, are composed of large particles with a high porosity that facilitates gas diffusion. The functional layers consist of small particles with a low porosity that increases the triple phase boundary (TPB) reaction area and reduces the activation overpotential. In the model, the particle diameter and functional layer thickness are adjusted and analyzed. The effects of the two parameters on the performance of the functional layer are monitored in the contexts of several multilateral approaches. Most reactions occurred near the electrode–electrolyte interface; however, an electrode design that included additional TPB areas improved the electrode performance. The role of the functional layer in a flat‐tubular SOFC is examined as a function of the functional layer particle size and thickness. The performance of a cell could be enhanced by preparing a functional layer using particles of optimal size and thickness, and by operating the device under conditions optimized for these parameters.

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H2 and further H2O/CO2/H2/CO. The experimental test shows the stability of the Ni-patterned electrode in the H2O/H2 atmosphere was much poorer than that in the CO/CO2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700oC with the inlet H2/H2O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H2O and H2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H2 and H2O, especially showed higher sensitivity to the partial pressure of H2O. Further, a possible mechanism of the H2O/H2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O2-(YSZ)OH-(YSZ)+(Ni)+e- and H(Ni)+OH-(YSZ)H2O(YSZ)+(Ni)+e-. Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O2-(YSZ)→OH-(YSZ)+(Ni)+e-, however in SOEC mode, the rate-determining step of the H2O electrochemical reduction should be H2O(YSZ)+(Ni)+e-→OH-(YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. Figure 1

Solid oxide fuel cells (SOFCs) are getting more importance with their promising future, due to their high-energy conversion efficiency, low-emissions and flexibility of usable fuel type. The performance and the lifetime of SOFCs are keenly dependent on electrode microstructure. In order to recognize the microstructural evolution and its degradation kinetics in SOFC cermet cathodes during long exposure time (up to 20 000 h) under realistic operating conditions ( T= 850 °C, J= 190-250 ), investigations on porous LSM/YSZ cathodes were conducted. The 3D-tomography technique (FIB/SEM) offers extensive data about the microstructures of various cathode aged during different operating times (2 500 h, 15 000 h and 20 000 h), which allows an exact quantification of particle size distribution, phase-connectivity, tortuosity factor and Triple Phase Boundary Length (TPBL). With the increasing operating time no significant 3D microstructural changes in the cathode were noticed in the obtained data. However, additional qualitative X-ray fluorescence measurement, indicate a clear presence of chrome contamination on aged cathodes, which may be the main degradation mechanism in the SOFC cathode. Keywords: chrome poisoning, long operating time, particle size distribution, SOFC, triple phase boundary, SEM, XRF

Functionally graded electrodes (FGEs) have been recently proposed to show a significant potential to improve the performance of solid oxide fuel cells (SOFC) as compared to conventional electrodes using a uniform composite layer. A mathematical model is used to mimic the FGE of a SOFC at the micro-scale level. This model is based on a comprehensive approach including the coupled mass transfer and electrochemical reactions in both the anode and cathode of a SOFC during operation. Overpotentials are solved by: the dusty-gas model (DGM) for porous-media transport in the electrodes; the modified Butler-Volmer equations for the electrochemical reactions at the triple phase boundary (TPB) layers; and ohmic law for polarization losses in the electrolyte. The current model makes use of a formulation for the active surface area such that three different kinds of graded electrodes, namely, composition graded, particle size graded, and porosity graded can be properly described. The results are then compared with available experimental data with encouraging results. Additional numerical studies explored optimum parameters for uniform conventional electrodes, the effect of the addition of an anode or cathode interlayer on cell performance, and optimum grading profiles for continuously graded electrodes. The present study reveals that micro-scale graded electrodes have the potential to enhance SOFC efficiency.

Implementation of nano-catalyst materials into solid oxide fuel cell (SOFC) electrodes to improve performance and stability has been widely studied. Addition of the nano-catalysts into an electrode structure serves to enhance the electrochemical performance of the SOFC by increasing the Triple Phase Boundary (TPB) area, improving redox stabilization, and modifying reaction kinetics of hydrocarbon gases that cause anode degradation due to carbon deposition.Typical Ni-based cermet anodes suffer from anode deactivation due to carbon build up under hydrocarbon flows. Carbon builds up and covers the TPB area which results in poor electrochemical performance. Larger carbon deposits can block pores within the anode microstructure which leads to gas diffusion issues. Carbon buildup also causes mechanical stresses to the electrode due to volumetric changes which can lead to fracture and complete failure of the cell.This work studied nano-catalyst decoration of the Ni-based cermet anodes with catalysts that promote internal reforming to protect against coking. The addition of active metal components and ceramic reforming promoters were investigated. Multi-component systems with several catalysts were also examined. Uniform incorporation of nano-catalyst into the anode microstructure was achieved through a patented liquid phase surfactant assisted process (using various catechol surfactants). Deposition loading densities and distribution of nanoparticles was controlled by altering the surfactant and catalyst solution concentrations.This work defined initial morphology and studied the coarsening/annealing kinetics of nano-catalysts used for enhanced solid oxide fuel cell (SOFC) operation. Nano-catalyst depositions were performed on flat single crystal surfaces using a bio-inspired surface modification technique. 3D profiles of the nanoparticles at various coarsening stages were obtained using atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) was used to track the changes in composition. Figure 1 shows AFM imaging of single component cerium oxide and cobalt oxide depositions on flat single crystal yttrium stabilized zirconia (YSZ) before and after 24 h of annealing in reducing conditions.Impregnated SOFCs were tested for 200+ h in constant load while operating under high methane fuel streams. Fuel compositions were selected to have high hydrocarbon content to accelerate degradation. Deposition techniques were optimized for multi-component internal reforming catalysts to obtain sustained performance enhancement. Current-voltage-power (I-V-P) measurements and electrochemical impedance spectroscopy (EIS) results were used for evaluation. Post-mortem microstructure and chemistry characterizations were used in analysis.Impregnated SOFCs demonstrated greater than 50% sustained improvement in harsh environments in long-term tests where the anode was subjected to 40% CH4 for 200+ h. Acknowledgements: Work supported by a subcontract under the GE Aerospace’s US DOE- ARPA-E project (DE-AR0001344) entitled “FueL CelL Embedded ENgine (FLyCLEEN)”. Material characterization and imaging work was made possible with the support of the West Virginia University Shared Research Facilities. Figure 1

Nano-scale erbia stabilized bismuth oxides (ESBs) were successfully synthesized by a wet chemical reverse strike co-precipitation. Due to homogenous, molecular level mixing, the desired cubic fluorite structure was formed at a dramatically reduced temperature of 500 °C, which was confirmed by X-ray diffraction and Raman spectroscopy. Moreover, this low calcine temperature led to nano-scale ESB powders with a crystallite size of ∼20 nm and a specific surface area of ∼13.2 m2 g−1. Due to the high surface area, the nano-sized ESB powders show high functionality for solid oxide fuel cell (SOFC) applications. As an SOFC electrolyte, the high sinterability of the co-precipitated ESB was demonstrated, achieving over 98% density after sintering at only 750 °C for 30 min. Moreover, the total conductivity of the sample was identical to that obtained by conventional methods after sintering at 890 °C (for 16 h), regardless of the different grain boundary densities. In addition, the co-precipitated ESB was used in a composite cathode with lanthanum strontium manganite (LSM), achieving significantly reduced cathodic ASRs, 0.55 and 0.03 Ω cm2, at 550 and 700 °C by extending triple phase boundary (TPB) lengths in the cathode bulk and at the cathode–electrolyte interface.

A simple, rapid spray method for preparing anode-supported solid oxide fuel cells with GDC electrolyte thin films