Composite Solid Oxide Fuel Cell Research Articles

Sulfur poisoning and the regeneration of the solid oxide fuel cell with metal catalyst-impregnated La0.75Sr0.25Cr0.5Mn0.5O3-δ anode

The sulfur resistance and regeneration ability of Ni- or Co-impregnated La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCrM) composite anodes are investigated in this study. The sulfur resistance of the composite anodes are assessed after a 5 h sulfur poisoning process. Interestingly, the composite anodes exhibit weak sulfur resistance due to the excellent catalytic activity of catalyst to fuel oxidation. Both chemical and electrochemical oxidative methods are attempted to realize the regeneration of the composite solid oxide fuel cells (SOFCs) after the sulfur poisoning process. Notably, opposite results are exhibited by the two regeneration methods for the composite SOFCs. The chemical oxidative method can restore the electrochemical properties of the composite SOFCs, but the electrochemical oxidative method cannot. The regeneration and failure mechanisms of the Ni- or Co-impregnated LSCrM anodes are evaluated in detail.

Interface and grain boundary degradation in LSM-YSZ composite Solid Oxide Fuel Cell cathodes operated in humidified air

The interfaces between the different phases and the associated triple phase boundaries (TPBs) in solid oxide fuel cell (SOFC) cathodes are critical for the oxygen reduction reaction, and their degradation impacts SOFC performance and durability. This work examines nanostructure degradation of composite LSM/YSZ cathodes induced by electrochemical operation in humidified air. Three commercial button cells operated in humidified air for various durations at 800 °C exhibited more severe performance degradation than the cell operated in dry air. Microscopy imaging reveals nanostructure degradation within the cathode active layer, especially in the regions nearest to the electrolyte. Newly formed Mn-enriched nano-precipitates accompanied by nano-voids initiate at the original TPBs and propagate along the LSM/YSZ interface. The abundance of the nano-precipitates at LSM/YSZ interfaces increases with operation time, and prolonged operation in humidified air further promotes the formation of the Mn-enriched precipitates along YSZ/YSZ grain boundaries. The formation mechanism of those precipitates and their influence on the cathode performance are discussed.

Determination of the Activation Energy and Master Sintering Curve for NiO/YSZ Composite Solid Oxide Fuel Cell Anodes

Master sintering curves (MSCs) were constructed for tape cast NiO, YSZ and NiO/YSZ composite materials using apparent activation energies obtained through dilatometry experiments. A rule of mixtures approach was demonstrated to be a useful analysis tool in understanding the activation energy of the NiO/YSZ composites since NiO and YSZ do not affect the sintering of the other phase. Prediction of the 48 vol% NiO composite was less accurate because of competing sintering modes near the percolation threshold.

Effective transport properties of 3D multi-component microstructures with interface resistance

A three-dimensional image analysis tool has been developed to compute the effective conductivity of a particulate composite made of pores (material 0) and two materials (1) and (2). Three types of resistances are considered: the two resistances between materials (1)–(1) and (2)–(2) and the charge transfer resistance at the interface between materials (1) and (2). The numerical tool uses voxelised 3D numerical microstructures generated by discrete element simulations to mimic the particulate nature of the tested material. The method is validated by comparing its results to a resistance network model that incorporates interface resistance. The validation is conducted for both homogeneous and composite materials. Our tool has the advantage, compared with a resistance-network model, of allowing 3D tomography images to be treated. The method is applied to a microstructure typical of a composite Solid Oxide Fuel Cell cathode, however it is general enough to be a versatile tool for computing effective transport properties of any composite material where interface properties must be taken into account.

Simulation of sintering kinetics and microstructure evolution of composite solid oxide fuel cells electrodes

A three-dimension (3D) kinetic Monte Carlo (kMC) model is developed to study the sintering kinetics and microstructure evolution of solid oxide fuel cell (SOFC) composite electrodes during the co-sintering processes. The model employs Lanthanum Strontium Manganite (LSM) – Yttria-stabilized Zirconia (YSZ) composites as the example electrodes but can be applied to other materials. The sintering mechanisms include surface diffusion, grain boundary migration, vacancy creation, and annihilation. A morphological dilation method is used to generate the initial LSM–YSZ compacts as the input structures for the kMC simulation. The three-phase boundary (TPB) length, porosity, and tortuosity factor of the composite cathodes are calculated during kMC sintering. Simulation results are compared with literature data and good agreement is found. Parametric study is conducted to investigate the effects of particle size, size distribution, and sintering temperature on sintering kinetics as well as the evolution of electrode microstructures. The kMC model is capable of simulating the initial and a part of intermediate sintering stages of SOFC electrodes by considering various sintering mechanisms simultaneously. It can serve as a useful tool to design and optimize the sintering processes for composite SOFC electrodes.

Monte Carlo Investigation of Particle Properties Affecting TPB Formation and Conductivity in Composite Solid Oxide Fuel Cell Electrode-Electrolyte Interfaces

A previously developed microstructure model of a solid oxide fuel cell (SOFC) electrode-electrolyte interface has been applied to study the impacts of particle properties on these interfaces through the use of a Monte Carlo simulation method. Previous findings that have demonstrated the need to account for gaseous phase percolation have been confirmed through the current investigation. In particular, the effects of three-phase percolation critically affect the dependence of TPB formation and electrode conductivity on (1) conducting phase particle size distributions, (2) electronic:ionic conduction phase contrast, and (3) the amount of mixed electronic-ionic conductor (MEIC) included in the electrode. In particular, the role of differing percolation effectiveness between electronic and ionic phases has been shown to counteract and influence the role of the phase contrast. Porosity, however, has been found to not be a significant factor for active TPB formation in the range studied, but does not obviate the need for modeling the gas phase. In addition, the current work has investigated the inconsistencies in experimental literature results concerning the optimal particle size distribution. It has been found that utilizing smaller particles with a narrow size distribution is the preferable situation for electrode-electrolyte interface manufacturing. These findings stress the property-function relationships of fuel cell electrode materials.

Microstructure and Connectivity Quantification of Complex Composite Solid Oxide Fuel Cell Electrode Three‐Dimensional Networks

Composite electrodes extend the electrochemically active region for solid oxide fuel cells. The complex microstructural and chemical composition of composite cathodes often make them difficult to fully characterize. The discrimination between the two oxide phases of the composite cathode as well as between them and the pore phase has been achieved using epoxy impregnation. The active regions of composite cathodes were analyzed at various length scales using focused ion beam/scanning electron microscope (FIB/SEM) and transmission electron microscope techniques. Dual beam FIB/SEM three‐dimensional (3D) reconstructions provided information of various microstructural parameters. Over 5900 nodes were evaluated in these complex 3D networks. Topological connectivity of the composite LCM/ScSZ system was evaluated with the average connectivity ranging between 2.69 and 2.94 for the various phases. Topological length of the composite cathode network ranged between 1.5 and 2.7 μm, with average composite cathode particle sizes between 1 to 4 μm. Such connectivity quantification provides the opportunity for an advanced understanding of the transport processes in composite materials.

Connected Three-Phase Boundary Length Evaluation in Modeled Sintered Composite Solid Oxide Fuel Cell Electrodes

A numerical methodology for evaluating the three-phase boundary length (TPBL) in sintered composite solid oxide fuel cell electrodes is developed. Three-dimensional models of a representative volume element of sintered composite electrodes are generated for which the mean particle diameter, composition, and total porosity may be specified as input parameters. Tomographic methods are used to reconstruct the modeled electrode and the percolation for each phase is evaluated. The connected TPBL is calculated for a range of electrode designs and comparisons are made with calculated TPBL values available in the literature. The maximum connected TPBL occurred at a porosity of 0.21 and at equal solid volume fractions of ionic and electronic conducting phases for particles having the same mean diameter and particle size variance. A cubic envelope having a minimum length of 14 times the mean particle diameter was necessary to adequately represent the electrode structure.

Three-phase Boundary Length Evaluation in Modeled Sintered Composite Solid Oxide Fuel Cell Electrodes

A numerical methodology for evaluating the three-phase boundary length in sintered composite electrode layers is developed. Three dimensional models of sintered composite electrodes are generated for which the mean particle diameter, composition, and total porosity may be specified as input parameters. Tomographic methods are used to reconstruct the modeled electrode and the percolation for each phase is evaluated. The connected three-phase boundary length is calculated for a range of electrode designs.

Connected Three Phase Boundary Length in Modeled Sintered Composite Solid Oxide Fuel Cell Electrodes

not Available.

Finite-Element Modeling of Idealized Infiltrated Composite Solid Oxide Fuel Cell Cathodes

The polarization resistance of idealized, branched, composite cathodes was modeled using a two-dimensional finite element calculation. The model structures consisted of micrometer-scale columns and nanoscale branches of ionically conducting materials that were coated with a mixed-conducting material. The structures approximate an ionic-conductor matrix infiltrated first with the same ionic-conductor material and then with a mixed conductor. Increasing the length of the ionically conducting nanobranches, and hence, the surface area of the infiltrated mixed conductor, resulted in a factor of polarization resistance decrease compared to mixed-conductor-coated columns without ionically conducting nanobranches. For many solid oxide fuel cell relevant temperatures , cathode geometries, and materials, the cathode resistance was limited by surface oxygen exchange and hence, was inversely proportional to the mixed-conductor surface area. However, for cathode columns or branches with large enough aspect ratios, ionic-conduction losses also limited the polarization resistance. The characteristic length of nanostructured cathodes was found to depend on the cathode surface area ratio in addition to the traditional bulk diffusion constant to surface exchange constant ratio . Lastly, the effects of materials properties, particularly ionic conductivity and surface resistance, were investigated and discussed for common cathode materials.

Finite Element Modeling of Idealized Infiltrated Composite Solid Oxide Fuel Cell Cathodes

Using a two-dimensional finite element approach, the polarization resistance of idealized, branched, nano-particulate, composite cathodes was determined. Porous CGO, LSGM, or YSZ networks infiltrated with additional ionic conductor and subsequently infiltrated with LSCF or BSCF were modeled. For fixed mixed conductor particle size, dual nano-particle infiltrations (ionic + mixed conductor) resulted in an order of magnitude polarization resistance decrease, compared to single component mixed conductor infiltrations. For most SOFC relevant temperatures (500-900C), geometries, and material combinations, cathode performance was limited by the charge transfer reaction occurring at the mixed conductor interface and therefore scaled with the cathode surface area, as long as the cathode thickness was <~10microns. For cathodes thicker than ~10microns, losses within the ionic conducting network determined performance, resulting in a breakdown of the linear performance dependence on cathode surface area. The boundary between these regimes varied with ionic conducting cathode arm width, column width, and ionic conductivity.