Microstructural Optimization of Anode-Supported Solid Oxide Fuel Cells by a Comprehensive Microscale Model

Abstract

The microstructural optimization of two-layer cathodes in anode-supported solid oxide fuel cells ͑SOFCs͒ was numerically performed by a comprehensive microscale model developed by the authors. The single-cell performances of anode-supported SOFCs, recently reported by Zhao and Virkar, were studied to provide the detailed information on the electrochemical processes occurring in the SOFCs. Good agreements between the numerical and experimental results were observed which also ensured the validity of the present calculation. The dependence of the electrochemical reaction and the mass transport on the particle size of each layer in two-layer cathodes was then studied. The optimal microstructure for two-layer cathodes was determined as a mean particle diameter of 0.5 m and a thickness of 15 m for cathode functional layers, and a mean particle diameter of 4.0 m and a thickness of 85 m for cathode current collector layers. The stack-cell performances of anode-supported SOFCs were also studied by fully considering the effect of interconnect rib geometry, e.g., the contact resistance, in-plane ohmic loss, and nonuniformity of current generation. Based on the simulation results, the geometrical criterion for interconnect rib geometry to obtain better performance was discussed.Solid oxide fuel cells ͑SOFCs͒ are high-temperature fuel cells that utilize the ionic conductivity of solid electrolytes at high temperatures ͑650-1000°C͒. 1,2 The high operation temperature enables the use of natural gas, liquidified petroleum gas ͑LPG͒, and methanol as fuels of SOFCs after internal reforming in anodes to produce hydrogen from the hydrocarbons. The high efficiency of SOFCs is primarily due to the fast electrochemical reaction at those elevated temperatures. In addition, the overall efficiency can be even more enhanced when high-quality waste heat is utilized by hybrid or cogeneration systems combined with SOFCs.Many researchers are currently working on the development of SOFCs that can operate at intermediate temperature range, less than 800°C. 3-7 The relatively low operation temperature is believed to enhance the long-term stability of SOFCs with reduced thermal degradation of electrodes due to operational sintering. However, low ionic conductivities of the electrolyte at those reduced temperatures hinder the use of thick electrolytes as in electrolyte-supported SOFCs. Anode-supported SOFCs with thinner electrolytes attracted much research interest for their relatively low operation temperature and also for their high current densities and low fabrication costs. 6 Typically, an anode-supported SOFC is fabricated to have a thick cermet anode made of nickel ͑Ni͒ and yttria-stabilized zirconia ͑YSZ͒, and a thin composite cathode made of strontium-doped lanthanum manganite ͑LSM͒ and YSZ, attached to either side of a very thin YSZ electrolyte. In general, the thicknesses of SOFC components are ϳ1 mm for the anode, ϳ50 m for the cathode, and ϳ10 m for the electrolyte. The electrodes of anodesupported SOFCs are frequently made to have mu...

Percolation Theory in Solid Oxide Fuel Cell Composite Electrodes with a Mixed Electronic and Ionic Conductor

“…Equations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) can be used to predict the effective properties of the composite electrode using the mono-particle size distribution of each type of material. Equations (20)(21)(22)(23)(24)(25)(26)(27)(28)(29) can be used to predict the effective properties of the composite electrode using the poly-dispersed particle sizes. To provide a general form of the extended percolation theory, all of the calculated properties will be presented in non-dimensional forms.…”

“…where the number of LSCF particles per unit volume within the entire composite electrode structure can be estimated using Equation (4) [12,20]. It is generally accepted that the global reaction rate will be governed by many multiple elementary chemical steps and by the transport processes in the gas phase, in the solid bulk and on the surface, such as the adsorption/desorption/spillover and the charge transfer reactions [33].…”

“…The cathodic electrochemical reaction will occur not only within the composite cathode body but also at the interface of the composite cathode and the dense electrolyte [20]. The percolated electrochemical reaction site per unit electrolyte surface area can be estimated by:…”

“…The effects of the pore former is further considered into the percolation theory by Bertei et al [18]. Furthermore, the percolation micro-models are generally agreed and widely incorporated into the cell-level models to evaluate the effects of the electrode microstructure parameters on the SOFC performance [19][20][21][22].…”

Optimization of the Interconnect Ribs for a Cathode-Supported Solid Oxide Fuel Cell

“…Ji et al [16] showed that the terminal output of a SOFC stack depended strongly on the contact resistance. Jeon et al [17] proposed a detailed microstructural model and examined systematically the influence of the rib width, pitch width and the contact area specific resistance (ASR contact ) on the stack-cell performance. Noh et al [18] showed that by modifying the current collection configuration, cell performance was improved by more than 30%.…”