(378d) Predictive Modeling Insights for Active Three-Phase Boundary Sites in Solid Oxide Fuel Cell Infiltrated Electrodes

A Solid Oxide Fuel Cell (SOFC) is an electrochemical device that converts chemical energy into electrical energy by directly and efficiently oxidizing hydrocarbon fuels at high temperature.  Fuel oxidation and oxygen reduction reactions occur in the electrodes.  For an electrode to be functional the following four properties must present: 1) porosity for mass transport, 2) catalytic activity for the reaction, 3) oxygen ion conductivity, and 4) electronic conductivity.  Infiltrated electrodes, where electronic conductor/catalyst particles are infiltrated into an oxygen ion conducting porous scaffold, can result in outstanding electrochemical performance. In addition, infiltration allows one to calcine the infiltrated particles at much lower temperatures than the scaffold sintering temperature. The benefits conferred include less electronic conducting material to form an interconnected conductive pathway, reduced dimensional instabilities within the fuel cell, and greater control over the infiltrate morphology. While the infiltration method provides great flexibility in composite composition and structure, the influence of the controllable parameters are not well understood. Furthermore, fabrication and testing of anode composites representing the full range and number of working parameters is time consuming. Thus, a model to guide experiments is desired. 

In previous work, we have demonstrated the ability of our mechanistic model to predict effective conductivity of the fuel cell electrode as a function of important processing variables such as scaffold particle and pore size, infiltrate size and loading, porosity, and initial scaffold film formulation. In this talk, we will highlight our recent work, building on this model, to predict the density of active three phase boundary (TPB) sites in the electrode as a function of the same processing variables.  These TPB sites, the location where the electrochemical reaction occurs, are located at the intersection of (1) void space, allowing for the presence of gas-phase reactants, (2) electronically conducting/catalytically active particles and (3) oxygen ion conducting backbone.  The model itself contains no adjustable parameters and is based upon the underlying physics of the entire fabrication process, which includes the formation of a presintered film, evaporation of the solvent in that film, burning off of the pore formers in that film, sintering of the ceramic scaffold and infiltration of conducting particles.  In addition to reporting the resulting predicted TPB densities and their relation to corresponding electronic conductivities, we identify the key underlying limiting structural feature for TPB density, interfacial surface areas of each pair of phases.  Further, we provide insight into the way these limiting features as well as the other modeling results suggest areas for expanding the experimental research design space.  Finally, we will discuss the opportunities to use this type of modeling to incorporate other SOFC properties leading to overall design optimization.