The Current Density Distribution in a Segmented-in-Series SOFC

A common tubular solid oxide fuel cell (SOFC) design consists of segmented-in-series electrochemical cells fabricated onto the outside of a porous support tube. Predicting the performance of this type of SOFC requires a detailed understanding of the current density distribution within each cell. This distribution is strongly coupled to the activation, concentration, and Ohmic losses, which occur as a result of the physical transport processes within the cell. A new computer code, known as the SOHAB code, has been developed to simulate these physical processes and thus make predictions of cell performance. The simulation results show how the magnitude of each loss varies spatially within the cell, causing the calculated current density distribution to be very different from that predicted by the established purely Ohmic models. At low currents the cell behavior is dominated by activation losses producing a very flat distribution. At moderate currents the Ohmic losses become more important, and the distribution is peaked at the edges of the electrolyte. At high currents the increased concentration losses flatten the distribution in the middle of the cell but not near its edges where gases flow from the surrounding inactive regions and the losses remain small. At low and moderate currents, the calculated current density distribution is sufficiently flat that the assumption of a uniform distribution can be used in conjunction with a one-dimensional model. However, at high currents this simplified model overestimates the concentration loss as it cannot account for the improved mass transport near the electrolyte edges.