Significant Influences of Operating Temperature and Water Vapor Distribution on the Power Density Reduction of Tubular Solid Oxide Fuel Cells during Scale-Up Processes

The power density of a solid oxide fuel cell (SOFC) can be reduced by 70% as it scales up from a button cell to a kW-class stack. The significant power density reduction constrains wider adoption in powering vehicles. In this study, we first experimentally examined the power density reduction of a tubular SOFC under different operating conditions. A numerical model is then constructed to conduct quantitative analysis since the model can eliminate the experimental uncertainty caused by cell performance degradation from repeated trials. The influences of operating temperatures and inlet fuel flow rates on the power density reduction of tubular cells with three lengths are examined. In this article, as the first step, we assume a uniform cell temperature profile and a constant oxygen mass fraction at the cathode surface. We show that the impact of the operating temperature on the power density reduction is greater than that of the inlet fuel flow rate. As the cell length is increased from 1 to 9 cm, the average power density is reduced by 28% at 600 °C, and it is increased to 44% at 700 °C. However, simply increasing the inlet fuel flow rate from 80 mL/min to 240 mL/min can only improve the power density of a 9 cm-long tubular cell from 0.148 W/cm2 to 0.180 W/cm2, which is still 35% less than the theoretical prediction. A possible reason for more severe power density reduction in solid oxide fuel cells compared with proton exchange membrane fuel cells is that water vapor is generated at the anode instead of the cathode, which is a crucial factor for power density reduction during a scale-up process. An appropriate method to remove the water vapor generated from the electrochemical reactions can effectively solve the problem of the power density reduction.