The effect of anode microstructure and fuel utilization on current relaxation and concentration polarization of solid oxide fuel cell under electrical load change

Abstract

The effect of anode microstructure (i.e., porosity, tortuosity, and thickness) and fuel utilization (i.e., the flow rate ratio of fuel consumed to fuel supply) on the potentiodynamic response of a solid oxide fuel cell is elucidated by resolving thermo-electrochemical parameters temporally. To investigate physical and electrochemical processes occurring at electrodes upon electrical load change, a high-fidelity physicochemical model is used in this study. Locally distributed thermo-fluidic flow field and thermodynamic variables are resolved spatially and temporally by performing dynamic, three-dimensional numerical modeling. Results show that relaxation time is required for current density to asymptotically recover from its excessive response to potential steps to its original magnitude upon potentiodynamic conditions. This is predominantly attributed to anodic concentration polarization, indicating that the overall dynamic characteristic is primarily governed by diffusive transport phenomena in the anode. A parametric study for the anode microstructure and fuel utilization, which may influence the species transport in the anode, is conducted to find out methodologies to control the relaxation time. The parametric study shows that the microstructure has a trivial effect on the species diffusion velocity and the transient behavior upon electrical load change. On the other hand, the relaxation time is substantially influenced by fuel utilization such that it increases by 240% (from 0.4 s to 1.36 s) when raising the fuel utilization from 40% to 80%. Its sensitivity coefficient is nearly 2.0 which is substantially larger than −0.03 to 0.4 of anode microstructure. This implies that the relaxation time under electrical load change can be primarily controlled by selecting the optimal operating conditions, in particular in the fuel side.