[en] Highlights: • Anode concentration polarization plays a key role in determining dynamic response. • Microstructure of an anode support has a negligible impact on transient behavior. • Raising the fuel utilization from 40% to 80% increases the relaxation time by 240%. • Local fuel depletion towards fuel outlet induces current undershoot and relaxation. • Controlling the local fuel concentration is critical for improving dynamic response. -- 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.

[en] Highlights: • Hydrocarbon-fueled SOFC operation may induce lower power output and reliability. • Anode thermochemical reaction zones are concentrated in the vicinity of fuel inlet. • Transport properties alter heat and mass transfer and electrochemical conditions. • Higher concentration and activation overpotentials reduce an overall cell voltage. • Larger temperature gradient and heat transfer with a neighboring unit are expected. - Abstract: Key features of hydrocarbon-fueled solid oxide fuel cell stack operation are elucidated by examining its local thermodynamic states with an aid of three-dimensional numerical simulations. A high-fidelity physical model, which resolves the coupling between thermo-chemical reactions and heat and mass transport, is developed and validated. To elucidate important reactions and transport phenomena, local thermodynamic state variables of hydrocarbon-fueled operation are compared with those estimated by assuming pure-hydrogen-supplied operation. Results show that thermochemical reactions proceed at high rates through the thick anode support layer. This induces complete methane conversion as soon as it is introduced to the anode and thermochemical reaction zones concentrated in the vicinity of the fuel inlet. In spite of the fast reaction processes, hydrocarbon-fueled operation has the same electrical current density profile as pure-hydrogen-supplied operation, resulting from changing its local thermodynamic states. Given that the presence of carbon substances and thermochemical reactions, in hydrocarbon-fueled operation, local chemical and electrical conditions are substantially different from those of pure-hydrogen-supplied operation. A lower hydrogen concentration induces a higher concentration overpotential and decreases a reversible electrochemical potential. A lower exchange current density is offset by increasing an activation overpotential at a given applied current. All these reduce the overall cell voltage, as compared to pure-hydrogen-supplied operation. Variation of transport properties such as diffusivities and viscosities influences heat and mass transport such that substantial stresses can be imposed on cell materials. In addition, thermal conditions result in lower incoming-gas heating and a larger heat transfer rate to a neighboring repeating unit. A larger temperature gradient near the fuel inlet may also impose stresses cell materials. A lower power output, attributed to the electrochemical losses in a form of activation and concentration overpotentials, and materials degradation can be accompanied in hydrocarbon-fueled stack operation.