Effect of Exchange Current Density on Current Distribution at Planar-Type SOFC Anodes

Introduction It has been reported for SOFC systems that high electrical efficiency of more than 70-80% could be achieved theoretically in a recent study [1]. However, their electrochemical performance would become lower due to an increase in ohmic and nonohmic resistances and possible fuel starvation. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena in fuel cells. Here, we focus on numerical analysis on electrode kinetics. Since exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at an SOFC electrode and in simulating the performance, the exchange current density of methane-fueled pre-reformed SOFC anodes was measured and compared with that of hydrogen-based fuel. Furthermore, the spatial distribution of current density was simulated for given average current density in case anode exchange current density i 0 is either constant or variable. Experiment In this study, electrolyte-supported cells Ni-ScSZ/ScSZ/LSM-ScSZ (ScSZ:10mol%Sc 2 O 3 - 1mol%CeO 2 -89mol% ZrO 2 ) were used. The electrode area was 8×8mm 2 for i 0 measurements. Pt mesh was used as a current collector. In this study, by changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each (average) current density (0.25, 0.3, 0.35, and 0.4 cm 2 ) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values. Results and discussion Anode exchange current density for hydrogen-fueled SOFCs may be described by Eq. (I) after Hosoi et al. [2]. Anode exchange current density could also be described by Eq. (II) in case vacant surface adsorption sites are predominant [2,3]. Figure 1(a) shows experimental values derived for the pre-reformed fuel in this study. The values derived from Eq. (i) are also shown with solid lines in Fig. 1(a) and (b), while the values derived from Eq. (II) are shown with dashed lines in Fig. 1(b). All these figures indicate gradual decrease in anode exchange current density in the high fuel utilization (Uf) region. i 0,a = 3.5・exp(-6.2×10 4 [Jmol -1 ] / RT)・(P H2 / P H2,ref ) 0.41 ・(P H2O /P H2O,ref ) 0.4 Eq. (I) i 0,a = a ’ ・exp(b ’ [Jmol -1 ] /RT)・P H2 0.5 ・P H2O 0.5 Eq. (II) 3D-numerical simulation has been made using the software COMSOL to derive 2D current density of the cell shown i