Effects of Current Collector Positions on Temperature and Reactive Current Distributions in a Micro-Tubular Solid Oxide Electrolysis Cell
Solid oxide electrolysis cells (SOECs) are devices which can generate hydrogen from high temperature steam by electrolysis techniques. For a micro-tubular SOEC, a current collector position affects the temperature and the reactive current distributions, and these distributions affect performance and durability. Understanding the distributions is important, however, researching the distributions by only experimental methods is difficult because of thin components, small radius and high operating temperature. Therefore, combining the experiment and numerical analysis is better way to understand phenomena in the SOEC.
In this study, the latter method is mainly reported. the two-dimensional tubular model of micro-tubular SOEC based on heat, mass and electric charge transfers was developed. The results of numerical analysis were compared with experimental data of micro-tubular SOEC. In addition, the conditions of co-flow and counter-flow were calculated and compared for the temperature distributions. Here, the co-flow means electron current flow is in parallel with gas flow. The counter-flow means electron current flow face gas flow. In this report, the anode gas flow is in parallel with the cathode gas flow.
2. Numerical model and analytic conditions
In the model, heat, mass and electric charge transfers were calculated simultaneously. Heat and mass transfers were calculated as two-dimensional models. On the other hand, the electronic charge transfer in anode electrode (the outside of the cylinder) and all ionic charge transfer were calculated as quasi-two-dimensional models to reduce computation time. The quasi-two-dimensional models mean that one-dimensional models for radius direction were lined up to direction of length. Electronic charge transfers in the cathode electrode (the inside of the cylinder) were calculated as two-dimensional models.
The anode electrode was mixed material of nickel (Ni) and yttrium stabilized zirconium (YSZ). The electrolyte was YSZ. The cathode electrode was mixed material of lanthanum strontium manganite (LSM) and YSZ. Current collectors were platinum (Pt). The thicknesses of anode electrode, electrolyte and cathode electrode were 200um, 20um and 20um, respectively. Supplied anode gas was mixed gas of hydrogen and steam (hydrogen/steam ratio = 1). Supplied cathode gas was air. The operating temperature was 1123K. Experiments for micro-tubular SOEC were carried out under the similarly conditions.
3. Results and discussions
Fig. 1 shows the relationship between current density and overpotential. Fig. 2 shows the relationship between overpotential and temperature change. In Fig. 1 and 2, both experimental data and results of numerical analysis are shown. In Fig. 2, experimental data and the results of numerical analysis is different, however, tendency of temperature changes was similar. In the part of lower overpotential, temperature changes were negative because of heat absorption with electrolysis reaction. On the other hand, in the part of higher overpotential, temperature changes were positive because amount of heat generation from overpotential was higher than that of heat absorption.
Fig. 3 is temperature distributions in a micro-tubular SOEC. Fig. 3(a) is positions of SOEC’s components. The current collector at the cathode was located at z = 4.0cm. Fig. 3(b) shows the distribution with co-flow condition. Fig.3(c) shows the distributions with counter-flow conditions. The maximum temperature change of co-flow condition was larger than that of counter-flow because the reactive current of the cathode was concentrated to the current collector side of the reactive area. The reactive area is range that z position is from 1.0cm to 3.0cm.