Results and Discussion

The mathematical SOEC model is then employed to investigate the cell performance. In particular, the study will determine how the size and location of delaminations at electrode/electrolyte interface affect SOEC performance. A delamination failure occurring at oxygen electrode/electrolyte interface is a gas filled gap, in a plane which is perpendicular to the main ionic/electronic current direction. Because of solid material discontinuity induced by delamination, the ionic/electronic conducting path is cutoff locally, the possible TPB sites are also ruined out. Obviously, delamination will influence local mass/charge transport and electrochemical reactions, and consequently affect SOEC performance. It is assumed that charges are not able to jump over the delamination site, as a result, the boundary conditions (BCs) for charge transport are treated as insulated BCs at this site.

3.4.1 Delamination effects with parallel flows

In this case study, the flows in anode and cathode channels are set as parallel flows. Hydrogen mixed with vapor is used as inlet carrier gas and reactant in cathode channel respectively while air is employed as carrier gas in anode channel. The simulation results are shown in Figure 3.3. Due to the electrolysis effect, hydrogen and oxygen are generated in cathode and anode electrodes respectively, as a result, the mass

fraction of hydrogen increases from 0.0846 to 0.167 along the cathode channel while that of oxygen increases from 0.243 to 0.34 along the anode channel.

The corresponding ionic current density distribution is shown in the base case of Figure 3.4. We then introduce delaminations at oxygen electrode/electrolyte interface. The delamination 1 is set at left side near the inlet as shown Figure 3.4(b). The

Delamination 1 (b)

Delamination 2 (c)

Delamination 3 (d)

×10-3

Figure 3.4 Ionic current density distributions: (a) Base case without delamination, (b) With delamination 1, (c) With delamination 2, (d) With delamination 3

Base Case (a) _{Base Case (a)}

Figure 3.5 SOEC performance sensitivity to delaminations in parallel flow

delamination 2 is set at the same site of delamination 1 but the delamination size is doubled as shown in Figure 3.4(c). The size of delamination 3 is the same as that of delamination 2, however, the location of delamination 3 is set at the center of the cell along the flow direction as shown in Figure 3.4(d). These three delaminations are individually introduced. Comparing ionic current density distribution in base cases and delamination cases, one can see that delaminations cut off the charge transport path and significantly influence local current density distributions. The effect of delaminations on cell polarization performance is shown in Figure 3.5. Both the delamination location and size affect the cell performance. For the parallel flow considered in this case, the cell performance is more sensitive to the delamination at the center of the cell (delamination 3) than that at the inlet of the cell (delamination 1 and 2). For the same delamination location, increasing the delamiantion size (delamiantion 1 to delamination 2) will make the cell performance a little bit worse.

3.4.2 Delamination effects with counter flows

As a comparison, counter flow is utilized in this case study, where the mixture of hydrogen and vapor flows through the hydrogen channel from the left side to the right side while air flows through the oxygen channel from the right side to the left side. Without any delaminations, the mass fraction distributions are shown in Figure 3.6. Basically the hydrogen mass fraction increases from 0.0849 to 0.163 along the flow

direction while the oxygen mass fraction increases from 0.243 to 0.336 along the air flow direction.

The corresponding ionic current density distributions are shown in Figure 3.7. We then individually introduce three delaminations. Delamination 4 with size 4.0e-4m is set at the oxygen electrode/electrolyte interface at the left end of the cell, delamination 5 with the same size (4.0e-4m) is introduced at the oxygen electrode/electrolyte interface but at the right end of the cell. Finally delamination 6 with size 8.0e-4m is set at the center of the cell along the oxygen electrode/electrolyte interface. The corresponding ionic current density distributions are shown in Figure 3.7(b)(c)(d), respectively. Comparing the base case in Figure 3.7(b)(c)(d), one can see that the local ionic current density distributions are significantly influenced since the charge transport path is cutoff at the delamination site. To highlight the sensitivity of cell performance to the delaminations, the cell V-I curve is obtained under different delamination settings as shown in Figure 3.8. It is clear to see that without delamination, the cell obtains the best performance. Once delamination is introduced, the cell performance gets worse, among

Figure 3.7 Ionic current density distribution: (a) base case, (b) with delamination 4, (c) with delamination 5, (d) with delamination 6

Base Case (a)

Delamination 4 (b) _{Delamination 6 (d)}

Base Case (a)

Delamination 5 (c)

which the cell with delamination 4 obtains similar performance to that with delamination 5 and the cell shows a slight high sensitivity to delamination 5 at relatively high current density conditions. When the delamination 6 is introduced, the cell obtains the worst performance.

3.5 CONCLUSION

In this research, a 2-D CFD model is developed for a planar SOEC. The model is validated using experimental data of a button cell under different temperatures. The model is utilized to investigate the sensitivity of electrolysis performance to deliminations occurred at oxygen electrode/electrolyte interface. Results indicate that delaminations significantly influence local charge current density distributions since the charge transport path is cutoff. In both parallel flow and counter flow settings, electrolysis performance is more sensitive to the delamination occurred at the center of the cell than those occurred at the edges of the cell.

Figure 3.8 SOEC performance sensitivity to