Electrochemical performance of composite anodes with a CGO layer

A set of button cells was prepared by tape casting and impregnation method like described in chapter 2.1 with a skeleton of YSZ fired at 1400 °C. The anode skeleton of a part of the cells was impregnated with CGO and all of them with an A-site deficient perovskite, either LST44 or LST44Mn6. The cathode was impregnated with La0.8Sr0.2FeO3-δ (LSF82) and a palladium catalyst was added to the anode side. The

four cells with four different composite anodes YSZ/LST44, YSZ/CGO/LST44, YSZ/LST44Mn6 and YSZ/CGO/LST44Mn6 were electrochemically tested using a test jig, a Solartron cell test system and a frequency response analyser, recording IV curves and EIS spectra.

Figure 4.24: IV curves of cells with composite anodes of YSZ/LST44, YSZ/CGO/LST44, YSZ/LST44Mn6 and YSZ/CGO/LST44Mn6.

The IV curves of all four cells are close to linear over the whole range of current density, as shown in figure 4.24. The superiority in the electrochemical performance of the YSZ/LST44Mn6 cell over the YSZ/LST44 cell has been discussed in chapter 4.2.3. The gradient of the IV curves show hardly any difference between the electrochemical performance of the YSZ/LST44 cell and the YSZ/CGO/LST44 cell with their maximal power densities at 750 °C being 90.1 mW/cm2 and 88.5 mW/cm2, respectively. The electrochemical performance of the YSZ/LST44Mn6 cell, with a maximal power density at 750 °C of 156.2 mW/cm2, deteriorates with an additional layer of CGO, the maximal power density of the YSZ/CGO/LST44Mn6 cell is only 90.6 mW/cm2.

Figure 4.25: Nyquist plots of the EIS curves of cells with composite anodes of YSZ/LST44, YSZ/CGO/LST44, YSZ/LST44Mn6 and YSZ/CGO/LST44Mn6.

Both the ohmic resistance RS and the non ohmic polarisation RP of the cells with

composite anodes of YSZ/LST44 and of YSZ/CGO/LST44 have similar values of around 1.4 to 1.6 Ω*cm2 for RS and around 3.8 to 4.3 Ω*cm2 for RP at 700 °C. The

CGO layer over the skeleton seems not to be of great influence on the impedance of the YSZ/LST44 anode, but, as figure 4.25 shows, does enlarge both the ohmic resistance and the non ohmic polarisation of the cell with the YSZ/LST44Mn6 anode. The value for RS is around 0.51 Ω*cm2 for the YSZ/LST44Mn6 cell and around 2.1

Ω*cm2 for the YSZ/CGO/LST44Mn6 cell at 700 °C, and at the same temperature RP

is 3.4 Ω*cm2 for the YSZ/LST44Mn6 cell and 5.1 Ω*cm2 for the YSZ/CGO/LST44Mn6 cell.

In the Nyquist plots of both cells with a CGO layer the shapes of the curves show two clearly separated depressed semicircle arcs, making it clearly possible to distinguish a low frequency polarisation RP2 with a characteristic frequency of around

0.2 Hz and a medium frequency polarisation RP1 with a characteristic frequency of

10-20 Hz at 700 °C. These two separated arcs can’t be observed in the cells without CGO layer, YSZ/LST44 and YSZ/LST44Mn6. The shape of the Nyquist curves with two arcs for RP1 and RP2 resembles the shapes of Nyquist plots of cells with

4.4.3. Conclusions

YSZ skeletons were covered with CGO by impregnation with an aqueous precursor and firing before impregnating them with an additional phase of the A-site deficient perovskites LST44 and LST44Mn6, in a way that the composition of the final composite anode was around 54 wt.% YSZ, 6 wt.% CGO and 40 wt.% perovskite.

Even if the composite anodes consisted around 6 wt.% CGO, no CGO peaks showed in the XRD, the diagram of the composite anode YSZ/CGO/LST44Mn6 shows exactly the same peaks as the diagram of the composite anode YSZ/LST44Mn6, but the peaks assigned to YSZ are broader and show shoulders on their left sides, which probably are caused by diffusion of Ce4+ into the YSZ lattice as described in literature [16]. The additional doping with Ce4+ also inhibits the formation of a secondary phase of tetragonally distorted YSZ after impregnation and calcination.

The microstructure under oxidizing conditions is very similar in the composite anodes YSZ/LST44Mn6 and YSZ/CGO/LST44Mn6. After reduction at 1050 °C both composite anodes show exsolution of nanoparticles, but while the surface of the grains are evenly decorated with nanoparticles in the YSZ/CGO/LST44Mn6 anode, the nanoparticles are concentrated on certain areas in the YSZ/LST44Mn6 anode. The exsolution of nanoparticles under the same reduction conditions can also be observed in composite anodes with perovskites not doped with manganese, YSZ/LST44 and YSZ/CGO/LST44, so it is not dependent on a doped B-site of the perovskite. As stated in literature the exsolved nanoparticles are MnO in case of the reduction of the manganese containing perovskite LST44Mn6 and Ti2O3 after the

The CGO layer did not improve the elctrochemical performance of the cells. There was hardly any difference in RS or RP between the YSZ/LST44 cell and the

YSZ/CGO/LST44 cell, and the performance of the YSZ/CGO/LST44Mn6 had considerably higher RS and RP values than the YSZ/LST44Mn6 cell, probably

because the electrochemical performance of the YSZ/LST44Mn6 cell was extraordinarily good without CGO layer.

The shape of the Nyquist plots of the cells with CGO the containing anodes YSZ/CGO/LST44 and YSZ/CGO/LST44Mn6 showed two depressed semi arcs unlike both cells without CGO, YSZ/LST44 and YSZ/LST44Mn6, which both only show one arc. This way the shape of the Nyquist plots resemble the shape of the cells with a CGO skeleton in chapter 3, even if the anodes of these cells only contained 6 wt.% CGO.

4.5. The influence of the calcination temperature on the