CALIDAD DE VIDA

The fabrication and the composition of the cell with the composite anode made of YSZ and LST44Mn6, which was investigated in chapter 4.2, is similar to that of one of the cells investigated in chapter 3, apart from the fact, that in this case a different perovskite was impregnated. Instead of the A-site deficient perovskite titanate with 6 % of the B-site titanium substituted by manganese, the impregnated perovskite LSTM4646 in chapter 3 was not A-site deficient and 60 % of the B sites were occupied by manganese.

The skeletons of both cells, the impregnation of the cathodes, the percentage of the impregnated phases, the firing temperature of the skeleton, the calcination temperatures after the impregnation of the anode and the cathode and the material and application of the additional catalyst for the oxidation of hydrogen were the same or very similar in both cells, so comparing the two cells could help to provide insight into the difference between composite anodes of YSZ and A-site deficient titanates on the one side and non A-site deficient titanates on the other side.

4.3.1. Differences in the microstructure

The YSZ/LSTM4646 composite anode shows a very smooth coating of the perovskite over the YSZ skeleton, neither the irregular grains of the YSZ nor any other grain structures are visible. On 10-15 wt.% LST44Mn6 impregnated into the YSZ skeleton the irregular shaped YSZ grains are changed to round and more regular grains, and this new microstructure is covered by a flat blanket layer if the level of the impregnated perovskite is increased to the 45 wt.% necessary for the percolation in the anode of a working fuel cell. The development of the microstructure of this composite anode is described in chapter 4.1.1 and in figure 4.2. The differences between the microstructure of the composite anodes YSZ/LSTM4646 and YSZ/LST44Mn6 are shown in SEM micrographs and schematics in figure 4.16.

Figure 4.16: SEI images of the microstructure of composite anodes of YSZ and 45 wt.% of La0.4Sr0.6Ti0.4Mn0.6O3 (LSTM4646) and La0.4Sr0.4Mn0.06Ti0.94O3-γ (LST44Mn6)

at 7500x magnification and corresponding schematic.

After 6 hours reduction at 900 °C in an atmosphere of 5 % hydrogen and 95 % argon the YSZ/LSTM4646 composite showed no changes of the microstructure. The composite anode of YSZ/LST44Mn6 showed an increase of the size of the grains visible below the blanket layer, and the formation of terraces on the surface of these grains, as shown in figure 4.17.

Figure 4.17: SEI images of the microstructure of composite anodes of YSZ and 45 wt.% of La0.4Sr0.6Ti0.4Mn0.6O3 (LSTM4646) and La0.4Sr0.4Mn0.06Ti0.94O3-γ (LST44Mn6)

after reduction at 7500x magnification.

4.3.2. Differences in the electrochemical performance

The IV curves of the two tested cells with composite anodes of YSZ and LSTM4646 and of YSZ and LST44Mn6 show almost linear shape over the whole range of current density. The IV curve of the cell containing LST44Mn6 shows a lower gradient, which indicates a lower total resistance and better electrochemical performance.

Figure 4.18: Nyquist plots of EIS curves of cells with composite anodes of YSZ and LSTM4646 and of YSZ and LST44Mn6.

The Nyquist plots of the EIS curves of the two cells at 700 °C and 750 °C show that the superiority of the cell with the A-site deficient perovskite is due to a reduced ohmic resistance part of the total resistance. While the non ohmic polarisation is very similar with around 3.0 Ω*cm2 for the LSTM4646 cell and around 3.4 Ω*cm2 for the LST44Mn6 cell at 700 °C and around 2.1 Ω*cm2 for both cells at 750 °C, the linear resistance of the cell with the A-site deficient LST44Mn6 is only around 0.5 Ω*cm2 compared to around 3.9 Ω*cm2 for the LSTM4646 cell at 700 °C.

4.3.3. Conclusions

The microstructure of the composite anode composed of YSZ and the A-site deficient perovskite La0.4Sr0.4Mn0.06Ti0.94O3-γ (LST44Mn6) does not show the smooth

coating observed in the microstructure of the composite anode made of YSZ and La0.4Sr0.6Ti0.4Mn0.6O3 (LSTM4646). Since this smooth coating can be ascribed to an

interaction between manganese in the coating perovskite and the YSZ skeleton [10], the reason for this coating missing in the YSZ/LST44Mn6 composite, could be that the B-sites in LST44Mn6 are only doped with manganese to a level of 6 %, compared to 60 % of the B-sites occupied with manganese in LSTM4646.

While reduction at 900 °C does not change the appearance of the microstructure of the composite anode YSZ/LSTM4646, the grains partly visible under the blanket layer of the YSZ/LST44Mn6 anode seem to increase their size upon reduction and develop a terrace structure.

While the non ohmic polarisation of cells with composite anodes of YSZ/LSTM4646 and of YSZ/LST44Mn6 are very similar, the ohmic resistance of the cell with the A- site deficient perovskite is with around 0.5 Ω*cm2 at 700 °C only a fraction of the ohmic resistance measured in the YSZ/LSTM4646 cell, resulting in a much lower total resistance and better electrochemical performance. These results seem to indicate that the ohmic resistance in the anode dominates over the ohmic resistance in the cathode and the electrolyte, and that the electrochemical performance of test cells can be greatly enhanced by using a better conductive composite anode. The higher conductivity in A-site deficient perovskites compared to non A-site deficient perovskites has been described in literature [1] [7] [11], and this higher conductivity seems to improve the conductivity of the composite anode with YSZ considerably.