7.3.2.1. Air Atmosphere
Conductivity measurements were performed on samples of La0.6MnTi0.4O3, La0.7MnTi0.3O3 and La0.8MnTi0.2O3 (x = 0.4, 0.3 and 0.2). Initial measurements were taken in air, and these results showed semiconductor behaviour with conductivities in the
range 1.6 to 12.5 S cm-1 at 850 °C; the conductivities were lowest for samples with higher Ti content, as shown in Table 23, Figure 52 and Figure 53.
x 0.2 0.3 0.4
Conductivity (S cm-1) at 800 °C 11.91 7.62 1.44 Conductivity (S cm-1) at 700 °C 10.42 6.60 1.13
Table 23. Conductivity data in air for the series La1-xMnxMn1-xTixO3
In addition, the activation energies also increased across the series as the Ti content (x) increased, Ea = 46.5, 49.2, 59.5 kJ mol-1 for x = 0.2, 0.3, 0.4 respectively as calculated from Figure 53. The decrease in conductivity in air with increasing Ti content is to be expected due to the disruption of the Mn-O network. These activation energies are higher than other similar systems: for example,SrFeO3-δ doped with Mo shows values between 19 - 34 kJ mol-1 in air, while for La0.75Sr0.25Cr0.5Mn0.5O3-δ has activation energies of 20.2 kJ mol-1 in air and 54 kJ mol-1 in H2 [49, 176, 207].
Value of x
0.15 0.20 0.25 0.30 0.35 0.40 0.45
Log10 Conductivity (S cm-1 )
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Conductivity in Air Conductivity in 5%H2/N2
Figure 52. Variation in the conductivities of the series La1-xMnxMn1-xTixO3-δ in air and 5%H2/N2 at 850 °C
Figure 53. Log10 σ vs 1000/T for the series La1-xMnxMn1-xTixO3-δ in air
7.3.2.2. Reducing Atmosphere
The conductivities were then measured under 5%H2/N2. Samples were heated in this atmosphere up to 850 °C and held for 1 hour before making measurements on cooling.
The data showed that the electronic conductivities of all three samples dropped by several orders of magnitude, to be in the range of 0.015 to 0.4 S cm-1.
x 0.2 0.3 0.4 Conductivity (S cm-1) at 800 °C 0.33 0.20 0.01 Conductivity (S cm-1) at 700 °C 0.22 0.11 0.01
Table 24. Conductivity data in 5%H2/N2 for the series La1-xMnxMn1-xTixO3 in 5%H2/N2
Figure 54. Graph of Log10 σ vs 1000/T of series La1-xMnxMn1-xTixO3-δ in 5%H2/N2
As before, the lowest conductivities were observed for the samples with the higher Ti content. The range of difference between the samples is again apparent, with over an
order of magnitude difference between the x = 0.2 and 0.3 and the x = 0.4 samples at higher temperatures. The activation energies displayed the same trend once more, rising as x increases; with Ea = 188.0, 154.3, 114.8 kJ mol-1 for x = 0.4, 0.3, 0.2 respectively at 800 °C, considerably higher than alternative materials [49]. Thus in addition to the lower conductivities, the activation energies are considerably higher than in air, especially for the high Ti content samples. The samples were analysed by XRD after the measurement, which confirmed that partial decomposition had occurred. In addition, the observed conductivities are too low for SOFC anode applications regardless of material redox stabilities.
7.4. Conclusions
In this work, a new series of perovskite systems has been synthesised, La1-xMnxMn
1-xTixO3-δ (x ≤ 0.4), stable under SOFC cathode operating conditions and displaying partial decomposition in SOFC anode operating conditions. While reasonable conductivities were observed in air, the conductivities decreased substantially in reducing environments, indicating that these systems are not suitable for anode applications.
Work within this Chapter has been published in the International Journal of Low-Carbon Technologies, Volume 7, Issue 1, Pages 60-62, 2011.
Chapter 8. Conclusions and Further Work
Current research suggests that perovskite materials have the potential to display suitable properties and characteristics as electrode materials for use within SOFCs. The doping of several systems proved moderately successful, with improvements over the original, undoped materials being recorded.
Oxyanion doping has proved a particularly viable strategy towards improving the performance of transition metal containing perovskite materials. In the present case of doping into SrFeO3-δ, this can partially be attributed to the stabilisation of the cubic phase in reducing environments, leading to a large increase in conductivity. Further work on this particular series could include more complete testing within a SOFC, further compatibility studies with existing electrolyte materials or doping with alternative oxyanions to study the effect on material characteristics in more detail. This oxyanion doping strategy has also proved successful in material systems other than perovskites, such as Ruddlesden Popper phases [64, 65, 67, 68, 171, 173, 208]. There is therefore wide scope for further work in this field, with a large range of materials potentially suitable for oxyanion doping.
For the Sr0.8Ti0.6Nb0.4O3-δ materials, vanadium doping was shown to have a positive effect on conductivity. The best performance was in the material Sr0.8Ti0.6Nb0.2V0.2O3-δ, where the electronic conductivity in 5%H2/N2 was shown to be fairly stable across a range of temperatures from 800 to 300 °C, in the region of 2 - 6 S cm-1, after reduction at 930 °C. The undoped sample had previously shown good stability in reducing
conditions, albeit with a resistance to reoxidisation caused by slow oxygen transport [146]. The V doped samples, in contrast, displayed far faster reoxidisation, even with as little as 5% vanadium on the B-site, suggesting higher oxide ion conductivity. Thermal expansion coefficients were shown to be comparable with existing electrolyte materials such as YSZ, however further work on chemical compatibility with these existing SOFC materials is needed. Additional work investigating V doping of other Sr1-x/2Ti1-xNbxO3-δ systems may also give further insight into some of the unexpected cell parameter changes of Sr0.8Ti0.6Nb0.4-xVxO3-δ.
The La1-xMnxMn1-xTixO3-δ series of materials proved less successful. It proved difficult to obtain phase pure samples in many cases, with small secondary peaks visible at higher x values. In addition, after reduction at 800 °C in 5%H2/N2 it can be seen that the structure is degraded, indicating instability under these conditions. Nevertheless, reasonable conductivities were obtained in air, with the highest values recorded for the x
= 0.2 sample. Some questions remain over whether there is indeed the incorporation of Mn onto the A-site, with the possibility of the formation of A-site deficient structures instead. This would require further verification by other techniques such as combined X-ray and neutron diffraction studies in order to accurately determine site occupancies.
The extrusion work also proved problematic, with issues surrounding repeatability and cell performance. While the work carried out appears to indicate the successful formation of tubes, these suffered from irregularities in wall thickness. In addition, many variations around material layering and cell preparation failed to produce a successful cell. In order for additional work to be viable, the extrusion die may need redesigning to allow greater
rigidity and so more regular tube sizes, and layering techniques would require further investigation to provide more repeatable results.
Moving forward, work into perovskite-based SOFCs could be driven forward through further fabrication of cells in order to form stacks, allowing increased testing profiles to be carried out on completed cells to better characterise the materials and properties. A wider selection of fuels, including the ability to introduce potential contaminants and poisons into the fuel mix, would also allow much greater in-situ analysis of materials under various performance conditions. The ability to introduce known contaminants into fuel streams, or even to run directly off the national gas grid, has the potential to prove the suitability of novel materials and demonstrate in-situ performance.
If in-situ analysis of novel materials was to progress on a larger scale, the ability to fabricate planar cells would simplify the process and, if a uniform cell size was established, cell sealing could be made more straightforward which would allow faster changeover of cells analysed. It would also facilitate more straightforward stack building if gasket materials and cell interconnects could be established, allowing far greater range and duration of testing protocols. This work could compliment work on m-SOFCs which could provide faster verification of suitability of materials due to their better thermal characteristics.
Integration of SOFCs into buildings as part of CHP systems would allow in-situ testing of systems and principals for widespread use of fuel cell systems as part of a microgeneration energy solution. Development of protocols for start-up, shut down and
load cycling, amongst other things, could help to build up the requirements for future progress in the area. In addition, data could be transmitted and processed in real time, allowing remote monitoring of the system and easing the expansion of such a program by facilitating centralised system control and development.
Many of these goals could be achieved through the creation of a spin-out company, supported directly by universities, allowing for transfer of knowledge and access to facilities while enabling the pursuit of more business- and production-oriented development. This would allow greater autonomy and the potential to build business arrangements with larger companies better positioned in the market to roll out future technologies, possibly to a degree above and beyond that which a university could support directly. In addition, by focussing less on the development of novel materials, cells and systems, and more on marketisation, it would have the potential to increase the speed at which products become commercially viable.