Lanthanum-based perovskites are widely used cathode materials. Normally, very high sintering temperature is usually used for densification of the YSZ. Consequently, such temperatures prevent the use of many potential La-based perovskites, as they tend to react with YSZ to form the insulating La2Zr2O7, or worse, cause chemical/physical process instability at such temperatures. Several measures can be taken to avoid exposure of these more reactive materials to the high temperature sintering process.
One interesting measure that has caught the attention of many scholars is the impregnation of perovskites materials into a porous well-sintered YSZ scaffold. This technique offers a feasible way to fabricate the composite electrode at low temperatures. This approach has been explored far and wide, with different investigators reporting varying or conflicting results. Within the haystack, the needle has been found and it involves optimization of the microstructure of the YSZ Scaffold. Better still, the optimization of microstructure, across length scales has proven to be particularly useful in promoting and understanding the performance of electrodes. However, thorough characterization of porous scaffolds for building SOFC cathodes is still required.
Recently, University of St Andrews researchers Dr. Chengsheng Ni, Dr. Mark Cassidy and Professor John Irvine conducted a study high-resolution SEM imaging in a bid to better understand the effect of particle size of YSZ on the microstructure of porous scaffold. In addition, the electrochemical performance of the impregnated LSF-YSZ composite electrode was also evaluated by AC impedance. Their work is currently published in Journal of the European Ceramic Society.
In their studies two types of YSZ powders, U1 and U2, were used to quantify the porous scaffolds. Specifically, AC impedance on symmetrical cells was also applied to evaluate the performance of the electrode impregnated with 35-wt.% La0.8Sr0.2FeO3. Lastly, scanning electron microscopy (SEM) images were analyzed.
The authors observed that the powder with 9 vol. % particles smaller than 0.3μm and bimodal distribution in the powder (U1 powder) induced the faster YSZ grain growth and decreased in surface area when compared to powders (U2 powder). Moreover, the electrode using scaffolds made from U2 powder with narrow particle size distribution showed the respective polarization and series resistance that were in agreement with the prediction of the image analysis.
In summary, University of St Andrews study presented the application of two types of powders, of different sintering behaviors, in the preparation of porous YSZ scaffolds for impregnation. Generally, SEM image quantification was reported to be consistent with the sintering behavior of the YSZ powder as was the performance of the impregnated electrode. Altogether, the quantitative study on image of the sintered scaffold indicates that U2 powder is better at producing architecture of high porosity or long triple phase boundary (TPB), which is attributed as the reason for the higher performance of the LaFeO3 (LSF)-impregnated electrode.
Chengsheng Ni, Mark Cassidy, John T.S. Irvine. Image analysis of the porous yttria-stabilized zirconia (YSZ) structure for a lanthanum ferrite-impregnated solid oxide fuel cell (SOFC) electrode. Journal of the European Ceramic Society, volume 38 (2018), page 5463–5470.Go To Journal of the European Ceramic Society