Solid oxide fuel cells (SOFCs) have high energy conversion efficiencies and entail low manufacturing costs. In fact, SOFCs can work on both standard and hydrogen fuel, in contrast to other fuel cell designs. These factors make SOFCs as one of the most researched fuel cell designs. However, production processes are particularly challenging and the present manufacturing costs must improve in order to achieve economic viability.
SOFC anodes are triple-phase materials that contain yttria-stabilized zirconia, nickel and gas-conduction (pore) phases. Electrochemical reactions for energy production take place at the junction of all three phases dubbed triple phase boundaries (TPB).
Tools to Visualize SOFC Microstructures
Several tools are available which can be used to view the intricate SOFC microstructures in three-dimensional format at adequate resolution. Focused ion beam milling along with scanning electron microcopy is employed to acquire three-dimensional data by continually milling thin slices and imaging. The ensuing images are rebuilt into a three-dimensional volume. Although this technique delivers high resolution, it has two major issues; the field of view is extremely limited and the sample is completely destructed. Such destruction prevents repeatable measurements of micro and nanostructure as a virtue of varying conditions like pressure or temperature. These data are important to gain a better insight into aging and failure mechanisms.
Xradia UltraXRM 3D X-ray Microscopes
The UltraXRM 3D X-ray nanotomography system designed by Xradia overcomes these problems by providing non-destructive, high resolution imaging. Along with synchrotron X-ray sources, the UltraXRM-S200 allows instant and non-destructive measurements of key aspects driving fuel cell performance, including electronic conductivity, TPB lengths and SOFC porosity.
The UltraXRM-S200 X-ray nanotomography system improves two important aspects of SOFC anode study.
REV Quantification by Measuring Porosity
The REV or representative elementary volume refers to the minimum volume which needs to be taken into account to differentiate the structure entirely. To calculate the REV of a SOFC anode, the Xradia UltraXRM-S200 was used to examine the porosity of a minute section. This allowed the pore space volume to be divided by the total volume. Then, measurement was carried out on two individual samples and the common REV expected to be reached was about 80 voxels, as illustrated in the figure given below.
Figure 1. Convergence of porosity measurements corresponded to a cube with side lengths of 4.6 µm.
A high resolution, three-dimensional image of a SOFC anode sample with a field of view of 30 µm helped in visualizing about 40 REVs in a single scan with a resolution of 30 nm. The sample was not affected during and after imaging and can be used for further testing and investigation purposes.
Quantifying Key Factors in Performance: Porosity, TPB and Connectivity
Different absorption levels among the SOFC phases provide a better understanding about the models employed to calculate performance parameters such as electronic conductivity, connected porosity, TPB, etc. After the beamline monochromator is adjusted above and below the Ni K-absorption edge and the tomographies at both photon energies are collected, the UltraXRM-S200 system displays different absorption rates in the granular anode. This helped in detecting the locations of the nickel phase, separated from the yttria-stabilized zirconia and pore phases. The figure given below demonstrates the result of one such measurement, which is useful in predicting the performance of end device and improving the design and manufacturing procedures.
Figure 2. (A) X-ray absorption spectroscopy enables precise identification of the three different phases – nickel phase (red), yttria- stabilized zirconia (blue), and pore (empty) – making up the SOFC anode. (B) Using this result, a variety of different parameters may be calculated that directly indicate the device performance.
Combined with a synchrotron X-ray source, the Xradia UltraXRM-S200 provides a large field of view and offers high spatial resolution, thus allowing non-destructive measurement of micro and nanostructures as well as accurate measurement of key device parameters. Performance parameters like electronic conductivity, connected porosity and TPB length can now be determined and could help in improving the manufacturing processes and economic viability of SOFC technology.
This information has been sourced, reviewed and adapted from materials provided by Xradia.
For more information on this source, please visit Xradia.