Thank you for your question.
-
It is true that there are limitations in using thin film geometries for approximating bulk degradation mechanisms. For solid oxide fuel cell electrodes, however, the limiting factor on performance is the catalytic reaction on the surface, since at high temperatures bulk diffusion is faster than the surface catalysis reaction. Thus, highly porous electrodes that maximize the gas-electrode interface area have superior performance.
-
However, contaminants in the gas will block active sites for catalysis on the surface, so the degradation of the material at the surface is ultimately responsible for the overall loss in cell performance. We have observed that the operating voltage, a measure of the cell’s performance, drops rapidly with the introduction of gas contaminants and recovers when the gas contaminant is removed, although not as quickly, which indicates that the surface is the important region of interest. If the bulk degradation was responsible for the overall cell degradation, a gradual loss of performance would be expected.
-
A dense thin film, compared to a thick, porous electrode, limits the surface reactions to a single plane at which we can concentrate the x-rays. For this reason and those outlined above, the dense thin film LSCF geometry and the glancing incidence were chosen to specifically target the surface.

Thank you for your question.
-
Due to the nature of synchrotron x-ray work, there are no opportunities to reproduce the results immediately. An experimental proposal only receives a few days of synchrotron time every four months, so reproducing results must be balanced with the need to perform new experiments. This data is particularly recent, so my next synchrotron time will be a similar experiment to further prove the technique and acquire new data.
-
During the synchrotron experiment, several measures are undertaken to minimize experimental error, like using a metal foil as energy reference, constantly monitoring and recalibrating detector sensitivity levels, and having an experienced beamline scientist on hand to ensure the x-ray beam is in working order. In the data processing, the recorded fluorescent x-ray intensity is normalized by the incident x-ray intensity at that moment so that variations and gradual decay in the beam intensity are accounted for. I am responsible for the specific SOFC parts of the experiment, and experimental error there also cannot be fully eliminated. However, working on high stakes experiments, like those at a synchrotron, has cultivated a habit of double- and triple-checking the procedure outlined and the actual experimental actions taken.

Thank you for your question.
-
This experiment is the first of a series of experiments that will explore more complicated electrode compositions and architectures. LSCF itself is one of the most well studied SOFC cathode materials and the composition has been optimized to the point of commercial production. I don’t believe there is much room for improvement in that specific area. Rather than recommend changes to the initial composition of the cell, my goal is to investigate how thin film coatings or decorated nanoparticles on the surface of LSCF could improve performance or robustness.
-
For example, one of my previous experiments examined how the electronic and local structure of LSCF cations changed when a thin film coating of a similar material (LSMn) was applied. This composite cathode showed superior performance to plain LSCF. At high temperature, we could see that there were interactions between the LSCF and the LSMn that changed which elements were involved in the oxygen reduction reaction (the important catalytic surface reaction). We could also observe ordering in the Mn local structure which we assert is a result of Mn diffusion and doping into LSCF. The result is that we have a better understanding of the interaction between the thin film coating and the substrate and have new ideas for synthesizing new materials, such as LSCF doped with Mn.
-
Fortunately, that kind of experiment was possible without necessarily requiring the operando setup I described here. This particular setup is intended for the faster processes occurring at the surface and in the presence of electrical polarization. Being able to demonstrate the success of the operando setup is part of my findings here. For future experiments, the surface will be a thin film coating on LSCF or distributed nanoparticles on the LSCF surface.

Thank you for your question and for reading the other responses.
-
Traditional techniques in materials science that we also employ are scanning electron microscopy and x-ray diffraction, which provide some basic morphological and crystallographic information. For electrochemical performance, the primary technique is electrochemical impedance spectroscopy (EIS), which measures impedance as a function of ac frequency. For our purposes, we mostly use EIS to track the value of the polarization resistance as a function of operating time or gas condition, which provides a sense of the “health” of the fuel cell.
-
For surface and chemical information, one of my colleagues and collaborators uses Raman spectroscopy, for which he helped develop a similar operando testing setup. Raman spectroscopy is quite useful because of its high spatial resolution and for its ability to detect phases/species that have Raman-active vibrational modes. I am also a user of x-ray photoelectron spectroscopy (XPS) for more element-specific surface chemical information. Building competency in XPS has also proven useful for me, as the synchrotron also is capable of its own variety of XPS.
-
I hope that answers your question. Please feel free to ask follow-up questions.

Thank you for your question.
-
For x-ray absorption spectroscopy (XAS), we typically discuss x-rays with respect to energy, but the conversion is trivial via Planck’s relation (E = h*c/lambda). The energy of the x-ray we use depends on the element of interest and its absorption edges. So, for example, in this experiment, I was interested in Fe and Co. For Fe, the K-edge absorption energy is about 7112 eV (about 1.7 Å in wavelength) and for Co, the K-edge absorption energy is about 7710 eV (about 1.6 Å in wavelength). The actual absorption edge energy will vary with the valence state of atom. During XAS, we vary the energy of the x-ray above and below the absorption edge energy by several hundred eV and measure the calculated absorption. These experiments were performed at beamline X18B.
-
We only used one angle (about 2 or 3 degrees) and it was not exact because the endstation did not have an angle-resolved stage. It was mostly trial and error to get the beam to be properly splayed out over the sample, which was confirmed by using x-ray fluorescence paper.
-
Our results had a few lessons in design. For example, it is important to consider the geometry and location of the detector. While we gave it some thought in the design, we didn’t realize how difficult it would be in execution and this caused a few speedbumps during the experiment. Having versatility and flexibility with different types of detectors can also improve the signal-to-noise ratio, a challenge we encountered and had to work around. Also, size and space can be limited inside beamline endstations, mostly in between the ion chambers. Our setup was able to fit but a more compact design would have made the fit easier and saved time, although compact designs introduce additional complexity in of themselves. These are optimization issues that will be ironed out through iterative design.
-
In terms of recommendations for improvement, there are a few but they are primarily driven by the scientific experiments of interest rather than lessons we learned above. The most important would be achieving a gas-tight design for testing full fuel cells instead of symmetrical cells shown here. Other features I would like to add are a dedicated, portable gas manifold to adjust the gas atmosphere easily and an accompanying humidity control. It might also be worth upgrading the heating elements to achieve higher temperatures but some material properties may limit the maximum temperature.
-
I hope that answers all of your questions. Please let me know if anything was unclear.

Thanks for compliment, Federica! I really enjoyed how entertaining your video was. And yet, also very intriguing at the same time.

;) thanks a lot!!!