Doctors use X-rays as a matter of routine, looking inside the human body for broken bones or other abnormalities. Postdoc Salvatore de Angelis and his colleagues at DTU Energy do the same with fuel cells, using novel X-ray solutions to get better, more detailed images and even record movies down to the micro- and nanoscale.
“Our interest lies in the fundamental workings of fuel cells, how they work and especially how and why the cells break down. While doctors can use 2D and 3D pictures to check the human body, we are able to add a 4th dimension, namely time, and look at a cell at microscale while it works”, explains Salvatore de Angelis.
Solid oxide fuel cells and electrolysis cells offer great prospects for the efficient and reversible conversion of chemical to electrical energy. Therefore, they are expected to play a key role in the renewable energy landscape, but their limited lifetime under operating conditions hinders their widespread usage.
Degradation mechanisms are attributed to changes in the microstructure from the micro- to the nanoscale. Salvatore de Angelis recently defended his PhD thesis “Tracking Solid Oxide Cell Microstructure Evolution by High Resolution 3D Nano-Tomography” where he explained how precise tracking of 3D microstructural evolution during operation is crucial to understand the complex phenomena occurring in a degrading solid oxide cell. He and his team adopted and refined a new methodology involving the use of X-rays to obtain 3D images of the sample. He can resolve features approx. 800 times smaller than the width of a human hair. The new technique (called ptychographic tomography) has been used to observe the effect of oxidation and reduction on a solid oxide cell anode, revealing new insights on these two important processes. The same procedure has been applied to study coarsening of nickel particles, a major cause of cell degradation.
A novel way to do X-ray images
Being able to follow the time evolution of a fuel cell during operation, watching it degrade and finally break helps researchers enormously. Now they are able to pinpoint what happens and when it happens instead of – as is usually done – cutting up a broken cell and trying to reconstruct the chain of events leading to the degradation. However, ptychography is time consuming, says Salvatore de Angelis, with a single scan taking hours of work and they were only able to analyze the initial and final stage of oxidation, reduction and coarsening. So the DTU Energy team adopted a new X-ray imaging technique called X-ray holo-tomography.
Watch the 4D-film of a fuel cell degrading here
“X-ray holo-tomography allows obtaining 3D images of SOC materials in a few minutes at the cost of lower resolution, and for the first time we were able to watch the evolution of oxidizing nickel particles live in 3D. Now we have a clearer understanding of “how it happens” and “why it happens”, enabling us to try to avoid undesired effects in the cell design process”, explains Salvatore de Angelis. Part of his job was making and handling the samples, and that wasn’t easy, since the technique needs samples five times smaller than the width of a human hair.
Please don’t sneeze
“It can be quite challenging handling samples five times thinner than a human hair, especially when you are handling them with tweezers under a microscope. The samples are very very small, we are talking about microns, so breathing too strongly make the sample flies away. You also have to take care to what you are wearing . Static electricity can transfer to the tweezers, making them attractive and causing the sample to stick to the tweezer. Every time your sample acts up, it is three days of work down the drain.”
During his PhD project, Salvatore de Angelis travelled to large facilities with very big particle accelerators called synchrotrons to study the fuel cells, taking good care not to sneeze, not to wear wool and not in any other way disturb the very small and fragile samples. He ended up with mountains of data, tens of terabytes (1 TB = 1000 GB), that he afterwards made into films that he and his colleagues could shift through to find new interesting findings. A tedious, but also very rewarding process, says Salvatore de Angelis.
“When having so much data you easily lose focus. Often it becomes a matter of gut feeling and intuition. Here, to find that particular sign that starts a degradation process, we are helped a lot by being able to film it.”
Part of a bigger effort
His PhD project was part of a project funded by Innovation Fund Denmark, the Cinema-project which brings together leading European universities and companies to develop imaging and modelling methods. “Being part of that was nice and very rewarding”, says Salvatore de Angelis.
Having defended his PhD thesis Salvatore de Angelis is now a postdoc at DTU Energy, continuing his research on degradation mechanisms, but this time in batteries as part of the MAX4ESSFUN-project.