Catching hydrogen in the act: Tracking the absorption process over time
If you're looking for hydrogen on the elemental chart, it won't take you long to find it. It is right there at the beginning, the lightest possible material. One electron, one proton—that's it. Simple, minimalistic, the Marie Kondo of the elemental chart, but with enormous potential in terms of possible technological applications.
A very prominent example interests every single one of us: Let's look into the daytime sky.
If we think of the sun as a furnace, then hydrogen atoms are the coal ingots.
The sun's gravitational force pushes hydrogen atoms together so violently that they fuse, generating an incredible amount of energy (nuclear fusion) that is irradiated (partly through visible light as well), travels to Earth, allows life on this planet and lets you enjoy a cold drink at the beach on a warm summer day (which, incidentally, also contains hydrogen).
Nuclear fusion reactors are naturally of great interest today. They would provide a clean and renewable source of energy—a big deal these days—and gaining full knowledge of how hydrogen atoms interact with other materials, for example with the metallic components of nuclear reactors, is of paramount importance.
This is a major example, but the question "How does hydrogen modify the structural and electrical properties of the materials it interacts with?" is also important when it comes to other crucial technologies for the production and storage of hydrogen. Hydrogen atoms can combust and generate propulsive power without producing toxic or greenhouse exhaust gases.
Our study
This is what motivated me and my collaborators to work on this topic.
Our guiding questions throughout the years have been: How do we track the interaction of hydrogen atoms with their surroundings? How does hydrogen penetration and absorption change the structural properties of a material?
I'm not sure if this is of general concern, but I wouldn't want my hydrogen-based car engine to break down because this question still has not been clarified.
Of course, we were not the first to ask this question. This field has already sparked the interest of quite a few people in the scientific community, and I always found it fascinating how ingenious the experimental techniques to investigate hydrogen absorption were.
The best part is that it all comes down to something that is not short of a game.
In the case of hydrogen, the game is similar to "find the seven differences between the two pictures."
The atoms in a material generate a scattering potential, a sort of wall that hinders the motion of particles through the material. If hydrogen starts permeating the material, creating vacancies and dislocations, it inevitably alters the scattering potential.
The technique that is usually used to determine the hydrogen concentration in a material is called Neutron Reflectometry (NR if you're a friend). You shine a sample with a beam of neutrons and measure the reflected intensity. Shoot and analyze, as simple as that.
The reflectivity of your sample tells you about the strength of the scattering potential. This is the first picture, the one on the left of our riddle book. Now hydrogen starts permeating the material, and you perform the experiment again. The intensity of the reflected neutron beam changes. This is our picture on the right, and all that's left for us to do is compare the two pictures.
This technique has already been widely used, but it has one major limitation. Data acquisition takes such a long time that what we see is an average of what happens over time while hydrogen permeates the material and alters its properties.
Have you ever taken a picture using too long an exposure? The result here is similar: a washed-out time average of the hydrogen absorption process.
A blurry view of absorption
That's the question my colleagues and I were asking ourselves: How can we track in real time the processes of hydrogen absorption? At this point, I wish I could say that exactly at that moment when we were discussing the issue, my cat walked into the living room and one of my colleagues started playing with him using a laser pointer. I wish I could attribute the idea to an anecdote, but, unfortunately, intellectual honesty obliges me to say that this wasn't the case.
However, the idea we came up with with my colleagues was as simple and effective as the one behind the laser pointer that drives cats crazy.
We thought: What if we enhance the reflected beam by sandwiching the material we want to analyze between two others?
All we would need to do would be to add a cap and a substrate to create the sandwich (or a resonant chamber, to be more exact).
This should create a standing wave of neutrons that, like in a laser, amplifies the signal. The amplification of the signal also means that we don't need such a long "exposure time" for our neutron detector as before. This is exactly the protocol we used, and now this technique is called Resonant Neutron Reflectometry (again, RNR if you're a friend).
The advantage? This trick brings down the time sensitivity for the analysis from two hours to a few seconds.
But how do we get the full picture? For that to happen, we had to bring out other items from our toolbox. I'm talking about the XRR wrench (X-Ray Reflectivity) and the resistance measurements.
XRR served to keep track of the structural changes of the material, while the resistance measurements gave us an insight into how the metallic properties of the materials change during the hydrogen absorption process. This was the first time neutron and X-ray reflectivities were used simultaneously! (We don't mean to Bragg about it.)
Four stages, one cascade
But now what you're all asking: What did we see? What happens to the hydrogen?
Well, now you're ready to know.
We were able to distinguish four different regimes of hydrogen absorption, each a consequence of the preceding one. A vertigo-inducing cascade of events that left the spectators breathless.
First, hydrogen penetrates the material through grain boundaries, voids and pre-existing defects.
From these defects, hydrogen enters the grains and changes the crystal structure of the material, inducing a large expansion of the spacing between crystalline planes. This expansion induces large stresses in the film, which are released through further plastic deformations and dislocation formation. Finally, because of the presence of new defects, more hydrogen penetrates the thin layer up to concentrations exceeding the amounts that can be attained in the corresponding hydride material.
A chain of events that clarifies the different stages of the absorption process and might be of use when it comes to designing materials to be used in hydrogen-based technologies.
My hope for the future is that our technological and energy infrastructure will increasingly include hydrogen-based technologies as main actors, and I'm happy that, together with my colleagues, we could place at least one cobblestone to pave the way. After all, that's how science works: It needs every possible new idea, and everybody can contribute.
Our work is published in the journal Advanced Functional Materials.
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Publication details
Laura Guasco et al, Kinetic Regimes of Hydrogen Absorption in Thin Films, Advanced Functional Materials (2026). DOI: 10.1002/adfm.75202
Who's behind this story?
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Guasco is a postdoctoral fellow at Max Planck Institute for Solid State research in the department of Solid State Spectroscopy lead by Prof. Bernhard Keimer who is extremely passionate about clean energy and sustainability. Guasco has tried to devote a scientific career to finding new solutions through innovative materials and characterization tools, starting from undergraduate studies in Material Sciences in a joint Erasmus Program at the University of Montpellier and LMU in Munich. Guasco then pursued studies in Physics and earned a Ph.D. at the Stuttgart University (Germany), with a focus on the hydrogen incorporation in thin films. Over the years, Guasco has gained expertise in techniques like X-ray and neutron scattering, electrical and optical methods, thin film growth and magnetism and believes in the importance of science outreach to share a passion for science and inspire future generations of scientists.
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