Novel Approach Visualizes Hydrogen on Atomic Scales in Storage Materials
High-energy ion beams determine the exact location and motion of hydrogen in crystal lattices, enabling the development of new materials for efficient hydrogen storage.
Hydrogen as energy carrier plays a critical role in shaping a fossil-free sustainable future. The storage in its liquid form is, however, difficult and ineffective. Metal systems absorbing hydrogen are viable alternatives as storage medium. Yet, can we find a material that exhibits a high and reversible storage capacity and fast cycling? Unfortunately, hydrogen is hard to detect by most analytical probes such as electrons or X-rays and even harder to quantify and locate without modifying the material itself.
To overcome these challenges, researchers from Uppsala University in Sweden use energetic nitrogen ions specifically sensitive to hydrogen nuclei as probe.
“We use a rare isotope of nitrogen, 15N, as a probe. After we bring the particles to specific high energies using a tandem accelerator nitrogen and hydrogen can undergo a nuclear reaction emitting characteristic radiation, which is readily detected”, explains Kristina Komander, PhD-student at the Department of Physics and Astronomy and first author of the study.
When the ion beam is oriented along major crystal axes or planes, the ions experience pronounced steering effects. The position of hydrogen atoms relative to the rows of atoms of the host matrix can subsequently be related to the probability of a nuclear reaction for specific experimental geometries.
“First, we measure the amount of the characteristic radiation emitted from the reaction between nitrogen and hydrogen for a number of different orientations of sample and probing beam. Then we use Monte-Carlo simulations to trace the ion trajectories, which allows us to unambiguously and quantitatively reveal both location and swinging motion of hydrogen on a true atomic scale”, says Daniel Primetzhofer, professor at the department and director of the Tandem laboratory, which hosts the accelerator employed for the study.
The method has subsequently been used by the researchers to investigate layered systems of transition metals absorbing hydrogen; a property making them interesting as potential storage media.
This research provides new tools for the optimization of material properties improving storage capacities and refueling speed. The measured datasets are also highly relevant as a benchmark for the numerous computational approaches which have been developed to model the behavior of hydrogen in different classes of materials.
K. Komander, et al. (2021); Interstitial hydrogen in Fe/V superstructures: Lattice site location and thermal vibration, Physical Review Letters 127 (2021) 136102.