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Scientists at the Institute of Industrial Science (IIS) at the University of Tokyo have uncovered the role of crystal symmetry in the quantum behavior of hydrogen. The findings will help in the design of a new generation of materials that will shape the next phase of clean energy technologies using hydrogen.
The move away from fossil fuels has seen a rise in renewable energy technologies like wind and solar. Even though countries have invested heavily in these, there are multiple areas where such intermittent energy sources cannot be deployed. Energy generated from wind and solar is great for maintaining the grid and for daily household use, but for heavy-duty industrial applications, an alternative energy source is needed.
Hydrogen is touted as the answer to these problems, since it can be combusted like fossil fuels to generate large amounts of energy but without any carbon emissions. However, being a highly flammable fuel also brings with it safety risks during storage and transportation. If hydrogen is to replace fossil fuels for daily use in passenger vehicles or even for large-scale industrial applications, it needs to be stored and transported in an extremely safe manner.
Working with vanadium
Over the years, scientists have determined that vanadium is a promising candidate for the safe storage of hydrogen. When used in a solid-state alloy, vanadium readily absorbs hydrogen. Estimates suggest that the metal can hold up to 3.8 percent of its weight in hydrogen.
This happens because the hydrogen molecule (H2) splits inside the solid-state alloy and slides into empty spaces within the crystal lattice structure of the alloy. The holding ability of vanadium is so strong that scientists recommend mixing other metals like iron, titanium, and chromium with these alloys, so that hydrogen can be recovered when required.
While much is known about vanadium, scientists have not be able to explain the variable behavior that hydrogen shows in the presence of vanadium. By combining measurements of hydrogen’s structure and diffusion with quantum mechanical calculations inside vanadium’s crystal structure, the researchers at the IIS found out how the crystal lattice structure of vanadium controls the behavior of hydrogen inside.
What did the researchers find?
The team of scientists found that hydrogen atoms hop between interstitial spaces inside the crystal lattice. Sometimes, they do this by behaving like a classical particle that has to overcome energy barriers between neighboring sites. At times, the atoms take a ‘quantum shortcut’ and tunnel through the sites, moving much like a wave.
“Our results show that crystal symmetry is key to controlling hydrogen’s quantum behavior,” explained Takahiro Ozawa, a research associate at IIS, who was involved in the work. “Highly symmetric structures allow hydrogen to tunnel, while distorted structures suppress this effect.”
According to the research team, when the hydrogen concentration is low, the vanadium crystal maintains its symmetrical structure, allowing hydrogen atoms to tunnel through. As hydrogen concentration increases, the crystal becomes distorted, forcing hydrogen to behave more like a classical particle.
“Crystal symmetry is the underlying switch that turns quantum behavior on or off,” added Katsuyuki Fukutani, professor of surface and interface physics at IIS, in a press release. “In a symmetric structure, hydrogen finds equivalent pathways that allow it to tunnel between sites.”
“Distort that symmetry — as happens at higher hydrogen concentrations — and tunneling is suppressed, forcing hydrogen to rely on thermal energy to hop between sites instead.”
Knowing how vanadium structure affects hydrogen storage can help in designing new materials that allow hydrogen’s quantum behavior and carry clean energy sources safely and herald a future of clean energy.
The research findings were published in the journal Nature Communications.
