A new study carried out at the Department of Energy’s Lawrence Berkeley National Laboratory “Berkeley Lab” could revolutionize the next generation of nanocrystal fuel cells by enabling them to have enhanced hydrogen storage properties. It’s a move that shows real promise for the safe storage of hydrogen in fuel cells suitable for passenger vehicles among other uses.

A new study explains how an ultrathin oxide layer (oxygen atoms are shown in red) coating graphene-wrapped magnesium nanoparticles (orange) still allows in hydrogen atoms (blue) for hydrogen storage applications. (Credit: Berkeley Lab)

The study takes a closer look at to the atomic details of the crystals’ ultrathin coating as well as its role in making hydrogen storage more efficient. Using their expertise, the researchers managed to synthesize and coat the magnesium crystals while studying their chemical composition. They were also able to create computer simulations to help better understand how the crystals and their coating work together.

Results from the study will enable the team to see how other coatings may also enhance performance and show promise in creating a more efficient hydrogen storage facility. This project is just one of several that are being implemented under the umbrella name of Hydrogen Materials – Advanced Research Consortium (HyMARC).

Reduced graphene oxide (rGO) is similar to traditional graphene, and it allows hydrogen in through its nanoscale holes while keeping the larger molecules out. Its carbon coating prevents magnesium from interacting with its environments as any exposure could potentially cause oxidation to occur and stop the hydrogen from getting in. However, this latest study has demonstrated that oxidation does occur automatically on the crystals while they’re being prepared, yet thankfully has no detrimental effect on the material’s performance.

“Previously, we thought the material was very well-protected,” said Liwen Wan, lead author of the study, and a postdoctoral researcher at Berkeley Lab’s Molecular Foundry. “From our detailed analysis, we saw some evidence of oxidation. Most people would suspect that the oxide layer is bad news for hydrogen storage, which it turns out may not be true in this case. Without this oxide layer, the reduced graphene oxide would have a fairly weak interaction with the magnesium, but with the oxide layer the carbon-magnesium binding seems to be stronger.”

David Prendergast is the director of the Molecular Foundry’s Theory Facility as well as being a participant in the study. He confirmed how current hydrogen fuelled vehicles use compressed hydrogen gas in which to power the fuel celled engines, but this is not the best way to do it. “This requires bulky, heavy cylindrical tanks that limit the driving efficiency of such cars.” Prendergast is confident that nanocrystals can offer a better alternative solution for storing hydrogen.

The study also demonstrated how just a thin oxide layer doesn’t affect the hydrogen getting through which is vital when needing to refuel quickly. So, essentially this means that wrapped nanocrystals can absorb more hydrogen at a much faster rate than any compressed hydrogen gas fuel tank was running an at the same pressure level. Wan’s models suggest that the oxidation that’s formed around the crystals remains thin over time, indicating that no progression occurs.

Part of the study was carried out at Berkeley Lab’s Advanced Light Source (ALS), which is a synchrotron that’s been used previously to determine how nanocrystals and hydrogen gas interact in real time. Wan believes that a vital party of the study involved interpreting the ALS X-ray data by creating simulations of hypothetical atomic models and then choosing those that best fit the bill.

The next move for Wan and colleagues will be to use other materials that are more fit for real-world hydrogen storage applications, which will include complex metal hydrides. “By going to complex metal hydrides, you get intrinsically higher hydrogen storage capacity and our goal is to enable hydrogen uptake and release at reasonable temperatures and pressures,” said Wan. “Now that we have a good understanding of magnesium nanocrystals, we know that we can transfer this capability to look at other materials to speed up the discovery process.”

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