A powdery mix of metal nanocrystals wrapped in single-layer sheets of carbon atoms, developed at the Molecular Foundry, shows promise for safely storing hydrogen for use with fuel cells for passenger vehicles and other uses. And now, a new study provides insight into the atomic details of the crystals’ ultrathin coating and how it serves as selective shielding while enhancing their performance in hydrogen storage.
The study, led by Foundry researchers, drew upon a range of Lab expertise and capabilities to synthesize and coat the magnesium crystals, which measure only 3-4 nanometers (billionths of a meter) across; study their nanoscale chemical composition with X-rays; and develop computer simulations and supporting theories to better understand how the crystals and their carbon coating function together.
The science team’s findings could help researchers understand how similar coatings could also enhance the performance and stability of other materials that show promise for hydrogen storage applications. The research project is one of several efforts within a multi-lab R&D effort known as the Hydrogen Materials—Advanced Research Consortium (HyMARC) established as part of the Energy Materials Network by the U.S. Department of Energy’s Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy.
Reduced graphene oxide (or rGO), which resembles the more famous graphene (an extended sheet of carbon, only one atom thick, arrayed in a honeycomb pattern), has nanoscale holes that permit hydrogen to pass through while keeping larger molecules at bay.
This carbon wrapping was intended to prevent the magnesium – which is used as a hydrogen storage material – from reacting with its environment, including oxygen, water vapor and carbon dioxide. Such exposures could produce a thick coating of oxidation that would prevent the incoming hydrogen from accessing the magnesium surfaces. But the latest study suggests that an atomically thin layer of oxidation did form on the crystals during their preparation. And, even more surprisingly, this oxide layer doesn’t seem to degrade the material’s performance.
The study also helped to show that the thin oxide layer doesn’t necessarily hinder the rate at which this material can take up hydrogen, which is important when you need to refuel quickly. This finding was also unexpected based on the conventional understanding of the blocking role oxidation typically plays in these hydrogen-storage materials.
That means the wrapped nanocrystals, in a fuel storage and supply context, would chemically absorb pumped-in hydrogen gas at a much higher density than possible in a compressed hydrogen gas fuel tank at the same pressures.