Sep 15 2017
Researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have synthesized a powdery mixture of metal nanocrystals enveloped in single-layer carbon atom sheets.
The mixture seems to be propitious for safe storage of hydrogen to be used in fuel cells of passenger vehicles and other applications. Moreover, at present, an innovative research offers in-depth knowledge related to atomic composition of the ultrathin coating of the crystals and the way they function as selective shielding at the same time also improving their performance in storing hydrogen.
A new study explains how an ultrathin oxide layer (oxygen atoms shown in red) coating graphene-wrapped magnesium nanoparticles (orange) still allows in hydrogen atoms (blue) for hydrogen storage applications. Credit: Berkeley Lab.
The research was headed by scientists from the Berkeley Lab and involved a wide array of lab proficiency and potentials to develop and coat the magnesium crystals with a width of just 3-4 nm (where 1 nm is one-billionths of a meter), to analyze their nanoscale chemical composition by using X-rays, and to develop supportive theories and computer simulations to have a better knowledge of the way in which the crystals and their carbon coating act in congruence.
The discoveries made by the researchers can provide in-depth understanding of the way similar coatings can improve the stability and performance of other materials just known to be propitious for hydrogen storage applications. The study is one among various attempts made by a multi-lab R&D effort called as the Hydrogen Materials—Advanced Research Consortium (HyMARC) formed 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 (i.e. rGO), which looks very similar to the very well known graphene (an extended, single-atom-thick carbon sheet arranged in a honeycomb pattern), includes nanoscale holes that allow only hydrogen to pass through and block larger molecules.
Such a carbon envelope was aimed to prevent reaction of magnesium (used as the hydrogen storage material) molecules such as water vapor, oxygen, and carbon dioxide in its surroundings. Such reactions can generate a thick oxidation coating that will block the incoming hydrogen from contacting the magnesium surfaces.
However, the recent research proposes that an atomically thin oxidation layer was formed on the crystals while developing the crystals. Astonishingly, the oxide layer did not hinder the performance of the material.
“Previously, we thought the material was very well-protected,” stated Liwen Wan, lead author of the research, who is a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, a DOE Nanoscale Science Research Center. The research was reported in the journal Nano Letters. “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.
Wan
“That’s a benefit that ultimately enhances the protection provided by the carbon coating,” she added. “There doesn’t seem to be any downside.”
David Prendergast, director of the Molecular Foundry’s Theory Facility who took part in the research, stated that the fuel cell engines in prevalent hydrogen-powered vehicles are powered by using compressed hydrogen gas. “This requires bulky, heavy cylindrical tanks that limit the driving efficiency of such cars,” stated Prendergast, and the nanocrystals provide one probability for abolishing bigger tanks by using other materials to store hydrogen.
The research also assisted in demonstrating that the thin oxide layer actually does not restrict the rate at which the material can store hydrogen—a significant point when it comes to swift refueling. This outcome was also unanticipated with regard to the traditional knowledge of the hindering role usually played by oxidation in the hydrogen-storage materials.
That is, under fuel storage and supply conditions, the enveloped nanocrystals will chemically absorb pumped-in hydrogen gas at greater densities than it is feasible in a compressed hydrogen gas fuel tank at the similar pressure conditions.
The models developed by Wan to elucidate the experimental data indicate that the oxidation layer formed on the crystals is atomically thin and gets stabilized gradually, meaning that the oxidation is not advanced further.
The investigation was partially based on experiments carried out at Berkeley Lab’s Advanced Light Source (ALS), an X-ray source known as a synchrotron used even before to investigate real-time interaction of the nanocrystals with hydrogen gas.
According to Wan, an important point was to interpret the ALS X-ray data by carrying out X-ray measurement simulation for hypothetical atomic models of the oxidized layer, and then choosing the models that are closely in relevance with the data. “From that we know what the material actually looks like,” stated Wan.
Wan stated that although majority of the simulations are dependent on highly pure materials that have clean surfaces, here, the simulations were expected to widely represent the real-time imperfections of the nanocrystals.
According to Wan, the following step as part of both the experiments and the simulations would be to use materials that are highly optimal for real-time hydrogen storage applications, for example, complex metal hydrides, or hydrogen-metal compounds, that will be enveloped in a protective graphene covering.
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.
Wan.
Certain complex metal hydride materials such as these are reasonably time-consuming to be simulated, and the scientists propose to apply the supercomputers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) for this study.
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.
Wan.
The Advanced Light Source, Molecular Foundry, and National Energy Research Scientific Computing Center are DOE Office of Science User Facilities.
The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy’s Fuel Cell Technologies Office supported the study.