A new study explains how an ultrathin oxide layer (oxygen atoms shown in red) around graphene-wrapped magnesium nanoparticles (gold) still allows hydrogen atoms (blue) to access the nanoparticles for storage. Image: Berkeley Lab.
A new study explains how an ultrathin oxide layer (oxygen atoms shown in red) around graphene-wrapped magnesium nanoparticles (gold) still allows hydrogen atoms (blue) to access the nanoparticles for storage. Image: Berkeley Lab.

A powdery mix of metal nanocrystals wrapped in single-layer sheets of carbon atoms, developed at the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab), shows promise for safely storing hydrogen for use with fuel cells in passenger vehicles and other applications. Now, a new study provides insight into the atomic details of this ultrathin coating and how it serves as selective shielding while enhancing the crystals’ performance in hydrogen storage.

The study, led by Berkeley Lab researchers and reported in a paper in Nano Letters, drew upon a range of expertise and capabilities to synthesize and coat the magnesium crystals, which measure only 3–4nm across. The expertise and capabilities were further required to study the crystals’ nanoscale chemical composition with X-rays, and to 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 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 DOE's Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy.

The carbon layers are made from 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) and possesses 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 certain components of the atmosphere, particularly oxygen, water vapor and carbon dioxide. Such exposures can produce a thick coating of oxidation that prevents 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.

"Previously, we thought the material was very well-protected," said Liwen Wan, a postdoctoral researcher at Berkeley Lab's Molecular Foundry, a DOE Nanoscale Science Research Center, who served as the study's lead author. "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.”

"That's a benefit that ultimately enhances the protection provided by the carbon coating," she explained. "There doesn't seem to be any downside."

David Prendergast, director of the Molecular Foundry's Theory Facility and a participant in the study, noted that the current generation of hydrogen-fueled vehicles power their fuel cell engines using compressed hydrogen gas. "This requires bulky, heavy cylindrical tanks that limit the driving efficiency of such cars," he said. The nanocrystals offer one possibility for eliminating these bulky tanks by storing hydrogen within other materials.

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. It 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.

The models that Wan developed to explain the experimental data suggest that the oxidation layer that forms around the crystals is atomically thin and stable over time, which means the oxidation does not progress far. This analysis was based, in part, on experiments performed at Berkeley Lab's Advanced Light Source (ALS), an X-ray source called a synchrotron that was earlier used to explore how the nanocrystals interact with hydrogen gas in real time.

Key to the study, said Wan, was interpreting the ALS X-ray data by simulating X-ray measurements for hypothetical atomic models of the oxidized layer, and then selecting those models that best fit the data. "From that we know what the material actually looks like," she said. While many simulations are based around very pure materials with clean surfaces, in this case the simulations were intended to be more representative of the real-world imperfections in the nanocrystals.

A next step, in both experiments and simulations, is to use materials that are more ideal for real-world hydrogen storage applications, such as complex metal hydrides (hydrogen-metal compounds) that would also be wrapped in a protective sheet of graphene. "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 said.

Some of these complex metal hydride materials are fairly time-consuming to simulate, and the research team plans to use the supercomputers at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC) for this work.

"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 said.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.