New nanocomposite for high-capacity hydrogen storage with rapid kinetics

Schematic of hydrogen storage composite material: high-capacity Mg NCs are encapsulated by a selectively gas-permeable polymer. Credit: Jeff Urban. Click to enlarge.

Scientists with the US Department of Energy (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have synthesized an air-stable composite material that consists of metallic Mg nanocrystals (NCs) in a gas-barrier polymethyl methacrylate (a polymer related to Plexiglas) matrix that enables both the storage of a high density of hydrogen (up to 6 wt% of Mg, 4 wt% for the composite) and rapid kinetics (loading in <30 min at 200 °C).

The pliable nanocomposite rapidly absorbs and releases hydrogen at modest temperatures without oxidizing the metal after cycling. In addition, nanostructuring of the Mg provides rapid storage kinetics without using expensive heavy-metal catalysts. A paper on their work is published in the journal Nature Materials.

Transmission electron micrographs of an air-stable composite comprised of metallic magnesium nanocrystals in a gas-barrier polymer matrix that enables the high density storage and rapid release of hydrogen without the need for heavy, expensive metal catalysts. Credit: Images from National Center for Electron Microscopy. Click to enlarge.

Producing materials capable of simultaneously absorbing hydrogen and releasing it on-demand is challenging, the authors note—i.e., and requires simultaneously optimizing two conflicting criteria: absorbing hydrogen strongly enough to form a stable thermodynamic state, but weakly enough to release it on-demand with a small temperature rise.

Some materials under development such as metal–organic frameworks (MOFs), nanoporous polymers, and other carbon-based materials, physisorb only a small amount of hydrogen at room temperature. Metal hydrides have high bond formation enthalpies (for example MgH₂ has a ΔH_f~75 kJ mol^-1), thus requiring unacceptably high release temperatures resulting in low energy efficiency. However, the authors note, recent theoretical calculations and metal-catalyzed thin-film studies have shown that microstructuring of these materials can enhance the kinetics by decreasing diffusion path lengths for hydrogen and decreasing the required thickness of the poorly permeable hydride layer that forms during absorption.

This work showcases our ability to design composite nanoscale materials that overcome fundamental thermodynamic and kinetic barriers to realize a materials combination that has been very elusive historically. Moreover, we are able to productively leverage the unique properties of both the polymer and nanoparticle in this new composite material, which may have broad applicability to related problems in other areas of energy research.

—Jeff Urban, Deputy Director of the Inorganic Nanostructures Facility at the Molecular Foundry, a DOE Office of Science nanoscience center and national user facility located at Berkeley Lab

Urban, along with coauthors Ki-Joon Jeon and Christian Kisielowski used the TEAM 0.5 microscope at the National Center for Electron Microscopy (NCEM), another DOE Office of Science national user facility housed at Berkeley Lab, to observe individual magnesium nanocrystals dispersed throughout the polymer.

With the high-resolution imaging capabilities of TEAM 0.5, the world’s most powerful electron microscope, the researchers were also able to track defects—atomic vacancies in an otherwise-ordered crystalline framework—providing insight into the behavior of hydrogen within this new class of storage materials.

To investigate the uptake and release of hydrogen in their nanocomposite material, the team turned to Berkeley Lab’s Energy and Environmental Technologies Division (EETD), whose research is aimed at developing more environmentally friendly technologies for generating and storing energy, including hydrogen storage.

Resources

• Ki-Joon Jeon, Hoi Ri Moon, Anne M. Ruminski, Bin Jiang, Christian Kisielowski, Rizia Bardhan & Jeffrey J. Urban (2011) Air-stable magnesium nanocomposites provide rapid and high-capacity hydrogen storage without using heavy-metal catalysts. Nature Materials doi: 10.1038/nmat2978