Hydrogen storage in advanced solid state materials has been an intense area of research due to many drawbacks in conventional high pressure or cryogenic liquid hydrogen storage methods. A practical hydrogen storing material is required to have high storage capacity and fast dehydrogenation kinetics. Among many solid state materials for hydrogen storage, magnesium hydride (MgH2) combines a hydrogen capacity of 7.6 wt % with the benefit of the low cost of production and abundance. The main difficulties for implementing MgH2 are slow absorption/desorption kinetics and high reactivity towards air and oxygen, which are also common issues in most lightweight metal hydrides. Previously, improvements in hydrogen storage and release properties have been reported by using nanostructured magnesium that can be obtained through various fabrication methods including ball-milling, mechanical alloying, and vapor transport. Nanostructured metal hydrides are expected to provide superior hydrogen storage properties mainly because of:
- Large surface area to volume ratio: Due to the large surface area and small diameters of the nanostructured metal hydrides, the rate of absorption and desorption will be faster.
- Crystal orientation of the structures can be optimized through the deposition parameters of GLAD for the maximum hydrogen absorption/desorption rates.
- Due to the lower oxidation rate of single crystal GLAD nanostructures, hydrogen permeability will be further enhanced.
- Porous structure will improve the mechanical stability of the metal hydride system that will be needed during volumetric changes due to the absorption/desorption of hydrogen.
In our work, we investigate the hydrogen absorption and desorption properties of magnesium “nanotrees” fabricated by glancing angle deposition (GLAD) technique, and also conventional Mg thin films deposited at normal incidence. Mg nanotrees are about 15 µm long, 10 µm wide, and incorporate “nanoleaves” (Fig. 1) of about 25 nm in thickness and 1,2 µm in lateral width. A quartz crystal microbalance (QCM) gas absorption/desorption measurement system has been used for our hydrogen storage studies. Nanostructured and thin film Mg have been deposited directly on the surface of the gold coated unpolished quartz crystal samples. QCM hydrogen storage experiments have been performed at temperatures ranging between 100-300 oC, and at H2 pressures of 10 and 30 bars. Our QCM measurements revealed that Mg nanotrees can absorb hydrogen at lower temperatures and faster compared to Mg thin film. In addition, Mg nanotrees can reach hydrogen storage values of about 7.0 wt %, which is close to the theoretical maximum storage value, at temperatures as low as 100 oC. The significant enhancement in hydrogen absorption properties of our Mg nanotrees is believed to originate from novel physical properties of their nanoleaves. These nanoleaves are very thin (~25 nm) and both surfaces are exposed to hydrogen enhancing the diffusion rate of hydrogen together with a decreased diffusion length. Based on X-ray diffraction measurements, individual nanoleaves have non-close-packed crystal planes that can further enhance the hydrogen absorption kinetics. In addition, our nanostructured Mg have been observed to quite resistant to surface oxidation, which is believed to due to the single crystal property of the Mg nanoleaves, that further improves the absorption kinetics of hydrogen.