Hydrogen Storage Materials
Future transport applications using hydrogen (H2) as a clean energy-carrier requires hydrogen storage in lightweight compact tanks under moderate conditions that are compatible with normal road-vehicle operation. Among the goals set for 2010 by the US Department of Energy (DOE) for the storage system is a 6 wt% hydrogen storage capacity at a density of 45 kg H2/m3 which can be refueled in 3 minutes. The main focus of the hydrogen storage research in our group is the chemical bonding and nanoscale structure of new lightweight metal hydrides, and hydrogen molecular adsorption in nano-porous compounds such as metal organic frameworks (MOF’s), nanocarbons, and clathrate hydrates. In combination with ab-initio simulations, new insight in their hydrogen storage properties can be gained down to atomic length scales using neutron scattering, XRD, electron microscopy and positron depth profiling, important in order to develop better hydrogen storage materials.
The combined demands of reversibility of hydrogen loading, high capacity, low volume, low weight, price, safety and ease of operation make that, currently, no material yet meets the constraint for reversible hydrogen storage under near-ambient conditions. In the graph below a summary of some relevant materials is given, showing their gravimetrical hydrogen capacity versus the chemical potential involved in the binding of hydrogen. The latter energy is closely related to the operation conditions: a larger chemical potential signifies in general a higher temperature of operation at ambient pressures. Ideally, a chemical potential of about 40 kJ/mole H2 is combined with a sufficiently high reversible hydrogen storage capacity, while the hydrogen can be cycled on a time scale of minutes or less. In practice, the surface adsorption materials like nanocarbons and MOF’s have relatively small chemical adsorption potentials and for that reason adsorb at temperatures below 77K. The metal hydrides like MgH2 and NaAlH4 on the other hand require elevated temperatures in order to operate, related to the stronger hydrogen binding and slow kinetics. In clarifying the atomic scale function of added catalysts, one of the goals for metal hydrides is to reach improved kinetics and to match operation temperatures to those of fuel cells.
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Main experimental techniques:
- Neutron scattering: structure and dynamics and directly observe the hydrogen inside the storage materials.
- X-ray diffraction: structural characterisation.
- Electron microscopy (TEM, SEM): nanoscale structural characterisation.
- Positron depth profiling: influence of defects on hydrogen kinetics and storage
- Macroscopic sorption measurements: capacity and thermal behaviour.
- Permeation measurements: kinetics and revealing the rate limiting processes.
- Physical synthesis of thin film metal hydrides using pulsed laser deposition and plasma sputter deposition.
- Chemical synthesis in collaboration with others.
- First principles modeling and molecular dynamics.