Final Report: Metal Perhydrides for Hydrogen Storage
Hydrogen is a promising energy source for the future economy due to its environmental friendliness. One of the important obstacles for the utilization of hydrogen as a fuel source for applications such as fuel cells is the storage of hydrogen. In the infrastructure of the expected hydrogen economy, hydrogen storage is one of the key enabling technologies. Although hydrogen possesses the highest gravimetric energy content (142 KJ/g) of all fuels, its volumetric energy density (8 MJ/L) is very low. It is desired to increase the volumetric energy density of hydrogen in a system to satisfy various applications. Research on hydrogen storage has been pursed for many years. Various storage technologies, including liquefaction, compression, metal hydride, chemical hydride, and adsorption, have been examined. Liquefaction and high pressure compression are not desired due to concerns related to complicated devices, high energy cost and safety. Metal hydrides and chemical hydrides have high gravimetric and volumetric energy densities but encounter issues because high temperature is required for the release of hydrogen, due to the strong bonding of hydrogen in the compounds. Reversibility of hydrogen loading and unloading is another concern. Adsorption of hydrogen on high surface area sorbents such as activated carbon and organic metal frameworks does not have the reversibility problem. But on the other hand, the weak force (primarily the van der Waals force) between hydrogen and the sorbent yields a very small amount of adsorption capacity at ambient temperature. Significant storage capacity can only be achieved at low temperatures such as 77K. The use of liquid nitrogen in a hydrogen storage system is not practical. Perhydrides are proposed as novel hydrogen storage materials that may overcome barriers slowing advances to a hydrogen fuel economy. In conventional hydrides, e.g. metal hydrides, the number of hydrogen atoms equals the total valence of the metal ions. One LiH molecule contains one hydrogen atom because the valence of a Li ion is +1. One MgH2 molecule contains two hydrogen atoms because the valence of a Mg ion is +2. In metal perhydrides, a molecule could contain more hydrogen atoms than expected based on the metal valance, i.e. LiH1+n and MgH2+n (n is equal to or greater than 1). When n is sufficiently high, there will be plenty of hydrogen storage capacity to meet future requirements. The existence of hydrogen clusters, Hn+ (n = 5, 7, 9, 11, 13, 15) and transition metal ion-hydrogen clusters, M+(H2)n (n = 1-6), such as Sc(H2)n+, Co(H2)n+, etc., have assisted the development of this concept. Clusters are not stable species. However, their existence stimulates our approach on using electric charges to enhance the hydrogen adsorption in a hydrogen storage system in this study. The experimental and modeling work to verify it are reported here. Experimental work included the generation of cold hydrogen plasma through a microwave approach, synthesis of sorbent materials, design and construction of lab devices, and the determination of hydrogen adsorption capacities on various sorbent materials under various electric field potentials and various temperatures. The results consistently show that electric potential enhances the adsorption of hydrogen on sorbents. NiO, MgO, activated carbon, MOF, and MOF and platinum coated activated carbon are some of the materials studied. Enhancements up to a few hundred percents have been found. In general, the enhancement increases with the electrical potential, the pressure applied, and the temperature lowered. Theoretical modeling of the hydrogen adsorption on the sorbents under the electric potential has been investigated with the density functional theory (DFT) approach. It was found that the interaction energy between hydrogen and sorbent is increased remarkably when an electric field is applied. This increase of binding energy offers a potential solution for DOE when looking for a compromise between chemisorption and physisorption for hydrogen storage. Bonding of chemisorption is too strong and requires high temperature for the release of hydrogen. Bonding for the physisorption is too weak for sufficient uptake of hydrogen. Electric field potentials can enhance the physisorption and can be adjusted to yield reversibility required in a system at room temperature.
- Research Organization:
- Michigan Technological Univ., Houghton, MI (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- FG36-05GO15003
- OSTI ID:
- 1019416
- Report Number(s):
- DOE/GO15003-F; 030939; TRN: US201209%%249
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
30 DIRECT ENERGY CONVERSION
ACTIVATED CARBON
ADSORPTION
AMBIENT TEMPERATURE
ATOMS
BINDING ENERGY
BONDING
CAPACITY
ELECTRIC CHARGES
ELECTRIC FIELDS
ELECTRIC POTENTIAL
ENERGY ACCOUNTING
ENERGY DENSITY
ENERGY SOURCES
FUEL CELLS
HYDRIDES
HYDROGEN
HYDROGEN FUELS
HYDROGEN STORAGE
STORAGE
SURFACE AREA
TRANSITION ELEMENTS
VALENCE
VAN DER WAALS FORCES
hydrogen
electric field
clusters
plasma
microwave
sorbent
chemisorption
physisorption
reversibility
enhancement
room temperature
electric potential
perhydride
activiated carbon
MOF