| Reversible cycle for aluminum hydride. All components of the electrochemical process can be recycled to continually afford a viable solid-state storage material. Source: SRNL. Click to enlarge. |
Researchers at the US Department of Energy’s Savannah River National Laboratory (SRNL) have demonstrated a reversible electrochemical route to generate aluminum hydride (alane, AlH3), a high-capacity hydrogen storage material. A paper on their work appeared in the Royal Society of Chemistry journal Chemical Communications.
Aluminum hydride is one of the promising bulk materials still under investigation at the US Department of Energy’s Metal Hydride Center of Excellence (MHCoE, based at SRNL) following the downselection of a number of other potential materials over the past year, according to a presentation given by MHCoE director Lennie Klebanoff at the recent 2009 DOE Merit Review.
| 5 Primary MHCoE Performance Criteria for Go/No-Go Materials Decisions |
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Aluminum hydride has a hydrogen gravimetric capacity of 10 wt% and volumetric capacity of 149 g/L. Its desorption temperature (~60 °C to 175 °C, depending on particle size and the addition of catalysts) meets the 2010 DOE targets for desorption.
Although alane possesses the desired qualities for on-board hydrogen storage, it has been considered impractical because of the high pressures required to combine hydrogen and aluminum to reform the hydride material.
Alternate methods of production using chemical synthesis have typically produced stable metal chloride byproducts that make it practically impossible to regenerate the alane. The electrochemical cycle demonstrated by principal investigator Ragaiy Zidan and the SRNL team for production of alane avoids both of these issues.
The SRNL approach utilizes electrolytic potential to drive chemical reactions to form AlH3. Once the alane is generated in the electrochemical cell, it has to be recovered in pure form. To avoid the concern of Al and AlH3 oxidizing in an aqueous environment, the researchers use non-aqueous electrolytes.
A number of attempts have been made in the past to make alane electrochemically; none of these previous attempts were totally successful. Earlier calculations by the SRNL team found that their electrochemical synthesis of alane was 75% efficient, compared to an ideal process efficiency of 85% (i.e., 25% energy consumption in the actual case, compared to 17% in the ideal).
By comparison, other work has shown that chemically recycling AlH3 by splitting LiCl requires a minimum of 70% of fuel energy for regeneration.
The SRNL team is collaborating with researchers from Brookhaven National Laboratory, the University of Hawaii, the University of New Brunswick, Argonne National Laboratory, and Toyota on the project.
Ongoing work includes optimizing the efficiency of the process and identifying more efficient separation solvents.
In conjunction with this research, the SRNL team discovered novel ways to facilitate separation and formation of aluminum hydride that also apply to the formation of other complex metal hydrides and have the potential to cost-effectively regenerate other high capacity hydrogen storage materials.
The SRNL results are expected to accelerate the development of a whole class of similar materials needed for hydrogen, batteries and other energy storage applications. In addition, this work will impact other fields including those of thin films, adduct based syntheses, and the recycling and regeneration of other materials.
Resources
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Ragaiy Zidan, Brenda L. Garcia-Diaz, Christopher S. Fewox, Ashley C. Stowe, Joshua R. Gray and Andrew G. Harter (2009) Aluminium hydride: a reversible material for hydrogen storage. Chem. Commun., 3717 – 3719, doi: 10.1039/b901878f
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Electrochemical Reversible Formation of Alane (Zidan, 2009 DOE Hydrogen Program Review)