by David Thorn, Frances Stephens, Karl Jonietz, William Tumas, R. Tom Baker
Replacing petroleum-powered vehicles with hydrogen-powered vehicles requires a number of technological breakthroughs, but perhaps the greatest challenge is storing enough hydrogen on board to achieve an adequate driving range.
Without adequate range, hydrogen-powered vehicles will be confined to major cities, and consumers will be less likely to relinquish gasoline-powered cars for hydrogen vehicles. To help meet this challenge, Los Alamos National Laboratory (LANL) and several partners have come together to form the Chemical Hydrogen Storage Center of Excellence (The Center).
The FreedomCAR and Fuel Partnership—a collaboration between the Department of Energy (DOE), the U.S. Council for Automotive Research, and energy companies—has developed on-board hydrogen storage targets with the goal of "achieving similar performance and cost levels as current gasoline fuel storage systems."
In order to provide a 300-mile driving range between refueling without unacceptably bulky storage vessels, the DOE has established targets for year 2010 of storing 0.045 kg hydrogen per liter of storage system volume and 0.06 kg hydrogen per kg of storage system mass.
Longer-term targets are even more demanding: 0.081 kg hydrogen per liter, and 0.09 kg hydrogen per kg of storage mass by the year 2015. The DOE has also established targets of 60% for the overall energy efficiency of storing hydrogen.
To put the storage density target numbers into context, note that compressed hydrogen gas at 200 atmospheres pressure is only 0.017 kg hydrogen per liter, and the pressure tank requires additional volume and mass. The greatest density of hydrogen that can ever be practically achieved is that of liquid hydrogen itself, only 0.07 kg per liter. Thus, compressed or liquefied hydrogen can never meet the DOE's 2015 target density.
Many researchers and industry partners believe that the key to reaching, and even exceeding, the DOE target hydrogen storage densities is to store not pure elemental hydrogen, but hydrogen that is physically or chemically bound to materials that can be made to release the hydrogen under well-defined conditions. One such means of storing hydrogen is by using metals that react with hydrogen at room temperature to form a hydride phase and release the hydrogen when heated.
Another such means of storing hydrogen is chemical hydrogen storage, whereby the chemically bound hydrogen is released not by heat alone, but by a more tangible chemical reaction or catalytic chemical process.
One of the best-known means of chemical hydrogen storage is called Hydrogen On Demand™ by its developer, Millennium Cell, and involves the catalyzed reaction between sodium borohydride and water to release hydrogen and form sodium borate.
Even though hydrogen-storing materials and hydrogen-releasing reactions have been known for some time, none presently meets both the DOE storage density and energy efficiency targets.
In 2003, the DOE issued the grand challenge storage call, asking for Centers of Excellence to develop new materials and processes that could meet all targets for storing hydrogen.
Three Centers of Excellence have been formed in response to this grand challenge.
Sandia National Laboratories (California) and its partner institutions are working to develop reversible metal hydride storage materials that use light elements to meet storage density targets.
The National Renewable Energy Laboratory (Colorado) and its partner institutions are working to develop high-capacity hydrogen-sorbing materials based on carbon.
LANL, in partnership with Pacific Northwest National Laboratory, seven universities and four companies, is working to develop chemical hydrogen storage materials.
The Chemical Hydrogen Storage Center of Excellence, which started work in early 2005, is pursuing a multi-pronged research approach aimed at meeting DOE targets. Center partners are focusing on enabling the Hydrogen On Demand™ system by developing new routes for regenerating sodium borohydride from the product sodium borate.
Fundamental thermodynamic properties of sodium borohydride establish the maximum possible energy efficiency of storing hydrogen in this system as 70%, meeting the DOE efficiency target and considerably greater than the actual energy efficiency of the present manufacturing process.
The Center is currently addressing the hard question by carefully scrutinizing what is known about sodium borohydride chemistry: can we realistically expect to devise a new cost-effective manufacturing process that approaches 70% energy efficiency?
At the same time, the Center is pursuing a second tier of research aimed at developing the chemistry of other boron-hydrogen materials, materials that may provide an even greater density of stored hydrogen and more energy-efficient regeneration than sodium borohydride. It is also exploring how hydrogen can be stored using other reaction and material concepts, including novel organic compounds and reaction mixtures as well as nanostructured materials based on light elements.
At this stage the decisions for the Center have focused on what materials and concepts to pursue based on a combination of thermodynamic considerations and DOE target criteria.
The Hydrogen On Demand™ fuel system developed by Millennium Cell as it appears in DaimlerChrysler's fuel cell research car, the Natrium. (Click image to enlarge)
