Sandia researcher Ethan Hecht
Moving Deeper Into Hydrogen and Finding Challenges
August / September 2007
Volume 5 Number 4
In 2006 President Bush announced the Advanced Energy Initiative, a comprehensive plan for energy security, environmental and economic benefits. This reinforced support for the Hydrogen Fuel Initiative, which called for $1.2 billion over five years to elevate hydrogen and fuel cell research and development. Congress agreed, and the Department of Energy brought money to the table for that promise in the form of a five-year Grand Challenge program for hydrogen.
Hydrogen's advantages over fossil fuels include its lack of polluting emissions at the point of use and the fact that it can be produced anywhere from renewable energy resources such as solar electricity or biomass. Hydrogen can also be produced from high-temperature water splitting using solar or nuclear sources or from coal with carbon sequestration, a process of uptake and storage. But there are significant technical challenges to fueling cars with hydrogen.
First, it's difficult to store enough hydrogen onboard to allow the vehicle to travel as far as a conventional vehicle on a full tank of fuel. While hydrogen releases more energy per mass than gasoline or diesel, it is a gas at ordinary temperatures and only a small amount (in terms of weight) can be stored in onboard fuel tanks of a reasonable size. Second, getting hydrogen to consumers will require new facilities and systems for hydrogen production, delivery and storage. It will take significant time and money to develop a hydrogen transportation and storage infrastructure. Third, hydrogen-fueled cars must win consumers' confidence before their benefits can be realized. Consumers may have concerns about the dependability and safety of these vehicles, just as they have with many other game-changing technologies. Finally, using the current technologies, hydrogen-powered vehicles are much too expensive for most consumers to afford. Vehicle performance, durability and cost (particularly fuel cell durability and cost) must all be improved.
DOE is addressing the first of those concerns with three Centers of Excellence and multiple independent projects in the National Hydrogen Storage Project. Researchers will search for breakthrough technologies in onboard reversible metal hydrides that can absorb and release hydrogen on the vehicle, chemical hydride systems that require recharging with hydrogen off the vehicle and adsorbent materials that retain hydrogen by sorption—€”soaking up or attracting hydrogen. The centers are joint university, industry and federal laboratory partnerships. The nineteen partners—€”nine universities, four companies and six federal labs—€”in the Metal Hydride Center of Excellence, which Sandia leads, concentrate on the development of advanced, on-board reversible metal-hydride materials, which if successful will help meet DOE's goal of a 300-mile per fill-up vehicle without compromising space onboard.
Currently, no material exists that safely and efficiently stores hydrogen fuel with the storage efficiencies that can meet the demanding DOE goals. Complex metal hydrides, however, are highly promising for developing on-board hydrogen storage systems due both to their high hydrogen storage weight capacities and high volumetric densities. Typically, the storage materials must be heated to release hydrogen and it is the goal of the R&D program to reduce the required energy input and improve the speed with which the material takes on hydrogen during re-fueling at a hydrogen filling station.
The metal hydride center focuses on new materials discovery guided by state-of-the-art theory. In addition, the materials are explored with the engineering requirements firmly in mind. The center is organized into five R&D project areas, with each project led by a different team member.
The Destabilized Metal Hydrides Project (Project A) is led by Professor Ian Robertson of the University of Illinois at Urbana/Champaign. This project focuses on improving the performance of solid-state materials consisting of light elements. Although light-metal hydrides have compelling weight and volume capacity advantages, two major hurdles must be overcome before those materials are acceptable for on-board reversible hydrogen storage applications. First, the chemical bonds in light-metal hydrides are often strong, resulting in low steady-state hydrogen pressures. Second, the bonds are highly directional, resulting in limited reversibility.
Ewa Ronnebro of Sandia is leading the Complex Anionic Materials project (Project B),which focuses on discovering new materials and synthesis routes because there are currently no hydrogen storage materials that meet the DOE's targets.
Ronnebro says, "Our group is synthesizing novel, lightweight, high-capacity metal hydrides as potential candidates for on-board materials. We are closely interacting with the center's theory group to obtain guidance on material features and insights into reaction mechanisms."
The Amides/Imides Project (Project C) is led by Professor Zak Fang of the University of Utah. This effort focuses on the discovery and characterization of materials and reactions that involve metal nitrides containing compounds and alanate materials. The development of hydrogen storage materials containing amides faces the same hurdles as do other categories of candidate materials: the reversible hydrogen storage capacity is still unsatisfactory, the temperature of the hydrogen release reaction is still too high, and the kinetic rates of reversible take-up and release reactions are too slow for automobile applications.
Jim Wegrzyn, of Brookhaven National Laboratory, is the leader for Project D, the Alane Project. Wegrzyn says that aluminum hydride (also known as Alane) is a fascinating material that has recently attracted attention for its potential as a hydrogen storage medium for low-temperature fuel cells. Alane is stable at room temperature and that stability is generally attributed to a surface oxide layer, which acts as a kinetic barrier to decomposition. However, because Alane is not a reversible hydride at moderate hydrogen pressures, the utility of this material will depend on the development of new techniques to regenerate it from spent aluminum powder in a cost-effective manner.
The Engineering, Analysis and Design Project (Project E) is led by Donald L. Anton of the Savannah River National Laboratory. The objective is to quantitatively demonstrate solid-state metal-hydride storage-system capabilities and progress toward DOE's system performance goals. The team's approach includes developing and evaluating heat and mass transfer analysis methods to predict performance of hydrogen storage systems based on the various metal hydrides of interest, designing and testing advanced heat-exchange systems, measuring engineering properties such as heat capacity, expansion stresses, and thermal conductivity and developing handling and use assessments for solid-state hydride materials.
Sandia's Lennie Klebanoff, the center's director, says "We are tightly focused on achieving the DOE goals for storage performance. Our center is highly collaborative, and everyone is aggressively pursuing the DOE performance targets." Over 200 technical tasks across the centers of excellence are tracked quarterly, with progress against the milestones assessed. Go/no-go decisions are made as the work progresses. However, while acknowledging that the DOE targets necessitate practical resource-allocation decisions, Klebanoff says that identifying the right storage material is not so clear cut.
"We are still heavily involved in materials discovery," Klebanoff says, "and there are a variety of materials properties (storage capacity, kinetics, thermodynamics) that need to be understood and improved on. We have yet to decide on a particular material."
To meet the Department of Energy's goals and to enable the widespread commercialization of hydrogen technologies by industry, scientists and engineers currently working on the National Hydrogen Storage Project have their work cut out for them. When the difficult research and development efforts generate commercializable technologies, and the auto and energy industries begins to apply those technologies to their products, we will then have vehicles that improve urban air quality, decrease greenhouse gases and reduce our dependence on foreign oil.
Margaret Lovell covers Sandia National Laboratories for Innovation.
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