November 23rd, 2009

"Let's Clear the Air"

Progress in Hydrogen Storage Technologies

by Eric Underwood, NAFTC Curriculum Developer

An advanced method of “recycling” hydrogen-containing fuel materials could enhance the development of viable hydrogen-based vehicles. The Los Alamos National Laboratory and University of Alabama researchers, working within the U.S. Department of Energy’s (DOE) Chemical Hydrogen Storage Center of Excellence, state that they have made progress in hydrogen storage technologies.

Although hydrogen is abundant in nature, it is associated with other compounds. The key is obtaining the hydrogen from lightweight materials that maintain overall fuel efficiency and pack high energy content into a small volume. Pure hydrogen in its gaseous form exhibits a low energy density per unit volume. Hydrogen storage technology will have to move beyond conventional compressed hydrogen cylinders. The DOE has established a goal for hydrogen vehicles to reach 300 miles or more to enable the vehicles to have a similar fuel economy range as compared to conventional gasoline and diesel vehicles. With this in mind, the general public might gain an increased interest in hydrogen technology.

The Chemical Hydrogen Storage Center of Excellence is focusing on using chemical hydrides such as ammonia borane to address the energy density issues associated with pure hydrogen. The hydrogen can be freed from the hydrides and potentially be used to run a fuel cell. Hence, the chemical compounds can be referred to as “chemical fuel tanks” because of their hydrogen storage capability. The new technology uses nano structure approaches to enhance the properties of conventional hydrogen storage materials used to supply hydrogen to fuel cells.

The term “chemical hydrogen storage” is used to describe storage technologies in which hydrogen is generated through a chemical reaction. The hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. In the first stage, slurry of an inert stabilizing liquid protects the hydride from contact with moisture and makes the hydride pumpable. The slurry is then mixed with water, and the consequent reaction produces high-purity hydrogen.


Ammonia Borane Hydrogen Process Schematic. Image courtesy of DOE/Los Alamos National Laboratory

However, the lack of energy-efficient methods to reintroduce hydrogen back into the spent fuel is an obstacle. Typically, the reactions are not easily reversible on-board a vehicle. Hence, the “spent fuel” and/or byproducts must be removed from the vehicle and regenerated off-board. In other words, after hydrogen has been released, the ammonia borane cannot be satisfactorily recycled.

To combat the issue, the Los Alamos researchers along with their University of Alabama colleagues are working on developing techniques for the efficient recycling of the hydrides. The research team made an advancement when it discovered polyborazylene, a specific form of dehydrogenated fuel that could assist the ammonia borane recycling process with modest energy input. This step can lead to using ammonia borane as a probable energy carrier for transportation purposes. Dr. Gene Peterson, leader of the Chemistry Division at Los Alamos, states that the research represents a breakthrough in the field of hydrogen storage and has significant practical applications.

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