One challenge in using hydrogen as a transportation fuel–besides finding a clean, cheap source of the fuel itself–is how to safely and reversibly store it without taking up too much space. Hydrogen has a low density, so it’s necessary to confine it either under pressure, which presents a safety hazard, or chemically or in an absorptive material.
In chemical storage, hydrogen is bonded to the molecules in a solid material such as ammonia borane. The advantage of chemical storage is that these materials are inert solids, and the hydrogen can be readily removed for reaction in a fuel cell. But the materials under development for chemically storing hydrogen have a major limitation: refueling them once they’re spent takes a large amount of energy. Now researchers have developed a series of reactions for refueling the high-density hydrogen-storage material ammonia borane at lower temperatures through a process that consumes much less energy.
The U.S. Department of Energy (DOE) has set a goal of a hydrogen fuel-cell car that can travel 300 miles on a single fuel tank using chemical hydrogen storage. The cars would be taken to a center to exchange the spent tanks for fresh ones, with the spent tanks regenerated at a plant.
The capacity of a material to chemically store hydrogen is measured as the percentage of its weight taken up by the element; in order to meet its goals, the DOE benchmark for hydrogen-storage materials is 6 percent by weight by 2010 and 9 percent by 2015. “The good news about ammonia borane is it can hit or surpass the volume and weight targets” set by the DOE, says Jamie Holladay, a senior research engineer at the Pacific Northwest National Laboratory. Ammonia borane contains 19.6 percent hydrogen by weight. “The challenge is regeneration of the spent fuel,” he says.
“Once you get the hydrogen out of the ammonia borane, you can’t just pressurize it with more hydrogen to regenerate the fuel,” because this is too energy-intensive, says John Gordon, a research chemist at Los Alamos National Laboratory in New Mexico. In order to find out which reactions were likely to work best without having to test hundreds on the bench, chemists at Los Alamos collaborated with David Dixon, a professor of chemistry at the University of Alabama, who developed algorithms to predict the energetics of the reactions. The group then tested the most promising chemistries and found that using a tin catalyst and regenerating the material in several steps required much less energy than driving the reaction directly.
Of course, a major problem remains before hydrogen fuel-cell cars become practical: developing improved methods for making hydrogen fuel in the first place, a challenge other researchers are working on.