In what may prove to be a long stride toward swapping hydrogen fuel cells for gas tanks in the clean-running cars of the future, researchers have found a better way to efficiently store hydrogen fuel.
The storage problem is one obstacle that has blocked hydrogen-powered cars from becoming commercially realistic. Now researchers have found a way to recycle spent fuel from one of the most promising hydrogen-storage compounds, a lightweight solid known as ammonia borane. Ammonia borane can store large amounts of hydrogen in a compact, lightweight form — a necessity in the confined space of a vehicle.
While previous research has shown that hydrogen can be harvested from ammonia borane for use in a fuel cell, the process leaves behind spent fuel. But researchers at Los Alamos National Laboratory and the University of Alabama have shown that the byproduct can be efficiently converted back into usable fuel through a series of chemical reactions.
”The stumbling block to this point has been: how do you regenerate the spent fuel? To date, this fuel recycle scheme represents the most promising result in this area,” said John Gordon, a chemist at Los Alamos National Laboratory and a lead author of the study, published in September, in the journal Angewandte Chemie.
Hydrogen has been sought as an alternative to fossil fuels because it can be used in a fuel cell to produce electrical energy without releasing harmful pollutants. But gallon for gallon it’s not as energetic as gasoline, so it hasn’t become a viable transportation fuel.
A vehicle must carry large volumes of hydrogen, usually in the form of compressed gas. Bulky fuel tanks add extra weight and take up cargo space in addition to being a fire risk in an accident. The U.S. Department of Energy (DOE) has warned that the technology must improve to facilitate a more lightweight, low-volume system — one that allows a car to drive at least 300 miles on a single fill-up.
Ammonia borane is an attractive solution to the storage problem. It is made of lightweight elements that can bond to lots of hydrogen atoms. Since the bonded hydrogen can be removed, ammonia borane can act like a fuel tank that stores more hydrogen atoms at lower volumes than hydrogen gas.
The new technique is an “elegant solution,” said University of Ottawa chemist R. Tom Baker, but there are still too many steps for it to be practical at the industrial level. Though not part of this study, Baker’s contribution to previous ammonia borane research laid the groundwork for the hydrogen-removal method the group employed. “This is a big advance, and it allows them to now think about the next advance, which is to reduce the number of operations in the process to get a more direct regeneration to happen,” he said.
Still, a simple, more direct recycle process would represent just one piece of the larger hydrogen car puzzle, albeit an important one.
Ideally, the fuel could be recycled without having to be removed from the vehicle, said John Hemminger, a chemist at the University of California, Irvine, and chair of the Basic Energy Sciences Advisory Committee, which provides independent advice to the DOE. While this finding does not solve that problem, he said, “The authors do describe interesting new chemistry that may eventually lead to an efficient regeneration process. A simple fuel regeneration process that could be carried out on-board a vehicle would be a huge step forward for hydrogen vehicles.”