The (Lightweight) Heavy Hitter

Los Alamos chemist Andrew Sutton discovered a new method for refueling cars that could go the distance. Sutton revealed a novel single-stage method for recharging hydrogen-rich ammonia borane (AB), a potential onboard hydrogen-storage compound for fuel-cell-powered vehicles.

A fuel cell converts hydrogen in the presence of oxygen into electricity and water, and can do so efficiently without generating polluting emissions—the sole byproduct is water. Hydrogen is a common fuel used in fuel cells and provides substantial energy for its low weight, but it requires energy to produce and takes up a lot of space. In addition, it is dangerous to transport due to its explosive nature. Therefore, a material that can store and release hydrogen safely would be very attractive.

How do you store hydrogen? Transport and storage of hydrogen can be dangerous in pure form, but when hydrogen is stored as AB, it is thermally stable at ambient temperatures and is easily transportable. AB is a particularly attractive hydrogen storage material because it is nontoxic and can liberate large quantities of hydrogen, potentially enough to propel hydrogen-fueled vehicles 300 or more miles per "tank," a federal research goal. The challenges to this goal include controlled dehydrogenation—getting the energy out—and regeneration of the spent fuel.

In an ideal AB-based fuel cell, the hydrogen-depleted spent fuel residue is composed of polyborazylene (PB). Thermodynamically, there is no way to convert the PB back into AB using hydrogen directly. A Los Alamos team led by Ben Davis developed a method to recycle PB with minimal energy input by introducing tin hydride as a reductant—providing hydrogen to the spent PB. First, the spent fuel is "digested" with a thiol, then a tin hydride is added, followed by the addition of ammonia, which reproduces AB. This is a huge breakthrough in itself, but unfortunately, tin hydride would be too expensive to implement on a large scale due to the high costs of handling it.

Carrying this work forward, Sutton decided to investigate a much lighter reductant in the form of hydrazine. He added hydrazine in liquid ammonia to PB, resulting in AB (the hydrogen source) and nitrogen. That is, he was able to devise a simple method to regenerate the hydrogen-storing compound from its spent-fuel form in a single container with just one step. Regeneration takes place offboard, but the researchers envision vehicles with interchangeable hydrogen storage tanks that can be swapped as needed.

The next target is to efficiently make hydrazine, as current manufacturing methods for hydrazine use energetically expensive precursors. Hydrazine is also potentially hazardous to transport across the nation in large loads, so Los Alamos researchers anticipate making it on-site during the AB recycling process by combining the byproduct nitrogen with hydrogen to make new AB. This is not a trivial process, but if hydrazine synthesis can be refined, the use of AB in the transportation sector could become viable. The achievements by Sutton and Davis greatly improve the characteristics of AB as a hydrogen storage material, potentially facilitating the large-scale implementation of hydrogen fuel cells that provide safe, renewable energy.

—Kirsten Fox

Fuel-cell powered car photo

Fuel-cell-powered cars such as this Chevrolet use hydrogen and oxygen to power the vehicle's electric motor, and the only emission is water. Los Alamos researchers have discovered a way to recycle the spent fuel.

 

 

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