Ammonia borane (H₃NBH₃) is of interest as a hydrogen storage material because of its high hydrogen content (19.6 wt%) and low molecular weight (30.7 gmol^-1). However, the material suffers from a number of drawbacks, including the lack of energy-efficient methods to reintroduce hydrogen back into the spent fuel once it has been released.

Other issues have included the temperature required for dehydrogenation; releasing the hydrogen from AB usually requires temperatures of more than 100 °C, making it too hot for polymer-based fuel cells. Other issues with its use are the release of other gases which could poison the hydrogen and instability, e.g., rapid expansion or turning into foam. (Earlier post). Work at LANL is also addressing the release and impurities issues.

Although spent fuel composition depends on the dehydrogenation method, we have focused our efforts on the spent fuel resulting from metal-based catalysis, which has to date shown the most promise to meet the DOE H₂ storage requirements for release rate and extent. Although the first transition-metal-catalyzed dehydrogenation of AB generated many products, more recent metal catalysts have produced single products, the fastest rates for a single equivalent of H₂ released from AB, and the greatest extent of H₂ release (up to 2.5 equiv of H₂ can be produced within 2 h).

While ongoing work is being carried out to tailor the composition of spent AB fuel, we have developed a method for regenerating the predominant product, polyborazylene, resulting from dehydrogenation by nickel carbene catalysts.

—Davis et al.

Their approach uses benzenedithiol as a reagent to avoid the formation of thermodynamically stable B-O bonds and the subsequent need for high-energy reducing agents. Heating PB and benzenedithiol produces a set of intermediate products (including ammonia).

A feature of the first step in this cycle requires highlighting: some of the nitrogen in the spent fuel is transformed into NH₃, which is subsequently retained by (C₆H₄S₂)B–H·(NH₃). This process contrasts with other proposed methods, which solely generate NH₄ + salts (which require thermal cracking to release and recycle NH₃).

—Davis et al.

Tributylstannane (Bu₃SnH) is used as a reductant on the products resulting from the first step, transforming them into a new boron-containing product. Bu₂SnH₂ then generates the AB. Using this methodology, including the stepwise addition of excess reductant, the team generated an overall yield of 67% isolated AB on their first attempt.

Future papers will provide more detail on the complete regeneration style, including the recycling of the tin by-product, use of hydrogen as the energy source, and improved overall efficiencies.

This research represents a breakthrough in the field of hydrogen storage and has significant practical applications. The chemistry is new and innovative, and the research team is to be commended on this excellent achievement.

—Dr. Gene Peterson, leader of the Chemistry Division at Los Alamos

The Chemical Hydrogen Storage Center of Excellence is one of three Center efforts funded by DOE. The other two focus on hydrogen sorption technologies and storage in metal hydrides. The Center of Excellence is a collaboration between Los Alamos, Pacific Northwest National Laboratory, and academic and industrial partners.

Resources

• Benjamin L. Davis, David A. Dixon, Edward B. Garner, John C. Gordon, Myrna H. Matus, Brian Scott, Frances H. Stephens (2009) Efficient Regeneration of Partially Spent Ammonia Borane Fuel. Angewandte Chemie International Edition Volume 48, Issue 37 , Pages 6812 - 6816 doi: 10.1002/anie.200900680

• Chemical Hydrogen Storage R&D at Los Alamos National Laboratory (2009 Merit Review)