Ammonia Borane as Hydrogen Storage Materials

• Among these B-N adducts, ammonia borane (H3N-BH3) is one of the most promising hydrogen storage materials because of its high hydrogen content (19.6% wt.), high stability under ambient conditions, non-toxicity, and high solubility in common solvents such as water and methanol.

• The safe and efficient storage of Hydrogen is the major obstacle in its wide applications

• Therefore,much attention has been paid to the development of new materials which can provide hydrogen with high gravimetric and volumetric densities with suitable thermodynamic and kinetic properties

• B-N adducts such as ammonia borane, dimethyl ammonia borane, hydrazine borane are considered as ideal hydrogen storage materials due to their high H2 content.

• The most effective and safest way of storing hydrogen is to use solid media such as sorbent materials or hydrides. Chemical hydrides provide a higher energy density for hydrogen storage as compared to the gas or liquid H2 tank systems.

• Recent reports have shown that B-N adducts need to be considered as hydrogen storage materials because of their high content of hydrogen with multiple nature, the protic N-H and hydridic B-H hydrogen.

Hydrolysis or methanolysis are two practical ways of hydrogen generation from ammonia borane for hydrogen fuel cell applications at ambient temperature. Hydrolysis can release

3 mol H2.

The efficient release of hydrogen stored in the storage materials is another challenge in the hydrogen economy.

Ammonia borane is relatively stable against hydrolysis in aqueous solution (Eq. (1)).

Therefore, the hydrolytic dehydrogenation can be achieved at an appreciable rate only in the presence of suitable catalyst at room temperature.

Hydrogen can be released from ammonia borane by thermolysis or solvolysis in the presence of suitable catalysts. The former process has some drawbacks.

• it requires long induction time (~3 h) and high temperature,

• various by-products, such as ammonia (NH3) and borazine (B3N3H6) can be formed during reaction.

Transition metal nanoparticles (NPs) have been widely used as catalysts in releasing H2 from AB.

Since the metal nanoparticles tend to agglomerate causing a significant loss in catalytic activity they need to be stabilized either by using surfactants or ligands in solution or by using supporting materials with large surface area in solid state. The results reported in literature shows that;

• metal nanoparticles dispersed in solution or supported on suitable solid materials with large surface area can catalyze the release of H2 from AB at room temperature and

• how to improve the catalytic activity and durability of metal nanoparticles by selecting suitable stabilizer or supporting materials for a certain metal or by modifying them appropriately.

TEM image of the water soluble laurate-stabilized Rh(0) NPs obtained from the reduction of rhodium(III) ions by dimethylamine borane in the presence of laurate anion in aqueous solution.

Fig. 3 e Results of the reusability tests for the water soluble laurate-stabilized Rh(0) NPs ([Rh] ¼ 0.25 mM) displayed as the percentage of initial catalytic activity retained in the successive runs of the hydrolytic dehydrogenation of AB (100 mM) at 25.0 ± 0.1 C.

Volume of H2 (mL) versus time (s) plot for the hydrolytic dehydrogenation of AB (100 mM) catalyzed by laurate-stabilized Rh(0) NPs in aqueous solution at 25.0 ± 0.1 C with different rhodium concentrations:

[Rh] ¼ 0.25, 0.50, 1.0, 1.5, and 2.0 mM.

► A novel H2 generator fueled by ammonia borane is developed.

► Dehydrogenation of ammonia borane is achieved by solvent-mediated thermolyses.

► The H2 generator shows fast load-following capability and rapid response time.

► The H2 generator produces H2 autothermally to operate a 200 We PEMFC stack.