(28e) High and Rapid Hydrogen Release From Thermolysis of Ammonia Borane near PEM Fuel Cell Operating Temperatures

The concerns over diminishing resources and the environmental impact of burning fossil fuels have generated attention on the development of alternative and sustainable energy sources for transportation applications. In this context, hydrogen is a potential clean and environmentally-friendly energy carrier because, with oxygen in fuel cells to generate electricity, its only product is water. A major obstacle for the development of hydrogen powered vehicles is the lack of safe, light weight and energy efficient means for on-board hydrogen storage.

There are targets for hydrogen storage materials so that a vehicle can travel >500 km on single hydrogen fill. The 2015 system volumetric and gravimetric targets are 4.7 MJ/L and 5.5 wt %, respectively. Evaluation of a hydrogen storage system includes all associated components (tank, valves, piping, insulation, reactants, etc.). Currently, no technology is able to meet the 2015 system targets. In order to meet the system targets, material-based H₂ yield must exceed the system targets by a typical factor of 2 or more.

Ammonia borane (NH₃BH₃, AB) has attracted considerable interest as a promising hydrogen storage candidate because of its high hydrogen content (19.6 wt %), hydrogen release under moderate conditions, and stability at room temperature.

Our experiments show promising results that high H₂ yield (~14 wt %, which is significantly higher than by any other method reported in the literature) can be obtained by neat AB thermolysis at or near proton exchange membrane fuel cell (PEM FC) operating temperatures along with rapid kinetics, without the use of either catalyst or additives. In addition, it was found that even small amount of boric acid as additive can decrease onset temperature of AB thermolysis to below PEM FC operating temperatures. Further, only trace amount of NH₃ was detected in the gaseous product from the above-mentioned approaches. Our results show that both neat AB thermolysis and AB thermolysis in the presence of boric acid have the potential to meet DOE target and even surpass it. To our knowledge, on a material basis, these high yield values are higher than by any other method reported in the literature, using AB near PEM FC operating temperatures.

Based on the results obtained from the above-mentioned approaches, we also developed, constructed and tested a continuous-flow hydrogen generation system and details will be presented in this paper. The results indicate that the proposed methods are promising for hydrogen storage, and could be used in PEM FC based vehicle applications.