Mar 03 2011
Boron- and nitrogen-based chemical hydrides are expected to be potential hydrogen carriers for PEM fuel cells, because of their high hydrogen contents. Among them, sodium borohydride (NaBH4, denoted SB) and ammonia borane (NH3BH3, denoted AB) have attracted much attention as promising hydrogen storage materials.
There are many similarities between SB and AB in their features and applications. Nevertheless, SB and AB as hydrogen storage materials do not compete. Rather, SB is intended more to portable technologies, while AB to vehicular applications.
Due to the fact that the US DOE has in a way compared SB and AB, it may be beneficial and interesting to compare these hydrides in terms of their basic and their state-of-the-art for either application.
SB is a versatile boron hydride. It is widely utilized in industrial processes (e.g. pharmaceutical, paper blenching and wastewater treatment). Accordingly it is produced in large amounts. Ever since the late 1990s it has also been suggested as a promising source of hydrogen because it contains 10.8 wt% of hydrogen. Stored hydrogen can be released by thermolysis or hydrolysis. Otherwise, SB can be oxidized to liberate eight electrons and thus it can power the direct borohydride fuel cell (denoted DBFC).
Unlike SB, the utilization of AB is not widespread. For example, it finds a use in organic chemistry as an air-stable derivative of diborane. This is especially detrimental for its production cost. Actually AB is a promising material by virtue of its gravimetric hydrogen storage capacity of 19.5 wt%. Besides this, it is the fuel of the direct ammonia borane fuel cell (denoted DABFC).
For both boron hydrides, finding cost-effective production routes with the prospect of application is one of the main objectives. Investigations are in progress but none of the proposed routes has reached sufficient efficiency. One way to reduce the cost is to recycle the spent fuel (i.e. the reaction by-products) back to the hydrides. Both hydrides suffer from their high cost and AB is even more expensive than SB.
Safety is important for chemicals intended for large-scale utilization. Information on safety is generally available in the chemicals’ material safety data sheet. The few about AB are incomplete. The ones on SB are much more complete. SB and AB are white solids. They are recognized as being safe and stable at room temperature if stored in a closed vessel and anhydrous medium. Both are moisture sensitive, hydrolyzing and generating hydrogen; to be stable and safe, AB must be of high purity. The main difference between SB and AB is the thermal stability. AB decomposes at very low temperatures in relation to SB.
Neither boron hydride is mature enough to envisage applications, especially automotive applications. Both suffer from low (not-optimized) hydrogen storage capacities, inefficiency in spent fuel recycling and catalyst inefficiency in terms of durability. With respect to SB, the most efficient process is hydrolysis; with respect to AB, the most efficient hydrogen release reaction is thermolysis. This has consequences with regard to their potential applications. It appears that SB is more suitable for portable applications (commercially available) if it is hydrolyzed, while AB is suitable for automotive applications if it is thermally decomposed. In other words, SB is not really competing with AB because they are intended for different, specific applications.
The question that arise is: what are the effective capacities of SB and AB for such application?
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