Navy

Metal borohydride tablets for on-demand hydrogen generation

Green chemistry solution for H2 pelletized storage and use

Energy

The U.S. Navy has invented a novel way to create hydrogen using a three-ingredient tablet. Businesses can commercialize the technology by licensing the patent.

The Navy patent application describes mixing sodium borohydride with cellulose and citric acid to form a solid tablet. Adding water to the tablet causes hydrogen gas to be released. (Wikimedia)

The reaction of metal borohydrides with water is a well-known technique to release hydrogen gas (H2) under controlled conditions. This reaction allows the borohydride salts to be used as hydrogen storage – as NaBH2 pellets – prior to combination with water. This is more efficient and safer than the storage of H2 gas. However, there are several issues with the release of hydrogen during the reaction of the salts with water. In the absence of a catalyst or other additive, the borohydride-water reaction consumes protons and creates a high pH condition. This leads to the formation of undesirable products and inhibits the complete borohydride-water reaction. Cobalt chloride has been used as a catalyst but it has been observed that the reaction of doped NaBH4 doesn’t consume the entire amount of catalyst thus causing additional problems such as increased temperature, catalyst degradation, and waste build up. Additionally, commonly used catalysts (Ru, Ni as well as Co) are hazardous and carcinogenic.

Addressing these issues, Navy researchers have developed a method to store and generate hydrogen while minimizing any excess solution volume or chemical handling.

The approach maintains ease of use and simplifies integration into complete H2 gas production systems. The method for H2 production begins with a relatively stable mixture of a NaBH2 and a non-hazardous solid acid such as mineral acids (hydrochloric acid and sulfuric acid) and organic acids (citric acid and acetic acid). An inert binder such as cellulose is the third component used to create a pellet. This resulting pellet is dropped into a container holding water allowing for hydrolysis to take place.

The process replaces potentially toxic and environmentally harmful catalysts in the promotion of borohydride hydrolysis and demonstrates that hydrogen release can be performed successfully with several different acid accelerators.

Of further benefit, neutral pH values are maintained even after the reaction is complete. The method represents an alternative approach to generating hydrogen using green chemistry.

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