Feb 03 2011
The increasing energy demand and the contemporary concern about urban environment raised the necessity to develop a hydrogen-based economy; it requires the development of hydrogen storage technologies.
Hydrogen can be stored and transported in different ways, each one showing advantages and disadvantages: as compressed gas, in liquid form, as metal hydride. In carbon nanotubes, in metal-organic frameworks. In each system, hydrogen is reversibly loaded and released; the temperature and pressure of the tank control the kinetic of the absorption/desorption process..
The chemical hydrogen storage represents an alternative approach: the highly efficient production of pure hydrogen by hydrolysis of chemical hydrides, such as sodium borohydride (NaBH4), could make the transition to a “hydrogen economy” a reality.
Hydrogen from borohydride hydrolysis can be supplied to a PEMFC on demand to power mobile electronic devices, such as mobile phones, laptop computers and so on.
Among the different chemical hydrides, sodium borohydride is very stable and easy to handle and it has been selected for its high hydrogen content. Moreover, it can be obtained from borax, which is a globally abundant natural substance.
Solid state sodium borohydride belongs to a group of chemical hydrides based on metal-hydrogen complex, that can react spontaneously and exothermically with water, in the presence of catalysts or acids, and release pure hydrogen via hydrolysis reaction.
A sodium metaborate (analogous to borax) could be formed as a by-product during the reaction, but it is water-soluble and environmentally benign.
Hence, sodium borohydride is regarded as a promising hydrogen source with the prediction that the water vapor presented in the hydrogen gas stream can be used to humidify the PEMFC membranes.
However, sodium borohydride hydrolysis reaction is very fast in acidic media, while slows down with increasing pH, so that aqueous sodium borohydride solutions may be considered stable at pH > ca. 13.
In fact, the most favoured technology for hydrogen generation from sodium borohydride is, to date, contacting an alkaline solution of sodium borohydryde (eg, NaOH + NaBH4) with suitable catalysts that accelerate the reaction to practically acceptable rates. This technology has some drawbacks: handling of caustic solutions; using transition metal catalysts, which may be expensive and make the disposal of by-products difficult; catalyst deactivation due to agglomeration and borate film deposition on its surface.
A different approach is based on hydrolysis of NaBH4 in acidic medium: the acids varied greatly in effectiveness but, in general, the stronger acids produced the greater acceleration. The reaction with some organic acids doesn’t involve any harsh solution and results in environmentally benign by-products.
What do you think is the best way to generate hydrogen from hydrolysis of sodium borohydride? In the presence of catalysts or acids? And why?
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