Tuesday, December 25, 2012

Will Hydrogen be the Sustainable Solution for World Fuel Crisis?

Hydrogen fuel cell (HFC) vehicles are currently becoming a popular choice among the people who prefer  driving their car under an 'emission free' label. In fact, today leading automobile manufacturers such as  Toyota, Honda and BMW are now in this business and conduct further research to develop much more efficient hybrid HFC cars. This recent report in CNN shows that the commercial HFC cars will emerge in near future not only to cater this niche market but also for general public.
In a HFC, chemical energy in hydrogen is converted into electricity by combining with oxygen in a  electrochemical cell and  producing water as the only by product. Hence, it has zero emissions unlike in the case of fossil fuel driven vehicles, which produces CO2 and various other harmful chemicals. It has also revealed that fuel efficiency of HFC driven vehicles are much higher than conventional petroleum vehicle. Despite the immense research carried out during the last couple of decades, there are  still a number of major challenges for  practical onboard HFC applications including  production and storage of hydrogen.

Even though hydrogen is one of the most abundant elements on earth (in the form of water), molecular hydrogen (H2 gas)  is not freely available, hence has to be produced using another energy source. Therefore, hydrogen used in HFCs is not described as a energy source but only as a energy carrier.  Currently most hydrogen is produced from coal and natural gases, which should not be a sustainable long term solution, because they produce green house gases in the process.  Use of nuclear power to produce hydrogen is another possibility despite the inherent risk associated with nuclear plants.  Nevertheless most research is focused on the production of hydrogen from renewable sources such as biomass, photolytic and fermentative micro-organism systems, and photoelectrochemical systems.1,2 Another speculated method is to split water by electrolysis in order to produce hydrogen using the solar energy. 1,2,3 

Jena, P.;  J. Phys. Chem. Lett.  2011, 2, 206-211

Currently, hydrogen is stored in HFC driven vehicle  in high pressure tanks which is not a efficient way of storing hydrogen because it has much less energy density.  Onboard hydrogen storage systems must have reasonable storage capacity [5.5 wt% , accordingly to US Department of Energy (DOE) 2017 target], reversibility, fast kinetics, safety, and cost effectiveness. Furthermore, mild operating conditions such as a temperature between -40 to 60 0C and delivery pressure not exceeding 12 bar have been recommended by the DOE for such a system.1,2    

Hydrogen storage research mainly focuses on developing chemical hydrogen storage materials. Sorbent materials and complex hydrides are two main categories of chemical hydrogen storage. Sorbent materials, physisorb molecular hydrogen through weak dispersion forces, hence it has a low hydrogen binding energy typically in the range of 4 - 8 kJ/mol.4 As a result, these materials have good reversibility and fast kinetics, but have limited hydrogen storage capacity at ambient temperature. Examples in this category are fullerines, graphenes, carbon aerogels, metal organic frameworks, etc.  Recent research revealed that imposing heterogeneity in the porous structure by adding transition metals  (e.g. Ni, Pt, Ru, Ti and etc), alkali and other metal ions, and surface modification with various functional groups  can enhance the hydrogen binding energy. Further improvements in hydrogen storage can be obtained by optimizing pore sizes and increasing the specific surface area.

In contrast to hydrogen physisorption in the sorbent materials, hydrogen is chemisorbed (dissociated atomic hydrogen is chemically bonded) into the bulk of the material in complex hydrides. This requires much more energy to desorb hydrogen and results in slower sorption kinetics. Hence, much higher temperatures are required for sorption process (adsorption / absorption and desorption). However, none of the storage systems studied so far have satisfied the entire onboard hydrogen storage requirements listed above.

Scientists and engineers in various laboratories are still working to develop solid state materials for on-board hydrogen storage, to find the most efficient way to produce hydrogen using HFC technology. If everything goes well there can there be a day in future, when water (hydrogen) powered car is a reality.

1. accessed 04 July 2012
2.      Broom, D. P., Hydrogen Storage Materials The Characterisation of Their Storage Properties, 1st ed.; Springer: New York, 2011.
3.      Jena, P., Materials for Hydrogen Storage: Past, Present, and Future. The Journal of Physical Chemistry Letters 2011 2, 206-211.
4.      Bhatia, S. K.; Myers, A. L., Optimum Conditions for Adsorptive Storage. Langmuir 2006, 22, 1688-1700.

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