Researchers at the universities of Oxford, Cambridge and Cardiff in the UK, and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia have shown that benign, readily-available heavy alkane hydrocarbon wax is capable of rapidly releasing large amounts of hydrogen—sufficient to meet the 7 wt% target set by the US DOE—through microwave-assisted catalytic decomposition.
This discovery, reported in an open-access paper in Scientific Reports, offers a new material and system for safe and efficient hydrogen storage and could facilitate its application in a hydrogen fuel cell vehicle. Hydrocarbon wax is the major product of the low temperature Fischer-Tropsch synthesis process from syngas and is currently thermally “cracked” to produce various fuels.
The F-T wax can be manufactured using renewable energy and any carbon-containing resources including biomass, CO2, natural gas, coal—and the resulting carbon residue from the hydrogen-depleted wax.
Despite strenuous efforts over the past decades covering a vast range of hydrogen storage materials, no single material has met simultaneously the critical requirements for a viable hydrogen storage and hydrogen releasing material suitable for use in HFCVs and other fuel cell applications.
… Chemical, complex and metallic hydrides all present significant difficulties of pyrophoricity, accidental hydrogen release (from accidental reaction with atmospheric moisture) and heat management problems, together with troublesome changes in particle morphology in the hydrogen charging/discharging processes. Similarly, rechargeable organic liquids—used to store hydrogen in a liquid carrier form—must be handled with great care as several react violently with strong oxidants, have fire and explosion hazards and, in certain cases, carry toxicity concerns from occupational exposure. Similar safety issues and concerns surround the potential use of ammonia as an on-board hydrogen storage material.
Important efforts have been made previously to liberate pure hydrogen from the catalytic decomposition of the lightest alkane, methane. Against this backdrop, we sought to investigate heavy alkane hydrocarbon waxes as cheap, safe, readily producible and widely accessible hydrogen storage materials. We reasoned that such materials—if suitably activated to rapidly release hydrogen, or hydrogen-rich mixtures—could exhibit many desirable features and yield hydrogen gravimetric densities approaching a theoretical value of ca. 14 wt%.—Gonzalez-Cortes et al.
Dehydrogenating a hydrocarbon wax so as to release hydrogen effectively and rapidly while minimizing unwanted by-products is a challenge. The research team developed highly selective catalysts that, with the assistance of microwave irradiation, can extract hydrogen from hydrocarbons instantly through a non-oxidative dehydrogenation process.
The researchers selected a representative wax, C26H54, and found that around 7 wt% hydrogen is rapidly produced by microwave radiation-assisted catalysis involving ruthenium nanoparticles on a carbon support (CS), intimately dispersed within paraffin wax (PW).
A thermodynamic analysis showed that the deep dehydrogenation reactions necessary for efficient hydrogen formation reactions become more favorable with increasing reaction temperature and with increasing number of carbon atoms in the lineal alkane.
|Schematic representation of one scenario for the decarbonization of a transportation fuel economy. Gonzalez-Cortes et al. Click to enlarge.|
Clearly, considerable engineering work is needed to adapt this laboratory discovery to large-scale hydrogen storage applications. However, we believe that the storage of hydrogen in, and rapid evolution from, paraffin wax could usher in a new and attractive path towards a decarbonized, hydrogen economy.—Gonzalez-Cortes et al.
S. Gonzalez-Cortes, D. R. Slocombe, T. Xiao, A. Aldawsari, B. Yao, V. L. Kuznetsov, E. Liberti, A. I. Kirkland, M. S. Alkinani, H. A. Al-Megren, J. M. Thomas & P. P. Edwards (2016) “Wax: A benign hydrogen-storage material that rapidly releases H2-rich gases through microwave-assisted catalytic decomposition” Scientific Reports 6, Article number: 35315 doi: 10.1038/srep35315