Technical Proceedings of the 2010 Clean Technology Conference and Trade Show

Clean Technology 2010

Chapter 5: Hydrogen & Transportation

I.N. Tsimpanogiannis, A.K. Stubos
National Center for Scientific Research “Demokritos”, GR
240 - 243
hydrogen storage, clathrate hydrates, cavity occupancy
The adoption of “Hydrogen Route” is considered as a promising alternative to meet our long-term energy needs. In this approach energy is stored/transferred and used in the form of hydrogen. Hydrogen is a versatile molecule and can be a fuel for direct combustion, a means of producing electricity in fuel cells for stationary use and transport, and a medium for storing energy. However, significant scientific, technological and socio-economic barriers need to be surpassed before the transition from the “carbon-based” to “hydrogen-based” economy is successfully completed. Moving towards the so-called “Hydrogen Economy” requires the secure supply of large amounts of hydrogen. Significant progress has been achieved towards hydrogen-production issues. However, a major research issue that has not yet been solved, in a satisfactory manner, is the temporary storage (and recovery) of hydrogen. As a result of their capacity to store large volumes of gas, hydrates have been considered as an alternative material for storing and transporting hydrogen. Gas hydrates are a class of crystalline, non-stoichiometric, inclusion compounds. They are composed of a framework of hydrogen bonded water molecules that forms cavities where small gas molecules can be enclosed (“enclathrated”). Depending on their crystal structure and the type and number of cavities present in the unit cell, different hydrates structures are known to exist in nature. The most common structures being sI, sII and sH. Clathrate hydrates can be considered a special kind of nanoporous materials (with pores of diameter 0.7-1.2 nm) that mainly consist of water. The major advantages of hydrates as hydrogen-storage materials, comparatively to other materials investigated for the same purpose, include complete reversibility, improved kinetics and cycle life, low cost, almost not any environmental hazards, and safety (in terms of toxicity and flammability). The synthesis of pure H2 hydrate (sII) was considered a breakthrough in the hydrogen storage research. Although pure H2 hydrate can achieve hydrogen content up to 5.0 wt. % (which is very close to the specifications required by the automobile industry for applications of hydrogen in transportation), this material is only stable at very high pressures (200 MPa at 280 K). The next major step was the stabilization of the sII hydrate by using a promoter, a substance that occupies some of the hydrate cavities, thus offering stability to the hydrate at moderate pressure conditions, when compared to the pure gas hydrate. Use of a promoter, however, results in reducing the hydrate storage capacity. The current study is a review. The main objective of this work is to evaluate the hydrogen storage capacity of hydrates based on the progress achieved in the recent years. All available experimental and simulations studies are reviewed in order to delineate the possible range of applications for which storing hydrogen in hydrates would be suitable. A detailed discussion is presented regarding the possible use of sH hydrates as an alternative to increase the hydrogen storage capacity. The issue of “hydrate tuning” is also revisited in light of the recent experimental findings.
Evaluation of Hydrogen Storage in Clathrate Hydrates: Review of Recent Advances