baudrunner's space: MOF's: an alternative for hydrogen storage
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Wednesday, February 27, 2008

MOF's: an alternative for hydrogen storage

I published in an earlier post a discussion outlining the various projects which are undergoing research in the area of on-board hydrogen generation for hydrogen gas powered vehicles versus storing the hydrogen in tanks. The main drawback to current hydrogen gas storage technologies is the tank's capacity - something in the order of the size of a car's trunk - and the relatively short distance that a full tank of the gas would allow one to travel, topping out at about a couple of hundred miles. An option is to liquefy the gas but that's expensive and difficult. Therefore, most of the research effort in hydrogen fueled automobiles has gone into on-board production of the gas rather than into the development of a tank to carry it around in.

That does not mean that research into hydrogen storage has abated to the point that no progress is being made. A technique which uses a small amount of titanium added to sodium alenate, a metal hydride, actually doubles the capacity over conventional storage techniques. This was achieved because the presence of titanium actually cools the hydrogen, thereby making it more dense with a greater capacity per unit of volume.

Another approach to the problem of storage density has led to the introduction of metal-organic compounds which essentially trap hydrogen within a three dimensional crystalline framework. Combining zinc with hetero-cyclic and octo-cyclic aromatic compounds such as benzene and octylene creates large cubic cavities which act as traps for the hydrogen gas. The unusual arrangement of these crystals cause an overlapping with an open access channel allowing permeation of the hydrogen gas, which is readily attracted to the carbon molecules in the aromatic hydrocarbons. Pumping the hydrogen into and out of the storage traps does not alter the structure of the framework. At a pressure of about 48 bar, the amount of hydrogen by weight that can be stored in this material is about equal to the weight of the material itself.

The material is called a Metal Organic Framework, or MOF. They promise not just a greater storage capacity over conventional pressurised cylinders overall but an MOF canister storing hydrogen at 30 atmospheres can hold the same amount of the gas as a similarly sized pressurised cylinder of the gas at 200 atmospheres, making them much safer.

The U.S. Department of Energy has set targets for the development of alternative fuels storage capacity. A group led by Hong-Cai Zhou has developed a novel way to store methane using a specially designed MOF, reviving interest in the development of methane powered vehicles. The methane storage capacity of the material, an Anthracene Derivative called PCN-14, has exceeded the DOE's 2003 goal by 28%.

nano-cage - credit: Shengqian Ma, Miami University

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