Hydrogen storage vs on-board generation

Hydrogen shows promise as an excellent fuel for powering motorized vehicles and mobile devices. Five kilograms of hydrogen has the energy equivalence of forty litres of regular gasoline. Problems lie not so much in utilisation as in its containment and in its transportation and delivery.

The density of hydrogen is so low that keeping it in useful quantities presents challenges. It is the objective of many research projects to develop practical and cost effective ways to store hydrogen. For example, it would be required to store four times the volume of hydrogen to match the energy content of gasoline if it were stored as a liquid under great pressure. The energy density of stored hydrogen is too low to consider using this method to deliver power effectively to a conventional automobile. This method is also potentially dangerous.

In 2003 the U.S. Department of Energy set a target year of 2010 for the development of materials capable of storing and releasing enough hydrogen to make up 6% of their total weight without the need for pressurisation. All materials developed so far have fallen short of the DOE goal. More recent research indicates that there are other alternatives to porous materials. One of these is to bind the hydrogen to other elements to make a liquid fuel that could be pumped using the commercial fuel delivery infrastructure that already exists, but most of the research in this area is still in the theoretical stage with result data obtained through computer modelling. The alternative is hydrogen generation on demand.

One interesting research project being carried out to fruition at Arizona State University involves development of a borohydride fuel cell to compete with the rechargeable batteries that power laptops and other electronic portable devices. So far, a hydrogen generator containing a 15% solution of borohydride in water has been constructed. Hydrogen is released as the solution flows over a ruthenium catalyst. The hydrogen is then passed through a membrane and combined with oxygen in the fuel cell, generating electricity. The process is clean, with water as the byproduct. To keep the cells from becoming clogged with insoluble boron oxide the system uses ethylene glycol to dissolve the compound. The work thus far has raised hydrogen storage to the equivalent of about 600 watt-hours per litre, according to chemist Don Gervasio and colleague Sonja Tasic. This is two to three times better than any battery. The ultimate goal of the Arizona State University team is to raise the concentration in solution to 30% hydrogen boride in water. This could theoretically deliver an energy density of 2200 watt-hours per litre, compared to 200 watt-hours per litre for a lithium polymer battery. The goal of commercially manufacturing these batteries for general use is still some years away.

The five kilograms of hydrogen having the energy equivalence of 40 litres of conventional fuel could deliver that energy using 18 kilograms of boron reacting with 45 litres of water. The products of the reaction of boron with water are hydrogen and boron oxide, which can be collected and reprocessed into boron, then returned to the system. To generate hydrogen in this way water vapor is heated to a few hundred degrees and passed through tanks containing powdered boron. The heat generated by the reaction of water with boron can be used to heat the incoming water once the process is jump-started. The hydrogen powers an internal combustion engine or reacts in a fuel cell producing water which can be reused, making this a zero-emission process. This research is being conducted by Tareq Abu-Hamed, now at the University of Minnesota, and colleagues at the Weizmann Institute of Science in Rehovot, Israel.

Another method for producing hydrogen on-board is the brainchild of Engineuity R&D Ltd of Ashkelon, Israel. In this process hydrogen is liberated from water when the tip of a metal wire such as aluminum is inserted into the water and ignited, producing steam and hydrogen which is piped after mixing with air into a modified internal combustion engine. The byproduct is aluminum oxide which can be recovered and reprocessed.

Generating Hydrogen on-board would remove the need to develop the costly infrastructure of pipelines and distribution technologies as well as remove the hazards associated with storing highly flammable liquids and compressed gases. It is apparent that the ways of the future will be crowded with environmentally friendly automobiles having no dependence on carbon fuel products other than to lubricate moving parts.

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Update: August 29/07

Science Daily ran an article today describing the latest research in hydrogen generation using an alloy made up of 80% aluminum and 20% gallium. This alloy reacts exothermically with water to yield hydrogen, aluminum oxide, and heat. The high aluminum concentration results from slow cooling of the molten alloy, versus the 28% aluminum - 72% gallium alloy produced by rapid cooling. The gallium is essential to prevent the oxide of aluminum from accumulating on the surface of aluminum as a skin, preventing further oxidation of the metal. The presence of gallium allows all of the aluminum to be oxidized in a constant reaction. A process which recycles at least 50% of the water produced as waste back into the reaction using the 80-20 alloy would allow the aluminum-gallium alloy to meet the DOE's 2010 goal of a material which has a hydrogen mass density of at least 6%.

Obtaining the gallium is not a problem since aluminum is refined from bauxite which also contains gallium and the refinement process actually produces gallium as a waste product.

A paper is to be presented on September 7 during the 2nd Energy Nanotechnology International Conference in Santa Clara, California written by Jerry Woodall, professor of electrical and computer engineering at Purdue University and by Charles Allen and Jeffrey Ziebarth, both doctoral students in Purdue's School of Electrical and Computer Engineering.