Molecular hydrogen (H2) is an ideal fuel to be used for transportation, since the energy content of hydrogen is three times greater than that of gasoline and four times greater than that of ethanol. Hydrogen power powered rockets launched by the National Aeronautics and Space Administration (NASA) for many years. Today, a growing number of automobile manufactures around the world are making hydrogen-powered vehicles. Because of depleting supplies and growing demand for oil, H2 may become an alternative to gasoline.

The idea of hydrogen as the fuel of the future was expressed long ago by Jules Verne in his novel L'Ile mysterieuse (1874-1875; The Mysterious Island, 1875). However, compared to oil, H2 is not abundant on Earth. Its atmospheric concentration is only 0.00001 percent, and there is even less of it in the oceans. Though many microorganisms produce H2 during fermentation, it is such a good source of energy that it is used almost immediately by other microbes. Thus, in order for humans to use hydrogen as fuel, it must be generated using other energy sources.

While molecular hydrogen is rare, the chemical element hydrogen is the most basic and plentiful element in the universe. It also forms a part of the most abundant chemical compound on Earth, water. Therefore, the challenge posed is to find a cost-effective and environmentally friendly way to generate H2 from water or other chemical compounds. At present,H2 is obtained mainly from natural gas (methane and propane) via steam reforming.

Although this approach is practically attractive, it is not sustainable. Molecular hydrogen can be also produced by electrolysis. In this process, electric energy is employed to split water into H2 and O2. The requisite electricity can be obtained using clean, sustainable energy technologies such as wind and solar power. However, the process is not efficient, requiring significant expenditure of energy and purified water.

There are other technological and economic obstacles to hydrogen power. These obstacles include safety issues, as well as the lack of effective solutions for storage and distribution of H2. Hydrogen has gained an unwarranted reputation as a highly dangerous substance among the general public. Like all fuels,H2 may produce an explosion, but it has been used for years in industry and earned an excellent safety record when handled properly.

Hydrogen is the lightest chemical, so it has a much lower energy density by volume than do other fuels. As a gas, it requires three thousand times more space for storage than does gasoline. Thus, hydrogen storage, especially in cars, represents a challenge for scientists and engineers. For storage, H2 is pressurized in cylinders or liquefied in cryotanks at -253œ Celsius. Both processes require a significant expenditure of energy and generate large quantities of waste carbon dioxide (CO2). In most contemporary hydrogen-powered vehicles,H2 is stored as compressed gas. Because compressed gas cannot be delivered in the same fashion as liquid fuel, gasoline stations and pumps cannot simply be converted into hydrogen stations (Service, 2004). Thus, the distribution system for the new fuel would have to be constructed from scratch, requiring considerable monetary investment.

Fossil fuels generate CO2, contributing to the greenhouse effect. Switching from fossil fuels to H2 would eliminate that source of greenhouse gas (GHG) emissions, provided that the new fuel could be produced without carbon-emitting technologies. Burning H2 for an energy source produces only water as a by-product, and H2 is also a renewable fuel, since it can be made from water again. Unfortunately, current methods of H2 production from natural gas also generate CO2. The ultimate goal is to generate H2 without emitting GHGs into the atmosphere, perhaps by using wind or solar power.

One promising green method of H2 production is a biological approach: A great number of microorganisms produce H2 from inorganic materials, such as water, or from organic materials, such as sugar, in reactions catalyzed by the enzymes hydrogenase and nitrogenase. Hydrogen produced by microorganisms is called biohydrogen (Ogden, 2006). The most attractive biohydrogen for industrial applications is that produced by photosynthetic microbes. These microorganisms, such as microscopic algae, cyanobacteria, and photosynthetic bacteria, use sunlight as an energy source and water to generate hydrogen. Hydrogen production by photosynthetic microbes holds the promise of generating a renewable hydrogen fuel using the plentiful resources of solar light and water.

It is possible to use hydrogen to fuel internal combustion engines. Doing so produces at least one GHG, nitrogen oxide, because the burning of hydrogen requires air, which is almost 80 percent nitrogen (N2). To use hydrogen power in climate-friendly ways, it will be necessary to replace the internal combustion engine with fuel cells, which would produce electricity to power vehicles without motors. Fuel cells are like batteries: They generate electricity via chemical reactions between H2 and O2. Fuel cells emit water and heat, not CO2 or other GHGs. In addition, fuel cells are 2.5 to 3 times more efficient in converting H2 energy than are internal combustion engines. Hydrogen fuel-cell cars could even provide power for homes and offices if necessary.

For hydrogen power to become a reality, tremendous research and investment efforts are necessary. The fate of hydrogen power technology will also depend on consumers' willingness to spend money on climate-friendly technologies.

References

1. Ogden, Joan. "High Hopes for Hydrogen." Scientific American, September, 2006, 94-99.

2. Service, Robert F. "The Hydrogen Backlash." Science 305, no. 5686 (August 13, 2004): 958-961.

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