Hydrogen is not a energy source in the conventional sense. There are no large supplies of free hydrogen under ground, in the oceans, or in the air ready to use to produce power for the economy. Rather, hydrogen should be viewed as an intermediary, a way of chemically storing energy for distribution between the place it is produced and the place it is consumed. Its major benefit is that the conversion of hydrogen into energy produces only water as a waste product, generating no chemical pollutants in the localities in which it is consumed. If cars were to run on hydrogen instead of gasoline, the air quality in urban areas would improve significantly.
The development of a hydrogen infrastructure will not in and of itself relieve the world of its dependence on fossil fuels, or even curb the rise in atmospheric levels of greenhouse gases. By far, the the most widespread method for the production of hydrogen in industry today is via "cracking" or reformation of fossil fuels, specifically natural gas, and the major byproduct is carbon dioxide, a greenhouse gas. Without an active program in carbon sequestration, greenhouse gas levels will continue to rise. However, a carbon sequestration program will be much more efficient and successful if there are only relatively few hydrogen generating facilities at fixed locations producing the greenhouse gases that need to be captured, rather than the current situation where hundreds of millions of individual automobiles spew CO2 everywhere they go. Thus, if breakthroughs in carbon sequestration technology are made, generation of hydrogen from fossil fuels may indeed slow the rise in atmospheric levels of greenhouse gases.
Furthermore, hydrogen can also be produced from renewable resources, reducing dependence on fossil fuels and without any net addition of carbon to the ecosystem. This includes through the reforming of hydrocarbon Biofuels and the extraction of hydrogen from water molecules through electrolysis. The environmental benefit of hydrogen production via electrolysis would of course then depend on whether the electricity used to power the electrolysis cell comes from conventional nonrenewable energy sources, such as coal, oil or nuclear power plants, or from alternative renewable energy sources, such as wind, solar, hydroelectric, or geothermal power generation.
Storage and Distribution
On-board an automotive vehicle, the hydrogen gas would either have to be under compression or cryogenically liquified. In either scenario, designing a storage device that is safe in the event of a crash is a challenge. Another possibility is through the use of metal hydrides, into which hydrogen can dissolve and be later recovered.
The California Hydrogen Highway project is currently engaged in an effort to make hydrogen available at many points in northern and southern California to potential hydrogen consumers. Current natural gas pipelines cannot be converted for use with hydrogen, which is a smaller and much more permeable gas. It is anticipated that the first hydrogen stations that the consumer will see will utilize on-site hydrogen generation. Along with this effort is the need to educate communities and train first responders in issues related to hydrogen safety.
For stationary applications, a hydrogen-generating device can be integrated into the device which then converts the hydrogen into energy. The device could use off-peak operating times to produce and store hydrogen, most likely via electrolysis using electricity from the grid, for later use.
The most commonly discussed method for the conversion of hydrogen into energy is via the generation of electricity in a fuel cell. A fuel cell is distinct from a battery in that it is not a self-contained device which stores electricity in the form of chemical energy. Instead, a fuel (most commonly hydrogen, although others such as methanol, natural gas and propane are possible) is supplied to the fuel cell, which then uses oxygen from the air to convert the chemical energy stored in the supplied fuel into electricity via an electrochemical reaction. With hydrogen as the fuel, the only waste product is water vapor. As long as as fuel and oxygen are supplied to the fuel cell (and the water removed), it will continue to operate and produce electricity. Lifetime of the device would depend on the size of the fuel reservoir built into the same device which also contains the fuel cell, and "recharging" involves refilling the fuel reservoir, not plugging into the electric grid. In theory, the fuel cell would not even require a local fuel reservoir, but be continuously supplied fuel via a yet-to-be-envisioned infrastructure.
In the current state of the art, hydrogen fuel cells do not contain hazardous or toxic components, such as corrosive acids or heavy metals which are found in other types of battery. They are envisioned for use in large-scale power plants as well as in small scale stationary backup generating systems.
Their most attractive application, and where they would make the greatest environmental impact, is as the power source of a clean electric vehicle. This is a long term goal and require the devlopment of a hydrogen storage and distribution infrastructure, as noted above. At the very least, this technology would then reduce pollution in high density urban areas. The more global overall environmental impact would then depend on the the means used to produce the hydrogen in the first place.
There are still many technical problems before fuel cells will replace the gasoline-powered engines in people's automobiles. Fuel cells will only operate on highly pure hydrogen gas and are not yet reliable over a wide range of operating temperatures. Furthermore, in the current state of the art, platinum metal is required as the electrocatalyst in the system, and it unclear whether there exists enough platinum deposits in the world for the world's fuel cell needs. Even if it did, the world would then be beholden to the government of South Africa, which controls 80% of the world's known platinum deposits.
Finally, the costs associated with the materials involved and the engineering necessary currently make fuel cell vehicles not economically competitive with vehicles powered by gasoline-based internal combustion engines. However, they are in theory less complex than today's hybrid vehicles in that they will not require both a gasoline-powered and an electric motor on board the vehicle.
Because there are doubts about whether fuel cells will ever meet the cost and technological needs of the automotive marketplace, an alternative technology for the conversion of hydrogen fuel into energy is through the Hydrogen Combustion Engine. Much like a gasoline-powered ICE, a hydrogen-based one would not generate electricity, but produce propulsion directly via combustion. As in a fuel cell, the only waste product would be water. One would still experience Carnot-cycle limitations and therefore be fundamentally less efficient than running an electric motor powered by a fuel cell. This fundamental limitation makes the electrochemical conversion of hydrogen far more attractive, but combustion of hydrogen does not require the same degree of purity of the hydrogen fuel as does a fuel cell and therefore may be a nearer-term solution. The hydrogen storage and distribution issues facing fuel cells, however, are still a limitiation to be overcome.
Finally, heavy isotypes of hydrogen can also be used for fusion reactions. This is such a different infrastructure and engineering time horizon that it is covered in a separate article.
Although there are technological hurdles to overcome, the talk of a hydrogen economy has bipartisan appeal as a future where there is less pollution but abundant energy. While a hydrogen infrastructure does not guarantee any environmental benefits or even a reduced dependency on fossil fuels, it makes those goals more attainable.