Hydrogen has long been championed, by former U.S. President George W. Bush among others, as the ultimate environmentally-friendly fuel, potentially offering zero-emissions motoring and freedom from oil addiction.

Hydrogen is readily available, in natural gas or in water. Burning hydrogen in an internal combustion engine or, more commonly, combining it with oxygen in a fuel cell to produce electricity, produces only heat and water vapor.

The lure of CO2-free driving has persuaded governments and the auto industry to invest billions of dollars in the last decade researching hydrogen fuel cells, developing demonstration cars and, more recently, installing hydrogen refueling stations.

For example, the Clean Energy Partnership consortium of German car makers and energy companies has started building a test network in Germany which will, if all goes well, be expanded to 1000 fuel stations.

Toyota, Honda, and Nissan have started work on a similar network in Japan, while the California Fuel Cell Partnership had, as of March 2011, 20 stations in operation, with eight more planned.

These follow pilot projects in ten cities in Europe, China and Australia under the European Commission-backed HyFLEET:CUTE project between 2006 and 2009 which set up fuelling stations for hydrogen-powered public buses.

London, Madrid, Hamburg and Perth have all gone on to introduce regular hydrogen bus services, although mass production of hydrogen buses is not expected until after 2012.

The same sort of timeframe applies to hydrogen cars and in that sense hydrogen has not yet delivered on its promise. Vehicles are very far from widespread public use, and there is a lot of doubt and skepticism about hydrogen’s viability as an alternative fuel.

Renault-Nissan is one carmaker that has cancelled its hydrogen car research in favor of electric cars, while others have scaled back their plans.

No (emissions) free lunch

One problem is the hydrogen production process itself, which requires energy and therefore undermines the claim that hydrogen cars are really emissions-free vehicles.

The leading way to produce hydrogen is to make it from methane gas through a process called ‘steam reforming’, either onsite at hydrogen filling stations or at large biomass or coal plants from which the hydrogen is distributed by truck or pipeline.

According to the U.S. National Renewable Energy Laboratory, producing a kilogram of hydrogen by steam reformation generates emissions equivalent to 11.9 kilograms of carbon dioxide.

Two hydrogen fuel cell prototypes, General Motors’ Chevy Equinox and Honda’s FCX Clarity, can travel 63 kilometers and 109 kilometers on a kilogram of hydrogen, respectively.

Powering these cars using hydrogen produced by steam reformation would actually create more CO2 emissions per kilometer than the gasoline-electric powered Toyota Prius hybrid and many small conventional cars, although this does not take into account the emissions generated by the production and distribution of the gasoline.

However, if hydrogen is produced from natural gas at 60 percent efficiency, the UN Intergovernmental Panel on Climate Change (IPCC) calculates that total ‘well-to-wheel’ CO2 emissions could be reduced by 50 to 60 percent compared to current gasoline vehicles.

And if the CO2 produced by steam reformation were to be sequestered by carbon capture and storage technology, the numbers would be even more promising.

An even greener alternative being researched is using renewable electricity to extract hydrogen from water via electrolysis. Using solar, wind, or nuclear energy to produce hydrogen, the IPCC says, has the potential to be truly CO2-free.

Surplus energy from variable renewable sources such as wind could be stored in the form of hydrogen and then used later.

Distribution and storage

Hydrogen faces even more severe distribution and energy storage problems than electric vehicles; there is no equivalent ‘hydrogen grid’ to tap into, it must be constructed.

Storing hydrogen is a challenge as compressed hydrogen tanks are not yet suitable for mass-produced vehicles as they require a lot of space and high pressures to provide enough fuel for the car.

Honda hopes to get around the distribution problem with its Home Energy Station, which drivers can use to make hydrogen at home via electrolysis by tapping into the domestic natural-gas supply. The mini-power plant can also provide electricity and heat for the home, but is still in the experimental stage.

Another issue is that fuel cells remain too expensive for mass production, partly because they contain scarce platinum as a catalyst for the reaction between hydrogen and oxygen.

In 2009, GM CEO Fritz Henderson told the Washington Post newspaper that each hydrogen fuel cell car cost 400,000 dollars to build compared to 40,000 dollars for each Chevy Volt plug-in electric car.

Meanwhile, despite billions of dollars in subsidies from governments worldwide, only a few dozen hydrogen filling stations exist.

For these reasons the U.S. Secretary of Energy Stephen Chu stated in 2009 when announcing the government’s energy plans that fuel cell hydrogen vehicles "will not be practical over the next 10 to 20 years".

On the other hand, the U.S. Congress overturned Chu’s plans to slash public funding for hydrogen fuel research, suggesting there are still some who believe in the hydrogen dream.