This would be an environmental advantage over the current generation of hydrogen-powered cars, where the hydrogen is most commonly made from fossil fuels. Even though the cars themselves emit no climate-warming greenhouse gases, the manufacture of their fuel does.

"This is a method of using renewable organic matter, using anything that's biodegradable and being able to generate hydrogen from that material," Logan said by telephone.

In research published in the Proceedings of the National Academy of Sciences, Logan and colleague Shaoan Cheng used naturally-occurring bacteria in an electrolysis cell with acetic acid, the acid found in vinegar.

The bacteria slurp up the acetic acid and release electrons and protons creating up to 0.3 volts of electricity. When a bit more electricity is added from an outside source, hydrogen gas bubbles up from the liquid.

WATER IS THE ONLY EMISSION

This is far more efficient than water hydrolysis, where an electric charge is run through water to break it down into its constituent parts of oxygen and hydrogen.

"It uses about a tenth as much energy as water electrolysis," Logan said.

That is because the bacteria do most of the work, breaking the organic material into subatomic particles, so all the electricity does is juice these particles to form hydrogen.

The resulting fuel is a gas, not a liquid, but could still be used to power vehicles. This process could be used with cellulose, glucose, acetate or other volatile acids, Logan said. The only emission is water.

Although it sounds futuristic, microbial fuel cell technology is available now. The researchers have filed for a patent on this work.

These cells are too large to be put into cars, so the gaseous hydrogen fuel they produce must be made in a factory.

"You could put one of these reactors at a food processing plant and take the waste water and make hydrogen out of it," Logan said. "Or you could go to a farm, where there's lot of cellulose or ... agricultural cellosic residues, take that and make hydrogen there."

This would be unlikely to work in big cities but might well be effective in rural areas.