Image caption An example of a microbial fuel cell has gone on display at London's Science Museum

US researchers say they have demonstrated how cells fuelled by bacteria can be "self-powered" and produce a limitless supply of hydrogen.

Until now, they explained, an external source of electricity was required in order to power the process.

However, the team added, the current cost of operating the new technology is too high to be used commercially.

Details of the findings have been published in the Proceedings of the National Academy of Sciences.

"There are bacteria that occur naturally in the environment that are able to release electrons outside of the cell, so they can actually produce electricity as they are breaking down organic matter," explained co-author Bruce Logan, from Pennsylvania State University, US.

"We use those microbes, particularly inside something called a microbial fuel cell (MFC), to generate electrical power.

"We can also use them in this device, where they need a little extra power to make hydrogen gas.

"What that means is that they produce this electrical current, which are electrons, they release protons in the water and these combine with electrons."

Prof Logan said that the technology to utilise this process to produce hydrogen was called microbial electrolysis cell (MEC).

"The breakthrough here is that we do not need to use an electrical power source anymore to provide a little energy into the system.

Image caption Hydrogen has long been hailed a transport fuel of the future but has yet to fulfil its potential

"All we need to do is add some fresh water and some salt water and some membranes, and the electrical potential that is there can provide that power."

The MECs use something called "reverse electrodialysis" (RED), which refers to the energy gathered from the difference in salinity, or salt content, between saltwater and freshwater.

In their paper, Prof Logan and colleague Younggy Kim explained how an envisioned RED system would use alternating stacks of membranes that harvest this energy; the movement of charged atoms move from the saltwater to freshwater creates a small voltage that can be put to work.

"This is the crucial element of the latest research," Prof Logan told BBC News, explaining the process of their system, known as a microbial reverse-electrodialysis electrolysis cell (MREC).

"If you think about desalinating water, it takes energy. If you have a freshwater and saltwater interface, that can add energy. We realised that just a little bit of that energy could make this process go on its own."

Early days

He said that the technology was still in its infancy, which was one of the reasons why it was not being exploited commercially.

"Right now, it is such a new technology," he explained.

"In a way it is a little like solar power. We know we can convert solar energy into electricity but it has taken many years to lower the cost.

"This is a similar thing: it is a new technology and it could be used, but right now it is probably a little expensive. So the question is, can we bring down the cost?"

The next step, Prof Logan explained, was to develop larger-scale cells: "Then it will easier to evaluate the costs and investment needed to use the technology.

The authors acknowledged that hydrogen had "significant potential as an efficient energy carrier", but it had been dogged with high production costs and environmental concerns, because it is most often produced using fossil fuels.

Prof Logan observed: "We use hydrogen for many, many things. It is used in making [petrol], it is used in foods etc. Whether we use it in transportation... remains to be seen."

But, the authors wrote that their findings offered hope for the future: "This unique type of integrated system has significant potential to treat wastewater and simultaneously produce [hydrogen] gas without any consumption of electrical grid energy."

Prof Logan added that a working example of a microbial fuel cell was currently on display at London's Science Museum, as part of the Water Wars exhibition.