NTT DoCoMo is showing at this week's Ceatec show in Japan a prototype fuel cell that it's developing with a Japanese start-up for use as a mobile phone recharger.

The polymer electrolyte fuel cell (PEFC) uses hydrogen gas as a fuel to produce electricity and is a compact 24mm square, 70mm long and weighs 45g.

The fuel cell is under development by Aquafairy, a startup spun out of Nitto Denko to work on the technology. NTT DoCoMo recently acquired a 36.5 percent stake in the company for an undisclosed sum.

The fuel cell can deliver enough power to recharge a mobile phone three times before requiring a hydrogen refill.

While it's still some way from being a commercial product, the fuel cell is completely different to what NTT DoCoMo showed at Ceatec 2005. A year ago the company was displaying a direct methanol fuel cell (DMFC) that was developed by Fujitsu and built into a cradle, into which a mobile phone could be placed for recharging.

DMFCs are based on a different chemistry from the PEFC on show this year. They typically work by mixing methanol with air and water to produce electrical power.

For companies like NTT DoCoMo a future goal remains a fuel cell small enough and safe enough to be built directly into a mobile phone. But the technology isn't at that stage yet so the first commercial fuel cell devices will likely be recharges that replenish the lithium-ion battery inside of the phone but offer the advantage of being able to do so anywhere - not just within a couple of metres of a wall socket.