For now, the addition of a fuel cell means the phone's thickness is about twice that of the equivalent commercial version of the phone. But at 22 millimeters this latest version is about half the thickness of a prototype shown several years ago and KDDI said it continues to work towards a thinner handset.

The phone is based on a fuel cell developed by Toshiba.

Toshiba has been promising a commercial battery charger based on the same fuel-cell technology for several months but has yet to announce a commercial product. Two months ago, the company's new president said it would be launched by the end of September but that deadline passed a few days ago without anymore information on the device.

The charger will be a portable device that can be used to charge the batteries in portable gadgets such as cell phones, music players and portable game devices instead of plugging them into an electrical outlet.