Speecys Corp, based in Tokyo and headed by former Sony robot engineer Tomoaki Kasuga, will on Friday begin selling the 50-centimeter (20-inch) tall, 4.2-kilogram (9.24-pound) robot carrying a built-in hydrogen cylinder.

Fuel cells produce electricity through a chemical reaction between hydrogen and oxygen, leaving water as the only by-product.

Fuel-cell batteries in cartridges can be easily replaced in contrast to conventional batteries that take hours to recharge.

"Fuel cells are a promising material as the source of energy for operating robots in the future," Speecys said in a statement.

"We believe that it is more suitable for (humanoid) robots to get fuel in cartridges as if they were having meals rather than to get batteries recharged," it said.

Speecys-FC is priced at 2.5 million yen (22,730 dollars) a unit, five times as pricy as a previous version run by conventional nickel metal hydride batteries.

The company aims to sell 10 units of Speecys-FC a year for research and display purposes.