Power
Toshiba has produced a prototype of a highly compact (22x56x4.5mm) direct methanol fuel cell (DMFC) that can be integrated into devices as small as digital audio players and wireless headsets for mobile phones. The total weight is 8.5grams (g). An MP3 music player could be supplied with power for as long as 20 hours on a single 2cc charge of highly concentrated methanol. The fuel cell output is 100mW.
The new DMFC adopts a "passive" fuel supply system that feeds methanol directly into the cell. Toshiba found a solution to the potential problem of "methanol crossover", in which methanol and oxygen combine without an energy-producing reaction. The company has optimised the structure of the fuel cell's electrodes and polymer electrolyte membrane that trigger the reaction. The new DMFC outperforms its predecessors by a factor of five in terms of power output. Toshiba expects to commercialise DMFC for PCs in 2004 and for smaller handheld devices in 2005.
Active systems use a pump and fan to feed methanol and oxygen into a cell stack, where the oxygen reacts with the methanol to produce electricity. Active systems are more complex and output more energy than passive systems, and are better suited for larger fuel cells. A passive DMFCs has a simpler structure that requires no pump or fan, using the concentration gradient to deliver and circulate methanol and oxygen in the cell stack.
Methanol in a fuel cell delivers power most efficiently when mixed with water in a concentration of less than 10% - a dilution requiring a fuel tank that is much too large for use with portable equipment. Toshiba overcame this and developed a system that allows a higher concentration of methanol to pass into the cell stack and produce electricity efficiently. As the result, Toshiba's new DMFC realises a fuel tank less than one tenth the size of that required for storing the same volume of methanol in a concentration of less than 10%.
