Move over, rechargeable batteries. Researchers at silicon-chip maker STMicroelectronics, Geneva (www.st.com), are developing tiny fuel cells for cell phones that can be refilled with fuel as needed. They are grappling with the fact that the phone consumes 300 mA of current at 3.6 V and its power source cannot occupy more than 12 cm3. The output current of a fuel cell is directly related to the common surface area between the electrodes and membrane and, to obtain 300 mA using conventional fuel-cells, would require a surface area of around 60 cm2, too large for a cell phone.
To overcome this issue, one approach is to implement the fuel cell as a 3D structure containing thousands of microchannels that maximize the contact area between the gases, catalysts, and electrodes. In the same vein, the research team has fabricated a nanoporous layer of silicon containing millions of pores, each measuring a few nanometers in diameter. The small pores give the layer a large effective surface area, boosting the catalysis efficiency. Also, the research team is working with the University of Naples to develop membranes that are more permeable and cheaper than membrane materials currently available.
Prototypes of fuel cells will find their way into notebook computers before the end of next year, according to NEC Corp., Kawasaki, Japan, and Toshiba Corp., Tokyo, Japan.
Direct-methanol fuel cells weigh about 900 grams and can power a system for five hours with one methanol fuel fill up. NEC's fuel cell has an output density of 50 mW/cm2 and consists of a polymer electrolyte sandwiched by carbon electrodes with catalyst particles. NEC engineers use single-walled carbon nanohorn aggregates, a type of carbon nanotube, to adsorb the platinum-catalyst particles. The platinum-catalyst particles are supported on the carbon nanohorns at about 2 nm, which the company claims is much finer than possible with conventional carbon-black materials. The prototype cell has an average output of 14 W (24-W maximum) at 12 V, and a fuel concentration of about 10% methanol.
Toshiba's prototype cell generates 12-W average and 20-W maximum. It powers a notebook computer for nearly five hours on a single, 50-cc cartridge of methanol. Methanol generates power most efficiently in a fuel cell when its concentration in water is 3 to 6%, according to Toshiba. However, thin methanol liquid has little energy, so a bulky fuel tank has been necessary for long electricity generation. Toshiba engineers solved this issue by making use of water, a byproduct of the cell itself. A dilution device makes it possible to store highly concentrated methanol and reduce the tank size and weight to 33 X 65 X 35 mm and 72 gm in the case of a 50-cc methanol cartridge. Toshiba has also prepared a 100-cc cartridge, which it claims can power a PC for 10 hr. Engineers there are working on bringing down the size of the fuel cells and cartridges.