Previously the power sources for micromachines have only worked at high temperatures and have emitted toxic by-products. Moreover, the devices contain high-speed moving parts that must be operated within tight limits. Now, Zhang and co-workers have fabricated a micro fuel cell that uses methanol. This material has a high energy density and it is easier to store in a miniature device than the hydrogen gas traditionally used in fuel cells.

The UCLA-Penn State team first made a proton exchange membrane assembly by sandwiching a solid-state electrolyte layer between a cathode and an anode. Then they integrated this electrode assembly into a silicon-based micro fuel cell that contained channels 750 microns wide and 400 microns deep.

An aqueous solution of methanol is fed into the anode via the microchannels, where it generates electrons and protons. The electrons flow through an external circuit to deliver current while the protons migrate through the exchange membrane to the cathode. The protons then combine with electrons from the circuit and oxygen in the air to produce water as a harmless by-product at the cathode. The higher the methanol concentration, the more protons it can supply and the higher the current produced. Carbon dioxide is also produced at the anode

Using a 1 molar solution, the researchers observed a power density of 47 milliwatts per square centimetre at a temperature of 60 degrees centigrade, which they say is among the highest densities ever achieved in a micro fuel cell. At room temperature, the density was 14.3 milliwatts per square centimetre. Furthermore, the design of the new cell also means that moving parts can be eliminated.

The team now plans to improve the performance of the exchange membranes and hopes to further increase the power density of the device by making a three-dimensional fuel cell. “We have already increased operating times of the cell by using 8 molar methanol without loss of performance,” team member David Yen told PhysicsWeb.