: Diagram of the fuel cell construction.
On-chip air-breathing, membraneless micro fuel cells prove pumps are not required.
The operation of devices such as on-chip micro-sensors necessitates miniature on-chip power sources. Tiny fuel cells show potential but their performance is poor due to structural problems.
Tetsuya Osaka and colleagues at Waseda University, Tokyo,1 have addressed this problem by removing the need for membranes in the structure of fuel cells. Their devices can generate power without pumps, with an overall performance greater than other on-chip fuel cells.
Conventional fuel cells need an electrolyte membrane to separate the anodic and the cathodic compartments, and to enable ionic conduction between them. This results in a structure where the electrolyte membrane sandwiches two electrodes. But, directly miniaturising this structure has proven to be troublesome.
Osaka and colleagues eliminated the membrane by using a selective catalyst for the oxygen reduction reaction in the cathode. This enabled them to produce fuel cells that are smaller and more flexible in design—they do not incorporate face-to-face electrodes.
The researchers developed cells on silicon substrates by etching shallow channels with deeper trenches running down the middle (Fig. 1). The anode was deposited in the trench—a platinum-ruthenium alloy—and the trench was filled with the fuel and electrolyte, in this case methanol and either sulphuric acid or sodium sulphate. The two cathodes were made of porous palladium-cobalt alloy.
Protons generated from the methanol oxidation on the anode transfer to the porous cathode through the electrolyte. Exposing the porous cathode to air resulted in the reaction of the protons with oxygen, which produced water. The electrons that were created in the methanol oxidation must pass through the external circuit from the anode to the cathode, thus generating power. In endurance tests, even without covering the cells to prevent fuel evaporation, the devices continued to generate power for more than an hour.
Although on-chip power was emphasized in the paper, Osaka is also optimistic about other applications. “Since the cells are fabricated on a substrate with nothing on the underside, we think that the most exciting application is a flexible fuel cell that can be pasted everywhere—like scotch tape by replicating cells on a polymer substrate,” he says.