Fuel cells are an environmentalist's dream: They generate electricity from fossil fuels without burning them and spewing pollutants. The devices haven't become a commercial success, however, in part because fuel cells either consume hydrogen, which can be dangerous and difficult to cart around, or they run at very high temperatures. But a new design, reported in this week's issue of Science, uses hydrocarbon fuels such as ethane while running 100°C cooler than its closest competitor.
Fuel cells work something like batteries. Within cells that run directly on hydrocarbons, a ceramic membrane separates a negatively charged electrode (the cathode) from a positively charged electrode (the anode). The cathode feeds electrons to oxygen molecules, and the resulting negatively charged oxygen ions pass through the membrane to the anode. At the anode, the oxygen ions help tear apart hydrocarbons to produce electricity, water, and carbon dioxide. For many hydrocarbon-consuming cells, this process is not pretty, because the anode often welds carbon atoms together to make soot that kills the cell. Running the cells below about 800°C prevents the soot from building up. But that causes its own problem, as typical zirconium-based membranes require temperatures near 1000°C to rattle the ions through.
To get around this problem, Takashi Hibino of the National Industrial Research Institute of Nagoya, Japan, and his colleagues at Nagoya University designed a cell with a cerium-dioxide membrane that lets ions through at lower temperatures. One side of the membrane is dabbed with nickel, which serves as the anode. The other side, the cathode, is a ceramic composite of samarium, strontium, cobalt, and oxygen. The wafer sits in a single chamber and air mixed with hydrocarbon fuel swirls around it. The chemical composition of the membrane and the geometry of the cell allow the cell to run at a cool 500°C, whereas the nearest ceramic competitor bottoms out at roughly 600°C.
Because the novel cell works at lower temperatures, engineers should be able to build additional fuel cell components from steel rather than expensive heat-resistant alloys, and that should drop the prices of ceramic cells considerably. However, warns Subash Singhal, head of fuel cell research at Pacific Northwest National Laboratory in Richland, Washington, it may take years of engineering tinkering to scale up the design for industrial use.