Solid oxide fuel cells (SOFCs) are currently under development for small-scale and emergency power sources. To make these power supplies useful for wider applications, however, new innovations are required. For example, SOFCs currently operate at high temperature, over 700 °C, which means they can take a long time to start up and shut down.

Now, Toshio Suzuki and co-workers at the National Institute of Advanced Industrial Science and Technology and the Fine Ceramics Research Association in Nagoya, Japan1, have designed SOFCs that can operate at considerably lower temperature by introducing a new type of porous anode.

“SOFCs are flexible in terms of the fuels they can use, and have higher efficiency than other types of fuel cells, making them a strong candidate for portable power sources,” says Suzuki. “Our project aims to realize highly efficient SOFCs with quick start-up and shut-down capability.”

Fig. 1: A microtubular solid oxide fuel cell, pictured next to a US quarter-dollar coin. The performance of the device is improved by the use of a porous electrode.Copyright © Toshio Suzuki 2009

SOFCs work by directly oxidizing a fuel at an anode, where the fuel enters the system. Oxide ions are supplied through a ceramic electrolyte wedged between the anode and a cathode, where oxygen enters. One promising way to improve the performance of SOFCs is to build tiny, tubular devices (Fig. 1), which provide very high power per volume and remain stable under rapid heating.

Suzuki and his co-workers expanded on this design by remodeling the cell anodes, which are made of nickel and zirconia, to be over 50% porous. This allows more fuel to diffuse through the anode. What’s more, the nickel particles in the anode are smaller, thus improving the catalytic performance of the device.

Many challenges remain to make these high-performance SOFCs a practical technology for the future — such as bundling the cells together in an efficient stack design. “So far, we have succeeded in developing a ‘cube’ bundle with nine microtubular SOFCs, providing a volumetric power density of over three watts per cubic centimeter at 550 °C,” says Suzuki.

With more work, Suzuki hopes that SOFCs should eventually be able to operate at more manageable temperatures, and therefore start up faster for use in portable devices or cars. “We are collaborating with several companies to develop power sources for household appliances and automobiles,” he says, “and we have started optimizing the microtubular SOFCs for hydrocarbon fuels.”