Startup company SiEnergy Systems has overcome a major barrier to commercializing solid-oxide fuel cells with a prototype that operates at temperatures hundreds of degrees lower than those on the market today. Working with Harvard materials science professor Shriram Ramanathan, SiEnergy Systems, based in Boston, has demonstrated a solid-oxide fuel cell that can operate at 500 degrees Celsius, as opposed to the 800 to 1,000 degrees required by existing devices. This allows the cell, which uses a thin-film electrolyte mechanically supported by a metal grid, to be much larger than similar devices fabricated before—on the order of centimeters in area, the size needed for practical applications, rather than micrometers.

Solid-oxide fuel cells, which can run a variety of fuels including diesel or natural gas, bring in oxygen from the air to be reduced at the cathode, and then pass the oxygen ions through a solid-oxide electrolye membrane to the anode, where the fuel is oxidized to produce electrons that are drawn out of the device. Their high operating temperatures are dictated by the fact that the ions move more quickly through the electrolyte at higher temperatures.

If the electrolyte is very thin—just a few hundred nanometers thick—a solid-oxide fuel cell can operate at lower temperatures. Such electrolytes can power very small demonstration devices, but until SiEnergy and Ramanathan's work, no one had been able to make an ultrathin solid-oxide membrane large enough for practical devices, says Harry Tuller, professor of materials science and engineering at MIT. "The challenge has been that the films, being so thin, are fragile and easily tear during processing or during heating and cooling cycles," says Tuller. When heated and cooled, the different materials of which they are made expand and contract at different rates, damaging the delicate film. "We and others have tried to support the films by one or more structural supports," he says, "but have not succeeded in doing so over as large an area."