Cooling Down Solid-Oxide Fuel Cells

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.”

In a paper published in the journal Nature Nanotechnology, the researchers describe making an electrolyte membrane that is more stable both thermally and mechanically. They started with a 100-nanometer-thick electrolyte membrane made up of zirconia and yttrium. They deposited a supportive metallic grid on top of it, to hold the membrane in place while it was heated and cooled and, since the grid was made of conductive material, to act as the anode. They combined this with a dense, high-performance cathode previously developed by Ramanathan. In their published work, SiEnergy has demonstrated arrays of fuel cells each about five millimeters square. Ramanathan says the method can be scaled up to the centimeter-scale areas needed for devices.

SiEnergy’s general manager, Vincent Chun, says this is just a first demonstration and the company is now working on integrating the thin fuel cells into full systems and testing fuels. Chun hopes the company’s fuel cells will save on materials costs because they are so thin.Chun says the company plans to offer replacements for diesel generators and home heating and power-generation systems.