It’s not enough for a new alternative energy technology to work. It also has to be cheap enough to compete with traditional fossil fuels. That’s been a high hurdle for devices called solid oxide fuel cells (SOFCs) that convert fuels—such as methane and hydrogen—directly to electricity without burning them. But now researchers report that they’ve come up with a new recipe for making key components in one type of SOFC more cheaply, which could sharply lower its overall cost.
“This is an excellent technological demonstration,” says Sossina Haile, a materials scientist and fuel cell expert at Northwestern University in Evanston, Illinois, who was not involved in the work. “I think it’s going to generate a lot of excitement.”
A fuel cell works much like a battery. Within it, two electrodes are separated by a charge-conducting electrolyte. In the case of SOFCs, the electrolyte consists of a solid ceramic membrane. In the typical setup, air is fed to the negatively charged electrode, or cathode, where oxygen molecules pick up extra electrons. These oxygen ions then travel through the membrane to the positively charged anode. There, they react with molecules in the fuel, generating water, carbon dioxide, and electricity. The electricity is fed through a circuit where it powers our devices, and then is returned to the anode. As long fuel as is fed in, the SOFC continues pumping out electricity.
SOFCs have some promising capabilities. The devices make electricity at an efficiency that can match a large natural gas-based power plant. But whereas a power plant is huge and costs hundreds of millions of dollars to build, SOFCs can be made to be any size. That makes them attractive as backup power sources for hospitals and manufacturing plants, as well as for producing distributed power systems not connected to the grid.
But SOFCs also have their drawbacks. In conventional, oxygen-conducting SOFCs, the membrane is made from a ceramic called yttria-stabilized zirconia, or YSZ, and the cells operate most efficiently at 800°C to 1000°C. That means they must be made using heat-resistant materials, which makes them too expensive for most applications. In recent years, researchers have begun exploring alternative membranes made from ceramics called yttrium-doped barium zirconates (BZY). These devices work best at converting hydrogen gas and oxygen to water and electricity, and even work at lower temperatures around 600°C. Unlike conventional SOFCs the BZY membranes allow the flow not of negatively charged oxygen ions toward the anode, but positively charged hydrogen ions, the opposite way, toward the cathode. But they’ve never matched the power output of the oxygen conducting SOFCs.
Now, Ryan O’Hayre, a materials scientist at the Colorado School of Mines in Golden, and his colleagues have found a way to boost the power from BZY fuel cells. The researchers suspected one problem with the BZY membranes was in the way they were made. Mixing the different ceramic components typically requires heating them to temperatures as high as 1700°C. But at that extreme temperature, barium vaporizes into a gas, which makes it harder to mix it uniformly throughout the ceramic. O’Hayre and his colleagues have recently helped pioneer an alternative mixing scheme called solid state reactive sintering, which lowers the blending temperature. And now they've found that adding trace amounts of what O’Hayre calls “magic pixie dust”—actually copper oxide and nickel oxide—to their ceramic mixture enables them to reduce the temperature to 1450°C, below the threshold where barium vaporizes. That, in turn, led to more uniform BZY mixtures and improved performance of their devices.
The change nearly doubled the power output produced by a single hydrogen SOFC, the researchers report in the current issue of Science. Not only that, but the new cells also matched the output of oxygen ion–conducting cells. What's more, the new BZY cells work best at about 500°C, the sweet spot temperature targeted by the fuel cell industry.
O’Hayre and Haile caution that the new advance won’t revolutionize the SOFC industry overnight. Thus far, O’Hayre’s group has produced just individual cells. Commercial devices, by contrast, work by wiring many such devices together into what’s known as a fuel cell “stack” that generates more power. If future BZY-based SOFC stacks work as well as the individual devices, then it could finally produce the tipping point the fuel cell industry has been looking for.