For several years now the Department of Energy (DOE) has been urging the fuel cell community to solve a major problem in the design of solid oxide fuel cells (SOFCs): heat. Such fuel cells could someday provide reliable power for homes and industry, dramatically cutting greenhouse gas emissions as well as other pollutants.
But SOFCs run hot, at temperatures as high as 1000 degrees Celsius (about 1800 degrees Fahrenheit). They're efficient at such temperatures, but only a few costly materials can withstand the heat. Using such materials makes things expensive, and is the reason for the push for lower temperatures by the DOE.
Sossina Haile, an associate professor of materials science and chemical engineering at the California Institute of Technology, is an expert in fuel cells, and she has been whittling away at the heat problem for years. Now she and her colleagues have not only solved the problem, they've smashed it. They've brought the temperature down to about 600 degrees Celsius (1100 degrees Fahrenheit), while achieving more power output than others are achieving at the higher temperatures--about 1 watt per square centimeter of fuel cell area.
They accomplished this by changing the chemical composition of one component of a fuel cell called the cathode. The cathode is where air is fed in to the fuel cell, and it's where the oxygen is electrochemically reduced to oxygen ions. The oxygen ions then migrate across the electrolyte (which conducts electricity), to react with fuel at the anode, another fuel cell component. The electrochemical reduction of oxygen is an essential step in the fuel cell's process of generating power. But the problem with running solid oxide fuel cells at 500 to 700 degrees Celsius is that the cathode becomes inactive when the temperature is less than about 800 degrees Celsius.
Haile and postdoctoral scholar Zongping Shao's insight was to switch out the conventional cathode and replace it with a compound that has a long chemical formula guaranteed to strike fear into the heart of every undergraduate, but is abbreviated as "BSCF" for short.
What BSCF can do that standard cathodes can't is to allow the oxygen to diffuse through it very rapidly. "In conventional cathodes, the oxygen diffuses slowly, so that even if the electrochemical reaction is fast, the oxygen ions are slow in getting to the electrolyte," says Haile. "In BSCF the electrochemical reaction is fast and the oxygen ion transport is fast. You have the best combination of properties." This combination is what gives the very high power outputs from Haile's fuel cells.
The work was reported in a recent issue of the journal Nature. Because they are using relatively conventional anodes and electrolytes with this new cathode, says Haile, it would be easy to switch out cathodes in existing fuel cells. That will probably be their next step, says Haile: to partner with a company to produce the next generation of solid-oxide fuel cells.
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