By Jeffery Kahn
Solving a vexing materials obstacle, Berkeley Lab researchers report they have made an important breakthrough in the research effort to develop potentially low cost fuel cell technology.
Focusing their efforts on solid oxide fuel cells (SOFCs), which have been on the verge of commercial viability for years, the researchers are using a deceptively simple ceramic process to develop a thin-film electrolyte that both doubles the power output and significantly reduces the cost of SOFCs.
Fuel cells, which transform hydrocarbons into electricity without combustion, are highly fuel-efficient and almost nonpolluting. But the cost to build them has been a barrier to broad commercialization. Other than for a few demonstration projects, their use to generate electricity and to power vehicles has not been practical. Researcher Steve Visco believes that is about to change.
"We have developed a low-cost means of fabricating a high-performance cell. In one fell swoop we have increased performance and reduced cost. Undoubtedly, this should accelerate the commercialization of SOFCs."
Visco and Berkeley Lab colleagues Selmar de Souza and Lutgard De Jonghe published the details of their work in the proceedings of the European Solid Oxide Fuel Cell Forum. Their research is funded by the Department of Energy and the Electric Power Research Institute.
Researchers Steven Visco (left), Lutgard De Jonghe (rear), and Selmar de Souza with their newly developed fuel cells.
SOFCs, like other types of fuel cells, produce electricity by electrochemically combining hydrogen with oxygen. All fuel cells can run on natural gas, but some require that the gas be preprocessed first (reformed), providing a supply of hydrogen to the cell. Some high-temperature cells, like SOFCs, don't require this expensive pretreatment; they can "burn" natural gas directly.
Up until now, SOFCs have been most fuel-efficient operating at 1000 degrees Centigrade. Unfortunately, this high temperature increases the cost of materials and decreases the lifetime of the cell. For some years, researchers around the world have been on a quest to drop the operating temperature without sacrificing performance. They have been stumped.
Says de Souza, "SOFCs are solid-state devices. We know how to drop their operating temperature, but the problem has been the electrolyte. It conducts ions between electrodes, and when you drop the temperature, you lose conductivity (and electrical output drops). One way to deal with this is by making the electrolyte thinner."
Like researchers elsewhere, Berkeley Lab scientists have looked for a way to thin down the electrolyte, from a 100 micron film down to about 10 microns. In theory, this should allow the cell to operate at 800 degrees Centigrade without any loss in performance. Thinner electrolytes, lower temperatures, cheaper materials¬this has been the recipe for a commercial-grade SOFC.
"Researchers have tried multiple approaches to come up with a thin electrolyte that still is functional," said de Jonghe. "Magnetron sputtering, plasma spray, chemical vapor deposition, electrochemical vapor deposition ¬ any number of expensive, high-tech approaches have not worked out."
Visco, de Souza and De Jonghe hit paydirt with an inexpensive and simple ceramic process. They have devised a technique that doesn't just preserve performance at the lower temperature of 800 degrees but actually doubles the power output.
The team reports that cells with the new, ultra-thin ceramic electrolyte generate two watts per square centimeter of cell surface area. This is double the record power for SOFCs. Additionally, during testing, the electrical output remains absolutely constant over 700 hours. While no guarantee that the cell can last the five to ten years necessary for commercial purposes, the test results are extremely promising.
Berkeley Lab's electrolyte, a yttria-stabilized zirconia film, starts out as a ceramic powder suspended in solution. Much like a latex paint, the solution is painted onto the anode (electrode) and then fired or sintered. The process readily lends itself to assembly line usage.
Although the Berkeley team always believed their approach had merit, success had been elusive. With concerted effort over the past three years, they ultimately hit upon the right combination of ceramic powders and processing. They modified the processing of both the anode and the electrolyte, and found a materials match-up that will help push the fuel cell out of the laboratory and into the energy marketplace.
"Like many things," said Visco, "once you know how, its a relatively simple process."