Fuel cells are far greener than gas-powered engines because they produce electricity without burning up the hydrogen (or other fuel) that powers them. But they’re often impractical on a commercial scale because they’re so much more expensive to make. Now, researchers report that by creating a fuel cell that can run at a midrange temperature, they’ve made an inexpensive, powerful version that could boost the prospects for plentiful green energy.
Most fuel cells run at temperatures too hot or too cool to make at a reasonable price. One class, the polymer electrolyte membrane (PEM) cells that power cars and buses, run at about 100°C. Another class, the solid oxide fuel cells (SOFCs) that power backup generators for hospitals and other buildings, typically run at 1000°C. The lower temperature of PEM cells makes the essential chemical reactions sluggish, requiring the use of expensive metal catalysts, such as platinum, to speed them up. But the feverish temperatures of SOFCs means that even if they don’t need the pricy catalysts, they need to be built from expensive metal alloys that can handle the scorching operating temperatures.
So in recent years, fuel cell researchers have pursued a Goldilocks strategy, looking for midrange temperature fuel cells that operate at about 500°C. That’s warm enough for reactions to proceed quickly, but cool enough to allow them to be built from cheaper metals, such as stainless steel. Initially, scientists tried doing so with catalysts borrowed from SOFCs. The devices worked, but they generated just 200 milliwatts of power per square centimeter (mW/cm2) of electrode surface area, well behind the performance of PEM fuel cells and SOFCs. To make it commercially, such fuel cells would need to produce at least 500 mW/cm2, according to the U.S. Department of Energy (DOE).
Two teams have gotten close. One group, led by Ryan O’Hayre, a materials scientist at the Colorado School of Mines in Golden, reported last year in Science that it had produced an intermediate temperature fuel cell capable of producing 455 mW/cm2. Another group, led by Ji-Won Son, a materials chemist with the Korea Institute of Science and Technology in Seoul, reported last year in Nature Communications that it got a similar result at the ideal operating temperature of 500°C.
Now, a group led by Sossina Haile, a chemical engineer at Northwestern University in Evanston, Illinois, has crossed the goal line. Haile and her colleagues figured out that one key problem was occurring as soon as the reaction started. Both PEM fuel cells and SOFCs, like batteries, have two electrodes separated by an ion-conducting electrolyte. At one electrode, fuel molecules are broken apart and stripped of negatively charged electrons, which pass through an external circuit to a second electrode. Meanwhile, positively charged ions ripped from the fuel molecules travel through the electrolyte to the second electrode where they recombine with the traveling electrons.
Haile discovered that the connection point between the first electrode—called an anode—and the electrolyte was weak, blocking protons from zipping through to the second electrode, or cathode. So Haile and colleagues added a thin but dense layer of catalyst material atop the bulk of their anode catalyst, creating an easier transition for protons to move into the electrolyte. The researchers also tweaked the composition of their ceramic electrodes to make them more stable in the presence of steam and carbon dioxide. As they report today in Nature Energy their devices produced nearly 550 mW/cm2 at 500°C. They were stable for hundreds of hours of operation with few signs of degradation.
O’Hayre says the new work is “a great contribution,” and calls the performance “impressive.” But he notes that there are still a few issues that need to be solved before these devices are ready for market. For starters, the current cells are small, just a few centimeters in diameter. Researchers would need to find a way to make much larger versions, which could be tricky. That’s because the dense coating on the anode was formed by a technique called pulsed laser deposition, which is difficult to do large-scale on a commercial assembly line.
Another challenge, adds David Tew, a program manager at DOE’s Advanced Research Projects Agency-Energy in Washington, D.C., is that the all-ceramic electrodes and electrolyte are extremely brittle, which could make them less durable for use in real-world conditions.
Haile doesn’t disagree with those concerns. But she says her team’s advance should encourage researchers to solve those problems. If they do, intermediate range fuel cells could transform renewable energy, because they can also be used to convert electricity—say from a wind turbine—into hydrogen and other fuels for storage, and later turn them back into electricity. That would solve renewable energy’s biggest challenge: storing energy when the sun isn’t shining and the wind is still. That’s a combination that even Goldilocks might say could be just right for the future of fuel cells.