JUST RIGHT — Sandia researchers Cy Fujimoto, right, and Michael Hibbs show the clarity of their recent membranes. Click image to download a larger file. (Photo by Randy Montoya)
Not too hot, not too cold
Sandia fuel cell membrane outperforms market
Fuel cells provide power without pollutants. But, like the Goldilocks story, membranes in automobile fuel cells work at temperatures either too hot or too cold to most effectively power automobiles. A polyphenyline membrane patented by Sandia, though, seems to work just about right, says chemist Cy Fujimoto (1853).
The membrane, which operates over a wide temperature range, lasts three times longer than comparable commercial products, say Cy and colleagues in the Aug. 21 issue of Nature Energy.
Fuel-cell PEMs (proton-exchange membranes) allow the excretion of protons — the husk, in a sense, of the material providing electrons that form the fuel cell’s electrical output. If the protons are not readily provided passage within the cell, the fettered flow reduces the electrical generating output.
Currently commercialized PEMs in most fuel-cell-powered vehicles require water, which means their operating temperature range can’t get higher than water’s boiling point. Higher temperatures dry out the membrane, increase cell resistance, and reduce performance, says Cy.
“One of the issues with the current PEMs is that you need to hydrate the hydrogen fuel stream for high performance and the fuel cell can’t run effectively at temperatures higher than the boiling point of water,” he says. “This problem can be solved by employing hydrated fuel streams and having a larger radiator to more effectively dissipate waste heat. Automakers are doing this now. But if PEM fuel cells didn’t need water to run it would make things a lot simpler.”
Another problem is that material costs for the current membrane of choice can be approximately $250 to $500 per square meter. “DOE would like to see $5 to $20 a square meter,” says Cy.
Researchers have attempted to mitigate these problems with a high-temperature method that uses phosphoric acid to dope a polybenzimidazole membrane at 180 degrees C. But the membrane can’t operate below 140 degrees without degrading the phosphoric acid. Thus the membrane is unsuitable for automotive applications, where water condensation from cold engine start-ups and other normal reactions at the fuel cell cathode unavoidably bring the temperature down into undesirable ranges that leach the acid out of the reaction.
Now comes the first ammonium ion-pair fuel cell — created at Los Alamos National Laboratory — to combine phosphates with the Sandia-patented membrane. The ammonium-biphosphate ion pairs have exhibited stable performance over a wide range of temperatures from 80-160 degrees C, responded well to changes in humidity, and lasted three times longer than most commercial PEM fuel cell membranes.
“There probably will be industrial interest in this discovery,” says Cy. “Our polymer contains a tethered positive charge that interacts more strongly with phosphoric acid, which improves acid retention. Heating the fuel cell and adding humidity doesn’t reduce performance.”
The fuel cell work was supported by the Fuel Cell Technologies Office of DOE’s Office of Energy Efficiency and Renewable Energy.