Membranes in automobile fuel cells typically operate at temperatures either too hot or too cold for them to be maximally effective. However, researchers at the U.S. Energy Department's Sandia National Laboratories say a polyphenyline membrane they have developed can operate over a wide temperature range and lasts three times longer than comparable commercial products.

Sandia researchers Michael Hibbs (l) and Cy Fujimoto demonstrate the clarity of the new membrane. Image credit: Randy Montoya.Sandia researchers Michael Hibbs (l) and Cy Fujimoto demonstrate the clarity of the new membrane. Image credit: Randy Montoya.Fuel-cell proton-exchange membranes (PEMs) allow the excretion of protons of the material providing the electrons that form the fuel cell’s electrical output. If the protons can’t pass easily within the cell, the fettered flow reduces the electrical output.

Currently, commercial PEMs in most fuel cell-powered vehicles require water, so their operating temperature can’t get higher than water’s boiling point. Higher temperatures dry out the membrane, increase cell resistance and reduce performance.

“This problem can be solved by employing hydrated fuel streams and having a larger radiator to more effectively dissipate waste heat,” says Sandia researcher Cy Fujimoto. He co-authored a paper describing the technology that has been published in Nature Energy. “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-$500 per square meter. “The DOE would like to see $5 to $20 a square meter,” Fujimoto says.

Researchers have tried to solve these problems with a high-temperature method that uses phosphoric acid to dope a polybenzimidazole membrane at more than 350 degrees F. But the membrane can’t operate below 284 F 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 phosphoric acid out of the reaction.

Results are now being reported from 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 176-320 F, 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,” Fujimoto says. “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.”

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