Sandia National Laboratories researchers Cy Fujimoto (right) and Michael Hibbs (left) demonstrate the clarity of their recent membranes. Photo: Randy Montoya, Sandia National Laboratories.Fuel cells can provide power without pollutants. But, as in the Goldilocks story, membranes in automobile fuel cells work at temperatures that are either too hot or too cold to be maximally effective. A novel polyphenyline membrane developed and patented by Sandia National Laboratories seems to work just about perfectly, though, says Sandia chemist Cy Fujimoto.
The membrane, which operates over a wide temperature range, lasts three times longer than comparable commercial products, say Fujimoto and his co-authors in a paper in Nature Energy.
The proton-exchange membranes (PEMs) in fuel cells allow protons to pass through them while blocking the passage of the electrons that form the fuel cell's electrical output. If protons can't pass easily through the PEM, this reduces the electrical output of the fuel cell.
The commercial PEMs currently used in most fuel-cell-powered vehicles require water, which means their operating temperature can't get higher than water's boiling point. Higher temperatures dry out the membrane, increase cell resistance and reduce performance, said Fujimoto.
"Part 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 explained.
"This problem can be solved by employing hydrated fuel streams and having a larger radiator to more effectively dissipate waste heat," Fujimoto continued. "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 PEM of choice can be $250–$500 per square meter. "The DOE [Department of Energy] would like to see $5 to $20 a square meter," Fujimoto said.
Researchers have tried solving these problems by doping a polybenzimidazole membrane with phosphoric acid, producing a membrane that can operate at temperatures up to 180°C. The problem, however, is that this doped membrane can't operate at temperatures below 140°C without degrading the phosphoric acid. This makes it unsuitable for automotive applications, where water condensation from cold engine start-ups and other normal reactions at the fuel cell cathode unavoidably lower the temperature below this level.
Now comes an ammonium ion-pair fuel cell – created at Los Alamos National Laboratory – that combines phosphates with Sandia’s polyphenyline membrane. The ammonium-biphosphate ion pairs exhibit stable performance over a wide range of temperatures from 80–160°C, respond well to changes in humidity and last three times longer than most commercial PEM fuel cell membranes.
"There probably will be industrial interest in this discovery," Fujimoto said. "Our polymer contains a tethered positive charge which interacts more strongly with phosphoric acid, which improves acid retention. Heating the fuel cell and adding humidity doesn't reduce performance."
This story is adapted from material from Sandia National Laboratories, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.