Researchers at Sandia National Laboratories, have developed a new membrane for fuel cells based on quaternary ammonium-biphosphate ion pairs that can operate under conditions unattainable with existing fuel cell technologies. A paper describing the Sandia-patented technology is published in the journal Nature Energy.
Fuel cells with this membrane technology exhibit stable performance at 80–160 ˚C with a conductivity decay rate more than three orders of magnitude lower than that of a commercial high-temperature PEM fuel cell. By increasing the operational flexibility, this class of fuel cell can simplify the requirements for heat and water management, and potentially reduce the costs associated with the existing fully functional fuel cell systems, the researchers said.
Currently, commercial PEMs in most fuel-cell-powered vehicles use a water-swollen co-polymer film for the critical proton exchange. Electrons are peeled off by oxidation of the hydrogen atoms and hydrated protons pass through the film to combine with oxygen on the other side to form water as a byproduct. The efficiency of the exchange process depends upon water, so efficiency—measured as proton conductivity—goes down as humidity goes down. (Earlier post.) Further, this means that the operating temperature can’t get higher than water’s boiling point. Higher temperatures dry out the membrane, increase cell resistance and reduce performance.
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. 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.—Sandia chemist Cy Fujimoto
Another problem is that material costs for the current membrane of choice can be approximately $250-$500 per square meter. The US Department of Energy (DOE) would like to see $5 to $20 a square meter, Fujimoto said.
Researchers have tried to solve these problems with a high-temperature method that uses phosphoric acid to dope a polybenzimidazole membrane at more than 180 ˚C. But the membrane can’t operate below 140 ˚C 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.
The Sandia ammonium-biphosphate ion pairs have exhibited stable performance over a wide range of temperatures.
There probably will be industrial interest in this discovery. 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.—Cy Fujimoto
The fuel cell work was supported by the Fuel Cell Technology Office of the Department of Energy’s Office of Energy Efficiency and Renewable Energy.
Kwan-Soo Lee, Jacob S. Spendelow, Yoong-Kee Choe, Cy Fujimoto & Yu Seung Kim (2016) “An operationally flexible fuel cell based on quaternary ammonium-biphosphate ion pairs” Nature Energy 1, Article number: 16120 doi: 10.1038/nenergy.2016.120