‘Goldilocks’ membrane perfect for fuel cells

RESEARCHERS in the US have developed a new membrane which works at just the right temperature to be maximally efficient for use in hydrogen fuel cells for automotive use.

Conventional Nafion fuel cell membranes require water to work, so are limited to 100?C, the temperature of boiling water. Higher than this and the membrane begins to dry out and the fuel cell stops working. In a car, the excess heat must be dissipated with a larger radiator. Much better, would be a fuel cell membrane which didn’t require water. Some researchers have experimented with using phosphoric acid to dope a polybenzimidazole membrane, but these can only work at temperatures above 140?C, and degrade when exposed to water below this temperature, which is not suitable for cold engine start-ups.

Cy Fujimoto and the team at Sandia National Laboratories say that their new membrane works at just the right temperature, neither too hot nor too cold, much like Baby Bear’s porridge in the Goldilocks story. The membrane is made from a polymer called polyphenylene, doped with phosphoric acid. The biphosphate ionic pair complexes formed within the membrane are very stable and aid the passage of protons through the cell membrane. The membrane can operate at temperatures between 80–180?C, a far wider range than other such proton exchange membranes.

The researchers tested their membrane against phosphoric acid doped polybenzimidazole membrane in a number of tests. In one such test, they exposed the two membranes to temperatures of 80?C and 40% humidity for 44 hours. The Sandia membrane retained 90% of its phosphoric acid doping, while the polybenzimidazole membrane retained just 47%. At 200?C, the two membranes retained 77% and 67% respectively. The researchers also tested membrane electrode assemblies, mimicking a fuel cell, in accelerated stress tests, ramping up the temperature for 80–160?C at a rate of 10?C/minute. The Sandia membrane retained 60% of its original phosphoric acid doping, lasting around three times longer, while the polybenzimidazole membrane retained just 15%.

“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,” write the researchers.

Fujimoto believes that there will be industrial interest in the discovery, and the team at Sandia has patented the membrane.

Nature Energy DOI: 10.1038/nenergy.2016.120