Polymer electrolyte improves fuel cell performance

Energy

May 8, 2008

A 5 µm thick standalone film suitable for DMFCs that has been assembled by layer-by-layer deposition. (Courtesy of Paula Hammond.)

The high ionic conductivity of novel polymer electrolyte thin films developed by scientists at Massachusetts Institute of Technology (MIT) promises much for improved fuel cells, batteries, and dye-sensitized solar cells [Argun et al., Adv. Mater. (2008) 20, 1539].

Such electrochemical devices are dependent on electrolytes to facilitate charge transport between electrodes. But processing difficulties and safety concerns with liquid or gel electrolytes has tended to limit widespread use of these devices. In contrast, polymer electrolytes can provide mechanical strength and more flexibility in manufacture, but they must still offer fast ion conduction.

Paula T. Hammond and colleagues have used the versatility of layer-by-layer (LBL) assembly to build thin films of two different polymers that when brought together offer both high conductivity and mechanical stability.

“We have generated an LBL-assembled solid-state thin film with ionic conductivity values over three orders of magnitude higher than the previous best performing multilayer films,” says Hammond. “These new ionic conductors have proton conductivities close to those of much more expensive specialized polymers, making this technique a competitor for fuel cell and other solid-state electrolyte applications.”

The team found that by sulfonating a thermally and mechanically stable aromatic polyether and pairing it with a complementary functionalized polymer, they could achieve ionic conductivity values up to 35.3 mS cm^–1. Furthermore, the assembled multilayer films show low methanol permeability. This makes the films of immediate interest in direct methanol fuel cells (DMFCs).

Nafion is the standard membrane material used in DMFCs, but it is highly permeable to methanol, hindering the working of the cell and significantly reducing its performance. “We have shown that incorporating these LBL materials into a DMFC improves power output by over 50%,” Hammond says.

While the LBL films were previously grown on a Nafion substrate, the group has now successfully produced standalone films (shown), Hammond tells Materials Today.

Jonathan Wood