In her lab at MIT, chemical-engineering professor Paula Hammond pinches a sliver of what looks like thick Saran wrap between tweezers. Though it appears unremarkable, this polymer membrane can significantly increase the power output of a methanol fuel cell, which could make that technology suitable as a lighter, longer-lasting, and more environmentally friendly alternative to batteries in consumer electronics such as cell phones and laptops.
Methanol is a promising energy source for fuel cells because it is a liquid at room temperature, so it’s easier to manage than hydrogen. But so far, its commercial applications have been limited. One reason has to do with the properties of the proton-conducting membranes at the heart of fuel-cell technology.
On one side of a methanol fuel cell, a catalyst causes methanol and water to react, yielding carbon dioxide, protons, and free electrons. The protons pass through a membrane to a separate compartment, where they combine with oxygen from air to form water. The electrons, which can’t cross the membrane, are forced into wires, generating a current that can be used to power electronic devices.
The more protons cross the membrane, the more power is generated. But the polymers that conduct protons well also tend to let the methanol solution into the other compartment. The resulting loss of fuel lowers the cells’ power output. To limit such “methanol crossover,” researchers have to either use polymers that don’t conduct protons as well or make thicker membranes. But both of those options decrease efficiency, too.
In work published last spring in Advanced Materials, Hammond used an elegant, inexpensive process to reduce methanol crossover in a commercial fuel-cell membrane, increasing the efficiency of a methanol fuel cell by more than 50 percent. “What we’ve done is generate a very thin film that actually prevents the permeation of methanol but at the same time allows a rapid rate of proton transport,” says Hammond. Encouraged by this success, her team is now working to build such membranes from scratch, which could make them less expensive.
A Modified Process
A layer-by-layer assembly technique is the key to Hammond’s membranes. In earlier work, her team altered a membrane made of Nafion, a polymer manufactured by DuPont that is commonly used in fuel cells. It conducts protons well but also permits some methanol leakage, and it’s relatively expensive to make.
To begin the new process, Avni Argun, a postdoc in the lab and lead author on the Advanced Materials paper, mounts a specially treated silicon disc in a lab hood and starts the disc slowly rotating. Facing the membrane are four sprayer nozzles. Each nozzle is connected to a separate container. One contains a positively charged polymer solution and one a negatively charged polymer solution; two hold water.
Argun starts the sprayer system, which mists the disc with the positive solution for a few seconds, then with a water rinse, then with the negatively charged polymer, and finally with water again. A two-layer film forms within about 50 seconds. The thickness of this “bilayer” depends on the polymers and can range from 3 to 50 nanometers. In about six hours, the sprayer can apply between 400 and 600 bilayers, creating a membrane about 20 micrometers thick. The membrane described in Advanced Materials was made up of three bilayers on top of a Nafion membrane, adding only 260 nanometers to its thickness. By using a combination of positive and negative polymers, the researchers maintained Nafion’s high conductivity while reducing its methanol crossover.