Other researchers have tried to reduce membrane permeability by using new polymers or blending two different polymers. Blending often doesn’t work well, though, because polymers with different structures tend to separate, making the membrane less stable. With the layer-by-layer assembly process–common in other areas of materials science–“we combine two different materials, but on a nanometer-length scale so they’re really intermingled,” Hammond says.
After the membrane dries, Argun carefully peels it off the disc and tests its permeability and electrical resistance, which allows him to calculate its conductivity. With a large clip, he fastens the membrane between a plastic chip and a base that holds platinum wires that will measure resistance. After putting the assembly in a sealed plastic box that allows him to control temperature and humidity, he manipulates the membrane using a pair of gloves that reach through the box and into the chamber. Most membranes perform better under high temperature and humidity, so both conditions must be noted. Argun connects the assembly to an external analyzer to test the membrane’s resistance. Measuring its permeability is more straightforward; he simply notes the amount of methanol that diffuses through it over a specific amount of time.
If a membrane fares well in these initial tests, Argun couples it to a positive and a negative electrode (where the electricity-producing reactions take place) to see how it would perform in an actual fuel cell. He places the electrodes–two black, circular carbon cloths studded with particles of platinum and a metal alloy–on either side of the membrane. Then he sandwiches the whole apparatus inside an insulating gasket that looks like thin cardboard. Finally, he seals the unit using a hot press.
Graduate student Nathan Ashcraft takes over from here. Ashcraft puts the membrane-electrode assembly into an active fuel cell, into which air and methanol are carefully pumped. Two square slabs of steel, about the size of slices of bread, make up the outside of the cell; they contain heaters that allow Ashcraft to precisely control the temperature of the reaction. Between the steel slabs, two gold-plated electrodes sandwich graphite blocks with small channels etched into them. Ashcraft places the membrane-electrode assembly between the blocks and secures it with screws. He then pumps methanol and air through the channels to either side of the assembly. He measures and records the resulting current, along with the system’s temperature.
Hammond’s team has not yet devised a completely new membrane that conducts as well as Nafion. However, “we feel like we’re very close,” she says. The team is also experimenting with membrane thickness; if a membrane is too thin, it will tear in the fuel cell, but thicker membranes don’t conduct protons as well. The membrane that the lab ends up with will probably be about 50 micrometers thick, Ashcraft says. Hammond also plans to try building membranes that incorporate additional polymers.