"Good Conduct," Feature Article, August 2007

FOCUS ON NANOTECHNOLOGY

good conduct

A researcher says that a new membrane textured on the nanoscale will let fuel cells triple the current they can carry.

by Michael Abrams, Contributing Editor

THE WORLD IS FLAT. The world of the fuel cell, that is. For years, the technology has held out the promise of a future with a more efficient and environmentally friendly source of energy. Laptops, automobiles, and space stations, we've been assured, will all some day be powered with these blocks that are free of moving parts, and produce mere H₂O as waste. We're not there yet, if you haven't noticed, and flatness is to blame.

The key to how a fuel cell works is the membrane that's sandwiched between the electrodes. When protons of hydrogen atoms are separated from their fellow electrons, it's this barrier that determines just how efficiently the cell will work. The higher the quantity and speed of the protons traveling from anode to cathode, the stronger the juice. Until now, however, the journeying protons had a difficult voyage from one side to the other, and not enough of them could move at the same time—all because that membrane has been a smooth one, and necessarily so.

But Joseph M. DeSimone, a professor of chemistry and chemical engineering at the University of North Carolina in Chapel Hill and at North Carolina State University, said he has found a way to give that membrane some texture and more than triple its conductivity.

Limits of Materials

"It started with trying to understand the limitations of the benchmark material," DeSimone said. That benchmark material is a sulfonated tetrafluorethylene copolymer called Nafion, produced by DuPont. Nafion, like Teflon, another DuPont product, is a linear polymer, meaning its molecules are long chains that don't branch out and can be dissolved away. Protons make their way through a fuel cell membrane by hopping from one acid group to another. But the more acid groups you attach to a linear polymer, the more water soluble it becomes—hardly an ideal situation for a technology that produces water as waste.

DeSimone had already been working on what he called the "holy grail of the fluoropolymers industry"—making a Teflon-like material that was both easy to process and had desirable surface properties. Teflon, of course, is known for its nonstick properties that make it ideal as a surface and a pain in the acid group to use in manufacturing. No one tries to make tables out of Teflon, and it's notoriously difficult to process into various products.

What DeSimone and his colleagues came up with was something they called "liquid Teflon," and it was photo-curable. "So we said, 'What would it take to make liquid Nafion?' People think of Nafion as a Teflon-like backbone with acid groups on it. So we took the liquid Teflon material and started putting acid groups on it."

More elegant than sandpaper: This nano-patterned polymer may be able to increase the surface area of a fuel cell's membrane as much as 50 times.

The resulting substance is made of two monomers, the liquid Teflon-like backbone and the acid groups, stitched together. They have to be chemically attached or the acids would wash out. The material is a thermoset, unlike Nafion, a thermoplastic, so it cures into a three-dimensional network and cannot flow again when heat or pressure is applied. And it's not water soluble.

The photo-cured mesh has many more acid groups per cubic unit, and they're more evenly dispersed than Nafion's acid groups, which tend to be clustered. In Nafion, protons can easily move around within one of those clusters, but have a harder time hopping from one cluster to another. According to DeSimone, they breeze right through the thermoset.

Not only is the material more conductive, DeSimone said, but it also performs better under more extreme conditions.

The Department of Energy has been hoping to find a way to make fuel cells work at 120°C, a temperature where catalysts do well. That's well above the temperature where Nafion breaks down—not much use to a soldier in the desert. And the desert's low humidity is another problem for Nafion, despite its near water-solubility. Since the acid groups are clustered in Nafion, protons need to ride with a water molecule. DeSimone said that his polymer, with its "sea of acid groups" can take the heat and its conductivity isn't dependent on the presence of a hydronium.

"So we basically have an easily fabricable liquid precursor to a proton exchange membrane that when cured is almost three times higher in proton conductivity than the benchmark standard," DeSimone said. "That was our big breakthrough."

'Nano Patterning'

That breakthrough, though, would lead to another one with the potential to increase a fuel cell's power per pound by much more than three times. The fact that the new polymer was photo-curable meant that the membrane could be made with the same photolithography used to make transistors on a microprocessor. This, in turn, meant that the membrane could be made into a pattern with nanoscale details.

"We saw a paper by some Stanford researchers who basically sandpapered their fuel cell—roughened it up," said DeSimone. "Seemed pretty simple. We thought an elegant approach would be to do nano patterning." Since the reaction that separates proton from electron and creates energy happens on the surface of the membrane, the more surface area, the higher the performance. "And now, instead of a flat membrane, which is what Nafion and all other fuel cells have, we increase the surface area because of the nanoscale embossing."

After etching a pattern in silicon, the crystal-clear liquid is poured onto this template. "Because of its low viscosity and excellent wetting characteristics, the liquid will wet every nook and cranny," said DeSimone. "Then we shine a light on it and peel it off."

DeSimone and his team have managed so far to increase the surface area by more than seven times, which means seven times the performance. And DeSimone said he may be able to bring that multiple up to as much as 50.

"My colleagues in the applied math department are collaborating with us to optimize the topographical pattern," DeSimone said. "Looking at the different ways of modeling and folding, it looks to be a straightforward exercise."

As is so often the case, solving one problem means creating others. Until now, everything in a fuel cell has been flat. Since the membrane was flat, the electrodes were flat. And there's no easy way to put a pattern on the electrodes.

"So we started running into mass transport problems," DeSimone said. He is now turning to organic chemistry and suspects that a solution may be found by using conductive patterned carbons.

Once these kinks are worked out, or worked in, as it were, the entire method of manufacturing fuel cells may change.

Currently, everything starts with the membrane and the rest of the fuel cell is built out from there, coating each side repeatedly. Since DeSimone's membrane is a liquid polymer, it may be possible to build a fuel cell from the outside in. Electrodes could be placed near each other and the membrane could be injected between them. The process would be a minor revolution in how fuel cells are made, and would significantly reduce their cost. "When you look at cost per watt, we can start getting in the range you need," he said.

DeSimone also hopes to increase the material's performance in humidity and test how it responds to a cycle of low and high humidity. He also hopes to look into ways of pre-aligning acid groups so protons move through channels to even further increase conductivity. Maybe the acid groups could be made even stronger than they are.

"This field has needed some orthogonal thinking," said DeSimone. "Finally, some new concepts are coming out. It's overdue."