"Running on Air," Feature Article, January 2004

running on air

Consider a fuel cell car stuck in heavy traffic. Will it choke to death?

by Lisa Kosanovic

Three years ago, Eivind Stenersen attended a meeting of the Fuel Cell Standards Committee of the Society of Automotive Engineers. Stenersen, an engineer at Donaldson Co., was working on air filters for fuel cells.

When Stenersen introduced himself, the committee chairman, a retired auto industry executive, looked surprised. "What are you guys doing here?" he asked. "Fuel cells don't need air filters."

People at Donaldson say they're used to hearing that, even from those who should know better. But, according to the firm, nothing could be further from the truth.

Donaldson, a Minneapolis-based developer of filtration products, says air filters keep a fuel cell free of airborne contaminants, which can kill a proton exchange membrane fuel cell quickly. Sulfur dioxide at 5 parts per million, for example, can kill one in about three and a half hours, according to Stenersen. PEM cell manufacturers are aiming for products that will last 5,000 hours in automobiles and between 40,000 and 50,000 hours in power plants.

Five parts per million is uncommonly contaminated air, but Donaldson points out that a fuel cell powering a car might be exposed to that level of sulfur dioxide in New York City traffic near a truck, for example.

Although much attention has been paid to fuel stream cleanliness, there has been little focus on the cleanliness of intake air, which is critical to PEM fuel cell operation in two ways. First, air supplies the oxygen needed to complete the electrochemical reaction that produces electricity. Second, air carries water, a byproduct of the process, out of the fuel cell. Otherwise, water would flood the cell and prevent oxygen from doing its part.

Donaldson, in cooperation with Los Alamos National Laboratory, is trying to determine how different air contaminants affect a PEM cell: how quickly they degrade its performance, and the mechanism by which they damage it, for example. The research, which is funded by the Department of Energy and Donaldson, covers a list of 15 contaminants, including hydrocarbons, seawater, diesel soot, and base gases such as ammonia.

Since the study began about a year ago, researchers have mostly completed their work on dust, sulfur dioxide, nitrogen dioxide, and salt. The full report is expected next spring, but because Donaldson is funding much of the work, details on the procedures are confidential until then.

Stenersen has been willing to say only that the study establishes baseline performance for a cell, by running it on clean, filtered air. Once the baseline is established, he said, researchers expose it to one of the contaminants under study, at various concentrations. If the cell's performance degrades, researchers then expose it to clean air to determine whether it can recover.

NATURALLY ELEGANT

Los Alamos scientist Francisco Uribe believes that proton exchange membrane fuel cells are much more elegant than combustors, because they operate the way nature operates. Burning is an inefficient, brute-force method of extracting energy from fuel, Uribe said, but using electrochemical reactions to draw chemical energy out of the fuel is similar to what the human body does to get energy. And like the human body, a PEM cell can recover if it is exposed to fresh air after poisoning from certain contaminants, such as carbon monoxide.

Recovery is an important feature for proton exchange membranes, in part because they are considered the best type of fuel cell for cars. Since cars move through different environments while they are operating, a fuel cell that can recover after hours in heavily polluted air—in bumper-to-bumper traffic, for example—is a much more practical product than one that can't.

One of the goals, then, is to determine whether a PEM fuel cell can recover from each of the contaminants that damages it. The Los Alamos team is finding that some surprising things matter when it comes to recovery.

A PEM cell that has been poisoned with sulfur dioxide, for example, cannot recover if it is exposed to fresh air afterward, no matter what the concentration of sulfur dioxide and the time of exposure were, according to Ken Stroh, the lab's program manager for hydrogen, fuel cell, and transportation programs.

Donaldson says its filters clean cathode air of SO_x, NO_x, NH₄, and volatile organic compounds.

But concentration and time seem to play a role when the contaminant is nitrogen dioxide, Stroh said. A PEM cell stopped working after it was exposed to nitrogen dioxide at 400 parts per billion for 500 hours, and did not recover after it was given clean air for 70 hours. But the same cell, which stopped working after exposure to more than 1,000 times the concentration of nitrogen dioxide, 5 parts per million, for 18 hours, recovered fully after an hour or two of exposure to clean air.

Stroh said that researchers were surprised by the result, and are studying it in more detail. They believe the difference comes from the kill mechanism: Short-term exposure to higher concentrations of nitrogen dioxide appears to have a surface effect on the PEM cell, while long-term exposure to lower concentrations might allow the contaminant to migrate more deeply into the membrane.

"Of all the things that we're doing, this is the most interesting to the automotive companies because they recognize it is critical to commercialization," Stroh said.

When fresh air doesn't get a PEM cell back on its feet, the team uses cyclic voltammetry, an analytical technique that surveys the status of the catalyst surface and indicates which contaminant has compromised the cell. It also cleans away the poison from the catalyst surface, using electrochemical oxidation. A cell that was poisoned with 5 ppm sulfur dioxide recovered full performance after cyclic voltammetry at the cathode.

Uribe said it is conceivable that auto mechanics in a fuel cell world could use cyclic voltammetry to get an automobile up and running, but it is not the best method because the equipment is expensive.

A cheaper, easier method is to simply apply a positive voltage to the cell, but that requires knowing the offending contaminant. That's because different contaminants require different cleaning voltages. Hydrogen sulfide, for example, requires a higher potential than carbon monoxide. Uribe said the group is still exploring other possible means, though any solution would probably require a driver to leave his car at the garage.

Researchers at Los Alamos and Donaldson have been working on the problem of intake-air contamination for years. In August 2002, Donaldson was awarded a patent for an air filter that uses an impregnated activated carbon adsorption medium to remove ammonia, amines, acidic gases, and organic materials, and oxidizes contaminants with a proprietary catalyst.

According to Donaldson, it makes filters of all sizes that clean air-streams up to 10,000 standard cfm of particulates and unwanted chemicals.

Despite all its work, though, Donaldson is only producing prototype filters because fuel cells are so new, according to Ric Canepa, director of Donaldson's fuel cell contamination control division. The company has already produced about 50 different prototype filters, ranging from an 8-millimeter spherical filter for a fuel cell that replaces a laptop battery, to a large filter that handles over 10,000 cfm for a small power plant.

Every application has its own requirements and its own challenges, Canepa said. In the auto industry, for example, filters must be small because of space limitations.

Another important issue for cars is packaging. Cars have typically been designed so that the air filter is readily accessible, since standard air filters must be replaced frequently. But Donaldson aims to develop a fuel cell air filter that can last about 150,000 miles, and that would make accessibility a much lower priority.

In turn, that opens the door to all sorts of new and interesting automotive designs, Canepa said.

All the components under the hood of a combustion-engine automobile could be replaced with electrical systems that fit neatly under the car, as with General Motors' Hy-Wire design, unveiled in 2002. The Hy-Wire has been compared to a skateboard that holds all the working parts of the car underneath the passenger compartment. There is no engine in front, no pedals, and no steering wheel, so the car has greater visibility and more legroom.

"You have to think about what they'll need 15 to 20 years from now," Canepa said. "If you look at the future of what fuel cells could be, you want them to have that flexibility."

CLEAN COOLING

In addition to its role in the electrochemical reaction, air plays an important supporting role in fuel cell operation. Perhaps half the energy released by a fuel cell's electrochemical reaction takes the form of heat, and cooling air is needed to prevent the cell from overheating and shutting down. As with intake air, cooling air must be clean.

That's where a different sort of air filter comes in.

Universal Air Filter in Sauget, Ill., is making prototypes that clean cooling air for fuel cells. Unlike the Donaldson filter, which must filter chemical and particulate contaminants, the cooling air filter need only address particulates, which it removes in the range down to 5 or 10 microns, according to Dan Krupp, UAF's director of engineering.

The test bench at Los Alamos where Donaldson's fuel cell air filter is under study.

Moreover, the filter need not have a long life, since it is generally used in stationary applications. It can simply be cleaned every three to six months, and replaced every two to three years, Krupp said.

Fuel cell filters were a natural transition for UAF, which specializes in air cooling of electronics. The filtered air must meet ANSI standards for electronic equipment, Krupp said, but there are no requirements particular to fuel cells.

Like Donaldson's filters, the UAF product is still in the prototype stage because the fuel cell industry is in its infancy. Each application is different, according to Krupp, with its own heat dissipation, power output, and enclosure requirements.

Lisa Kosanovic is a freelance science writer, specializing in energy. She has a master's degree in mechanical engineering from the University of Massachusetts and lives in Amherst, Mass.