Fuel Cells in Your Future?
by Marcia Goodrich
There's a reason we're not all driving around in hydrogen-powered SUVs just yet. The perfect fuel cell design is a finicky as Goldilocks. It can't be too hot or too cold, too wet or too dry. And, with water as the only by-product, being too wet is proving to be a real problem.
"A fuel cell can drown in a teaspoon of water," says Jeffrey Allen, "that's all it takes."
Allen, an assistant professor of mechanical engineering-engineering mechanics, is Michigan Tech’s lead investigator on a $2.7-million contract with the US Department of Energy. He and his team are using nanotechnology, physics, materials science, and fluid dynamics to build a better fuel cell. In particular, they are focusing on water management. “It’s about getting rid of the liquid,” says Allen.
Under the DOE contract, they are collaborating with scientists and engineers at the Rochester Institute of Technology and General Motors. The RIT researchers will incorporate Michigan Tech’s findings into a fuel cell design. If the design shows improved performance, then GM will build their innovations into a small, working fuel cell.
Fuel cells work by using a catalyst such as platinum at the anode (see diagram) to split hydrogen gas into its component electrons and protons. The electrons are diverted through a circuit, creating the electricity used to drive the motor. The protons hook up with the oxygen in air at the cathode to produce water, the fuel cell’s exhaust.
This process depends on keeping one very important component damp but not soggy: the polymer electrolyte membrane, or PEM, the center layer, which allows protons (hydrogen without its electrons) to pass through, but blocks the electrons that have to move through the electric circuit outside of the fuel cell. If the PEM gets dry spots, it essentially shorts out, diverting electricity from the power system and developing hot spots that damage the fuel cell.
Not Too Dry, Not Too Wet
On the other hand, if the PEM gets too wet, it can leak too much water out through the gas diffusion layers into the bipolar plates. These two plates are networked with tiny channels that bring in hydrogen on one side and air on the other. If the gas diffusion layers and the bipolar plates get flooded with water, that can stop the flow of the gases—and stop the engine in its tracks.
Too much water in a fuel cell poses another problem, especially in winter. “GM doesn’t want one of their engines to freeze in the parking lot,” Allen notes.
In a lab setting under perfect conditions, a fuel cell can keep on working trouble-free for 25,000 hours, about five times longer than the automotive industry standard. However, under real driving conditions, fuel cells fail one way or another much faster, in about 1,500 hours.
“When you stomp on the accelerator, you generate a lot of water,” Allen explains. “Then, in idle mode, there’s no air flow to push all that water out.” The anode and cathode can flood, grinding the reaction to a halt.
Allen’s specialty is capillary action, the movement of liquids through very tiny channels, such as those in the gas diffusion layers and the bipolar plates. Holding a bipolar plate, he points out the intricate web of narrow channels that carry hydrogen, air, and water throughout a fuel cell. If you were designing a system to trap water, you couldn’t do much better.
“I’m trying to understand the basic phenomena,” he says. “The passages that the water has to move through are only about a millimeter across, so water is clinging all over the place.
“What most engineers are doing is trying to fight the water, because capillary forces are causing it to hang up. However, we think we can take advantage of those same forces to move the water around.”
Associate Professor Jason Keith examines the fuel cells on the eGator, which Alternative Fuels Enterprise students have been testing.
To do that, Allen’s team is investigating the “wetability” of the channel walls, to make them more slippery, and changing their geometry to facilitate flow.
“In the last twenty years, we’ve overcome many problems in fuel cell design,” says Allen. “Now water management is percolating up; it’s the tall pole. I think we’re in a position now where we can make a huge impact in the technology.”
You Can't Stack Them Too High
Jason Keith, an associate professor of chemical engineering, researches fuel cells and explains them in simple language. To power a car, a Dagwood sandwich of fuel cells is required, he says.
“To power many electric motors, you need about 300 volts, and each cell in a fuel cell generates less than one volt. So, you may need about 430 cells to run a car, which is too huge,” he says.
Since 2004, he has taught a course on the topic, and he has also been developing modules to introduce fuel cell concepts into all the courses in the chemical engineering curriculum. “It’s a hot topic, and it’s not in most texts,” he notes.
He also advises undergraduates in the Alternative Fuels Enterprise, which adapted an electric utility vehicle, a well-used John Deere eGator (pictured on page 9) whose batteries had finally died, to run on a hydrogen fuel cell.
With zero greenhouse gas emissions, fuel cells offer an intriguing alternative to internal combustion engines. But that doesn’t make them sustainable, says Keith. Currently, you can’t do much with the hundreds of bipolar plates in a fuel cell stack once they fail other than put them in a landfill. Keith and his colleague Julia King, a professor of chemical engineering, are trying to develop better bipolar plates that can be recycled.
“We’re using a thermoplastic polymer, with multiple carbon fillers, that can be remelted,” Keith says. By mixing carbon black, synthetic graphite, and carbon fiber in the filler, they are also creating bipolar plates that are better at cooling the cell and provide improved electrical conductivity.
Mini-Robot Vehicles Provide More Answers (And Questions)
Fuel cells aren’t limited to passenger vehicles. Supported by the Army Research Lab, Jay Meldrum, director of the Keweenaw Research Center, built a small, unmanned vehicle (“Little Brother”) for them and powered it with hydrogen.
Meldrum is now working on a second project for the Army Research Lab, a 5,000-pound hybrid vehicle (“Big Brother”) powered by diesel and hydrogen fuel cells. For this, he is tricking out a Bobcat loader of the type often seen at construction sites, but powered by an off-the-shelf fuel cell engine.
Working with them firsthand, Meldrum can count off the problems with fuel cells. Pound for pound, they don’t pack the power of petroleum or even batteries. They don’t work well in cold weather because the watery exhaust freezes, along with the engine. “And how do you refuel with hydrogen?”
But even though the technology has a ways to go, there are niches where fuel cells can shine even now. “The fact that they don’t emit any toxic gases is a very good thing, and it makes them especially useful for inside work, as in warehouses,” Meldrum says.
Too Expensive To Save the World?
The value of fuel cells, both as an alternative to petroleum and as an antidote to global warming, depends on your assumptions, says Allen.
Assume, for instance, that you are producing hydrogen with electricity from a coal-fired power plant. Under that scenario, fuel cell technology simply pushes the emissions up the energy chain. Then your fuel cell-powered vehicle becomes a de facto producer of greenhouse gases, Allen admits.
Little Brother (left) and Big Brother (right) unmanned vehicles, operated by fuels cells, are being designed, built, and tested at the Keweenaw Research Center.
“But if you are using wind, solar, or nuclear power, then making hydrogen is very clean,” he says. “There’s no carbon monoxide, no carbon dioxide, and no soot.”
Then there’s the matter of sticker shock. Fuel cell vehicles are probably not yet gracing the lots at your local dealership, but once they get there, prepare to open wide your wallet. Honda has announced that it will lease a limited number of its fuel cell sedans in Los Angeles for monthly payments of $600.
Assume that the value of fuel cell vehicles can only be calculated in the cost of driving one, and they don’t stack up, even in an era of three-dollar-a-gallon gas. Plus, there is the matter of hydrogen stations (what hydrogen stations?).
Heavy considerations, but broaden your economic assumptions, and the penalties paid for not moving to fuel cell vehicles could be much weightier, says Allen. The bottom line of climate change, from record droughts to flooded coastlines, could make hydrogen seem like the bargain of the century.