LIQUID CONTROL: By using its waste water to regulate the size of the reaction, scientists have created a fuel cell that produces more power when it is given more hydrogen. The fuel cell itself is pictured as a little gray box in the middle. Image: COURTESY OF JAY BENZIGER
Fuel cells powered by hydrogen are delicate systems. Too little water in the system and the special membranes that allow electricity to be generated dry up, shutting the chemical reaction down. Too much water and the droplets can block the hydrogen gas from interacting with the electrodes, shutting the chemical reaction down. But researchers at Princeton University have discovered ways to make such fuel cells hardier by making them both self-draining and self-regulating, according to a paper in the February Chemical Engineering Science.
Claire Woo, an undergraduate student in the lab of chemical engineer Jay Benziger, attempted to find a way to make the lab's patented self-draining fuel cell 100 percent efficient, using just as much hydrogen gas as was supplied. Typical commercial fuel cells only use up to 40 percent of the hydrogen and require recycling systems to bring unused gas back into the reaction chamber. Instead of such a recycling system, Woo added a water tank to collect any excess formed during the chemical reaction. Because gravity already pulls any forming water down into a pool at the bottom of the reaction chamber, Woo could control the overall size of the reaction by the amount of hydrogen gas she allowed to flow into the chamber.
When she increased the flow of hydrogen, it pushed excess water out of the chamber and freed up more of the cell for power production. Conversely, when she added less fuel, more water filled the chamber, blocking part of the reactive surface. "The water would flow in and out of the fuel cell to give a variable area where its size was matched to the power," Benziger says. "Instead of throwing away all of the water, we're letting some of it continue to accumulate."
Of course, this closed fuel cell system is still a prototype on a laboratory bench, but already some of its elements have been found in larger, commercial efforts. For example, Honda's new FCX prototype also relies on a vertical alignment to allow gravity to drain water away. "Honda only had it half right," Benziger notes. "Honda still had to humidify their feeds."
The Princeton prototype does not have to add water because it does not rely on channels to guide the hydrogen gas where it needs to go. Instead, it flows through a series of pillars on its way across the electrode and membrane. By combining such "dry" feeds with no need for recycling—and a way to match power output to fuel input—Benziger and his colleagues have developed a fuel cell that is economical across a broad range of uses. Plus, Benziger says, it has plenty of room to grow: "There is no inherent problem with scaling it up."