A new polymer, shown in powdered form, can be used to make stable fuel-cell membranes that conduct negatively charged ions.
Fuel cells are, in principle, the most efficient way to convert hydrogen fuel into electricity. But they require expensive catalysts such as platinum to split hydrogen into ions and electrical current. Cheaper metals simply can’t withstand the harsh acidic environment of the fuel cell. Now researchers in China have developed a fuel cell that uses a new membrane material to operate in alkaline conditions, eliminating the need for an expensive catalyst. The power output of the new prototype, which uses nickel as a catalyst, is still relatively low, but it provides a first demonstration of a potentially much less expensive fuel cell.
Conventional fuel cells consist of two electrodes coated with a platinum catalyst that splits hydrogen fuel into acidic hydrogen ions and electrons. The electrodes are separated by a polymer membrane that conducts acidic hydrogen ions from one side to the other, creating an external electrical current. The new fuel cell, developed by researchers led by Lin Zhuang, a professor of chemistry at Wuhan University, in Wuhan, China, uses a new membrane that conducts alkaline ions called hydroxyl groups. Alkaline fuel cells work by reacting hydrogen and oxygen to create hydroxyl ions and water, a reaction catalyzed in the Wuhan University fuel cell by the nickel anode. The hydroxyl ions are conducted across the polymer membrane, generating an external electrical current.
Most researchers have been focused on acidic fuel cells because membranes that work well under such conditions have already been developed. A stable hydroxyl-conducting membrane has been “the holy grail of electrochemistry,” says Robert Savinell, a professor of chemical engineering at Case Western Reserve University, in Cleveland. Such a membrane would allow researchers to build fuel cells and batteries that don’t require precious-metal catalysts but can use cheaper ones like nickel.
Zhuang’s polymer is comparable in structure to the highly conductive polymer Nafion that’s used in conventional acidic fuel cells. It may prove to be less expensive than Nafion, which must be fortified with fluorine groups to protect it from acidic conditions. Other researchers are working on improving the power output and lowering the cost of acidic fuel cells by developing alternatives to Nafion, but these cells still require expensive catalysts.
Zhuang’s group demonstrated the new membrane in an alkaline fuel cell that uses a silver cathode and a hydrogen-splitting nickel anode as the catalyst. The nickel catalysts used in previously developed alkaline fuel cells weren’t very efficient because they quickly got oxidized, so alkaline fuel cells have used the same platinum catalysts as their acidic counterparts. The Wuhan researchers created an anode coated with nickel nanoparticles decorated with chromium that’s more tolerant to oxidation than previous nickel catalysts.
The power output of the new fuel cell–about 50 milliwatts per square centimeter at 60 ºC–is modest. But as the first demonstration of an alkaline fuel cell that doesn’t require expensive metal catalysts, it’s an important proof of principle, researchers say. Fuel cells have a long way to go in terms of efficiency, long-term stability, and expense, says Frank DiSalvo, a professor of physical science at Cornell University, in Ithaca, NY. “This work enhances the research tool kit of materials we can explore to see if we can deliver on fuel-cell efficiency,” he says.
Zhuang says that he and his group are working on improving the cell’s power output by further tuning the catalyst and the membrane. They’ll also have to demonstrate the long-term stability of the cell. “We believe that catalysts with higher activity and lower cost will soon be realized,” he says.