Small steps toward big energy gains

New studies with different fuel cell catalysts show promising results

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As the automotive industry is betting that hydrogen can become the fuel of the future, technology is taking steps to bring that hope closer to reality. Three papers being published by the journal Science promise to fill some of the most significant gaps in what could someday be an environmentally friendly cycle of hydrogen production and consumption.

In a paper published online July 31 in Science and in an upcoming issue of the journal, Matthew Kanan and Daniel Nocera of the Massachusetts Institute of Technology report the discovery of a chemistry trick that could make it vastly more efficient than it is now to extract hydrogen from water. Nocera says that the method should make it feasible to store large amounts of energy obtained from renewable sources that are not available 24/7, such as solar or wind. The hydrogen could also be used as a clean fuel for cars, whose exhaust would be just water vapor.

Kanan and Nocera improve on the time-honored method of electrolysis, which uses an electrical current to split water into oxygen and hydrogen. The current runs between two electrodes dipped in water. At one electrode, a platinum coating facilitates the break up of water molecules, which forms oxygen molecules, electrons and protons (hydrogen nuclei). The electrode scoops up the electrons, which then travel to the other electrode via a wire. Meanwhile, the protons travel through the water to the other electrode, where they recombine with the electrons to form hydrogen gas.

Once the hydrogen fuel has been made, it can be recombined with oxygen and made into water again, in a fuel cell for example. This making of water molecules produces an electrical current that can power a car.

In principle, electrolysis could store almost as much energy in hydrogen — as the energy that would be released when hydrogen combines again with oxygen to form water — as it uses to split water. But existing electrolytic cells are inefficient, Nocera says, and end up wasting too much energy as heat. Also, platinum, the catalyst most commonly used, is scarce and very expensive.

Nocera and Kanan have found that replacing platinum with cobalt dramatically lowers the energy needed to split water molecules at the positive electrode. “It’s making oxygen at the bare minimum energy required,” Nocera says, although the chemistry involved is still not completely clear.

Nocera says he can’t precisely quantify the efficiency of a cobalt-based device until he builds a fully engineered electrolysis cell. “Give us a few more weeks,” he says.

Thomas Moore of ArizonaStateUniversity in Tempe says the team’s result is the type of breakthrough that’s needed “if humans are ever going to achieve a sustainable production of energy.”

Platinum is also commonly used on the consumption side, in the fuel cells that turn hydrogen back into water and produce electric currents. In Science‘s August 1 issue, researchers at Monash University in Australia propose replacing metal-based catalysts in the electrodes of hydrogen fuel cells with Gore-Tex membranes coated with an electrically conducting polymer.

The polymer acts as the catalyst, while the microscopic pores that make Gore-Tex a breathable fabric increase the membrane’s surface area, and thus the rate of the chemical reaction on the catalyst surface.

Polymers have an advantage over metallic catalysts such as platinum, says study coauthor Douglas MacFarlane. As air flows into the fuel cell to deliver oxygen, carbon monoxide also seeps in and degrades a metal catalyst’s performance. “Almost any metal will have problems with carbon monoxide,” he says. But the polymer his team used did not.

In their tests, the team’s polymer electrodes performed head-to-head with platinum electrodes, producing comparable amounts of electrical currents.

But the conditions of the tests somehow seem to have made platinum into a lousy competitor, according to Piotr Zelenay of Los Alamos National Laboratory in New Mexico, so that the benchmark was platinum electrodes with severely impaired catalytic activity. In his view, it is doubtful that the team’s approach could produce viable catalysts for fuel cells.

Also in Science’s August 1 issue, Jacobo Santamaría of the ComplutenseUniversity in Madrid and his collaborators describe an advancement made on a different type of fuel cell called a solid oxide cell. Solid oxide cells typically use ceramic materials to transfer electricity between electrodes. They are among the most efficient fuel cells, but they only work at temperatures of 700° Celsius or more, which limits their applications. Now Santamaría’s team has created a new material that’s based on the ceramics’ same elements — zirconium, strontium and titanium — but has a more orderly, crystal structure. Alberto Rivera-Calzada, one of Santamaría’s colleagues, says the new material works at just 84° C.

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