US researchers have found a class of non-precious metal derivatives that can catalyse hydrogen fuel-cell reactions at a much lower cost than platinum.

Cobalt nitride can be used in the oxygen reduction reaction (ORR) as an electro-catalyst in alkaline fuel cells. Cobalt is 475 times less expensive than platinum, which is conventionally used in fuel cells, says Professor Hector Abruna of the Department of Chemistry and Chemical Biology at Cornell University. Most fuel cells are acidic.

Abruna and his colleagues published their findings in a paper, titled ‘Nonprecious Transition Metal Nitrides as Efficient Oxygen Reduction Electrocatalysts for Alkaline Fuel Cells’, in the peer-reviewed journal Science Advances.

Platinum or other precious metals are used in proton-exchange-membrane (PEM) fuel cells to speed up the notoriously slow oxygen reduction reaction (ORR). Platinum accelerates the process and is sufficiently stable to withstand the acidic environment of PEM fuel cells.

“Having lower-cost fuel cells would enable their broad deployment and use in numerous applications” Abruna, Cornell

But the precious metal contributes some 40pc of the fuel-cell stack cost, say the researchers. Because of the cost of platinum, researchers are studying the potential of alkaline fuel cells—which enable the use of non-precious metals.

The Cornell scientists considered transition metal nitrides as good candidates to catalyse the sluggish ORR. They are made with cobalt, manganese, iron and other transition metals.

When exposed to air, they form a thick oxide shell several nanometres thick on the conductive metal nitride core, serving as a highly active catalyst architecture. Each material was tested in a fuel cell, and cobalt nitride was the winner.

Bryan Pivovar, senior research fellow at the US’ National Renewable Energy Laboratory, cautions that it is easier to make a membrane for an acid fuel cell that is durable and high-performing than it is for an alkaline fuel cell.

Membrane problem

This ‘membrane problem’ is well-known in fuel cells. Pivovar says that, if the membrane problem were solved, alkaline fuel cells would take over. “But so far, no-one has been able to solve it,” he says

Abruna agrees and adds that membranes for alkaline fuel cells have been in development for a considerably shorter time than those for acidic environments. “Yes, we need an improvement, but I do not see it as a real roadblock,” he says.

He suggests the needed improvement will occur within five years. “Once we have the membrane problem solved, we are ready to rock and roll,” Abruna says.

Another problem for alkaline fuel cells is that they are sensitive to carbonation issues related to the presence of CO₂, says Pivovar. “The sensitivity to CO₂ will always be true, but we can mitigate it—it is not a show-stopper,” says Abruna.

The CO₂ is from the atmosphere. The best solution, Abruna says, would be to design new membrane materials and develop a protocol in which the accumulated carbonates are ejected or released at prescribed intervals. He concludes: “Having lower-cost fuel cells would enable their broad deployment and use in numerous applications including automotive, residential and industrial.”

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Author: Ros Davidson