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Process Replaces Platinum with Iron as Fuel Cell Catalyst

H2 hydrogen icon

(Energy.gov)

Chemists at Pacific Northwest National Laboratory in Richland, Washington designed a process that makes it possible for iron to replace expensive platinum as a catalyst to make electricity in hydrogen fuel cells. The team led by PNNL’s Morris Bullock published its findings online in yesterday’s issue of the journal Nature Chemistry (paid subscription required).

Bullock, who directs the Center for Molecular Electrocatalysis at PNNL — a division of the U.S. Department of Energy — says the research can help make fuel cells a more feasible option for renewable energy. Fuel cells generate electricity out of a chemical fuel, usually hydrogen.

The bond inside hydrogen molecules store electricity, where two electrons connect two hydrogen atoms. One problem with fuel cells is that they need a catalyst to loosen that bond and release the electricity, and up to now, that catalyst has been the expensive metal platinum, which Bullock notes is “more than a thousand times more expensive than iron.”

In today’s fuel cells, platinum cracks open hydrogen molecules, releasing the electrons for capture. Platinum has a chemical composition to perform this function, but not inexpensive elements like iron, which keeps iron from simply replacing platinum in fuel cells. Some elements in nature, called hydrogenases, work with iron to perform this function, which is where the PNNL team began its inquiry.

The PNNL team created several potential hydrogenase molecules to test. For these tests, the researchers manipulated the iron and hydrogen molecules at the atomic and sub-atomic levels, by interacting the iron, hydrogen, and hydrogenase molecules. Those tests found a compound that induced iron to break off one of the hydrogen protons — a hydrogen molecule has two protons and two electrons — and sending it to a proton acceptor molecule. In a real fuel cell, the acceptor would be oxygen.

Breaking off that proton makes it easier for the corresponding electron to be attracted to a fuel cell’s electrode. The process then is repeated for the second pair of hydrogen proton and electron. The team tested catalysts of different shapes and sizes, as well as various proton acceptors, to engineer a process as fast and efficient as today’s commercial catalysts.

The optimum design placed iron in the middle of the cell, with arms hanging around the edges to draw away the protons. With this design, the device processed two molecules per second, which the researchers say is thousands of times faster than the closest, non-electricity making iron-based competitor. They also gauged the design’s overpotential, a measure of efficiency, at 160 to 220 millivolts that the team says is similar in efficiency to most commercially available catalysts.

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