Nanoscale ‘leaves’ produce hydrogen for fuel cells
30 December 2017

Replacing your everyday gas guzzler with a hydrogen-fueled car could drastically reduce your carbon footprint. So why don't we all make the switch? One of reason is the expensive platinum catalyst required to operate hydrogen fuel cells efficiently.

Research led by scientists at Sandia National Laboratories and the University of California (UC), Merced aims at bringing down the cost of hydrogen fuel cells by replacing expensive platinum catalysts with a dirt-cheap compound that benefits from an uneven surface resembling a plant's leaves. This additional surface area helps the compound to catalyze hydrogen almost as efficiently as platinum.

Lead researchers Stanley Chou, a Sandia materials scientist, and UC Merced's Vincent Tung have applied for a joint patent for the spray-printing process that produces the ‘leaves’ from inexpensive molybdenum disulfide. The increased surface area of the rippling ‘leaf’ creates three times as many catalytic contact points as other molybdenum disulfide structures, and the new creation can handle higher temperatures than platinum without sintering and gumming up the cell.

This work, reported in a paper in Advanced Materials, is part of an effort to power hydrogen-fueled cars more cheaply; these cars are desirable because they emit water rather than carbon monoxide or carbon dioxide.

The production method uses nature as an ally rather than a hindrance, Chou said. "In traditional thinking, forces such as gravity, viscosity and surface tension must be overcome to achieve the manufactured shapes you desire. We thought, instead of thinking of these forces as limitations, why not use them to do something useful? So, we did."

Tung said the method uses natural processes to produce materials for extremely inexpensive fuel cell terminals that liberate hydrogen. "The printing process also allows for continued deposition, with the ability to scale for industry," he said.

The team mix molybdenum disulfide with water and use the printing process to expel micrometer-size droplets into an enclosed area about two feet high. As they drop, the droplets first separate into nanoscopic subunits. These dry further as they fall, their shrinking volume producing an uneven three-dimensional (3D) surface much like the leaves of plants, with tiny ridges, hills, canals, caves and tunnels.

Landing on a substrate and on each other, the ‘leaves’ are still moist enough to bond as though attached at critical points by tiny droplets of glue. Thus, the nanostructures do not lose their individuality but instead, by maintaining their identities, create tiny tunnels within and between them that permit extraordinary access for atoms of hydrogen to seek their freedom from chemical bonds.

The inspiration for creating a bio-inspired 3D form arose from studying the cuticle folding process, a mechanism used by plants for controlling diffusion and permeability on leaf surfaces, Chou said.

"We see our catalyst as an inorganic material acting like a plant. The nanostructures, like leaves, are varied in shape, with tiny rises and falls," he explained. "The structures take in an external material to produce hydrogen rather than oxygen, and one day may be powered by sunlight." Right now, very low-voltage electricity does the job.

Doubts about the strength of the structures formed in such a serendipitous manner, Tung recounted, were settled when a 170-pound student unwittingly trod upon one of the first molybdenum disulfide-catalyst creations when it accidentally fell on the floor. A few hundred nanometers thick, it rested upon a centimeter-square carbon substrate but was otherwise unprotected. Elecromicroscopic investigation showed the tiny structure to be undamaged. The ‘leaves’ have also proved to be long lasting, continuing to produce hydrogen for six months.

This story is adapted from material from Sandia National Laboratories, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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