How doped nanomaterials could cut fuel cell costs

10 January 2018

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Research indicates that effective, less costly replacements for expensive platinum in fuel cells are nitrogen-doped carbon nanotubes or modified graphene nanoribbons.

In fuel cells, platinum is used for fast oxygen reduction, the key reaction that transforms chemical energy into electricity.

The research findings come from computer simulations scientists created to see how carbon nanomaterials could be improved for fuel cell cathodes. Their study reveals the atom-level mechanisms by which doped nanomaterials catalyse oxygen reduction reactions (ORR).

Doping with nitrogen

Boris Yakobson, a professor of materials science and nanoengineering and of chemistry at Rice University and his colleagues are among many researchers looking for a way to speed up ORR for fuel cells, which were discovered in the 19th century but not widely used until the latter part of the 20th. Fuel cells have since powered transportation modes ranging from cars and buses to spacecraft.

The researchers used computer simulations to discover why graphene nanoribbons and carbon nanotubes modified with nitrogen and/or boron, long studied as a substitute for expensive platinum, are so sluggish and how they can be improved.

Doping, or chemically modifying, conductive nanotubes or nanoribbons changes their chemical bonding characteristics. They can then be used as cathodes in proton-exchange membrane fuel cells. In a simple fuel cell, anodes draw in hydrogen fuel and separate it into protons and electrons. While the negative electrons flow out as usable current, the positive protons are drawn to the cathode, where they recombine with returning electrons and oxygen to produce water.

The models showed that thinner carbon nanotubes with a relatively high concentration of nitrogen would perform best, as oxygen atoms readily bond to the carbon atom nearest the nitrogen.

Tubes versus ribbons

Nanotubes have an advantage over nanoribbons because of their curvature, which distorts chemical bonds around their circumference and leads to easier binding, the researchers found.

The challenging part is making a catalyst that is neither too strong nor too weak as it bonds with oxygen. The curve of the nanotube provides a way to tune the nanotubes’ binding energy, according to the researchers, who determined that ‘ultrathin’ nanotubes with a radius between 7 and 10 angstroms would be ideal. (An angstrom is one ten-billionth of a meter; for comparison, a typical atom is about 1 angstrom in diameter.)

The researchers report their findings in the journal Nanoscale.

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