Nanotechnology Now - Press Release: "Graphene/nanotube hybrid benefits flexible solar cells: Rice University labs create novel electrode for dye-sensitized cells"

The Rice lab of materials scientist Jun Lou created the new cathode, one of the two electrodes in batteries, from nanotubes that are seamlessly bonded to graphene and replaces the expensive and brittle platinum-based materials often used in earlier versions.

The discovery was reported online in the Royal Society of Chemistry's Journal of Materials Chemistry A.

Dye-sensitized solar cells have been in development since 1988 and have been the subject of countless high school chemistry class experiments. They employ cheap organic dyes, drawn  from the likes of raspberries, which cover conductive titanium dioxide particles. The dyes  absorb photons and produce electrons that flow out of the cell for use; a return line  completes the circuit to the cathode that combines with an iodine-based electrolyte to refresh the dye.

While they are not nearly as efficient as silicon-based solar cells in collecting sunlight and transforming it into electricity, dye-sensitized solar cells have advantages for many applications, according to co-lead author Pei Dong, a postdoctoral researcher in Lou's lab.

"The first is that they're low-cost, because they can be fabricated in a normal area," Dong said. "There's no need for a clean room. They're semi-transparent, so they can be applied to glass, and they can be used in dim light; they will even work on a cloudy day.

"Or indoors," Lou said. "One company commercializing dye-sensitized cells is embedding them in computer keyboards and mice so you never have to install batteries. Normal room light is sufficient to keep them alive."

The breakthrough extends a stream of nanotechnology research at Rice that began with chemist Robert Hauge's 2009 invention of a "flying carpet" technique to grow very long bundles of aligned carbon nanotubes. In his process, the nanotubes remained attached to the surface substrate but pushed the catalyst up as they grew.

The graphene/nanotube hybrid came along two years ago. Dubbed "James' bond" in honor of its inventor, Rice chemist James Tour, the hybrid features a seamless transition from graphene to nanotube. The graphene base is grown via chemical vapor deposition and a catalyst is arranged in a pattern on top. When heated again, carbon atoms in an aerosol feedstock attach themselves to the graphene at the catalyst, which lifts off and allows the new nanotubes to grow. When the nanotubes stop growing, the remaining catalyst (the "carpet") acts as a cap and keeps the nanotubes from tangling.

The hybrid material solves two issues that have held back commercial application of dye-sensitized solar cells, Lou said. First, the graphene and nanotubes are grown directly onto the nickel substrate that serves as an electrode, eliminating adhesion issues that plagued the transfer of platinum catalysts to common electrodes like transparent conducting oxide.

Second, the hybrid also has less contact resistance with the electrolyte, allowing electrons to flow more freely. The new cathode's charge-transfer resistance, which determines how well electrons cross from the electrode to the electrolyte, was found to be 20 times smaller than for platinum-based cathodes, Lou said.

The key appears to be the hybrid's huge surface area, estimated at more than 2,000 square meters per gram. With no interruption in the atomic bonds between nanotubes and graphene, the material's entire area, inside and out, becomes one large surface. This gives the electrolyte plenty of opportunity to make contact and provides a highly conductive path for electrons.

Nanotechnology Now - Press Release: "Graphene/nanotube hybrid benefits flexible solar cells: Rice University labs create novel electrode for dye-sensitized cells"