A Graphene-Carbon Nanotube Composite

December 3, 2014

When I worked in corporate research, one business principle that came into play often when considering mergers and acquisitions was the idea of synergy. Synergy, from the Greek word, for "working together" (synergos, συνεργος), was the concept that "the whole is greater than the sum of its parts;" that is, the combination of business entities would lead to efficiency and the possibility of combined product offerings.

Before you scoff at an operation that makes 1 + 1 = 3, you should check out the Banach–Tarski paradox. This mathematical proof, published in 1924 by the eminent pair of Polish mathematicians, Stefan Banach (1892-1945) and Alfred Tarski (1901-1983), showed that it's possible to break a sphere into a finite number of pieces, and then reassemble these pieces to make two spheres the same size as the original.[1] (Artist's impression of the Banach–Tarski Paradox by Benjamin D. Esham, via Wikimedia Commons.)

Joining dissimilar materials into composites is one way that materials scientists make a whole that's better than the sum of its parts. A common example of this is fiberglass, which is a composite of the inexpensive materials, glass fiber and epoxy. Glass, itself, is a brittle and dense material that's not much suited for the production of structural components, but drawn glass fiber in an epoxy matrix becomes a lightweight, moldable material of reasonable strength.

Glass fiber composites are being replaced in high end applications by carbon-fiber-reinforced polymer composites. Carbon is the wonder material of the past few decades, with its various allotropic forms, such as nanotubes, bucky-balls and graphene, showing unique and useful properties. This carbon revival comes just after the similarly impressive feat of commercial production of diamond-like carbon layers.

Materials scientists from Rice University (Houston, Texas), the University of Akron (Akron, Ohio), and Tsinghua University (Beijing, China) have made a composite of the nanotube and graphene carbon allotropes to form a better cathode for dye-sensitized solar cells.[2-3] As shown in the figure, this cathode is formed from vertically-aligned nanotubes that are bonded to graphene sheets.[3] I wrote about dye-sensitized solar cells, also called Graetzel cells, in a previous article (Dye-Sensitized Solar Cells, June 23, 2010).

Carbon nanotubes, bonded to a graphene substrate. (Image: Tour Group/Rice University.)[3]

Dye-sensitized solar cells are not as efficient as other types of solar cells, but they can be constructed from environmentally benign materials, such as a dye extracted from raspberries, and titanium dioxide; and, they don't need any exotic processing equipment, such as vacuum chambers and cleanrooms. The purpose of the dye is to absorb light at solar wavelengths and transfer electrons so electrical current can flow in the cell. The composite electrode replaces the brittle, and expensive, platinum electrode on tin oxide coated glass usually used in these cells since it doesn't degrade in the electrolyte.[2-3]

Research always builds on what was discovered previously, and this electrode is no exception. In 2009, Rice University chemist, Robert Hauge, discovered a method of growing vertically-aligned carbon nanotubes using catalytic particles that are raised by the nanotubes during growth to promote further growth. Then, in 2012, the method of doing the same on graphene was perfected to produce nanotubes bonded to the graphene substrate,[3] and graphene was subsequently produced on metal grid as a transparent electrode.[4]

Left, construction of a Graetzel cell using the graphene-carbon nanotube electrode. Right, scanning electron microscope image of a group of vertically-aligned carbon nanotubes. (Left image and right image, N3L Research Group/Rice University.)[3]

This composite electrode has a large surface area, which is estimated as more than 2,000 square meters per gram.[3] Since the carbon atoms of the nanotubes are bonded to the carbon atoms of the graphene substrate, the entire area is highly conductive.[3] The charge-transfer resistance of the cathode is twenty times smaller than that of platinum-based cathodes.[2-3] Solar cells fabricated at Rice had thickness up to 350 micrometers, which is the thickness of two sheets of paper, so they could be flexed easily and repeatedly.[3]

Cathodes with the longest nanotubes performed the best, and these allowed a current of nearly 18 milliamps per square centimeter. The platinum electrodes that they replace have a current density of 14 mA/cm². The dye-sensitized solar cells produced with the new electrode material had efficiencies of up to 8.2%, as compared with the 6.8% for the platinum-electrode cells.[3]

As is so much research in Texas, this research was funded by the Welch Foundation. Other support came from the Air Force Office of Scientific Research, theDepartment of Energy, Lockheed Martin, Sandia National Laboratory, and the Office of Naval Research.[3]

References:

1. Stefan Banach and Alfred Tarski, "Sur la décomposition des ensembles de points en parties respectivement congruentes". Fundamenta Mathematicae, vol. 6 (1924 ), pp. 244-277. (1.7 MB PDF File).
2. Pei Dong, Yu Zhu, Jing Zhang, Feng Hao, Jingjie Wu, Sidong Lei, Hong Lin, Robert H. Hauge, James M. Tour, and Jun Lou, "Vertically Aligned Carbon Nanotubes/Graphene Hybrid Electrode as a TCO- and Pt-Free Flexible Cathode for Application in Solar Cells," Journal of Materials Chemistry A, vol. 2, no. 48 (Oct 31, 2014), pp. 20902-20907, DOI: 10.1039/C4TA05264A.
3. Mike Williams, "Graphene/nanotube hybrid benefits flexible solar cells," Rice University Press Release, November 17, 2014.
4. Pei Dong, Yu Zhu, Jing Zhang, Cheng Peng, Zheng Yan, Lei Li, Zhiwei Peng, Gedeng Ruan, Wanyao Xiao, Hong Lin, James M. Tour, and Jun Lou “Graphene on Metal Grids as the Transparent Conductive Material for Dye Sensitized Solar Cell,” J. Phys. Chem. C, vol. 118, no. 45(October 8, 2014), pp 25863-25868, DOI: 10.1021/jp505735j.