Editorial Feature

The Enhanced Potential of Graphene Foam

Graphene is an allotrope of carbon, in which densely packed carbon atoms make up the vertices of a hexagonal lattice resembling a honey comb to form a thin single layered sheet. Layers of graphene stacked on top of each other with an inter-planar distance of 0.335 nm is called graphite1.

The sp2 orbital hybridization of s, px and py makes each carbon atom of graphene to form σ bond with the neighboring carbon atoms in the two-dimensional plane, where the electron in the pz orbital makes the p bond, which sticks out of the plane. The length of each C-C bonds is 1.42 A° and the μ bonds hybridize together to form a μ- band and μ* bands, which is the main reason for graphene’s special electronic properties2.

Graphene, whose single carbon-atom thickness makes this material extremely thin and light weight, with one square meter weighing only 0.77 mg, is discovered to be 100 – 300 times stronger than steel with a tensile strength of 150,000,000 psi1.

Graphene’s great mechanical strength, inordinate ability to conduct thermal and electrical energies, unusual levels of light absorption and ecofriendly nature interest researchers in the fields of electrical, electronics and material sciences to investigate its potential applications. In fact, it is currently being used in supercapacitors, lithium ion batteries, transparent conductive films, catalytic systems, and much more3. Recently, researchers at Rice University’s Department of Chemistry have developed three-dimensional rebar graphene foam (3D rebar GF).

The research group used multi-walled carbon nanotubes (MWCNT) to reinforce their previously developed 3D graphene foam (3D GF) to increase the modulus, which is necessary for its applications in fields requiring high electrical conductance and mechanical properties. Carbon nanotubes (CNT) are tube-shaped materials made of hexagonal carbon mesh, with diameters in the nanometer (nm) scale. CNTs could be either single-walled (SWCNTs), which resembles a straw with a single layer of nanotube making the wall, or multi-walled nanotubes (MWCNT), which are comprised of two or more layers of cylindrical nanotubes enclosed in one another of increasing diameter. These nanotubes have low weight, exceptional strength, great flexibility and high conductance of electrical and heat energy4.

James Tour’s lab at Rice University used a Powder Metallurgy Template method to develop 3D rebar GF. This technique involves the mechanical mixing of nickel (Ni) powder particles (2 – 3 mm), which act as a template and catalyst, sucrose, as a carbon source and multi-walled carbon nanotubes (MWCNT) plus surfactant, serving as reinforcing bars with deionized water. The water is then evaporated and the resulting mixture is cold pressed in a steel die at 1120 MP to form a pellet. This pellet is further loaded in a quartz tube furnace and heated at 1000° C under the inert atmosphere of hydrogen and argon and the pellets were further annealed at 1000 ° C for 30 min. A thin layer of carbon from the sucrose is formed around the Ni particles, which are then etched by a ferric chloride (FeCl3) solution, leaving the carbon skeleton. The matrix is further subjected to critical point drying to form the 3D rebar GF4.

Morphological characterization of the 3D rebar GF by Scanning Electron Microscopy (SEM) revealed the carbon shells and graphene sheets to be connected by MWCNTs, which was then further confirmed by Transmission Electron Microscopy (TEM). Comparing the 3D rebar GF with the previously developed 3D GF showed that MWCNTs improved the thermo-stability, conductivity and storage modulus (290 kPa Vs 18 kPa) 4. In fact, the 3D rebar GF can support more than 3,150 times its own weight with no irreversible height change, where the only height change was measured at 25% upon supporting 8,500 times its weight.  The lithium ion capacitor (LIC) produced from 3D rebar GF delivered a maximum energy density of 32 Wh/kg, which was stable for 500 cycles. Furthermore, the lithium ion capacitor showed a 78 % energy density retention at a high current density, displaying its robustness in serving as efficient and mechanically stable 3D electrodes4. Researchers are hopeful that this 3D rebar GF will inspire others to develop new designs and more robust 3D composite materials.

References:

  1. "Graphene - What Is It?" Graphenea. Web. https://www.graphenea.com/pages/graphene#.WKYtFhiZPdQ.
  2. Cooper, Daniel R.; D’Anjou, Benjamin; Ghattamaneni, Nageswara; Harack, Benjamin; Hilke, Michael; Horth, Alexandre; Majlis, Norberto; Massicotte, Mathieu; Vandsburger, Leron; Whiteway, Eric; Yu, Victor (3 November 2011). "Experimental Review of Graphene" (PDF). ISRN Condensed Matter Physics. International Scholarly Research Network. 2012: 1–56.
  3. Junwei Sha, Rodrigo V. Salvatierra, Pei Dong, Yilun Li, Seoung-Ki Lee, Tuo Wang, Chenhao Zhang, Jibo Zhang, Yongsung Ji, Pulickel M. Ajayan, Jun Lou, Naiqin Zhao, James M. Tour. Three-Dimensional Rebar Graphene. ACS Applied Materials & Interfaces, 2017; DOI: 10.1021/acsami.6b12503
  4. "What Are Carbon Nanotubes?" Nanoscience. Web. http://www.nanoscience.com/applications/education/overview/cnt-technology-overview/.

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