Sep 8 2015
A group of researchers has developed an innovative, one-step process for fabricating perfect carbon-based nanomaterials, which maintain excellent mechanical, electrical and thermal characteristics in 3D.
The latest breakthrough holds promise for achieving better energy storage in supercapacitors and high efficiency batteries, thus improving energy conversion efficiency in lightweight thermal coatings, solar cells, etc. The research has appeared in the online journal Science Advances.
In prior experimental testing, a 3D supercapacitor resembling a fiber and made with continuous fibers of graphene and carbon nanotubes equaled or surpassed the record-high capacities reported for this kind of device. In a dye-sensitized solar cell, the material was utilized as a counter electrode that not only allowed the cell to change power with 6.8% efficiency, but also considerably increased the performance of a similar cell, which utilized a costly platinum wire counter electrode.
Carbon nanotubes can be extremely conductive along 2D graphene sheets in the 2D plane and 1D nanotube length. However, with respect to the 3D world, the materials prove unsuitable because of reduced interlayer conductivity, similar to dual-step processes used for bonding graphene and nanotubes into 3D.
"Two-step processes our lab and others developed earlier lack a seamless interface and, therefore, lack the conductance sought," stated Liming Dai, who headed the study and is also the Kent Hale Smith Professor of Macromolecular Science and Engineering at Case Western Reserve University. "In our one-step process, the interface is made with carbon-to-carbon bonding so it looks as if it's one single graphene sheet," Dai said. "That makes it an excellent thermal and electrical conductor in all planes."
For almost four years, Dai worked with Yong Ding, a senior research scientist from Georgia Institute of Technology; Zhong Lin Wang, the Hightower Chair in Materials Science and Engineering, Ajit Roy, principal materials research engineer in the Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton; Zhenhai Xia, professor of materials science and engineering from the University of North Texas and others on a U.S. Department of Defense-Multidisciplinary University Research Initiative or MURI program (Joycelyn Harrison, Program Manager).
Dai also worked with Yuhua Xue, CWRU’s Research Associate and also visiting scholar from the Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical University, along with professors Hao Chen and Jia Qu from the Wenzhou Medical University.
In order to fabricate the 3D material, the research team first etched radially aligned nanoholes along the circumference and length of a small aluminum wire and then utilized chemical vapor deposition (CVD) process to coat the surface with graphene without using any metal catalyst which may possibly remain in the structure.
"Radially-aligned nanotubes grow in the holes. The graphene that sheathes the wire and nanotube arrays are covalently bonded, forming pure carbon-to-carbon nodal junctions that minimize thermal and electrical resistance," Wang said.
According to the researchers, the design results in a large surface area that adds to the transport properties. They subsequently used the Brunauer, Emmett and Teller theory to determine this architectural surface area and found it to be almost 527m2/g of material.
Following testing, the material was found to be a suitable electrode for high-efficient energy storage. Capacitance by length reached up to 23.9 millifarads/cm and by area as high as 89.4 millifarads/cm2 in the fiber-resembling supercapacitor.
It is possible to customize the properties. In fact, through the one-step process, the material can be converted into a tube with a narrower/wider diameter or can be made relatively long. The nanotubes’ density can be changed to yield materials having different properties to meet different requirements.
The material thus obtained could be utilized for charge storage in batteries and capacitors or the huge surface area could allow for hydrogen storage.
"The properties could be used for an even wider variety of applications, including sensitive sensors, wearable electronics, thermal management and multifunctional aerospace systems", Roy said.
The researchers are studying the properties, which can be obtained from the 3D graphene layer fibers and also devising a procedure for fabricating multilayer fibers.