Dec 8, 2011
Out-of-plane growth of CNTs on graphene for supercapacitor applications
A new class of graphene-based structures is essential for maximizing the material's performance in energy-storage devices. A graphene-carbon nanotube (CNT) hybrid structure has been fabricated using an atmospheric chemical vapour deposition (APCVD) process to directly grow out-of-plane CNT arrays on graphene. Electrochemical testing of the graphene-CNT hybrid structure shows high capacitance and rate performance.
While graphene shows exciting promise as a next generation material for energy-storage applications such as lithium ion batteries and supercapacitors, a modified graphene-based structure is necessary to realize its full potential as an electrode material. Publishing their results in the journal Nanotechnology, researchers at Stevens Institute of Technology have fabricated a graphene-CNT hybrid structure in order to solve the current problem of self-aggregation of graphene flakes.
Controlled suppression of carbon etching
An out-of-plane, direct growth of CNTs on planar graphene was achieved via an APCVD process by controlling competing reactions of carbon formation and hydrogenation, which allow and repress CNT growth via hydrogen etching of carbon, respectively.
In this work, cyclic voltammetry results for the graphene-CNT hybrid structure exhibited larger capacitance (490.3 µF/cm2) than graphene-only electrodes (99.6 µF/cm2) at scan rates of 300 mV/s, indicating high rate performance.
Because the CNTs were directly grown on planar graphene layers, the bulk of the exposed surface was used for electrical double layer (EDL) formation compared with the limited active surface area for EDL formation in chemically synthesized graphene-based electrode materials.
In the hybrid configuration both graphene and the CNT array serve as active materials for charge storage. In addition, the outstanding electrical properties of both materials ensure good electrical contact, a direct result of the direct growth of the CNTs on the graphene layer.
This work demonstrates the potential of graphene-CNTs hybrid structures for energy-storage applications and represents a milestone towards the group's ultimate goal of 3D multi-stack graphene structures for supercapacitor applications.
More details can be found in the journal Nanotechnology.
About the author
The work was performed in the Mechanical Engineering Department at Stevens Institute of Technology, Hoboken, New Jersey, US. The team was led by Prof. E H Yang, associate professor of Mechanical Engineering and director of the Multi-User Micro Devices Laboratory at Stevens Institute of Technology. Dr Youn-su Kim is a postdoctoral scholar under the supervision of Profs E H Yang and Frank Fisher. He is currently working on graphene-based energy-storage applications including supercapacitors and lithium ion batteries based on graphene and CNTs. Kitu Kumar is a doctoral student who is currently working on graphene and CNT-based structures for photovoltaic applications under the supervision of Profs E H Yang and M G Prasad. Dr Fisher is associate professor of Mechanical Engineering at Stevens Institute of Technology and is currently serving a two-year term as the Chair of the Polymers Technical Committee of the American Society of Mechanical Engineering (ASME) Materials Division.