In a research funded by a U.S. Department of Defense-Multidisciplinary University Research Initiative grant and Wenzhou Medical University, an international team of scientists has developed what is referred to as the first one-step process for making seamless carbon-based nanomaterials that possess superior thermal, electrical and mechanical properties in 3D. The research may hold potential for increased energy storage in high efficiency batteries and supercapacitors, increasing the efficiency of energy conversion in solar cells, for lightweight thermal coatings and more. 

The group's early testing showed that a 3D fiber-like supercapacitor made with uninterrupted fibers of carbon nanotubes and graphene matched or even surpassed bettered the reported record-high capacities for such devices. When tested as a counter electrode in a dye-sensitized solar cell, the material enabled the cell to convert power with up to 6.8% efficiency and more than doubled the performance of a similar cell that used an expensive platinum wire counter electrode. 

Both CNTs and graphene seem to perform better in their 2D forms, but fall somewhat short in a 3D scheme due to factors like poor interlayer conductivity, as do two-step processes that attempt to combine nanotubes and graphene into three dimensions but often lack a seamless interface and, therefore, lack in conductance. But in this new one-step process the interface is made with carbon-to-carbon bonding so it looks as if it’s one single graphene sheet, gaining excellent thermal and electrical conduction in all planes. 

Making the 3D material involved etching radially aligned nanoholes along the length and circumference of a tiny aluminum wire, then using CVD to cover the surface with graphene using no metal catalyst that could remain in the structure. The scientists explain that 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. The architecture yields a large surface area, adding to the transport properties, the researchers say. 

Another interesting fact is that the material's properties can be customized. With the one-step process, the material can be made very long, or into a tube with a wider or narrower diameter, and the density of nanotubes can be varied to produce materials with differing properties for different needs. The scientists believe that the material can be used for a huge variety of applications, including the aforementioned batteries, supercapacitors and solar cells but also sensitive sensors, wearable electronics, thermal management and multifunctional aerospace systems. The scientists are further exploring the properties that can be derived from these single 3D graphene layer fibers and are developing a process for making multilayer fibers.

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