Chakrapani V. Varanasi

Synergistic effects from graphene and carbon nanotubes enable flexible and robust electrodes for high-performance supercapacitors.

Flexible and lightweight energy storage systems have received tremendous interest recently due to their potential applications in wearable electronics, roll-up displays, and other devices. To manufacture such systems, flexible electrodes with desired mechanical and electrochemical properties are critical. Herein we present a novel method to fabricate conductive, highly flexible, and robust film supercapacitor electrodes based on graphene/MnO(2)/CNTs nanocomposites. The synergistic effects from graphene, CNTs, and MnO(2) deliver outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. These flexible electrodes allow highly active material loading (71 wt % MnO(2)), areal density (8.80 mg/cm(2)), and high specific capacitance (372 F/g) with excellent rate capability for supercapacitors without the need of current collectors and binders. The film can also be wound around 0.5 mm diameter rods for fabricating full cells with high performance, showing significant potential in flexible energy storage devices.

Improving the performance of cobalt–nickel hydroxide-based self-supporting electrodes for supercapacitors using accumulative approaches

In this paper we describe an accumulative approach to move beyond simple incorporation of conductive carbon nanostructures, such as graphene and carbon nanotubes, to improve the performance of metal oxide/hydroxide based electrodes in energy storage applications. In this approach we first synthesize Co–Ni double hydroxides/graphene binary composites through a co-precipitation process. We then assemble these composites into films (∼6 mg cm−2) by integrating with carbon nanotubes that can be used directly as electrodes. Experimental results indicate that the synergistic contributions from nanotubes, graphene and cobalt substitution enabled electrodes with substantially improved energy storage performance metrics. With 50% Co and 50% Ni (i.e. Co0.5Ni0.5(OH)2), the composite exhibited a remarkable maximum specific capacitance of 2360 F g−1 (360 mA h g−1) at 0.5 A g−1 and still maintained a specific capacitance as high as 2030 F g−1 at 20 A g−1 (∼86% retention). More importantly, the double hydroxides exhibited tunable redox behavior that can be controlled by the ratio between cobalt and nickel. These results demonstrate the importance of the rational design of functional composites and the large-scale assembly strategies for fabricating electrodes with improved performance and tunability for energy storage applications.

Highly Efficient Oxygen Reduction Electrocatalysts based on Winged Carbon Nanotubes

Developing electrocatalysts with both high selectivity and efficiency for the oxygen reduction reaction (ORR) is critical for several applications including fuel cells and metal-air batteries. In this work we developed high performance electrocatalysts based on unique winged carbon nanotubes. We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes. After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance. While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

Effect of interlayer spacing on sodium ion insertion in nanostructured titanium hydrogeno phosphates/carbon nanotube composites

In sodium ion batteries, the ease of insertion and extraction of sodium ions in the electrode materials is one of the key parameters for the overall performance. In this article, the electrochemical sodium ion insertion in layered titanium hydrogeno phosphates (TiP) has been studied. In this material, the interlayer spacing and the particle morphology can be controlled by the choice of synthesis methods. Both nanostructured TiP and the coarse grained bulk counterpart were synthesized and properties were compared. While the specific capacity of nanostructured TiP materials was found to be not sensitive to the interlayer spacing, the specific capacity of coarse grained bulk TiP materials was significantly increased as the interlayer spacing was increased with the intercalation of water molecules in the layered host structure. These results indicate that interlayer spacing may not be the primary factor for Na-ion diffusion in nanostructured materials, where many interstitials are available for Na-ion diffusion. It is shown that nanostructured TiP materials can deliver excellent rate capability, and long term cycle stability with stable reversible capacity without the need of interlayer spacing expansion. The electrochemical properties of nanostructured materials were further enhanced when prepared as composites with carbon nanotubes that enhance the overall conductivity of the electrode materials.