Transparent films of carbon nanotubes can accommodate strains of up to 150% and demonstrate conductivities as high as 2,200 S cm−1 in the stretched state.

Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes

Metal oxide nanostructures are promising electrode materials for lithium-ion batteries and supercapacitors because of their high specific capacity/capacitance, typically 2-3 times higher than that of the carbon/graphite-based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications. It is therefore urgent to improve their overall device performance, which depends on not only the development of advanced electrode materials but also in a large part "how to design superior electrode architectures". In the article, we will review recent advances in strategies for advanced metal oxide-based hybrid nanostructure design, with the focus on the binder-free film/array electrodes. These binder-free electrodes, with the integration of unique merits of each component, can provide larger electrochemically active surface area, faster electron transport and superior ion diffusion, thus leading to substantially improved cycling and rate performance. Several recently emerged concepts of using ordered nanostructure arrays, synergetic core-shell structures, nanostructured current collectors, and flexible paper/textile electrodes will be highlighted, pointing out advantages and challenges where appropriate. Some future electrode design trends and directions are also discussed.

Recent Advances in Metal Oxide-based Electrode Architecture Design for Electrochemical Energy Storage

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors This review provides comprehensive knowledge to this field We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors

Design and Mechanisms of Asymmetric Supercapacitors.

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost-effective microwave-assisted method. In order to achieve high energy and power densities, a high-voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)2/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g−1 and high energy density of 77.8 Wh kg−1. Furthermore, the Ni(OH)2/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.

Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Adv. Mater. 2009, 21, 4793–4797 2009 WILEY-VCH Verlag G N Stretchable electronic devices, such as p–n diodes, photovoltaic devices, transistors, and functional electronic eyes, have been fabricated using buckled single-crystal (e.g., Si, GaAs) thin films supported by elastomeric substrates. Recently, carbon nanotube (CNT)-based highly conducting elastic composites and stretchable graphene films have been reported, which are suitable as interconnects in stretchable electronic devices. As an indispensable component of stretchable electronics, a stretchable power-source device should be able to accommodate large strains while retaining intact function. Of various power-source devices, supercapacitors have attracted great interest in recent years due to their high power and energy densities compared with lithium-ion batteries and conventional dielectric capacitors, respectively. The most active research in supercapacitors is the development of new electrode materials. Recently, CNTs have been studied as good candidates for electrode materials because of several advantages, including a high surface area, nanoscale dimensions, and excellent electrical conductivity. Here, we report stretchable supercapacitors based on periodically sinusoidal single-walled carbon nanotube (SWNT) macrofilms (a 2D network of randomly oriented SWNTs). The stretchable supercapacitors comprise two sinusoidal SWNT macrofilms as stretchable electrodes, an organic electrolyte, and a polymeric separator. Electrochemical tests were performed and the fabricated stretchable supercapacitors are found to possess energy and power densities comparable with those of supercapacitors using pristine SWNT macrofilms as electrodes. Remarkably, the electrochemical performance of the stretchable supercapacitors remains unchanged even under 30% applied tensile strain. The preparation of the periodically sinusoidal SWNT macrofilms is of primary importance for stretchable supercapacitors. The synthesis of high-quality, purified, and functionalized SWNT macrofilms is, thus, an important preprocess, which has been presented elsewhere. The purified SWNT macrofilm was then shaped to a sinusoidal form by following the steps shown in Figure 1a. The procedure introduced here (step i in Fig. 1a) involves the uniaxial prestretching (epre) of an elastomeric substrate of a poly(dimethylsiloxane) (PDMS) slab (epre1⁄4DL/L for length changed from L to LþDL), followed by a chemical surface treatment to form a hydrophilic surface (see Experimental Section). The exposure of UV light introduces atomic oxygen, an activated species that reacts with PDMS and, thus, changes the

Stretchable supercapacitors based on buckled single-walled carbon-nanotube macrofilms.

The effect of temperature on the kinetics and the diffusion mechanism of the ions in a supercapacitor assembled with single-walled carbon nanotube (SWNT) film electrodes and an organic electrolyte were thoroughly investigated. An improved room temperature performance of the supercapacitor was observed due to the combined effects of an increase in the conductivity of the SWNT films and surface modifications on the SWNT films by repeatedly heating and cooling the supercapacitor between the temperatures of 25 and 100 °C. Modified Randles equivalent circuit was employed to carry out an extensive analysis of the Nyquist spectra measured at different temperatures between 25 and 100 °C in order to understand the fundamentals of the capacitive and resistive variations in the supercapacitor. The experimental results and their thorough analysis will have significant impact not only on the fundamental understanding of the temperature-dependent electrode/electrolyte interfacial properties but also on supercapacitor d...

Effect of temperature on the capacitance of carbon nanotube supercapacitors.

Carbon nanotubes (CNTs) are considered to be excellent candidates for high performance electrode materials in Li ion batteries. The nanometer-sized pore structures of CNTs can provide the hosting sites for storing large numbers of Li ions. A short diffusion distance for the Li ions may bring about a high discharge rate. The long-cycle performance of aligned multiwalled carbon nanotubes (MWNTs) directly synthesized on stainless-steel foil as an anode material in lithium battery is demonstrated. An increase in the specific capacity with an increase in the cycle number is observed. Starting at a value of 132 mA hg -1 in the first cycle at a current rate of 1 C, the specific capacity increased about 250% to a value of 460 mA hg -1 after 1 200 cycles. This is an unusual but a welcoming behavior for battery applications. It is found that the morphology of the MWNTs with structural and surface defects and the stainless-steel substrate play an important role in enhancing the capacity during the cycling process.

Long-Cycle Electrochemical Behavior of Multiwall Carbon Nanotubes Synthesized on Stainless Steel in Li Ion Batteries

Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric

High capacity silicon based anode active materials are described for lithium ion batteries. These materials are shown to be effective in combination with high capacity lithium rich cathode active materials. Supplemental lithium is shown to improve the cycling performance and reduce irreversible capacity loss for at least certain silicon based active materials. In particular silicon based active materials can be formed in composites with electrically conductive coatings, such as pyrolytic carbon coatings or metal coatings, and composites can also be formed with other electrically conductive carbon components, such as carbon nanofibers and carbon nanoparticles. Additional alloys with silicon are explored.

Charan Masarapu

Papers

Stretchable supercapacitors based on buckled single-walled carbon-nanotube macrofilms.

Effect of temperature on the capacitance of carbon nanotube supercapacitors.

Long-Cycle Electrochemical Behavior of Multiwall Carbon Nanotubes Synthesized on Stainless Steel in Li Ion Batteries

Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric

High capacity anode materials for lithium ion batteries