Yongyi Zhang

Elastic, Conductive, Polymeric Hydrogels and Sponges

As a result of inherent rigidity of the conjugated macromolecular chains resulted from the delocalized π-electron system along the polymer backbone, it has been a huge challenge to make conducting polymer hydrogels elastic by far. Herein elastic and conductive polypyrrole hydrogels with only conducting polymer as the continuous phase have been simply synthesized in the indispensable conditions of 1) mixed solvent, 2) deficient oxidant, and 3) monthly secondary growth. The elastic mechanism and oxidative polymerization mechanism on the resulting PPy hydrogels have been discussed. The resulting hydrogels show some novel properties, e.g., shape memory elasticity, fast functionalization with various guest objects, and fast removal of organic infectants from aqueous solutions, all of which cannot be observed from traditional non-elastic conducting polymer counterparts. Whats more, light-weight, elastic, and conductive organic sponges with excellent stress-sensing behavior have been successfully achieved via using the resulting polypyrrole hydrogels as precursors.

Carbon‐Nanotube Fibers for Wearable Devices and Smart Textiles

Carbon-nanotube (CNT) fibers integrate such properties as high mechanical strength, extraordinary structural flexibility, high thermal and electrical conductivities, novel corrosion and oxidation resistivities, and high surface area, which makes them a very promising candidate for next-generation smart textiles and wearable devices. A brief review of the preparation of CNT fibers and recently developed CNT-fiber-based flexible and functional devices, which include artificial muscles, electrochemical double-layer capacitors, lithium-ion batteries, solar cells, and memristors, is presented.

Dry-Processable Carbon Nanotubes for Functional Devices and Composites

Assembly of carbon nanotubes (CNTs) in effective and productive ways is of vital importance to their application. Recent progress in synthesis of CNTs has inspired new strategies for utilizing the unique physiochemical properties of CNTs in macroscale materials and devices. Assembling CNTs by dry processes (e.g., directly collecting CNTs in the form of freestanding films followed by pressing, stretching, and multilayer stacking instead of dispersing them in solution) not only considerably simplifies the processes but also avoids structural damage to the CNTs. Various dry-processable CNTs are reviewed, focusing on their synthesis, properties, and applications. The synthesis techniques are organized in terms of aggregative morphologies and microstructure control of CNTs. Important applications such as functional thin-film devices, strong CNT films, and composites are included. The opportunities and challenges in the synthesis techniques and fabrication of advanced composites and devices are discussed.

Strong and Conductive Dry Carbon Nanotube Films by Microcombing.

In order to maximize the carbon nanotube (CNT) buckypaper properties, it is critical to improve their alignment and reduce their waviness. In this paper, a novel approach, microcombing, is reported to fabricate aligned CNT films with a uniform structure. High level of nanotube alignment and straightness was achieved using sharp surgical blades with microsized features at the blade edges to comb single layer of CNT sheet. These microcombs also reduced structural defects within the film and enhanced the nanotube packing density. Following the microcombing approach, the as-produced CNT films demonstrated a tensile strength of up to 3.2 GPa, Youngs modulus of up to 172 GPa, and electrical conductivity of up to 1.8 × 10(5) S m(-1) , which are much superior to previously reported CNT films or buckypapers. More importantly, this novel technique requires less rigorous process control and can construct CNT films with reproducible properties.

Bio-Inspired Aggregation Control of Carbon Nanotubes for Ultra-Strong Composites

High performance nanocomposites require well dispersion and high alignment of the nanometer-sized components, at a high mass or volume fraction as well. However, the road towards such composite structure is severely hindered due to the easy aggregation of these nanometer-sized components. Here we demonstrate a big step to approach the ideal composite structure for carbon nanotube (CNT) where all the CNTs were highly packed, aligned, and unaggregated, with the impregnated polymers acting as interfacial adhesions and mortars to build up the composite structure. The strategy was based on a bio-inspired aggregation control to limit the CNT aggregation to be sub 20–50 nm, a dimension determined by the CNT growth. After being stretched with full structural relaxation in a multi-step way, the CNT/polymer (bismaleimide) composite yielded super-high tensile strengths up to 6.27–6.94 GPa, more than 100% higher than those of carbon fiber/epoxy composites, and toughnesses up to 117–192 MPa. We anticipate that the present study can be generalized for developing multifunctional and smart nanocomposites where all the surfaces of nanometer-sized components can take part in shear transfer of mechanical, thermal, and electrical signals.

Oxygen Evolution Assisted Fabrication of Highly Loaded Carbon Nanotube/MnO2 Hybrid Films for High-Performance Flexible Pseudosupercapacitors.

To date, it has been a great challenge to design high-performance flexible energy storage devices for sufficient loading of redox species in the electrode assemblies, with well-maintained mechanical robustness and enhanced electron/ionic transport during charge/discharge cycles. An electrochemical activation strategy is demonstrated for the facile regeneration of carbon nanotube (CNT) film prepared via floating catalyst chemical vapor deposition strategy into a flexible, robust, and highly conductive hydrogel-like film, which is promising as electrode matrix for efficient loading of redox species and the fabrication of high-performance flexible pseudosupercapacitors. The strong and conductive CNT films can be effectively expanded and activated by electrochemical anodic oxygen evolution reaction, presenting greatly enhanced internal space and surface wettability with well-maintained strength, flexibility, and conductivity. The as-formed hydrogel-like film is quite favorable for electrochemical deposition of manganese dioxide (MnO2 ) with loading mass up to 93 wt% and electrode capacitance kept around 300 F g(-1) (areal capacitance of 1.2 F cm(-2) ). This hybrid film was further used to assemble a flexible symmetric pseudosupercapacitor without using any other current collectors and conductive additives. The assembled flexible supercapacitors exhibited good rate performance, with the areal capacitance of more than 300 mF cm(-2) , much superior to other reported MnO2 based flexible thin-film supercapacitors.

Smart and flexible supercapacitor based on a porous carbon nanotube film and polyaniline hydrogel

Smart and flexible supercapacitors are of ever increasing importance for energy-storage devices. Herein, a simple and effective strategy to make a supercapacitor in a “flexible” and “electrochromic” way was based on the direct and convenient immobilization of a polyaniline (PANI) hydrogel onto a porous carbon nanotube (CNT) film. The resultant symmetric all-solid-state supercapacitor showed a favorable specific capacitance of 315 F g−1 and a relatively high cycling stability (92% capacitance retention after 1500 cycles). The bending test indicated that the obtained supercapacitor was flexible and its performance was only decreased by less than 7.0% after 150 bending cycles with a bending angle of 180°. Furthermore, the supercapacitor displayed a rapid and reversible chromatic transition between different working stages, and its level of stored energy could be quickly determined by the naked eye in a predictable and noticeable manner. This work may open a simple and convenient avenue for the fabrication of smart, flexible and all-solid-state supercapacitors.

Hydrothermal deposition of a zinc oxide nanorod array on a carbon nanotube film as a piezoelectric generator

Piezoelectric generators based on zinc oxide (ZnO) nanowires/nanorods require not only an aligned assembly morphology but also Schottky contacts between ZnO and electrodes to rectify the piezoelectric signals. Here we demonstrate that two-dimensional carbon nanotube (CNT) assembly films can serve as highly efficient electrode materials to meet these two requirements. The flexibility, porosity and pore size distribution, and intimate contact with ZnO of CNT films have advantages in controllable hydrothermal deposition to generate highly aligned ZnO nanorods with high crystallinity at a high density. Due to the Schottky characteristics between ZnO and CNT, aluminum and titanium are suggested to serve as the negative electrode of a piezoelectric energy supply, by using their ohmic contacts with ZnO. The three-layered piezoelectric generator outputs a signal of 50–60 mV by using bending deformations, one order of magnitude larger than the signal generated by fast pressing deformations. This study presents a method to design ZnO-based piezoelectric generators without using precious metals or rare earth elements.

A new insight into the rechargeable mechanism of manganese dioxide based symmetric supercapacitors

Pseudocapacitive symmetric supercapacitors, where both the cathode and the anode have the same pseudocapacitive material, have been widely investigated for developing high-performance supercapacitors. However, being different from electrochemical double-layer (EDL) capacitive electrodes, the charge storage of pseudocapacitive materials relies on reversible redox reactions that change the ion valence status, which is not the case for EDL capacitors (EDLCs). In this research, as a typical inorganic pseudocapacitive material for supercapacitors, a manganese dioxide (MnO2) based symmetric supercapacitor was carefully investigated by using a flexible and ultra-light carbon nanotube (CNT) film as the current collector and substrate for MnO2 electrodeposition. The results indicated that the pristine active material on the positive electrode showed no change after cyclic charging/discharging, but only served as a stable counter electrode and reference electrode. The main redox reaction for the energy storage of the supercapacitor occurred on the negative electrode. Furthermore, the dissolved Mn2+ ions on the negative electrode were deposited onto the positive electrode, which induced an increase in mass of the positive electrode and a decrease in mass of the negative electrode. This research could give new insight into the working mechanism of MnO2 electrodes and other pseudocapacitive materials in symmetric supercapacitors.