Wenjun Meng

From Industrially Weavable and Knittable Highly Conductive Yarns to Large Wearable Energy Storage Textiles

Wearable electronic textiles that store capacitive energy are a next frontier in personalized electronics. However, the lack of industrially weavable and knittable conductive yarns in conjunction with high capacitance, limits the wide-scale application of such textiles. Here pristine soft conductive yarns are continuously produced by a scalable method with the use of twist-bundle-drawing technique, and are mechanically robust enough to be knitted to a cloth by a commercial cloth knitting machine. Subsequently, the reduced-graphene-oxide-modified conductive yarns covered with a hierarchical structure of MnO2 nanosheets and a polypyrrole thin film were used to fabricate weavable, knittable and wearable yarn supercapacitors. The resultant modified yarns exhibit specific capacitances as high as 36.6 mF cm(-1) and 486 mF cm(-2) in aqueous electrolyte (three-electrode cell) or 31 mF cm(-1) and 411 mF cm(-2) in all solid-state two-electrode cell. The symmetric solid-state supercapacitor has high energy densities of 0.0092 mWh cm(-2) and 1.1 mWh cm(-3) (both normalized to the whole device) with a long cycle life. Large energy storage textiles are fabricated by weaving our flexible all-solid-state supercapacitor yarns to a 15 cm × 10 cm cloth on a loom and knitting in a woollen wrist band to form a pattern, enabling dual functionalities of energy storage capability and wearability.

Magnetic-Assisted, Self-Healable, Yarn-Based Supercapacitor

Yarn-based supercapacitors have received considerable attention recently, offering unprecedented opportunities for future wearable electronic devices (e.g., smart clothes). However, the reliability and lifespan of yarn-based supercapacitors can be seriously limited by accidental mechanical damage during practical applications. Therefore, a supercapacitor endowed with mechanically and electrically self-healing properties is a brilliant solution to the challenge. Compared with the conventional planar-like or large wire-like structure, the reconnection of the broken yarn electrode composed of multiple tiny fibers (diameter <20 μm) is much more difficult and challenging, which directly affects the restoration of electrical conductivity after damage. Herein, a self-healable yarn-based supercapacitor that ensures the reconnection of broken electrodes has been successfully developed by wrapping magnetic electrodes around a self-healing polymer shell. The strong force from magnetic attraction between the broken yarn electrodes benefits reconnection of fibers in the yarn electrodes during self-healing and thus offers an effective strategy for the restoration of electric conductivity, whereas the polymer shell recovers the configuration integrity and mechanical strength. With the design, the specific capacitance of our prototype can be restored up to 71.8% even after four breaking/healing cycles with great maintenance of the whole devices mechanical properties. This work may inspire the design and fabrication of other distinctive self-healable and wearable electronic devices.

Proton-Insertion-Enhanced Pseudocapacitance Based on the Assembly Structure of Tungsten Oxide

The capacitances of supercapacitors with carbon and metal oxides as electrodes are usually associated with the available surface areas of the electrode materials. However, in this paper, we report that proton insertion, an unusual capacitive mechanism, may effectively enhance the capacitance of metal oxides with low surface area but specific structures. Tungsten trioxide (WO3) as the electrode material for supercapacitors has always suffered from low capacitance. Nevertheless, enhanced by the proton insertion mechanism, we demonstrate that electrodes fabricated by an assembly structure of hexagonal-phase WO3 (h-WO3) nanopillars achieve a high capacitance of up to 421.8 F g(-1) under the current density of 0.5 A g(-1), which is the highest capacitance achieved with pure WO3 as the electrodes so far, to the best of our knowledge. Detailed analyses indicate that proton insertion dominates the electrochemical behavior of h-WO3 and plays the key role in reaching high capacitance by excluding other mechanisms. In addition, a thorough investigation on the temperature-dependent electrochemical performance reveals excellent performance stability at different temperatures. This study provides a new approach to achieving high capacitance by effective proton insertion into ordered tunnels in crystallized metal oxides, which is primarily important for the fabrication of compact high-performance energy storage devices.

Polymer composites of boron nitride nanotubes and nanosheets

Hexagonal boron nitride (h-BN) is a layered material with planar networks of BN hexagons, which is flexible to form various nanostructures. This feature article begins with an overall introduction of BN nanostructures and their novel properties, such as electrical insulating properties, high thermal conductivity, great mechanical strength, optical properties, and so on. Then a comprehensive review of polymer composites of BN nanostructures with distinguished properties for different applications is presented. Finally, the problems of using BN nanostructures for the fabrication of polymer composites are discussed.

Enhanced tolerance to stretch-induced performance degradation of stretchable MnO2-based supercapacitors.

The performance of many stretchable electronics, such as energy storage devices and strain sensors, is highly limited by the structural breakdown arising from the stretch imposed. In this article, we focus on a detailed study on materials matching between functional materials and their conductive substrate, as well as enhancement of the tolerance to stretch-induced performance degradation of stretchable supercapacitors, which are essential for the design of a stretchable device. It is revealed that, being widely utilized as the electrode material of the stretchable supercapacitor, metal oxides such as MnO2 nanosheets have serious strain-induced performance degradation due to their rigid structure. In comparison, with conducting polymers like a polypyrrole (PPy) film as the electrochemically active material, the performance of stretchable supercapacitors can be well preserved under strain. Therefore, a smart design is to combine PPy with MnO2 nanosheets to achieve enhanced tolerance to strain-induced performance degradation of MnO2-based supercapacitors, which is realized by fabricating an electrode of PPy-penetrated MnO2 nanosheets. The composite electrodes exhibit a remarkable enhanced tolerance to strain-induced performance degradation with well-preserved performance over 93% under strain. The detailed morphology and electrochemical impedance variations are investigated for the mechanism analyses. Our work presents a systematic investigation on the selection and matching of electrode materials for stretchable supercapacitors to achieve high performance and great tolerance to strain, which may guide the selection of functional materials and their substrate materials for the next-generation of stretchable electronics.

An electrochromic supercapacitor and its hybrid derivatives: quantifiably determining their electrical energy storage by an optical measurement

In this article, an electrochromic supercapacitor was developed with the electrode material active for both electrochromism and energy storage. The detailed measurements of the optical spectra of the device revealed that the normalized optical density, a concept in electrochromic studies, depended linearly on the electrical energy storage (EES) of the supercapacitor. This enabled the precisely quantifiable determination of a solid-state supercapacitors EES by simple optical transmission measurement, which is demonstrated here for the first time, to the best of our knowledge. One step further, parallel-structured hybrid supercapacitors were designed to integrate the developed smart function with high-performance supercapacitors using polypyrrole (PPy) and manganese oxide (MnO2) as electrode materials. The developed hybrid supercapacitors exhibited excellent capacitive performance and maintained the ability of electrochromic EES indicators well. Different calibration curves can be produced for different types of hybrid supercapacitors. With these curves, the EES of hybrid supercapacitors can be precisely determined using a simple optical transmission measurement. Our study paves the way for the integration of electrochromic EES indicators in various energy storage devices, as well as the prompt and quantitative determination of the EES of various types of supercapacitors using a simple optical transmission measurement.

Facile synthesis of α-Fe2O3 nanodisk with superior photocatalytic performance and mechanism insight

Abstract Intrinsic short hole diffusion length is a well-known problem for α-Fe2O3 as a visible-light photocatalytic material. In this paper, a nanodisk morphology was designed to remarkably enhance separation of electron-hole pairs of α-Fe2O3. As expected, α-Fe2O3 nanodisks presented superior photocatalytic activity toward methylene blue degradation: more than 90% of the dye could be photodegraded within 30 min in comparison with a degradation efficiency of 50% for conventional Fe2O3 powder. The unique multilayer structure is thought to play a key role in the remarkably improved photocatalytic performance. Further experiments involving mechanism investigations revealed that instead of high surface area, ·OH plays a crucial role in methylene blue degradation and that O·2− may also contribute effectively to the degradation process. This paper demonstrates a facile and energy-saving route to fabricating homogenous α-Fe2O3 nanodisks with superior photocatalytic activity that is suitable for the treatment of contaminated water and that meets the requirement of mass production.

Cytosine-phosphodiester-guanine oligodeoxynucleotide (CpG ODN)-capped hollow mesoporous silica particles for enzyme-triggered drug delivery

We designed, for the first time, an enzyme-triggered drug delivery system that is based on cytosine-phosphodiester-guanine oligodeoxynucleotide (CpG ODN)-capped hollow mesoporous silica (HMS) particles as carriers. Fluorescein dye was used as a model drug, and the fluorescein loading, amino-grafting and CpG ODN capping were evaluated by UV/Vis analysis, zeta potential and N(2) adsorption-desorption measurements and gel electrophoresis. The fluorescein loading capacity and CpG ODN capping amount were 37.7 and 39.6 μg mg(-1), respectively at the weight ratio of 10 Dye/HMS-NH(2)/CpG ODN. Importantly, fluorescein release can be triggered by the addition of deoxyribonuclease I (DNase I) for CpG ODN degradation, and the release rate can also be controlled by changing the DNase I concentration. Therefore, it might be a promising controlled drug delivery system for application in the field of biomedicine and cancer therapy.

Robust reduced graphene oxide paper fabricated with a household non-stick frying pan: a large-area freestanding flexible substrate for supercapacitors

Inspired by cooking omelettes, a facile, low-cost and scalable method involving the use of a readily available household non-stick frying pan is introduced to fabricate large-area freestanding reduced graphene oxide (RGO) paper. The as-fabricated RGO paper is robust enough to bear sandpaper polishing, bending/folding, and hydrothermal and electrochemical deposition processes without obvious structure/performance degradation. As a demonstration, the as-obtained RGO papers were directly used as universal flexible substrates for high performance supercapacitors (SCs). Thus, WO3 and PPy, which are two distinctive active materials, were loaded onto the RGO paper via a hydrothermal process and electrodeposition process, respectively, which are two typical fabrication methods for high performance SC electrodes. The resultant WO3– and PPy–RGO paper, which act as negative and positive electrodes, respectively, were further assembled into a flexible asymmetric supercapacitor (ASC), achieving a high energy density of 0.23 mW h cm−3 at a power density of 7.3 mW cm−3 when normalized to the whole volume. Moreover, benefiting from the robust flexible RGO substrate, the performance of the ASC showed great stability under different bending angles and after repeated bending/folding. These exciting results demonstrate that our robust RGO paper is an ideal universal substrate for different active materials synthesized via various processing methods, which show its great potential in an all-solid energy storage system with excellent flexibility and robustness.