Liubing Dong

Flexible electrodes and supercapacitors for wearable energy storage: a review by category

Supercapacitors are important energy storage devices capable of delivering energy at a very fast rate. With the increasing interest in portable and wearable electronic equipment, various flexible supercapacitors (FSCs) and flexible electrodes (FEs) have been investigated widely and constantly in recent years. Currently-developed FEs/FSCs exhibit myriad physical forms and functional features and form a complicated and extensive system. Herein, we summarize the recent results about FEs/FSCs and present this review by categories. According to different micro-structures and macroscopic patterns, the existing FEs/FSCs can be divided into three types: fiber-like FEs/FSCs; paper-like FEs/FSCs; and three-dimensional porous FEs (and corresponding FSCs). Subsequently each type of the FEs/FSCs is further sub-classified based on their construction rules, and mechanical and electrochemical properties. To our best knowledge, this is the first time such a hierarchical and detailed classification strategy has been propose. We believe it will be beneficial for researchers around the world to understand FEs/FSCs. In addition, we bring up some fresh ideas for the future development of wearable energy storage devices.

Breathable and Wearable Energy Storage Based on Highly Flexible Paper Electrodes.

Breathable and wearable energy storage is achieved based on an innovative design solution. Carbon nanotube/MnO2 -decorated air-laid paper electrodes, with outstanding flexibility and good electrochemical performances, are prepared. They are then assembled into solid-state supercapacitors. By making through-holes on the supercapacitors, breathable and flexible supercapacitors are successfully fabricated.

High-performance compressible supercapacitors based on functionally synergic multiscale carbon composite textiles

High-performance compressible supercapacitors were realized based on functionally synergic multiscale carbon composite textiles. The composite textiles were fabricated by introducing nanoscale carbon fillers, i.e., carbon nanotube (CNT) or graphene (GN), on activated carbon fiber felt (ACFF) body material using a “simple impregnation and freeze-drying method”. The prepared CNT/ACFF and GN/ACFF composite textiles preserved the advantages of the ACFF body material in structure, such as being foldable and windable, whereas functionally they showed a synergic effect of the body material and nano-fillers, by integrating the excellent electrical conductivity of the nano-fillers with the high specific surface area and appropriate pore structures of ACFF. Thus, their electrochemical performances were significantly enhanced in the assembled symmetric supercapacitors, compared with those of the existing studies on textile electrodes. Areal capacitance, energy density and power density could be as high as 3350 mF cm−2, 112 μW h cm−2 and 4155 μW h cm−2, respectively. Most interestingly, our composite textiles can be compressed into a much smaller-sized and windable supercapacitive device, providing opportunities for their application as portable textile supercapacitors.

Combination effect of physical drying with chemical characteristic of carbon nanotubes on through-thickness properties of carbon fiber/epoxy composites

The present work studied the combination effect of physical drying with chemical modification of carbon nanotubes (CNTs) on some through-thickness properties of carbon fiber/epoxy composites. Different drying methods of heat drying and freeze drying were utilized to affect CNT organization form in carbon fiber/CNTs preforms and composites: The adoption of heat-drying method made CNTs more inclined to form aggregates accompanied with randomly scattered CNTs, while continuous CNT networks could always be assembled when freeze drying method was employed. The formation mechanism of such CNT networks was discussed, and could be described as “freeze drying within confined space.” Chemical characteristic of CNTs was controlled by choosing different solutions of non-functionalized CNTs (NOCNTs) or hydroxyl-modified CNTs (OHCNTs). As a consequence, CNT networks modified composites, especially that with OHCNTs formed networks, displayed significantly better electrical performance than composites with CNT aggregates and scattered CNTs; NOCNT networks and scattered OHCNTs made the corresponding composites possess higher interlaminar shear strength (ILSS) value, whereas OHCNT networks impaired ILSS while enhancing flexural strength and modulus of composites.

High-performance supercapacitors based on graphene/MnO2/activated carbon fiber felt composite electrodes in different neutral electrolytes

Graphene/MnO2 composites are introduced into activated carbon fiber felt (ACFF) to fabricate composite textile electrodes. Their micro-structure, electrical properties and electrochemical performance for supercapacitor applications in different neutral electrolytes (1 M NaNO3 and Ca(NO3)2 aqueous solutions) have been studied. The composite electrodes have similar pore features to original ACFF textiles, but show notably enhanced electrical and electrochemical performance. The composite textile electrodes show low electrical resistance, high specific capacitance (up to 1516 mF cm−2 in neutral electrolytes) and excellent cycling stability (no capacitance decay after 5000 charge–discharge cycles). Besides, electrochemical capacitance of composite textile electrodes in Ca(NO3)2 electrolyte is higher than that in NaNO3 electrolyte at low scan rates (1–5 mV s−1), but the situation is reversed when scan rates are higher than 10 mV s−1. Above all, the results show that our low-cost composite textile electrodes are high-performance in neutral electrolytes, which is helpful for developing large-scale energy storage devices.

Stacking up layers of polyaniline/carbon nanotube networks inside papers as highly flexible electrodes with large areal capacitance and superior rate capability

Developing high-performance flexible film-like electrodes is still a primary task for the practical applications of wearable/portable planar supercapacitors. In this work, a facile and effective approach, i.e., stacking up layers of polyaniline (PANI)/carbon nanotube (CNT) composite networks inside air-laid papers, is proposed to fabricate highly flexible paper electrodes with large areal capacitance and superior rate capability. The layer-by-layer deposition of PANI/CNT networks endows the fabricated paper electrodes with high loading and uniform distribution of PANI; meanwhile, the good electrical conductivity and porous structure of these introduced PANI/CNT networks guarantee sufficient paths for electron movement and ion transportation in the electrodes. Consequently, when 4 layers of PANI/CNT networks (with optimal PANI content) are stacked inside papers, the areal capacitance of the prepared electrode is as high as 1506 mF cm−2 at a charge/discharge current of 10 mA cm−2 and 1298 mF cm−2 at 100 mA cm−2; the electrode also exhibits high flexibility and good cycling stability (with 82% capacitance retention after 11 500 charge/discharge cycles). These merits make our PANI/CNT/papers promising candidates for flexible planar supercapacitor electrodes. Besides, this work is believed to provide a new thought for producing high-loading and high-energy wearable/portable energy storage devices.

A hollow spherical carbon derived from the spray drying of corncob lignin for high-rate-performance supercapacitors

Controlling the microstructure of biomass-derived carbon is of essential importance for directing its use. Herein, a hollow spherical carbon (HSC) was prepared from corncob lignin through spray drying and subsequent heat treatment. The HSC, which is characterized by its hierarchically porous structure, delivers high rate capability when it is directly used as electrode material for supercapacitors. This strategy that uses lignin as the precursor avoids the intrinsic difficulty in tuning the microstructure of the biomass-derived carbons and is suitable for mass production for practical use.

One-Step Preparation of Long-Term Stable and Flexible CsPbBr3 Perovskite Quantum Dots/Ethylene Vinyl Acetate Copolymer Composite Films for White Light-Emitting Diodes

CsPbBr3 perovskite quantum dots (PQDs)/ethylene vinyl acetate (EVA) composite films were prepared via a one-step method; on the basis of this, both supersaturated recrystallization of CsPbBr3 PQDs and dissolution of EVA were realized in toluene. The prepared films display outstanding green-emitting performance with high color purity of 92% and photoluminescence (PL) quantum yield of 40.5% at appropriate CsPbBr3 PQD loading. They possess long-term stable luminescent properties in the air and in water, benefiting from the effective protection of CsPbBr3 PQDs by the EVA matrix. Besides, the prepared CsPbBr3 PQDs/EVA films are flexible enough to be repeatedly bent for 1000 cycles while keeping unchanged the PL intensity. The optical properties of the CsPbBr3 PQDs/EVA films in white light-emitting diodes were also studied by experiments and theoretical simulation. Overall, facile preparation process, good long-term stability, and high flexibility allow our green-emitting CsPbBr3 PQDs/EVA films to be applied in lighting applications and flexible displays.

Microhoneycomb Monoliths Prepared by the Unidirectional Freeze-drying of Cellulose Nanofiber Based Sols: Method and Extensions

Monolithic honeycomb structures have been attractive to multidisciplinary fields due to their high strength-to-weight ratio. Particularly, microhoneycomb monoliths (MHMs) with micrometer-scale channels are expected as efficient platforms for reactions and separations because of their large surface areas. Up to now, MHMs have been prepared by a unidirectional freeze-drying (UDF) method only from very limited precursors. Herein, we report a protocol from which a series of MHMs consisting of different components can be obtained. Recently, we found that cellulose nanofibers function as a distinct structure-directing agent towards the formation of MHMs through the UDF process. By mixing the cellulose nanofibers with water soluble substances which do not yield MHMs, a variety of composite MHMs can be prepared. This significantly enriches the chemical constitution of MHMs towards versatile applications.

Editable asymmetric all-solid-state supercapacitors based on high-strength, flexible, and programmable 2D-metal–organic framework/reduced graphene oxide self-assembled papers

Although some progress has been made in flexible supercapacitors (SCs), their high energy density, mechanical robustness, and device-level editability and programmability are still highly desirable for the development of advanced portable and miniaturized electronics, especially considering the fact that these flexible devices are likely to experience some mechanical impact and potential damage. Herein, we demonstrate the fabrication of hybrid electrodes containing self-assembled 2D metal–organic framework (MOF)/reduced graphene oxide (rGO) papers, which not only efficiently alleviate the self-restacking of rGO and the MOF but also maintain high electrical conductivity (0.32 Ω cm), excellent flexibility and mechanical properties with a Youngs modulus of 34.4 GPa and a tensile strength of 89.9 MPa. In addition, a one-for-two strategy is introduced to construct two types of porous electrodes for flexible asymmetric SCs via a one MOF-derived synthesis route with simply changing metal ion precursors. As a consequence, the flexible asymmetric SCs possess a high volumetric energy density of 1.87 mW h cm−3 and an outstanding volumetric power density of 250 mW cm−3. More importantly, the all-solid-state asymmetric SCs exhibit high editability and bending-tolerance properties and perform very well under various severe service conditions, such as being seriously cut, bent, and heavily loaded. Particularly, the operations of micro-SCs with artistically designed patterns are demonstrated. Being high-strength, easily programmable and connectable in series and in parallel, the editable supercapacitor is promising for developing stylish energy storage devices to power various portable, miniaturized, and wearable devices.