High-performance Flexible Electrode Research Articles

Electrodes derived from carbon fiber-reinforced cellulose nanofiber/multiwalled carbon nanotube hybrid aerogels for high-energy flexible asymmetric supercapacitors

Developing electrodes with excellent electrochemical and mechanical properties is the key to achieve state-of-the-art flexible asymmetric supercapacitors (ASCs). This work proposes a facile and scalable strategy to prepare high-performance flexible electrodes based on carbon fiber-reinforced cellulose nanofiber/multiwalled carbon nanotube-hybrid aerogels (CF-CNF/MWCNT-HAs). The three-dimensional (3-D) porous structure, excellent conductivity, binder-free nature, and high strength of the CF-CNF/MWCNT-HAs enable it to serve as a powerful platform for constructing flexible electrodes with outstanding capacitive performance. The as-prepared CF-CNF/MWCNT/MnO2 positive electrode and CF-CNF/MWCNT/active carbon (AC) negative electrode display remarkably high areal-specific capacitances (1745 and 1273 mF/cm2 at a current density of 1 mA/cm2, respectively). In addition, the carbon fiber-reinforced electrode also presented an excellent mechanical property with a maximum stress up to 27.9 MPa. The as-fabricated CF-CNF/MWCNT/MnO2//CF-CNF/MWCNT/AC flexible ASC device exhibits significant capacitance and energy density of 19.4 F/cm3 and 8.93 mWh/cm3, respectively. Additionally, this flexible ASC device can retain more than 96.7% of its capacitance after 3000 charge-discharge cycles. These results will enable us to fabricate flexible solid-state ASC devices from CF-CNF/MWCNT-HAs, and to expand its applications in the fields of portable and wearable electronics.

NiCo-LDH nanowires@nanosheets core-shell structure grown on carbon fiber cloth for high performance flexible supercapacitor electrode

In this paper, a facile two-step method was adopted to in-situ develop a flexible electrode with NiCo-LDH nanowires@NiCo-LDH nanosheets core-shell structure on carbon fiber cloth (CFC). The electrochemical properties of the composite materials with different Ni/Co molar ratios nanosheets were systematically studied. It is found that the NiCo-LDH nanowires@Ni2Co1-LDH nanosheets (NiCo@Ni2Co1) composite material achieves a high areal capacitance of 9.67 F/cm2 at 5 mA/cm2 and a good cycling stability (75% capacitance retention after 2000 cycles). The excellent performance benefits from the distinct core-shell structure and the synergistic effect of NiCo-LDH nanosheets and NiCo-LDH nanowires. Moreover, a solid-state flexible asymmetric supercapacitor (ASC) assembled with NiCo@Ni2Co1/CFC as positive electrode and activated carbon on CFC as negative electrode can work stably at a voltage of 1.6 V. This ASC displays a high areal energy density of 298.6 uWh/cm2 at a current density of 1 mA/cm2 and with a corresponding areal power density of 0.80 mW/cm2.

Self-assembled three-dimensional Ti3C2Tx nanosheets network on carbon cloth as flexible electrode for supercapacitors

In recent year, flexible energy storage materials have attracted extensive attention in portable and wearable devices. MXenes, a new family of two-dimensional material, which exhibits excellent energy storage properties, has been widely used in flexible energy storage system. In this investigation, a Ti3C2Tx/CC (Carbon cloth) flexible electrode is successfully synthesized by a simple method of dipping and fast freezing. The results indicate that the prepared electrode shows a areal capacitance of 191 mF·cm−2 at current density of 1 mA·cm−2 and it could be increased to 312 mF·cm−2 by further surface treatment (obtaining ak-Ti3C2Tx/CC electrode). Furthermore, it exhibits a 97% capacitance retention at a current density 4 mA·cm−2 after 8000 charge-discharge cycles. The excellent electrochemical performance can be explained by the open three-dimensional network structure of self-assembled Ti3C2Tx nanosheets. This unique structure effectively enhances the access of electrolyte ions and facilitates the fast electron transfer during charge and discharge processes. The electrode is characterized by low cost, simple preparation, high flexibility and unique open network structure, which is expected to be used as flexible substrate to produce high-performance flexible electrode by combining with other materials such as oxides and conducting polymers.

Breaking 12% efficiency in flexible organic solar cells by using a composite electrode

The performance of flexible organic solar cells (OSCs) significantly relies on the quality of transparent flexible electrode. Here, we used silver nanowires (AgNWs) with various weight ratios to dope high-conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PH1000) to optimize the optical and electronic properties of PH1000 film. A high-quality flexible composite electrode PET/Ag-mesh/PH1000:AgNWs-20 with smooth surface, a low sheet resistance of 6 Ω/sq and a high transmittance of 86% at 550-nm wavelength was obtained by doping 20 wt% AgNWs to PH1000 (PH1000:AgNWs-20). The flexible OSCs based on the PET/Ag-mesh/PH1000:AgNWs-20 electrode delivered a power conversion efficiency (PCE) of 12.07% with an open circuit voltage ( V oc) of 0.826 V, a short-circuit current density ( J sc) of 20.90 mA/cm2 and a fill factor (FF) of 69.87%, which is the highest reported PCE for the flexible indium-tin oxide (ITO)-free OSCs. This work demonstrated that the flexible composite electrodes of PET/Ag-mesh/PH1000:AgNWs are promising alternatives for the conventional PET/ITO electrode, and open a new avenue for developing high-performance flexible transparent electrode for optoelectronic devices.

Functional graphene film macroscopic assemblies for flexible supercapacitor application

The ever-increasing demand for flexible/wearable electronic devices has stimulated the rapid development of flexible supercapacitors with enhanced electrochemical performance and mechanical flexibility. Graphene-based materials are promising candidates for high-performance flexible supercapacitor electrodes due to their large surface area, high conductivity, low weight and superior mechanical flexibility. In this review, after a brief introduction, we mainly discuss different strategies to enhance performances of graphene-based electrode in recent years. Their fabrication process and electrochemical performances are discussed and compared. Then two types of flexible supercapacitors, namely, fiber-type supercapacitor and micro-supercapacitor, are also discussed in detail. We also present our point of view in our discussion.

Highly Flexible Hydrogen Boride Monolayers as Potassium-Ion Battery Anodes for Wearable Electronics.

The rapid development of wearable electronics has revealed an urgent need for low-cost, highly flexible, and high-capacity power sources. In this sense, emerging rechargeable potassium-ion batteries (KIBs) are promising candidates owing to their abundant resources, low cost, and lower redox potential in nonaqueous electrolytes compared to lithium-ion batteries. However, the fabrication of flexible KIBs remains highly challenging because of the lack of high-performance flexible electrode materials. In this work, we investigated the mechanical properties and electrochemical performance of a recently developed hydrogen boride (BH) monolayer as a high-performance anode material on the basis of density functional theory formalism. We demonstrated that (i) BH presents ultralow out-of-plane bending stiffness, rivaling that of graphene, which endows it with better flexibility to accommodate the repeated bending, rolling, and folding on wearable device operation; (ii) high in-plane stiffness (157 N/m along armchair and 109 N/m along zigzag) of BH makes the electrode stable against pulverization upon external and internal strains. More importantly, a BH electrode delivers a low voltage of ∼0.24 V in addition to desired K-ion affinity and hopping resistance, which remains very stable with the bending curvature. Emerged H vacancies in electrodes were found to improve both the K-ion intercalation and K-ion hopping, yielding a high theoretical capacity (1138 mAh/g), which was among the highest reported values in the literature for K-ion anode materials. All of the presented results suggested that a BH electrode could be used as a brand-new flexible and lightweight KIB anode with high capacity, low voltage, and desired rate performance.

Conjugated Nitroxide Radical Polymers: Synthesis and Application in Flexible Energy Storage Devices

The synthesis and electrochemical behavior of nitroxide radical conjugated polymers (NCPs) have long been an intriguing topic in redox polymer-based energy storage. However, common (electro)chemical oxidation polymerization methods have proved difficult in the synthesis of well-defined NCPs, and many of these polymers have been difficult to process into thin films. In addition to these drawbacks and coupled with the complex charge-transfer and storage mechanisms, the use of NCPs as electrodes has been significantly limited. The aim of this work is to provide mechanistic insights into this complex charge-transfer and storage process using a new and well-defined NCP synthesized using an ultrafast cyclopolymerization with the Grubbs 3rd generation catalyst. The monomer, consisting of a 1,6-heptadiyne group and a TEMPO (i.e. 2,2,6,6-tetramethylpiperidine-1-oxy) radical, through the cyclopolymerization produced a well-defined NCP with a five-membered ring-containing polyene backbone. This polymer demonstrated excellent film formation properties, allowing the study of their thin-film electrochemical behavior. We found that the electrochemical oxidation of the conjugated backbone and its internal charge transfer to the nitroxide radicals were strongly affected by the applied potential window, current densities, and cycle numbers. Using these new insights, we successfully utilized our NCPs in a flexible energy storage device by fabricating high-performance NCP-coated carbon cloth-based flexible electrodes.

Entirely solution-processed and template-assisted fabrication of metal grids for flexible transparent electrodes

Photopatterned seamless polymer meshes coated chemically with aluminum are successfully demonstrated as metal grids for high-performance flexible transparent electrodes.

A core–shell MnO2@Au nanofiber network as a high-performance flexible transparent supercapacitor electrode

A highly transparent core–shell MnO2@AuNF network electrode is fabricated for a flexible portable energy storage device

High-Performance Transparent and Flexible Electrodes Made by Flash-Light Sintering of Gold Nanoparticles

Metallic nanowire-based transparent electrodes (TEs) are potential alternatives to indium tin oxide (ITO). To achieve a high performance [sheet resistance (Rs) 90%]...

Recent Advances in Nanowire‐Based, Flexible, Freestanding Electrodes for Energy Storage

The rational design of flexible electrodes is essential for achieving high performance in flexible and wearable energy-storage devices, which are highly desired with fast-growing demands for flexible electronics. Owing to the one-dimensional structure, nanowires with continuous electron conduction, ion diffusion channels, and good mechanical properties are particularly favorable for obtaining flexible freestanding electrodes that can realize high energy/power density, while retaining long-term cycling stability under various mechanical deformations. This Minireview focuses on recent advances in the design, fabrication, and application of nanowire-based flexible freestanding electrodes with diverse compositions, while highlighting the rational design of nanowire-based materials for high-performance flexible electrodes. Existing challenges and future opportunities towards a deeper fundamental understanding and practical applications are also presented.

Carbonized cotton fabric in-situ electrodeposition polypyrrole as high-performance flexible electrode for wearable supercapacitor

A polypyrrole/carbonized cotton fabric (PPy/CCF) flexible bind-free electrode was firstly created by simple carbonization and in-situ electrodeposition method for supercapacitors. The PPy/CCF-1:2 not only exhibits a high specific capacitance of 3596 mF cm−2 at a current density of 2 mA cm−2, which is almost 9 times higher than the pristine CCF (420 mF cm−2), but also shows good rate capability and high stability with 96.5% capacitance retention after 4000 cycles. The excellent electrochemical properties can be attributed to CCF substrate with a hierarchical 3D porous structure, which can provide conductive pathways for fast electron transfer and large accessible area for PPy loading. Meanwhile, PPy with hornlike microstructure can shorten ion diffusion paths, and ensure adequate contact area between the electrode and the electrolyte, leading to an dramatical increase in the specific capacitance and fast charge/discharge capability of the hybrid electrodes. In addition, the assembled flexible supercapacitor possesses a high volumetric energy density of 1.18 mWh cm−3 at power density of 17 mW cm−3, expressing no any decline in the capacitance under different bending situations. This device can be easily sewn into a garment for powering an electronic watch, with the low-cost, lightweight and easy fabrication features, which will be considerate as a promising candidate for wearable energy storage device.

Low diffuse reflection of silver nanowire/ruthenium oxide nanosheet hybrid films for high-performance transparent flexible electrodes

Transparent conducting electrodes (TCEs) based on silver nanowire (AgNW) networks possess high conductance, transmittance, and mechanical flexibility. However, due to the relatively high diffuse reflection of incident light on AgNWs, they cannot be practically implemented in displays requiring low pattern visibility. One promising strategy for solving this problem is to place an optical stack with high refractive index underneath the AgNW layer. In this work, AgNW-RuO2 nanosheet hybrid TCEs with low diffuse reflections are fabricated using metallic RuO2 nanosheets as undercoats. As predicted by theoretical simulations, RuO2 nanosheets with high refractive indices reduce the diffuse reflections of AgNWs by almost 8%. Moreover, after the partial etching of AgNWs, the difference in the diffuse reflections of their etched and non-etched regions becomes equal to about 0.003, leading to the formation of an invisible pattern. The film consisting of micro-sized RuO2 nanosheets is not damaged during etching, but instead forms a current path between different AgNWs broken by cyclic bending, resulting in a tenfold decrease in the resistance of the AgNW TCE after 170 000 cycles. Further, RuO2 nanosheets suppress the diffusion of humid air from the outside, thus improving the environmental stability of the AgNW-RuO2 nanosheet hybrid TCEs.

Polyhedron carbon-scale stacking foldable fibrous film electrode with high capacitance performance from chitin fiber cloth for super flexible supercapacitors

This work is concerned with exploring high-performance flexible carbon electrode materials for supercapacitors derived from low-cost commercially-available fabrics. Herein, through direct carbonization without any additional pre-treated process and without any activators as well as any templates, we explored different carbonization temperatures to develop a super flexible carbon film derived from chitin fiber textile. The as-obtained carbon fibrous films are found to possess a novel unique microstructure and ultra-high specific surface areas (up to 382.2 m2 g−1), formed via irregular stacking and inter-connection of numerous polyhedron carbon-scales with much crevices and grooves. They present super high mechanical flexibility capable of being repeatedly folded and exhibited a very high capacitance performance with a maximum specific capacitance of up to 114.9 F g−1 at 1 A g−1 and an excellent cycling performance with about 94.0% of capacitance retention after 5000 charge-discharge cycles at 1 A g−1. The carbon fibrous film from biomass textiles shows great promise for applications as flexible electrode in energy storage.

“Bubble-in-nanorod” hierarchical hybrid fiber: A highly-efficient design for pyrophosphate-based freestanding cathodes towards fast sodium/lithium intercalation

Tailoring electroactive materials into diverse functional architecture are triggering unprecedented innovations in the promotion of energy storage devices. Herein, we introduced a general strategy to prepare “bubble-in-nanorod” hierarchical hybrid fibers for fabrication of flexible pyrophosphate cathodes with superior lithium/sodium storage properties. The hollow spheres, which are composed of pyrophosphate nanoscale crystals and carbon matrix, are uniformly distributed within the porous carbon-based fiber. The prepared “bubble-in-nanorod” hierarchical structure not only can provide a bicontinuous conductive skeleton for fast electron transport, but also can effectively protect the crystal against structural deterioration, and thus is favorable to fast ion transport and stable structure integrity. Two kinds of pyrophosphates, i.e. Na3.12Fe2.44(P2O7)2 and Li2FeP2O7, are employed as examples in this study. For the first time, the mechanism on the combustion-assisted formation of the bubble-in-nanorod architecture is specified, and the effects of the functional structure on the ion intercalation chemistry of pyrophosphates are elaborated. Both pyrophosphate-based hybrid fibers can achieve better rate capability and longer cycling durability than the reference ones. Moreover, they are capable of long-term high-rate cycling. After six hundred cycles at alternate 20 and 3 C rates, the Na3.12Fe2.44(P2O7)2 and Li2FeP2O7 based fibers retain 94.4% and 95.1% of the initial capacity. Therefore, this work not only introduces a highly efficient architecture to realize the superior ion intercalation chemistry, but also provides a novel general strategy to fabricate high-performance flexible electrode for advanced lithium/sodium batteries.

A high-performance flexible supercapacitor electrode material based on nano-flowers-like FeS2/NSG hybrid nanocomposites

Novel FeS2/nitrogen-sulfur double doped graphene (NSG) flexible hybrids electrode materials have been successfully prepared via a rapid microwave method followed by tube furnace heating process. Small size nano-flower-like structure of FeS2 uniformly supported on multi-fold NSG, which promote electron transport and electrochemical reaction. The material shows good electrochemical performance in a three-electrode system, such as high specific capacitance (528.7 F g−1 at the current density of 1 A g−1), excellent charge/discharge stability and long-term cycling life (89% of capacitance retention after 10,000 cycles at 10 A g−1). These results demonstrate great potential of FeS2/NSG flexible hybrid nanocomposites for supercapacitor application.

All-MXene-Based Integrated Electrode Constructed by Ti3C2 Nanoribbon Framework Host and Nanosheet Interlayer for High-Energy-Density Li-S Batteries.

High-energy-density lithium-sulfur (Li-S) batteries hold promise for next-generation portable electronic devices, but are facing great challenges in rational construction of high-performance flexible electrodes and innovative cell configurations for actual applications. Here we demonstrated an all-MXene-based flexible and integrated sulfur cathode, enabled by three-dimensional alkalized Ti3C2 MXene nanoribbon (a-Ti3C2 MNR) frameworks as a S/polysulfides host (a-Ti3C2-S) and two-dimensional delaminated Ti3C2 MXene (d-Ti3C2) nanosheets as interlayer on a polypropylene (PP) separator, for high-energy and long-cycle Li-S batteries. Notably, an a-Ti3C2 MNR framework with open interconnected macropores and an exposed surface area guarantees high S loading and fast ionic diffusion for prompt lithiation/delithiation kinetics, and the 2D d-Ti3C2 MXene interlayer remarkably prevents the shuttle effect of lithium polysulfides via both chemical absorption and physical blocking. As a result, the integrated a-Ti3C2-S/d-Ti3C2/PP electrode was directly used for Li-S batteries, without the requirement of a metal current collector, and exhibited a high reversible capacity of 1062 mAh g-1 at 0.2 C and enhanced capacity of 632 mAh g-1 after 50 cycles at 0.5 C, outperforming the a-Ti3C2-S/PP electrode (547 mAh g-1) and conventional a-Ti3C2-S on an Al current collector (a-Ti3C2-S/Al) (597 mAh g-1). Furthermore, the all-MXene-based integrated cathode displayed outstanding rate capacity of 288 mAh g-1 at 10 C and long-life cyclability. Therefore, this proposed strategy of constructing an all-MXene-based cathode can be readily extended to assemble a large number of MXene-derived materials, from a group of 60+ MAX phases, for applications such as various batteries and supercapacitors.

Ultraflexible and High-Performance Multilayer Transparent Electrode Based on ZnO/Ag/CuSCN.

Driven by huge demand for flexible optoelectronic devices, high-performance flexible transparent electrodes are continuously sought. In this work, a flexible multilayer transparent electrode with the structure of ZnO/Ag/CuSCN (ZAC) is engineered, featuring inorganic solution-processed cuprous thiocyanate (CuSCN) as a hole-transport antireflection coating. The ZAC electrode exhibits an average transmittance of 94% (discounting the substrate) in the visible range, a sheet resistance ( Rsh) of 9.7 Ω/sq, a high mechanical flexibility without Rsh variation after bending 10 000 times, a long-term stability of 400 days in ambient environment, and a scalable fabrication process. Moreover, spontaneously formed nanobulges are integrated into ZAC electrode, and light outcoupling is significantly improved. As a result, when applied into super yellow-based flexible organic light-emitting diode, the ZAC electrode provides a high-current efficiency of 23.4 cd/A and excellent device flexibility. These results suggest that multilayer thin films with ingenious material design and engineering can serve as a promising flexible transparent electrode for optoelectronic applications.

Free-standing and flexible organic cathode based on aromatic carbonyl compound/carbon nanotube composite for lithium and sodium organic batteries

Free-standing and flexible organic cathode based on aromatic carbonyl compound/carbon nanotubes (CNTs) has been successfully synthesized by a simple vacuum filtration strategy. The obtained flexible and free-standing film could be directly used as the binder-, additive- and current collector-free cathode for lithium ion batteries (LIBs) and sodium ion batteries (SIBs). Benefitting from the synergistic effect provided by the aromatic carbonyl compound and CNTs, the flexible organic cathode shows excellent lithium and sodium storage properties, including high reversible capacity (∼150 mAh g−1 at 50 mA g−1 for LIBs and 57.8 mAh g−1 at 25 mA g−1 for SIBs), excellent cycling stability (over 500 cycles for LIBs and 300 cycles for SIBs) and good rate capability (48 mAh g−1 even at 2000 mA g−1 for LIBs and 48 mAh g−1 even at 1000 mA g−1 for SIBs). In view of the simple preparation process and excellent performance, the proposed strategy might open new avenues for the design of high-performance flexible organic electrode for the application in energy storage and conversion.

Core-Shell Composite Fibers for High-Performance Flexible Supercapacitor Electrodes.

Core-shell nanofibers containing poly(acrylic acid) (PAA) and manganese oxide nanoparticles as the core and polypyrrole (PPy) as the shell were fabricated through electrospinning the solution of PAA and manganese ions (PAA/Mn2+). The obtained nanofibers were stabilized by Fe3+ through the interaction between Fe3+ ions and carboxylate groups. Subsequent oxidation of Mn2+ by KMnO4 produced uniform manganese dioxide (MnO2) nanoparticles in the fibers. A PPy shell was created on the fibers by immersing the fibers in a pyrrole solution where the Fe3+ ions in the fiber polymerized the pyrrole on the fiber surfaces. In the MnO2@PAA/PPy core-shell composite fibers, MnO2 nanoparticles function as high-capacity materials, while the PPy shell prevents the loss of MnO2 during the charge/discharge process. Such a unique structure makes the composite fibers efficient electrode materials for supercapacitors. The gravimetric specific capacity of the MnO2@PAA/PPy core-shell composite fibers was 564 F/g based on cyclic voltammetry curves at 10 mV/s and 580 F/g based on galvanostatic charge/discharge studies at 5 A/g. The MnO2@PAA/PPy core-shell composite fibers also present stable cycling performance with 100% capacitance retention after 5000 cycles.