High-performance Flexible Electrode Research Articles

Overcoming the Tradeoff of Visible Transparency and Electrical Conductance via Dual Smoothing of Dielectric/Metal Interfaces in Cu-Thin-Layer-Based Transparent Electrodes.

The rapid advancement of flexible optoelectronic devices, such as light-emitting diodes, solar cells, and electrochromic devices, necessitates the development of high-performance flexible transparent electrodes (TEs). Dielectric/metal/dielectric (DMD)-type TEs are promising alternatives to conventional indium tin oxide (ITO) due to the high electrical conductance, excellent visible transparency, and sufficient mechanical flexibility. However, the tradeoff between electrical conductance and visible transparency poses a challenge to performance enhancement. This study introduces an Ar-ion-mediated interface modification method to address this tradeoff by dual smoothing of dielectric/metal interfaces in TiOx/Cu/ZnO TEs. Implementing this dual smoothing methodology significantly enhances both electrical conductance and visible light transmittance, achieving a Haacke figure of merit 200% higher than that of an unmodified otherwise identical structure. The highest figure of merit is 0.113 Ω-1, a record high for Cu-thin-layer-based DMD TEs, far surpassing ITO electrode values. Further, the enhanced optoelectronic performance remains highly durable under severe and simultaneous electrical, thermal, and mechanical stresses, showcasing the potential for significant advances in flexible optoelectronics.

Autonomous self-healing strategy for flexible fiber lithium-ion battery with ultra-high mechanical properties and volumetric energy densities

The development of wearable devices urgently requires flexible, lightweight, and high-performance power solutions. Flexible fiber batteries are flexible and deformable, which holds great potential in this field. However, it still face significant challenges in achieving excellent electrochemical performance and good mechanical properties. Herein, we developed a new method for preparing flexible fiber lithium-ion batteries by surface etching and in-situ chemical cross-linking strategies using direct ink writing-based 3D printing technology. On the one hand, the surface of graphene oxide undergoes oxidation and etching to form pores. This unique pore structure provides additional channels for ions, which increases the ion transmission rate. On the other hand, polyvinyl alcohol/sodium metaborate is introduced into the printing ink/coagulation bath. In-situ chemical cross-linking occurs during the printing and curing process, which exhibits self-healing properties. The flexible fiber electrode has excellent strain (∼30 %) at the macro level, and the assembled fiber lithium-ion battery exhibits impressive volumetric energy density (157.9 mWh cm−3), which exceeds previously reported flexible fiber batteries. And it is also integrated into wearable smart watches for use in daily life. This work proposes an innovative method to fabricate high-performance flexible fiber electrodes, which is of great significance for promoting the development of future portable/wearable electronics.

Growth of CQD/PPy on cotton cloth as a binder-free high-performance flexible electrode for symmetric supercapacitor

Benefited from the continuous development of nanomaterials, wearable microelectronics with flexible and miniaturized features are booming, and smart textiles are attracting more attentions because of their superior flexibility and perfect safety. The combination of conductive polymers and flexible substrates has created a new field for flexible supercapacitors. Herein, cotton cloth (CC), which is commonly used in daily life, was selected as the flexible substrate and a layer of CQDs was uniformly grown on the CC fibers, and then pyrrole was directly grown on CC by in-situ polymerization under Fe3+ initiation to realize the tight bonding between the conductive polymer and the flexible substrate. The electrochemical properties of the composite electrode could be controlled by changing the amount of CQD. The obtained CC@CQD@PPy electrode has a high specific capacitance (C) of 537.9 F/g at 0.5 A/g. The assembled symmetric CC@CQD@PPy supercapacitor exhibits an excellent energy density (E) of 18.7 Wh/kg at a power density (P) of 125.0 W/kg, and a supreme cycling stability with a capacitance retention of 77.9 % after 10,000 cycles. In addition, the CC@CQD@PPy supercapacitor has excellent flexibility, with almost unchanged electrochemical performance under different bending angles, and the assembled all-solid-state supercapacitor has good electrochemical properties.

Flexible Graphene Paper Modified Using Pt&Pd Alloy Nanoparticles Decorated Nanoporous Gold Support for the Electrochemical Sensing of Small Molecular Biomarkers.

The exploration into nanomaterial-based nonenzymatic biosensors with superb performance in terms of good sensitivity and anti-interference ability in disease marker monitoring has always attained undoubted priority in sensing systems. In this work, we report the design and synthesis of a highly active nanocatalyst, i.e., palladium and platinum nanoparticles (Pt&Pd-NPs) decorated ultrathin nanoporous gold (NPG) film, which is modified on a homemade graphene paper (GP) to develop a high-performance freestanding and flexible nanohybrid electrode. Owing to the structural characteristics the robust GP electrode substrate, and high electrochemically catalytic activities and durability of the permeable NPG support and ultrafine and high-density Pt&Pd-NPs on it, the resultant Pt&Pd-NPs-NPG/GP electrode exhibits excellent sensing performance of low detection limitation, high sensitivity and anti-interference capability, good reproducibility and long-term stability for the detection of small molecular biomarkers hydrogen peroxide (H2O2) and glucose (Glu), and has been applied to the monitoring of H2O2 in different types of live cells and Glu in body fluids such as urine and fingertip blood, which is of great significance for the clinical diagnosis and prognosis in point-of-care testing.

Construction of novel coaxial electrospun polyetherimide@polyaniline core-shell fibrous membranes as free-standing flexible electrodes for supercapacitors

Designing porous and flexible electrodes with excellent electrochemical properties is crucial to fabricate high-performance flexible supercapacitors. Conductive polymer-based electrospun fibers are expected to be an ideal material for constructing flexible electrodes due to the enormous specific surface area, unique pore structure and inherent flexibility. However, the exploration of such electrospun fibers with facile preparation methods and high areal capacitance remains challenging. Herein, a novel polyetherimide@polyaniline core-shell fibrous membranes (PEI@PANI CFMs) is fabricated via simply coaxial electrospinning and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) secondary doping. With the aid of polyetherimide, highly concentrated emeraldine base polyaniline (EB-PANI) can be perfectly wrapped on the surface of fibers without any spinning assistants, ensuring the well-developed pore structures, superior hydrophilicity, good electrical conductivity and outstanding mechanical flexibility. Benefiting from the synergistic effect of structure and constituents, such flexible PEI@PANI CFMs as electrode materials can provide more accessible electrochemical active sites and decrease the ion diffusion resistance. Therefore, the PEI@PANI CFMs present outstanding areal capacitance of 1159.9 mF cm−2 and remarkable energy density of 46.91 μW h cm−2 in the electrodes and symmetric supercapacitors, demonstrating great potential for applications in flexible energy storage field. This fabrication strategy provides a simple and practicable way to manufacture polyaniline-based fibrous membranes for high-performance flexible electrode materials.

A novel CoS/Ti3C2Tx MXene conductive filler effectively improves the pseudocapacitive performance of conductive hydrogel electrode materials for all-solid-state supercapacitors

It is currently an essential research direction to develop a low-cost, cost-effective, and environmentally friendly high-performance flexible electrode material. The electrode materials prepared from Ti3C2Tx MXene based hydrogels showed favorable physical and chemical properties (e.g., extreme flexibility, excellent mechanical strength, and electrical conductivity), and CoS (cobaltous sulfide) nanoparticles were grown by in-situ hydrothermal growth on the “accordion-like” Ti3C2Tx MXene surfaces, which were then synthesised into PCCT (PAA/chitosan/CoS/Ti3C2Tx) conductive hydrogels with physical crosslinked networks in a two-step process. This 3D porous structure containing highly conductive materials can provide suitable geometrical space and electronic structure, which can help to suppress the accumulation of active material at high mass loading and improve the specific capacitance of the electrode material. With a current density of 4 mA/g, the specific capacitance of the PCCT conductive hydrogel electrode was 3.6F/g. At a current density of 10 mA/g, the cycling stability was still 80 % after 2000 cycles. The elongation at break of PCCT conductive hydrogel wass about 349 %. After 48 h of healing at room temperature, the self-healing efficiency can reach 66.88 %. A foundation has been laid for obtaining flexible all-solid-state supercapacitors with stable structures and excellent electrochemical properties.

Enhancing the performance of quasi-solid-state flexible supercapacitors with Ag and MnO2 co-decorated carbon nanofibrous electrodes

Flexible supercapacitors are gaining increasing popularity due to their potential in powering portable, wearable, and lightweight electronic devices. In this study, we have successfully developed a high-performance flexible electrode for quasi-solid-state supercapacitors by incorporating Ag into carbon nanofibers (CNF) through electrospinning, and growing MnO2 as an active material on the electrode using an in-situ redox method. The Ag and MnO2 co-decorated CNF electrode offers several advantages over conventional CNF electrodes, including an enlarged reaction area, higher charge storage capacity, and improved resistance to brittleness issues. Among all the quasi-solid-state supercapacitors tested, the MnO2-decorated electrode with an optimized Ag incorporation exhibited the highest specific capacitance of 184 F/g at a current density of 1 A/g, representing a remarkable 17-fold increase compared to the reference CNF electrode. Furthermore, the Ag and MnO2 co-decorated electrode demonstrated an energy density of 16.3 Wh/kg and a power density of 400 W/kg at the same current density. Notably, the electrode exhibited excellent flexibility, with a capacitance retention rate of 87 % when subjected to bending with a curvature ranging from 5 to 1.25 cm. Our results suggest that the Ag and MnO2 co-decorated CNF electrode holds great promise for flexible energy storage applications.

Self-assembled high polypyrrole loading flexible paper-based electrodes for high-performance supercapacitors

Despite the intriguing features of freestanding flexible electronic devices, such as their binder-free nature and cost-effectiveness, the limited loading capacity of active material poses a challenge to achieving practical high-performance flexible electrodes. We propose a novel approach that integrates multiple self-assembly and in-situ polymerization techniques to fabricate a high-loading paper-based flexible electrode (MXene/Polypyrrole/Paper) with exceptional areal capacitance. The approach enables polypyrrole to form a porous conductive network structure on the surface of paper fiber through MXene grafting via hydrogen bonding and electrostatic interaction, resulting in an exceptionally high polypyrrole loading of 10.0 mg/cm2 and a conductivity of 2.03 S/cm. Moreover, MXene-modified polypyrrole paper exhibits a more homogeneous pore size distribution ranging from 5 to 50 μm and an increased specific surface area of 3.11 m2/g. Additionally, we have optimized in-situ polymerization cycles for paper-based supercapacitors, resulting in a remarkable areal capacitance of 2316 mF/cm2 (at 2 mA/cm2). The capacitance retention rate and conductivity rate maintain over 90 % after undergoing 100 bends.The maximum energy density and cycling stability are characterized to be 83.6 μWh/cm2 and up to 96 % retention after 10,000 cycles. These results significantly outperform those previously reported for paper-based counterparts. Overall, our work presents a facile and versatile strategy for assembling high-loading, paper-based flexible supercapacitors network architecture that can be employed in developing large-scale energy storage devices.

Nylon binder enables high-performance flexible ultra-thick electrode preparation in a water-based environment

A water-based method for producing ultra-thick, flexible lithium-ion battery cathodes and anodes with good deformation resistance and ultra-high areal capacity is proposed, in which nylon (PA) provides flexible support and forms fast ion channels.

Nickel-Doped Tungsten Disulfide for Energy Storage Applications

Nickel-doped hexagonal pyramid-like tungsten disulfide (WS2) has been synthesized via a simple hydrothermal synthesis method and offers great promise for use in electrochemical energy storage devices such as supercapacitors. The nickel concentration of 3, 4.5, and 6 at. % was added to WS2. Three-electrode and two-electrode configurations are used to test the electrochemical performances of the materials as prepared. The nickel doping of 4.5 at. % exhibits a high specific capacitance of 144 F g−1 at a current density of 1 A g−1 with an energy density of 13.3 Wh kg−1 and power density of 802 W kg−1 for the symmetric supercapacitor device. Increased electrochemically active surface area and decreased impedance are attributed to the improved supercapacitive performances. Finally, high specific power retention (92.1%) and Coulomb efficiency (86.0%) were demonstrated after 3,000 cycles. These results demonstrate that the synthesized nickel-doped WS2 can be used for high-performance supercapacitor flexible electrodes.

Multiscale Structural Design of 2D Nanomaterials-based Flexible Electrodes for Wearable Energy Storage Applications.

2D nanomaterials play a critical role in realizing high-performance flexible electrodes for wearable energy storge devices, owing to their merits of large surface area, high conductivity and high strength. The electrode is a complex system and the performance is determined by multiple and interrelated factors including the intrinsic properties of materials and the structures at different scales from macroscale to atomic scale. Multiscale design strategies have been developed to engineer the structures to exploit full potential and mitigate drawbacks of 2D materials. Analyzing the design strategies and understanding the working mechanisms are essential to facilitate the integration and harvest the synergistic effects. This review summarizes the multiscale design strategies from macroscale down to micro/nano-scale structures and atomic-scale structures for developing 2D nanomaterials-based flexible electrodes. It starts with brief introduction of 2D nanomaterials, followed by analysis of structural design strategies at different scales focusing on the elucidation of structure-property relationship, and ends with the presentation of challenges and future prospects. This review highlights the importance of integrating multiscale design strategies. Finding from this review may deepen the understanding of electrode performance and provide valuable guidelines for designing 2D nanomaterials-based flexible electrodes.

Freestanding, flexible and interfused V2O5 nanowire/rGO composite fiber nonwoven fabric for high-capacity binder-free aqueous zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) have emerged as a promising alternative to traditional lithium-ion batteries due to their high safety, environmental friendliness, and cost-effectiveness, as well as their high theoretical capacity. However, developing freestanding cathodes with excellent electrochemical performance, including high specific capacity and energy density, still faces significant challenges. Herein, we present a freestanding, flexible, and interfused V2O5 nanowire/reduced graphene oxide (rGO) composite fiber nonwoven fabric for the cathode of aqueous ZIBs via a novel wet-spinning and wet-fusing assembly strategy. The resulting V2O5 nanowire/rGO composite fiber nonwoven fabric can be directly utilized as the cathode for aqueous ZIBs, demonstrating outstanding electrochemical performance, including high specific capacity (402.5 mAh g−1 at 0.1 A g−1), energy density (286.3 Wh kg−1 at 71.4 W kg−1), excellent rate capability, and long cycling life (4000 cycles). This study provides new insights into the development of high-performance flexible freestanding electrode materials for ZIBs

Conductive Nanofiber Web Film with Polydimethylsiloxane Sidewalls Selectively Coated through a Plasma Process for High Performance Flexible Transparent Electrodes.

Transparent electrodes are commonly used in various applications, such as solar cells, touch screens, smart windows, wearable electronic devices, and rollable flexible displays. Currently, indium tin oxide (ITO) is widely used as a transparent electrode material. However, ITO is not suitable for next-generation transparent electrodes that require flexibility; therefore, alternative nanomaterials, such as carbon nanotubes, conductive polymers, and metal nanowires, are being studied. However, these nanomaterials have poor mechanical strength and limited substrate availability. In this study, we developed a high-performance transparent electrode web film fabrication process based on conductive nanofibers, in which metal nanofibers are semiembedded in polydimethylsiloxane (PDMS). The mechanical strength of the conductive nanofibers was improved through the PDMS coating on the entire surface of the film, and the semiembedded structure of the nanofibers was realized using the reactive ion etching (RIE) process. In this study, we confirmed through transparency/conductivity analysis and bending, cycle, and taping tests that the transparent electrode fabricated using our approach has excellent mechanical strength and conductivity. Finally, the transparent electrode fabricated using our method can be widely applied as a next-generation transparent electrode because the process is easy and simple and requires inexpensive equipment and materials.

Deposition of MnO2 on KOH-activated laser-produced graphene for a flexible planar micro-supercapacitor

The rapid development of flexible supercapacitors has been impeded by the difficulty of preparing flexible electrodes. We report the fabrication of a highly flexible and conductive microporous graphene-based substrate obtained by direct laser writing combined with KOH activation, which we call activated laser-produced graphene (a-LPG), which is then decorated with electrochemically deposited MnO2 to form a flexible a-LIG/MnO2 thin-film electrode. This hybrid electrode has a high areal capacitance of 304.61 mF/cm2 at a current density of 1 mA/cm2 in a 1 mol/L Na2SO4 aqueous electrolyte. A flexible asymmetric supercapacitor with a-LIG/MnO2 as the anode, a-LIG as the cathode and PVA/ H3PO4 as a gel electrolyte was assembled, giving an areal energy density of 2.61 μWh/cm2 at a power density of 260.28 μW/cm2 and an ultra-high areal capacitance of 18.82 mF/cm2 at 0.2 mA/cm2 with 90.28% capacitance retained after 5 000 cycles. It also has an excellent electrochemical performance even in the bent state. This work provides an easy and scalable method to design high-performance flexible supercapacitor electrodes and may open a new way for their large-scale fabrication.

Fabrication of transparent flexible electrodes on polyethylene terephthalate substrate with high conductivity, high transmittance, and excellent stability

Transparent flexible electrodes based on silver nanowires (AgNWs) have great potential for practical applications and many studies have been carried out to improve their conductivity, transparency, and surface roughness. In this study, we demonstrate a new approach for the fabrication of high-performance transparent flexible electrodes based on AgNWs and graphene oxide (GO) on polymer substrates using the pressing method. The surface morphology of the pressed AgNW/GO electrode was characterised by atomic force microscopy (AFM) and observed by scanning electron microscope (SEM). The electrode had a low surface roughness with the root mean square (Rq) of 7 nm. The electrode exhibits a low sheet resistance of 22 Ω/sq, a high transmittance of ~88%, resulted in a figures-of-merit (FoM) value of ~12.6. The optical and electrical properties of the electrode are comparable to that of the flexible ITO electrode. In addition, the electrodes also displayed outstanding durability and mechanical stability. This simple and scalable fabrication method is expected to contribute to future studies on flexible transparent conductive electrodes.

Regenerated silk protein based hybrid film electrode with large area specific capacitance, high flexibility and light weight towards high-performance wearable energy storage

Planar wearable supercapacitors (PWSCs) have sparked intense interest owing to their hopeful application in smart electronics. However, current PWSCs suffered from poor electrochemical property, weak flexibility and/or large weight. To relieve these defects, in this study, we fabricated a high-performance PWSC using silk protein derived film electrodes (PPy/RSF/MWCNTs-2; RSF, PPy and MWCNTs represent regenerated silk film, polypyrrole and multi-walled carbon nanotubes, respectively, while 2 is the mass ratio of silk to MWCNTs), which were developed by ‘dissolving-mixing-evaporating’ and in situ polymerization. In three-electrode, PPy/RSF/MWCNTs-2 showed a superb area specific capacitance of 8704.7 mF cm−2 at 5 mA cm−2, which surpassed numerous reported PWSC electrodes, and had a decent durability with a capacitance retention of 90.7 % after 5000 cycles. The PPy/RSF/MWCNTs-2 derived PWSC showed a largest energy density of 281.3 μWh cm−2 at 1660.1 μW cm−2, and a power density as high as 13636.4 μW cm−2 at 125.6 μWh cm−2. Furthermore, impressive capacitive-mechanical stability with a capacitance retention of 92 % under bending angles from 0 to 150 was depicted. Thanks to the rational and affordable preparation, our study for the first time prepared RSF electrode that had great capacitive property, high mechanical flexibility and light weight, simultaneously. The encouraging results can not only open up a new path to manufacture high-performance flexible electrodes, but may also help to realize the high-value-added utilization of silk.

Poly (3, 4-ethylenedioxythiophene) engineered hollow Bi2O3 core-shell architectures for long cycle performance of flexible supercapacitors

The booming development of electronic devices has promoted the in-depth research on flexible supercapacitors. The structural design of core@shell can be regarded as an effective method of achieving excellent electrochemical performance and outstanding flexibility of electrode materials. Herein, a special structure of core@shell hybrid Bi2O3-x@carbon fiber@poly (3, 4-ethylenedioxythiophene) (Bi2O3-x@CF@PEDOT) electrode derived from a Bi-metal-organic framework (Bi-MOF) is fabricated by using the electrospinning technique, and using the stabilization, pyrolyzation and polymerization procedures. The amount of Bi-MOF is regulated to obtain an optimized flexible substrate (Bi2O3-x@CF). The free space of the hollow Bi2O3 microrod can effectively alleviate the volume expansion during long cycling processes and further promote ion diffusion. The effective optimizations for the structure and content could significantly improve the conductivity and electrochemical performance of the constructed electrode. The prepared Bi2O3–0.5@CF@PEDOT electrode exhibits a satisfied specific capacitance of 460 F g−1 (1 A g−1) and great cycling stability. The assembled symmetric supercapacitor yields a desired energy density (i.e., 16.4 Wh kg−1) and power density (i.e., 500.34 W kg−1), and remarkable cycling performance (i.e., 99 % capacitance retention after 8500 cycles). Moreover, the excellent flexibility of the device is demonstrated by folding the supercapacitor into different angles and without obvious capacitance loss. This work provides a special structural design method of constructing high-performance flexible electrodes.

Bi-MOF-Derived Carbon Wrapped Bi Nanoparticles Assembly on Flexible Graphene Paper Electrode for Electrochemical Sensing of Multiple Heavy Metal Ions.

The development of nanohybrid with high electrocatalytic activity is of great significance for electrochemical sensing applications. In this work, we develop a novel and facile method to prepare a high-performance flexible nanohybrid paper electrode, based on nitrogen-doped carbon (NC) wrapped Bi nanoparticles (Bi-NPs) assembly derived from Bi-MOF, which are decorated on a flexible and freestanding graphene paper (GP) electrode. The as-obtained Bi-NPs encapsulated by an NC layer are uniform, and the active sites are increased by introducing a nitrogen source while preparing Bi-MOF. Owing to the synergistic effect between the high conductivity of GP electrode and the highly efficient electrocatalytic activity of Bi-NPs, the NC wrapped Bi-NPs (Bi-NPs@NC) modified GP (Bi-NPs@NC/GP) electrode possesses high electrochemically active area, rapid electron-transfer capability, and good electrochemical stability. To demonstrate its outstanding functionality, the Bi-NPs@NC/GP electrode has been integrated into a handheld electrochemical sensor for detecting heavy metal ions. The result shows that Zn2+, Cd2+, and Pb2+ can be detected with extremely low detection limits, wide linear range, high sensitivity, as well as good selectivity. Furthermore, it demonstrates outstanding electrochemical sensing performance in the simultaneous detection of Zn2+, Cd2+, and Pb2+. Finally, the proposed electrochemical sensor has achieved excellent repeatability, reproducibility, stability, and reliability in measuring real water samples, which will have great potential in advanced applications in environmental systems.

High mass loading paper-based electrode material with cellulose fibers under coordination of zirconium oxyhydroxide nanoparticles and sulfosalicylic acid

With the rapid expansion of the flexible electronics market, it is critical to develop high-performance flexible energy storage electrode materials. Cellulose fibers, which are sustainable, low cost, and flexible, fully meet the requirements of flexible electrode materials, but they are electrically insulating and cause a decrease in energy density. In this study, high-performance paper-based flexible electrode materials (PANI:SSA/Zr-CFs) were prepared with cellulose fibers and polyaniline. A high mass loading of polyaniline was wrapped on zirconia hydroxide-modified cellulose fibers under metal-organic acid coordination through a facile in situ chemical polymerization process. The increase in mass loading of PANI on cellulose fibers not only improves the electrical conductivity but also enhances the area-specific capacitance of the flexible electrodes. The results of electrochemical tests show that the area specific capacitance of the PANI:SSA/Zr-CFs electrode is 4181 mF/cm2 at 1 mA/cm2, which is more than two times higher than that of the electrode with PANI on pristine CFs. This work provides a new strategy for the design and manufacture of high-performance flexible electronic electrodes based on cellulose fibers.

Free-standing ultra-thin film with semi-embedded metal nanofiber web for high-performance flexible transparent electrodes

We develop a novel manufacturing process for freestanding conductive nanofiber web films in which metal nanofibers are semi-embedded in polydimethylsiloxane (PDMS) to realize high-performance flexible transparent electrode films. This concept utilizes a sacrificial layer to induce a very thin, free-standing film, and the semi-embedded structure maintains the nanofiber web structure. The excellent adhesion properties of PDMS as well as improved mechanical strength mean that it can be applied to large area devices. In this study, we demonstrate that our approach has improved mechanical strength and excellent conductivity through transparency/conductivity analysis and bending, cycle, and taping tests.