Flexible Free-standing Electrodes Research Articles

Cactus-like NiCo2O4@Nickel-plated fabric nano-flowers as flexible free-standing supercapacitor electrode

Binary transition-metal oxides have caused wide public concern for energy storage as a result of its higher electrical conduction and electrochemical activity compared to single metal oxides in the past few years. However, its practical applications are severely limited by poor specific capacitance and cycling stability. Recently, an effective strategy for enhancing the electrochemical performance of supercapacitors has been demonstrated by proper structural design and optimization of material. Herein, cactus-like NiCo2O4@NPF nano-flower flexible supercapacitor electrodes assembled from 1D NiCo2O4 nanoneedles on 2D hollow metal networks by simple hydrothermal and calcination treatment. The unique nano-flower structure can construct multi-dimensional channels, which facilitate the transport of electrons and ions and significantly enhances the reaction kinetics at the interface between the electrolyte and the active materials. The NiCo2O4@NPF-1 electrode displays an outstanding electrochemical energy storage performance and the specific capacitance is 1106 F g−1 at 1 A g−1. Besides, the composite electrode exhibits a distinguished rate capability (633 F g−1 at 50 A g−1) and favorable cyclability (capacity retention of 92.2% after 2000 cycles). These exceptional results recommend that as-assembled NiCo2O4@NPF nano-flower hybrid material could be a hopeful contender for multifunctional flexible supercapacitor electrodes.

Simultaneous detection of dihydroxybenzene isomers in the environment by a free-standing flexible ZnCo2O4 nanoplate arrays/carbon fiber cloth electrode

The simultaneous determination of dihydroxybenzene isomers is highly valuable for early environmental monitoring, but it is still a challenge. In this work, a free-standing flexible electrode was prepared for the simultaneous detection of hydroquinone (HQ), catechol (CC), and resorcinol (RC). The bimetallic zinc/cobalt zeolitic imidazolate frameworks nanoplate arrays (Zn/Co-ZIF NPAs) grown in situ on the carbon fiber cloth (CFC) was fabricated by a facile static synthesis method, and the porous ternary ZnCo2O4 NPAs derived from Zn/Co-ZIF NPAs were formed by annealing in air. Due to the fast electron transmission, abundant active sites and excellent electrocatalytic properties with enzyme-like kinetic performance of the ZnCo2O4/CFC electrode, the as-proposed sensor showed a wilder linear response (2–500 μM), a lower detection limits (0.03 μM HQ, 0.06 μM CC and 0.15 μM RC) and a higher sensitivity (23.58 μA μM−1 cm−2 HQ, 17.72 μA μM−1 cm−2 CC, and 15.18 μA μM−1 cm−2 RC), respectively. More importantly, the proposed electrochemical sensor exhibited excellent detection performance in complex water samples, providing a strategy for the detection of other toxic substances in the ecological environment.

Few‐Atomic‐Layered Co‐Doped BiOBr Nanosheet: Free‐Standing Anode with Ultrahigh Mass Loading for “Rocking Chair” Zinc‐Ion Battery

Insertion host materials are considered as a candidate to replace metallic Zn anode. However, the high mass loading anode with good electrochemical performances is reported rarely. Herein, a few-atomic-layered Co-doped BiOBr nanosheet (Co-UTBiOBr) is prepared via one-step hydrothermal method and a free-standing flexible electrode consisting of Co-UTBiOBr and CNTs is designed. Ultrathin nanosheet (3 atomic layers) and CNTs accelerate Zn2+ and electron transfer respectively. The Co-doping is conducive to the reduced Zn2+ diffusion barrier, the improved volume expansion after Zn2+ intercalation, and the enhanced electronic conductivity of BiOBr, verified by experimental and theoretical studies. An insertion-conversion mechanism is proposed according to ex situ characterizations. Benefiting from many advantages, Co-UTBiOBr displays a high capacity of 150 mAh g-1 at 0.1 A g-1 and a long-term cyclic life with ≈100% capacity attention over 3000 cycles at 1 A g-1 . Remarkably, excellent electrochemical performances are maintained even at an ultrahigh mass loading of 15mg cm-2 . Co-UTBiOBr//MnO2 "rocking chair" zinc-ion battery exhibits a stable capacity of ≈130 mAh g-1 at 0.2 A g-1 during cyclic test and its flexible quasi-solid-state battery shows outstanding stability under various bending states. This work provides a new idea for designing high mass loading anode.

Sb Nanoparticles Embedded in the N-Doped Carbon Fibers as Binder-Free Anode for Flexible Li-Ion Batteries.

Antimony (Sb) is considered a promising anode for Li-ion batteries (LIBs) because of its high theoretical specific capacity and safe Li-ion insertion potential; however, the LIBs suffer from dramatic volume variation. The volume expansion results in unstable electrode/electrolyte interphase and active material exfoliation during lithiation and delithiation processes. Designing flexible free-standing electrodes can effectively inhibit the exfoliation of the electrode materials from the current collector. However, the generally adopted methods for preparing flexible free-standing electrodes are complex and high cost. To address these issues, we report the synthesis of a unique Sb nanoparticle@N-doped porous carbon fiber structure as a free-standing electrode via an electrospinning method and surface passivation. Such a hierarchical structure possesses a robust framework with rich voids and a stable solid electrolyte interphase (SEI) film, which can well accommodate the mechanical strain and avoid electrode cracks and pulverization during lithiation/delithiation processes. When evaluated as an anode for LIBs, the as-prepared nanoarchitectures exhibited a high initial reversible capacity (675 mAh g−1) and good cyclability (480 mAh g−1 after 300 cycles at a current density of 400 mA g−1), along with a superior rate capability (420 mA h g−1 at 1 A g−1). This work could offer a simple, effective, and efficient approach to improve flexible and free-standing alloy-based anode materials for high performance Li-ion batteries.

Integrating Co3O4 nanocubes on MXene anchored CFE for improved electrocatalytic activity: Freestanding flexible electrode for glucose sensing

Developing facile strategies to construct free-standing flexible electrodes with extremely sensitive and stable electrochemical response towards analytes is of huge demand and it is still highly challenging. Herein, we design a unique and cost-effective hybrid electrode comprising of cobalt oxide (Co3O4) directly grown on highly conducting MXene based carbon fiber electrode (CFE) as a self-standing electrode using a simple hydrothermal-annealing method, where MXene nanosheets on electrode support facilitate the growth of uniform-sized Co3O4 nanocubes. Our results demonstrate that the synergetic effects of conductive MXene support and catalytically active Co3O4 nanocubes provide a superior response towards glucose oxidation in an alkaline medium (0.1 M KOH). The resultant Co3O4/MXene@CFE demonstrates remarkable sensitivity (19.3 μA mM−1 cm−2), wide linear range (0.05 μM to 7.44 mM) and lower detection limits (10 nM), thus providing excellent selectivity over interfering species and good electrochemical stability for an extended shelf-life. Therefore, the obtained self-standing Co3O4/MXene@CFE flexible electrode establishes a promising platform for glucose oxidation and it could be envisioned for the highly sensitive detection of various target molecules for their widespread applications in clinical diagnosis and health care monitoring.

Co-doped 1T‐MoS2 nanosheets anchored on carbon cloth as self-supporting anode for high-performance lithium storage

The widespread use of wearable devices has triggered a huge demand for flexible power source. Flexible lithium-ion batteries (LIBs) are widely regarded as an ideal power source for wearable devices because of their high energy density and long cycle life, but their application is limited by traditional rigid electrodes. A promising strategy is to prepare flexible electrodes by synthesising active materials with high capacity on flexible carbon substrates. In this study, Co-doped 1T‐MoS2 nanosheets were successfully anchored on a pretreatment carbon cloth (CC) by using a facile one-step solvothermal method, which is denoted as Co-MoS2/CC. Benefiting from the high conductivity and large layer spacing of the metallic 1T‐MoS2, self-supporting CC framework, doping of cobalt and their synergistic effect, when used as a self-supporting anode for LIBs, Co-MoS2/CC delivers a high reversible capacity of 1392 mAh g−1 at 0.1 A g−1 and retains 611 mAh g−1 at 2 A g−1 over 300 cycles. Furthermore, it delivers a remarkable rate performance of 230 mAh g−1 at 10 A g−1. The facile and scalable synthesis method can also be applied to other similar flexible free-standing electrodes for the application of electrochemical energy storage.

An Ultrafast and Stable High-Entropy Metallic Glass Electrode for Alkaline Hydrogen Evolution Reaction

A new type of high-entropy alloy with a composition of Pt25Pd25Ni25P25 (at.%) and an amorphous structure, referred to as a high-entropy metallic glass (HEMG), was developed by a scalable metallurgical technique for efficient hydrogen evolution reaction (HER). The achieved overpotential was as low as 19.8 mV at a current density of 10 mA cm–2 while maintaining an ultrareliable performance for 60 h in 1.0 M KOH solution, exhibiting 5- and 10-times higher performance than those of traditional Pt60Ni15P25 and Pd40Ni40P20 metallic glasses, respectively, and also surpassing the benchmark performance of commercial Pt/C nanoparticles and pure Pt sheet. Experimental and theoretical results revealed that the enhanced HER activity was ascribed to a synergistic function of multiprincipal components that optimized the electronic structure to accelerate the rate-determining steps in HER. Moreover, the unique long-range disordered structure provided a high density of unsaturated atomic coordination that was able to improve the amount of electrochemically active sites. This bulk HEMG strategy paves the way for the development of flexible freestanding electrodes for water splitting applications.

3D porous H-Ti3C2Tx films as free-standing electrodes for zinc ion hybrid capacitors

Flexible zinc ion hybrid capacitor (ZICs) as a kind of emerging energy storage devices have great prospect for grid-scale energy storage and wearable electronics due to some advantages such as appropriate density, superb power output, abundant zinc resources, and nontoxic nature. Currently, there are still great demands to seek and design novel electrodes for zinc ion hybrid capacitors, especially for flexible free-standing electrode. Herein, a novel rechargeable aqueous Zn-ion hybrid capacitor using 3D porous H-MXene film (3D-PHMF) as the cathode was reported. The 3D porous H-MXene film was constructed with an interlayer interaction engineering via freeze-casting method, wherein the hydrogen ion (H+) was introduced to weaken electrostatic repulsion of MXene interlayer. The aqueous Zn//3D-PHMF capacitor delivers a specific capacitance of 105.6 mAh g−1 at 0.2 A g−1 and good rate performance of 61.0 mAh g−1 at 5 A g−1 with remarkable cycling stability, which remains 90% of specific capacitance after 20,000 cycles. Besides, the capacitor also shows outstanding anti-self-discharge ability in which the self-discharge rate is 1.87 mV h−1. Remarkably, the as-obtained flexible cathode can be designed into a flexible quasi-solid-state device, which shows excellent flexibility and stability. This work demonstrates the promising possibility of MXene as free-standing electrode for energy storage applications.

Fabrication of NiFeB flexible electrode via electroless deposition towards selective and sensitive detection of dopamine.

A novel cost-effective and eco-friendly flexible electrochemical sensor was designed for highly selective and sensitive detection of dopamine to deal with the problems related to serious neurological disorders. The novel flexible paper electrode with NiFeB synthesised by simple dip-coating exhibits a high sensitivity of 35.35 μA μM-1 cm-2, 2.36 μA μM-1 cm-2 and 0.215 μA μM-1 cm-2 in the linear ranges from 10 nM to 1 μM, 5 μM to 50 μM and 100 μM to 400 μM, respectively, with an ultra-low detection limit of 2.1 nM. Besides, the free-standing flexible electrode for the detection of DA, the flexible paper sensor displayed superior selectivity towards various interferents, such as ascorbic acid, glucose and uric acid, as well as stability under various deformations.

Insights into MWCNTs/polyimide nanocomposites: from synthesis to application as free-standing flexible electrodes in low-cost micro-supercapacitors

In the pursuit to enlarge the library of polyimide materials for energy applications, new polyimide/MWCNTs composite films have been developed by MWCNTs-assisted polycondensation reaction of a hydroxyl and triphenylmethane-containing diamine with benzophenone tetracarboxylic dianhydride targeting to highlight their electrical storage capability as flexible electrodes in micro-supercapacitors (mSCs). The Fourier-transform infrared spectroscopy, proton nuclear magnetic resonance, UV–vis, fluorescence, and Raman spectroscopies were used to demonstrate the evolution of interfacial interactions between MWCNTs and the precursors (diamine monomer and intermediate polyamidic acid) and polyimide matrix that proved to be the origin of MWCNTs homogeneous dispersion. Thus, composite films incorporating 1, 3, 5, and 10 w.t.% MWCNTs were obtained and thoroughly investigated with regard to their morphology, mechanical behavior, thermal stability, and electrical conductivity. The electrochemical performance of these composites was first analyzed in a classical three-electrode cell by cyclic voltammetry and galvanostatic charge-discharge in both aqueous and organic electrolyte systems. By far, the best electrical storage capacity was obtained with the composite polyimide film containing 10% MWCNTs that was further used as both active material and current collector in a flexible symmetric mSC realized by a straightforward and low-cost procedure. In the attempt to better exploit the advantages of this composite film, it was layered with a graphite-containing paint and tested as an electrode in a flexible mSC, which provided satisfactory results. To our knowledge, this is the first report on the electrical charge storage capability of a polyimide/MWCNTs free-standing film as a flexible electrode in mSCs, which do not require time- and resource-consuming processing steps.

Porous Carbon Tubes Constructing Freestanding Flexible Electrodes for Symmetric Potassium-Ion Hybrid Capacitors

Developing electrode materials to apply in anodes and cathodes with superior kinetics and high specific capacity still remains a serious challenge for potassium-ion hybrid capacitors (PIHCs). Herein, a well-designed flexible freestanding electrode consisting of porous carbon tubes (FSF-PCTs) that functioned as both an anode and a cathode for PIHCs presents favorable energy storage performance. Based on this fully carbon-based FSF-PCTs electrode, symmetrical freestanding potassium-ion hybrid capacitors (SPIHCs) are developed. Moreover, SPIHCs deliver good practical application in terms of a high power density of 8109.9 W kg–1 (at 39.2 W h kg–1), a high energy density of 117.8 W h kg–1 (at 450 W kg–1), and a great cycle stability of 51 W h kg–1 at 1 A g–1 after 1500 cycles as well as ultrafast charge and slow discharge characteristics (full charge in only 1 min and the discharge time lasting for 15 min). Moreover, systematic characterization analysis in half potassium-ion batteries (PIBs) reveals that the K+ storage mechanism in anode FSF-PCTs is a synergistic effect of intercalation and surface capacitance, and the operating principle of FSF-PCTs as a cathode is an adsorption behavior of PF6– on its surface.

Graphene paper with electrodeposited NiCo2S4 nanoparticles as a novel flexible sensor for simultaneous detection of folic acid and ascorbic acid

We studied the production and electrochemical performance of NiCo2S4 electrodeposited reduced graphene oxide (rGO) paper for use as a freestanding flexible electrode for the simultaneous determination of folic acid (FA) and ascorbic acid (AA). Production of rGO paper was achieved through vacuum filtration of the aqueous dispersion of graphene oxide, followed by chemical reduction in hydrogen iodide solution. NiCo2S4/rGO paper material was fabricated with a simple approach using electrodeposition of hydrothermal synthesized NiCo2S4 nanoparticles directly on rGO paper. The physicochemical properties of as-prepared flexible NiCo2S4/rGO paper were characterized by means of field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray powder diffraction patterns. This freestanding paper electrode demonstrated high sensitivity, a wide linear range, and a low detection limit towards the simultaneous detection of FA and AA. NiCo2S4/rGO paper sensor, possessing easy preparation, low cost, and high flexibility, was tested in real samples with promising recoveries. This study exhibited that the approach of the electrodeposition of bimetallic sulfides on freestanding graphene paper substrates has great potential to develop high-performance flexible electrochemical sensors.

Porphyrin-assisted synthesis of hierarchical flower-like polypyrrole arrays based flexible electrode with high areal capacitance

Flexible free-standing electrode with special hierarchical structure have received widespread attentions due to the increased specific surface area, shortened transport path of electrolyte ions and accelerated ion transmission. Herein, for the first time, the flexible free-standing three-dimensional (3D) flower-like polypyrrole/5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin copper (II) (TF-PPy/Cu-TCPP) nanocomposite electrode with hierarchical PPy micro-flower arrays on the surface has been fabricated via the combination of electrophoretic deposition and electrochemical polymerization. The growth mechanism is involved in the following aspects. I) The wrinkled Cu-TCPP nanosheets provide abundant porous channels to facilitate the immersion of pyrrole molecules and uniform cross-linking of PPy. II) The rough surface of the PPy/Cu-TCPP composite film provides rough sites for the growth of hierarchical TF-PPy arrays under the assistance of H2 bubbles. The collaborative advantages of 3D conductive networks and the hierarchical TF-PPy arrayed on the rough surface of electrode facilitate the fast ion/electron transport for the enhanced electrochemical performances. The corresponding TF-PPy/Cu-TCPP electrode shows an areal capacitance of 593 mF cm−2 at 0.5 mA cm−2, which is superior to the PPy/Cu-TCPP electrode (351 mF cm−2) and the pristine PPy (287 mF cm−2). The symmetric supercapacitor exhibits the energy density of 5.94 μW h cm−2 at a highest power density of 25 μW cm−2. This work will further enrich the valuable design concepts of flexible free-standing electrode with hierarchical structure for energy storage and conversion.

Electrochemical fabrication of polyaniline/graphene paper (PANI/GP) supercapacitor electrode materials on free-standing flexible graphene paper

Free-standing flexible supercapacitive electrodes have practical application for wearable energy storage devices. In this paper, graphene paper (GP), a flexible electrode substrate, was prepared by one-step reduction of graphene oxide (GO) using HI solution. GP can be used independently as a flexible electrode with specific capacitance of 227 F/g. In order to make up for the shortage of GP specific capacitance storage, polyaniline (PANI) with high specific capacitance and good electrical conductivity was selected to composite with GP by electrochemical polymerization approach. This method to fabricate electrode material by direct electrochemical polymerization avoids the use of conductive binder and organic solvent. Owing to the specific capacitance contribution of PANI and GP, the PANI/GP composites exhibit higher specific capacitance when the polymerization time is 30 s and the polymerization voltage is 0.8 V. At 1 A/g current density, the specific capacitance of composite is up to 759 F/g, which is 3.34 times of neat GP.

Construction of flexible V3S4@CNF films as long-term stable anodes for sodium-ion batteries

The vanadium sulfides draw comprehensive attention as alternative anode materials for sodium ion batteries (SIBs) owing to their layered structural features and high theoretical capacities, but the unsatisfactory cycle life and rate performance caused by the huge volume change hinder their commercial application. Herein, we fabricate a flexible film with layered V3S4 embedded in carbon nanofiber (V3S4@CNF) via a facile and scalable industrialized electrospinning method followed by the thermal sulfuration. When evaluated as anodes for SIBs, V3S4@CNF delivers brilliant sodium storage performance (400 mAh g−1 at 0.1 A g−1; 185 mAh g−1 at 10 A g−1; capacity retention of 98% after 3500 cycles). Moreover, the assembled full cell performs remarkable capacity of 265 mAh g−1 and superior energy density of 400 Wh kg−1 at 0.1 A g−1. The carbon nanofibers-constructed film supported the integrity of the structure and constructed the cross-linked conductive skeleton to boost the electron transfer and reaction kinetic, and the layered V3S4 in the carbon nanofibers gave the mostly capacity, endowing the excellent sodium storage performance. This work not only provides a casual engineering to devise flexible freestanding electrode, but also designs a high energy density anode with great potential for practical application in SIBs.

All-polymer free-standing electrodes for flexible electrochemical sensors

Abstract Flexible free-standing electrodes with inherent conductivity and superior mechanical property are highly desired for flexible electrochemical sensor. Herein, we report the design of all-polymer flexible film electrodes for dopamine (DA) detection, in which polypyrrole (PPy) doped with pentaerythritol ethoxylate (PEE) are grown together with PPy doped with 2-naphthalene sulfonate (2-NS-PPy) by adopting sandwiched structure. This all-polymer electrode is free-standing and breaks through the bottleneck of traditional flexible electrodes that requires elastic substrate and conductive additives, demonstrating high mechanical tensile strength (81.9 MPa), good flexibility (elongation-at-break of 41.6 %), and inherent and stable conductivity. When used as electrochemical sensor for DA detection, this PPy composite film electrode shows high sensitivity (343.75 μA mM−1 cm−2). Moreover, this all-polymer electrode can bear various deformation (bending, twisting, folding, and knotting) with negligible effect on the sensing performance. This electrochemical sensor demonstrates strong electrochemical stability with 97.3 % current response remaining at 25 % strain for DA detection. The strong sensing stability (mechanical deformation and long-time storage), in conjunction with superior mechanical property and high sensitivity, proves that this all-polymer free-standing film electrode exhibits giant potential in application of flexible electrochemical sensors.

Fabrication of rGO/CoSx-rGO/rGO hybrid film via coassembly and sulfidation of 2D metal organic framework nanoflakes and graphene oxide as free-standing supercapacitor electrode

Porous cobalt sulfide (CoSx)/reduced graphene oxide (rGO) hybrid films has been fabricated as a flexible freestanding supercapacitor electrode via the co-assembly and sulfidation of 2D metal organic framework (MOF) nanoflakes and graphene oxide (GO). Firstly, zeolitic imidazolate-67 (ZIF-67) nanocubes were added into the aqueous solution of GO to yield a mixed dispersion of ZIF-67 and GO (ZIF-GO). It was found that the morphology of ZIF-67 changed from nanocubes to 2D nanoflakes owing to the concentration change, which favored the formation of hybrid film. Secondly, the sandwich-like GO/ZIF-GO/GO hybrid film was fabricated by the successive vacuum membrane filtration of GO, ZIF-GO, and GO solutions. Finally, the hybrid film was sulfidized via a hydrothermal process using thioacetamide as the sulfur source. This process also led to the reduction of GO to rGO. The resulting rGO/CoSx-rGO/rGO hybrid film was shown to have good electrochemical performance because it combined the good pseudocapacitor property of cobalt sulfide as well as the good conductivity and electric double layer capacitor property of rGO. In addition, an all-solid-state asymmetric supercapacitor (aSC) was assembled using rGO/CoSx-rGO/rGO hybrid film and active carbon as the positive and negative electrodes, respectively. It exhibited an energy density of 10.56 Wh kg−1 and a power density of 2250 W kg−1. Also, it retained 92.8% of initial capacitance after 10,000 cycles. The good electrochemical performance revealed that the resulting aSC has great potential in the practical application of supercapacitors.

Direct Coating Pen Ink Carbon on a Carbonized Melamine Sponge as a Flexible Free-Standing Electrode

A facile and environmental-friendly method is developed to enhance the mechanical properties and electrochemical performance of a carbonized melamine sponge (CMS) via commercial pen ink coating. Th...

Freestanding and Flexible β-MnO2@Carbon Sheet for Application as a Highly Sensitive Dimethyl Methylphosphonate Sensor.

Research on wearable sensor systems is mostly conducted on freestanding polymer substrates such as poly(dimethylsiloxane) and poly(ethylene terephthalate). However, the use of these polymers as substrates requires the introduction of transducer materials on their surface, which causes many problems related to the contact with the transducer components. In this study, we propose a freestanding flexible sensor electrode based on a β-MnO2-decorated carbon nanofiber sheet (β-MnO2@CNF) to detect dimethyl methylphosphonate (DMMP) as a nerve agent simulant. To introduce MnO2 on the surface of the substrate, polypyrrole coated on poly(acrylonitrile) (PPy@PAN) was reacted with a MnO2 precursor. Then, phase transfer of PPy@PAN and MnO2 to carbon and β-MnO2, respectively, was induced by heat treatment. The β-MnO2@CNF sheet electrode showed excellent sensitivity toward the target analyte DMMP (down to 0.1 ppb), as well as high selectivity, reversibility, and stability.

Ultrafine Co0.85Se nanocrystals dispersed in 3D CNT network as a flexible free-standing anode for high-performance lithium-ion battery

A flexible free-standing film electrode with high pseudocapacitive contribution and strengthened cyclic stability was successfully prepared.