Flexible Supercapacitor Electrodes Research Articles

Nanocellulose-based advanced materials for flexible supercapacitor electrodes

Nanocellulose is sustainable, has excellent properties and a unique structure that can be used to create environmentally friendly supercapacitor electrodes with pore structures. Supercapacitor, as a high-performance electrochemical energy storage device, has the advantages of high power density, long cycle life, fast charging and discharging, and high energy density. Currently, the problem of energy depletion is becoming more and more serious, and in order to solve this problem, people are beginning to replace traditional petroleum-based materials with environmentally friendly biomass-based materials. Nanocellulose, as a derivative of cellulose, the most widely distributed biomass in nature, has high specific surface area, biodegradability, high strength and good dispersibility. This review discusses and summarizes the latest research on nanocellulose-based supercapacitor electrode materials. First, the physical and chemical properties of nanocellulose and the preparation strategy are presented. Second, the types of electrodes are introduced in terms of electrodes with nanocellulose as substrate and those with other support materials. Next, recent studies on the use of these electrodes for flat supercapacitors and fiber-based supercapacitors are presented. Finally, the future prospects and challenges of using nanocellulose in supercapacitor electrode materials are outlined.

Construction and electrochemical properties of carbon nanotube composite Mn-MOFs materials electrode for high-performance flexible supercapacitors

Metal organic frameworks (MOFs) as electrode materials for supercapacitors have attracted extensive attention due to their porous structure. However, the poor conductivity of MOFs limits the charge transfer. In this work, Mn-MOF/carbon nanotube (CNT) composites were grown directly on carbon cloth (CC) in one step by the traditional hydrothermal synthesis of MOFs to improve the electrical conductivity of Mn-MOFs. The structural and performance tests show that CNTs, as a conductive backbone, can connect Mn-MOFs to each other, which can obviously solve the problem of poor conductivity of MOF materials. The Mn-MOFs/CNTs/CC electrodes show a larger specific capacitance and excellent cycling stability by electrochemical tests. Compared to the original Mn-MOF electrode, the surface capacitance obtained increases from 188.8 mF·cm−2 to 385.8 mF·cm−2 at a current density of 0.8 mA·cm−2 and maintains a high capacitance retention of 118% after 5000 charge/discharge cycles at a current density of 5 mA·cm−2. This work confirms that the in- situ synthesis of MOFs and CNTs has promising applications in fabric-based flexible supercapacitors.

Robust double-network polyvinyl alcohol-polypyrrole hydrogels as high-performance electrodes for flexible supercapacitors

The growing demands of flexible and wearable electronic devices boost the rapid development of flexible supercapacitors (FSCs). Conductive hydrogels are considered to be one type of promising electrode materials for FSCs due to their good processability and electrochemical properties. However, the poor mechanical properties of conductive hydrogels hinder their practical applications. Building robust cross-linked network structures is a feasible way to enhance their mechanical properties. Herein, the double-network polyvinyl alcohol (PVA)-polypyrrole (PPy) conductive hydrogels are synthesized by the freeze–thaw and in-situ polymerization method. The double-network structure not only enhances mechanical properties of the hydrogels, but also promotes their electrolyte ion transport. The maximum elongation at break of the optimized PVA-PPy hydrogels can reach 156.4%, and the specific capacitance is 1718.7 mF cm−2 at 0.5 mA cm−2. Furthermore, the energy densities of the symmetrical PVA-PPy FSCs are 46.7 and 13.3 μWh cm−2 at power densities of 200.0 and 2000.0 μW cm−2. Such excellent electrochemical performances and mechanical properties make the synthesized PVA-PPy hydrogels a promising candidate for FSCs.

Hierarchical porous 3D Ni3N-CoN/NC heterojunction nanosheets with nitrogen vacancies for high-performance flexible supercapacitor

In the realm of everyday existence, the allure of flexible portable electronic devices has captured widespread attention. Consequently, the imperative to conceive and fabricate flexible energy storage/conversion systems assumes paramount significance. Transition metal nitrides (TMNs), distinguished by their extraordinary electrochemical properties, emerge as compelling candidates for electrode materials in high-performance energy storage devices. This study undertakes the synthesis of three-dimensional Ni3N-CoN/NC heterojunction nanosheets via a meticulous two-step in situ growth process involving metal organic frameworks (MOFs) on carbon cloth (CC), succeeded by annealing in an ammonia-rich environment. The ensuing nitridation engenders copious ion pathways, engrossing a considerable specific surface area of 233.2 m2 g−1 and endowing nitrogen vacancies in abundance. The augmented conductivity of the nitrogen-vacancy-rich heterojunction is corroborated through density functional theory calculations. Employed as a flexible freestanding supercapacitor electrode, the Ni3N-CoN/NC/CC configuration evinces a marked enhancement in electrochemical performance attributable to its superior intrinsic conductivity and heightened active sites. Notably, the synthesized Ni3N-CoN/NC/CC flexible electrode showcases a remarkable specific capacitance of 468.3 mA h g−1 at a current density of 3 A g−1. Moreover, the integration of the Ni3N-CoN/NC/CC cathode with an activated carbon (AC) anode in a flexible asymmetric supercapacitor (ASC) yields an impressive energy density of 0.2144 mWh cm−2 and a maximum power density of 80 mW cm−2, exhibiting exceptional cycling stability of 92.3% even after 15,000 cycles. The exceptional performance exhibited by the synthesized freestanding Ni3N-CoN/NC/CC composite underscores its tremendous potential in the development of flexible energy storage devices.

“Improving with using” effect and mechanism analysis of electrodeposited poly(1,5-diaminoanthraquinone)/carbon cloth electrode for high-performance flexible supercapacitors

As a robust and conductive substrate, carbon cloth (CC) has been modified with various pseudocapacitive materials to boost its electrochemical performance in flexible supercapacitors. Here, poly(1,5-diaminoanthraquinone) (PDAA) electrodeposited CC electrodes were developed and much higher areal specific capacitance was obtained in comparison with the functionalized CC (FCC). Most importantly, an unusual phenomenon of “improving with using” was found for the optimized one, FCC@PDAA-3, which exhibited the increased capacitance retention from 150.4% to 194.8% with increasing the number of cycles from 10,000 to 50,000. Such extraordinary cyclic life was mainly ascribed to the doping and electropolymerization of the encapsulated 1,5-diaminoanthraquinone (DAA) in the immobilized PDAA during the electrochemical cycles. These findings are expected to contribute to a deeper understanding of the pseudocapacitive materials evolution during long charging/discharging cycles, favoring the design of long-life supercapacitors for practical application.

Rational design of N-doped porous biomass carbon nanofiber electrodes for flexible asymmetric supercapacitors with high-performance

Flexible supercapacitors have become a concern of flexible energy storage devices. In the process of exploring, the squid ink as N-doped biomass carbon was introduced to self-supporting flexible supercapacitors by electrospinning for the first time. By regulating the content of squid ink, the activation temperature, and the mass ratio of KOH-assisted activation, a N-doped porous biomass carbon@polyacrylonitrile (NPBC-1.5@PAN-800-2) with an ideal capacity and an excellent specific surface area (636.68 m2 g−1) was prepared. In addition, ZnO further optimized the capacity (422.7 F/g at 1 A g−1) and as an adhesive-free electrode for flexible asymmetric supercapacitor provided a relatively high specific capacitance, a satisfactory capacitance retention, and a wonderful energy density (48.01 Wh kg−1 at 1124.43 W kg−1). The capacitance was thought to originate from a suitable microporous structure, N-doped, and metal-derived pseudocapacitance propertie.

Solvent‐free synthesis of ZIF‐67 thin films as the catalyst for the direct growth of carbon nanotubes for the fabrication of electrical double‐layer capacitors

AbstractObjectiveThis work is focused on the direct growth of vertically aligned carbon nanotubes (CNTs) onto the flexible stainless‐steel mesh (SM) substrate using ZIF‐67 as the catalytic layer.Materials & MethodsThe CNTs/SM electrode is prepared by two‐step chemical vapor deposition (CVD) method using ZIF‐67 thin films as the catalytic layers. The time duration for the deposition of ZIF‐67 thin films by an atmospheric CVD process is controlled for direct, vertically aligned, dense and uniform growth of CNTs by a low‐pressure CVD process at a low temperature of 450 °C.Results & DiscussionThe electrochemical performance of CNTs/SM electrode is studied by cyclic voltammetry and galvanostatic charge/discharge measurements using KOH aqueous electrolyte which showed their better EDLC features and outstanding rate capability. The binder‐free directly grown CNTs/SM electrode displayed a low interfacial resistance between CNTs and SM substrate which comes from their synergistic effect of lower charge loss and better charge collection. Moreover, vertically aligned one‐dimensional CNTs could be rendered as the charge highway in the axial direction, which can promote the rapid charge transportation.ConclusionsThe directly grown CNTs on the commercial SM substrate using the ZIF‐67 catalytic layer fabricated via the solvent free technology can be regarded as a promising EDLC electrode for flexible super capacitors.

Super-Hydrophilic Coal-Based Carbon Fibers as Flexible Electrodes for Supercapacitors

Super-Hydrophilic Coal-Based Carbon Fibers as Flexible Electrodes for Supercapacitors

Hydrothermal Synthesis of a Cellular NiO Film on Carbon Paper as a Promising Way to Obtain a Hierarchically Organized Electrode for a Flexible Supercapacitor.

The formation of a cellular hierarchically organized NiO film on a carbon paper substrate under hydrothermal conditions using triethanolamine as a base has been studied. The thermal behavior of the carbon paper substrate with the applied semi-product shell was studied using synchronous thermal analysis (TGA/DSC) and it was demonstrated that such modification of the material surface leads to a noticeable increase in its thermal stability. Using scanning electron microscopy (SEM), it was shown that the NiO film grown on the carbon fiber surface is characterized by a complex cellular morphology, organized by partially layered individual nanosheets of about 4-5 nm thickness and lateral dimensions up to 1-2 μm, some edges and folds of which are located vertically relative to the carbon fiber surface. The surface of the obtained material was also examined using atomic force microscopy (AFM), and the electronic work function of the oxide shell surface was evaluated using the Kelvin probe force microscopy (KPFM) method. The electrochemical parameters of the obtained flexible NiO/CP electrode were analyzed: the dependence of the specific capacitance on the current density was determined and the stability of the material during cycling was studied, which showed that the proposed approach is promising for manufacturing hierarchically organized electrodes for flexible supercapacitors.

Boosting the Capacitive Performance of Supercapacitors by Hybridizing N, P-Codoped Carbon Polycrystalline with Mn3O4-Based Flexible Electrodes.

Chitosan, a biomass raw material, was utilized as a carbon skeleton source and served as a nitrogen (N) atom dopant in this study. By co-doping phosphorus (P) atoms from H3PO4 and nitrogen (N) atoms with a carbon (C) skeleton and hybridizing them with Mn3O4 on a carbon fiber cloth (CC), an Mn3O4@NPC/CC electrode was fabricated, which exhibited an excellent capacitive performance. The N, P-codoped carbon polycrystalline material was hybridized with Mn3O4 during the chitosan carbonization process. This carbon polycrystalline structure exhibited an enhanced conductivity and increased mesopore content, thereby optimizing the micropore/mesopore ratio in the electrode material. This optimization contributed to the improved storage, transmission, and diffusion of electrolyte ions within the Mn3O4@NPC electrode. The electrochemical behavior was evaluated via cyclic voltammetry and galvanostatic charge-discharge tests using a 1 M Na2SO4 electrolyte. The capacitance significantly increased to 256.8 F g-1 at 1 A g-1, and the capacitance retention rate reached 97.3% after 5000 charge/discharge cycles, owing to the higher concentration of the P-dopant in the Mn3O4@NPC/CC electrode. These findings highlight the tremendous potential of flexible supercapacitor electrodes in various applications.

Porous prussian blue analogs derived nickel-iron bimetallic phosphide nanocubes on conductive hollow mesoporous carbon nanospheres for stable and flexible high-performance supercapacitor electrode

Nickel-iron bimetallic phosphide (Ni-Fe-P) is the ideal battery-type materials for supercapacitor in virtue of high theoretical specific capacitance. Nevertheless, its actual adhibition is astricted on account of inferior rate capability and cyclic stability. Herein, we constructed hierarchical core–shell nanocomposites with hollow mesoporous carbon nanospheres (HMCS) packaged via prussian blue analogs derived Ni-Fe-P nanocubes (Ni-Fe-P@HMCS), as a positive electrode for hybrid supercapacitor (HSC). Profiting from the cooperative effects of Ni-Fe-P nanocubes with small size and good dispersibility, and HMCS with continuously conductive network, the Ni-Fe-P@HMCS composite electrode with abundantly porous architectures presents an ultrahigh gravimetric specific capacity for 739.8 C g−1 under 1 A g−1. Specially, the Ni-Fe-P@HMCS electrode presents outstanding rate capability of 78.4% (1 A g−1 to 20 A g−1) and cyclic constancy for 105% after 5000 cycles. Density functional theory implies that the composite electrode possesses higher electrical conductivity than bare Ni-Fe-P electrode by reason of the incremental charge density, and the electrons transferring from NiFe3P4 to HMCS layers. Additionally, the assembled Ni-Fe-P@HMCS//HMCS HSC facility delivers the high energy density for 64.1 Wh kg−1, remarkable flexibility and mechanical stability. Thus, this work proffers a viable and efficacious measure to construct ultra-stability electrode for high-performance portable electronic facilities.

High specific surface area MXene/SWCNT/cellulose nanofiber aerogel film as an electrode for flexible supercapacitors

Transition metal carbonitrides (MXene) with two-dimensional layered structures are a recent research topic in the field of supercapacitors. However, because MXene can easily self-stack, its capacitance performance is affected. To this end, one-dimensional cellulose nanofibers (CNF) and carboxylic single-walled carbon nanotubes (SWCNT) were used to intercalate MXene to obtain well-dispersed MXene/SWCNT/CNF aqueous suspensions and their hybrid hydrogels, and to obtain hybrid aerogels through supercritical drying. CNF gives aerogel excellent porosity, a high specific surface area (301.03 m2 g−1), good electrolyte infiltration, and gentle mechanical flexibility, while SWCNT imparts high conductivity to aerogel film. We assembled a symmetrical all-solid-state flexible supercapacitor and interdigitated micro-supercapacitors using an aerogel film as a flexible electrode. The area specific capacity of the above electrode reaches 746.68 mF cm−2 and 244.50 mF cm−2, respectively. This study provides a fabrication method of polymer–inorganic hybrid nanocomposite aerogel film electrodes for flexible supercapacitors.

High mass loading and additive-free prussian blue analogue based flexible electrodes for Na-ion supercapacitors

Supercapacitor electrodes often suffer from the low mass loading of active substances and the unsatisfactory ion/charge transport features due to the use of various additives. Exploring high mass loading and additive-free electrodes is of huge significance to develop advanced supercapacitors with commercial application prospects, which still remains challenging. Herein, high mass loading CoFe-prussian blue analogue (CoFe-PBA) electrodes are developed by a facile co-precipitation method using activated carbon cloth (ACC) as the flexible substrate. The homogeneous nanocube structure, large specific surface area (143.9 m2 g−1) and appropriate pore size distribution (3.4 nm) of the CoFe-PBA endow the as-prepared CoFe-PBA/ACC electrodes with low resistance and appealing ion diffusion characteristics. Typically, the high areal capacitance (1155.0 mF cm−2 at 0.5 mA cm−2) is obtained for high mass loading CoFe-PBA/ACC electrodes (9.7 mg cm−2). Furthermore, symmetrical flexible supercapacitors (FSCs) are constructed using CoFe-PBA/ACC electrodes and Na2SO4/polyving alcohol (Na2SO4/PVA) gel electrolyte, achieving superior stability (85.6% capacitance retention after 5,000 cycles), maximum energy density of 33.8 μWh cm−2 at 200.0 μW cm−2 and promising mechanical flexibility. This work is expected to offer inspirations for the development of high mass loading and additive-free electrodes for FSCs.

Copper oxide-based high-performance symmetric flexible supercapacitor: potentiodynamic deposition

Flexible supercapacitors have gained significant attention in recent times due to their many advantages such as high specific capacitance, lightweight, long lifespan, high energy density, high flexibility, and high-power density. These benefits make them ideal for various high-power applications in various industries. Copper oxide is particularly attractive as an electrode material because of its high theoretical specific capacitance, low cost, and eco-friendliness. Copper oxide is the most promising electrode material in energy storage systems among metal oxides due to its higher theoretical value of specific capacitance (1800 F/g). In the present study, the synthesis of a thin film of copper oxide on a flexible copper substrate through electrodeposition was carried out to produce a flexible and lightweight supercapacitor. The supercapacitor's performance was evaluated using cyclic voltammetry (CV) and galvanostatic charge–discharge analysis in a 1 M KOH electrolyte. The results showed that the copper oxide/copper-based supercapacitor had a large specific capacitance of 983.3 F/g and good performance even after 2200 cycles, with a capacity retention of 89.70%. The flexibility of the electrode was measured at various bending angles. The electrode showed a capacity retention of 87.5% after a 180° bending angle with a good coulombic efficiency of 79.15%. Hence, it could be a promising material for flexible supercapacitor electrodes. This demonstrates that copper oxide has great potential as a material for flexible supercapacitor electrodes. The newer applications for supercapacitors in industries such as wearable electronics, flexible displays, and energy harvesting systems can be explored.

Electrodeposition of polypyrrole as binder-free and high mass-loading electrodes for flexible supercapacitors

Flexible or bendable energy storage devices have drawn great attention recently due to the great demand for the development of soft portable equipment. Conducting polymers (polypyrrole, polyaniline, polythiophene) are particularly interesting powder form candidates for active materials in such devices. A traditional slurry-based technology with low mass loadings (1 mg/cm2) is usually used. However, poor interparticle or particle-substrate connections influence the performance of devices during bending. Furthermore, increasing the mass loading (10 mg/cm2) of commercial-level electrode materials often has the problems of poor electrical and ionic conductivity, which will hinder their practical applications. The electrodeposition process of conducting polymers provides an alternate useful method to synthesize uniform and binder-free electrodes with high mass loadings. Here, electrochemically synthesized polypyrrole is proposed and investigated to increase the energy storage performance with high mass loadings. The electrodeposition polypyrrole electrode with a mass loading of 10 mg/cm2 demonstrates a competitive gravimetric capacitance of 265 F/g at a current density of 1 A/g. In the case of a symmetrical supercapacitor with E-PPy, the differences in the capacitance obtained with different electrode masses are insignificant (59.1 F/g for 2.4 mg/cm2 and 54.9 F/g for 10 mg/cm2). The assembled supercapacitor also possesses outstanding flexibility (as high as 96.3% capacitance retention at 2 A/g) upon bending to 140° for 10 cycles.

PEDOT: PSS for Reinforced Performances of Co/Ni-MOF as Flexible Supercapacitor Electrodes

PEDOT: PSS for Reinforced Performances of Co/Ni-MOF as Flexible Supercapacitor Electrodes

Synthesis of tungsten disulfide for electrochemical energy applications

One-dimensional tungsten disulfide (WS2) was synthesized by a simple hydrothermal method for electrochemical energy applications, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts and flexible supercapacitors. The synthesized WS2 exhibits a low overpotential of −230 mV at −10 mA cm−2 and 290 mV at 10 mA cm−2 with low Tafel slopes of 127 mV dec-1 and 163 mV dec-1 for HER and OER, respectively. In addition, the synthesized WS2 performed high specific capacitance of 152F g−1 at a current density of 1 A g−1 with an energy density of 13.5 Wh kg−1 and power density of 408 W kg−1 for the flexible supercapacitor. Ultimately, high specific capacitance retention (93 %) and coulombic efficiency (87 %) after 5,000 cycles with reliable flexibility were shown. These results demonstrated that the synthesized WS2 could be used for electrocatalysts and flexible supercapacitor electrodes with high performance.

A morphology control engineered strategy of Ti3C2Tx/sulfated cellulose nanofibril composite film towards high-performance flexible supercapacitor electrode

2D Ti3C2Tx MXene is an ideal material for fabricating supercapacitor electrodes due to its excellent physical-chemical properties. However, the inherent self-stacking, narrow interlayer spacing, and low general mechanical strength limit its application in flexible supercapacitors. Herein, facile structural engineering strategies by drying (vacuum drying, freeze drying, and spin drying) were proposed to fabricate 3D high-performance Ti3C2Tx/sulfated cellulose nanofibril (SCNF) self-supporting film supercapacitor electrodes. Compared with other composite films, the freeze-dried Ti3C2Tx/SCNF composite film exhibited a looser interlayer structure with more space which was conducive to charge storage and ion transport in the electrolyte. Therefore, the freeze-dried Ti3C2Tx/SCNF composite film exhibited a higher specific capacitance (220 F/g) compared to the vacuum-dried Ti3C2Tx/SCNF composite film (191 F/g) and the spin-dried Ti3C2Tx/SCNF composite film (211 F/g). After 5000 cycles, the capacitance retention rate of the freeze-dried Ti3C2Tx/SCNF film electrode was close to 100 %, showing excellent cycle performance. Meanwhile, the tensile strength of freeze-dried Ti3C2Tx/SCNF composite film (13.7 MPa) was much greater than that of the pure film (7.4 MPa). This work demonstrated a facile strategy for control of Ti3C2Tx/SCNF composite film interlayer structure by drying for fabricating well-designed structured flexible and free-standing supercapacitor electrodes.

Interconnected Vanadyl Pyrophosphate Nanonetworks as a Flexible Electrode for High-Voltage and Long-Life Li-Ion Supercapacitors.

Engineering vanadium-based materials with high conductivity, superior redox performance, and high operating voltage has attracted widespread attention in energy storage devices. Herein, we demonstrated a simple and feasible phosphorization technique to design three-dimensional (3D) network-like vanadyl pyrophosphate ((VO)2P2O7) nanowires on flexible carbon cloth (CC) (VP-CC). The phosphorization process enabled the VP-CC to increase the electronic conductivity, and the interconnected nano-network of VP-CC opens pathways for fast charge storage during the energy storage processes. Specifically, the 3D VP-CC electrodes and LiClO4 electrolyte designed as a Li-ion supercapacitor (LSC) demonstrate a maximum operating window of 2.0 V with a superior energy density (Ed) of 96 μWh cm-2, power density (Pd) of 10,028 μW cm-2, and outstanding cycling retention (98%) even after 10,000 cycles. In addition, a flexible LSC assembled utilizing VP-CC electrodes with a PVA/Li-based solid-state gel electrolyte exhibits a high capacitance value of 137 mF cm-2 and excellent cycling durability (86%) with a high Ed of 27 μWh cm-2 and Pd of 7237 μW cm-2. Considering excellent energy storage features, the highly conductive vanadium-based material has been utilized as an ideal electrode for various flexible/wearable energy storage devices with superior performance.

A homologous strategy to parallelly construct doped MOFs-derived electrodes for flexible solid-state hybrid supercapacitors

A homologous strategy to parallelly construct doped MOFs-derived electrodes for flexible solid-state hybrid supercapacitors