Flexible Supercapacitors Research Articles

Polypyrrole wrapped‐carbon nanotube composites with self‐assembled core‐shell structure for high‐performance flexible supercapacitors

AbstractThe polypyrrole wrapped‐carbon nanotube composites with the core‐shell design are prepared via two‐step self‐assembly originated from the multi‐interactions of dopant, zeolitic imidazolate framework, carbon nanotube and pyrrole. The core composed of carbon nanotube effectively improves the conductivity of the composite and provides stable double‐layer capacitance. The shell composed of polypyrrole and tightly wrapped on carbon nanotubes effectively prevents the volume expansion of polypyrrole while undergoing the process of doping/de‐doping as well as endows supercapacitors with high pseudocapacitance. The polypyrrole‐coated carbon nanotube displays a remarkable specific capacitance of 510 F g−1 when operating at a current density of 1 A g−1. Surprisingly, the assembled polypyrrole wrapped‐carbon nanotube//activated carbon asymmetric supercapacitor not only possesses a high capacitance preservation of 88.5% after 10,000 charge/discharge cycles at 5 A g−1, but also shows superior flexibility and stable electrochemical behaviors when subjected to various bending angles. Furthermore, the flexible polypyrrole wrapped‐carbon nanotube//activated carbon supercapacitor achieves the superb energy density of 86.3 Wh kg−1 and the power density of 800 W kg−1 when operating at 1 A g−1. These results highlight the significant potential of this new generation of flexible supercapacitor devices.Highlights Polypyrrole is wrapped on carbon nanotube to form a core‐shell structure CNTs improve the conductivity and provides double‐layer capacitance The core‐shell design prevents the volume expansion of polypyrrole The assembled asymmetric supercapacitor shows superior flexibility

Sulfonated polyaniline/MXene composite electrode with high cycling stability for anti-freezing flexible supercapacitor

Polyaniline (PANi) is widely used in supercapacitors owing to its unique structural flexibility and redox behavior. However, due to the limitations of the inherent molecular structure, the expansion and contraction of the PANi skeleton will be triggered during the charging and discharging process, leading to decreased stability and rapid degradation. For this reason, we design a novel sulfonated polyaniline structure of aniline copolymerized with 2-aminobenzenesulfonic acid, thus immobilizing the sulfonic acid group (−SO3H) side chain onto the PANi main chain. The −SO3H acts as a proton reservoir to provide protons for PANi, accelerating the occurrence of redox reactions and providing pseudocapacitance, effectively suppressing the deprotonation phenomenon. Then SPANi is grown in-situ on the interlayers of MXene, which induces the disordered polymer to be ordered on the MXene substrate, forming a 3D interconnected network composite with both high specific capacitance and excellent stability. SPANi/MXene composite has an outstanding specific capacitance of 512.45 F g−1 at 1 A/g and retains up to 92.16 % of the initial capacitance after 10,000 cycles. Assembled with PHEAA-PAA-MXene double-crosslinked organogel electrolyte to form an anti-freezing device with a specific capacitance of 158.63 F g−1, it retains a high specific capacitance of 118.26 F g−1 even under the extreme conditions of −40 °C. The capacitance retention after 10,000 cycles is 95.2 % (20 °C) and 81.82 % (−40 °C), respectively. This work provides simple and effective methods to fabricate high-performance PANi-based electrodes and anti-freezing supercapacitors.

Enhancing electrochemical performance of carbon fiber with lignin for flexible supercapacitors in PVA/H3PO4 matrix

Lignin is an unlimited resource of an aromatic natural polymer useful for multiple applications, though its use remains limited due to the lack of technology developments. The introduction of lignin as coating of fibers and textile for composites is being recently studied with very promising results. Here, commercial woven carbon fibers (WCFs) have been superficially modified following an easy approach to introduce lignin and carbonize it to enhance the electrochemical performance of the fibers. The modified carbon fibers containing lignin were tested as electrodes using KCl 3M as electrolyte, reaching 5 F/g at 10mVs-1. A symmetric supercapacitor was fabricated using these electrodes, and a proof of concept based on a red and yellow light-emitting diode (LED) powered on and a bending test were done. The supercapacitor was able to recover 99% of its capacitance when returned to its original unbent position.

Chitosan-based high-performance flexible supercapacitor via “in-situ co-doping/self-regulation-activation” strategy

The construction of N, P co-doped hierarchically porous carbons (NPHPC) by a facile and green approach is crucial for high-performance energy storage but still an enormous challenge. Herein, an environment-friendly “in-situ co-doping, self-regulation-activation” strategy is presented to one-pot synthesize NPHPC using a phytic acid-induced polyethyleneimine/chitosan gel (PEI-PA-CS) as single precursor. NPHPC displayed a specific surface area of up to 1494 m2 g−1, high specific capacitance of 449 F g−1 at 1 A g−1, outstanding rate capability and cycling durability in a wide temperature range (−20 to 60 °C). NPHPC and PEI-PA-CS electrolyte assembled symmetric quasi-solid-state flexible supercapacitor presents superb energy outputs of 27.06 Wh kg−1 at power density of 225 W kg−1. For capacitive deionization (CDI), NPHPC also exhibit an excellent salt adsorption capacity of 16.54 mg g−1 in 500 mg L−1 NaCl solution at a voltage of 1.4 V, and regeneration performance. This study provides a valuable reference for the rational design and synthesis of novel biomass-derived energy-storage materials by integrating phytic acid induced heteroatom doping and pore engineering.

Architecting MxNy-MOF@MXene composites for excellent supercapacitor electrodes

The increasing demand for efficient energy storage systems has prompted the exploration of novel electrode materials, such as bimetallic metal-organic frameworks (MOFs). This study examines the synthesis of a variety of bimetallic MxNy-MOF materials, including Ni1Co1-MOF, Ni1Co3-MOF, Ni3Co1-MOF, Cu1Co1-MOF, and Ni1Cu1-MOF, as well as their integration with MXene to produce MOF@MXene composite electrode materials for supercapacitors. These MxNy-MOF materials display distinct morphologies and uniform particle sizes. MXene serves as an effective substrate, enhancing the surface coverage with the MOF phase. In interactions with MXene, Co2+ and Cu2+ demonstrate stronger binding than Ni2+, which prefers binding with ligands. Electrochemical testing reveals that Ni and Co-based composites, particularly Ni1Co1-MOF@MXene, exhibit superior performance and achieving a high specific capacitance of 1493.6 F g−1 at 1 A g−1. These materials also show impressive rate performance and cycle stability, with a retention of 57.2 % after 5,000 cycles. An all-solid-state flexible supercapacitor using Ni1Co1-MOF@MXene demonstrates an energy density of 73.9 Wh·kg−1 at a power density of 750 W·kg−1, maintaining 81.8 % capacity and 97 % Coulombic efficiency after 10,000 cycles. The energy storage mechanism primarily involves Faradaic redox reactions of Co2+/Co3+ and Ni2+/Ni3+, coupled with OH- adsorption/desorption processes. This study not only elucidates the kinetic dynamics and storage mechanisms of these composite materials but also highlights their potential applications in advanced energy storage technologies.

Electrodeposition of Graphene/Polypyrrole Electrode for Flexible Supercapacitor with Large Areal Capacitance

Electrodeposition of Graphene/Polypyrrole Electrode for Flexible Supercapacitor with Large Areal Capacitance

Flexible Asymmetric Supercapacitors Constructed by Reduced Graphene Oxide/MoO3 and MnO2 Electrochemically Deposited on Carbon Cloth.

A flexible asymmetric supercapacitor (ASC) is successfully developed by using the composite of MoO3 and graphene oxide (GO) electrochemically deposited on carbon cloth (CC) (MoO3/rGO/CC) as the cathode, the MnO2 deposited on CC (MnO2/CC) as the anode, and Na2SO4/polyvinyl alcohol (PVA) as the gel electrolyte. The results show that the introduction of the GO layer can remarkably increase the specific capacitance of MoO3 from 282.7 F g-1 to 341.0 F g-1. Furthermore, the combination of such good electrode materials and a neutral gel electrolyte renders the fabrication of high-performance ASC with a large operating potential difference of 1.6 V in a 0.5 mol L-1 Na2SO4 solution of water. Furthermore, the ASCs exhibit excellent cycle ability and the capacitance can maintain 87% of its initial value after 6000 cycles. The fact that a light-emitting diode can be lit up by the ASCs indicates the device's potential applications as an energy storage device. The encouraging results demonstrate a promising application of the composite of MoO3 and GO in energy storage devices.

Venice’s macroalgae-derived active material for aqueous, organic, and solid-state supercapacitors

In this study, self-doped porous activated biochar derived from Venice lagoon’s Sargassum brown macroalgae (ABS) has been successfully prepared through thermochemical carbonization (pyrolysis) followed by CO2 physical activation and used as electrodes for supercapacitor (SC) applications. The ABS exhibits a remarkable specific surface area of 821 m2g–1 and heteroatoms (N, O, and S) doping, both key features to attain high-performance carbon-based SC electrodes. The electrochemical performances of ABS-based SCs were assessed in three different electrolytes. Two are aqueous (i.e., 1 M H2SO4 and 8 M NaNO3), while the third one is the prototypical organic, namely 1 M TEABF4 in acetonitrile. In these three electrolytes, the ABS-based electrodes exhibited specific capacitance values (Cg) of 109.5, 79.0, and 64.3Fg–1, respectively, at a current density of 0.1 Ag–1. The capacitive performance resulted in SC energy densities of 3.45 Wh kg−1 at 22.5 W kg−1, 6.3 Wh kg−1 at 36.1 W kg−1, and 12.4 Wh kg−1 at 57.4 W kg−1 and maximum power densities of 147, 222, and 378 kW kg−1 in the acidic, quasi-neutral aqueous electrolyte and organic electrolyte, respectively. The ABS electrodes were used to realize a flexible solid-state SC based on the sulfonated polyether ether ketone (SPEEK):functionalized niobium disulfide flakes (f-NbS2) composite membrane. The flexible solid-state SC displayed a remarkable 97% Cg retention even under various mechanical stresses, including bending up to 1000 times and folding angles up to 180°, while keeping a Coulombic efficiency above 98%. This study reveals ABS as a promising sustainable source of active materials for SCs. The remarkable performance of ABS-based SCs can be attributed to their multi-scale porosity, heteroatom doping, and enhanced surface wettability, providing abundant active sites for charge accumulation, and efficient electrolyte diffusion, thus highlighting its potential as a sustainable solution for energy storage applications.

PANI-grafted boron, nitrogen co-doped carbon fiber: An outstanding, high-performance supercapacitor electrode

The rapid development of wearable electronic devices demands light-weight, flexible, high performance and eco-friendly supercapacitors. However, an efficacious design and fabrication of a steady structure for the same is still a challenge. Herein, a novel composite of polyaniline (PANI) grafted Boron, Nitrogen co-doped cotton derived carbon fiber (BNCF) has been synthesized via in-situ chemical polymerization. The detailed structural characterization confirms the successful incorporation of granular PANI over the BNCF surface. The supercapacitor performance of the as-synthesized BNCF-PANI electrode was investigated as electrode material for supercapacitor and the capacitance was achieved as high as 370 F g−1 at a current density of 1 A g−1. Moreover, BNCF-PANI based electrode exhibited the energy density of 12.8 Wh kg−1 at current density of 1 A g−1 and power density 2500 W kg−1 at current density of 10 A g−1.

Biomimetic and electrostatic self-assembled nanocellulose/MXene films constructed with sequential bridging strategy for flexible supercapacitor

Despite the two-dimensional titanium carbide MXene (Ti3C2Tx) has shown promising applications in energy storage, the inherent self-stacking and weak connectivity among nanosheets present a challenge in manufacturing robust MXene films for emerging flexible supercapacitors. Herein, inspired by the robust “soft-hard” structured exoskeletons of crustaceans, the biomimetic cellulose nanofibers (CNFs)/MXene-Al (CM-Al) film with integrated high mechanical toughness, electroconductivity and electrochemical behavior, sequential bridging with hydrogen and ionic bonds, is crafted via the facile electrostatic self-assembly strategy. The embedded CNFs guide the in-plane orientation of MXene through hydrogen bonding, forming a “soft-hard” microstructure and dramatically enhancing the mechanical toughness of CM-Al. The rigid ionic bonds established between the oxygen-containing groups on CNFs and MXene with Al3+ further elevate the mechanical strength (163.88 MPa) and act as additional electron transfer channels to impart the reinforced electroconductivity (82.63 S cm−1). Benefiting from the more active site exposure induced by the expanded MXene layer spacing after CNFs embedding and the weakened intrinsic resistance caused by the constructed ionic bonding, the assembled flexible quasi-solid-state symmetric supercapacitor with CM-Al as electrode delivers a high area capacitance (679 mF cm−2), energy density (16.2 μWh cm−2), cycling capability (85.3 % capacitance retention after 10,000 cycles) and intrinsic tolerance to various non-stretching deformations. Moreover, the selective principles for metal ions are summarized that involved a comprehensive consideration of ionic radius, charge density and basic chemical properties. This work demonstrates an accessible and feasible construction strategy for flexible composite films and provides an alternative pathway for wearable energy storage devices.

Redox-active “Structural Pillar” molecular doping strategy towards High-Performance polyaniline-based flexible supercapacitors

To coordinate flexibility to electrodes without sacrificing their electrochemical properties is critical for the development of wearable supercapacitors (SCs). Polyaniline (PANI) is well-known pseudocapacitive electrode material due to its high conductivity and different oxidation states upon switchable structures. However, its rigid conjugated backbone and structural instability caused by repeated doping/de-doping during cycling severely impede its utilization in flexible SCs. Herein, we deploy a directional freezing and redox-active “structural pillar” molecular doping strategy to boost PANI-based flexible SCs with high performance. The directional freezing strategy constructs an interconnected 3D honeycomb hydrogel structure with PANI nanofibers, which guarantees fast ion diffusion and electron transport and meanwhile exposes abundant active sites. The large-sized dopant 2-amino-4-bromoanthraquinone-2-sulfonic acid sodium (AQNS) is used as the structural pillar to alleviate the structural instability of PANI during cycling and provides additional pseudocapacitance arising from its redox active quinone groups. Moreover, the negatively charged −SO3− on AQNS can further interact with H+ in the electrolyte to act as an internal proton reservoir, assisting the protonation of –NH- and −N = in PANI to facilitate its charge storage process. Consequently, the PANI-AQNS electrode achieves a high specific capacitance of 578 F g−1 at 1 A g−1 and its symmetric SCs exhibit a specific capacitance of 199 F g−1 at 0.5 A g−1 with an energy density of 13.61 W h kg−1 at a power density of 175 W kg−1. Upon 2000 cycles of dynamic deformations, the SCs can still maintain above 90 % of the initial capacitance, verifying their excellent flexibility-relevant property.

Graphene heterostructure fiber with enhanced physicochemical properties for flexible solid-state supercapacitors

The graphene/pseudocapacitive composite electrode materials used for energy storage usually benefit from the advantages of each component, however, they also face significant challenges in terms of structural integrity and rate performance due to the weak interactions and limited interface area between different phases. To tackle these issues, we propose a facile strategy that integrates 2D/2D van der Waals forces and hydrogen bonds to effectively and closely bind each component together. We illustrate this concept by designing and constructing a heterojunction fiber that immobilizes protonated g-C3N4 onto GO nanosheets through electrostatic self-assembly. The resulting g-C3N4/RGO heterostructure fiber exhibits enhanced electrical conductivity and improved mechanical properties. When utilized as a supercapacitor electrode, it demonstrates high specific capacitances of 70.6 F cm−3 at a current density (Id) value of 0.1 A cm−3, in addition to exceptional cyclic stability (Retaining 99.4 % after 15000 cycles), rate capability (Retaining 55.8 % at Id = 2 A cm−3), and enduring mechanical flexibility. We anticipate that this study will inspire the fabrication of heterostructure fibers for durable and high-rate energy storage devices.

Ultrafast gelation of multifunctional β-cyclodextrin based hydrogel electrolyte for self-healing supercapacitor

The increasing attention towards flexible supercapacitors with self-healing ability can be attributed to their capacity to endure deformation and damage. However, the development of self-healing and adhesive hydrogel electrolytes, which serve as substrate materials for supercapacitors, remains a challenge. This work reports an ultrafast gelation method for fabricating multifunctional PAA-SL-CD hydrogels with ultrahigh stretchability, fast self-healing ability, superior adhesion, and high ionic conductivity (6.15 S/m). These excellent properties can be attributed to the hydrogen bonding between β-CD, the catechol groups and PAA chains, as well as metal-catechol coordination, metal-carboxyl coordination, and the unique truncated cone structure of the β-CD molecule. A flexible supercapacitor, fabricated using PAA-SL-CD hydrogel as electrolyte, demonstrates high areal specific capacitance (950 mF/cm2) and a capacitance retention rate of 76.3 % even after five cutting and healing cycles. With the well-designed adhesion of the hydrogel electrolyte, the device exhibits impressive specific capacitance retention after cyclic bending. Notably, a series supercapacitor retains the ability to power a clock for 28 min.

Graphene-wrapped (Ni,Co)Se2 carbon nanowires toward high performance hybrid supercapacitors

To probe the next generation of flexible supercapacitors for practical application, small size, excellent electrical conductivity, low cost, Eco-friendly and excellent capacitance are prerequisites. However, due to dissatisfied electrical conductivity, limited surface area and low porosity, which results in few active sites and long ion transport channels, electrochemical improvements are needed for bimetallic selenide-based flexible supercapacitors. Hence, novel three dimensional graphene (3DG) encapsulated cobalt-nickel selenide carbon nanowires [3DG/(Co,Ni)Se2 CNWs] composite aerogels was successfully fabricated through a modified self-templating approach. The assembled 3DG/(Co,Ni)Se2 CNWs electrode showcases outstanding electrochemical performance, achieving a high specific capacitance of 1286 F g−1 at 1 A g−1 attributed to its combination structure of carbon based materials and nanowires. It demonstrates excellent cycling stability, with a capacity retention of 88 % after 3000 cycles at 2 A g−1. The prepared 3DG/(Co,Ni)Se2 CNWs//active carbon (AC) asymmetric flexible all-solid-state supercapacitors demonstrates outstanding electrochemical properties, achieving an energy density of up to 28 Wh kg−1 at a power density of 800 W kg−1. Even after 6000 cycles at 5 A g−1, it retains 84.15 % of its specific capacitance, thus presenting a new direction for energy storage applications.

Electroactive polylactic acid nanofiber multilayer membranes as “Green” flexible electrode for supercapacitor

Against the backdrop of the post-COVID-19 era, the demand for masks has become increasingly steady, discarded masks have brought about new environmental problems due to the lack of effective means of disposal as well as recycling mechanisms. To solve this problem, we make secondary use of discarded polylactic acid (PLA) masks. The nanofiber multilayer membranes PLA/PDA/GO/PPy were synthesized by layer-by-layer self-assembly for flexible supercapacitors (SCs). The multiple coating on PLA significantly increases the capacitive performance. Optimization of the PLA/PDA/GO/PPy demonstrates capacitance up to 1331 mF cm−2. Symmetric aqueous SCs using PLA/PDA/GO/PPy electrodes show higher energy density than other literature-reported SCs based on nanofiber multilayer membranes. In addition, we also explored the effects of discarded PLA/PDA/GO/PPy on the growth of ryegrass and canola in the soil. The exceptional combination of remarkable electrochemical properties and excellent environmental friendliness makes the PLA membrane promising for supercapacitors.

Tailoring the ion storage of MXene by aramid nanofibers towards self-standing electrodes for flexible solid-state supercapacitors

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) are a class of 2D nanomaterials that can offer excellent properties for high-performance supercapacitors. Nevertheless, irreversible restacking of MXene sheets decreases the interlayer spacing, which inhibits the ion intercalation between the MXene nanosheets and finally deteriorates the electrochemical performance of supercapacitors. Herein, aramid nanofibers (ANFs) are mixed with Ti3C2T x MXene to prepare MXene/ANFs composite films. The restacking of MXene sheets is inhibited by the electrostatic repulsion between ANFs and MXene. The ANFs act as intercalation agents to increase the interlayer spacing of the composite films, which can improve the ion storage ability of supercapacitors. Furthermore, the ANFs enhance the mechanical strength of the composite films due to the strong hydrogen bonding interaction and nanomechanical interlocking between ANFs and MXene, endowing the composite films with self-standing property. The resultant composite films are used as electrodes for flexible solid-state supercapacitors to achieve high specific capacitance (996.5 mF cm−2 at 5 mV s−1) and outstanding cycling stability. Thus, this work provides a potential strategy to regulate the properties of 2D nanomaterials, which may expand the application of them in energy storage, ionic separation, osmotic energy conversion and beyond.

High-performance positive electrode material of MXene/FeNi2S4 nanocomposite for flexible supercapacitor with large potential window

Developing heterostructured nanomaterials with abundant active sites and excellent electronic conductivity is essential to enhance the electrochemical activity of supercapacitors. Herein, MXene/FeNi2S4 composites, as a high-performance electrode material for the flexible supercapacitor, were fabricated using a simple probe sonication method. The formation of a 3D architecture can create abundant electrochemical active sites, promoting the charge transfer as well as the redox reactions. Accordingly, a supercapacitor electrode based on MXene/FeNi2S4 exhibited excellent electrochemical performances, with a high specific capacitance of 673 F/g at 1 A/g. When MXene/FeNi2S4 was used as the electrode material of an asymmetric supercapacitor, the device showed an outstanding specific capacitance of 141 F/g at 1 A/g in a large potential window of 1.8 V. Up to 90 % of this high specific capacitance was retained after 2000 charge–discharge cycles. Furthermore, the device displayed high values of both energy density (63.37 Wh/kg) and power density (900.98 W/kg). The device also exhibited stable electrochemical performances under high flex at the bending angle of 135°. The exceptional electrochemical performances of MXene/FeNi2S4 highlight its great potential as an excellent electrode material for the next generation of flexible energy storage devices.

Resilient 3D porous self-healable triple network hydrogels reinforced with graphene oxide for high-performance flexible supercapacitors

The application of gel polymer electrolyte (GPE) in flexible supercapacitor (FSC) has attracted much attention due to its excellent mechanical properties and chemical stability. However, traditional polymer substrates have high crystallinity, resulting in sluggish ion migration and low ionic conductivity. This study reports the successful fabrication of a resilient highly porous triple hydrogel network (TNH) comprising the natural polymer of sodium alginate and biocompatible components of polyvinyl alcohol and graphene oxide (GO), exhibiting remarkably high ionic conductivity and astonishing mechanical strength, self-healing, and flame retardant properties. The physical and electrochemical features of the GPEs were optimized by tuning the GO wt% within the polymeric matrix. Adding a low wt% of GO (0.8 wt%) led to remarkable improvements in ionic conductivity (83.33 mS cm−1), mechanical strength (2.21 MPa), and self-healing and flame-retardant properties of the resulting PVA/SA/GO GPE than the pristine GPE. This was attributed to the formation of a microporous three-dimensional polymeric network based on reversible non-covalent bonds. The carbon cloth-based symmetric SC prepared based on this GPE exhibited a long cycle life, high specific capacitance of 633.3 mF cm−2 at 0.5 mA cm−2, and an energy density of 84 mWh cm−2 at a high power density of 500.2 mW cm−2 (at 1 mA cm−2)), which surpassed the values reported in previous studies. The electrochemical performance of the FSC prepared based on PVA/SA/GO GPE remained constant when subjected to bending deformation, indicating the harmonious physical, chemical, and electrochemical properties of the developed GPE and its suitability for flexible energy storage devices, and wearable electronics

High toughness antifreeze conductive double network zwitterionic gel electrolyte for flexible supercapacitors

AbstractFlexible wearable supercapacitors based on hydrogel electrolyte are considered as flexible energy storage device with great potential. However, polymer gel electrolytes contain a large number of water molecules, which can easily freeze at sub‐zero temperatures, thus severely inhibiting ion transport. The ionic conductivity, cyclic stability and mechanical properties of gel electrolytes decrease at low temperatures (below −25°C), which seriously hinders the application of flexible wearable supercapacitors. In order to improve the mechanical properties and ensure the excellent ionic conductivity of the electrolyte, a freezing‐resistant gel electrolyte constructed by supramolecular self‐assembly, which has a wide temperature range from 25 to −80°C, stable ionic conductivity and ultra‐high mechanical strength. Low‐temperature‐resistant gel electrolytes, called flexible supercapacitor‐SBMA‐co‐PVA‐AA/CaCl2 (FSAPC), have been designed and prepared by molecular‐scale supramolecular self‐assembly of rigid [2‐(methacryloyloxy)ethyl]dimethyl(3‐sulfopropyl) polymer chains and flexible poly(vinyl alcohol) polymer chains, and the test results showed that the gel electrolyte ‐SBMA‐co‐PVA‐AA/CaCl2 (PSAPC) has excellent conductivity of 34.07 mS cm−1 at 25°C, the flexible supercapacitor has a specific capacity of 8.57 Wh kg−1, and the capacity retention rate is 91.2% even after 5000 cycles at −25°C, excellent conductivity at −25°C for 31.88 mS cm−1 and at −80°C for 35.34 mS cm−1. The potential applications of the amphiphilic ionic gel electrolyte in the industrial development of low‐temperature resistant supercapacitors are noteworthy.

Rational design of bilayer heterogeneous poly(ionic liquid)s electrolytes for high-performance flexible supercapacitors with ultraslow self-discharge rate

Supercapacitors have high power density and long cyclic life, but always face a great challenge of rapid self-discharge. Herein, we design a series of bilayer heterogeneous poly(ionic liquid)s electrolytes (BHPE) with anionic and cationic poly(ionic liquid)s (PolyILs), to construct high-performance supercapacitors with ultraslow self-discharge rate. Zeta potential and ionic conductivity of cationic PolyILs are adjusted by changing the volume of counter anions (BF4-, PF6-, TFSI-) and the length of alkyl side chain (ethyl, butyl and hexyl). Both experimental and simulated results reveal that the interaction energy between the cationic PolyILs and counter anion greatly depends on the sizes of anions and the lengths of alkyl side chains of cationic PolyILs, which has intrinsic effect on the zeta potential difference between anionic and cationic PolyILs of the BHPEs, and further can efficiently regulate the self-discharge rate of supercapacitors. The developed supercapacitor based on BHPE with using cationic PolyILs with small anion of BF4- and long alkyl side chain of hexyl exhibits an ultraslow self-discharge rate of −0.04 V h−1 and a high energy density of 34.8 Wh kg−1, which shows great promise for further practical applications in the future.