Electrolytes For Supercapacitors Research Articles

Functional hydrogel-based supercapacitors for wearable bioelectronic devices

Functional hydrogel-based electrodes and electrolytes for multifunctional supercapacitors for wearable bioelectronic devices.

Ion regulation of ionic liquid electrolytes for supercapacitors

As a member of the liquid electrolyte family, ionic liquids (ILs) possess distinctive chemical and electrochemical stability, offering a pathway to realize supercapacitors (SCs) with both outstanding energy density and high safety.

A study on the preparation of polycation gel polymer electrolyte for supercapacitors.

The polycation gel polymer electrolyte (PGPE) is a promising electrolyte material for supercapacitors due to its high ionic conductivity and great flexibility. Herein, we report a novel flexible PGPE film, which is prepared by thermal copolymerization. The superiority of PGPE is attributed to the existence of charged groups in the polymer skeleton. Consequently, the crystallinity of the polymer is effectively reduced, and the migration of the lithium ion is evidently promoted. Moreover, the liquid retention capacity of the film is improved, which enhances its ionic conductivity as well. The reported PGPE exhibits a high ionic conductivity of 57.6 mS cm−1 at 25 °C and a potential window of 0–1.2 V. The symmetrical PGPE supercapacitor (AC/AC) shows 95.21% mass-specific capacitance retention after 5000 cycles at 2 A g−1 with a maximum energy density of 12.8 W h kg−1 and a maximum power density of 5.475 kW kg−1. This study confirms the exciting potential of PGPE for high performance supercapacitors.

Identification of self-discharge mechanisms of ionic liquid electrolyte based supercapacitor under high-temperature operation

Ionic liquids (ILs) are promising electrolytes for supercapacitors (SCs) aimed for high-temperature applications, where increased ionic conductivity results in superior capacitive performance compared to room temperature (RT) performance. However, an increased temperature also accelerates the self-discharge rate that adversely affects energy retention and restricts the usage of SCs in standalone applications. In this study, a detailed electrochemical investigation on the self-discharge behaviour of carbon-based SCs containing an IL, 1-Ethyl-3-methylimidazolium acetate (EMIM Ac), has been carried out in the temperature range RT - 60 °C, and the underlying self-discharge mechanisms are identified. The results reveal that at a high voltage of 1.5 V, the self-discharge is characterized by a combination of charge redistribution and diffusion at both RT and 60 °C. At 60 °C, the diffusion-controlled mechanism dominates at lower voltages over the charge redistribution effect, while at RT both mechanisms contribute to a similar extent. The observed difference in the self-discharge mechanism between RT and 60 °C is explained in terms of a decreased RC time constant (τRC) at elevated temperature, and the same conclusions are potentially applicable to other IL-containing SCs as well.

Facile Preparation of Low-Cost and Cross-Linked Carbon Nanofibers Derived from PAN/PMMA/Lignin as Supercapacitor Electrodes

To develop economical and high-performance water electrolyte supercapacitor electrodes, low-cost and cross-linked carbon nanofibers (CNFs) were prepared with an electrospinning method by using PAN,...

Understanding the Charge Storage Mechanism of Redox-Active Ionic Liquid Based Supercapacitors

Redox-active ionic liquids are emerging as promising new electrolytes for supercapacitors, which provide higher capacitance and energy density than organic or ionic liquid electrolytes. The fundamental studies of charge storage mechanism in supercapacitors are of critical importance for the development and applications of devices. Solid-state NMR (SS-NMR) methodology that has the ability to give atomic information on local environments within electrodes has been recently developed to study the charge storage mechanism of supercapacitors at molecular level. The charge storage mechanisms in supercapacitors with organic or ionic liquid electrolytes have been studied by SS-NMR. However, there is until now no research on supercapacitors with redox-active electrolyte published. Therefore, the study of charge storage mechanism in supercapacitors with redox-active electrolyte are highly required. In this context, we employed SS-NMR techniques combined with electrochemical dilatometry measurements that are associated with charge induced strain of electrode to investigate in depth the charge storage during charging process in supercapacitors with redox-active ionic liquid electrolyte. It is revealed that the charging process of supercapacitors with redox-active ionic liquid electrolyte EMIM FcNTf/ACN is driven by different charge regimes for different voltages, that is, co-ion desorption at low voltage range and subsequently counter-ion adsorption at higher voltage range. The electrochemical dilatometry measurements show macroscopic change of the electrode during charging and further confirm the proposed mechanism obtained from SS-NMR. The results give a detailed picture of the charge storage mechanism of supercapacitors with redox ionic liquid electrolyte, providing new insights on the charge storage of supercapacitors.

Electrochemical stability of steel, Ti, and Cu current collectors in water-in-salt electrolyte for green batteries and supercapacitors

The electrochemical behaviour of steel, copper, and titanium current collectors was studied in aqueous solutions of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at various concentrations, from 0.5 up to 20 m. As the concentration of the electrolyte increases, the electrochemical window of water stability widens according to the “water-in-salt” concept. The metal grids have been studied electrochemically, both under anodic and cathodic conditions, by means of cyclic voltammetry and chronoamperometry. Subsequently, a microscopic analysis with SEM and compositional analysis with XPS was carried out to evaluate the surface modifications following electrochemical stress. We found that copper is not very suitable for this kind of application, while titanium and steel showed interesting behaviour and large electrochemical window.

Polyvinyl fibers as outperform candidature in the solid polymer electrolytes

Solid polymer electrolytes (SPEs) are the best choice to replace liquid electrolytes in supercapacitors, fuel cells, solar cells and batteries. The main challenge in this filed is the ionic conductivity and thermal stability of SPEs which is still not up to mark, therefore more investigations are needed to address these issues. In this study, PVA/salt based SPEs was fabricated using both solution cast and electro-spinning methods to probe the effect of different salts such as (NaCl, KCl and KI) and their concentrations on the ionic conductivity. Scanning electron microscopy (SEM) x and Fourier Transform Infra-Red (FTIR) have been employed to study the morphology as well as the different functional groups of SPEs, respectively. It was noted that small addition of NaCl, KCl and KI salts in SPEs dramatically increased the ionic conductivity to 5.95×10−6, 5.31×10−6 and 4.83×10−6 S/cm, respectively. Importantly, the SPEs obtained with NaCl via electro-spinning have higher ionic conductivity (5.95×10−6 S/cm) than their casted SPEs (1.87×10−6 S/cm). Thermal stability was also studied at two different temperatures i.e. 80 °C and 100 °C. The weight loss percentage of electrospun SPEs have zero percent weight loss than the solution based SPEs. The combined results clearly indicated that the nature of salt, concentration and fabrication process play a vital role in the ionic conductivity. Also, the NaCl salt with low molecular weight at low concentrations shows an enhanced ionic conductivity.

Chitosan-supported deep eutectic solvent as bio-based electrolyte for flexible supercapacitor

The aim of this work is demonstration of a solid polymer electrolyte based on choline chloride/lactic acid deep eutectic solvent (DES) entrapped in chitosan (CS) matrix as potential separator for electrochemical energy storage devices. This full bio-based material demonstrates the high ionic conductivity up to 2.5 mS cm−1 and electrochemical stability window of 1.6 V. The polypyrrole-based film electrodes were prepared using CS/DES films as a supporting matrix for vapor phase polymerization of pyrrole. That allows assembling of a supercapacitor with the specific capacitance of 194 mF cm−2 at the current density of 1 mA cm−2.

Comparing the performance of sulfonium and phosphonium ionic liquids as electrolytes for supercapacitors by molecular dynamics simulations

Ionic liquids (ILs) are generally considered good candidates for supercapacitors electrolytes due to their wide electrochemical windows (EW), which results in high energy densities. Yet they generally offer poor power delivery compared to the conventional organic electrolytes. Here we performed molecular dynamics simulations of slit porous electrode in contact with two ionic liquids with similar viscosities and ionic conductivities, but different electrochemical stability. They are formed by sulfonium and phosphonium cations sharing the same anion. The supercapacitor built with sulfonium-based IL can store up to 50 % more energy at high voltage (ΔΨ = 3.8 V) than the one involving the phosphonium-based IL. Contrarily to previous works with ILs, the improved performance in terms of energy density is accompanied with a faster charging. The properties are rationalized by comparing the structure of the liquids in the two systems. These results show that it is not always necessary to compromise the power delivery for enhanced energy density in ionic liquids-based supercapacitors.

A lignocellulose-based neutral hydrogel electrolyte for high-voltage supercapacitors with overlong cyclic stability

A lignocellulose biomass-based membrane (LC-FD) with ultra-high porosity of 95% and high electrolyte uptake of 1100 wt% is prepared by freeze-drying method to fabricate a Li2SO4-based neutral hydrogel electrolyte (LC-FD-Li2SO4). LC-FD-Li2SO4 has a high degree of gelation, exhibits an ionic conductivity similar to that of the liquid electrolyte, and maintains better electrochemical stability than liquid electrolyte at a high operating voltage of 1.8 V. The specific capacitance of LC-FD-Li2SO4-based SC at 1 A g-1 is 138.4 F g-1, higher than that of liquid electrolyte-based SC. This is due to the ultra-high porosity of LC-FD membrane not only for electrolyte storage provides enough space, and also provides more channels for ion transmission. More importantly, LC-FD-Li2SO4-based SC has the 88% retention after 10,000 cycles at a working voltage of 1.8 V, much higher than the 53% retention of liquid electrolyte-based SC. This is mainly due to the fact that LC-FD membrane inhibits the activity of water molecules in LC-FD-Li2SO4 through both physical and chemical effects. Moreover, LC-FD-Li2SO4-based SC can output a 16.08 Wh Kg-1 maximum energy density. These results indicate that LC-FD-Li2SO4 is a neutral hydrogel electrolyte that is suitable for preparing high-voltage aqueous SCs with high safety performance and environmental friendliness.

Polymer gel electrolytes for flexible supercapacitors: Recent progress, challenges, and perspectives

With the rapid development of portable electronic products, wearable flexible energy storage devices such as flexible supercapacitors (FSCs) have attracted much attention. FSC devices possess various advantages, such as small size, flexibility and lightness, smooth operation, wide operating temperature range, and high energy density and power density. However, there are still significant challenges to make FSC devices in large-scale applications, such as poor gel electrolyte/electrode interface compatibility, poor cyclic stability under bending test conditions and low ionic conductivity of the gel electrolyte. Therefore, to develop FSC devices with outstanding characteristics, many efforts have been devoted to the development of polymer gel electrolytes which have excellent mechanical properties and electrochemical performance. This review aims to provide an overview of recent progress towards the development of advanced FSCs based on aqueous, non-aqueous, ionic liquid-based and redox gel electrolytes. We systematically discussed the challenges and future development of various gel electrolytes for FSC devices. Moreover, the development and performance evaluation of FSCs are also discussed.

New types of hybrid electrolytes for supercapacitors

The state-of-the-art types of hybrid electrolytes for supercapacitors (SCs) are reviewed, and the effects of the different hybrid systems on the performance of SCs and the underlying mechanisms are also summarized. Supercapacitors (SCs) are emerging as efficient energy storage devices but still suffering from limited energy density compared with batteries. Electrolytes have been regarded as the key to determine the energy storage performance of SCs. However, none of the conventional electrolytes can fully meet the increasing requirements of SCs in terms of high ion conductivity, excellent stability, wide voltage window and operating temperature range, as well as environmentally friend concerns. To this end, hybrid electrolytes have sprung up in recent years, which are believed to be the candidate to solve these shortcomings. Herein, the state-of-the-art types of hybrid electrolytes for SCs, including the combination of aqueous and organic, aqueous and gel polymer, ionic liquids (ILs) and organic, and ILs and gel polymer hybrid electrolytes, are reviewed. The effects of different hybrid systems on the performance of SCs and the underlying mechanisms are among the focal points of the review, and prospects and possible directions are discussed as well to provide further insight into the future development of this field.

Conductivity/Electrochemical Study of Polyvinyl pyrrolidone-Poly(vinyl alcohol)/I3\u2212 Thin Film Electrolyte for Integrated Dye-Sensitized Solar Cells and Supercapacitors

The current era focuses not only on producing solar energy but also preserving it for future use. Dye-sensitized solar cells (DSSC) and supercapacitors (SC) are such energy-based devices. DSSCs capture the solar energy and SCs store this captured energy. A natural anthocyanin dye extracted from Garcinia indica (kokum fruit) was used in the DSSCs. SnO2, one of the promising electrode materials for DSSC, was synthesized via a microwave technique. Blend polymer electrolytes (BPE) were prepared through a solution casting technique. A polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA) blend with varying concentrations of potassium iodide, along with iodine dopant, was prepared as a BPE electrolyte composition. The best of the PVA-PVP/KI composition was chosen using Nyquist plots of electrochemical impedance spectroscopy (EIS). Varying the temperature, the dielectric and conductivity study of the chosen composition was studied in detail. A fast/single-step synthesis technique, namely a laser-engraved approach, was used for few-layer graphene synthesis. This graphene serves as a common platform for the DSSC-SC integrated device: as a counter electrode in DSSC and graphene-graphene symmetric electrode in SC. A DSSC-SC integrated device was fabricated and characterized using various analytical and microscopy techniques. The integrated device showed a 0.42 fill factor and 0.56% efficiency. The discharge time for integrated DSSC-SC cells was found to be increased threefold.Graphical

Can polyoxometalates enhance the capacitance and energy density of activated carbon in organic electrolyte supercapacitors?

Polyoxometalates (POMs) have been shown to work as faradaic additivesto activated carbon (AC) in acidic aqueous electrolytes. Yet, their use in organic media allows not only for added capacity but also higher voltage. Here we show that the tetraethylammonium derivative of phosphotungstate [PW12O40]3− (PW12) can be homogeneously distributed throughout the pores of activated carbon (AC) in organic solvents such as N,N’-dimethylformamide (DMF) and demonstrate the use of this hybrid electrode material in an organic electrolyte (1 M TEABF4 in acetonitrile) supercapacitor. Our results show the efficient electroactivity of the PW12 cluster even in the absence of protons, providing a higher voltage than aqueous electrolytes and fast and reversible redox activity. The hybrid material shows a combination of double-layer (AC) and redox (PW12) capacities leading to an increase (36%) in volumetric capacitance with respect to pristine AC in the same organic electrolyte (1 M TEABF4 in acetonitrile). Remarkably, we were able to quantify this increase as coming predominantly from non-diffusion-limited processes thanks to the utterly dispersed nature of POMs. Moreover, the hybrid material delivers a good rate capability and excellent cycle stability (93% retention of the initial capacitance after 10,000 cycles). This study has a profound significance on improving capacitance of carbon-based materials in organic electrolytes.

Electrosorption mechanism and nonequilibrium thermodynamics model of room-temperature ionic liquids within graphene nanochannels

<p indent=0mm>Supercapacitor, an advanced energy-storage technique based on ion electrostatic adsorption at the solid–liquid interface, exhibits fast charge–discharge rates, high power density, high rate ability, and long cycle life. Two-dimensional nanomaterials (e.g., graphene) and room-temperature ionic liquids (RTILs) have been considered potential electrode and electrolyte candidates, respectively, for supercapacitors. However, their high concentration, nanosized confined space, and complex molecular structure/intermolecular interactions make the ion packing structure and electrosorption process substantially different from that in conventional dilute electrolytes (e.g., aqueous solution and organic electrolyte) near the nonconfined space, which cannot be interpreted by the Gouy–Chapman–Stern model and Debye–Huckel approximation based on the continuous medium hypothesis. In this study, molecular dynamics simulations are employed to comprehensively explore the in-depth mechanisms of ion distribution at equilibrium and ion transport at nonequilibrium within the multilayer graphene nanochannel/RTILs system. The ion layer structure and its number density inside the graphene nanochannels are significantly enhanced compared with the nonconfined graphene plane. Moreover, the position of counter-ion within the graphene nanochannel is much closer to the charged wall, indicating better charge-storage capability. These phenomena are mainly attributed to the more prominent interactions between graphene and ions inside the confined space. In addition, the ion transport behaviors inside the graphene nanochannel are strongly dependent on the electrode potential and channel widths. In particular, an anion adsorption-dominated charging mechanism is recognized on the graphene plane, whereas confined graphene nanochannels exhibit more complex ion transport behaviors, i.e., the anion adsorption-dominated and cation desorption-dominated behaviors alternate with increasing potential. The ion transport mechanism is highly related to the confinement, potential, ion–ion interaction, and ion–electrode interaction. Finally, an interfacial ion transport model based on nonequilibrium thermodynamics theory is proposed to analyze the real-time flux of anions and cations within the electrodes in the nonequilibrium state. It is found that the decreased charge storage at high charge–discharge rates is mainly attributed to the significant hindrance of co-ion transport, which is quite different from the traditional view (i.e., the hindrance/hysteresis of counter-ion adsorption). Such a phenomenon can be interpreted as follows. First, the transient formation of crowded counter-ions and dilute co-ions inside the graphene nanochannel leads to slower transport of the co-ions. Second, the decreasing number density of co-ions inside the graphene nanochannel also hinders the flux of co-ions in and out of the electrode. These insights reveal the energy and mass transfer mechanisms during the electrostatic adsorption process within complex graphene nanochannel/RTILs systems, which can potentially guide the design and fabrication of electrode materials and electrolytes for high-performance supercapacitors.

Poorly-crystallized poly(vinyl alcohol)/carrageenan matrix: Highly ionic conductive and flame-retardant gel polymer electrolytes for safe and flexible solid-state supercapacitors

Gel polymer electrolyte (GPE) based on poly(vinyl alcohol) (PVA) is widely reported due to its excellent mechanical strength, low cost, nontoxicity, and chemical stability. However, the low ionic conductivity that attributed to semi-crystalline structure and the high flammability of PVA limits its application in solid-state supercapacitors (SSCs). Herein, we use self-flame-retardant carrageenans (Car) to form hygrogen bonds with PVA molecular chains and improve their energy of nucleation. The obtained blend PVA/Car GPE displays low crystallinity, high ionic conductivity and excellent flame-retardant performance. The SSCs fabricated by PVA/Car GPE and activated carbon electrodes exhibit a high specific capacitance (406 mF cm−2 at 5 mA cm−2) and an excellent rate performance (99.9% capacitance retention after 5500 cycles). Furthermore, the flexible SSC in the bent state does not show any change and can drive a light-emitting diode successfully. The present work highlights a novel biomass strategy to reduce the crystallinity of PVA and obtain flame-retardant hybrid GPE simultaneously, which provides opportunities for flexible electronics with greatly enhanced ionic conductivity and safety.

Preparation of Graphene/Mn3O4 by Flash Irradiating for High Voltage Aqueous Supercapacitors

Graphene/Mn3O4 composites were synthesized by a facile flash irradiation method at room temperature. The flash irradiating triggers the deoxygenation reaction of graphene oxide and the decomposition reaction of manganese nitrate. Additionally, the graphene/Mn3O4 positive electrodes showed a wide operating potential of 0–1.2 V (vs SCE) in aqueous electrolyte for supercapacitors. The extended potential windows further expand the stable cell voltage (2.2 V) for aqueous asymmetric supercapacitors with a high energy density of 18.5 Wh kg−1 at power density of 0.28 kW kg−1.

Multifunctional supramolecular gel polymer electrolyte for self-healable and cold-resistant supercapacitor

Self-healability and cold-resistance will greatly improve the reliability of an energy-storage device in case of physical damage and subzero environment, which bring higher and higher requirement to electrolyte. Herein, a multifunctional supramolecular gel polymer electrolyte is prepared, which includes the first cross-linking network between PVA and TA through hydrogen bond cross-linking, and the second PVA network via the freezing-thawing cycle. The synergistic effects of two networks endow the gel polymer electrolyte with outstanding mechanical properties such as high tensile strengths (1.19 MPa), elongations (18.35 mm mm−1), self-recovery and excellent self-healability. Moreover, a high conductivity of gel polymer electrolyte is also obtained. A supercapacitor based on the gel polymer electrolyte can significantly restore its electrochemical performances after 10 cutting-healing cycles without external treatment at room temperature. In addition, the supercapacitor presents great controllability and can be tailored into different patterns without property deterioration. Besides, even at −20 °C, the SC is able to maintain 68.3% of its room temperature capacitance. The investigation offers a facile and versatile strategy to construct an intrinsically self-healable and cold-resistant energy-storage device which has potential applications for portable and wearable electronics, smart apparels or flexible robots, and so on.

Bio-inspired redox mediated electrolyte for high performance flexible supercapacitor applications over broad temperature domain

For practical supercapacitors, recent advances in electrolytes have diminished long-standing problems such chemical stability, low ionic conductivity and limited electrochemical performance. Herewith, we communicate a facile method to produce anhydrous redox-active Bio-inspired electrolytes for flexible supercapacitors with application over a broad temperature range. The glycerol (Gly) gel electrolyte consists of various fractions of potassium hydroxide (KOH) and redox active unit, i.e., ammonium molybdate, (NH4)2MoO4 (abbreviated as Mo), in the anhydrous state. The doped glycerol matrix acts as a framework that provides a stable gel electrolyte, forming hierarchical pathway for ion diffusion. The developed gel network with Gly-KOH-MoX (X = 3, 5, 10, and 15%, w/w) provides an effective route to carry the ions onto the pores of the electrodes. The supercapacitor with Gly-KOH5-Mo10 gel provides a specific capacitance of 328 F g−1, and the same device achieved an energy density of 45.6 Wh kg−1 at a power density of 497 W kg−1. In addition, the supercapacitor maintained extraordinary cyclic robustness and wide temperature tolerance. Thus, the low cost and environmentally benign, cyclic durability of Gly-KOH-Mo makes these gel electrolytes a potential candidate for use in various energy storage devices.