Electrolytes For Supercapacitors Research Articles

Ionic Liquid‐Based Redox Active Electrolytes for Supercapacitors

AbstractSupercapacitors (SCs) are fascinating energy storage devices due to their delivery of exceptional power density and long cycling stability. Unfortunately, their practical applications are still impeded by relatively inferior energy density, which is proportional to the square of the operating voltage of SCs. Ionic liquid (IL) electrolytes have a wide electrochemical stability window and thus can be used to significantly increase the energy density of SCs. The introduction of redox active species into IL‐based electrolytes effectively contributes to pseudocapacitance. Accordingly, IL‐based redox active electrolytes (IL‐REs) for SCs are springing up rapidly in recent years. This review provides an overall insight into various IL‐REs, including the ILs mixed with other redox active species, the ILs possessing redox active groups themselves, and the IL‐based redox gel polymer electrolytes for SCs. The basic understanding of IL electrolytes and IL‐REs is introduced and discussed as well as the application of the IL‐REs in SCs. Then, the energy storage mechanisms of these IL‐REs are discussed, and finally, current challenges and perspectives are highlighted for future research in this promising field.

Understanding the Capacitive Charge in Bulk Porous Electrodes by Mathematical Modeling

Supercapacitors exhibit immense potential in electric vehicles and national grids due to their high power density. The power and energy density of supercapacitors largely depend on the design of electrodes and the composition of electrolytes, which dominate the capacitive charging process. However, the availability of only a few mathematical models for capacitive charging in bulk porous electrodes has limited the rational design of a supercapacitor in practice. Herein, a mathematical model is proposed to predict the charging process in a porous electrode and is then experimentally validated. In this model, the dependence of capacitive behavior on the conductivity of both the electrode and electrolyte, as well as the porosity and thickness of electrode, are systematically discussed. Moreover, different charging modes, e.g., potentiostatic charging, potentiodynamic charging, and galvanostatic charging, are studied. The results of this model indicate that the conductivity of the electrolyte (even though it is an aqueous electrolyte) is the bottleneck that limits the charging rate of a supercapacitor, in particular, in thick electrode films. This mathematical model could be a powerful tool for optimizing the design of both electrodes and electrolytes for high-performance supercapacitors.

Last developments in polymers for wearable energy storage devices

Last developments in polymers for wearable energy storage devices

Polymer Derived Carbon Nanostructure Electrodes for Solid-State Supercapacitor

To resolve the energy storage challenges in portable electronic devices, flexible and solid-state supercapacitor are need of the hour. These devices are appealing as replacements to conventional lithium-ion batteries due to their high power density, long cycle life, chemical stability, and safety. We have synthesized sp2 hybridized porous carbon nanostructure using poly-paraphenylenediamine (PpPD) as a polymer and activated the prepared material via chemical activation for supercapacitor application. To the best of our knowledge, this is the first report on the demonstration of poly-paraphenylenediamine (PpPD) derived N-enriched porous activated carbon (PNAC) electrode with PVA/H2SO4 electrolyte for solid-state supercapacitor (SSC) device with high energy density and excellent cyclic stability. PNAC electrodes-based SSC electrochemical studies exhibit a high specific capacitance value of 39 F g−1 at 0.2 A g−1 current density. The solid-state supercapacitor attained energy density of 21.66 Wh kg−1 at power density of 199 W kg−1 and 0.2 A g−1 current density. The symmetric solid-state supercapacitor displays an outstanding cyclic stability up to 10,000 galvanostatic charge-discharge (GCD) cycles. High energy density and stability of the prepared device suggests it utility in portable energy storage application.

Mixture of non-ionic and organic ionic plastic crystals immobilized in poly(vinylidene fluoride-co-hexafluoropropylene): A flexible gel polymer electrolyte composition for high performance carbon supercapacitors

Flexible and quasi-solid-state gel polymer electrolytes (GPEs) comprising plastic crystals are emerged as new class of materials for the energy storage applications. Here, a novel composition of a flexible matrix of GPEs comprising a mixture of a non-ionic plastic crystal (succinonitrile) and an organic-ionic plastic crystal 1-ethyl-1-methyl-pyrrolidinium bis(trifluoromethyl sulfonyl)imide, immobilized in a polymer poly(vinylidine fluoride-co-hexafluoropropylene) for application in carbon supercapacitors. Different structural, thermal and electrochemical properties including high ionic conductivity (σ ~3.3 × 10−3 S cm−1 at room temperature) and a wide electrochemical stability window (~3.1 V versus Ag) make the GPE films attractive as potential electrolyte for supercapacitors. Effect of addition of Li-salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in GPE is also studied and found improvement in supercapacitors' performance. Supercapacitors are fabricated using the mixed plastic crystals-based GPE films separated by activated carbon electrodes, optimized via different techniques namely cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge measurements. The optimized capacitor with GPE containing LiTFSI offers the specific capacitance of ~370.7 F g−1 with specific energy and maximum power density of ~80.5 Wh kg−1 and ~70.6 kW kg−1, respectively. The supercapacitor offers stable cyclic performance for ~10,000 charge-discharge cycles with ~93% of retention and safe performance over a temperature range from −40 to 80 °C.

A solvent-exchange strategy to develop stiff and tough hydrogel electrolytes for flexible and stable supercapacitor

Toughness of flexible energy storage devices and a stable energy output during dynamic deformation are two challenges to be solved in their practically application. Herein, we reported a stiff and tough poly(vinyl alcohol) (PVA) hydrogel electrolyte for flexible supercapacitor with toughness and stable energy output. A one-step solvent-exchange strategy was developed to replace the good solvent dimethyl sulfoxide (DMSO) to a poorer one water and simultaneously introduce kosmotropic ions into the PVA pre-hydrogels. This triggered the PVA chains to self-assemble and forming a homogeneous network crosslinked by the PVA crystalline domains and hydrophobic interactions. Benefiting from the excellent energy-dissipating ability of the homogeneous network, the PVA hydrogel electrolytes exhibited remarkable stiffness and toughness with tensile strength 16.54 MPa, elongation at break 1203%, and toughness 111.21 MJ m−3, which surpassed the most reported hydrogels. By utilizing polyaniline as electrodes, a flexible supercapacitor was assembled with superior areal specific capacitance of 156.50 mF cm−2 at 1.0 mA cm−2. The supercapacitor delivered high resistance to mechanical stimuli such as bending and pressure. Moreover, it could well maintain stable energy output even being dynamically deformed. The developed supercapacitor showed great potential in the wearable applications.

Highly Deformable, Conductive Double-Network Hydrogel Electrolytes for Durable and Flexible Supercapacitors.

Developing flexible energy storage devices with the ability to retain capacitance under extreme deformation is promising but remains challenging. Here, we report the development of a durable supercapacitor with remarkable capacitance retention under mechanical deformation by utilizing a physical double-network (DN) hydrogel as an electrolyte. The first network is hydrophobically associating polyacrylamide cross-linked by nanoparticles, and the second network is Zn2+ cross-linked alginate. Through soaking such a DN hydrogel into a high concentration of ZnSO4 solution, a highly deformable electrolyte with good conductivity is fabricated, which also shows adhesion to diverse surfaces. Directly attaching the hydrogel electrolyte to two pieces of an active carbon cloth facilely produces a flexible supercapacitor with a high specific capacitance and theoretical energy density. Remarkable capacitance retention under tension, compression, and bending is observed for the supercapacitor, which can also maintain above 87% of the initial capacitance after 4000 charge-discharge cycles. This study provides a simple way to fabricate hydrogel electrolytes for deformable yet durable supercapacitors, which is expected to inspire the development of next-generation flexible energy storage devices.

Direct observation of the CO2 formation and C–H consumption of carbon electrode in an aqueous neutral electrolyte supercapacitor by in-situ FTIR and Raman

Electrical double-layer capacitors (EDLCs) consist of energy storage devices that present high-power and moderate energy density. The electrolyte and electrode physicochemical properties are crucial for improving their overall energy storage capabilities. Therefore, the stability of the EDLCs’ materials is the primary focus of this study. Since energy storage depends on the specific capacitance, and also on the square of the maximum capacitive cell voltage (UMCV). Thus, electrodes with high specific surface area (SSA) and electrolytes with excellent electrochemical stability are commonly reported in the literature. Aqueous electrolytes are safer and green devices compared to other organic-based solutions. On the other hand, their UMCV is reduced compared to other electrolytes (e.g., organic-based and ionic liquids). In this sense, spanning the UMCV for aqueous-based electrolytes is a ‘hot topic’ research. Unfortunately, the lack of protocols to establish reliable UMCV values has culminated in the publishing of several conflicting results. Herein, we confirm that multiwalled carbon nanotubes (MWCNTs) housed in cells degrade and produce CO2 under abusive polarisation conditions. It is probed by employing electrochemical techniques, in-situ FTIR and in-situ Raman spectroscopies. From these considerations, the current study uses spectro-electrochemical techniques to support the correct determination of the electrode and electrolyte stability conditions as a function of the operating electrochemical parameters.

Heteroatoms self-doped porous carbon from cottonseed meal using K2CO3 as activator and DES electrolyte for supercapacitor with high energy density

Biomass had been extensively explored and applied in many fields due to their abundance, attractive structure, low cost, renewability, and environmental friendliness. Cottonseed meal (CM), one of the by-products of cotton, consisted of much crude protein, fiber, and inorganic ions, was a potential carbon precursor. In this work, CM was used to prepare N, S, and O self-doped carbon materials by two steps (hydrothermal pre-carbonization and K2CO3 carbonization–activation) processes. The optimized material displayed high capacitive performance, which benefited from the large surface area (2361 m2/g), hierarchical porous structure and rich multi-heteroatoms doping of the prepared porous carbon. What's more, we prepared a new-type K2CO3-based deep eutectic solvent (DES) electrolyte. The assembled symmetric device using DES electrolyte displayed a superior energy density (34.4 Wh/kg) at room temperature. Furthermore, the energy density could reach 36.5 Wh/kg when the temperature rose to 50 °C. Even under extreme conditions, the device delivered a not particularly bad energy density (11.8 Wh/kg at −25 °C and 8.6 Wh/kg at 105 °C). This study provided an efficient and simple method to prepare CM-based heteroatoms self-doped porous carbon materials and uncovered a new possibility for the exploitation of carbon-based supercapacitors with high energy density.

Designing of two dimensional lanthanum cobalt hydroxide engineered high performance supercapacitor for longer stability under redox active electrolyte

Redox active electrolyte supercapacitors differ significantly from the conventional electrolytes based storage devices but face a long term stability issue which requires a different approach while designing the systems. Here, we show the change in layered double hydroxides (LDHs) systems with rare earth elements (lanthanum) can drastically influence the stability of two dimensional LDH systems in redox electrolyte. We find that the choice of rare earth element (lanthanum) having magnetic properties and higher thermal and chemical stability has a profound effect on the stability of La–Co LDHs electrode in redox electrolyte. The fabricated hybrid device with rare earth based positive electrode and carbon as negative electrode having redox electrolyte leads to long stable high volumetric/gravimetric capacity at high discharge rate, demonstrates the importance of considering the rare earth elements while designing the LDH systems for redox active supercapacitor development.

In-situ electrochemical and operando Raman techniques to investigate the effect of porosity in different carbon electrodes in organic electrolyte supercapacitors

The analyzes of micro-, meso‑ and macro-porous carbon electrodes in a symmetrical supercapacitor filled with a 1.0 M TEABF4 EC-DMC electrolyte were reported in this work. The activated carbon (AC), multi-walled carbon nanotubes, and graphite materials were studied. The results indicate AC is the best electrode material for the application with high specific capacitance and energy. The Raman operando analyzes of AC show that the D- and G- bands shift their positions and intensities. The cause of this phenomenon is the electrostatic ionic adsorption inside very narrow pores, affecting the vibrational mode and the work function of the material, resulting in the shift of the Fermi level of the material. For mesoporous carbon, this phenomenon is much less pronounced. In principle, graphite has enough specific capacitance for competitive application. However, its overall surface area is reduced. Graphite's Raman operando analyzes revealed the BF4−insertion into graphene layers. The G-band splits while the in-situ electrochemical data support a capacitance with three orders of magnitude higher than expected for a purely electrostatic adsorption process on graphite. Overall, this fundamental study evidences that in-situ electrochemistry and Raman spectroscopy combined could offer important information on the energy storage process for supercapacitors.

An ultrahigh‐energy density and wide potential window aqueous electrolyte supercapacitor built by polypyrrole/aniline 2‐sulfonic acid modified carbon felt electrode

An ultrahigh‐energy density and wide potential window aqueous electrolyte supercapacitor built by polypyrrole/aniline 2‐sulfonic acid modified carbon felt electrode

A tailor-made deep eutectic solvent for 2.2 V wide temperature-tolerant supercapacitors via optimization of N,N-dimethylformamide/water co-solvents

A high-performing electrolyte plays an imperative role in the pursuit of high energy density, green, low-cost, and safe supercapacitors. To this end, a novel deep eutectic solvent (DES) composed of N-methylacetamide and lithium perchlorate has been engineered by the addition of co-solvents including water and N,N -dimethylformamide (DMF). Interestingly, each co-solvent makes a specific contribution to DES performance as water for flame resistance and DMF for antifreeze. The optimized hybrid DES is tested as the electrolyte for symmetric supercapacitors with polyaniline/reduced graphene oxide composite electrodes (PANI-rGO). The hybrid DES with high ionic conductivity imparts a high operation voltage of 2.2 V to devices whereas the PANI-rGO electrode with the pseudocapacitive behavior provides the high capacitances. Due to the above synergistic effects, our device delivers the maximum energy density of 28.2 W h kg −1 at a power density of 1.1 kW kg −1 and shows moderate cyclic stability at room temperature. More importantly, the supercapacitors can well function in a low temperature range from −20 to 25 °C. The device operated at 25 °C achieves the maximum capacitance of 41.9 F g −1 due to improved ionic conductivity of the DES electrolyte. The current study opens a new route towards sustainable and affordable energy storage devices. • DES electrolyte with N,N -dimethylformamide as co-solvent shows 2.2V window. • PANI-rGO exhibits pseudocapacitive behavior in the hybrid DES electrolyte. • Fabricated symmetric supercapacitor has potential to operate from −20 to 70 °C. • It delivers an energy density of 28.2 W h kg −1 at a power density of 1.1 kW kg −1

Coherent Integration of Organic Gel Polymer Electrolyte and Ambipolar Polyoxometalate Hybrid Nanocomposite Electrode in a Compact High-Performance Supercapacitor.

We report a gel polymer electrolyte (GPE) supercapacitor concept with improved pathways for ion transport, thanks to a facile creation of a coherent continuous distribution of the electrolyte throughout the electrode. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was chosen as the polymer framework for organic electrolytes. A permeating distribution of the GPE into the electrodes, acting both as integrated electrolyte and binder, as well as thin separator, promotes ion diffusion and increases the active electrode–electrolyte interface, which leads to improvements both in capacitance and rate capability. An activation process induced during the first charge–discharge cycles was detected, after which, the charge transfer resistance and Warburg impedance decrease. We found that a GPE thickness of 12 μm led to optimal capacitance and rate capability. A novel hybrid nanocomposite material, formed by the tetraethylammonium salt of the 1 nm-sized phosphomolybdate cluster and activated carbon (AC/TEAPMo12), was shown to improve its capacitive performance with this gel electrolyte arrangement. Due to the homogeneous dispersion of PMo12 clusters, its energy storage process is non-diffusion-controlled. In the symmetric capacitors, the hybrid nanocomposite material can perform redox reactions in both the positive and the negative electrodes in an ambipolar mode. The volumetric capacitance of a symmetric supercapacitor made with the hybrid electrodes increased by 40% compared to a cell with parent AC electrodes. Due to the synergy between permeating GPE and the hybrid electrodes, the GPE hybrid symmetric capacitor delivers three times more energy density at higher power densities and equivalent cycle stability compared with conventional AC symmetric capacitors.

Mesoporous electrode from human hair and bio-based gel polymer electrolyte for high-performance supercapacitor

This work reports a facile and scalable method for assembling a supercapacitor. Cost-effective solid-state supercapacitor contrived by combining waste biomass human hair derived activated carbon (HHCK) as an electrode and albumen (egg white gel polymer) in 1 M NaCl electrolyte (1 M NaCl-EWG). Human hair as an electrode precursor and egg white as a gel polymer precursor provide a sustainable approach for biomass-based material's cost-effective and eco-friendly utilization. Human hair derived interconnected three-dimensional hierarchal porous carbon of human hair possessed a high specific surface area of 1466 m2 g−1 along with the incorporation of ~23% heteroatoms into the carbon matrix. The solid-state supercapacitor (HE-SC) exhibits high specific capacitance of 491 F g−1 at 1 A g−1 within a wide electrochemical stable potential window of 1.5 V. HE-SC provided a high energy density of 38.4 W h kg−1 at 0.374 kW kg−1 power density, quick charge-discharge ability (time constant was 4.3 s), and high cyclic stability (86% capacitance retention with 95% coulombic efficiency after 6500 cycles). This current research provides a feasible and effective strategy for preparing low-cost and high-performance supercapacitors utilizing waste biomass.

Energy and power performances of binary mixtures of ionic liquids in planar and porous electrodes by molecular dynamics simulations

Binary mixtures of ionic liquids are interesting alternatives to tailor ionic liquids properties. Some of them have been proposed as electrolytes for supercapacitors. Herein, we performed molecular dynamics simulations of planar and porous electrodes with binary mixtures of ionic liquids comprising the 1-ethyl–3-methyl–imidazoliumbis(trifluoromethane–sulfonyl)imide ([EMIM][NTf2]) and 1-ethyl–3-methyl–imidazoliumtetrafluoroborate ([EMIM][BF4]), increasing the mole fraction of the latter (xBF4) from 0 to 1. The energy densities calculated from the simulations increase monotonically with xBF4, accordingly with available experimental results [Angew. Chem. Int. Ed. 2015, 54, 1 – 5]. Considering that the electrode simulations were dealt with the constant potential model, the characteristic charging times were calculated for planar and porous electrodes. The devices containing only [EMIM][NTf2] charge slower than electrolytes with xBF4 > 0.0, and the fastest charging happens in xBF4 = 1.0 V. These features are related to the higher [NTf2]-electrode interaction and also to the smaller size of [BF4]−. For porous electrode, the calculated Ragone plot indicates that [EMIM][BF4] outperforms all the other electrolytes with a voltage of 3.0 V in terms of energy and power densities. In the system of best performance, there are considerable contributions of co-ion desorption and counter-ion adsorption mechanisms in the negative and positive electrodes, respectively.

Novel Electrode Materials and Redox‐Active Electrolyte for High‐Performance Supercapacitor

AbstractHeteroatom doping has been increasingly applied to enhance the electrochemical performance of carbon materials when used as supercapacitor electrodes. However, the use of doped electrode by itself often does not provide sufficiently high performance for aqueous‐based supercapacitor. Here, a high‐performance supercapacitor was developed, using nitrogen‐doped microporous carbon spheres‐based materials and a redox‐active electrolyte as electrodes and electrolyte, respectively. The doped electrode, which was produced via template and self‐assembly routes, exhibited a well‐developed microporous structure, large specific surface area, and abundant heteroatoms. The combination of the heteroatom‐doped electrode and KI‐H2SO4 electrolyte in supercapacitor substantially increased the specific capacitance, from 230 F g−1 to 532 mAh g−1. Such a large enhancement was attributed to the addition of KI as a redox mediator in the aqueous electrolyte. The NCS‐800 supercapacitor has a specific capacity as high as 68 mAh g−1 (at a 1 A g−1 current density) in KI‐H2SO4 electrolyte. It could retain 57 % of capacitance even after maintaining a constant voltage of 1 V for 5 hours, highlighting its good electrochemical performance of NCS‐800 as a supercapacitor electrode material.

Sustainable and Printable Nanocellulose-Based Ionogels as Gel Polymer Electrolytes for Supercapacitors.

A new gel polymer electrolyte (GPE) based supercapacitor with an ionic conductivity up to 0.32–0.94 mS cm−2 has been synthesized from a mixture of an ionic liquid (IL) with nanocellulose (NC). The new NC-ionogel was prepared by combining the IL 1-ethyl-3-methylimidazolium dimethyl phosphate (EMIMP) with carboxymethylated cellulose nanofibers (CNFc) at different ratios (CNFc ratio from 1 to 4). The addition of CNFc improved the ionogel properties to become easily printable onto the electrode surface. The new GPE based supercapacitor cell showed good electrochemical performance with specific capacitance of 160 F g−1 and an equivalent series resistance (ESR) of 10.2 Ω cm−2 at a current density of 1 mA cm−2. The accessibility to the full capacitance of the device is demonstrated after the addition of CNFc in EMIMP compared to the pristine EMIMP (99 F g−1 and 14.7 Ω cm−2).

Deep eutectic solvents as effective electrolyte from potassium iodide and ethylene glycol exhibiting redox behavior for supercapacitor application

In present work, an efficient deep eutectic solvents (DESs) system based on KI and ethylene glycol (EG) has been successfully constructed and used as an electrolyte for supercapacitors in wide temperature range. The DESs electrolytes represent good electrochemical performance and the maximum specific capacitance reaches 217 F g−1 and the energy density is 18.42 Wh kg−1 at 0.7 kW kg−1. It is noteworthy that the DESs electrolytes can produce pseudocapacitance. Consequently, the total pseudocapacitance of supercapacitors is contributed by the redox reaction of KI as well as the redox reaction of oxygen-containing functional groups on the surface of activated carbon. Meanwhile, iodine-related ions and unbonded hydroxyl group of DESs electrolytes form hydrogen bonds with the oxygen-containing functional groups, accelerating the migration of electrolytes ions. Additionally, it is found that the internal composition of DESs electrolytes plays a vital effect on its properties. Resultantly, DESs with a molar ratio of KI to EG of 1:7 delivers the highest conductivity (1.22 mS cm−1) and the lowest viscosity (39.77 cP), exhibiting best electrochemical performance. The present novel DESs electrolyte paves the way for application of non-aqueous redox additives in supercapacitors, simultaneously delivering hydrogen bonding between solvent and redox additive.

Concentration dependent electrochemical performance of aqueous choline chloride electrolyte

In this work we have analysed the performance of aqueous choline chloride as an electrolyte in supercapacitors. The aqueous choline chloride electrolyte is prepared in molar concentrations of 0.3, 0.5, 0.7, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4. The different concentrations of aqueous choline chloride electrolyte are used to fabricate activated carbon electrode based supercapacitors. The prepared electrolytes are characterized using FTIR technique and their ionic conductivity and electrochemical stability window are analysed using electrochemical impedance spectroscopy and cyclic voltammetry respectively. The electrochemical stability window is found 1.8 V for all concentration of choline chloride electrolytes. The highest conductivity is 129 mS/cm for 2.5 M aqueous choline chloride and two peaks feature is observed in ionic conductivity vs. concentration of electrolyte plot. The electrochemical performance of supercapacitors is analysed using cyclic voltammetry and electrochemical impedance spectroscopy, the supercapacitors showed good charge propagation and highest specific capacitance is reported for 1.0 M electrolyte (5.75F/g).