Asymmetric Electrochemical Capacitor Research Articles

Exploring polyaniline nanofiber synthesis at vegetable oil-water interfaces for high-performance hybrid electrochemical capacitors

Polyaniline (PANI) nanofibers are synthesized at five different vegetable oil-water interfaces by a modified interfacial polymerization method. A series of syntheses is performed to study the effect of vegetable oil- water interfaces on the sizes, morphologies and corresponding electrochemical performances of resultant PANI nanofiber samples. Five PANI nanofiber samples are synthesized and the corresponding electrochemical performances as electrochemical capacitor (EC) electrodes are evaluated. It is observed that the PANI nanofiber sample prepared at water-sunflower oil interface, PANI5 possesses the lowest diameter and highest specific capacitance among all the prepared samples. Herein, we have fabricated an all-solid-state asymmetric electrochemical capacitor (AEC) of PANI5 and bio-derived chemically activated carbon (AC). The electrochemical measurements show that PANI5 and AC exhibits specific capacitance of 395 F g−1 and 240 F g−1 respectively at current density 1 A g−1. The fabricated PANI5//AC AEC exhibits areal capacitance of 70 mF cm−2 at current density of 1 mA cm−2. The energy density (Ed) and power density (Pd) of constructed AEC are found to be 19 µW h cm−2 and 696 µW cm−2, respectively. It gives the cyclic stability of 62 % retention of initial areal capacitance after completing 5000 galvanostatic charge-discharge cycles.

Battery-Type-Behavior-Retention Ni(OH)2-rGO Composite for an Ultrahigh-Specific-Capacity Asymmetric Electrochemical Capacitor Electrode.

Nanosized battery-type materials applied in electrochemical capacitors can effectively reduce a series of problems caused by low conductivity and large volume changes. However, this approach will lead to the charging and discharging process being dominated by capacitive behavior, resulting in a serious decline in the specific capacity of the material. By controlling the material particles to an appropriate size and a suitable number of nanosheet layers, the battery-type behavior can be retained to maintain a large capacity. Here, Ni(OH)2, which is a typical battery-type material, is grown on the surface of reduced graphene oxide to prepare a composite electrode. By controlling the dosage of the nickel source, the composite material with an appropriate Ni(OH)2 nanosheet size and a suitable number of layers was prepared. The high-capacity electrode material was obtained by retaining the battery-type behavior. The prepared electrode had a specific capacity of 397.22 mA h g-1 at 2 A g-1. After the current density was increased to 20 A g-1, the retention rate was as high as 84%. The prepared asymmetric electrochemical capacitor had an energy density of 30.91 W h kg-1 at a power density of 1319.86 W kg-1 and the retention rate could reach 79% after 20,000 cycles. We advocate an optimization strategy that retains the battery-type behavior of electrode materials by increasing the size of nanosheets and the number of layers, which can significantly improve the energy density while combining the advantage of the high rate capability of the electrochemical capacitor.

Performance evaluation of electrochemical capacitors with activated carbon spheres as electrode material and aqueous electrolyte

The model activated carbon spheres (ACSs) were optimized to be used as negative electrode in asymmetric electrochemical capacitors (ECs). A microporous commercial activated carbon was used as positive electrode.Carbon spheres (CSs) were synthesized by precipitation polymerization at room temperature followed by pyrolysis and activation. By enhancing the carbon:KOH ratio from 1:2 to 1:4, an increase in the specific surface area from 380 m2 g−1 to 2835 m2 g−1 (1:4) was achieved along with an increase in pore volume/size. Consequently, the electrochemical performance in the aqueous electrolyte was improved. After activation, low-temperature-pyrolyzed polymer spheres (400 °C) result in random-like particles with macroporous structure, while intermediate- and high-temperature-pyrolyzed materials (550 °C and 700 °C) lead to partially and totally conserved spheres.The interaction between the electrode material and electrolyte is important and is related to the texture, morphology, and surface chemistry of ACS. In 1 mol L−1 H2SO4, capacitance and rate handling are mostly affected by electrode morphology and wettability with the electrolyte. In 1 mol L−1 Li2SO4 electrolyte solution, texture and surface chemistry of the electrode material are crucial to obtain high-performance EC. Regardless of morphology, poor wetting properties of synthesized materials were found in Li2SO4, compared to the H2SO4 solution.

Sustainable production of lignin-derived porous carbons for high-voltage electrochemical capacitors

• CuCl 2 activation mechanism is elucidated. • Lignin-derived porous carbon with high specific surface area. • Lignin-derived porous carbon with high yields up to 49.4%. • High-voltage electrochemical capacitors are fabricated. The high energy storage cost is a crucial factor limiting the wide application of electrochemical capacitors. Herein, we proposed a comprehensive strategy to reduce the cost of electrochemical capacitors with aqueous electrolytes, i.e. , reducing the cost of electrode materials and increasing the energy density of electrochemical capacitors. Low-cost lignin-derived porous carbon electrode materials with high specific surface area and hierarchical porous structure were prepared by CuCl 2 activation. The CuCl 2 activation agent was recycled and thus can be reused. What is more, we proposed that the mechanism of CuCl 2 activation is a solid-phase reaction in which the chloride ions in copper chloride deprive the hydrogen atoms in lignin, resulting in the loss of hydrogen atoms and the formation of pores. High energy density C//C symmetric electrochemical capacitors, Zn//C and Pb//C asymmetric electrochemical capacitors based on lignin-derived porous carbon were fabricated using sulfate electrolytes which endow high working voltage window. This work provides a comprehensive strategy for designing electrochemical capacitors with high energy densities and low cost of energy storage, which is expected to promote the industrial application of electrochemical capacitors with aqueous electrolytes.

Synthesis of Oxygen‐Deficient WO3−x Nanoplates and Hollow Microspheres Decorated on a Carbon Cloth for Supercapacitors

AbstractOxygen‐deficient CC−WO3−x nanoplates and hollow microspheres on carbon fabrics cloth were synthesized utilizing a simple solvent‐thermal route. The obtained CC−WO3−x possess an improved electronic conductivity because of the oxygen‐deficient effect and a higher specific surface area due to the unique nano‐structural features. The CC−WO3−x exhibits a high capacitance of 804 F g−1, excellent rate performance (>80 %) while the current density is enhanced five‐fold and superior cycling stability by retaining 109.5 % after 14000 cycles at 30 mV s−1. In addition, an asymmetric electrochemical capacitor (ASC) reveals a competitive energy storage performance (50.7 W h kg−1 at 1.3 KW kg−1). The reasons for its good electrochemical property are that the novel hybrid nanoplates and hollow microspheres structure with oxygen‐deficient and binder‐free, which is conducive to exposing more electroactive sites and enhancing electrical conductivity, therefore accelerating ion diffusion, facilitating charge transportation, and promoting the capacitive contribution for rapid charge‐discharge kinetics synchronously.The route is anticipated to reveal a new way to prepare hybrid nanostructure with oxygen‐deficient and binder‐free metal oxide‐based electrode material.

Tailored 3D Foams Decorated with Nanostructured Manganese Oxide for Asymmetric Electrochemical Capacitors

Tailored 3D (Ni and NiCo) metallic foam architectures were produced by electrodeposition and decorated via electrochemical routes with manganese oxide (MnOx) to serve as positive electrodes for supercapacitors. For comparative purposes, an electrode made of commercial Ni foam was also prepared. The foam-based electrodes were paired with a carbon cloth electrode and used to assemble asymmetric electrochemical cells. The electrochemical response of these cells was studied by applying different electrochemical techniques. In addition, two different protocols (cycling and floating) were applied to assess cells durability and fade. Despite the significant differences in the decorated foams morphology and structure their electrochemical responses revealed similar trends. The electrodes made of tailored foams showed higher specific capacitance, better capacitance retention at high current load and enhanced cycling stability compared to the electrodes made of commercial foam. The asymmetric cells made with the tailored foams revealed higher (maximum) specific energy (11–14 Wh kg−1) and specific power (1.3–1.4 × 104 W kg−1) compared to cells assembled with commercial foams (8.4 Wh kg−1 and 6.3 × 103 W kg−1). The durability tests evidenced that corrosion of the NiCo electrodeposited foams and electrochemical dissolution of MnOx are possible causes of cells degradation.

Enzymatic Hydrolysis Lignin-Derived Porous Carbons through Ammonia Activation: Activation Mechanism and Charge Storage Mechanism.

The low energy density and low cost performance of electrochemical capacitors (ECs) are the principal factors that limit the wide applications of ECs. In this work, we used enzymatic hydrolysis lignin as the carbon source and an ammonia activation methodology to prepare nitrogen-doped lignin-derived porous carbon (NLPC) electrode materials with high specific surface areas. We elucidated the free radical mechanism of ammonia activation and the relationship between nitrogen doping configurations, doping levels, and preparation temperatures. Furthermore, we assembled NLPC∥NLPC symmetric ECs and NLPC∥Zn asymmetric ECs using aqueous sulfate electrolytes. Compared with the ECs using KOH aqueous electrolyte, the energy densities of NLPC∥NLPC and NLPC∥Zn ECs were significantly improved. The divergence of charge storage characteristics in KOH, Na2SO4, and ZnSO4 electrolytes were compared by analyzing their area surface capacitance. This work provides a strategy for the sustainable preparation of lignin-derived porous carbons toward ECs with high energy densities.

Fabrication of Vertical-Standing Co-MOF Nanoarrays with 2D Parallelogram-like Morphology for Aqueous Asymmetric Electrochemical Capacitors.

Metal organic frameworks (MOFs) have been considered as one of the most promising electrode materials for electrochemical capacitors due to their large specific surface area and abundant pore structure. Herein, we report a Co-MOF electrode with a vertical-standing 2D parallelogram-like nanoarray structure on a Ni foam substrate via a one-step solvothermal method. The as-prepared Co-MOF on a Ni foam electrode delivered a high area-specific capacitance of 582.0 mC cm−2 at a current density of 2 mA cm−2 and a good performance rate of 350.0 mC cm−2 at 50 mA cm−2. Moreover, an asymmetric electrochemical capacitor (AEC) device (Co-MOF on Ni foam//AC) was assembled by using the as-prepared Co-MOF on a Ni foam as the cathode and a active carbon-coated Ni foam as the anode to achieve a maximum energy density of 0.082 mW cm−2 at a power density of 0.8 mW cm−2, which still maintained 0.065 mW cm−2 at a high power density of 11.94 mW cm−2. Meanwhile, our assembled device exhibited an excellent cycling stability with a capacitance retention of nearly 100% after 1000 cycles. Therefore, this work provides a simple method to prepare MOF-based material for the application of energy storage and conversion.

Significantly Enhanced Electrochemical Redox for High‐Performance Electrochemical Capacitor via Active Ion‐Tunnel Oriented BaCoF4 Electrodes

AbstractActive plane and specific morphology with reduced particle sizes have long been considered a promising strategy to achieve superior electrochemical properties, but the active sites involved may not be sufficiently utilized or the surface atomic configurations may obscure the activity. Herein, a novel structural “active orientation” strategy is developed to overcome the aforementioned shortcomings and improve the efficiency at active sites. A transition‐metal fluoride BaCoF4 is well controlled to thin the dimensions along an active orientation through a sodium dodecyl benzene sulfonate assisted solution chemistry route. The active orientation facilitates the opening of the ionic pathways, for example, OH– in the electrolyte, to take full advantage of the redox activity of the electrochemically active Co2+/Co3+ cations in ‐BaCoF4, resulting in significantly enhanced electrochemical redox performance. A high specific capacitance (692 F g−1 at 1 A g−1 in 6 m KOH electrolyte) is achieved owing to active‐tunnels orientation, ≈five‐fold higher compared to its bulk counterpart. Strikingly, the asymmetric electrochemical capacitor (AEC) fabricated with ‐BaCoF4 and activated carbon exhibits an ultrahigh energy density of 147.7 Wh kg−1 at a power density of 1.025 kW kg−1 (also >100 Wh kg−1 at 5 kW kg−1), much higher than the majority of existing AEC systems.

Reversible Intercalation of Al‐Ions in Poly(3,4‐Ethylenedioxythiophene):Poly(4‐Styrenesulfonate) Electrode for Aqueous Electrochemical Capacitors with High Energy Density

Due to the high capacity of the three‐electron redox mechanism, Al‐ions‐based energy‐storage devices have the potential to provide a viable solution to meet the growing demand for powering electronic products. However, discovering suitable electrode materials for reversible insertion of Al ions remains a difficult task. Herein, it is reported that a classical conductive polymeric material poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) can perform the reversible Al‐ions intercalation for aqueous electrochemical capacitors. The as‐prepared PEDOT:PSS film on a carbon cloth composite electrode exhibits a large magnitude of faradaic currents and sharp redox peaks in cyclic voltammetry (CV) curves in aluminum sulfate electrolyte, and delivers a high capacitance of 269 F g−1 (78 mAh g−1). Diffusion‐controlled Al‐ions intercalation/deintercalation as the charge‐storage mechanism is demonstrated here, which is not observed in other ions‐based electrolytes (H+, Mg2+, Li+, Na+). An asymmetric electrochemical capacitor based on Al ions, composed of such an electrode and activated carbon electrode is assembled and displays a high energy density of 41.6 Wh kg−1 at a power density of 0.24 kW kg−1, demonstrating a promising aqueous electrochemical capacitor with an advanced energy density via polyvalent ions intercalation.

A review of neutral pH polymer electrolytes for electrochemical capacitors: Transitioning from liquid to solid devices

Abstract The development of neutral pH polymer electrolytes has enabled high-performance solid-state, thin, and flexible electrochemical capacitors (ECs) to provide power for future consumer electronics and Internet-of-Thing devices. Notwithstanding their promising prospect, there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes. In this review, we provide an overview of state-of-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations. Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes, key considerations in developing neutral pH polymer electrolytes are discussed. The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices, as well as some enhanced functionalities are presented. The future directions for research on neutral pH polymer electrolytes are proposed, expected to provide reference for further enriching the fundamental knowledge and improving the device performances.

Electrochemical capacitance of intermetallic vanadium carbide

The polycrystalline intermetallic vanadium carbide (V8C7) is successfully synthesized and investigated as an electrode material for electrochemical capacitors. The V8C7 electrode has a large specific capacitance of 161.7 F g−1 in 1.0 mol L−1 Na2SO4 at a specific current density of 1 A g−1 and remains 92.8% of this value after 10,000 cycles. 83.3% of the initial value can maintain even at a high current density of 10 A g−1. The outstanding electrochemical performance is attribute to the high electrical conductivity of V8C7 which can ensure fastly charge transfer. Combined with the analysis of specific capacitances and specific surface areas, the charge storage mechanism belongs to the sub-surface space pseudocapacitive mechanism. Moreover, the V8C7//activated carbon asymmetrical electrochemical capacitor owns a high energy density of 16.9 W h kg−1 at a power density of 8.7 kW kg−1.

Electropolymerized poly(3-methylthiophene) onto high density vertically aligned carbon nanotubes directly grown on aluminum substrate: Application to electrochemical capacitors

Potentiostatic pulsed deposition of poly(3-methylthiophene) (P3MT) is carried out on vertically aligned carbon nanotubes directly grown on aluminum foil (VACNT/Al) in a mixture of ionic liquid in acetonitrile (25/75 v/v) as electrolyte to prepare pseudo-capacitive materials for electrochemical capacitors. VACNT with high density (2.1011 VACNT/cm2) and anisotropy, have high specific surface area (340 m2/g) and serve as template electrode to nanostructure the P3MT coating. The effect of the polymer weight in the nanocomposite on the morphology and electrochemical properties of nanocomposite electrodes is investigated. Up to 70% of P3MT is electrodeposited inside the carpet onto the VACNTs as evidenced by Scanning Electronic Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). A specific capacitance of 170F/g was obtained for the optimal nanocomposite composition (70%). The calculated areal capacitance is 380 mF/cm2 and the volumetric capacitance is 76 F/cm3. In addition, nanostructured electrodes present an excellent stability and capacitance retention after 19,000 cycles. Finally, asymmetric electrochemical capacitors were assembled and specific energy of 52 Wh/kg (14 Wh/L) and a specific power of 10 kW/kg were obtained. A comparison with EDLC symmetric device highlights the role of P3MT to improve energy while maintaining a high power due to fast diffusion in the channels perpendicularly oriented to the substrate.

High-performance wearable asymmetric electrochemical capacitors based on composite aramid nonwovens with unique surface architecture

High-performance flexible asymmetric electrochemical capacitors (ANW-AECs) for high energy-density wearable devices have been developed through assembling unique aramid nonwovens (ANWs) with different surface structures, that is, ANWs chemically coated with Ag particles and polypyrrole (ANW/Ag/PPy) act as a positive electrode, while ANWs chemically coated with Ag particles and multi-walled carbon nanotubes (ANW/Ag/CNT) play as a negative electrode. The influences of different compositions of surface modified ANWs on structure and integrated performances of ANW-AECs were intensively studied and compared with those of symmetric electrochemical capacitors (ANW-SECs). The prepared ANW-AECs not only have high energy density, but also process good wearability and cyclability. Their areal, volumetric or gravimetric energy density reaches 2.55 × 10−4 W h cm−2, 3.27 × 10−3 W h cm−3 or 8.61 × 10−3 W h g−1 at a high operating voltage of 1.6 V; their capacitance is still as high as 95.0% or 95.9% when the electrochemical or bending cycle reaches 1000 or 500. These outstanding performances originate from asymmetric design of devices as well as unique chemical and space structures of electrodes, including interactions between different components, high electrical conductivity and specific surface area.

A theoretical and experimental study of hexagonal molybdenum trioxide as dual-ion electrode for rechargeable magnesium battery

Abstract The hexagonal polytype of molybdenum trioxide (h-MoO3) was prepared through the hydrothermal method. The anisotropic growth of the particles yields to micro rods with prismatic geometry. Theoretical calculations were carried out to simulate the insertion of magnesium in the framework of h-MoO3. The electrochemical behavior of single-phase h-MoO3 in non-aqueous magnesium cells was studied, and for that purpose, Mg metal or activated carbon (A.C.) was used as a counter electrode. This is the first report about the insertion of a divalent cation into h-MoO3. The experimental capacity vs. Mg is only around 20–50 mAh g−1. Nevertheless, whether Mg metal is replaced by A.C. as the counter electrode, the electrochemical behavior of h-MoO3 is improved, and the reversible capacity is about 100 mAh g−1 after 130 cycles. The combination of h-MoO3 and A.C. forms a hybrid or asymmetric electrochemical capacitor. The mechanism of the reaction in the working electrode is more complex than a Mg2+-insertion. Anion (TFSI−) adsorption and redox of oxygen ions in the lattice of h-MoO3 also contribute to the reversible capacity. Consequently, h-MoO3 is a dual-ion electrode material. For higher mass ratio A.C./h-MoO3, the experimental maximum reversible capacity is up to 350 mAh g−1 (equivalent to nominal composition Mg0.94MoO3).

Hydrothermal synthesis of transition metal sulfides/MWCNT nanocomposites for high-performance asymmetric electrochemical capacitors

Achieving both high energy and power densities for electrochemical capacitors is always one of the greatest scientific and societal challenges. Owing to the outstanding electrochemical performances of transition metal sulfides, especially high theoretical capacities and wide potential windows, they have drawn extensive attention as promising electrode materials for electrochemical capacitors recently. Herein, the Ni–S/MWCNT composite as positive electrode and the Ni–Fe–S/MWCNT composite as negative electrode have been prepared by a hydrothermal method. The Ni–S/MWCNT electrode shows a high specific capacity of 265.6 mAh g−1 at 1 A g−1 and an excellent rate performance, which are attributed to a larger BET surface area and faster charge transfer rate due to an introduction of MWCNTs compared to the pure Ni–S electrode. The Ni–Fe–S/MWCNT electrode also exhibits a good electrochemical performance due to its rich redox reaction and high total pore volume after the integration of MWCNTs. The fabricated Ni–S/MWCNT//Ni–Fe–S/MWCNT asymmetric electrochemical capacitor delivers a high cell potential of 1.7 V, a high energy density of 42.2 Wh kg−1 at a high power density of 3.7 kW kg−1 (even 19.1 Wh kg−1 at 11.9 kW kg−1), and a relatively good cycling stability. Our work may provide an alternative protocol for the fabrication of high-performance electrochemical capacitors.

Metal Sulfide/Mxene Based Nanostructured Electrode Materials for High-Performance Supercapacitors

2D transition metal carbides and nitrides, known as MXenes, are an emerging class of 2D materials with a wide spectrum of potential applications, due to their unique layered structure, high surface area, remarkable chemical stability, and electrical conductivity for energy storage. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions is the key for high-rate pseudocapacitive energy storage in MXene electrodes. However, the low capacity of MXene electrodes limits their further application in ECs. In this study we developed the electrode materials comprised of titanium carbide MXene (Ti3C2Tx) decorated with MWCNTs and Metal Sulfide, which are both 2D layered materials that contribute to the fast ion diffusion in the interdigitated electrode architecture. The nanocomposite samples have been characterized by XRD, SEM, TEM, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements. Due to the synergistic effect between Metal sulfide and Ti3C2 matrix, the nanocomposite exhibits a superior capacitance and shows excellent cycling stability with capacitance up to several hundred cycles. These results show that pseudocapacitive MXene based negative electrodes exhibits excellent properties in asymmetric electrochemical capacitors, leading to an increased energy density.

Optimization of Design Parameters and Operating Conditions of Electrochemical Capacitors for High Energy and Power Performances

Theoretical expressions for performance parameters of different electrochemical capacitors (ECs) were optimized by writing MATLAB scripts to solve them and also via MATLAB R2014a optimization tool box. Performance of different kind of ECs at given circumstances were compared through theoretical equations and simulation of various models subject to conditions of device components, using optimal coefficient associated to battery-kind material and constant associated to electrolyte material KBMopt and KEopt , as well as our symmetric electric double layer capacitor (EDLC) experimental data. Estimations of performance parameters were feasible and achievable once details of electrodes mass ratio, operating potential range ratio and specific capacitance of electrolyte are known. Performances of asymmetric EC with suitable electrode mass and operating potential range ratios using aqueous electrolytes, and that with suitable electrode mass and operating potential range ratios using organic electrolyte with appropriate operating potential range and specific capacitance were factors of 2.2 and 5.56, respectively, greater than those of symmetric EDLC and asymmetric EC using the same aqueous electrolyte, respectively. This enhancement came along with reduction in cell mass and volume. Also, storable and deliverable energies of asymmetric EC with suitable electrode mass and operating potential range ratios using proper organic electrolyte were factor of 12.9 greater than those of symmetric EDLC using aqueous electrolyte, with reduction in cell mass and volume. Storable energy, energy density and power density of asymmetric EDLC with suitable electrode mass and operating potential range ratios using proper organic electrolyte were factor of 5.56 higher than those of similar symmetric EDLC using aqueous electrolyte with reduction in cell mass and volume by a factor 1.77. Also, introduction of asymmetric EDLC with the same type of electrode, and suitable electrodes mass ratio, working potential range ratios and proper organic electrolyte enhanced performance of conventional symmetric EDLC using aqueous electrolyte with reduction in cell mass and volume. These results can obviously reduce number of experiments to determine optimum manufacturing state of EC, and also demonstrated that institution of asymmetric electrode and organic electrolyte was very successful in improving performance of EC with reduction in cell mass and volume. These results can as well be a guideline for design, fabrication and operation of electrochemical capacitor with outstanding storable energy, energy and power densities. Key words: Electrodes symmetry; Modelling and simulation; Performance parameters; Electrolytes type; Working potential range; Asymmetric capacitors.

Low Temperature Performance of MnOx@Carbon Nanofoam-Based Ecs with Neutral pH-Based Aqueous Electrolytes

Electrochemical capacitors (ECs) comprising high-surface-area carbon electrodes and nonaqueous electrolytes offer many attractive performance attributes including long cycle life and the ability to operate at low temperatures (–40⁰C), yet their energy density is ultimately limited by reliance on the double-layer charge-storage mechanism. Asymmetric EC configurations that include pseudocapacitive materials (e.g., transition metal oxides) offer the opportunity to increase energy density while also using safer and cheaper aqueous electrolytes. We have shown that with the appropriate electrode structure and electrolyte additives, the additional charge stored via pseudocapacitance is delivered at EC-like rates over 1000 cycles with minimal capacitance fade. To further prove the practical feasibility of such aqueous-electrolyte ECs, we turned our attention to low-temperature performance, focusing on aqueous electrolyte compositions containing lithium or sodium cations and sulfate or nitrate anions. We characterized pertinent bulk properties (e.g., freezing point and ionic conductivity) of down-selected electrolytes and found that the anion dominates the freezing point, with sulfate-based electrolytes having a freezing point of –45oC, while nitrate-based electrolytes have a freezing point of –35oC. The ionic conductivities ranged from 63 to 136 mS cm-1 at 20oC, conductive enough to support high-rate operation. To assess the impact of temperature on performance as a function of electrolyte composition, we fabricated symmetric pouch cells comprised of MnOx@carbon nanofoam electrodes, evaluating the resulting devices via cyclic voltammetry and electrochemical impedance spectroscopy over a wide temperature range (25⁰C to –45⁰C). All ECs exhibited a gradual decrease in capacitance as the temperature decreased; however, capacitance was recovered upon warming to 25°C for all electrolyte compositions. We found that ECs with Li2SO4-based electrolytes (neat or mixed composition) were operational at temperatures as low as –35°C, revealing that these electrolytes are competitive with the nonaqueous electrolytes used in conventional double layer capacitors.

Fabrication of graphene-based electrochemical capacitors through reactive inverse matrix assisted pulsed laser evaporation

Electrodes constituted by nitrogen-doped reduced graphene oxide (NrGO) in combination with NiO nanostructures were fabricated by means of reactive inverse matrix assisted pulsed laser evaporation technique. The structure-composition of the electrode composites was tailored by laser-inducing chemical reactions of graphene oxide (GO) flakes with different precursor molecules (citric acid, ascorbic acid and imidazole) during GO deposition. Structural characterizations reveal the formation of wrinkles and nanoholes in the NrGO sheets, besides their coating with NiO nanostructures. Compositional studies disclose that imidazole precursor promotes the synthesis of NrGO with the largest degree of reduction and nitrogen doping (mainly with graphitic and pyridinic N). Electrochemical analyses of the obtained electrodes reveal that NiO nanostructures increase surface charge storage processes (double layer – pseudocapacitive) over diffusive ones, being the imidazole-based electrodes the ones exhibiting the best performance (up to 114 F cm−3 at 10 mV s−1). Symmetric and asymmetric electrochemical capacitors were also fabricated showing excellent robustness over 10,000 charge-discharge cycles at high specific currents.