Ionic Liquid Supercapacitors Research Articles

Scale-up biomass strategy to macro-microporous nitrogen-doped carbon aerogels for ionic liquid supercapacitors with high efficiency

Ionic liquids are commonly employed as supercapacitor electrolytes owing to their wide voltage windows. However, ion transport in a carbon electrode is limited by the larger viscosity and larger ion size of an ionic liquid, which reduces the energy density of supercapacitors. In this work, we fabricated macro-microporous nitrogen-doped carbon aerogels (H-A-C-N) as electrode materials by scale-up biomass strategy for high-energy-density supercapacitors. The macroporous structure ensures the rapid transport of ions in the ionic liquid, particularly under high-current charging and discharging conditions. We fabricated a symmetric supercapacitor using the H-A-C-N as the electrode and 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4) as the electrolyte. The developed supercapacitor presents an energy density of 68.8 Wh kg−1. The results of this study present considerable potential for the preparation of high-performance electrode materials in future studies.

A two-dimensional nanochannel facilitates ionic conductivity of a deep eutectic solvent for an efficient supercapacitor

As an environmentally benign promising emerging class of versatile solvents, deep eutectic solvents (DESs) are inexpensive, non-corrosive, compatible with electrodes and eco-friendly, and attractive for cost-efficient energy storage systems. However, the limited ionic conductivity and liquid state constrict the wide application of bulk DESs in energy storage. Herein, we present a nanoconfined system by confining a common DES, choline chloride-ethylene glycol, into the two-dimensional nanochannel of graphene oxide (GO) membranes. Under infusing DES into GO nanochannels, the ionic conductivity of DESs has increased 64 times as that of the bulk one. The molar ratio and temperature influence of DES components on the ionic conductivity of nanoconfined GO-DES membranes have been investigated. As a proof of conception, the resulting GO-DES membranes were utilized as quasi-solid liquid electrolytes for supercapacitors. The GO-DES-based supercapacitor demonstrates a 50.88% energy density improvement than bulk one. The nanoconfined system of GO-DES10 exhibits high specific capacitance of 120 F g−1 at 1 A g−1 and an energy density of 10.23 Wh kg−1 at power density of 1 kW kg−1. These are the best performance among similar ionic liquid supercapacitors. This enhanced ionic conductivity in nanoconfined DESs will offer a new way for DESs in the electrochemical area.

High-yield preparation of B/N co-doped porous carbon nanosheets from a cross-linked boronate polymer for supercapacitor applications

B/N Co-doped carbon nanosheets (BN-CSs) are considered as the promising candidates for high-performance supercapacitors, yet high-yield preparation of such carbon nanomaterials from a boronate polymer is rarely accessed. Herein, we have demonstrated a possibility of high-yield preparation of BN-CSs by using template-free CuBr2-mediated carbonation of a cross-linked boronate polymer, which is synthesized via a solvothermal polymerization reaction between 4-formylphenylboronic acid and p-phenylenediamine. The effect of activation temperature on the morphology, specific surface area (SBET), heteroatom-doping content, and electrochemial properties of BN-CSs is discussed. It is found that CuBr2, as a mild activation reagent, not only can relieve the over-etching of carbon skeleton and B/N-bearing species, but also can react with the carbon motif via redox reaction mechanism to generate numerous pores. Consequently, the as-prepared BN-CSs present high carbon yield (>52.7 wt%), large SBET value (up to 2714.4 m2 g−1), numerous micorpores coupled with abundant and evenly-distributed B/N species (B > 1.86 at.% and N > 6.26 at.%). All these benefits contribute to the desirable electrochemical performance in both aqueous and ionic-liquid supercapacitors. When 6 M KOH aqueous solution is used as the electrolyte, the BNC-800 electrode delivers a satisfactory capacitance of 258.5 F g−1 at 0.5 A g−1, showing a certain degree of advantages when compared to reported B/N co-doped carbon materials (See Table S4). Moreover, the supercapacitors adopted a ionic-liquid electrolyte achieve a high energy density of 43.7 Wh kg−1 under the power density of 400 W kg−1. These excellent electrochemical properties coupled with both facile polymerization method and versatile CuBr2-activation strategy offer numerous possibilities for further exploration of mass B/N-rich porous polymer and carbon materials, which can be extended to a diversity of functional applications, especially in gas storage, catalysis, and batteries.

Impedance Response of Ionic Liquids in Long Slit Pores

We study the dynamics of ionic liquids in a thin slit pore geometry. Beginning with the classical and dynamic density functional theories for systems of charged hard spheres, an asymptotic procedure leads to a simplified model which incorporates both the accurate resolution of the ion layering (perpendicular to the slit pore wall) and the ion transport in the pore length. This reduced-order model enables qualitative comparisons between different ionic liquids and electrode pore sizes at low numerical expense. We derive semi-analytical expressions for the impedance response of the reduced-order model involving numerically computable sensitivities, and obtain effective finite-space Warburg elements valid in the high and low frequency limits. Additionally, we perform time-dependent numerical simulations to recover the impedance response as a validation step. We investigate the dependence of the impedance response on system parameters and the choice of density functional theory used. The inclusion of electrostatic effects beyond mean-field qualitatively changes the dependence of the characteristic response time on the pore width. We observe peaks in the response time as a function of pore width, with height and location depending on the potential difference imposed. We discuss how the calculated dynamic properties can be used together with equilibrium results to optimise ionic liquid supercapacitors for a given application.

Fully periodic, computationally efficient constant potential molecular dynamics simulations of ionic liquid supercapacitors.

Molecular dynamics (MD) simulations of complex electrochemical systems, such as ionic liquid supercapacitors, are increasingly including the constant potential method (CPM) to model conductive electrodes at a specified potential difference, but the inclusion of CPM can be computationally expensive. We demonstrate the computational savings available in CPM MD simulations of ionic liquid supercapacitors when the usual non-periodic slab geometry is replaced with fully periodic boundary conditions. We show how a doubled cell approach, previously used in non-CPM MD simulations of charged interfaces, can be used to enable fully periodic CPM MD simulations. Using either a doubled cell approach or a finite field approach previously reported by others, fully periodic CPM MD simulations produce comparable results to the traditional slab geometry simulations with a nearly double speedup in computational time. Indeed, these savings can offset the additional cost of the CPM algorithm, resulting in periodic CPM MD simulations that are computationally competitive with the non-periodic, fixed charge equivalent simulations for the ionic liquid supercapacitors studied here.

Effects of dilution in ionic liquid supercapacitors

Room-temperature ionic liquids (RTILs) are synthetic electrolytes that have a large electrochemical stability window, making them attractive candidates for electric double-layer capacitor (EDLC) applications. Due to their high viscosities and low ionic conductivities, RTILs are often diluted with organic solvent for practical use. We study the effects of dilution on the performance of RTIL EDLCs using a simple mean-field model. We find that dilution diminishes the unfavorable hysteresis that results from a spontaneous surface charge separation (SSCS). As a result, the RTIL concentration can be used to modulate the proximity to the SSCS transition, and maximize capacitance. The interplay between the concentration and the correlation strength gives rise to complex zero-potential phase behavior, including a tricritical point and a λ-line, very similar to the Blume-Capel dilute Ising model. Additionally, electrodes that are solvophilic aid in the prevention of SSCS by drawing solvent molecules to the electrode and displacing ions. Solvophilic electrodes give rise to a phase transition at finite potential where the surface charge rapidly increases with a small increase in potential, leading to a substantial increase in capacitance and energy storage.

Scalable and Sustainable Ionic Liquid Supercapacitors with Engineered Nanocarbons

Scalable and Sustainable Ionic Liquid Supercapacitors with Engineered Nanocarbons

Solid-State, Single-Anion-Conducting Networks for Flexible and Stable Supercapacitor Electrolytes

Supercapacitors are a complementary energy storage technology to batteries and will be important for next-generation applications such as wearable sensors, soft robotics, roll-up displays, and electric vehicles. Here, we report a flexible supercapacitor with an all solid-state, single-ion-conducting polymer network electrolyte. The electrodes are comprised of reduced graphene oxide held together with a neutral polymer network to stabilize the dispersion and lend flexibility to the overall supercapacitor. Due to the high thermal and electrochemical stability of the network polymerized ionic liquid electrolyte, the resulting device can be charged to high voltages (3 V) and operated at high temperatures (120 °C) with excellent cycling stability. A 300 F/g specific capacitance is achieved by charging the device at 0.57 A/g and 90 °C or at 1.8 A/g and 120 °C. Due to the flexible network, the device can bend up to 180° without a substantial change in capacitance. The electrode/electrolyte interface, network architecture, and single-ion-conducting nature of the electrolyte are shown to be critical for high capacitance through a series of control experiments with ionic liquid supercapacitors.

Effects of Confinement and Ion Adsorption in Ionic Liquid Supercapacitors with Nanoporous Electrodes.

We investigate the effects of pore size and ion adsorption on the room-temperature ionic liquid capacitor with nanoporous electrodes, with a focus on optimizing the capacitance and energy storage. Using a recently developed modified BSK model accounting for both ion correlations and nonelectrostatic interactions, we find that ion crowding proximate to the electrode surface induced by the spontaneous charge separation due to strong ion correlations is responsible for the anomalous increase in the capacitance with decreasing pore sizes observed in experiments. Reducing the strength of ion correlations increases the capacitance and suppresses the anomalous size dependence. For a given pore size, the capacitance peak diverges when the ion correlation strength α reaches a critical value, αsc,L. The capacitance peak shifts to smaller pore size as α decreases because of rapid decrease of αsc,L with decreasing pore size. Asymmetric preferential ion adsorption is shown to lead to significantly enhanced energy storage close to the transition point for any pore sizes. For a given correlation strength, the energy storage is optimal at a pore size where α = αsc,L.

Boosting rate capability of ionic liquid supercapacitors by copolymer-derived activated hollow carbon nanospheres

Herein, a novel activated hollow carbon nanospheres (AHCNSs) with enlarged specific surface area (SSA) of 1796 m2ú g-1and pore volume (Vp) of 1.33 cm3ú g-1was synthesized from poly(aniline-co-pyrrole) hollow nanospheres via KOH activation method. The supercapacitor containing AHCNSs displayed high gravimetric capacitance (Cg) of 290 F ú g-1at 1 Aú g-1and 79% capacitance retention even at 20 Aú g-1in ionic liquid EMIMBF4, indicating its excellent rate capability. This study highlights the potential value of novel hollow structure activated carbon in the field of energy storage.

Nitrogen-doped mesoporous graphene nanoflakes for high performance ionic liquid supercapacitors

Mesoporous nitrogen-doped graphene nanoflakes (N-GNFs) with high nitrogen doping level of 5.0–10.7 at.% were produced by a facile template CVD synthesis using the mixture of acetonitrile and benzene as a precursor. The pore structure, morphology and physicochemical properties of N-GNFs were characterized by TEM, XPS, Raman spectroscopy, and low-temperature nitrogen physisorption. The electrochemical performance of N-GNFs was tested in electric double-layer capacitor (EDLC) with ionic liquid electrolyte (1.2 M N+Et4TFSI− solution in CH3CN) by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy. With increasing the nitrogen content in N-GNFs their specific capacitance increased and achieved 167 F g−1 at a scan rate of 5 mV s−1. The maximum delivered energy density was 46.3 Wh kg−1, which corresponded to a power density of 0.74 kW kg−1. The high electrochemical performance of N-GNFs was attributed both to the pseudo-capacitance of active nitrogen sites and to the developed mesoporosity. Different tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids were used to evaluate the cation size effect on the stability and performance of N-GNFs-based EDLCs. In contrast to N+Et4TFSI− and N+Bu4TFSI− electrolytes, the cycling performance of N-GNFs in N+Me4TFSI− was limited because of the electrode degradation resulted from the intercalation of N+Me4 ions between graphene layers and their exfoliation.

Facile synthesis of carbon with nanopore-network precisely controlled by zinc ions at the molecular level for Boosting the performance of supercapacitors

Three-dimensional hierarchically nanoporous carbons, drived from the ion-exchange resin of D113-type, are precisely synthesized by zinc ions uniform injection in molecular level. And a facile metal-gasification carbonization approach was performed for the preparation of the carbon material with a high surface area value of 2178.0 m2 g−1 and large pore volume of 1.445 cm3 g−1. Especially, this novel carbon electrode materials show high degree of graphitization and hierarchically porous structure, and largely boost the performance of ionic liquid supercapacitors, with an outstanding specific energy of 180.2 Wh kg−1, based on total mass of electroactive materials. Moreover, the Zn-promoted hierarchically nanoporous carbons (ZNCs)-based supercapacitors also possess an excellent cycling stability with 92.1% capacitance retention after 10,000 cycles with a high limited charge voltage of 4.0 V in neat 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) electrolyte at 298 K.

Improved Performance of Ionic Liquid Supercapacitors by using Tetracyanoborate Anions.

Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN)4] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9 mS cm−1). Herein, we report the use of ILs containing the [B(CN)4] anion for such an application. They presented a high maximum operating voltage of 3.7 V, and two‐electrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20 F g−1 @ 15 A g−1). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.

Single-Layered Mesoporous Carbon Sandwiched Graphene Nanosheets for High Performance Ionic Liquid Supercapacitors

Ionic liquid based supercapacitors generally using nanoporous carbon as electrode materials hold promise for future energy storage devices with improved energy density, but their power performances are limited by the high viscosity and relatively large size of ionic liquid electrolytes. Understanding the relationship between the pore size of nanoporous carbon, the ionic liquid electrolyte diffusivity, and the energy/power density is critical for the development of ionic liquid based supercapacitors with high performance. Herein, we report the synthesis of single-layered mesoporous carbon sandwiched graphene nanosheets (sMC@G) with mesopore-dominant (82% ∼89%) high surface area and tunable mesopore sizes (4.7, 6.8, 9.4, 10.6, and 13.9 nm). When using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) as the electrolyte with a cation size of 0.76 nm, it is demonstrated that the ion diffusion coefficient increases a little when the mesopore size is not larger than 6.8 nm, and then jumps dramatically in ...

Highly sp2 hybridized and nitrogen, oxygen dual-doped nanoporous carbon network: Synthesis and application for ionic liquid supercapacitors

The utilization of ionic liquids as electrolyte is beneficial for realization of high energy density supercapacitors due to their high operation voltage. Nevertheless, ionic liquids electrolytes suffer from sluggish ion diffusion and poor wetting behavior on porous electrodes as a result of large ion sizes and high viscosity, which has severely hindered their practical use. Herein, highly sp2 hybridized and nitrogen, oxygen dual-doped nanoporous carbon networks were prepared based on charge-induced self-assembly strategy using chitosan as carbon precursor with reduced graphene oxide (GO) in between as conductive scaffolds. The optimized material as supercapacitor electrodes exhibits an outstanding specific capacitance of 201 F g−1 in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) electrolyte at 0.5 A g−1 with a maximum energy density of 111 Wh kg−1, superior to numerous reported values, which are attributed to multiple synergistic effects of several beneficial characteristics, that is, excellent conductivity deduced from highly sp2 hybridized carbon originated from carbonized chitosan and graphene conductive scaffolds being in favor of rate performance and cyclic stability, nitrogen and oxygen functionalization improving the surface wettability with electrolyte as well as contributing to the capacitance, large specific surface areas providing abundant active sites to boost charge capacity, and sheet-like structures effectively shortening diffusion pathways to improve ion transports.

Carbonized Nanocellulose Sustainably Boosts the Performance of Activated Carbon in Ionic Liquid Supercapacitors

Carbonized cellulose nanofibrils (CNF) have been employed to improve the rate performance of activated carbon (AC) traditionally used in supercapacitors. Because of the large amount of surface functionalities, CNF form strongly interconnected composite with AC, which turns into a free-standing carbon nanofibers/AC film after carbonization. In the film, the carbon nanofibers are ‘welded’ on AC particles and integrate them into one piece of carbon. The interaction between AC and carbon nanofibers, originating from the strong AC-nanocellulose affinity, is much stronger than the traditional physically mixed AC/nanocarbon composite and also significantly reduces the contact resistance in the composite. Conductive atomic force microscope (C-AFM) analysis reveals that the network of carbonized CNF possesses markedly better electron transport efficiency than the AC particles. When tested as supercapacitor electrode at commercial level mass loading, the composite film exhibits 2 times slower capacitance fading at high current and 3 times higher maximum power density than the bare AC. In addition, using the nanocellulose, which is derived from renewable resources, increases the total electrode cost only by a small margin, thereby making the composite a competitive electrode material for electricity storage on a large scale.

Carbonized nanocellulose sustainably boosts the performance of activated carbon in ionic liquid supercapacitors

Carbonized cellulose nanofibrils (CNF) have been employed to improve the rate performance of activated carbon (AC) traditionally used in supercapacitors. Because of the large amount of surface functionalities, CNF form strongly interconnected composite with AC, which turns into a free-standing carbon nanofibers/AC film after carbonization. In the film, the carbon nanofibers are ‘welded’ on AC particles and integrate them into one piece of carbon. The interaction between AC and carbon nanofibers, originating from the strong AC-nanocellulose affinity, is much stronger than the traditional physically mixed AC/nanocarbon composite and also significantly reduces the contact resistance in the composite. Conductive atomic force microscope (C-AFM) analysis reveals that the network of carbonized CNF possesses markedly better electron transport efficiency than the AC particles. When tested as supercapacitor electrode at commercial level mass loading, the composite film exhibits 2 times slower capacitance fading at high current and 3 times higher maximum power density than the bare AC. In addition, using the nanocellulose, which is derived from renewable resources, increases the total electrode cost only by a small margin, thereby making the composite a competitive electrode material for electricity storage on a large scale.

Redox-active electrolyte supercapacitors using electroactive ionic liquids

Here we present redox ionic liquid supercapacitors (RILSCs) which use electrolytes made from ionic liquids modified with an electroactive function to increase the energy density of activated carbon electrodes via faradaic reactions. More specifically, two different ionic liquids were made by modifying either the imidazolium cation or the bis(trifluoromethanesulfonyl)imide anion with ferrocene in order to determine the importance of the electroactive ion's polarity. The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4M) and a large maximum operating voltage (2.5V). An energy density of up to 13.2Wh per kg (both electrodes) was obtained which represents an 83% increase vs. the unmodified ionic liquid. When the ionic liquid's anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. The results presented herein demonstrate that electroactive ionic liquids constitute a promising alternative to conventional solute in solvent electrolytes found in energy storage devices, and are particularly well-suited for redox-active electrolyte supercapacitors.

N-doped reduced graphene oxide aerogel coated on carboxyl-modified carbon fiber paper for high-performance ionic-liquid supercapacitors

Nitrogen-doped reduced graphene oxide aerogel (N-rGO aerogel) with high porosity and ionic conductivity were synthesized by a hydrothermal reduction of graphene oxide with hydrazine and following freezing-dry method. N-rGO aerogel was spray-coated on carboxyl-modified carbon fiber paper with a hydrophilic surface and used as the supercapacitor electrode. Not only can the N-rGO aerogel electrode accelerate the diffusion of the electrolyte but also it can store electronic charge via a surface redox reaction due to the N-containing groups. Among the electrolytes studied, the ionic liquid-based supercapacitor of the N-rGO aerogel provides a wide working potential of ca. 4.0 V and rather high specific capacitance of 764.53 F/g at 1 A/g with a capacity retention of 86% over 3000 charge–discharge cycles as well as maximum specific power and energy of 6525.56 W/kg and 245.00 Wh/kg, respectively. The device prototype fabricated in a single coin cell shape (CR2016) can supply electricity to red LED over 17 min.

Geometrically confined favourable ion packing for high gravimetric capacitance in carbon–ionic liquid supercapacitors

The specific energy of carbon–ionic liquid supercapacitors comparable to NiMH batteries has been achieved by a combined modeling and experimental approach.