Article pubs.acs.org/JPCC
Physicochemical Investigation of Adiponitrile-Based Electrolytes for Electrical Double Layer Capacitor Fouad Ghamouss,*,† Aymeric Brugère,†,‡ and Johan Jacquemin*,‡ †
Laboratoire de Physico-Chimie des Matériaux et des Electrolyte pour l’Energie (EA 6299), Faculté des Sciences et Technique, Université de Tour, Parc de Grandmont, 37200 Tours, France ‡ The QUILL Research Centre, School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom S Supporting Information *
ABSTRACT: Herein, we present the formulation and the characterization of novel adiponitrile-based electrolytes as a function of the salt structure, concentration, and temperature for supercapacitor applications using activated carbon based electrode material. To drive this study two salts were selected, namely, the tetraethylammonium tetrafluoroborate and the 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide. Prior to determination of their electrochemical performance, formulated electrolytes were first characterized to quantify their thermal, volumetric, and transport properties as a function of temperature and composition. Then, cyclic voltammetry and electrochemical impedance spectroscopy techniques were used to investigate their electrochemical properties as electrolyte for supercapacitor applications in comparison with those reported for the currently used model electrolyte based on the dissolution of 1 mol·dm −3 of tetraethylammonium tetrafluoroborate in acetonitrile. Surprisingly, excellent electrochemical performances were observed by testing adiponitrile-based electrolytes, especially those containing the 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide roomtemperature molten salt. Differences observed on electrochemical performances between the selected adiponitrile electrolytes based on high-temperature (tetraethylammonium tetrafluoroborate) and the room-temperature (1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide) molten salts are mainly driven by the salt solubility in adiponitrile, as well as by the charge and the structure of each involved species. Furthermore, in comparison with classical electrolytes, the selected adiponitrile +1ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide solution exhibits almost similar specific capacitances and lower equivalent serial resistance. These results demonstrate in fact that the adiponitrile +1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide mixture can be used for the formulation of safer electrolytes presenting a very low vapor pressure even at high temperatures to design acetonitrile-free supercapacitor devices with comparable performances. (≈6 °C) and emits toxic combustion products.6 To date, the alternative electrolytes proposed in the open literature can be split into two categories: ionic liquid (IL) and organic solvent based electrolytes. Several studies demonstrate that EDLCs containing an IL-based electrolytes have an operative voltage as high as 4 V, leading, in fact, to higher energy storage in comparison with conventional EDLCs.7−11 However, the maximum power of these IL-based EDLCs is often limited due to their relatively high viscosity especially at room temperature (RT), particularly, their equivalent series resistance (ESR), which is considerably higher than those reported for EDLCs based on conventional electrolytes.11 Concerning the organic solvent based electrolytes, the formation of low-viscosity electrolytes containing linear carbonates and PC for the realization of EDLCs has been also reported.2,12 In parallel, different types of solvents, such as sulfone, dimethylsulfone, and
1. INTRODUCTION Electrochemical double layer capacitors (EDLCs), also defined as supercapacitors, are today recognized as one of the most promising energy storage technologies mainly because EDLCs can be theoretically charged and discharged within a few seconds. To date, the commercially available EDLCs contain activated carbon as the active material and an electrolyte containing generally a quaternary ammonium salt, such as the tetraethylammonium tetrafluoroborate ([Et4N][BF4]), dissolved in propylene carbonate (PC) or acetonitrile (ACN).1,2 To date, the most common commercial electrolytes used in supercapacitors are ACN-based electrolytes. These conventional EDLCs have operative voltages in the range from 2.3 to 2.7 V, display a high power (up to 10 kW·kg−1), and have an extremely high cycle life (>500,000).3,4 These electrolytes, while having good conductivity and ion transport properties, also have high volatility, flammability, and toxicity, which raise safety and environmental concerns.5 Moreover, the use of acetonitrile generally limits their applications above 70 °C due to the risk of cell rupture. Additionally, ACN has also a very low flash point © 2014 American Chemical Society
Received: February 13, 2014 Revised: June 5, 2014 Published: June 6, 2014 14107
dx.doi.org/10.1021/jp5015862 | J. Phys. Chem. C 2014, 118, 14107−14123
The Journal of Physical Chemistry C
Article
therefore very influential, and the prior knowledge of its physical properties, such as volumetric, thermal, transport, and electrochemical properties, is essential to enhance the formulation of safer electrolytes for the realization of environmentally friendly EDLCs operating at high temperature. In the present work, we suggest combining ADN and 1-ethyl3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C2mIM][TFSI]), room temperature molten salt (or ionic liquid), to prepare a safe, and conductive electrolyte for EDLC applications. [C2mIM][TFSI] was selected because of its relatively low viscosity and its moderate ionic conductivity in comparison with other classical ionic liquids.22 Different ADN + [C2mIM][TFSI] mixtures were prepared, and their physicochemical and thermal properties were then determined. Vapor pressures and solid−liquid equilibrium diagrams of selected electrolytes were also predicted using the COSMOthermX program. For all investigated species, their σ profile and structure were calculated using the Gaussian 03-D1 package with the DFT/B3LYP/DGTZVP level of theory. In addition, the ADN + [C2mIM][TFSI] based mixture was used as electrolyte in an EDLC system and then characterized in term of its specific capacitance, energy, and power using cyclic voltammetry and galvanostatic charge−discharge profile. Electrochemical impedance spectroscopy was used to determine ESR and the behavior of the electrode/electrolyte interface. All results are discussed and compared to those obtained using ADN + [Et4N][BF4] or ACN + [Et4N][BF4] electrolytes.
ethyl methyl carbonate, have been also used during the formulation of EDLC-based electrolytes.13 By using these electrolytes, high operating voltage (close to 3.3 V), performance, and cycling stability are reported, showing in fact that these electrolytes can be used in EDLCs. Recently, nitrile- and dinitrile-based electrolytes have been investigated as an alternative to conventional electrolytes in Li-ion and EDLC devices.14−20 For example, adiponitrile (ADN) and sebaconitrile have been used in mixtures with ethylene carbonate (EC) for the formulation of novel electrolytes for Li-ion batteries.17,18,20 During these studies, authors have clearly shown that dinitriles, such as sebaconitrile or adiponitrile, are more appropriate and compatible with high-voltage Li-ion batteries thanks to their high electrochemical stability. In addition, Isken et al. used adiponitrile for the formulation of electrolytes free from volatile linear carbonate to improve their flash point.20 Concerning EDLC, Brandt et al. reported the use of the ADN-based electrolyte in EDLCs.14 In this study, this group demonstrates that this electrolyte family has good electrochemical performances and stability even after more than 5,000 cycles. The same team published a very promising result concerning the compatibility between ADN-based electrolyte and activated carbon electrode, and they suggested also that a CO2 treatment of activated carbon improves significantly the performances of EDLC.19 This treatment allows the authors to achieve EDLC with higher performances and stability up to 3.6 V using an ADNbased electrolyte. Besides having a large electrochemical window, the use of ADN in the formulation of EDLC-based electrolytes allows also the possibility to reach a high operating voltage (up to 3 V). Furthermore, this solvent has a very low vapor pressure and a moderated viscosity in comparison with low vapor pressure solvents such as ionic liquids, for example. However, the solubility of the current EDLC-based molten salts in the ADN is very limited. For example, at 25 °C the solubility of [Et4N][BF4] in ADN is lower than 0.8 mol·dm−3.21 This poor salt solubility limits, in fact, the formulation of ADN-based electrolytes, as well as their physical properties. For example, their ionic conductivity is generally 11 times smaller than that of ACN-based electrolytes.14,21 As this salt solubility is mainly driven by the structure and the melting temperature (Tm) of the dissolved salt and solvent, as well as their interactions in solution, the substitution of a classical molten salt (Tm > 100 °C) by a room-temperature molten salt, such as an IL, can potentially decrease the melting point and the eutectic temperature of the ADN-based electrolytes. Furthermore, due to their unique and attractive propertiessuch as negligible vapor pressure, nontoxicity, large liquid range temperature, and good electrochemical stabilityILs are very attractive for many electrochemical applications.8−10 Based on which, ILs have been used as electrolytes to build safe and stable supercapacitors and Li-ion batteries.5,9 However, the main drawbacks of ILs are their viscosity and cost, explaining in fact why their large-scale applications are still limited. In order to reduce the viscosity of ILs, which is typically many times higher than that of classical solvents,22 binary mixtures containing an IL dissolved in a molecular solvent, usually ACN or PC, have been optimized and used as electrolytes for several electrochemical applications,23−27 such as EDLCs, for example.9,27 Generally, these mixtures used as electrolytes lead to an increase of the electrochemical performances of EDLCs. Thus, in the case of supercapacitors, the ESR value is reduced when the viscosity is decreased, thereby leading to an increase of the maximum power of the device (Pmax = U2/4RESR). In other words, the choice of the electrolyte is
2. EXPERIMENTAL SECTION 2.1. Materials and Electrolytes Preparation. Adiponitrile (ADN, 99.0%), acetonitrile (ACN, 99.8%), and high-temperature molten salt (HTMS) tetraethylammonium tetrafluoroborate ([Et4N][BF4], 99.0%) used in this study were purchased from Sigma-Aldrich and were used without further purification. The room-temperature molten salt (RTMS) 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, ([C2mIM][TFSI], > 99.0%) was purchased from Solvionic, Toulouse, France. During this study, the selected RTMS was used after drying under high vacuum (1 Pa) at 60 °C overnight and was then stored under nitrogen atmosphere to avoid water contamination from the atmosphere. A porous Whatman membrane (thickness η = 675 μm and pore diameter Ø = 2.7 μm) filled with the electrolyte solution was used as the separator during electrochemical measurements, as the activated carbon with ≈5 mg of active material (10 mm diameter). Electrodes were prepared by mixing activated carbon (DLC 50) with carbon black (20% by weight) and poly(vinylidene fluoride) (PVDF) binder (10% by weight). The mixture was dispersed and mixed in ethanol to obtain a homogeneous paste, which is applied by doctor blading technique on an aluminum foil. Each electrolyte was then simply prepared by mass with an accuracy of ±1.10−3 g using an OHAUS pioneer balance by mixing at ambient temperature known quantities of the salt (HTMS or RTMS) and the adiponitrile under a dry atmosphere in a glovebox MBraun, with a water level lower than 1 ppm. During this study several electrolytes were made as a function of the added salt structure and composition in ADN. Interestingly, at 25 °C the selected [C2mIM][TFSI] RTMS is completely soluble in ADN, while the [Et4N][BF4] HTMS is partially soluble in ADN up to a salt molar concentration in solution close to 0.7 mol·dm−3. Herein, five electrolytes based on the ADN + [Et4N][BF4] mixture were prepared at 25 °C within a salt molar concentration in solution 14108
dx.doi.org/10.1021/jp5015862 | J. Phys. Chem. C 2014, 118, 14107−14123
The Journal of Physical Chemistry C
Article
Table 1. Source, Abbreviation, Purity, and Water Content for Each Chemical Sample Reported during This Work chemical name
source
abbreviation
mole fraction purity
water content, ppm
adiponitrile acetonitrile tetraethylammonium tetrafluoroborate 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide
Aldrich Aldrich Aldrich Solvionic
ADN ACN [Et4N][BF4] [C2mIM][TFSI]
0.99 0.998 0.99 >0.99
