Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors Naresh C. Osti, Alejandro Gallegos, Boris Dyatkin, Jianzhong Wu, Yury Gogotsi, and Eugene Mamontov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02521 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors Naresh C. Osti§,†,*, Alejandro Gallegos£,†, Boris Dyatkin‡,¶, Jianzhong Wu£, Yury Gogotsi‡, Eugene Mamontov§,* §

Neutron Scattering Division, Oak Ridge National Laboratory, PO BOX 2008 MS6455, Oak

Ridge, Tennessee 37831, United States £

Department of Chemical and Environmental Engineering, University of California, 900

University Ave, Riverside, California 92521, United States ‡

Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute,

Drexel University, 3141 Chestnut St, Philadelphia, Pennsylvania 19104, United States †

Equally contributed authors.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] [email protected] ¶

Current Address: U.S. Naval Research Laboratory, Washington, DC 20375, USA.

ACS Paragon Plus Environment

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ABSTRACT: Well-tailored mixtures of distinct ionic liquids can act as optimal electrolytes that extend the operating electrochemical window and improve charge storage density in supercapacitors. Here, we explore two room-temperature ionic liquids (RTILs), 1-ethyl-3methylimidazolium

bis(trifluoromethylsulfonyl)imide

(EmimTFSI)

and

1-ethyl-3-

methylimidazolium tetrafluoroborate (EmimBF4). We study their electric double layer behavior in the neat state and as binary mixtures on the external surfaces of onion-like carbon (OLC) electrodes using quasi-elastic neutron scattering (QENS) and classical density functional theory (cDFT) techniques. Computational results reveal that a mixture with 4:1 EmimTFSI: EmimBF4 volume ratio displaces the larger [TFSI-] anions with smaller [BF4-] ions, leading to an excess adsorption of [Emim+] cations near the electrode surface. These findings are corroborated by the manifestation of nonuniform ion diffusivity change, complementing the description of structural modifications with changing composition, from QENS measurements. Molecular level understanding of ions packing near electrodes provides insight for design of ionic liquid formulations that enhance the performance of electrochemical energy storage devices.

1. INTRODUCTION Growing demand for electrical energy requires sustainable solutions that rely on renewable energy sources and efficiently store and release electrochemical energy.1 Supercapacitors, which are also called ultracapacitors and electric double layer capacitors (EDLC), provide a balance between energy and power densities that allows them to charge at the same rate as traditional electrolytic capacitors and store charge at very high levels ( > 1 kWh/g).2-3 To store charge, supercapacitors rely on electrosorption of electrolyte ions onto electrode surfaces under applied potentials. Their energy densities, which are determined by the total amount of stored charge and the stable operating voltage window, partially depend on the structure of the electrolyte-electrode


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interface and arrangement of ions in the electric double layer (EDL).2 Since optimal energy densities can be directly attributed to the high accessible surface area of electrodes and minimal electrolyte breakdown under high potentials, there have been research efforts tailoring porous electrode architectures and matching them to the appropriate electrochemically stable electrolytes.4-6 Room temperature ionic liquid (RTILs) electrolytes have gained particular attention because of their high electrochemical window, high thermal stability, low volatility, and molecular tunability.7-9 Even though RTILs are less prone to electrochemical breakdown than other electrolytes,10-11 their working operational voltage window (OPW) is still limited by the different electrochemical activities of anions and cations. Consequently, working potentials of the positive and negative electrodes in an EDLC are inherently asymmetric.6,

12

Various methods have

attempted to address this asymmetry from an electrode perspective: e.g., electrode mass balancing,6,

13-14

and charge injection12 into one or both electrodes. Different positive and

negative electrode material compositions15 have also been proposed. However, none of existing methods completely resolves the unbalanced charge distribution issue. Recently, an optimized binary mixture of two ionic liquids in a model system of onion-like carbon (OLC) electrodes has balanced charge storage with an increase in the OPW of the supercapacitor.16 The mixture used ionic liquids with the same cation (1-ethyl-3-methylimidazolium, or [Emim+]) and two different anions (bis(trifluoromethylsulfonyl)imide, or [TFSI-], and tetrafluoroborate, or [BF4-]). This balance on the capacitance of the electrodes shows that properties of the mixed electrolyte diverge from those of its constituents, and the OPW is extended from 2.5 V to 3.5 V. However, fundamental molecular-level understanding of this beneficial mixing effect is still limited.


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Mixing an ionic liquid with other ionic liquids17-18 or co-solvents19 alters its bulk properties such as diffusivity, viscosity, and electrical conductivity. Furthermore, a combination of several types of co-ions and counterions is expected to change the resulting electrosorption densities on electrode surfaces. Recently, a classical density functional theory (cDFT) study on the EmimTFSI/EmimBF4 mixtures has pinpointed an optimal 4:1 ratio mixture of the respective constituents that yields a maximum capacitance.20 The effect was attributed to a significant reduction of the layering structures that enhanced the contact density of [Emim+] counterions on the electrode surfaces due to the presence of smaller [BF4-] anions. However, a rational system design should rely on fundamental understanding that incorporates and rationalizes both the experimental16 and computational results.20 Here, we examine the beneficial effect of mixing RTIL electrolytes for the most fundamental, basic system with zero applied potential. Interfacial phenomena involving various electrodes and electrolytes (aqueous and ionic liquids) have been explored using molecular dynamics simulations,21 Monte Carlo simulations22-23 and cDFT.24-25 Here, we utilize cDFT and quasielastic neutron scattering (QENS) to investigate the structure and microscopic dynamics of cations in the mixtures of EmimTFSI/EmimBF4 adsorbed on the external surfaces of onion-like carbons (OLCs), which are concentric graphitic spheres of 5-10 nm in diameter.26 We chose the OLC electrode model system because it facilitates a direct comparison of our findings against previous results, allows us to rely on a carbon structure that is commonly used in supercapacitors, and presents an optimal electrode/electrolyte interface that benefits from a high specific surface area without convolution from internal confinement of ions. In addition to their function as important model electrode systems, they are useful in a number of commercial-scale applications. In addition to operating high-rate devices,27 they act as substrates for grafted redox-


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active species and function as pseudocapacitors28 and are integral for energy storage devices that operate under extreme temperature conditions.29 Furthermore, other devices, such as microsupercapacitors, benefit from the external surface area structures of OLCs.30 We have found that a 4:1 vol./vol. mixture of EmimTFSI:EmimBF4 demonstrates highest cation diffusivity and corresponds with peak adsorption of cations on the OLC’s surface. This experimental observation is further supported by the computed excess adsorption of the cation on the electrode surface. These findings, which match previously reported RTIL mixture compositions that had extended the OPW of supercapacitors,16 reveal the microscopic dynamics of the cations in an ionic liquids mixture associated with the highest capacitance. Our findings provide further guidance for tuning electrolyte properties for electrochemical energy storage device design. 2. MATERIALS AND METHODS 2.1 Synthesis of Carbon Onions Onion-like carbons (OLCs) were synthesized according to the previously established procedure.31-32 UD-90 (Nanoblox, Inc.) nanocrystalline diamond powder (5-10 nm diameter particles) was placed into a graphite crucible and loaded into a vacuum furnace (Solar Atmospheres). The sample was outgassed for 24 hours at 25 °C. The furnace reached a high vacuum (10-6 torr) during this time. The furnace was ramped up to 1800 °C (at a rate of 10 °C min-1), held at that temperature for 8 hours, and cooled down to room temperature.26 The process fully converted sp3-bonded carbon in nano-diamond into concentric fullerene-type layers of sp2bonded graphitic carbon. The spherical structures exhibited only an external surface area (no internal porosity), and gas sorption measurements33 found the OLC’s specific surface area (SSA) to be 400 m2 g-1 and its external pore volume to be 1.16 cm3 g-1.


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2.2 Absorption of Ionic Liquid onto OLC Surfaces Room-temperature ionic liquids (RTIL) and various mixtures were absorbed onto surfaces of OLCs according to the previously established vacuum infiltration procedure.34-36 All ionic liquids were purchased from IoLiTec. Measured mass amounts of ionic liquids were dispersed in acetonitrile at room temperature (99.99% purity, Alpha Aesar) using magnetic stirring. This experiment investigated three different ionic liquid combinations: 1) pure EmimTFSI; 2) pure EmimBF4; 3) 80 vol. % EmimTFSI/ 20 vol. % EmimBF4 mixture.16 OLC powders were added to the solutions during stirring in ratios that would allow for the predetermined amounts of RTIL(s) to fully occupy the external pore volume of each OLC system. To determine the ratios, the bulk density of neat EmimTFSI was assumed to be 1.50 g cm-3, and the bulk density of neat EmimBF4 was assumed to be 1.25 g cm-3. Three additional RTIL-only dispersions (without added OLC) were prepared with identical ratios stated above using the same dispersion and stirring method. The mixtures continued to stir overnight until the acetonitrile evaporated, and the resulting materials were dried for 24 hours under a low vacuum (0.01 torr) at 80 ºC. 2.3 Loading into QENS Cans Samples were placed into cylindrical annular and flat-plate aluminum cans for quasi-elastic neutron scattering experiments and sealed with indium wire. OLC-containing samples used flat plate cans with 0.025 cm thick deep inserts, and RTIL-only samples used the annular cans with insets providing 0.005 cm thick samples to control the effects due to multiple scattering. All samples were sealed inside of an Ar-filled glovebox (Vacuum Atmospheres). 2.4 Classical Density Functional Theory (cDFT) For this work, we employ the primitive model of electrolytes to represent room temperature ionic liquids (RTILs). Although atomic details are lost in this model, it accounts for important characteristics including the ionic excluded volume and electrostatic correlation effects. Here,


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cations and anions are represented as charged hard spheres and the electrode surface is represented by a planar surface. The diameter of [Emim+] is 0.50 nm, [BF4-] is 0.40 nm, and [TFSI-] is 0.55 nm. The ion density in the bulk is fixed at 4.64 nm-3 for pure EmimTFSI and 7.78 nm-3 for pure EmimBF4, corresponding to the ionic liquids at 298 K and 1 bar. The number density of the mixture can be reasonably assumed to be a linear combination between the two pure RTILs based off the volume of mixing for ionic liquids being very small (< 0.1%).37 Within the primitive model, the pair potential (uij) between ionic species i and j is provided by a hard-core repulsion calculation plus Coulombic interaction:   ∞  uij =  2  Zi Z j e  4πε 0ε r

r< r≥

σi +σ

j

2 σi +σ

j

(1)

2

where r is the center-to-center distance, e is the unit charge, ε0 is the permittivity of free space, and σ and Z are the diameter and valence, respectively, of each specific species. Because all ionic species are accounted for explicitly in this work, the relative dielectric permittivity is set to that of a vacuum (ε=1). The electrode is modeled as a planar wall, which has been shown to be an effective representation of an OLC.20 The charged wall exerts an external potential on each ion given by  ∞  ext βVi ( z ) =  −2π l Z Qz / e B i 

z

Mixed Ionic Liquid Improves Electrolyte Dynamics in Supercapacitors

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