Article pubs.acs.org/JPCC
Ionic Liquid Dynamics in Nanoporous Carbon Nanofibers in Supercapacitors Measured with in Operando Infrared Spectroelectrochemistry Francis W. Richey,† Chau Tran,‡ Vibha Kalra, and Yossef A. Elabd*,§ Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Electric double-layer capacitors (EDLCs), or supercapacitors, rely on rapid electrosorption of ions into porous carbon electrodes to achieve high power densities and long lifetimes. Ionic liquid (IL) electrolytes offer large operating voltage windows and can potentially increase the energy density of EDLCs if the electrode/ electrolyte interface is properly optimized. Herein, we present molecular level measurements of ion dynamics of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) IL in an operating EDLC with freestanding electrodes composed of nanoporous carbon nanofibers (NCNFs) and potassium hydroxide (KOH)-activated NCNFs using in operando infrared spectroelectrochemistry. For non-KOH-activated NCNF electrodes, the concentrations of IL ions (both cations and anions) decrease as the ions enter the nanopores inside the nanofibers during charging. However, the concentration of the anions inside the positively charged pores is larger than the concentration of cations for voltage windows above 1 V. Conversely, when charging the KOH-activated NCNF electrodes, the cation concentration increases as the anion concentration decreases. The KOH activation process introduces oxygen functionalities on the surface of the nanofibers and increases the ionophilicity of the electrodes, which causes cations to desorb from the nanopores while anions adsorb into the nanopores. This provides direct experimental evidence that the charge storage mechanism of IL electrolytes in nanoporous carbon electrodes of EDLCs is directly affected by the surface chemistry and ionophilicity of the carbon material. The quantitative, species-specific molecular-level infrared spectroelectrochemical measurements presented here provide deep insights into the behavior of IL ions in EDLCs that will improve the design and performance of electrode materials.
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INTRODUCTION Electrochemical double-layer capacitors (EDLCs), also referred to as supercapacitors, are energy storage devices offering exceptionally large power densities that can be used to harvest energy rapidly, e.g., during automotive braking.1 The energy density of EDLCs is directly proportional to the square of the operating voltage window; hence, a key strategy to enhance charge storage capacity is to implement electrolytes and electrolyte/electrode combinations with larger operating voltage windows.2,3 Favorable physicochemical properties of room-temperature ionic liquids (ILs), such as high electrochemical stability with voltage windows of >4 V, low flammability, and large operating temperature ranges (−50 to +150 °C) make them well-suited for EDLCs.4,5 However, the physical understanding of the double-layer structure and molecular level dynamics of the highly coordinated IL ions is still not well understood.6,7 In addition to understanding the dynamics of ILs during electrosorption of ions into carbon pores,8,9 it is also important to understand how the surface chemistry,10−12 or ionophilicity,13,14 of the carbon pores affects the behavior of the IL ions during electrosorption into the carbon pores. However, these studies are typically limited to simulation and theory.13,15 Porous carbon materials composed of carbon fibers or fiber mats (woven or nonwoven) possess several advantages over © 2014 American Chemical Society
carbon particles for use in EDLCs, including the ability to fabricate them into free-standing electrodes without the addition of polymeric binding agents that are known to add dead-weight to the electrodes and reduce the specific capacitance and energy density of the device.16−19 Electrospinning is a simple and high-throughput technique to create carbon nanofibers ranging in diameter size from 50 to 500 nm. In order to create nanoporous nanofibers, a blend of two or more polymers is electrospun, where one of the polymers is carbonized and the other (sacrificial) polymer is removed via pyrolysis to form nanopores within the resulting carbon nanofibers.20,21 In this work, nanoporous carbon nanofibers (NCNFs) developed by Tran et al.19,22 with uniformly distributed nanopores were fabricated by electrospinning solutions consisting of a binary mixture of Nafion and poly(acrylonitrile) (PAN). PAN was carbonized, and the Nafion was sacrificially removed from the electrospun nanofibers via pyrolysis at 1000 °C, resulting in a free-standing device-ready NCNF electrode. In addition to the NCNF, potassium hydroxide (KOH)-activated NCNF electrodes were also fabricated for this study. KOH activation is commonly used Received: July 10, 2014 Revised: August 26, 2014 Published: August 28, 2014 21846
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analysis (TGA), which was carried out using a Perkintric analysis under replica conditions of the carbonizing process. It was run under air flow up to 280 °C and remained at 280 °C for 1 h. Then it was run under nitrogen flow up to 600 °C at a rate of 5 °C min−1. X-ray photoelectron spectroscopy (XPS) (ESCA PHI-5000 VersaProbe (ULVAC-PHI Inc., Japan)) showed that the carbonized Nafion:PAN nanofibers (i.e., nanoporous carbon nanofibers) and pure PAN nanofibers (i.e., nonporous carbon nanofibers) showed largely similar composition except for a small fraction of fluorine in the carbonized Nafion:PAN samples, possibly resulting from residual pyrolized Nafion. These samples were used as freestanding electrodes without any addition of binder for testing. The free-standing carbon nanofiber mats (electrodes) were also activated with potassium hydroxide (KOH, Amresco, reagent grade). They were soaked in 30 wt % KOH solution overnight, blotted with lint-free paper upon removing from the solution, and then heated to 800 °C at a rate of 5 °C min−1 under a nitrogen flow. The samples were held at 800 °C for 30 min. Materials Characterization. Specific surface area and porosimetry of NCNFs were determined using gas sorption using a Quadrasorb (Quantachrome Instruments) system. Isotherms were collected at 77 K using a N2 adsorbate. The Brunauer−Emmett−Teller (BET) specific surface area (SSA) was calculated for P0 values in the 0.05−0.10 range,34 and density functional theory-based evaluations of the SSA, pore volume, and pore size distributions were made using quenched solid density functional theory (QSDFT).35 Quantachrome’s Quadrawin software was used for all calculations. The external morphology of NCNF mats was characterized using scanning electron microscopy (SEM; Zeiss Supra 50VP). For high magnification of single nanofibers, NCNFs were embedded in epoxy (Electron Microscopy Science) and then microtomed into thin longitudinal and cross sections using a Leica EM UC6 ultramicrotome equipped with a diamond knife. The sections were transferred onto lacey carbon copper grids and characterized using transmission electron microscopy (TEM; JEOL JEM2100) operated at 200 kV. XPS measurements were obtained using an ESCA PHI-5000 VersaProbe (ULVAC-PHI Inc., Japan) with Al Kα anode (E = 1486 eV). Capacitor Fabrication and Electrochemical Testing. Each NCNF electrode (100−150 μm thick) is a freestanding film that does not require any additional binder and weighs 1.5−2 mg. Neat EMIm-TFSI (Iolitec) electrolyte was added to each electrode in a dropwise fashion, and the electrode was soaked in the electrolyte for 12 h. A Teflon Swagelok cell was used to construct a symmetric two-electrode capacitor for CV measurements in a glovebox. Stainless steel rods with a diameter of 0.5 in (1.3 cm) were used as current collectors. Two electrodes were separated by a layer of separator (Celgard 3501). All procedures were carried out in a glovebox with water content less than 1 ppm. Cyclic voltammetry was performed with various scan rates from 20 to 1 V s−1 in the voltage window from 0 to 3.5 V. Spectroelectrochemical Experiments. A rectangular capacitor with dimensions of 5 × 15 mm was clamped onto the surface of a diamond attenuated total reflectance (ATR) crystal with a surface area of 3.42 mm2 using the anvil from the Golden Gate accessory (Specac Inc.) at a constant load capacity of 36 kg. After clamping, the EDLCs with pure EMIm-TFSI electrolyte were allowed to equilibrate for 12 h before voltage was applied. All experiments were performed in a dry chamber, which was purged with moisture-free and CO2-free air, and the
to increase the surface area of carbon materials for enhanced capacitance. However, the activation process changes the chemical functional groups on the carbon surface, which will affect the affinity of the EMIm cation and TFSI anion to the carbon and change the behavior of the ions entering and exiting the nanopores of the nanofibers during charge storage.13,23,24 A deeper understanding of the charging/discharging behavior of ILs in the nanopores of NCNFs and KOH-activated NCNFs will require molecular-level in situ or in operando (i.e., in situ during device operation) measurements. Unfortunately, there are only a few in situ experimental techniques capable of measuring ion dynamics in carbon electrodes of EDLCs, including NMR spectroscopy25,26 and electrochemical quartz crystal microbalance (EQCM),27,28 of which only EQCM has been used to study ILs as the electrolyte.29 In this work, we present molecular level results on the ion dynamics of an IL in NCNF electrodes using an in operando infrared spectroelectrochemical technique recently developed in our laboratory.30,31 Spectroelectrochemical techniques combine standard electrochemical measurements with in situ infrared spectroscopy (i.e., infrared spectroelectrochemistry) and can be used to study EDLCs by coupling molecular level chemical changes with electrical inputs/outputs.32,33 Previously, we used in operando infrared spectroelectrochemistry with attenuated total reflectance (ATR) to study an entire functioning EDLC composed of carbon particles and showed qualitatively (time-resolved absorbance) that EMIm cations and TFSI anions together enter and exit the pores of carbide-derived carbon particles during charging and discharging, respectively.31 In this work, we extend this technique to quantitatively measure concentration (via absorbance−concentration calibration) changes of the EMIm cation and TFSI anion as they are adsorbed into the nanopores of NCNF electrodes. Specifically, concentration changes of the cation and anion of the IL (1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI)) in the working electrode of an entire functioning EDLC composed of NCNF and KOH-activated NCNF will be shown. Results show that KOH activation directly affects the molecular level charge storage mechanism of the individual EMIm cation and TFSI anion of the IL in the nanopores of the KOHactivated NCNFs, which has not been previously demonstrated.
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EXPERIMENTAL METHODS Nanoporous Carbon Nanofiber (NCNF) Fabrication. A detailed experimental procedure of the synthesis of the NCNF is reported here.19 Briefly, the Nafion powder (prepared by drying a LIQUION 1105 solution, Ion Power Inc.) and PAN (MW = 150 000 g mol−1, Sigma-Aldrich) were dissolved in N,Ndimethylformamide (DMF; ACS grade, BDH) under gentle heating and stirring for 1 h. A 22% (w/w) solution of 70:30 (wt:wt) Nafion:PAN in DMF was electrospun at room temperature with relative humidity below 20% RH. The distance between the tip of the needle (22 gauge needle; spinneret from Hamilton Company) and the grounded collector was 5−6 in (12.7−15.4 cm), and the applied voltage of 8−10 kV was used to obtain a stable Taylor cone. The flow rate was kept at a constant 0.2 mL h−1. The electrospun nanofibers were placed in a horizontal tube furnace and stabilized by heating to 280 °C at a rate of 5 °C min−1 under an air flow. The stabilized nanofibers were then carbonized by heating to 1000 °C at a rate of 5 °C min−1 and held at 1000 °C for 1 h under a nitrogen flow. The complete decomposition of Nafion was confirmed by thermogravimetric 21847
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Figure 1. (a) Schematic of the in operando infrared spectroelectrochemical experiment with supercapacitor on the ATR crystal of the FTIR spectrometer. (b) Scanning electron microscopy (SEM) image of supercapacitor electrodes composed of nanoporous carbon nanofibers (NCNFs). (c) Magnified view of the ATR crystal/electrode interface showing the sampling depth into the working electrode in the range of 1−2 μm.
drops were added to each electrode. The electrodes with electrolyte were subsequently placed in a vacuum oven at 80 °C and dynamic vacuum for 6 h. This process allowed for the removal of all of the ethanol solvent, leaving behind electrodes containing varying concentrations of EMIm-TFSI electrolyte. Following the ethanol removal, the electrodes were again weighed so that the ratio of electrode mass to EMIm-TFSI mass in the electrode could be calculated. Lastly, infrared spectra of electrodes with varying EMIm-TFSI concentrations were collected.
kinetics of relative humidity in the chamber after closing the chamber were reported previously30 (relative humidity decreases to a final value of 0.25%). A Fourier transform infrared (FTIR) spectrometer (Nicolet 6700 Series, Thermo Electron Corporation) was used to collect all spectra using a single reflection diamond ATR Golden Gate accessory. All infrared spectra were collected using a liquid nitrogen-cooled mercury−cadmium−telluride detector at 4 scans per spectrum and a resolution of 4 cm−1 resulting in a spectrum collected every 1.57 s. All spectra were corrected with a background subtraction of the ATR crystal spectrum. A potentiostat (Solartron SI 1287, Corrware Software) was used to perform cyclic voltammetry (CV), voltage step, and galvanic cycle experiments. The EDLC was clamped onto the ATR crystal in a two-electrode cell with aluminum contacts. The bottom electrode of the supercapacitor, which covers and is in intimate contact with the ATR crystal, extends beyond the ATR crystal surface and makes contact with the aluminum lead in an area that is not in contact with the ATR crystal. Hence, electrons must travel laterally from the electrode/aluminum contact to reach portions of the electrode that are in contact with the ATR crystal. This configuration may contribute to additional resistive effects. A two-electrode configuration was used because of the difficulty of inserting a reference electrode into the EDLC while it was clamped on the ATR crystal. The electrode in contact with the ATR crystal is the positively charged working electrode for all experiments. In general, CV experiments were performed from 0 to 2 V at variable scan rates, where one data point was collected every second. Within the voltage range explored in this study, no evidence of electrolyte decomposition was observed in either the infrared spectroscopy or cyclic voltammetry results. A minimum of three voltage sweeps were performed at each scan rate to ensure the reproducibility of the infrared spectra and CV data. FTIR Electrolyte Concentration Calibration. Nanoporous carbon nanofiber electrodes were cut into uniform square pieces weighing ∼3 mg. The exact mass of the electrode prior to electrolyte uptake was recorded. Ethanol and EMImTFSI were added to vials at varying concentrations ranging from pure EMIm-TFSI to pure ethanol. The ethanol/EMImTFSI solutions were vigorously stirred for ∼1 h before being added to the electrodes in a dropwise fashion where three
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RESULTS Figure 1 shows an experimental schematic of the infrared spectroelectrochemical method used in this work to study the ion dynamics of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm-TFSI) IL electrolyte in NCNF electrodes in a supercapacitor. The supercapacitor is composed of a Celgard separator, symmetric NCNF electrodes (∼100−150 μm), and EMIm-TFSI electrolyte. Figure 1b shows an SEM image of the NCNF electrodes used in this work. Transmission electron microscopy (TEM) images of single CNF and NCNF are shown in the Supporting Information, Figure S1. After sandwiching the separator between the NCNF electrodes the entire capacitor is clamped onto the ATR crystal for in operando infrared spectroelectrochemical measurements. In our previous work, we analyzed EMIm-TFSI dynamics in supercapacitor electrodes consisting of both carbide-derived carbon (CDC) and onion-like carbon (OLC) particles using a technique similar to the one presented here.31 Previously, thin gold current collectors were applied to the surface of the electrodes to create an equipotential surface and to act as a charge collector. In this work, the freestanding, binder-free NCNF electrodes were annealed at 1000 °C, which results in NCNF electrodes with a high enough electrical conductivity that the thin gold current collectors were no longer required. This simplifies the optical physics of the electrode/ATR crystal interface, where n1 and n2 in Figure 1 are the refractive index of the diamond ATR crystal and the working electrode, respectively. The depth of penetration of the evanescent wave is ∼1−2 μm into the working electrode36,37 (Figure 1c) and allows for the direct calibration of EMIm-TFSI electrolyte 21848
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concentration is evidenced and expected and has been reported recently for battery electrodes.38 At an IL concentration of ∼1 gEMIm‑TFSI/gelectrode, the volume of EMIm-TFSI is comparable to the theoretical BET pore volume of the electrode. The absence of an IL absorbance signal below this concentration indicates that the EMIm-TFSI is inside the pores of the nanofibers and cannot be detected due to the carbon blocking the IR signal. In other words, no absorbance is detected in Figure 2b,c below ∼1 gEMIm‑TFSI/gelectrode because all of the IL is within the nanopores of the NCNFs (see Supporting Information, Figure S2). Linear regressions in Figure 2b,c were used to convert cation and anion absorbance data from spectroelectrochemical experiments to cation and anion concentrations at each input voltage in the time-resolved experiments shown below. A Bruggeman approximation was used to calculate changes in the refractive index of the electrode as a function of EMImTFSI content and found that the refractive index (and in turn depth of penetration) changes minimally in the absorbance range of interest (see Supporting Information, Figure S3). Consequently, the changes in infrared absorbance (cation and anion) observed during voltage cycling (charging and discharging) can be related to the concentration of each ion in the electrode. The measurable absorbance and therefore concentration of each ion will change as ions move in and out of the nanopores of the NCNFs during charging and discharging. In order to prove that changes in absorbance or concentration of the cation and anion of EMIm-TFSI are due to ion sorption in the nanopores of the NCNFs and not another phenomenon, such as diffusion across the electrode thickness, a control experiment was performed on nonporous carbon nanofibers (i.e., no pores within nanofibers, but still macropores due to interfiber spacing between nanofibers) as shown in Figure 3. These samples were prepared by pyrolysis of pure PAN nanofibers without the sacrificial polymer, Nafion, and therefore the nanopores within NCNFs are not present, i.e., nonporous carbon nanofibers (CNFs). Due to the lack of nanopores, the surface area and capacitance of nonporous CNFs are low as shown in the Supporting Information, Figure S6. Figures 3a and 3b show the initialized concentration (C(t) − C0V, gEMIm‑TFSI/gElectrode) of the EMIm cation and TFSI anion, respectively, in the nonporous CNF working electrode as a function of time for a series of three CV cycles from −1.5 to +1.5 V at 5 mV/s scan rate. For the nonporous CNFs in Figures 3a,b, the EMIm cation and TFSI anion concentration is initialized to the concentration at open-circuit voltage (OCV, the voltage before starting CV cycles), which is referred to as C0 V here, in order to more quantitatively compare the response of the cation and anion to input voltages. The concentration for both the cation and anion changes minimally over the course of the CV cycles since there are no pores within the fibers for the ions to enter and exit during charging and discharging. The cation concentration has a lower signal-to-noise ratio compared to the anion concentration because the characteristic cation bond vibration has a different absorption coefficient than the characteristic anion bond vibration. Therefore, small changes in the measured integrated area of the cation band are more pronounced than for the anion band. The results presented in Figure 3 corroborate with the previously published results on nonporous onion-like carbon particles, where the absorbance of the cation and anion did not fluctuate significantly with applied voltage.31 Contrastingly, larger concentration fluctuations of the EMIm cation and TFSI anion are observed in the NCNF,
concentration to the infrared absorbance of the electrolyte in the electrode. Figure 2a shows the infrared spectra corresponding to NCNF electrodes containing varying equilibrium concentra-
Figure 2. (a) Infrared spectra of the NCNF electrodes with varying concentrations (concentration units of gEMIm‑TFSI/gelectrode) of EMImTFSI electrolyte. Concentration−absorbance calibrations of the (b) cation (CC stretching vibration, 1576 cm−1) and the (c) anion (SO2 stretching vibration, 1055 cm−1). The dotted lines represent linear regressions.
tions of EMIm-TFSI electrolyte, where the spectra with the largest absorbance corresponds to the largest concentration of EMIm-TFSI (8.8 gEMIm‑TFSI/gelectrode) and the spectra with the smallest absorbance corresponds to the lowest concentration of EMIm-TFSI (1.6 gEMIm‑TFSI/gelectrode) in the electrode. The chemical structures of the TFSI anion and EMIm cation are shown in Figure 2a, which highlight the characteristic chemical bonds of the anion (SO2 stretching, 1055 cm−1) and cation (CC stretching, 1576 cm−1). Figure 2b,c shows the integrated absorbances of the cation and anion of EMIm-TFSI as a function of the concentration of EMIm-TFSI in the electrode (gEMIm‑TFSI/gelectrode). The anion absorbance was integrated in the 1040−1060 cm−1 wavenumber region to avoid the tail of this peak. A separate calibration for the cation and anion is necessary due to different absorption coefficients and wavenumbers for each specific species. A linear relationship between absorbance and 21849
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Figure 3. In operando infrared spectroelectrochemical results: Initialized time-resolved concentration (concentration units g/g corresponds to gEMIm‑TFSI/gelectrode) of the (a) EMIm cation and (b) TFSI anion in nonporous CNF electrodes during three CV cycles from −1.5 to +1.5 V at a scan rate of 5 mV/s. (c) Illustration showing ions adsorbing on the CNF surface during charging of nonporous CNFs.
possessing nanopores within the fibers during CV cycles as shown in Figure 4. Figure 4 shows the in operando infrared spectroelectrochemical results for the initialized time-resolved concentrations of the EMIm cation and TFSI anion in the NCNF electrodes as a function of CV voltage window. The PAN/Nafion nanofibers were pyrolyzed at 1000 °C to decompose out the sacrificial polymer, Nafion, creating nanoporous carbon nanofibers (NCNFs) with a BET surface area of 1218 m2/g with an average pore size of 1.88 nm and the most common pore size of 0.78 nm (see Supporting Information, Figure S4). Figure 4a,b,c shows the initialized time-resolved concentration response of the ions during three CV scans over a voltage window from 0 to 1, 0 to 1.5, and 0 to 2 V, respectively. In Figure 4a, as the voltage increases from 0 to 1 V, the absorbance of both the EMIm cation and TFSI anion decreases indicating that the cations and anions do not separate and enter the nanopores of the NCNFs together. This result was expected due to the strong interaction between the EMIm cation and the TFSI anion and is similar to previous results on EMIm-TFSI ions entering CDC nanopores during charging.31 Interestingly, as the voltage window increases from 1 to 1.5 and to 2 V, a larger concentration of anions enters the positively charged pores compared to cations showing evidence of some cation−anion separation. When the voltage reaches 2 V, while the change in concentration of TFSI anions is −0.25 gTFSI/gelectrode, the concentration change of EMIm cations is only −0.1 gEMIm/ gelectrode indicating that a larger concentration of anions than cations is located in the nanopores of the nanofibers at larger voltages, although both ions are still present.39 A change of EMIm cation concentration of −0.1 gEMIm/gelectrode corresponds to a ∼1.6% decrease in the initial cation concentration, and a change of TFSI anion concentration of −0.25 gTFSI/gelectrode corresponds to a ∼5% decrease in the initial anion concentration. The small changes in electrolyte concentration (
