Carbon Capsules of Ionic Liquid for Enhanced Performance of

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Functional Nanostructured Materials (including low-D carbon)

Carbon Capsules of Ionic Liquid for Enhanced Performance of Electrochemical Double Layer Capacitors Qinmo Luo, Peiran Wei, Qianwen Huang, Burcu Gurkan, and Emily B. Pentzer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01285 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Carbon Capsules of Ionic Liquid for Enhanced Performance of Electrochemical Double Layer Capacitors Qinmo Luo,1,† Peiran Wei,1,† Qianwen Huang,2 Burcu Gurkan,2,* Emily B. Pentzer1,* 1. Department of Chemistry 2. Department of Chemical Engineering Biomolecular Engineering Case Western Reserve University, 10900 Euclid Ave. Cleveland, OH 44106

ABSTRACT: Ion-accessible, large surface area, and complete wetting of a carbonaceous electrode by the electrolyte are crucial for high-performance electrochemical double layer capacitors. Herein, we report a facile and scalable method to prepare electrode-electrolyte hybrid materials where an ionic liquid (IL) electrolyte is encapsulated within a shell of reduced graphene oxide (GO) nanosheets as the active electrode material (called rGO-IL capsules). These structures were templated using a Pickering emulsion consisting of a dispersed phase of 1-methyl-3-butylimidazolium hexafluorophosphate ([bmim][PF6]) and a continuous water phase; graphene oxide nanosheets were used as the surfactant, and interfacial polymerization yielded polyurea which bound the nanosheets together to form the capsule shell. This method prevents the aggregation and restacking of GO nanosheets and allows wetting of the materials by IL. The chemical composition, thermal properties, morphology, and electrochemical behavior of these new hybrid architectures are fully characterized. Specific capacitances of 80 F g−1 at 18 °C and 127 F g−1 at 60 °C were achieved at a scan rate of 10 mV s−1 for symmetric coin cells of rGO-IL capsules. These architected materials have higher capacitance at low temperature (18 °C) across many scan rates (10−500 mV s−1) compared to analogous cells with the porous carbon YP-50. These results demonstrate a distinct and important methodology to enhance the performance of electrochemical double layer capacitors by incorporating electrolyte and carbon material together during synthesis. KEYWORDS: self-assembly

supercapacitor,

ionic

liquid,

graphene

oxide,

Pickering

emulsion,

INTRODUCTION Electrochemical double layer capacitors (EDLCs) are characterized by excellent cycling stability and high power density, complimentary characteristics to batteries which provide high energy densities, but significantly slower charge and discharge rates.1–4 EDLCs store energy electrostatically on the surface of the active material, using electrolyte counterions to balance charge on the electrode. In a typical coin cell device, an anode and a cathode are in an electrolyte solution and are separated by a membrane that is permeable to the electrolyte solution.1,5–8 The stored energy is defined as E = 1/2CV2, (C = capacitance, V = voltage); further, capacitance is defined according to Helmholtz as C = (εrε0A)/d (εr = dielectric of the material, ε0 = dielectric constant, A = surface area, d = distance between charges).1,9 As such, much active research is focused on enhancing the performance of EDLCs by increasing the active surface area of the electrode and/or expanding the voltage window.

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In recent years, many methodologies have been developed to increase the active surface area of EDLC electrodes. Electrode materials are typically composed of carbon nanomaterials (allotropes of conjugated carbon) due to their relatively high electrical conductivity, high specific surface area, chemical stability, and scalability of synthesis.10 The varied carbon materials developed for EDLC electrodes include templated carbide-derived carbon,11,12 carbon nanotubes,6,13,14 and graphene,7,15 among others.16,17 Whereas graphene is attractive due to its high specific surface area (single-layer graphene can exhibit surface area as high as 2,675 m2 g-1), it is limited by poor solubility and aggregation from extensive π−π stacking of individual sheets, which decreases accessible surface area.8 Much research has focused on preparing hierarchical pores within carbon materials, such that mesopores (2−50 nm) and micropores (350 °C), favorable properties for next generation EDLCs.26 Limitations to the performance of IL-containing EDLCs have been attributed to relatively low ionic conductivity and high viscosity, which lead to slower rates of charging and discharging compared to aqueous and organic systems, in addition to poor interactions/wetting of a carbon electrode.8,27,28 Efforts to improve the performance of EDLCs with ILs include diluting the IL with organic solvents27,28 and modifying the surface of carbon electrode materials for favorable interactions.8,29 For example, Suh, Ruoff, and coworkers reported EDLCs based on poly(ionic liquid)-modified graphene-based electrode materials with a specific capacitance of 187 F g−1 at a scan rate of 40 mV s-1;8 this relatively high capacitance is presumably due to improved wettability of the modified carbon material with electrolyte. In a similar vein, Qingguo et al. thermally reduced graphene oxide nanosheets suspended in the ionic liquid 1-ethyl-3 methylimidazolium tetraﬂuoroborate;30 the resulting material had higher specific capacitance compared to rGO nanosheets blended with the IL (114 versus 63 F g−1). More


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recently, Zimin et al. developed a new approach to increase interactions between carbonaceous materials and IL by decorating the surface of GO with nanometer-sized droplets of IL.31 These structures showed a relatively high specific capacitance (302 F g−1 at 1 A g−1). Clearly, improved interactions between an IL electrolyte and active carbon electrode materials can improve performance, and a thorough understanding of how structure relates to performance is required. Herein, we present a distinctly new one-step synthetic approach to architecting higher order structures of graphene oxide (GO) nanosheets and IL electrolyte and demonstrate enhanced capacitance near room temperature. ILs are chosen for their wide operating voltage window, as discussed above, and GO nanosheets are chosen due to their ability to be processed and reduced to yield active electrode materials. rGO-IL capsules were templated using a Pickering emulsion with a dispersed IL phase and continuous water phase, stabilized by interfacial polymerization, and made electrically conductive by thermal reduction (Figure 1). While GO nanosheets can stabilize oil-in-water and oil-in-oil emulsions,32–36 they have not been used to prepare IL-containing emulsions. The chemical composition and thermal properties of the hybrid rGO-IL capsules are fully characterized and they are determined to contain ~80 wt% IL, and estimated to have ~0.6 wt% rGO nanosheets. Of note, the amount of active material (rGO) in these hybrid samples is low compared to state-of-the art electrodes, but could be controlled by changing emulsion composition. The electrochemical properties of rGO-IL capsules are evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in particular to determine the specific capacitance and the electrode-electrolyte interface resistance. Coin cells fabricated from rGO-IL capsules are compared to a commercial porous carbon (YP-50) and rGO-IL capsule shells themselves; the specific capacitances based on the active material mass (rGO for the hybrid material) were comparable to YP-50 at high voltage scan rates at 60 °C, and are superior at 18 °C for all voltage scan rates (10−500 mV s−1). This approach to architecting carbon nanomaterials and electrolyte in a single synthetic step represents a distinct and complimentary methodology for improving the performance of EDLCs for next generation energy management.

Figure 1. Illustration of rGO-IL capsule preparation and structure of coin-cell electrochemical capacitors. Capsules were formed by the interfacial polymerization of hexamethylenediamine and hexamethylene diisocyanate in an ionic liquid-in-water Pickering emulsion stabilized by GO nanosheets, followed by thermal reduction. Symmetric coin cells were prepared using standard protocols and the architected materials compared to YP-50.



RESULTS AND DISCUSSION Highly oxidized graphene oxide (GO) nanosheets were prepared as previously reported,37 and characterized by the standard techniques of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and FTIR spectroscopy. AFM revealed that the nanosheets deposited on mica had heights of ~1 nm (Figure S1), consistent with a single layer GO nanosheet and suggesting little-to-no aggregation.38 XPS further revealed that the GO nanosheets had a C:O ratio of ~2:1 (Figure S2), indicating that the graphene is highly oxidized, and thus insulating.39 The Raman spectrum of the GO nanosheets reveals the signature G and D bands at 1591 cm−1 and 1303 cm−1, respectively, with a ID/IG ratio of 5.2:3.2 (Figure S3).40 Further characterization of the GO nanosheets by FTIR spectroscopy shows the expected broad peak from 2145 cm−1 to 3645 cm−1, and peaks at 1700 cm−1 and 1610 cm−1, attributed to carboxylates, carbonyls and carbon-carbon double bonds, respectively (Figure S4).41 Architected GO-IL capsules were templated using a Pickering emulsion with water as the continuous phase and IL as the discontinuous phase (Figure 1). To prepare these emulsions, the IL used must not be miscible with water, and GO nanosheets must have similar wettability to the two fluids (i.e., water and IL). To this end, the IL [bmim][PF6] was selected due to the ability to prepare Pickering emulsions using only GO as the surfactant and due to its immiscibility with water. The [bmim][PF6] was added to an aqueous suspension of GO nanosheets, and agitated by vortex and sonication, which resulted in an IL-in-water emulsion. The templated GO-IL capsule structures were stabilized by interfacial polymerization between hexamethylene diamine in the aqueous phase and hexamethylene diisocyanate in the IL phase (see experimental for details). The resulting capsules of IL with a shell of polyurea and GO nanosheets were isolated by filtration and then thermally reduced to render the GO nanosheets conductive, yielding rGO-IL capsules. Assuming 100% incorporation of all reagents added, these structures are expected to be composed of ~20 wt% polyurea, ~0.6 wt% reduced graphene oxide (rGO), and ~80 wt% [bmim][PF6] (Table S1).


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Figure 2. Characterization of the [bmim][PF6] (red traces), rGO (black traces), rGO-IL capsules (green traces), and rGO-IL capsule shell (blue traces) by: A) FTIR spectroscopy; B) Raman spectroscopy; C) Differential scanning calorimetry (DSC); and D) thermogravimetric analysis (TGA).

Figures 2A and 2B show the characterization data of the chemical composition of IL, rGO, rGO-IL capsules, and rGO-IL capsule shells (after IL was removed by extraction with acetone). The FTIR spectra in Figure 2A show that both the IL and rGO-IL capsules have stretching frequencies at 3200 cm−1, 1163 cm−1, and 800 cm−1,42–44 which are absent from the rGO-IL capsule shell, indicating the presence and absences of IL, respectively. Further comparison of the spectra in Figure 2A shows that peaks consistent with polyurea at 1550 cm−1 (carbonyls), and 3300 cm−1 and 3400 cm−1 (N-H absorption) are only present in rGO-IL capsules and rGO-IL capsule shell but not IL, as expected. Empty capsules containing only polyurea, without GO or IL, were also prepared and have a similar FTIR spectrum to that of rGO-IL capsule shell (Figure S5). Taken together, these results support that [bmim][PF6] is present in rGO-IL capsules, but not in the shell, and that polyurea is the major component of the shell. From FTIR data, the presence of rGO cannot be ascertained, however Raman spectroscopy can identify the presence of rGO. As seen in Figure 2B, the Raman spectrum of rGO-IL capsules is dominated by the character of rGO, with the D band at 1324 cm−1 and G band at 1629 cm−1.40 The shallow penetration depth of the Raman laser (~3 µm) renders [bmim][PF6] unobservable. Further characterization of the chemical composition of rGO-IL capsules by X-ray photoelectron spectroscopy and X-ray diffraction are consistent with these results (Figure S6-S8). The encapsulated IL was extracted from rGO-IL capsules with acetone-d6 containing mesitylene as an internal standard; subsequent integration of the 1H NMR spectra revealed that rGO-IL capsules were ~80 wt% IL on average (see Figure S6 and Table S2 for details). The thermal properties of rGO-IL capsules and rGO-IL capsule shells give insight into the composition, as well as the impact of encapsulation on the IL. The differential scanning



calorimetry (DSC) thermal profiles of pure IL and rGO-IL capsules both show an endothermic peak at −37 °C attributed to the cold crystallization behavior of [bmim][PF6] (Figure 2C). Cold crystallization is a unique behavior for some viscous liquids which can subcool to a glass state and then crystalize upon heating.45 Both IL and rGO-IL capsules show an exothermic peak at 11 °C which corresponds to melting (Tm), and in line with previous reports of [bmim][PF6].45 The DSC thermogram of rGO-IL capsule shell is featureless, supporting that IL has been removed (Figure 2C, blue trace). The thermal stability of the three samples, as determined by thermogravimetric analysis (TGA), is shown in Figure 2D. The [bmim][PF6] is stable until ~350 °C with complete weight loss at 470 °C (Figure 2D, red trace), consistent with previous reports.42,45 The weight loss profile of the rGO-IL capsule shell (Figure 2D, blue trace) shows weight loss steps at 200 °C (~13%) and 310 °C (~73%), and residual mass of ~12%. This weight loss profile is similar to polyurea itself, except for a higher residue mass, which can be attributed to the presence of rGO, which itself has a residual mass of 45% at 500 °C (Figure S10). The weight loss profile of rGO-IL capsules (Figure 2D, green trace) shows a first weight loss at ~240 °C (~3%), which comes from polymer shell. A second, broad weight loss step occurs from ~350−450 °C, which is attributed to the polyurea and IL, contributing to ~93% weight loss of the capsules, and yielding a residual mass of ~3%. This decrease in the thermal stability of the IL is also observed by simply combining the rGO-IL capsule shell with IL (Figure S11), and is in line with previous reports which illustrate a decrease in thermal stability upon combination with other materials.46 We note that the thermal decomposition temperature is still well above that used for common operating temperature of the EDLCs.

Figure 3. A) Bright field and dark field (inset) optical images of rGO-IL capsules before pressing; B) Dark field and bright field (inset) optical images of rGO-IL capsules after pressing; C), D) SEM images of rGO-IL capsules with different magnification; E) Cross-sectional SEM image of a rGO-IL capsule etched by FIB.

The shape and morphology of rGO-IL capsules were characterized by optical microscopy and scanning electron microscopy (SEM), both of which support the spherical nature of the


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capsules, as well as the formation of discrete particles (i.e., no inter-particle cross-linking). Figure 3A shows the optical microscope bright field image of rGO-IL capsules cast on a glass slide, and the inset shows the dark field image. Figure 3B shows the same sample after physical compression with a second glass slide. It is clear that the IL is expelled from the shell upon mechanical compression. Comparison of Figures 3A and 3B shows that the liquid liberated from the capsules sits on the glass slide around the capsule shells (see Video S1). SEM images in Figure 3C and 3D show that rGO-IL capsules have diameters ranging from ~25−100 µm. Cutting a capsule using a focused ion beam (FIB) during characterization by SEM shows that the wall of the capsule is 10−15 µm thick (Figure 3E); further characterization of the IL leakage from the capsules was observed in SEM, including chemical characterization of the location of the IL (Figure S12). The electrochemical performance of these novel architectures was then evaluated and compared to the commercial porous carbon material YP-50 and rGO-IL capsule shell (after extraction of IL) using the same electrolyte, [bmim][PF6]. Microcavity electrodes were prepared and characterized by cyclic voltammetry (CV, see Figures S13 and S14). Symmetrical coin cells were prepared and characterized by electrochemical impedance spectroscopy (EIS). From CV, the measured electrochemical stability windows (EW) for rGO-IL capsules and YP-50 were −1.7 to 2.6 V vs. Ag quasi-reference with a cut-off current of 0.2 µA (see Figure S15A). This EW is consistent with previous reports for [bmim][PF6] and indicates encapsulation does not negatively impact the electrochemical stability of the electrolyte.47 The rGO-IL capsules display small redox currents within the voltage window, likely attributed to the residual functional groups of rGO, more apparent in the coin cell arrangement (Figure S15B). As determined by EIS, the nonconductive polymer within the rGO-IL capsule shell contributes significantly to the overall resistance, in comparison to YP-50 cell (compare Figure 4). Comparison of the EIS traces of the materials at different temperatures can be used to further understand resistance. The sloping line at 18 °C at lower frequencies for the rGO-IL capsule cell more closely resembles a vertical line, thus demonstrating more of a capacitive behavior (Figure 4B). At the same temperature, the YP-50 cell demonstrates diffusional impedance as a consequence of its porous structure. Porosity, and in particular the random pore size distribution in YP-50, results in a constant phase element (CPE) behavior.48 The EIS data for the materials at elevated temperatures were also evaluated, as the viscosity of ILs is known to decrease and diffusivity of ions improves.49,50 At 60 °C, the sloping line at low frequencies in the YP-50 cell (Figure 4A) takes the form of a capacitor, rather than a CPE, and is due to the enhanced diffusional processes such as ease of entering into pores. At 60 °C, the rGO-IL capsule cell also has improved impedance, however the slope at low frequencies does not show as dramatic of a change as in YP-50, since pore accessibility is not a limiting factor for rGO-IL capsules. Characterization of the surface area and porosity of YP-50, rGO-IL capsules, and rGO-IL capsule shells by BET analysis showed that YP-50 is porous and thus has a higher surface area than the other two materials, as expected (see Figures S16 and S17, Tables S3 and S4). We note that the intention of this work is to hybridize carbon material and IL in a single synthetic step and therefore the relative performance of the two materials illustrates the attractiveness of this approach.



Figure 4. EIS measurements at 18 °C and 60 °C presented in Nyquist plots for: A) YP-50, B) rGO-IL capsule shell, and C) rGO-IL capsule in symmetrical coin cells with [bmim][PF6]. Voltage amplitude: 10 mV at OCV. Frequency range: 2 MHz to 0.01 Hz.

Figures 5A shows the specific capacitances of YP-50 cells, rGO-IL capsule shell cells, and rGO-IL capsule cells as a function of the CV scan rate and at 18 °C and 60 °C. At the lower temperature, rGO-IL capsule capacitance is superior to YP-50 and rGO-IL capsule shells at all scan rates (e.g., 80 versus 70 F g−1 at 10 mV s−1 comparing YP-50 and rGO-IL capsules). The capacitance for the rGO-IL capsule cells at 60 °C is 127 F g-1 which is a ~60 % increase compared to 18 °C. For YP-50, the increase in capacitance was more dramatic: 176 F g-1 at 60 °C. This is in agreement with the EIS measurements where ion accessibility into pores for YP-50 is challenging due to the high viscosity of [bmim][PF6]. Such a highly nonlinear enhancement with temperature is not observed with the rGO-IL capsule cell because ion accessibility is not a limiting phenomenon for this material, as it is with YP-50. The CVs of the cell materials as a function of temperature at a scan rate of 10 mV s−1 are shown in Figures 5B and 5C. These data further support differences in ion accessibility for the types of materials evaluated. Specifically, the more rectangular CV shape of the YP-50 cell at 60 °C in Figure 5C, in comparison to 18 °C in Figure 5B, supports the improved capacitive behavior of YP-50 with increased temperature. Further, the CV shape of the rGO-IL capsule cell does not show a dramatic difference at different temperatures, with the exception of increasing current towards 2.5 V at 60 °C. This is possibly due to electrochemical reactions of the polymer and/or rGO at elevated temperature. It should be noted that the specific capacitances reported are differential capacitances based on each electrode mass. Comparison between the cell fabricated from the capsules and that from the capsule shell illustrates that encapsulation leads to better performance at lower temperature, again indicating that the architecture of the system impacts performance. Characterization of the morphology of the electrodes by SEM after performing CV shows that the rGO-IL capsule cell retains the spherical capsule structure (Figure S18). Durability tests show that the specific capacitance of YP-50 cells remained unchanged over 5,000 cycles whereas that of rGO-IL capsule cells decreases ~20% over the first 250 cycles and then remains unchanged (Figure S19). This fade in capacitance is likely due to the presence of the polymer. These results show that the assembly of carbon material and electrolyte in a one-step synthetic procedure is an attractive approach to improving the performance of electrochemical double layer capacitors, and ongoing work focuses on increasing the amount of the active carbon material in the capsules (rGO), decreasing the diameter of the capsules to increase IL-shell


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interactions, and decreasing the amount of insulating polymer used to bind the nanosheets together.

Figure 5. A) Specific capacitances (based on mass of active electrode material) for coin cell supercapacitors of rGO-IL capsules, rGO-IL capsule shells, and YP-50 as a function of scan rate; CVs of rGO-IL capsules, rGO-IL capsule shells, and YP-50 coin cells at 18 °C (B) and 60 °C (C).

CONCLUSION In summary, we have demonstrated a distinctly new approach to preparing EDLC electrode materials by architecting reduced GO nanosheets and IL in a single synthetic step. GO nanosheets are used for the first time to stabilize an IL-in-water Pickering emulsion which was used to template rGO-IL capsules stabilized by interfacial polymerization. The chemical composition, thermal properties, and morphology of the rGO-IL capsules were characterized by FTIR and Raman spectroscopies, DSC, TGA, and optical and electron microscopies. Extraction of the encapsulated IL using an internal standard and integration of 1H NMR spectra reveal that rGO-IL capsules are ~80 wt% IL. The hybrid architectures did not impact the operating electrochemical window of the IL. However, high resistivity due to the insulating polymer was observed by EIS. Fabrication of symmetric coin cells using rGO-IL capsules and comparison to cells composed of the commercially available porous material YP-50 reveal that the hybrid architecture gives higher capacitance at room temperature, based on the amount of active material. rGO-IL capsules have specific capacitances of 80 F g−1 at 18 °C and 127 F g−1 at 60 °C at 10 mV s−1 scan rate. The simple, scalable methodology presented herein is ideal for architecting dissimilar materials into hybrid structures and provides a complimentary approach to understanding and improving EDLC performance to traditional methods.

METHODS Materials: Graphite, sulfuric acid, hydrogen peroxide, sodium carbonate, Hexamethylenediamine, tetraethylammonium perchlorate (TEAP; 99 %) and hexamethylene diisocyanate were purchased from Sigma-Aldrich. Potassium permanganate was purchased from Alfa Aesar. Ammonium hydroxide, octane, and [bmim][PF6] were from Fisher. CR2032 coin cell cases, stainless steel spacer (15.6 mm diameter, 0.5 mm thickness), stainless steel wave spring (14.5 mm diameter, 0.3 mm thickness) and carboxymethyl cellulose (CMC) were purchased from MTI Corporation. Cellulose separator was purchased from KBB, Japan. The Teflon coated fiberglass tape was purchased from McMaster-Carr.



Instrumentation: TGA was performed using a TA Instruments Q500-TGA, heating from room temperature to 700 °C at 15 °C min−1 under a nitrogen gas. DSC was recorded using a Q100 DSC (TA Instruments) at a ramp mode (ramp 2 °C/min to −80 °C, isothermal for 1 min, and then ramp 5 °C/min to 50 °C, aluminum pans). The thermal properties were analyzed at the second heating cycle. FTIR spectra were obtained using an Agilent Cary 630 FTIR in ATR mode and diamond/ZnSe crystal. AFM was performed on a NX-10 Park System in tapping mode (topography, and amplitude). Centrifugation was completed using a Centrifuge 5804 by Eppendorf. Sonication was performed in a Branson 3800. SEM data was collected from a FEI Helios 650 Field Emission Scanning Electron Microscope with Focused Ion Beam. X-ray photoelectron spectra (XPS) were acquired using a PHI Versaprobe 5000 X-ray photoelectron spectrometer with Al Ka radiation and referenced to internal SiO2. NMR were collected on a Bruker Ascend III HD 500 MHz NMR instrument equipped with prodigy probe and shifts are reported relative to residual solvent peak (of CDCl3). Raman spectra were collected using Xplora (HORIBA Instruments Inc). All samples were measured directly on a glass substrate except for GO was drop cast from an aqueous solution onto a glass slide and dried in vacuum oven for 1 h at room. The 785 nm laser was excited with about 9 mW power, within the range of 1000 to 1800 cm−1. The spectral resolution was 6.5 cm−1. The electrochemical measurements were carried out on an electrochemical workstation (SP-240 potentiostat, Bio-Logic Science Instruments). The water content of [bmim][PF6] was measured by KF Titrator (899D Coulometer, Metrohm). Optical images were recorded using an AmScope M150C microscope with AmScope MU500-CK 5.0 MP USB Microscope Camera. The coin cells were sealed with an electronic cell crimper (MSK-160E, MTI) inside of an Argon-filled glovebox (VTI, H2O < 0.1 ppm; O2 < 0.1 ppm). Preparation of rGO-IL capsules. The aqueous phase was prepared by adding and aqueous solution of sodium carbonate (1 M, 0.010 mL) to an aqueous solution of GO (1 g/L, 1 mL). [Bmim][PF6] (0.2 mL) was mixed with hexamethylene diisocyanate (0.042 mL) to generate the ionic liquid phase. The aqueous phase (1 mL) and ionic liquid phase were then mixed in a vial, and agitated by three alternating cycles of vortex (10 s) and sonication (10 s) to emulsify the two phases. Then, a hexamethylenediamine aqueous solution (0.14 M, 0.25 mL) was added drop-wise to the emulsion solution, and the mixture was gently swirled by hand. The mixture was left unagitated for 72 hours, followed by addition of ammonium hydroxide (1 M, 7 mL) to remove unreacted isocyanate functional groups. The solid particles were collected by gravity filtration and washed with water to neutral. rGO-IL capsules were obtained by drying the isolated GO-IL capsules in the vacuum oven to a constant mass, then thermal reduction at 200 °C for 20 min, yielded as a black powder. Determination of ionic liquid content (wt% IL) by 1H NMR. Mesitylene was used as an internal standard in d6-acetone to determine the amount of [bmim][PF6] based on the relative integration in 1HNMR spectrum (0.039 M mesitylene in d6-acetone). rGO-IL capsules (20 mg) were weighed in a glass vial, and then the standard solution (0.7 mL) was added. The sample was sonicated for 30s, then passed through a PTFE syringe filter. Relative integration of the signal in 1H NMR peak attributed to the N-methyl of [bmim][PF6] (4.06 ppm) to the methyl group of mesitylene (2.22 ppm) was used to calculate the wt% IL in each sample (see Figure S6 and Table S2). This was repeated twice, using samples from different batches.


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Microcavity Electrode Measurements51,52: The microcavity electrode was fabricated by sealing a gold wire with a diameter, φ, of 200 µm and a length of 2 cm in a glass tube (φout/φin, 2.0/1.12 mm, length 3 mm) using a Bunsen burner. The gold wire was connected to a copper wire (φ 1 mm, length 4 cm) in the glass tube via ohm contact. The electrode tip was polished by abrasive paper and the micro cavity was achieved by potentiostatic electrodissolution of gold in an aqueous 1 M H2SO4 (1.5 V vs. Ag/AgCl electrode for ~600 s). The volume of the cavity was determined by microscopy to be 2.55 × 10−6 cm3. The other end of the glass tube was sealed with an epoxy resin to prevent leaking. Cavities were thoroughly washed with concentrated sulfuric acid, DI water, acetone and ethanol in ultrasonic bath. The electrolyte was introduced to the cavity via ultrasonic cleaner before sample loading to prevent trapping bubbles inside the cavity. A small amount of sample was inserted into the micro cavity electrode through tapping method. The electrode was handled with care to prevent fracturing. The schematics of the microcavity electrode setup is presented in Figure S9. The electrochemical stability of [bmim][PF6] was measured by cyclic voltammetry (CV) using the three-electrode setup in Figure S9 and a potentiostat. Pt mesh was the counter electrode and Ag wire was the quasi-reference electrode. The microcavity (working) electrode contained either rGO-IL capsules, YP-50 carbon or hollow rGO capsules. About 1.5 mL of [bmim][PF6] was used as the electrolyte. The CV scan rate was 20 mV s−1 and the voltage range was −1.7 to 2.6 V vs Ag quasi-reference. The cut-off current for the reported EWs was 0.2 µA. The electrochemically active surface area of both YP-50 and rGO-IL capsule shell were determined also by CV, shown in Figure S10 (10 mV s−1; 0.4−1 V vs Ag quasi-ref). A different electrolyte containing 0.1 M TEAP in acetonitrile was used with an internal standard of 2 mM Fc whose diffusion coefficient is reported to be 2 × 10−5 cm2/s at 18 °C in this electrolyte.53 Randles−Sevcik equation53 (Eqn. 1) was used to calculate the electrochemically active electrode area. ݅௣ = 0.4463݊‫ ܥܣܨ‬ቀ

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௡ிఔ஽ మ ቁ ோ்

Eqn. 1

݅௣ is the peak current due to Fc/Fc+ redox couple [A], n is the number of electrons transferred in the redox reaction, A is the electrode area [cm2], F is the Faraday constant [C/mol], D is the diffusion coefficient of Fc [cm2/s], C is concentration of Fc [mol/cm3], ν is the scan rate [V/s], R is gas constant [J/Kmol], and T is the temperature [K]. The available active surface area within the cavity volume was determined to be 1208 and 124 cm2/cm3 for YP-50 and rGO, respectively. Electrode Preparation for Coin Cells. An aqueous slurry containing 95 wt% rGO-IL capsules and 5 wt% CMC binder was prepared. The slurry was drop casted on the carbon pre-coated Al current collector (~0.05 mm thickness) with a glass rod. The wet film thickness was controlled by the Teflon coated fiberglass tape (0.010” thick). The film was dried at 80 °C overnight. The 15.6 mm diameter discs were punched out from the dried electrode film. The YP-50 electrodes were prepared following the same protocol. The surface of the disc electrodes was covered by [bmim][PF6] and further exposed to vacuum at 80 °C prior to cell assembly for 3 days. Coin Cell Assembly. The rGO-IL capsule disc electrodes were used as both the anode and the cathode in a CR2032 coin cell. 70 µL of [bmim][PF6] was introduced as the electrolyte to wet the cellulose separator. The water content of [bmim][PF6], which is less than 10 ppm, was



measured before using. The coin cell was sealed by applying 1.1 Torr of pressure with an electronic button cell crimper. The coin cells with YP-50 disc electrodes were fabricated similarly. Electrochemical Impedance Spectroscopy (EIS). To access cell impedance and specifically the electrode-electrolyte interface resistance, fabricated coin cells were perturbed with a 10 mV amplitude voltage at open circuit potential. The frequency range was 2 MHz to 0.01 Hz. The experiments were performed at room temperature, 18 °C, and at 60 °C (± 1 °C). Specific Capacitance Measurements. The capacitance of the fabricated coin cells were determined by CV at different scan rates (10−500 mV s−1) within the voltage range of 0 to 2.5 V. The specific capacitance, C [F/g], was calculated by eqn. 2: ‫=ܥ‬

ଶூ ௠௩

Eqn. 2

where I is the total current in units of [A], m is the active electrode mass on one electrode in units of [g] and v is the scan rate in [V/s]. Experiments were performed at 18 and 60 °C and repeated three times.

ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental details, materials characterizations, electrochemical analysis graphs (PDF) and video.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Peiran Wei: 0000-0001-7820-1716 Qinmo Luo: 0000-0003-4269-2641 Qianwen Huang: 0000-003-0346-468X Burcu Grukan: 0000-0003-4886-3350 Emily Pentzer: 0000-0001-6187-6135 Author Contributions † These two authors contribute equally to this paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS EP, PW, and QL would like to thank CWRU College of Arts and Sciences and NSF CAREER Award #1551943 for financial support. XPS measurements were performed at the Swagelok


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Center for Surface Analysis of Materials (SCSAM) at CWRU. The authors thank NSF MRI-1334048 for NMR instrumentation. QH and BC thank CWRU College of Engineering for funding and Ms. Ningjin Xu and Mr. Mirko Antloga for assistance with electrochemical measurements.

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Carbon Capsules of Ionic Liquid for Enhanced Performance of

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