Ionic Liquid Directed Mesoporous Carbon Nanoflakes as an Effiencient Electrode material

Supercapacitors are considered to be the most promising approach to meet the pressing requirements for energy storage devices. The electrode materials for supercapacitors have close relationship with their electrochemical properties and thus become the key point to improve their energy storage efficiency. Herein, by using poly (vinylidene fluoride-co-hexafluoropropylene) and ionic liquid as the dual templates, polyacrylonitrile as the carbon precursor, a flake-like carbon material was prepared by a direct carbonization method. In this method, poly (vinylidene fluoride-co-hexafluoropropylene) worked as the separator for the formation of isolated carbon flakes while aggregated ionic liquid worked as the pore template. The obtained carbon flakes exhibited a specific capacitance of 170 F/g at 0.1 A/g, a high energy density of 12.2 Wh/kg and a high power density of 5 kW/kg at the current of 10 A/g. It also maintained a high capacitance retention capability with almost no declination after 500 charge-discharge cycles. The ionic liquid directed method developed here also provided a new idea for the preparation of hierarchically porous carbon nanomaterials.

the removing of the IL template. However, up to now, only a few results were reported for the preparation of IL templated porous carbon materials through direct calcination treatment [22][23][24] .
Herein, we proposed the direct carbonization route to prepare mesoporous carbon nanoflakes by using polyacrylonitrile (PAN) as the carbon precursor, and using poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and ionic liquid (IL) as dual templates. In this method, PVDF-HFP worked as the separator for the formation of isolated carbon flakes while IL worked as the pore template. Moreover, their electrochemical capacitive properties, including specific capacitance, rate performance and cycling stability, were also studied. The method provided here offered a new way for the preparation of hierarchically porous carbon nanomaterials.
Preparation of Carbon Nanoflakes. 5 g PAN, 0.5 g PVDF-HFP and 1 g EMIBF 4 were dissolved in 20 mL DMF by stirring for more than 6 h. By casting 2 mL of the above solution on a 7.5 × 2.5 cm 2 area glass substrate and evaporated the solvent on a heating plate at 80 °C for 2 h, the composite membrane was obtained. In order to obtain the final arrayed carbon nanoflakes, the above composite membrane was calcined at 280 °C for 1 h and then at 700 °C for 1 h under N 2 atomosphere at a flow of 100 cm 3 min −1 with a 5 °C min −1 heating rate.
Electrochemical Measurement. In a three-electrode system, the test electrode was prepared by loading slurry consisting of 80 wt % active material, 10 wt % carbon black, and 10 wt % poly-(vinylidene fluoride) (PVDF) (in NMP) on a nickel foam and dried at 80 °C for 1.5 h under vacuum. As-formed electrodes were then pressed at a pressure of 10 MPa and further dried in a vacuum oven at 80 °C overnight. The loading mass of active materials on each current collector was 3.0-4.0 mg, and area was 1.0 cm 2 . In the three-electrode system, the sample was used as the test electrode, platinum foil as the counter electrode, Ag/Ag + electrode as reference electrode, and 6.0 M KOH aqueous solution as electrolyte. Cyclic voltammetry curves were obtained in the potential range of -1.0-0 V vs Ag/Ag + electrode. Galvanostatic charge-discharge measurements were carried out at 0.1-10.0 A g −1 over a voltage range of − 1.0-0 V vs Ag/Ag + electrode. The specific capacitance of the supercapacitor cell was calculated from the equation of C cell = IΔt/mΔ V , where I is the discharge current, Δ t is the discharge time, m is the total mass of active materials in the electrode, and Δ V is the voltage drop upon discharge. The energy density (E) and power density (P) of a supercapacitor cell in the Ragone plots were calculated following the equations of E = 1/2C cell Δ V 2 and P = E/Δ t, respectively.
Chracterizations. Transmission electron microscopy (TEM) and Field emission scanning electron microscopy (FESEM) images were recorded by FEI Tecnai G2 F20 S-Twin 200 KV and Hitach S-4800 equipped with an Energy Dispersive Spectrometer respectively. Raman spectra were measured on Horiba JY Labrain HR800 Raman spectroscopy (exciting source: 632 nm laser for Raman in air at room temperature). N 2 adsorption/desorption analyses were carried out at 77 K using Micromeritics ASAP 2050. Electrochemical performance analyses (cyclic voltammeter (CV) curves and galvanostatic charge-discharge test were recorded by CHI660C electrochemical work station. Thermogravimetric analysis (TGA) was conducted using asystem under dry air with a heating rate of 10 °C min −1 .

Results and Disccussions
Morphology and Formation Mechanism. The top view SEM images of as-prepared and calcined PVDF-HFP/PAN/IL films were shown in Fig. 1. As shown in Fig. 1A, the as-prepared PVDF-HFP/PAN/IL composite film exhibited a chapped morphology, which is composed of isolated "islands" and the gaps between them. After calcination, the isolated islands disappeared and some standed carbon flakes which existed in similar arrangement way with the gaps were left (Fig. 1). Moreover, among the carbon flakes, some small carbon dots were formed on the surface of lower-layer carbon substrate. According to these results, we proposed the suggested mechanism for the formation of these carbon flakes and in the manuscript below, we would prove our suggested mechanism by more experimental data. The suggested formation mechanism was as below: since PVDF-HFP and PAN were of different miscibility with IL, the composite film was separated into two phases after being dried at 80 °C. One phase is PVDF-HFP islands which have poor miscibility with IL and the other one is PAN/IL gaps as PAN has good miscibility with IL. The good miscibility originated from the hydrogen bond interaction between EMI + of IL and the -CN group of PAN. After calcination, PVDF-HFP greatly shrinked into small carbon dots while PAN/IL was carbonized into mesoporous carbon flakes (Fig. 2). As PAN shrinked less than PVDF-HFP upon calcination, the carbon flakes were a lot larger than the carbon dots. Since IL could be removed under high temperature, the IL existed in PAN leaded to the formation of the mesopores in the obtained carbon flakes ( Figure S1). By tunning the weight ratio of PVDF-HFP to PAN, the nanostructure could be well tuned from nanofibers to nanoflakes, and finally to nanorod-like products. Among them, since the carbon flakes exhibited the best electrochemical energy storage performance, we mainly concentrated our work on its formation mechanism and electrochemical property investigation.
In order to prove our suggested mechanism for the formation of carbon flakes, firstly, we compared the SEM morphologies of the carbon flakes with carbonized PVDF-HFP/PAN, PAN, PVDF-HFP, PAN/IL and PVDF-HFP/ IL films. From supplementary Figure S2, it could be clearly observed that no flake-like morphology appeared in the final products and only some small pores or irregular particles formed on their surface due to the evaporation of IL and shrinkage of the polymer. Secondly, we tuned the used amount of IL in the composite films to investigate their influence on the morphology of the final products. As shown in supplementary Figure S3, along with the increase of the concentration of IL in film-casting solution, the carbon flakes became larger and larger, taller and taller, and finally formed irregular short rod-like product when the concentration of IL reached to 100 mg/ mL. This was because IL worked as a template agent for the phase separation. In detail, when no IL was added in the PVDF-HFP/PAN solution, the two polymers were well mixed into a homogenous solution and the phase separation did not happen in the obtained composite film. Along with the increase of the added amount of IL, the phase separation became more and more obvious, and the PAN/IL gaps became deeper and larger. As a result, the final carbon flakes became larger and taller. When the concentration of IL reached to 100 mg/mL, the formed structure of PVDF-HFP/PAN/IL composite film was totally changed and the rod-like nanostructures instead of carbon flakes were observed in the final product. Moreover, in order to investigate the roles of PVDF-HFP and PAN played in the formation process of carbon flakes, the SEM images of the carbonized PVDF-HFP/PAN/IL films with different ratios of PAN to PVDF-HFP were given out in Fig. 3. As shown in Fig. 3, the carbon flakes could only be obtained when the ratios of PAN to PVDF-HFP were 1:1 and 1:2. At other ratio values, the product exhibited nanofibrous morphologies. This indicated that only when the concentrations of PAN and PVDF-HFP  were close, the composite film could exhibit the chapped morphology for the formation of carbon flakes. When anyone of them was overdose, the templated structure for the formation of carbon flakes would be destroyed. As a result, after calcination, only fibrous morphology could be obtained.
In order to further confirm the suggested mechanism, different shrinkage extents of PVDF-HFP and PAN upon calcination were proved by their DTA and TGA curves. As shown in Fig. 4, the weight retention of PAN after calcination was about 53% while that of PVDF-HFP was about 17% (right axis). Moreover, the weight retention of PAN/IL and PVDF-HFP/IL were 38% and 22% respectively. These results further confirmed the different shrinkage extents after calcination, which contributed to the formation of carbon flakes. Since the densities of calcined PVDF-HFP and PAN/IL were similar, high weight retention corresponded to a large volume. As a result, the carbon flakes were a lot larger than the carbon dots produced from PVDF-HFP.
As described in the part of formation mechanism, the carbon flakes were formed from PAN/IL gaps. As a result, it should exhibit similar Raman spectra with calcined PAN. On the other hand, the carbon substrate under the carbon flakes was mainly composed of calcined PVDF-HFP. From the Raman spectra tested on the carbon flake side and on the carbon substrate side (Fig. 5), it could be clearly observed that the results were in well accordance with our assumption. All of the calcined products exhibited two characteristic peaks of carbon materials, which were the well-documented D band at 1328 cm −1 and G band at 1587 cm −1 , corresponding to the disordered graphitic carbon and the E2g vibration of the sp 2 bonded carbon atoms 25 . The PVDF-HFP based carbon materials, including calcined PVDF-HFP and PVDF-HFP/IL exhibited a obviously higher D band and a relatively lower G band, indicating a low graphitization degree while the PAN based carbon materials, including calcined PAN and PAN/IL, exhibited a slightly higher G band than D band, which indicated a much higher graphitization degree than PAN based carbon materials. Since the Raman spectrum tested from the carbon flake side exhibited very similar intensity distribution of D band and G band to calcined PAN and PAN/IL, it is believed that the carbon flakes were mainly derived from PAN. Similarly, as the carbon substrate side exhibited analogous intensity distribution of D band and G band to PVDF-HFP based carbon materials, it could be inferred that the carbon substrate was mainly derived from PVDF-HFP. These results also proved our suggestion about the formation mechanism of the carbon flakes.
The surface area and pore diameter distribution of the as-prepared product was obtained by N 2 adsorption/ desorption analysis. Since the surface area of calcined PAN/IL was too small to be analyzed by Micromeritics ASAP 2050, only the analyzed results of calcined PVDF-HFP/PAN/IL and PVDF-HFP/IL were presented in the manuscript. As shown in Fig. 6, both the isotherms of carbon flakes and calcined PVDF-HFP/IL exhibited unclosed loops which were due to the irreversible adsorption of nitrogen during the analyzing process. According to the BDDT classification, both of the two samples exhibited type IV isotherms with H4 hysteresis loop at a relative high pressure which were due to the incomplete desorption of N 2 from narrow slit-like pores 26 . The hysteresis loop at middle pressure for carbon flakes indicated the presence of mesopores, which could not be seen in the isotherm of calcined PVDF-HFP/IL. This point was more obvious in the pore diameter distribution spectra shown in Fig. 6b. In detail, the calcined PVDF-HFP/IL exhibited only one peak centered at 2.37 nm, while the carbon flakes exhibited two high peaks centered at 2.93 and 4.00 nm respectively. The hierarchically porous structure of the calcined PVDF-HFP/PAN/IL originated to its unique mesoporous flake-like nano morphology. The calculated BET surface areas of calcined PVDF-HFP/PAN/IL and PVDF-HFP/IL were 317.01 m 2 /g and 494.47 m 2 /g. Although the latter sample exhibited a higher surface area, its monodispersed pore structure hindered the immigration of the electrolyte ions during the charge-discharge process and thus its specific capacitance was much lower than the carbon flakes.
Electrochemical Properties. The electrochemical capacitive properties of the mesoporous carbon flakes were evaluated by means of CV, galvanostatic charge-discharge and cycling-life tests in three-electrode systems,  and were compared with those of calcined PAN/IL and PVDF-HFP/IL. As shown in Fig. 7a, the CVs of all the electrode materials exhibited nearly rectangular shape in the potential range of -1 ~ 0 V, demonstrating their electrical double layer capacitive properties. Among them, the CV curve of mesoporous carbon flake based electrode exhibited larger enclosed area than the other two, indicating its better capacitive performance. Specific capacitances of the three calculated according to the galvanostatic charge-discharge curves (Fig. 7b-f) and the calculated results at different charge-discharge current densities were given in Fig. 7a. The galvanostatic charge-discharge tests were also performed in a potential range from − 1 to 0 V. The almost symmetrical charge and discharge curves demonstrated their good and stable capacitive behavior in the wide potential window. As shown in Fig. 7a, at the current density of 0.1 A/g, the carbon flake based electrode exhibited a specific capacitance of 170 F/g, which was much higher than calcined PVDE/IL and PAN/IL, which were 109.1 and 23.6 F/g respectively. Along with the increase of the charge-discharge current density, the specific capacitance of the electrode materials decreased due to the decreased charge-discharge time which was not enough for the immigration of all the electrolyte ions into the electrode. However, the specific capacitance of carbon flake based electrode was still kept a lot higher than the other two kinds of electrode materials. Their different capacitive properties were mainly determined by their different nanostructures. The mesoporous carbon flake exhibited exposed and hierarchically porous morphology which facilitated the immigration of the electrolyte ions into the pores of the electrode material, and thus the electrical double-layer capacitance was improved. Although the calcined PVDF-HFP/IL exhibited larger surface area and pore volume than mesoporous carbon flake, its monodispersed pore structure in the bulk carbon phase hindered the quick immigration of the electrolyte ions, and thus its electrical double-layer capacitance was much lower than that of the carbon flake. For calcined PAN/IL, its low surface area s well as its bulk phase morphology leaded to its low specific capacitance.
To understand the role of nitrogen functionalities in capacitive performance, the atom percentage of nitrogen in the obtained products calcined under different temperatures (600, 700 and 800 °C) were characterized by Energy Dispersive Spectrum. The results indicated that the atom percentages of nitrogen were 7.68%, 4.04% and almost undetectable in the products calcined under 600, 700 and 800 °C. High nitrogen content usually corresponded to high pseudo capacitance 16 . However, in our products, their capacitive performances were not in good correspondence with their nitrogen contents. The specific capacitances were 136.6, 170 and 123.3 F/g for the products obtained under 600, 700 and 800 °C ( Figure S4). This may be due to the increased conductivity of the obtained products under high calcination temperature. Considering the effects of both conductivity and nitrogen content, carbon flakes calcined under 700 °C exhibited the highest specific capacitance.
Ragone plots of the carbon flake electrode based supercapacitor was shown in Fig. 8b, which exhibited a high energy density of 23.6 Wh/kg at the current density of 0.1 A/g, with the power density of 50.2 W/kg. Along with the increase of the charge-discharge current density, the energy density decreased and the power density increased. Owing to the facile ion transportation process for the carbon flake electrode based supercapacitor, the energy density was still high under high charge-discharge current density, which was 12.2 Wh/kg at the current density of 10 A/g with the power density of 5 kW/kg. Moreover, the endurance galvanostatic charge-discharge experiment was also carried out to study the cyclic stability of the carbon flake based electrode. As shown in Fig. 8c,d, the capacitance exhibited almost no declination after 500 cycles.

Conclusions
In summary, a typical kind of mesoporous flake-like carbon materials was proposed by a direct carbonization method, in which the IL and PVDF-HFP were used as the surfactant-like agent and PAN was used as the carbon precursor. By using this method, the final product could exhibit a hierarchically porous structure which facilitated the ion transportation during the charge-discharge process and thus afforded them high electrochemical capacitive properties. Electrochemical studies demonstrated their specific capacitance of 170 F/g, as well as high energy density and power density, which were 12.2 Wh/kg and 5 kW/kg at a current density of 10 A/g. The carbon flake electrode also showed excellent cyclic stability, as its specific capacitance exhibited almost no declination after 500 charge-discharge cycles. The method proposed here may hold promise for preparing more kinds of carbon materials with hierarchically porous structures for low-cost and high-performance energy storage devices.