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Pore engineering of nanoporous carbon nanofibers toward enhanced supercapacitor performance
Applied Surface Science 497 (2019) 143693
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Pore engineering of nanoporous carbon nanofibers toward enhanced supercapacitor performance ⁎
Chang Hyo Kima, Cheol-Min Yanga, , Yoong Ahm Kimb, Kap Seung Yangb,
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a Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea b Alan G. MacDiarmid Energy Research Institute, Department of Polymer Engineering, Graduated School & School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous carbon nanofiber Pore size Surface functionality Supercapacitor Aqueous electrolyte
Porous carbon nanofibers (PCNFs) were prepared from electrospinning both without and with a pore generating inorganic material. Next, they were activated with different activation media (N2, H2O, or CO2). The pore size was tailored from 0.64 to 0.81 nm under various activation conditions, and the specific surface area ranged from 404 to 1624 m2·g−1. To determine the charging mechanism of the supercapacitor in an aqueous electrolyte, the normalized capacitance was calculated, and it was compared with the adsorption behavior of the solvent, H2O, separately. The normalized capacitance showed a trend similar to that of H2O adsorption at a low relative pressure (P/P0 = 0.1), which was expected to be driven by the filler–pore wall interaction, indicating that the ions were strongly solvated by the solvent, H2O. The highest normalized capacitance value (32 μF·cm−2 at 1 mA·cm−2) was achieved from PCNFs having a pore size of 0.64 nm, similar to the electrolyte solvated ion sizes. It was observed that both the capacitance and H2O adsorption were achieved near the pore size of 0.64 nm and at a high functionality. It was understood that the adsorption of the solvated ions was primarily driven by the interaction of the solvent, H2O, with the surface functional groups.
1. Introduction Porous carbon materials (e.g., activated carbon, carbon nanotubes, graphene, carbide-derived carbons, and carbon nanofibers) are widely used in various applications including renewable energy storage and generation owing to their excellent properties such as a large specific surface area (SSA) with microporosity, controlled pore structure, and high electrical conductivity [1–5]. Among the porous carbon materials, electrospun carbon nanofibers (CNFs) have received much attention owing to their one-dimensional nano-sized fibers, shallow pore depth, and ability to form a free-standing electrode without the use of any binder [6–10]. Moreover, these electrospun CNFs are easy to functionalize by introducing organic and inorganic materials in a spinning dope or by controlling the heat-treatment conditions [11–14]. Supercapacitors have been regarded as new energy storage devices because of their unique characteristics such as a high power density, fast response, and long-term stability [15]. Typically, a supercapacitor is composed of a separator placed between two electrodes, a current collector, and an electrolyte. The electrolyte is of two essential types: aqueous and organic. Although organic electrolytes have been widely
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used in commercial supercapacitor applications, aqueous electrolytes have several advantages to compared to the former, for example, low cost, high electrical conductivity, environmental friendliness, and easy construction [16–19]. There are several approaches for improving the specific capacitance of aqueous-electrolyte-based systems, such as the synthesis of new carbon materials with a large SSA and high electrical conductivity [16–23], doping heteroatoms (e.g., N, B, P, and S) in the carbon structures [24–28], and hybridization of the carbon materials with a metal oxide (e.g., Ru, Mn, Ni, Co, NieMn, NieCo, and V [13,29–33]). Recently, the capacitance behavior in nanopores, which have a size similar to those of the solvated ions of the electrolyte, have been investigated. Chmiola et al. reported that the electrochemical double layer capacitor (EDLC) property using carbide-derived nanoporous carbon electrodes in 1 M H2SO4 electrolyte depended on the pore size and SSA [34]. Kalluri et al. studied the effects of pore size and surface charge density on the capacitance of graphitic nanoporous carbon electrodes by performing electrochemical experiments together with atomistic simulations. They found a pronounced increase in the capacitance in the sub-nanometer pores and a threshold-like charging behavior in the pores of sizes comparable to the sizes of the hydrated
Corresponding authors. E-mail addresses: [email protected] (C.-M. Yang), [email protected] (K.S. Yang).
https://doi.org/10.1016/j.apsusc.2019.143693 Received 31 March 2019; Received in revised form 27 June 2019; Accepted 13 August 2019 Available online 14 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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electrolyte ions (0.65 nm pore for Na+ and 0.79 nm pore for Cl−) [35]. Zhou et al. achieved a highest specific capacitance of 223 F·g−1 at 0.2 A·g−1 by finely tuning ultramicroporous carbon materials with pore size range from 0.60 to 0.76 nm. These materials were prepared by the direct pyrolysis of carboxylic phenolic resins with alkali salts of Li+ to Cs+ having different ion sizes [36]. Kim et al. fabricated highly conductive and mesoporous CNFs by the electrospinning of polyacrylonitrile (PAN), poly (methyl methacrylate)(PMMA), and graphene; and successive heat-treatment of the stabilized nanofibers in an inert atmosphere, which have high SSA of 647 m2·g−1 and highest specific capacitance of 128 F·g−1 in 6 M KOH electrolyte [5]. However, the capacitance behavior in nanoporous materials, which have pore sizes similar to that of solvated ions of aqueous electrolyte, is still unclarified. Here in, we prepared finely tailored porous CNFs (PCNFs) to determine the charging mechanism of a supercapacitor with an aqueous electrolyte. The tailored pore structures were prepared by the electrospinning of PAN solution containing tetraethyl orthosilicate (TEOS, Si (OC2H5)4) as the pore generator and successive activation of the stabilized nanofibers in N2, H2O, and CO2 atmospheres at 800 and 900 °C for various time durations. Electrospinning with a pore generator is a useful method to obtain a fibrous network with a porous structure, which has favorable properties [14,37] for supercapacitor applications such as a free-standing structure without using any binder and high electrical conductivity without conducting additive materials [6–14]. The electrospun fibers were thermally treated in the pore-generating gases of N2, H2O, and CO2. A 6 M KOH aqueous solution was used as the electrolyte having solvated cation and anion sizes of 0.64 and 0.60 nm, respectively [38]. In addition, the adsorption analysis of the solvent, H2O, was performed to determine the solvent effect on the adsorption behaviors of the solvated ions present in confined pores with surface functionalities.
spectroscopy (XPS, K-alpha, Thermo Scientific). The electrical resistivity of the PCNFs was measured using the four-point probe method. The electrical conductivity of the PCNFs was calculated according to the following equation: σ = L/AR (σ: electrical conductivity, R: electrical resistance, A: cross-sectional area of the PCNFs (width × thickness), and L: distance between the electrodes). 2.3. Cell fabrication and electrochemical measurements The cells for measuring the electrochemical performance were built by assembling symmetric PCNFs electrodes cut into 1.5 cm × 1.5 cm sizes. Filter paper was positioned between two PCNFs electrodes for separating them, and Ni foil was used to as the current collector. The assembled cells were immersed and tested in 6 M KOH aqueous solution. Cyclic voltammetry (CV, IM6e, Zahner Electrik) of the cell was performed in the potential range from 0 to 1 V at a scan rate of 10 mV·s−1. The electrochemical impedance spectra (EIS, IM6e, Zahner Electrik) measurements were carried out in the frequency range from 1 MHz to 10 mHz with 10 mV amplitude. The capacitance (at a galvanostatic charge/discharge) of the PCNFs was measured by using a WBCS 3000 battery cycler system (Won-A Tech. Co., Korea) in the potential range from 0 to 1 V at a current density of 1–30 mA·cm−2. 3. Results and discussion Fig. 1 shows FE-SEM images of the PCNF webs obtained from the PAN spinning dope solution with or without TEOS by heat-treatment in various gas atmospheres. All the PCNF samples have a smooth fiber surface with an average fiber diameter from 89 nm to 212 nm, depending on the heat-treatment conditions. Generally, the TEOS-containing PAN-based PCNFs (T-PAN-PCNFs) samples have a smaller diameter than the PAN-based PCNFs (PAN-PCNFs) under the same condition: PAN-N2-800 (212 nm) > T-PAN-N2-800 (148 nm), PANH2O-800 (123 nm) > T-PAN-H2O-800 (109 nm), PAN-CO2-800 (167 nm) > T-PAN-CO2-800 (128 nm), PAN-CO2-900-1 (126 nm) > T-PAN-CO2-900-1 (113 nm), and PAN-CO2-900-3 (110 nm) > T-PANCO2-900-3 (89 nm). These results are owing to the lower viscosity of the TEOS-containing dope and pore accelerating property of TEOS, which can reduce the fiber diameter during post heat-treatment [14]. The activation reduces the fiber diameter in the order: H2O > CO2 > N2 at 800 °C, which is in good agreement well with the oxidation reactivity with carbon: H2O > CO2 > N2 at 800 °C [39–41]. The pore structures of the various PCNFs were evaluated by N2 adsorption measurements at 77 K. All the N2 adsorption/desorption isotherms (Fig. 2(a) and (b)) for the PCNFs samples show typical type I behavior. The N2 adsorption mainly occurs in the micropores (pore size < 2 nm) and is nearly completed at a low relative pressure (P/ P0 < 0.1). Particularly, only T-PAN-H2O-800 has a small hysteresis loop at a high relative pressure of 0.45, indicating the existence of mesoporosity. Fig. 2(c) and (d) exhibit the pore volume fractions of the PCNFs. The total pore volumes of the PCNFs increases in the following order: N2-800 < CO2-800 < CO2-900-1 < H2O-800 < CO2-900-3; this trend is the same for both the PAN- and T-PAN-PCNFs. T-PAN-H2O800 has the largest mesopore volume of 0.21, whereas the other PCNFs have smaller mesopore volumes in the range from 0.03 to 0.14, which is in good agreement with the N2 adsorption isotherm of T-PAN-H2O-800. All the PCNFs have large SSA values from 404 to 1624 m2·g−1, as shown in Fig. 3(a) and (b). Under the same activation conditions, most of the T-PAN-PCNFs have a larger SSA than the PAN-PCNFs because TEOS acts as a pore generator for creating micropores on the nanofiber surface. When the TEOS-containing nanofibers were heated under carbonization or activation conditions, the ethylene groups of TEOS were decomposed and they accelerated pore creation on the nanofiber surface [8,14]. The micropore size distributions and average micropore sizes obtained by the MP method and SPE method, respectively, are displayed in Fig. 3(c) and (d). The average micropore sizes of the PCNFs
2. Experimental 2.1. Preparation of PCNFs Finely tuned PCNFs were prepared by continuous electrospinning, stabilization, and pore development in a N2, H2O, or CO2 atmosphere. In detail, the spinning dope solution was prepared using 10 wt% PAN (molecular weight: 150,000 g·mol−1, Pfaltz and Bauer, Inc.) or by blending 20 wt% TEOS (reagent grade, 98%, Sigma-Aldrich) in PAN (TPAN) and dissolving in N, N′–dimethylformamide (DMF, extra pure grade, DUKSAN Pure Chemical Co., Ltd). Electro-spun nanofibers webs were obtained by electrospinning (NT-ESS-300, NTSEE Co., Korea) using the prepared electrospinning dopes. The electrospinning was performed from the tip to the collector distance of 18 cm with an applied voltage of 25 kV and feeding rate of the spinning dope of 4 mL·h−1. The electro-spun nanofibers were stabilized at 280 °C in air for 1 h. Then the stabilized nanofibers were heat-treated in three different atmospheres of N2, H2O, and CO2 at 800 °C for 1 h or 900 °C for 1–3 h (identified by CO2-900-1 or CO2-900-3) to control the pore structure of the nanofibers. The detailed experimental procedure and identification of the samples are summarized in Fig. S1. 2.2. Characterization A field-emission scanning electron microscope (FE-SEM, Helios NanoLAB 650 dual beam, FEI) was used to characterize the surface morphologies of the PCNFs. The pore characteristics of the PCNFs, such as SSA, micropore size distribution, and average micropore size, were evaluated by the Brunauer–Emmett–Teller (BET) method, micropore analysis (MP) method, and subtracting pore effect (SPE) method using N2 adsorption isotherms (BELSORP-max, MicrotracBEL) at 77 K, respectively. To analyze the accessibility of H2O molecules to the pores of the PCNFs, a H2O adsorption study was performed at 298 K. The surface chemistry of the PCNFs was characterized via X-ray photoelectron 2
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Fig. 1. FE-SEM images of the PCNFs; PAN-PCNFs (a) N2-800, (b) H2O-800, (c) CO2-800, (d) CO2-900-1, and (e) CO2-900-3, T-PAN-PCNFs (f) N2-800, (g) H2O-800, (h) CO2-800, (i) CO2-900-1, and (j) CO2-900-3.
PCNFs, SSA, and pore size were successfully controlled by the activation media and time. Fig. S2 shows the electrical conductivity of the PCNFs and commercial activated carbon (Nippon Graphite) electrodes. A highly porous commercial activated carbon electrode with a binder and conductive material shows an electrical conductivity of 1.38 S·cm−1. Even though, the electrical conductivities of the PCNF electrodes were measured without any conductive materials, the CO2-activated (900 °C) PCNF
are in the range from 0.64 nm to 0.81 nm. In the micropore size distributions, we observe that the maximum intensity position of the micropore size distribution curves is shifted to a larger pore size when the heat-treatment atmosphere changes from inert (N2) to oxidative atmospheres (H2O or CO2). Particularly for the CO2 activation, the activation temperature increase from 800 °C to 900 °C or the extension of the activation time from 1 h to 3 h increases the pore size of the PCNFs regardless of the presence of TEOS. The pore characteristics of the
Fig. 2. (a, b) N2 adsorption/desorption isotherms (77 K, closed symbol: adsorption, open symbol: desorption) and (c, d) pore volumes for (a, c) PAN-PCNFs and (b, d) T-PAN-PCNFs. 3
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Fig. 3. (a, b) BET SSAs and (c, d) micropore size distributions determined from the MP method for (a, c) PAN-PCNFs and (b, d) T-PAN-PCNFs.
PAN-N2-800 (3.3), PAN-H2O-800 (20.6), PAN-CO2-800 (4.1), PAN-CO2900-1 (0.4), PAN-CO2-900-3 (2.2), T-PAN-N2-800 (1.5), T-PAN-H2O800 (9.4), T-PAN-CO2-800 (1.9), T-PAN-CO2-900-1 (0.8), and T-PANCO2-900-3 (4.5). The charge transfer resistance induced by charge transfer at the interface of the electrolyte and electrode is associated with the pore structure and electrical conductivity of the electrode. The H2O-activated PCNFs with a large average micropore size have high charge transfer resistance values, indicating their low electrical conductivity, as shown in Fig. S2. In contrast, PAN-CO2-900-1 with the highest electrical conductivity has the lowest charge transfer resistance. Therefore, the charge transfer resistance values are strongly dependent on the electrical conductivity of the PCNF electrodes. It is well known that the specific capacitance is proportionally related to the SSA value for a supercapacitor electrode. The gravimetric specific capacitance values of the PCNF electrodes were plotted against the BET SSA to determine the charging mechanism, as shown in Fig. S4. The gravimetric specific capacitance increases with increasing SSA. However, the T-PAN-N2-800, T-PAN-CO2-800, PAN-CO2-800, and PANCO2-900-1 electrodes deviate from the trend compared with the other PCNF electrodes. Although the above four PCNF samples have a smaller SSA than the H2O-activated PCNFs, they show a higher or similar specific gravimetric capacitance compared to the latter. Such behavior cannot be explained by the general relationship between the SSA and specific capacitance. Accordingly, the gravimetric specific capacitance values of the PCNF electrodes were normalized by the SSA to evaluate the effect of the pore size. The normalized specific capacitance values of the PCNF electrodes (in Fig. 4(a) and (b)) show completely different results compared to the gravimetric specific capacitance values. The normalized specific capacitance values of the PCNF electrodes regardless of the addition of TEOS have the following order: N2-800 > CO2800 > CO2-900-1 > CO2-900-3 > H2O-800. To confirm the relationships between the average micropore size and normalized specific capacitance values, the latter at 1 mA·cm−2 and 30 mA·cm−2 were plotted against the average micropore size, as shown in Fig. 4(c) and
electrodes exhibit higher conductivities than commercial ones because of their fibrous networking and high-temperature activation [6–14]. However, the H2O-activated PCNF electrodes regardless of the presence of TEOS show the lowest electrical conductivity of 0.13 and 0.10 S cm−1 owing to the more porous structure from the high oxidation reactivity [14,39–41]. The CV, galvanostatic charge/discharge, and constant-power discharge profiles were obtained to evaluate the electrochemical performances of the PCNF electrodes without any binder and conductive materials such as super P. The CV curves of the PCNF electrodes obtained at the scan rate of 10 mV·s−1 are shown in Fig. S3(a) and (b). All the PCNF electrodes show an almost perfect rectangular shape, signifying an ideal capacitive behavior in the potential window ranging from 0 to 1 V. The gravimetric capacitances are calculated from the galvanostatic charge/discharge profiles according to the following equation:
C = 4(I × Δt)/(m × ΔV), where C is the gravimetric capacitance, I is the discharge current in A, Δt is the discharge time in s, ΔV is the discharge voltage in V, and m is the total weight (g) of the two electrodes. As shown in Fig. S3(c) and (d), the PCNF electrodes show high gravimetric capacitance values in the range from 112 F·g−1 to 220 F·g−1 at 1 mA·cm−2 discharge current. Even when the discharge current increases to 30 mA·cm−2, the PCNF electrodes still have high gravimetric capacitance values in the range from 98 to 161 F·g−1. Generally, the CO2-activated PCNFs show a higher capacity than the N2‑carbonized and H2O-activated PCNFs regardless of the presence of TEOS. Moreover, the T-PAN-PCNF electrodes show a higher capacitance than the PAN-PCNF ones, which is well consistent with the SSA and electrical conductivity of the PCNFs. Furthermore, the impedance spectra of the PCNF electrodes are shown as Nyquist plots in Fig. S3(e) and (f). All PCNF electrodes show a single semicircle in the high frequency range. The charge transfer resistance (Rct) values calculated from the semicircle diameter are as follows: 4
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Fig. 4. Normalized specific capacitances by the SSA of (a) PAN-PCNF and (b) T-PAN-PCNF electrodes, and correlation of the average micropore size and normalized specific capacitance by the SSA (c) at 1 mA·cm−2 and (d) at 30 mA·cm−2 discharge current density. The fitted lines were obtained by nonlinear regression based on the second-order polynomial model.
which is well matched with the BET SSAs obtained from the N2 adsorption isotherms at 77 K. Interestingly, the noticeable uptake point of H2O adsorption shifts to a low relative pressure with a decrease in the average micropore size. The PCNF samples with a large average micropore size such as H2O-800 and CO2-900-3 show a rapid increase in the H2O adsorption amount at a medium relative pressure (0.3 to 0.5). However, the N2-800, CO2-800, and CO2-900-1 samples with smaller average micropore size than H2O-800 and CO2-900-3 exhibit a steeply increasing slope of the adsorption curve at a lower relative pressure (< 0.3). This result indicates that the H2O adsorption behavior is strongly influenced by the pore size of the PCNFs. For more detailed examination, the H2O adsorption isotherms on the PCNF samples below a relative pressure of 0.1 are shown in Fig. 5(c) and (d). The H2O adsorption quantities of the PCNFs below the relative pressure of 0.1 show an opposite trend with the average micropore size of the PCNFs. Although the PAN-H2O-800 and CO2-900-3 samples have a larger average micropore size than PAN-N2-800, CO2-800, and CO2-900-1, their H2O adsorption quantities at the relative pressure of 0.1 are smaller than those of PAN-N2-800, CO2-800, and CO2-900-1. This trend is also observed in the T-PAN-PCNFs. Therefore, the micropore size strongly influences the H2O adsorption quantity at low relative pressures [42–44]. The H2O adsorption quantity at a relative pressure of 0.1 is plotted against the average micropore size and XPS atomic ratio of the PCNFs in Fig. 6(a) and (b), respectively. This is to elucidate the correlation of the H2O adsorption quantity at this relative pressure with the average micropore size and surface functionality of the PCNFs. The XPS atomic ratios of the PCNFs are the sums of the atomic % of O, N, and Si divided by the C atomic %, (O + N + Si)/C (Table S1). As shown in Fig. 6(a), the H2O adsorption quantity at the relative pressure of 0.1 has a
(d). The normalized specific capacitance gradually increases with the decrease in the average micropore size and has a maximum at 0.64 nm of average micropore size, which is close to the diameters of the solvated cations and anions of 6 M KOH electrolyte (ca. 0.64 and 0.60 nm, respectively) [38]. For the CO2-activated PCNFs prepared at different temperatures and time, the PCNF electrodes with pore sizes similar to the solvated ion diameters show higher normalized capacitance values compared with those with larger pore sizes. These tendencies are clearly observed at the discharge current of 1 mA·cm−2 and even 30 mA·cm−2, with correlation factor R2 values of 0.65 and 0.59, respectively. The low correlation factor R2 values are attributed to the unexpected normalized capacitance values of PAN-CO2-900-1, T-PANCO2-900-1, and T-PAN-CO2-800. When PAN-CO2-900-1, T-PAN-CO2900-1, and T-PAN-CO2-800 are omitted, the correlation factor R2 increases to 0.94 and 0.88 at 1 and 30 mA·cm−2, as shown by the red dotted lines in Fig. 4(c) and (d), respectively. We used the H2O adsorption technique (298 K) for the PCNF samples to elucidate the relationship between the H2O adsorption property and capacitive behavior of the supercapacitor built with the PCNF electrodes. H2O adsorption on porous carbons is sensitively influenced by both the pore size and concentration of the polar functional groups on the carbon surface because of the hydrogen bond between the H2O molecules and polar functional groups. The H2O adsorption isotherms of the PAN- and T-PAN-PCNFs are presented in Fig. 5. All the H2O adsorption isotherms of the PCNF samples exhibit an S-shaped type V isotherm, having a noticeable uptake at a medium relative pressure and a marked hysteresis loop (Fig. S5). The H2O adsorption quantity of the PCNF samples at saturated vapor pressure (P/P0 = 1) has the following order: CO2-900-3 > H2O-800 > CO2-900-1 > CO2-800 > N2-800, 5
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Fig. 5. H2O adsorption isotherms on various PCNF samples at 298 K in the relative pressure range from 0 to 1, (a) PAN-PCNFs, (b) T-PAN-PCNFs, and the low relative pressure range from 0 to 0.1, (c) PAN-PCNFs and (d) T-PAN PCNFs.
correlation factor R2 value of 0.78 with the average micropore size, similar to the normalized capacitance versus average micropore size trend in Fig. 4(c) and (d). Furthermore, for the plot of the H2O adsorption quantity at the relative pressure of 0.1 and XPS atomic ratio of the PCNFs, a relatively high correlation factor R2 value of 0.81 is obtained (Fig. 6(b)). These results can be explained by the combination effect of the increased interaction between the H2O molecules and confined ultramicropore surface and electrostatic interaction between the H2O molecules and surface functional groups of the PCNFs (Figs. S6 and S7) [45–47]. Additionally, the XPS atomic ratios of the PCNFs are plotted versus their average micropore size in Fig. 6(c) to confirm their relationship. Interestingly, the functional group content of the PCNFs shows a decreasing tendency with increasing micropore size owing to the higher temperature and longer activations, except for the H2O activation. By associating the results of the H2O vapor adsorption and normalized specific capacitance of the PCNFs, we find that the anomalously increased normalized specific capacitances of the PCNFs even low gravimetric specific capacitances (e.g., N2-800 and CO2-800 PCNFs regardless of the addition of TEOS) are caused by two effects: 1) The average micropore size being similar to the size of the solvated ions of the KOH electrolyte can induce a more densely packed layer of the adsorbed electrolyte ions in the micropores of the PCNFs. This suggests a high fractional filling, which is the ratio of the pore volume filled by the adsorbed electrolyte ions to the pore volume of the PCNF. 2) The high surface functionality of the PCNFs can promote the accessibility of the solvated ions of the KOH electrolyte to the micropore surface by electrostatic interactions, which are demonstrated in the H2O vapor adsorption properties.
4. Conclusion Finely tuned PCNFs for supercapacitors in 6 M KOH were fabricated using PAN- or TEOS/PAN-based electro-spun nanofibers by high-temperature activation at 800 or 900 °C in a N2, H2O, or CO2 atmosphere. The combination of activation and a pore generator was found to be useful to achieve a large SSA with a controlled average micropore size from 0.64 nm to 0.81 nm in the CNFs. The capacitive performance in 6 M KOH electrolyte was maximized by developing a large SSA with an optimum pore size of approximately 0.64 nm, similar to the solvated ion sizes of the electrolyte. Furthermore, the confined ultramicropores with a high surface functionality could assist the solvated ion migration into the pores by electrostatic interactions, resulting in a high normalized specific capacitance at an average micropore size similar to the solvated ion sizes. In summary, three important findings from the electrochemical evaluation in 6 M KOH electrolyte and H2O adsorption of the PCNFs were as follows: 1) A high gravimetric capacitance of 220 F·g−1 at 1 mA·cm−2 in 6 M KOH was obtained using free-standing PCNF electrode with a large SSA of 1624 m2·g−1.2) The normalized specific capacitance values were maximized at the average micropore size similar to the solvated ion sizes of 6 M KOH (0.64 and 0.60 nm). 3) The quantity of the H2O adsorption at a low relative pressure of 0.1 was highly correlated to the average micropore size and surface functionality of the PCNFs. Moreover, it was directly proportional to the normalized specific capacitance because of the combined effect of the confined ultramicropores and high surface functionality of the PCNFs. Our study clearly revealed that a suitable pore size for accommodating the electrolyte ions as well as a large SSA and high surface functionality in nanoporous carbons were key factors to enhance the supercapacitive performance. It was observed that both the capacitance and water adsorption were achieved near the pore size of 0.64 nm and at a high 6
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Fig. 6. Relationships between the H2O adsorption quantity at the relative pressure of 0.1 and (a) average micropore size and (b) XPS atomic ratio of (O + N + Si)/C of the PCNF samples. (c) Relationship between the XPS atomic ratio of (O + N + Si)/C and average micropore size of the PCNF samples. The fitted lines in (a) and (c) were obtained by nonlinear regression based on the second-order polynomial model, and the fitted line in (b) was obtained as a function of linear regression.
functionality. It was understood that the adsorption of the solvated ions was majorly driven by the solvent water interaction with the surface functional groups.
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Acknowledgements C.H. Kim acknowledges the financial support from National Research Foundation of Korea Grant funded by the Korean Government (Ministry of Education, Science and Technology) [NRF2015R1A6A3A01058919]. C.M. Yang acknowledges the financial support from the Korea Institute of Science and Technology (KIST) Institutional Program. K.S. Yang acknowledged the financial support from Basic Science Research Program (NRF2018R1D1A1A02046116) and Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT and Future Planning (NRF-2016M3A7B4905618). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143693. References [1] J. Gamby, P. Taberna, P. Simon, J. Fauvarque, M. Chesneau, Studies and characterisations of various activated carbons used for carbon/carbon supercapacitors, J. Power Sources 101 (2001) 109–116. [2] K.H. An, W.S. Kim, Y.S. Park, Y.C. Choi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Supercapacitors using single-walled carbon nanotube electrodes, Adv. Mater. 13 (2001) 497–500. [3] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, Activated graphene-based carbons as supercapacitor electrodes with macro-and mesopores, ACS Nano 7 (2013)
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Full length article
Pore engineering of nanoporous carbon nanofibers toward enhanced supercapacitor performance ⁎
Chang Hyo Kima, Cheol-Min Yanga, , Yoong Ahm Kimb, Kap Seung Yangb,
T
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a Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Chudong-ro 92, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea b Alan G. MacDiarmid Energy Research Institute, Department of Polymer Engineering, Graduated School & School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous carbon nanofiber Pore size Surface functionality Supercapacitor Aqueous electrolyte
Porous carbon nanofibers (PCNFs) were prepared from electrospinning both without and with a pore generating inorganic material. Next, they were activated with different activation media (N2, H2O, or CO2). The pore size was tailored from 0.64 to 0.81 nm under various activation conditions, and the specific surface area ranged from 404 to 1624 m2·g−1. To determine the charging mechanism of the supercapacitor in an aqueous electrolyte, the normalized capacitance was calculated, and it was compared with the adsorption behavior of the solvent, H2O, separately. The normalized capacitance showed a trend similar to that of H2O adsorption at a low relative pressure (P/P0 = 0.1), which was expected to be driven by the filler–pore wall interaction, indicating that the ions were strongly solvated by the solvent, H2O. The highest normalized capacitance value (32 μF·cm−2 at 1 mA·cm−2) was achieved from PCNFs having a pore size of 0.64 nm, similar to the electrolyte solvated ion sizes. It was observed that both the capacitance and H2O adsorption were achieved near the pore size of 0.64 nm and at a high functionality. It was understood that the adsorption of the solvated ions was primarily driven by the interaction of the solvent, H2O, with the surface functional groups.
1. Introduction Porous carbon materials (e.g., activated carbon, carbon nanotubes, graphene, carbide-derived carbons, and carbon nanofibers) are widely used in various applications including renewable energy storage and generation owing to their excellent properties such as a large specific surface area (SSA) with microporosity, controlled pore structure, and high electrical conductivity [1–5]. Among the porous carbon materials, electrospun carbon nanofibers (CNFs) have received much attention owing to their one-dimensional nano-sized fibers, shallow pore depth, and ability to form a free-standing electrode without the use of any binder [6–10]. Moreover, these electrospun CNFs are easy to functionalize by introducing organic and inorganic materials in a spinning dope or by controlling the heat-treatment conditions [11–14]. Supercapacitors have been regarded as new energy storage devices because of their unique characteristics such as a high power density, fast response, and long-term stability [15]. Typically, a supercapacitor is composed of a separator placed between two electrodes, a current collector, and an electrolyte. The electrolyte is of two essential types: aqueous and organic. Although organic electrolytes have been widely
⁎
used in commercial supercapacitor applications, aqueous electrolytes have several advantages to compared to the former, for example, low cost, high electrical conductivity, environmental friendliness, and easy construction [16–19]. There are several approaches for improving the specific capacitance of aqueous-electrolyte-based systems, such as the synthesis of new carbon materials with a large SSA and high electrical conductivity [16–23], doping heteroatoms (e.g., N, B, P, and S) in the carbon structures [24–28], and hybridization of the carbon materials with a metal oxide (e.g., Ru, Mn, Ni, Co, NieMn, NieCo, and V [13,29–33]). Recently, the capacitance behavior in nanopores, which have a size similar to those of the solvated ions of the electrolyte, have been investigated. Chmiola et al. reported that the electrochemical double layer capacitor (EDLC) property using carbide-derived nanoporous carbon electrodes in 1 M H2SO4 electrolyte depended on the pore size and SSA [34]. Kalluri et al. studied the effects of pore size and surface charge density on the capacitance of graphitic nanoporous carbon electrodes by performing electrochemical experiments together with atomistic simulations. They found a pronounced increase in the capacitance in the sub-nanometer pores and a threshold-like charging behavior in the pores of sizes comparable to the sizes of the hydrated
Corresponding authors. E-mail addresses: [email protected] (C.-M. Yang), [email protected] (K.S. Yang).
https://doi.org/10.1016/j.apsusc.2019.143693 Received 31 March 2019; Received in revised form 27 June 2019; Accepted 13 August 2019 Available online 14 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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electrolyte ions (0.65 nm pore for Na+ and 0.79 nm pore for Cl−) [35]. Zhou et al. achieved a highest specific capacitance of 223 F·g−1 at 0.2 A·g−1 by finely tuning ultramicroporous carbon materials with pore size range from 0.60 to 0.76 nm. These materials were prepared by the direct pyrolysis of carboxylic phenolic resins with alkali salts of Li+ to Cs+ having different ion sizes [36]. Kim et al. fabricated highly conductive and mesoporous CNFs by the electrospinning of polyacrylonitrile (PAN), poly (methyl methacrylate)(PMMA), and graphene; and successive heat-treatment of the stabilized nanofibers in an inert atmosphere, which have high SSA of 647 m2·g−1 and highest specific capacitance of 128 F·g−1 in 6 M KOH electrolyte [5]. However, the capacitance behavior in nanoporous materials, which have pore sizes similar to that of solvated ions of aqueous electrolyte, is still unclarified. Here in, we prepared finely tailored porous CNFs (PCNFs) to determine the charging mechanism of a supercapacitor with an aqueous electrolyte. The tailored pore structures were prepared by the electrospinning of PAN solution containing tetraethyl orthosilicate (TEOS, Si (OC2H5)4) as the pore generator and successive activation of the stabilized nanofibers in N2, H2O, and CO2 atmospheres at 800 and 900 °C for various time durations. Electrospinning with a pore generator is a useful method to obtain a fibrous network with a porous structure, which has favorable properties [14,37] for supercapacitor applications such as a free-standing structure without using any binder and high electrical conductivity without conducting additive materials [6–14]. The electrospun fibers were thermally treated in the pore-generating gases of N2, H2O, and CO2. A 6 M KOH aqueous solution was used as the electrolyte having solvated cation and anion sizes of 0.64 and 0.60 nm, respectively [38]. In addition, the adsorption analysis of the solvent, H2O, was performed to determine the solvent effect on the adsorption behaviors of the solvated ions present in confined pores with surface functionalities.
spectroscopy (XPS, K-alpha, Thermo Scientific). The electrical resistivity of the PCNFs was measured using the four-point probe method. The electrical conductivity of the PCNFs was calculated according to the following equation: σ = L/AR (σ: electrical conductivity, R: electrical resistance, A: cross-sectional area of the PCNFs (width × thickness), and L: distance between the electrodes). 2.3. Cell fabrication and electrochemical measurements The cells for measuring the electrochemical performance were built by assembling symmetric PCNFs electrodes cut into 1.5 cm × 1.5 cm sizes. Filter paper was positioned between two PCNFs electrodes for separating them, and Ni foil was used to as the current collector. The assembled cells were immersed and tested in 6 M KOH aqueous solution. Cyclic voltammetry (CV, IM6e, Zahner Electrik) of the cell was performed in the potential range from 0 to 1 V at a scan rate of 10 mV·s−1. The electrochemical impedance spectra (EIS, IM6e, Zahner Electrik) measurements were carried out in the frequency range from 1 MHz to 10 mHz with 10 mV amplitude. The capacitance (at a galvanostatic charge/discharge) of the PCNFs was measured by using a WBCS 3000 battery cycler system (Won-A Tech. Co., Korea) in the potential range from 0 to 1 V at a current density of 1–30 mA·cm−2. 3. Results and discussion Fig. 1 shows FE-SEM images of the PCNF webs obtained from the PAN spinning dope solution with or without TEOS by heat-treatment in various gas atmospheres. All the PCNF samples have a smooth fiber surface with an average fiber diameter from 89 nm to 212 nm, depending on the heat-treatment conditions. Generally, the TEOS-containing PAN-based PCNFs (T-PAN-PCNFs) samples have a smaller diameter than the PAN-based PCNFs (PAN-PCNFs) under the same condition: PAN-N2-800 (212 nm) > T-PAN-N2-800 (148 nm), PANH2O-800 (123 nm) > T-PAN-H2O-800 (109 nm), PAN-CO2-800 (167 nm) > T-PAN-CO2-800 (128 nm), PAN-CO2-900-1 (126 nm) > T-PAN-CO2-900-1 (113 nm), and PAN-CO2-900-3 (110 nm) > T-PANCO2-900-3 (89 nm). These results are owing to the lower viscosity of the TEOS-containing dope and pore accelerating property of TEOS, which can reduce the fiber diameter during post heat-treatment [14]. The activation reduces the fiber diameter in the order: H2O > CO2 > N2 at 800 °C, which is in good agreement well with the oxidation reactivity with carbon: H2O > CO2 > N2 at 800 °C [39–41]. The pore structures of the various PCNFs were evaluated by N2 adsorption measurements at 77 K. All the N2 adsorption/desorption isotherms (Fig. 2(a) and (b)) for the PCNFs samples show typical type I behavior. The N2 adsorption mainly occurs in the micropores (pore size < 2 nm) and is nearly completed at a low relative pressure (P/ P0 < 0.1). Particularly, only T-PAN-H2O-800 has a small hysteresis loop at a high relative pressure of 0.45, indicating the existence of mesoporosity. Fig. 2(c) and (d) exhibit the pore volume fractions of the PCNFs. The total pore volumes of the PCNFs increases in the following order: N2-800 < CO2-800 < CO2-900-1 < H2O-800 < CO2-900-3; this trend is the same for both the PAN- and T-PAN-PCNFs. T-PAN-H2O800 has the largest mesopore volume of 0.21, whereas the other PCNFs have smaller mesopore volumes in the range from 0.03 to 0.14, which is in good agreement with the N2 adsorption isotherm of T-PAN-H2O-800. All the PCNFs have large SSA values from 404 to 1624 m2·g−1, as shown in Fig. 3(a) and (b). Under the same activation conditions, most of the T-PAN-PCNFs have a larger SSA than the PAN-PCNFs because TEOS acts as a pore generator for creating micropores on the nanofiber surface. When the TEOS-containing nanofibers were heated under carbonization or activation conditions, the ethylene groups of TEOS were decomposed and they accelerated pore creation on the nanofiber surface [8,14]. The micropore size distributions and average micropore sizes obtained by the MP method and SPE method, respectively, are displayed in Fig. 3(c) and (d). The average micropore sizes of the PCNFs
2. Experimental 2.1. Preparation of PCNFs Finely tuned PCNFs were prepared by continuous electrospinning, stabilization, and pore development in a N2, H2O, or CO2 atmosphere. In detail, the spinning dope solution was prepared using 10 wt% PAN (molecular weight: 150,000 g·mol−1, Pfaltz and Bauer, Inc.) or by blending 20 wt% TEOS (reagent grade, 98%, Sigma-Aldrich) in PAN (TPAN) and dissolving in N, N′–dimethylformamide (DMF, extra pure grade, DUKSAN Pure Chemical Co., Ltd). Electro-spun nanofibers webs were obtained by electrospinning (NT-ESS-300, NTSEE Co., Korea) using the prepared electrospinning dopes. The electrospinning was performed from the tip to the collector distance of 18 cm with an applied voltage of 25 kV and feeding rate of the spinning dope of 4 mL·h−1. The electro-spun nanofibers were stabilized at 280 °C in air for 1 h. Then the stabilized nanofibers were heat-treated in three different atmospheres of N2, H2O, and CO2 at 800 °C for 1 h or 900 °C for 1–3 h (identified by CO2-900-1 or CO2-900-3) to control the pore structure of the nanofibers. The detailed experimental procedure and identification of the samples are summarized in Fig. S1. 2.2. Characterization A field-emission scanning electron microscope (FE-SEM, Helios NanoLAB 650 dual beam, FEI) was used to characterize the surface morphologies of the PCNFs. The pore characteristics of the PCNFs, such as SSA, micropore size distribution, and average micropore size, were evaluated by the Brunauer–Emmett–Teller (BET) method, micropore analysis (MP) method, and subtracting pore effect (SPE) method using N2 adsorption isotherms (BELSORP-max, MicrotracBEL) at 77 K, respectively. To analyze the accessibility of H2O molecules to the pores of the PCNFs, a H2O adsorption study was performed at 298 K. The surface chemistry of the PCNFs was characterized via X-ray photoelectron 2
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Fig. 1. FE-SEM images of the PCNFs; PAN-PCNFs (a) N2-800, (b) H2O-800, (c) CO2-800, (d) CO2-900-1, and (e) CO2-900-3, T-PAN-PCNFs (f) N2-800, (g) H2O-800, (h) CO2-800, (i) CO2-900-1, and (j) CO2-900-3.
PCNFs, SSA, and pore size were successfully controlled by the activation media and time. Fig. S2 shows the electrical conductivity of the PCNFs and commercial activated carbon (Nippon Graphite) electrodes. A highly porous commercial activated carbon electrode with a binder and conductive material shows an electrical conductivity of 1.38 S·cm−1. Even though, the electrical conductivities of the PCNF electrodes were measured without any conductive materials, the CO2-activated (900 °C) PCNF
are in the range from 0.64 nm to 0.81 nm. In the micropore size distributions, we observe that the maximum intensity position of the micropore size distribution curves is shifted to a larger pore size when the heat-treatment atmosphere changes from inert (N2) to oxidative atmospheres (H2O or CO2). Particularly for the CO2 activation, the activation temperature increase from 800 °C to 900 °C or the extension of the activation time from 1 h to 3 h increases the pore size of the PCNFs regardless of the presence of TEOS. The pore characteristics of the
Fig. 2. (a, b) N2 adsorption/desorption isotherms (77 K, closed symbol: adsorption, open symbol: desorption) and (c, d) pore volumes for (a, c) PAN-PCNFs and (b, d) T-PAN-PCNFs. 3
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Fig. 3. (a, b) BET SSAs and (c, d) micropore size distributions determined from the MP method for (a, c) PAN-PCNFs and (b, d) T-PAN-PCNFs.
PAN-N2-800 (3.3), PAN-H2O-800 (20.6), PAN-CO2-800 (4.1), PAN-CO2900-1 (0.4), PAN-CO2-900-3 (2.2), T-PAN-N2-800 (1.5), T-PAN-H2O800 (9.4), T-PAN-CO2-800 (1.9), T-PAN-CO2-900-1 (0.8), and T-PANCO2-900-3 (4.5). The charge transfer resistance induced by charge transfer at the interface of the electrolyte and electrode is associated with the pore structure and electrical conductivity of the electrode. The H2O-activated PCNFs with a large average micropore size have high charge transfer resistance values, indicating their low electrical conductivity, as shown in Fig. S2. In contrast, PAN-CO2-900-1 with the highest electrical conductivity has the lowest charge transfer resistance. Therefore, the charge transfer resistance values are strongly dependent on the electrical conductivity of the PCNF electrodes. It is well known that the specific capacitance is proportionally related to the SSA value for a supercapacitor electrode. The gravimetric specific capacitance values of the PCNF electrodes were plotted against the BET SSA to determine the charging mechanism, as shown in Fig. S4. The gravimetric specific capacitance increases with increasing SSA. However, the T-PAN-N2-800, T-PAN-CO2-800, PAN-CO2-800, and PANCO2-900-1 electrodes deviate from the trend compared with the other PCNF electrodes. Although the above four PCNF samples have a smaller SSA than the H2O-activated PCNFs, they show a higher or similar specific gravimetric capacitance compared to the latter. Such behavior cannot be explained by the general relationship between the SSA and specific capacitance. Accordingly, the gravimetric specific capacitance values of the PCNF electrodes were normalized by the SSA to evaluate the effect of the pore size. The normalized specific capacitance values of the PCNF electrodes (in Fig. 4(a) and (b)) show completely different results compared to the gravimetric specific capacitance values. The normalized specific capacitance values of the PCNF electrodes regardless of the addition of TEOS have the following order: N2-800 > CO2800 > CO2-900-1 > CO2-900-3 > H2O-800. To confirm the relationships between the average micropore size and normalized specific capacitance values, the latter at 1 mA·cm−2 and 30 mA·cm−2 were plotted against the average micropore size, as shown in Fig. 4(c) and
electrodes exhibit higher conductivities than commercial ones because of their fibrous networking and high-temperature activation [6–14]. However, the H2O-activated PCNF electrodes regardless of the presence of TEOS show the lowest electrical conductivity of 0.13 and 0.10 S cm−1 owing to the more porous structure from the high oxidation reactivity [14,39–41]. The CV, galvanostatic charge/discharge, and constant-power discharge profiles were obtained to evaluate the electrochemical performances of the PCNF electrodes without any binder and conductive materials such as super P. The CV curves of the PCNF electrodes obtained at the scan rate of 10 mV·s−1 are shown in Fig. S3(a) and (b). All the PCNF electrodes show an almost perfect rectangular shape, signifying an ideal capacitive behavior in the potential window ranging from 0 to 1 V. The gravimetric capacitances are calculated from the galvanostatic charge/discharge profiles according to the following equation:
C = 4(I × Δt)/(m × ΔV), where C is the gravimetric capacitance, I is the discharge current in A, Δt is the discharge time in s, ΔV is the discharge voltage in V, and m is the total weight (g) of the two electrodes. As shown in Fig. S3(c) and (d), the PCNF electrodes show high gravimetric capacitance values in the range from 112 F·g−1 to 220 F·g−1 at 1 mA·cm−2 discharge current. Even when the discharge current increases to 30 mA·cm−2, the PCNF electrodes still have high gravimetric capacitance values in the range from 98 to 161 F·g−1. Generally, the CO2-activated PCNFs show a higher capacity than the N2‑carbonized and H2O-activated PCNFs regardless of the presence of TEOS. Moreover, the T-PAN-PCNF electrodes show a higher capacitance than the PAN-PCNF ones, which is well consistent with the SSA and electrical conductivity of the PCNFs. Furthermore, the impedance spectra of the PCNF electrodes are shown as Nyquist plots in Fig. S3(e) and (f). All PCNF electrodes show a single semicircle in the high frequency range. The charge transfer resistance (Rct) values calculated from the semicircle diameter are as follows: 4
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Fig. 4. Normalized specific capacitances by the SSA of (a) PAN-PCNF and (b) T-PAN-PCNF electrodes, and correlation of the average micropore size and normalized specific capacitance by the SSA (c) at 1 mA·cm−2 and (d) at 30 mA·cm−2 discharge current density. The fitted lines were obtained by nonlinear regression based on the second-order polynomial model.
which is well matched with the BET SSAs obtained from the N2 adsorption isotherms at 77 K. Interestingly, the noticeable uptake point of H2O adsorption shifts to a low relative pressure with a decrease in the average micropore size. The PCNF samples with a large average micropore size such as H2O-800 and CO2-900-3 show a rapid increase in the H2O adsorption amount at a medium relative pressure (0.3 to 0.5). However, the N2-800, CO2-800, and CO2-900-1 samples with smaller average micropore size than H2O-800 and CO2-900-3 exhibit a steeply increasing slope of the adsorption curve at a lower relative pressure (< 0.3). This result indicates that the H2O adsorption behavior is strongly influenced by the pore size of the PCNFs. For more detailed examination, the H2O adsorption isotherms on the PCNF samples below a relative pressure of 0.1 are shown in Fig. 5(c) and (d). The H2O adsorption quantities of the PCNFs below the relative pressure of 0.1 show an opposite trend with the average micropore size of the PCNFs. Although the PAN-H2O-800 and CO2-900-3 samples have a larger average micropore size than PAN-N2-800, CO2-800, and CO2-900-1, their H2O adsorption quantities at the relative pressure of 0.1 are smaller than those of PAN-N2-800, CO2-800, and CO2-900-1. This trend is also observed in the T-PAN-PCNFs. Therefore, the micropore size strongly influences the H2O adsorption quantity at low relative pressures [42–44]. The H2O adsorption quantity at a relative pressure of 0.1 is plotted against the average micropore size and XPS atomic ratio of the PCNFs in Fig. 6(a) and (b), respectively. This is to elucidate the correlation of the H2O adsorption quantity at this relative pressure with the average micropore size and surface functionality of the PCNFs. The XPS atomic ratios of the PCNFs are the sums of the atomic % of O, N, and Si divided by the C atomic %, (O + N + Si)/C (Table S1). As shown in Fig. 6(a), the H2O adsorption quantity at the relative pressure of 0.1 has a
(d). The normalized specific capacitance gradually increases with the decrease in the average micropore size and has a maximum at 0.64 nm of average micropore size, which is close to the diameters of the solvated cations and anions of 6 M KOH electrolyte (ca. 0.64 and 0.60 nm, respectively) [38]. For the CO2-activated PCNFs prepared at different temperatures and time, the PCNF electrodes with pore sizes similar to the solvated ion diameters show higher normalized capacitance values compared with those with larger pore sizes. These tendencies are clearly observed at the discharge current of 1 mA·cm−2 and even 30 mA·cm−2, with correlation factor R2 values of 0.65 and 0.59, respectively. The low correlation factor R2 values are attributed to the unexpected normalized capacitance values of PAN-CO2-900-1, T-PANCO2-900-1, and T-PAN-CO2-800. When PAN-CO2-900-1, T-PAN-CO2900-1, and T-PAN-CO2-800 are omitted, the correlation factor R2 increases to 0.94 and 0.88 at 1 and 30 mA·cm−2, as shown by the red dotted lines in Fig. 4(c) and (d), respectively. We used the H2O adsorption technique (298 K) for the PCNF samples to elucidate the relationship between the H2O adsorption property and capacitive behavior of the supercapacitor built with the PCNF electrodes. H2O adsorption on porous carbons is sensitively influenced by both the pore size and concentration of the polar functional groups on the carbon surface because of the hydrogen bond between the H2O molecules and polar functional groups. The H2O adsorption isotherms of the PAN- and T-PAN-PCNFs are presented in Fig. 5. All the H2O adsorption isotherms of the PCNF samples exhibit an S-shaped type V isotherm, having a noticeable uptake at a medium relative pressure and a marked hysteresis loop (Fig. S5). The H2O adsorption quantity of the PCNF samples at saturated vapor pressure (P/P0 = 1) has the following order: CO2-900-3 > H2O-800 > CO2-900-1 > CO2-800 > N2-800, 5
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Fig. 5. H2O adsorption isotherms on various PCNF samples at 298 K in the relative pressure range from 0 to 1, (a) PAN-PCNFs, (b) T-PAN-PCNFs, and the low relative pressure range from 0 to 0.1, (c) PAN-PCNFs and (d) T-PAN PCNFs.
correlation factor R2 value of 0.78 with the average micropore size, similar to the normalized capacitance versus average micropore size trend in Fig. 4(c) and (d). Furthermore, for the plot of the H2O adsorption quantity at the relative pressure of 0.1 and XPS atomic ratio of the PCNFs, a relatively high correlation factor R2 value of 0.81 is obtained (Fig. 6(b)). These results can be explained by the combination effect of the increased interaction between the H2O molecules and confined ultramicropore surface and electrostatic interaction between the H2O molecules and surface functional groups of the PCNFs (Figs. S6 and S7) [45–47]. Additionally, the XPS atomic ratios of the PCNFs are plotted versus their average micropore size in Fig. 6(c) to confirm their relationship. Interestingly, the functional group content of the PCNFs shows a decreasing tendency with increasing micropore size owing to the higher temperature and longer activations, except for the H2O activation. By associating the results of the H2O vapor adsorption and normalized specific capacitance of the PCNFs, we find that the anomalously increased normalized specific capacitances of the PCNFs even low gravimetric specific capacitances (e.g., N2-800 and CO2-800 PCNFs regardless of the addition of TEOS) are caused by two effects: 1) The average micropore size being similar to the size of the solvated ions of the KOH electrolyte can induce a more densely packed layer of the adsorbed electrolyte ions in the micropores of the PCNFs. This suggests a high fractional filling, which is the ratio of the pore volume filled by the adsorbed electrolyte ions to the pore volume of the PCNF. 2) The high surface functionality of the PCNFs can promote the accessibility of the solvated ions of the KOH electrolyte to the micropore surface by electrostatic interactions, which are demonstrated in the H2O vapor adsorption properties.
4. Conclusion Finely tuned PCNFs for supercapacitors in 6 M KOH were fabricated using PAN- or TEOS/PAN-based electro-spun nanofibers by high-temperature activation at 800 or 900 °C in a N2, H2O, or CO2 atmosphere. The combination of activation and a pore generator was found to be useful to achieve a large SSA with a controlled average micropore size from 0.64 nm to 0.81 nm in the CNFs. The capacitive performance in 6 M KOH electrolyte was maximized by developing a large SSA with an optimum pore size of approximately 0.64 nm, similar to the solvated ion sizes of the electrolyte. Furthermore, the confined ultramicropores with a high surface functionality could assist the solvated ion migration into the pores by electrostatic interactions, resulting in a high normalized specific capacitance at an average micropore size similar to the solvated ion sizes. In summary, three important findings from the electrochemical evaluation in 6 M KOH electrolyte and H2O adsorption of the PCNFs were as follows: 1) A high gravimetric capacitance of 220 F·g−1 at 1 mA·cm−2 in 6 M KOH was obtained using free-standing PCNF electrode with a large SSA of 1624 m2·g−1.2) The normalized specific capacitance values were maximized at the average micropore size similar to the solvated ion sizes of 6 M KOH (0.64 and 0.60 nm). 3) The quantity of the H2O adsorption at a low relative pressure of 0.1 was highly correlated to the average micropore size and surface functionality of the PCNFs. Moreover, it was directly proportional to the normalized specific capacitance because of the combined effect of the confined ultramicropores and high surface functionality of the PCNFs. Our study clearly revealed that a suitable pore size for accommodating the electrolyte ions as well as a large SSA and high surface functionality in nanoporous carbons were key factors to enhance the supercapacitive performance. It was observed that both the capacitance and water adsorption were achieved near the pore size of 0.64 nm and at a high 6
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Fig. 6. Relationships between the H2O adsorption quantity at the relative pressure of 0.1 and (a) average micropore size and (b) XPS atomic ratio of (O + N + Si)/C of the PCNF samples. (c) Relationship between the XPS atomic ratio of (O + N + Si)/C and average micropore size of the PCNF samples. The fitted lines in (a) and (c) were obtained by nonlinear regression based on the second-order polynomial model, and the fitted line in (b) was obtained as a function of linear regression.
functionality. It was understood that the adsorption of the solvated ions was majorly driven by the solvent water interaction with the surface functional groups.
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