Flexible and Free-Standing Reduced Graphene Oxide and

Jun 26, 2019 - ... to keep the balance between the mechanical property and electrochemical performance. In this Article, a two-step simple method is d...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Flexible and Free-Standing Reduced Graphene Oxide and Polypyrrole Coated Air-Laid Paper-Based Supercapacitor Electrodes Chang Ma, Wen-Tao Cao, Wei Xin, Jing Bian, and Ming-Guo Ma* Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, People’s Republic of China

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ABSTRACT: Nowadays, the needs of bendable energy storage components are growing to match with the development of wearable devices. However, it is still a great challenge to fabricate flexible electrodes to keep the balance between the mechanical property and electrochemical performance. In this Article, a two-step simple method is developed to prepare a type of free-standing paper-based electrodes assembled by polypyrrole, reduced graphene oxide, and air-laid paper. The as-obtained hybrid paper electrodes are mechanical flexible and lightweight with outstanding specific capacitance (1685 mF cm−2 at 2 mA cm−2) and cycling stability (capacitance retention of 92.8% after cycling for 5000 times). The all-solidstate supercapacitor is assembled, which has a remarkable areal capacitance of 1408 mF cm−2, and a high areal energy density of 147 μWh cm−2. These results indicate that the hybrid paper has significant potential for the future wearable energy devices.

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thiophene) (PEDOT)).14−16 It is noticed that the highperformance SCs are usually assembled into a hybrid to take full advantage of the materials of EDLCs and pseudocapacitors.8 Therefore, the reduced graphene oxide and PPy were introduced to this system to improve the electrochemical performance.17−19 On the other hand, the mechanical property of film-like flexible SCs relied on the flexible basements. Papermaking technology was a vital invention in ancient China, which can date back to the West Han dynasty (AC 105), providing the foundation of material technology. All traditional paper is made of cellulose by the process of pulping, bleaching, and wet-forming technology.20 Cellulose is rich in functional groups (like hydroxyl), which also leads to the excellent hydrophilic property of paper.21 Therefore, ordinary cellulose paper was chosen as the substrate for the flexible electrode.22−26 PPy was coated on the common cellulose paper through a low-cost and simple method called “soak and polymerization” for assembling flexible solid-state SCs with a high energy density of 1.0 mW h cm−3 at the power density of 0.27 W cm−3, and a H3PO4/poly(vinyl alcohol) (PVA) membrane was used as the solid gel electrolyte and separator in this solid-state SC.27 Moreover, Alshareef et al.28 prepared an electrode by drop casting PEDOT:PSS followed by acid treatments on the common printing paper, and then PEDOT

INTRODUCTION During the recent decades, the global warming situation is becoming increasingly severe with the emission of greenhouse gases caused by burning fossil energy, and it is highly needed to find clean, renewable, and high-efficiency storage devices such as supercapacitors (SCs, also named as electrochemical capacitors or ultracapacitors) and lithium-ion batteries.1 Among these storage devices, the SCs have been considered as vital because of their characteristics such as high power density, low resistance, and fast charge/discharge cycle.2 In addition, the needs of the flexible, wearable, and stretchable SCs are growing to meet the demand and development of bendable electronic devices such as foldable mobile phones, implant heart sensors, and flexible OLED displays, etc.3,4 Therefore, it is still a great challenge to fabricate flexible SCs, especially for film-like electrodes with good mechanical property and electrochemical performance. The flexible film-like electrodes usually include the electrochemical active materials and the flexible substrates. The electrochemical performance of SCs depends on the electrochemical active materials. The SCs have been separated into two categories on the basis of the energy storage mechanism of active materials: electrical double-layer capacitors (EDLCs) and pseudocapacitors.5 Specifically, EDLCs are based on carbon materials including active carbon particles,6 carbon black,7 graphene,8 and carbon nanotubes (CNTs),9 while pseudocapacitors consisted of metal oxide or metal hydroxide (like MnO2, RuO2, CuO, Co3O4, and so on)10,11 and electronically conductive polymers (e.g., polyaniline (PANI),12 polypyrrole (PPy),13 and poly(3,4-ethylenedioxy© 2019 American Chemical Society

Received: Revised: Accepted: Published: 12018

April 16, 2019 June 13, 2019 June 14, 2019 June 26, 2019 DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) Sheet resistance of PPy/air-laid papers prepared by different concentrations of pyrrole in 0.010 g mL−1 (red cone), 0.015 g mL−1 (green cone), 0.020 g mL−1 (blue cone), and 0.025 g mL−1 (orange cone); (b) FT-IR spectra of the air-laid paper and the PPy/air-laid paper; and (c) XPS survey and (d) high-resolution XPS N1s spectra of the PPy/air-laid paper, respectively.

sized by a two-step method, in which the first step was the preparation of the precursor PPy/air-laid paper and the second step was the reduction of graphene oxide32 for deposition of rGO on the PPy/air-laid paper. In the first step, an interfacial polymerization method was adopted, and the Fe3+ was used as oxidant. As the reaction proceeded, the air-laid paper was gradually transformed from white to black, which indicated that polypyrrole had been synthesized on the both sides. To ensure the higher performance of assembled rGO/PPy/air-laid paper electrodes, precursor PPy/air-laid papers with the lowest possible sheet resistance should be selected. The PPy/air-laid paper whose volume concentration of pyrrole is 0.015 g mL−1 has the lowest sheet resistance, as shown in Figure 1a and Table S1. The lowest sheet resistance is 3.83 Ω □−1, and the value is obviously lower than that of similar conductive hybrid paper electrodes.33−35 For example, the sheet resistance of asreported carbon nanotube (CNT)/cellulose composite paper is 40 Ω □−1.36 In addition, the conductivity of the paper electrode could be calculated using the following equation:

was electrochemically deposited on PEDOT:PSS/paper substrates. This PEDOT/PEDOT:PSS/paper electrode was used to carry out solid-state SCs, whose capacitance is 11 mF cm−2 with an ion gel electrolyte and 32 mF cm−2 with the PVA/H2SO4 gel electrolyte. However, the mechanical strength of ordinary cellulose paper such as common printing paper or filter paper is low; especially paper electrode needs be treated by strong acid or alkali during the preparation process. A new kind of paper called air-laid paper was employed for solving this problem.29,30 The air-laid paper is manufactured by dry-forming technology and composed of cellulose and polyester without any binder or chemical additive. The polyester can cover the shortage of the simple cellulose, and improve the tensile strength of paper whether dry or wet. Therefore, the air-laid paper is a kind of tremendously important platform material for building flexible and high mechanical strength electrode.31 In this work, the objective goal was to find a way for improving the electrochemical and mechanical performance of the electrode. The air-laid paper was employed as the flexible substrate, and PPy and reduced graphene oxide (rGO) as the electrochemical active materials to fabricate a flexible electrode and all-solid-state SCs. The obtained electrodes not only possess the characteristics of mechanical flexibility and being lightweight but also exhibit outstanding specific capacitance (1685 mF cm−2 at a current density of 2 mA cm−2) and cycling stability (capacitance retention of 92.8% after cycling 5000 times). The all-solid-state SC assembled by this flexible hybrid paper was developed in detail.

i1y 1 (σ , S m−1) = jjjj zzzz = k ρ { dR

(1)

In eq 1, ρ is the resistivity, and d is the thickness of the sample. After calculating, the corresponding conductivity is 1652 S m−1, which is also at a high level in previously reported paper electrodes.34−39 The precursor PPy/air-laid paper with the lowest sheet resistance was characterized by Fourier transform infrared (FTIR). The FTIR spectra revealed the differences of molecular structure between air-laid paper and PPy/air-laid paper (Figure 1b). The FTIR spectrum of the air-laid paper shows the typical characteristic peaks of cellulose. For example, the peaks at 3400 and 2900 cm−1 are, respectively, related to

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RESULTS AND DISCUSSION Preparation and Characterization of PPy/Air-Laid Paper. The rGO/PPy/air-laid paper electrodes were synthe12019

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Industrial & Engineering Chemistry Research

the concentration of pyrrole is up to 0.020 g mL−1 or more, the PPy nanoparticles are becoming agglomerated and uneven (Figure 2d,e). It may result in that the electrical sheet resistance of specimen decreases at the beginning and next increases with the increasing concentration of the adding pyrrole.47 The FE-SEM image of the 0.015-PPy/air-laid paper at high magnification is shown in Figure 2f. High magnification FE-SEM image reveals that the PPy particles of the 0.015-PPy/ air-laid paper are relatively uniform, which is beneficial to enhance electrical conductivity. Preparation and Characterization of rGO/PPy/AirLaid Paper. In this paper, the traditional Hummer’s method was used for preparing the graphene oxide (GO) dispersions. Figure 3a and Figure S1 are the atomic force microscopy (AFM) image and transmission electron microscope (TEM) image of GOs, respectively, from which the appearance of the surface can be observed intuitively and the thickness of the exfoliated GO sheet can be quantified. It was found that the average thickness for GO sheet is 0.37 nm, which is close to the corresponding data of single-layer GO sheet in previous research.19 Therefore, the degree of exfoliation to graphene level is beneficial to insert into the gap of previous PPy nanoparticles. Figure 3b is the digital image of the Tyndall effect in GO dispersion solution, showing that the GO dispersion is a colloid system. The high-resolution XPS C1s spectra of the GO/PPy/air-laid paper (the precursor PPy/air-laid paper immersing in 2.4 g L−1 GO dispersion for 2 h) and rGO/PPy/air-laid paper (after reduction by ascorbic acid) are shown in Figure 3c and d. For the GO/PPy/air-laid paper sample, the XPS spectrum indicates the presence of four main types of carbon bonds of C−C, C−N, C−OH, and CO; they are at 284.5, 284.9, 287.1, and 288.0 eV, respectively. However, after reduction by ascorbic acid, the peak intensity of oxygen-containing functional groups decreases, which indicated that the reductive reaction is successful.48 As shown in Figure 4a−d, the FE-SEM images present the microscopic morphologic changes of rGO/PPy/air-laid papers as the increasing concentration of the graphene oxide dispersion. Figure 4b−d displays the RPA-0.24, RPA-0.48, and RPA-2.4, respectively, and the white circles signify the main distribution positions of rGO sheets. As compared to the FESEM image of PPy/air-laid paper (Figure 4a), it can be clearly seen that there is some amount of sheet-like rGO on the precursor PPy/air-laid paper.48 It is easily found that the proportion of sheet-like rGO increased with increasing the concentration of the graphene oxide dispersion. What is more, the rGO, unlike the simple covered on the PPy/air-laid paper, penetrated into the PPy layers, as shown in Figure 4e, and it is beneficial to obtain the stronger interactions, which originated from the π−π stacking interaction and strong van der Waal’s force of PPy and rGO. The sheet resistance of RPA-2.4 is 1.50 Ω □−1, 0.46 and 0.52 times lower than those of RPA-0.48 (2.80 Ω □−1) and RPA-0.16 (3.53 Ω □−1). The results show that the GO proportion is negatively correlated with the sheet resistance, which is probably due to the participation of highly conductive rGO sheets and the increase of connections between rGO and PPy.19 The rGO/PPy/air-laid paper electrodes assembled through this method have the characteristics of excellent flexibility, and Figure 4f−h reveals that the electrodes were folded into various shapes such as roll circle and plane. In addition, because the raw material is air-laid paper, the electrodes are ultralight, as shown in Figure 4i.

the O−H group and C−H stretching vibration in the pyranoid ring.40 As compared to the air-laid paper, the above-mentioned peaks of the PPy/air-laid paper are greatly reduced or even disappeared. The CC and C−C stretching vibrations of PPy ring appear at 1620 and 1540 cm−1, and the band at 1160 cm−1 may be assigned to the N−C stretching vibration and of pyrrole ring.41,42 The characteristic peaks at 1040 and 775 cm −1 correspond to C−H in-plane and out-of-plane deformation vibrations of PPy, respectively.42−44 The above results indicate that the PPy has successfully been coated on the air-laid paper substrate. X-ray photoelectron spectroscopy (XPS) caused the atoms or inner electrons to be excited to emit to obtain the composition of the test sample. Surface elemental analysis and chemical bonding were carried out by XPS, as shown in Figure 1b and c. Figure 1c presents three main peaks at binding energy (BE) of 532, 400, and 285 eV, respectively, representing O1s, N1s, and C1s, and this result indicates the existence of the nitrogen-containing compound. According to the high-resolution XPS N1s spectrum (Figure 1d), there are two fitted peaks at 401.5 and 399.8 eV, which stand for the chemical bonds positively charged nitrogen (−N+−) and amine (−NH−), respectively. It further illustrates the existence of polypyrrole on the surface of PPy/air-laid papers.45,46 The morphological changes on the surface between air-laid paper and PPy/air-laid paper can be observed by field-emission scanning electron microscopy (FE-SEM) intuitively. As can be seen from Figure 2a, the surface of original air-laid paper without PPy coverage is typically fibrous. However, the PPy/ air-laid paper was coated by the PPy nanoparticles in different size and scale. Figure 2b−e shows the 0.01-PPy/air-laid paper, 0.015-PPy/air-laid paper, 0.02-PPy/air-laid paper, and 0.025PPy/air-laid paper, respectively, and the scale of conductive loads increases with the increasing amount of the PPy. When

Figure 2. FE-SEM images of (a) air-laid paper, (b) 0.01-, (c) 0.015-, (d) 0.02-, (e) 0.025-PPy/air-laid paper, and (f) 0.015-PPy/air-laid paper at high magnification. 12020

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Figure 3. (a) AFM image of GO nanosheets on mica surface with height profile and (b) digital image of Tyndall effect in GO dispersion solution; and high-resolution XPS C1s spectra of (c) the GO/PPy/air-laid paper and (d) RPA-2.4.

is the potential window, and S (cm2) is the superficial area of the working electrode. As compared to the other electrodes, the RPA-2.4 electrode has a maximum specific capacitance, which is higher than that of many other paper-based or paperlike electrodes, such as PPy coated ordinary print paper (1.5 F cm−2)25 and PANI electrodeposited on Au/paper (0.8 F cm−2).39 Moreover, it also has a high rate capability with the capacitance of 1328 mF cm−2 at a current density of 5 mA cm−2 and 1295 mF cm−2 at a current density of 10 mA cm−2 for the RPA-2.4 electrode (Figure 6e). As shown in Figure 6f, the capacitance retention of the PPy/air-laid paper remains about 44.6% after 5000 times cycles, but the RPA-0.16, RPA0.48, and RPA-2.4 show cycling stability with a capacitance retention of 82.3%, 88.6%, and 92.8%, respectively, after 5000 cycles, which reveals that the deposition of rGO improves the electrochemical stability. What is more, the mass loading of the RPA-0.16, RPA-0.48, and RPA-2.4 electrodes is 3.28, 3.98, and 4.81 mg/cm2, respectively. By combining with the areal specific capacitance results, the reason why the RPA-2.4 electrode has such good electrochemical performance can be explained as follows: (1) The amount of active materials increases with the increasing of rGO amount gradually, leading to the reduction of the charge/discharge diffusion length, which could improve the electrochemical performance.19 (2) Because of the porous structure of the substrate, the electrolyte ions diffusion and adsorption are convenient, which is preferable for electrochemical performance in the aqueous electrolyte.50 (3) The lowest sheet resistance of PPy/air-laid paper could lead to the faster electron transport.19 (4) The synergistic effect of PPy and rGO may improve the electrochemical performance of the hybrid paper-based electrode.

Electrochemical Performance. The electrochemical performance of the rGO/PPy/air-laid paper electrode was studied through the cycle voltammetry (CV), the galvanostatic charge−discharge curves (GCD), and electrochemical impedance spectroscopy (EIS) measurements in 1 M HCl aqueous electrolyte with a three-electrode system. Figure 5 shows the CV curves with different scan rates from 5 to 100 mV s−1 and the GCD curves with various current densities from 5 to 20 mA cm−2 of PA, RPA-0.16, RPA-0.48, and RPA-2.4 for exploring the connection of rGO proportion and the electrochemical performance. The CV curves are carried out with potential windows ranging from −0.2 to 0.8 V and show the symmetric shuttle-like mirrored shapes at varied scan rate, indicative of the contribution of perfect both double-layer and pseudocapacitance behavior of rGO and PPy. According to the electrochemical principle, the area of CV loop is smaller, and the capacitance is lower. It is obviously found that with the increasing amount of rGO, the area of the CV loop becomes bigger, which indicates that the higher GO proportion contributes to improving capacitance.49 As shown in Figure 6a−d, the charging and discharging traces display nearly linear and symmetric triangle shape with a slight curvature, revealing the good electrochemical reversibility and charge−discharge properties. The areal capacitances calculated by GCD curves are, respectively, about 397, 782, 842, and 1685 mF cm−2 for the PPy/air-laid paper, RPA-0.16, RPA-0.48, and RPA-2.4 at a current density of 2 mA cm−2, which is calculated by the formula: CA = (I × Δt ) ÷ (ΔV × S)

(2)

where CA (mF cm−2) is the areal capacitance, I (A) is the charge−discharge current, Δt (s) is the discharge time, ΔV (V) 12021

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Figure 4. FE-SEM images of (a) PPy/air-laid paper, (b) RPA-0.24, (c) RPA-0.48, and (d) RPA-2.4; the white circles point out the distribution of rGO sheets. (e) An abstractive schematic diagram of rGO/PPy/air-laid paper. Digital images of (f) folded rGO/PPy/air-laid paper, (g) bent rGO/ PPy/air-laid paper, (h) rGO/PPy/air-laid paper into plane, and (i) rGO/PPy/air-laid paper into flower on a leaf.

consistent with the GCD results. Additionally, it is obvious that the PRA-2.4 shows a more vertical line at low frequency, which may indicate more ideal capacitive performance.47 In addition, for exploring the strength ability of RPA-2.4, the stress−strain test had been processed. The rGO/PPy/cellulose paper-2.4 stands for the sample, which is fabricated by the same method, only with the different substrate changed into ordinary printing paper. Figure 7b shows the stress−strain curves of the pure air-laid paper, RPA-2.4 electrode, and rGO/ PPy/cellulose paper-2.4 electrode. After the composite process, the strain and tensile stress of RPA-2.4 are slightly lower than those of pure air-laid paper, because of the acid treatment during the preparation process. Furthermore, it is obviously observed that the RPA-2.4 can bear a higher tensile stress than that of rGO/PPy/cellulose paper-2.4. Therefore, the air-laid paper is a better flexible substrate than the ordinary printing

Figure 7a depicted the Nyquist plots of the electrochemical impedance spectroscopy (EIS) spectrum of the RPA-2.4; from EIS the charge transfer between the interfaces of electrode and electrolyte can be evaluated. It is presented from Figure 7a that the spectrum is similar to a quasi-semicircular over a highfrequency range and linear at a low-frequency range. In the high frequency range, the solution resistance (Rs) is the point of intersection on the real axis, and from Figure 7a, it can be seen that the Rs value of RPA-2.4 is 1.7 Ω, the Rs value of PPy/ air-laid paper is 4.3 Ω, and the bigger Rs value means the smaller conductivity of the material. The diameter of the semicircular represents the charge transfer resistance (Rct), so the Rct value of RPA-2.4 is 2.3 Ω and the Rct value of PPy airlaid paper is 12.8 Ω. The knee frequencies of the PPy/air-laid paper and RPA-2.4 are 1.468 and 14.680 Hz, and a higher knee frequencies mean a better rate performance, which is 12022

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Industrial & Engineering Chemistry Research

Figure 5. CV curves of the (a) PPy/air-laid paper, (b) RPA-0.16, (c) RPA-0.48, and (d) RPA-2.4 at different scan rates of 5, 10, 20, 50, and 100 mV/s.

open-circuit condition. As can be seen from Figure 8c, RPA2.4-SSC undergoes a rapid self-discharge stage at the start and then tends to be smoother. After 4.8 h of self-discharging, the voltage leakage reaches 50% of the initial potential. Finally, the RPA-2.4-SSC still retains 0.36 V after 10 h. What is more, as shown in Figure 8d and Table S2, the areal energy density data of this work (147 μWh cm−2) were compared to other recently reported flexible supercapacitors, such as PPy-Paper supercapacitor (62.4 μWh cm−2, H3PO4/ PVA),47 CNT/CNF/rGO hybrid aerogel film supercapacitor (28.4 μWh cm−2, H2SO4/PVA),51 CNF/SWCNT nonwoven macrofiber mat supercapacitor (0.7 μWh cm−2, H3PO4/ PVA),52 N-doped BC-derived carbon nanofiber/rGO/BC supercapacitor (0.1 mWh cm−2, KOH),53 V2O5-graphene paper supercapacitor (89 μWh cm−2, LiClO4),54 MnO2graphene paper supercapacitor (35.1 μWh cm−2, Na2SO4)55 and MnO2-CNT-paper supercapacitor (4.2 μWh cm−2, KOH/ PVA).56 It is easily found that our PRA-2.4-SSC exhibits better areal energy density, which suggests that this flexible hybrid paper is a kind of potential material for future wearable energy storage devices.

paper, and there is no major loss in mechanical properties during the fabrication process. To further prove the excellent mechanical flexibility and stable conductivity of the as-fabricated electrodes, a series of tests were carried out under different bending states. As shown in Figure 7c and d, the sheet resistance variations of RPA-2.4 electrode under different bending angles and different cycles have been tested, and there was no conspicuous increase or decrease during the course of the test. The brightness of LED lamps did not lead to any observable change during bending. The great flexibility and stable conductivity of RPA-2.4 electrode indicated that it has a promising potential to be applied in wearable electronic devices. The poly(vinyl alcohol) (PVA)/H3PO4 gel was employed as the gel electrolyte between two symmetric RPA-2.4 electrodes for fabricating the flexible all-solid-state SC (RPA-2.4 SSC). The CV curves and GCD curves of RPA-2.4 SSC are shown in Figure 8a and b in the potential window from −0.2 to 0.8 V. It is obvious that the CV curves still maintain the quasirectangular shape at different scan rates. Through calculations, the specific capacitances of RPA-2.4-SSC are 1408, 993, and 569 mF cm−2 at the current densities of 2, 5, and 10 mA cm−2, respectively. Furthermore, the areal energy density (E) and areal power density (P) were calculated using the following equations: E=

1 CA(ΔV )2 2

P = E/t

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CONCLUSIONS In summary, a kind of hybrid paper-based electrodes was prepared by a two-step method including the preparation of the precursor PPy/air-laid paper and the reduction of graphene oxide for deposition of reduced graphene oxide on the PPy/ air-laid paper. After optimal experiment, RPA-2.4 has an outstanding specific capacitance (1685 mF cm−2 at current density of 2 mA cm−2) and rate abilities (capacitance retention of 92.8% after cycling 5000 times). In addition, the all-solidstate supercapacitor was assembled by using two pieces of hybrid air-laid paper as electrodes and a PVA/H3PO4 gel as electrolyte, which has a remarkable areal energy density of 147 μWh cm−2 and an areal power density of 0.63 mW cm−2.

(3) (4)

where CA is areal capacitance, ΔV is the potential window, and t is the discharge time. The results show that the RPA-2.4-SSC exhibits the maximum areal energy density of 147 μWh cm−2 and a power density of 0.63 mW cm−2. The self-discharge curve of voltage leakage of the RPA-2.4SSC is given in Figure 8c, which was measured for 24 h in an 12023

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Industrial & Engineering Chemistry Research

Figure 6. GCD curves of the (a) PPy/air-laid paper, (b) RPA-0.16, (c) RPA-0.48, and (d) RPA-2.4 at different current densities of 2, 5, and 10 mA cm−2. (e) Area capacitance of the PPy/air-laid paper, RPA-0.16, RPA-0.48, and RPA-2.4 at different current densities. (f) Cycling performance for different air-laid paper-based electrodes at a current density of 2 mA cm−2.

These results suggest that this flexible hybrid paper is a kind of potential material for future wearable energy storage devices.

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Synthesis of Graphene Oxide. The graphene oxide was obtained according to the modified Hummers’ method.32 Briefly, 3.0 g of graphite powder and 1.5 g of sodium nitrate were slowly added to the 70 mL of concentrated sulfuric acid and stirred for 15 min. Next, 9.0 g of potassium permanganate was slowly added to the mixture, and stirring was continued for 90 min in an ice water bath. At this time, the reaction system was transferred to a 35 °C water bath and stirred for 2 h. Next, 150 mL of deionized water was slowly added to the reaction system, then the temperature was adjusted to 90 °C, and the mixture was stirred for 20 min. At this point, 500 mL of deionized water and 15 mL of (30%) hydrogen peroxide were added and stirred for 10 min. After this, the reaction system was allowed to stand overnight, and the supernatant was removed. In the last stage, the substance after washing with 5 wt % hydrochloric acid and deionized water was dialyzed and centrifuged to obtain graphene oxide dispersion. Fabrication of Reduced Graphene Oxide (rGO)/PPy/ Air-Laid Paper Electrodes. The PPy/air-laid paper with the lowest sheet resistance was chosen to assemble the reduced grapheme oxide (rGO)/PPy/air-laid paper electrode. The PPy/air-laid paper was immersed in 15 mL of a graphene oxide

EXPERIMENTAL SECTION

Materials. Graphite powder was supplied by JinRiLai Co., Ltd., Qingdao, PR China. Air-laid paper was provided by MAXCLEAN Co. Other chemicals including pyrrole, ferric chloride, hydrochloric acid, and hydrazine were of analytical grade without further purification and purchased from Beijing LanYi Chemical Reagents and Consumables Agency, PR China. Preparation of PPy/Air-Laid Paper. Air-laid papers were cropped into 4 cm × 4 cm paper-squares. The cut-square airlaid paper was immersed in the solution of ferric chloride (FeCl3) for 30 min, whose molar concentration was 0.35 mol L−1. After draining, the above-mentioned soaked air-laid paper was then sufficiently immersed in an isopropanol solution of different volume percentages of pyrrole monomer in a freezer (4 °C) for 24 h. Finally, the as-obtained product was taken out and drained, washed with deionized water, and vacuum-dried at room temperature for 24 h. 12024

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Industrial & Engineering Chemistry Research

Figure 7. (a) Nyquist plots of the PPy/air-laid paper and RPA-2.4. (b) Stress−strain curves of the pure air-laid paper, RPA-2.4 electrode, and rGO/ PPy/cellulose paper-2.4 electrode. (c) Sheet resistance variation of RPA-2.4 electrode with bending test in different bending angles. (d) Sheet resistance variation of RPA-2.4 electrode with bending test in different cycles.

Figure 8. (a) CV curves at the scan rates of 5, 10, 20, 50, and 100 mV/s for RPA-2.4-SSC. (b) GCD curves of RPA-2.4-SSC at different current densities of 2, 5, and 10 mA cm−2. (c) Self-discharge curve (voltage leakage) of RPA-2.4-SSC. (d) Ragone plot of RPA-2.4-SSC, and the values reported for other SCs were added for comparison.

dispersion solution of a certain concentration for 2 h, and then 15 mL of ascorbic acid solution (0.15 mol L−1) was added into

the above mixture, followed by stirring for 12 h. After being washed with deionized water and vacuum dried at room 12025

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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Industrial & Engineering Chemistry Research ORCID

temperature for 24 h, rGO/PPy/air-laid paper composite flexible electrode materials were fabricated. Fabrication of All-Solid-State Flexible Supercapacitor. The PVA/H3PO4 gel was employed as the electrolyte that was prepared by adding H3PO4 (6.0 g) and PVA (6.0 g) to 60 mL of deionized water at 90 °C until they dissolved, and then they were cooled. Then two pieces of rGO/PPy/air-laid papers were immersed into the above solution for 1 h, and a PVA/ H3PO4 membrane was laid on each electrode for combining into a thin separator. Finally, for solidification of the electrolyte, the all-solid-state flexible SC was left in the fume hood at room temperature at least for 12 h. Characterizations. The morphological representations were carried out using the field-emission scanning electron microscope (FE-SEM, Hitachi, SU8010, and Japan). The atomic force microscope (AFM) images were recorded using a Bruker multimode 8 AFM, with the samples prepared by spincoating GO suspension onto freshly exfoliated mica substrates. Fourier transform infrared spectra (FTIR) were performed via a Bruker, Tensor II IR spectrometer. The sheet resistance (R) was measured by a four-point probe resistivity/square resistance tester (ST2258C, Jing Ge Electronic Technology, China). X-ray photoelectron (XPS) spectra were carried out on an ESCALAB 250Xi system (Thermo Scientific) probe spectrometer. The stress−strain tests were done on a tensile testing machine (Zwell/Roell, Germany) with the ordinary chuck distance of 10 mm. Electrochemical Characterization. The electrochemical characterizations were conducted at room temperature in 1 M HCl electrolyte using a three-electrode setup: the as-prepared rGO/PPy/air-laid paper electrode, a platinum sheet electrode, and an Ag/AgCl electrode, respectively, served as the working electrode, counter electrode, and reference electrode. CV, GCD, and EIS were examined with a CHI 760E (Chen Hua Technology, China) electrochemical workstation. The cyclic voltammetry (CV) tests were conducted in the potential window from −0.2 to 0.8 V for rGO/PPy/air-laid paper electrode (vs Ag/AgCl electrode) at different scan rates of 5, 10, 20, 50, and 100 mV s−1. The galvanostatic charge/ discharge (GCD) measurements of the rGO/PPy/air-laid paper electrode were done between −0.2 and 0.8 V at current densities of 2, 5, and 10 mA cm−2. The electrochemical impedance spectroscopy (EIS) measurements were tested in the frequency from 105 to 0.01 Hz under the alternate current amplitude of 5 mV.

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Jing Bian: 0000-0002-5183-895X Ming-Guo Ma: 0000-0001-6319-9254 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of this work by the Fundamental Research Funds for the Central Universities (no. 2017ZY49) and the Beijing Forestry University Outstanding Young Talent Cultivation Project (2019JQ03014).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02088. Transmission electron microscope (TEM) image of GO nanosheets; sheet resistance and conductivity of PPy/ air-laid paper; comparison of areal energy density and power density of reported flexible supercapacitor and this work; and a movie of electrochemical characterization (PDF)

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*Tel.: +0086-10-62337250. Fax: +0086-10-62336903. E-mail: [email protected]. 12026

DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027

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DOI: 10.1021/acs.iecr.9b02088 Ind. Eng. Chem. Res. 2019, 58, 12018−12027