Polypyrrole Aerogel

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11175−11185

pubs.acs.org/journal/ascecg

Cellulose Nanofibers/Reduced Graphene Oxide/Polypyrrole Aerogel Electrodes for High-Capacitance Flexible All-Solid-State Supercapacitors Yunhua Zhang,† Zhen Shang,† Mengxia Shen,*,† Susmita Paul Chowdhury,† Anna Ignaszak,‡ Shuhui Sun,§ and Yonghao Ni*,† †

Department of Chemical Engineering, University of New Brunswick, 3 Bailey Drive, Fredericton, New Brunswick E3B 5A3, Canada Department of Chemistry, University of New Brunswick, 3 Bailey Drive, Fredericton, New Brunswick E3B 5A3, Canada § Institut National de la Recherche Scientifique-É nergie Matériaux et Télécommunications, Varennes, Quebec J3X 1S2, Canada

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‡

S Supporting Information *

ABSTRACT: Flexible supercapacitors with considerable energy storage performance from green/sustainable materials have attracted significant attention in many fields, such as portable and wearable electronics. In this work, flexible cellulose nanofibers/reduced graphene oxide/polypyrrole (CNFs/rGO/PPy) aerogel electrodes with well-defined threedimensional porous structures are prepared using citric acid-Fe3+ (CA-Fe3+) complexes as oxidant precursors to command the deposition of PPy. The in situ gradual release of Fe3+ leads to the formation of thin and uniform polypyrrole in the composites. A flexible all-solidstate supercapacitor is then prepared by the CNFs/rGO/PPy aerogel film electrode and poly(vinyl alcohol) (PVA)/H2SO4 gel electrolyte and separator. Due to the porous structure, high electrical conductivity, and remarkable wettability of the electrodes, the assembled supercapacitors show excellent electrochemical properties with maximum areal capacitance of 720 mF cm−2 (405 F g−1 for single electrode) at 0.25 mA cm−2 and good cycle stability (95% retention after 2000 cycles). The device with maximum energy density of 60.4 μW h cm−2 also exhibits nearly constant capacitance under different bending conditions, suggesting their great potential for applications in flexible electronics. KEYWORDS: Cellulose nanofibers, Graphene oxide, Polypyrrole, In-situ oxidant releasing, Flexible supercapacitors

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INTRODUCTION

graphene and pseudocapacitive materials which have high specific capacitance, such as transition metal oxides/hydroxides (e.g., MoO2, Ni(OH)2, CoO, and CoMoO4),9−13 metal nitrides (e.g., VN and TiN),14,15 and conducting polymers (e.g., Polypyrrole (PPy) and polyaniline (PANI)).16−19 Among these pseudocapacitive materials, conducting polymers, such as PPy, have attracted substantial attention because of their low cost, good chemical stability, and large pseudocapacitance.20 However, large volumetric change during charge/discharge leads to the poor cycling stability of PPy which limits its practical application.21 It is also believed that the charge storage capability and cyclic stability of the PPy nanocomposite can improve significantly by combing carbon nanomaterials such as rGO, which causes an increase in the ion diffusion rate, the contact surface area, and the electrical conductivity.22−25 As a result, PPy/rGO has become a popularly studied material among the various conducting polymer hybrid electrodes for supercapacitors.26−30

Along with technological progress and tremendous energy consumption, high-performance energy storage devices are in increasing demand. Flexible all-solid-state supercapacitors, one kind of energy storage device, have received significant attention due to their high flexibility, fast charge−discharge rate, long cycle life, high power density, environmentalfriendliness, high safety, and low cost; moreover, these considerable advantages could make them applicable in many fields such as portable and wearable electronics.1−4 Among various promising electrode materials for supercapacitors, graphene-based porous carbon materials (including reduced graphene oxide, rGO) have received extensive interest in flexible energy storage devices in the past, due to many their superior properties such as high specific surface area (2675 m2 g−1), excellent chemical stability, and high electrical conductivity. Unfortunately, their relatively low capacity (resulting from the electrochemical double layer capacitor (EDLC) and unavoidable restacking of graphene nanosheets which leads to difficult electrolyte ion diffusion), has restrained their practical applications.5−8 To achieve better electrochemical performance of graphenebased supercapacitors, one effective method is to combine © 2019 American Chemical Society

Received: January 17, 2019 Revised: May 27, 2019 Published: May 30, 2019 11175

DOI: 10.1021/acssuschemeng.9b00321 ACS Sustainable Chem. Eng. 2019, 7, 11175−11185

Research Article

ACS Sustainable Chemistry & Engineering

PPy aerogel film-based supercapacitors have excellent performance.

Another effective approach is using one-dimensional materials (like carbon nanotubes CNT,31 or cellulose nanofibers CNFs4,32−36) as a nanospacer to restrain the ordered restacking of graphene nanosheets, which significantly increases the EDLC performance. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) oxidized one-dimensional CNFs are a kind of nanometer fiber having high aspect ratio, prominent flexibility, and superior hydrophilicity.37 The CNFs with an appropriate geometric structure can not only act as spacers between rGO layers to increase the electrolyte-accessible interface, but also construct the CNFs aerogels with 3D hierarchical networks used as a flexible porous substrate to support the rGO or PPy-based material.32,33,38 Moreover, the hydrophilic CNFs in the aerogels can absorb electrolytes acting as electrolyte nanoreservoirs and also provide electrolyte ions diffusion channels, which further improves the electrochemical performances of the supercapacitors.34,39 We believe it is possible to obtain an electrode material with good performance with the combination of CNFs, rGO, and PPy. A common method for preparing PPy-based electrodes is directly mixing the monomer with a substrate (such as carbon cloth, rGO array, paper, or even CNFs aerogel), followed by adding the oxidant agent (such as APS, Fe3+).23,40−44 Although many PPy-based electrodes with high-performance have been prepared, there are still some shortcomings for this method. For example, due to the fast formation of GO or CNFs hydrogel induced by hydrogen-bonding or multivalent metal ions cross-linked as well as the quick polymerization of PPy, it is hard to dominate the component distribution and morphology of the PPy-based hydrogels after adding the Fe3+ oxidizing agent.45−47 The formation of hydrogel and polymerization of PPy is too quick to form a uniform PPy/ CNFs/rGO hydrogel, especially the PPy/CNFs/GO aerogel. The complicated diffusion, gelation, and polymerization in the system make the uncontrollable deposition of PPy. From a performance perspective, a uniform gel with a moderately thin uninterrupted PPy coating is the best, while the nonuniform distribution of composite gel may block the pores, resulting in undesired concentration polarization.48 However, it is still a big challenge to prepare uniform PPy/CNFs/rGO aerogel film as an electrode. Controlled release is an effective way to control the release rate of active ingredients. The multivalent metal ions can be gradually released from metal ion-based complexes from acidification,43 for example, hydrogen chloride vapor treatment. The in situ released Fe3+ can then lead to the oxidative polymerization of Py. Gao et al.49 prepared CNFs-PPy composites by acidification of CNFs suspension containing pyrrole resulting in the formation of PPy deposited onto CNFs. Unfortunately, the maximum specific electrode capacitance of as-prepared CNFs/PPy aerogel film electrode is only 215 F g−1 at 0.19 mA cm−2, and below 30% capacitance retention at about 1 mA cm−2. In this project, ternary CNFs/rGO/PPy aerogel with welldefined structures has been developed using citric acid-Fe3+ (CA-Fe3+) colloid as the oxidant precursor to control the deposition of PPy.49 Then, the CNFs/rGO/PPy aerogel-based symmetric supercapacitors are prepared. The microscopic morphology and PPy layers of the CNFs/rGO/PPy aerogel can be influenced by the CA-Fe3+ colloid. Moreover, these supercapacitors exhibit high areal specific electrode capacitance, high gravimetric specific electrode capacitance, and excellent recyclability. These results indicate that CNFs/rGO/

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EXPERIMENTAL SECTION

Materials. Hardwood bleached Kraft pulps. 2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO), sodium hypochlorite solution (NaClO), NaBr, pyrrole (Py), NaOH, citric acid (CA), FeCl3.6H2O, vitamin C, poly(vinyl alcohol) (PVA), K2S2O8, P2O5, KMnO4, H2O2, HCl, and H2SO4 (98%, Sigma-Aldrich) were purchased from commercial suppliers. All analytical reagents were used without further purification, except for pyrrole which was distilled once before use. Preparation of the CA-Fe3+ Colloid. FeCl3·6H2O (0.01 mol) and citric acid (0.01 mol) were added into deionized water (100 mL) under continuous stirring until the solution became clear. NaOH solution was dipped into the above mixture to adjust its pH to about 8.5. Preparation of the CNFs/rGO/PPy Aerogel. GO nanosheets and CNFs were synthesized according to the literature reported by Isogai and Tung, respectively.50,51 The detailed information is given in the Support Information (SI). A variety of graphene oxide dispersions (6 mg/mL) were added to 6.66 g CNFs suspension (0.75 wt %) (the ratios of GO to CNFs were 10:100, 20:100, 30:100, and 50:100). The mixtures were ultrasonicated at room temperature until homogeneous CNFs/GO suspensions were obtained. Various amounts of pyrrole/ CA-Fe3+ solutions (the molar ratio of Py to CA-Fe3+ was about 1:1) were added to the above CNFs/GO suspensions (the mass ratios of Py to CNFs were 25:100, 50:100, and 100:100), and the obtained CNFs/GO/Py/CA-Fe3+ mixtures were ultrasonicated to reach uniform dispersions. The formation of PPy and CNFs/GO/PPy in the composites was carried out by following a hydrochloric acid vapor treatment. The well-dispersed CNFs/GO/Py/CA-Fe3+ suspensions obtained above, were transferred into beakers, then placed under a hydrochloric acid vapor for 12 h at about 4 °C. The GO was reduced to rGO by vitamin C, the CNFs/GO/PPy hydrogels were immersed in vitamin C (30 g L−1) aqueous solution for 24 h at 80 °C. The resultant CNFs/rGO/ PPy hydrogels were repeatedly rinsed using 0.2 M hydrochloric acid to remove inorganic ions, followed by distilled water to yield neutral hydrogels. Finally, the CNFs/rGO/PPy aerogels were prepared after solvent-exchanged in tert-butyl alcohol and freeze-dried. The obtained CNFs/rGO/PPy aerogels electrodes were labeled as CNFs/rGO/ PPy-25, CNFs/rGO/PPy-50, and CNFs/rGO/PPy-100, respectively, corresponding to the mass ratios of Py to CNFs used. The CNFs/PPy (mass ratio of Py to CNFs is 50:100), and CNFs/rGO (mass ratio of GO to CNFs is 30:100) aerogel film electrodes, for comparison, were prepared using the above-mentioned procedures, under otherwise the same conditions. Preparation of the All Solid-State Supercapacitor. The CNFs/rGO/PPy aerogel films were pressed from aerogels under 0.5 MPa (1 h). The pressed films were cut to the desired shape (1.5 × 0.5 cm2), then one side was coated with silver paste which one was pasted to the aluminum foil. Two pieces of film were immersed in poly(vinyl alcohol) (PVA)/H2SO4 solution for 15 min, which was used as both electrolyte and separator, and then dried at 25 °C for about 3 h to remove the excess water. Finally, the electrodes were gently pressed to assemble the sandwich structured supercapacitor (Figure S1). The working area was 0.5 cm2. Characterizations. Field-emission Scanning electron micrographs (FE-SEMs) were performed on a Hitachi S-4800 SEM. XRD was performed by Bruker D8 Advance spectrometer (Bruker, Germany) diffractometer using monochromatic Cu Ka 1 radiation (k 1/4 1.5406 °A) at 40 kV and 25 mA. Fourier transform infrared spectra (FTIR) were performed using a Nicolet iS5 spectrometer (U.S.A.). Raman spectra were characterized on a HORIBA Scientific LabRAM HR Evolution Raman spectrometer (532 nm). The surface elemental characterization was recorded by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The specific surface area of the samples was performed by N2 adsorption analyses (BELSORP-Mini II). The 11176


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Figure 1. Preparation of the CNFs/rGO/PPy composites electrode: mixing of CNFs, GO, Py, and CA-Fe3+; the controllable forming of CNFs/ GO/PPy hydrogel; reduction of GO; freeze-drying to form the CNFs/rGO/PPy aerogel; and pressing to obtain the CNFs/rGO/PPy hybrid aerogel film.

Figure 2. SEM image of (a) CNFs, (b) CNFs/rGO, (c) CNFs/PPy, (d) CNFs/rGO/PPy-25, (e) CNFs/rGO/PPy-50, and (f) CNFs/rGO/PPy100 aerogel film. Barrett−Joyner−Halenda (BJH) model was used to calculate the pore size distribution. The conductivity was characterized by 4-point probe resistivity measurement system using a Keithley 2750 multimeter/ switch system (Keithley Instruments Inc., U.S.A.). Contact angles were measured using an attention θ optical tensiometer (KSV Instruments, Ltd.). The electrochemical performances of supercapacitor were studied using a two-electrode system by cyclic voltammetry (CV, 0−0.8 V), galvanostatic charge/discharge (GCD, 0−0.8 V), and electrochemical impedance spectroscopy (EIS, 105 to 0.01 Hz) on a CHI 660E (USA).

containing functionalities (hydroxyl and carboxyl groups) on the surfaces of GO, and CNFs, stable CNFs/GO suspensions in water were formed under ultrasonication. Upon the addition pyrrole (Py) and CA-Fe3+ complexes, Py was interacting with GO nanosheets, and CNFs via H-bonds formed between −COOH groups in GO or CNFs, and N−H groups in Py. The water-soluble CA-Fe3+ complexes are stable at a slightly alkaline pH, but the Fe3+ oxidants are released upon acidification. The homogeneous dispersion of CA-Fe3+ and gradual release of Fe3+ from CA-Fe3+ in the system as a result of exposure to hydrochloric acid vapor, is critical to form thin and uniform PPy on the CNF and GO surfaces of the CNFs/ GO/PPy hydrogels. Vitamin C was used to reduce GO to rGO

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RESULTS AND DISCUSSION The schematic for preparing the CNFs/rGO/PPy composites electrode is shown in Figure 1. Due to the rich oxygen11177


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Figure 3. (a) FT−IR and (f) XRD spectra of the rGO film, CNFs, CNFs/PPy, CNFs/rGO, CNFs/rGO/PPy aerogel film; (b) Raman spectra of CNFs/PPy, CNFs/rGO, and CNFs/rGO/PPy-50 aerogel film; (c) XPS survey spectra of the CNFs, CNFs/PPy, CNFs/rGO, CNFs/rGO/PPy-50 aerogel film; (d) C 1s spectrum of the CNFs/rGO/PPy-50 aerogel film; (e) N 1s spectrum of the CNFs/rGO/PPy-50 aerogel film; (f) XRD spectra of the rGO film, CNFs, CNFs/rGO, CNFs/PPy, and CNFs/rGO/PPy aerogel film; (g) conductivity of CNFs/rGO/PPy aerogel film prepared with different GO content at 50% Py; and (h) active mass loading and conductivity of CNFs/rGO/PPy aerogel film prepared with different Py content at 30% GO. 11178


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ACS Sustainable Chemistry & Engineering so that CNFs/rGO/PPy aerogel films with well-defined 3D porous structures, were obtained after washing, freeze-drying, and pressing. SEM observations of CNFs, CNFs/PPy, CNFs/rGO, and CNFs/rGO/PPy aerogel are presented in Figure 2. All the SEM results show a three-dimensional (3D) porous network with nanoscale pore structure. Figure 2a reveals that the CNFs aerogel has a random oriented 1D fiber-like structure, which can connect with each other and form a well-defined 3D porous web-like structure. After adding rGO, the morphology of the CNF/rGO hybrid aerogel film (Figure 2b) exhibits the rGO nanosheets uniformly dispersed in the porous network. The morphology of the CNF/PPy hybrid aerogel film (Figure 2c), shows 3D porous networks also formed by connecting 1D fiber-like structures which look rougher than that of CNFs aerogel (composed of CNFs and PPy). As shown in Figure 2d−f, no obvious PPy (nano) particles can be seen in CNFs/rGO/PPy-25 and CNFs/rGO/PPy-50 aerogels; however, they are evident in the CNFs/rGO/PPy100 aerogels. The presence of PPy on the rGO sheet surface and 1D fiber-like pore walls are controllable, leading to the formation of homogeneous PPy coating on the surface of CNFs/rGO skeleton. The CA-Fe3+ colloid can control the deposition of PPy layers, responsible for the observed results. We further carried out the N2 absorption/desorption isothermal analysis. As shown in Figure S2, the isotherm of CNFs/rGO/PPy-50 aerogel exhibits a characteristic small hysteresis loop at a relatively high pressure (type-IV), which implies the presence of lots of mesopores. BJH results (insets of Figure S2) further confirm this observation. The pore diameter distribution curve indicates that the size of pores is in the 1.8−80 nm range and a shoulder peak appears at 11.8 and 23 nm. CNFs/rGO/PPy with 3D conductive porous networks which promote the electrolytes permeation and shorten the path of charge transport are promising for energy storage. The formation of CNFs/rGO/PPy hybrid aerogel was confirmed by FT-IR spectroscopy (Figure 3a). The PPy characteristic peaks were confirmed in these samples, supporting the notion that PPy are successfully polymerized and deposited on rGO sheets and CNFs. Those at 1543 cm−1, 1450 cm−1, 1309 cm−1, 1157 cm−1, and 895 cm−1 are assigned to C−C and C−N stretching in the pyrrole ring, C−H and C− N in-plane deformation modes, N−C stretching vibration of PPy ring, and C−H out of plane vibration, respectively. The characteristic peaks of CNFs around 3340 cm−1, 2900 cm−1, 1718 cm−1, and 1020 cm−1 are the typical stretching vibrations of −OH groups, asymmetrically C−H stretching, CO stretching, and C−O−C stretching vibrations, respectively. For rGO, only the weak peaks for CO stretching (1718 cm−1) and absorbed water molecules can be seen (1623 cm−1) The typical characteristic absorptions of CNFs and rGO are retained in the CNFs/rGO/PPy hybrid aerogel. The CNFs/rGO, CNFs/PPy-50, and CNFs/rGO/PPy-50 samples were further subjected to Raman analyses (Figure 3b). The spectrum of CNFs/rGO shows two characteristic peaks for D band (1345 cm−1) and G band (1584 cm−1).4 The CNFs/PPy-50 and CNFs/rGO/PPy-50 samples have the typical characteristic peaks for PPy.52 Compared with CNFs/ PPy-50, some characteristic peaks for PPy in the CNFs/rGO/ PPy-50 show a blue-shift, indicating interactions between rGO and PPy. XPS is used to study the surface elemental composition and electronic states between elements. The XPS survey spectra

(Figure 3c) of CNFs and CNFs/rGO exhibit the existence of C and O elements and two other elements (N and Cl) are found for CNFs/PPy and CNFs/rGO/PPy. The existence of N indicates the deposition of PPy, and the presence of Cl proves the Cl−-doped PPy.53 The high-resolution XPS C 1s spectrum of CNFs/rGO/PPy-50 (Figure 3d) exhibits five Gaussian peaks with different binding energies, namely, C−C (284.7 eV), C−N (285.5 eV), C−OH (286.6 eV), CO (288.0 eV), and O−CO (289.3 eV).54 The presence of C− N confirms the interaction between PPy and rGO, in agreement with the Raman results. The N 1s spectrum of CNFs/rGO/PPy-50 (Figure 3e) is divided into three component peaks, namely, −N (389.8 eV), −NH− (399.6 eV), and N+(400.7 eV).4 The presence of −N+ confirms the oxidation state of PPy, which can increase the conductivity of CNFs/rGO/PPy hybrid aerogel, and thus lead to high electrochemical performance. Due to the presence of excellent hydrophilic CNFs well dispersed in the pore walls of CNFs/rGO/PPy aerogels, the CNFs/rGO/PPy aerogel film such as CNFs/rGO/PPy-50 can completely adsorb the water droplet within 10 s (as seen in the dynamic water contact angle tests shown in Figure S3). The wettability of 1D fiber-like pore walls not only decreases the electrolyte ion diffusion resistance, but also improves the utilization efficiency of the surface for charge storage.55 Figure 3f shows XRD spectra of the rGO nanosheet, CNFs aerogel, CNFs/PPy, CNFs/rGO, and CNFs/rGO/PPy hybrid aerogels. The diffraction peak of CNFs aerogel at 16.1° and 22.9°, corresponds to the typical (101), and (200) diffraction planes of cellulose I crystalline structure, respectively. The XRD patterns of CNFs/rGO/PPy, CNFs/PPy, and CNFs/ rGO hybrid aerogels are similar to that of cellulose I crystalline structure, only the peaks become relatively weak and broad which demonstrates that CNFs are well preserved in the composite. In addition, the absence of two peaks of rGO at 25.9° and 43.2° attributed to the diffraction planes (002), and (100) in the CNFs/rGO/PPy hybrid aerogels also indicates that CNFs can act as nanospacers to effectively avoid the π−π stacking of graphene nanosheets, as reported elsewhere.32 In addition, the conductivity (Figure 3g) of CNFs/rGO/ PPy aerogel film increases with GO content until reaching a maximum of 30% GO. On the basis of these results, 30 wt % GO is chosen for the subsequent experiments. After PPy polymerization, active mass loading (the total mass of rGO and PPy) of CNFs/rGO/PPy (Figure 3h) prepared with different Py content always increases with Py content, but the conductivity of CNFs/rGO/PPy reaches its maximum value of 2.5 S cm−1 for CNFs/rGO/PPy-50, which is much higher than some other materials (1.61 × 10−4−3.9 S cm−1 for PPy/ cellulose fibers, 1.14 S cm−1 for pure PPy film).56 The high conductivity can be attributed to the synergy among CNFs, rGO, and PPy in the formation of 3D conductive porous networks. With the increase of PPy content, an effective conductive network with suitable thickness of PPy coating is formed and shows the highest conductivity. Further increasing the PPy content, led to the aggregates of PPy which results in decreased conductivity, because it prevents the effective charge transport along the conductive networks. The symmetric supercapacitors are prepared by two pieces of CNFs/rGO/PPy-25 aerogel film using H2SO4/PVA gel as both electrolyte and separator (Figure S1). The electrochemical behavior of CNFs/rGO/PPy-25 aerogel film-based supercapacitors (CNFs/rGO/PPy-SC-25) was carried out in a 11179


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Figure 4. (a) CV curves of CNFs/rGO, CNFs/PPy, and CNFs/rGO/PPy-25 aerogel supercapacitor at 5 mV s−1, (b) the corresponding GCD curves at 0.25 mA cm−2, (c) CV curves of CNFs/rGO/PPy-SC-25 at different sweep rates, (d) the corresponding GCD curves at different current densities. (e) Areal capacitance of the CNFs/rGO-SC, CNFs/PPy-SC, and CNFs/rGO/PPy-SC-25 at different current densities, and (f) the corresponding Nyquist impedance plots. The inset in (f) shows (i) an enlarged scale image at high frequency and (ii) equivalent electrical circuit from EIS data for CNFs/rGO/PPy-SC-25.

profile of CNFs/rGO/PPy-SC-25 (Figure 4c) is always smaller than that of CNFs/PPy-SC (Figure S5) at the corresponding sweep rate. The GCD curves of the supercapacitors are tested at different current densities (from 0.25 to 2.0 mA cm−2). The symmetrical triangles and small iR-drop indicate a good capacitance performance.57 The almost symmetrical triangular GCD curves of CNFs/rGO/PPy-SC-25 (Figure 4d) at high current density show good EDLCs behavior. The pseudocapacitance characteristics of CNFs/rGO/PPy-25 electrodes are more clearly seen at low current density (this phenomenon is more obvious in CNFs/PPy aerogel film electrode). It is also shows that at the studied current densities the iR-drops of CNFs/rGO/PPy-SC-25 are significantly smaller than that of CNFs/PPy-SC, this may be caused by the uninterrupted PPy coating around CNFs/rGO, which provides excellent electrical conductivity. The specific capacitances (Cs) of the symmetric supercapacitor are calculated from the discharge curves without iRdrop, and the results are shown in Figures 4e and S6a. The areal capacitance (CA) of CNFs/rGO/PPy-SC-25 is 400 mF cm−2 at 0.25 mA cm−2, which is 288% and 48% higher than that of the CNFs/rGO-SC (103 mF cm−2) and CNFs/PPy-SC (270 mF cm−2), respectively. The corresponding gravimetric specific capacitance (Cg) of single electrodes shown in Figure S6a for CNFs/rGO/PPy-SC-25, CNFs/rGO-SC, and CNFs/ PPy-SC is 317, 154, and 268 F g−1, respectively. The possible reason for the higher Cs (either CA or Cg) of CNFs/rGO/PPySC-25 is the combination of EDLC of rGO and pseudocapacitance of PPy.53,58 Furthermore, the CA of CNFs/rGO/PPySC-25 decreases from 400 to 288 mF cm−2 when the current density increases from 0.25 to 2 mA cm−2. The ∼72% initial capacitance retention at 2 mA cm−2 exhibits good capacitance retentions (rate capability). However, CNFs/PPy-SC displayed 58% capacitance retention by increasing the current

two electrode system by CV, GCD, and EIS at room temperature, and the results were compared with those of CNFs/rGO, and CNFs/PPy aerogel film-based supercapacitors (CNFs/rGO-SC, CNFs/PPy-SC). Figure 4a shows the CV curves at 5 mV s−1 within the potential window 0 to 0.8 V. CNFs/rGO-SC shows the symmetric and rectangular CV curves suggesting the typical EDLCs behavior. CNFs/PPy-SC exhibits obviously distorted symmetrical quasi-rectangular shapes, indicating a typical characteristic of pseudocapacitance behavior systems.20 By combining the PPy and rGO, a slightly distorted symmetric and quasi-rectangular CV curve of CNFs/rGO/PPy-SC-25 is observed. Moreover, the larger integral area of the CV curves for CNFs/rGO/PPy-SC-25 (compared to CNFs/PPy-SC and CNFs/rGO-SC) indicates that CNFs/rGO/PPy-SC-25 displays the highest capacitance, which may be due to the high conductivity, the combination of EDLCs and pseudocapacitance. Galvanostatic charging and discharging curves of CNFs/ rGO/PPy-SC-25, CNFs/rGO-SC, and CNFs/PPy-SC at 0.25 mA cm−2 are shown in Figure 4b. The nonideal straight GCD curve of CNFs/rGO/PPy-SC-25 with a slight plateau region at the end of charging curves is observed, in response to the redox reactions at the electrode/electrolyte interface. The typically symmetric triangular charge/discharge profile of CNFs/rGO, and the significantly distorted triangular GCD curve for CNFs/ PPy-SC with more obvious plateau regions show the corresponding results to the above CV test. Furthermore, the detailed electrochemical properties of CNFs/rGO/PPy-SC-25, CNFs/rGO-SC, and CNFs/PPy-SC are shown in Figure 4c and d, Figures S4 and S5, respectively. All CV curves show quasi-rectangular shape with some distortion as increase of the sweep rates from 5 to 200 mV s−1. In addition, it is obvious that the distortion of the CV 11180


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Figure 5. (a) CV curves of CNFs/rGO/PPy-SC-25, CNFs/rGO/PPy-SC-50, and CNFs/rGO/PPy-SC-100 at 5 mV s−1, (b) the corresponding GCD curves at 0.25 mA cm−2, and (c) the corresponding Nyquist impedance plots. (d) CV curves of CNFs/rGO/PPy-SC-50 at different sweep rates and (e) the corresponding GCD curves at different current densities. (f) Areal capacitance of the CNFs/rGO/PPy-SC-25, -50, and -100 at different current densities.

density. More importantly, the Cs of CNFs/rGO/PPy-SC-25 is always higher than that of the other two supercapacitors at all current densities studied, which is attributed to the synergetic contribution among the three components as shown in Figure 1: (i) the 1D CNFs prevents both the restacking of rGO and the agglomeration of PPy which increases the electrolyteaccessible interface, and the hydrophilic CNFs formed pore structure also provides electrolyte ions diffusion channels; (ii) PPy, which is uniformly coated on CNFs/rGO 3D porous network structures significantly increased the pseudocapacitance of the devices; and (iii) rGO, which is embedded in the porous structures of CNFs/rGO/PPy nanohybrid aerogel, increases the electric double layer capacitance of the devices, and acts as a conductive bridge to provide many more conductive paths between different layers of nanohybrid aerogel to form good interpenetrating conductive networks. To further evaluate the electrochemical performance of the different all solid-state supercapacitors, EIS performed from 0.01 Hz to 100 kHz is employed (Figure 4f). The simulated equivalent circuit was shown in inset (ii), where Rs is the electrolyte resistance, constant phase element CPE is doublelayer capacitance considering the actual morphology of aerogel, Rct is the charge transfer resistance in the surface of electrolyte/electrode, Warburg impedance Zw reflects ion diffusion/transport at the electrode/electrolyte interface, Cp is the pseudocapacitative of PPy, and Rp is a PPy resistance. The Rs (the intercept of the Nyquist plots on the Zres axis in inset (i)) of CNFs/rGO/PPy-SC-25, CNFs/PPy-SC, and CNFs/rGO-SC is 17, 30, and 43 Ω, respectively. In the low frequency region, the Nyquist plots of CNFs/rGO/PPy-SC-25 show the more vertical shape, exhibiting better capacitance behavior. The excellent electrochemical performance of CNFs/ rGO/PPy ternary aerogel film-based supercapacitor results

from the synergistic effects of the high wettability, the good electrical conductivity, and the porous structure of aerogel. To further study the effect of PPy prepared in this way on the device, the electrochemical properties of CNFs/rGO/PPy aerogel film-based supercapacitor with different PPy content were studied. Figure 5a shows the CV curves of CNFs/rGO/ PPy-SC-25, -50, and -100 at 5 mV s−1. The shape of the CV curve becomes most symmetrical and rectangular in CNFs/ rGO/PPy-50,while with more PPy added, they show slightly distorts. The largest integral area of CNFs/rGO/PPy-SC-50 shows the highest electrode capacitance among the three supercapacitors. The GCD results at 0.25 mA cm−2 (Figure 5b) agree with the above CV result. The EIS curves (Figure 5c) also exhibit the resistance of different supercapacitors. All the EIS curves consist of quasi-semicircular parts at high frequency and linear parts at low-frequency and can be simulated to the equivalent circuit which is shown in Figure 4f (inset (ii)). The inset in Figure 5c shows that Rs of CNFs/ rGO/PPy-SC-50, -25, and -100 is 10, 17, and 12 Ω, respectively, confirming the good conductivity of the CNFs/ rGO/PPy aerogel film-based electrode (accordance with the previous conductivities analysis). The semicircle diameter of the CNFs/rGO/PPy-SC-50 is the smallest, suggesting the lowest Rct. In addition, the more vertical plot of CNFs/rGO/ PPy-SC-50 in the low frequency region, indicates better capacitance performance. The detailed electrochemical properties of CNFs/rGO/PPySC-50, and -100 are shown in Figures 5d and e, and S7. All the CV curves are nearly symmetrical and quasi-rectangularly shaped at low sweep rates and show slight distortion even when increasing the sweep rate to 200 mV s−1. All the GCD curves at different current densities (from 0.25 to 2.0 mA 11181


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Figure 6. (a) Ragone plots of CNFs/rGO/PPy-SC-25, -50, and -100, (b) Cycling stability of the CNFs/rGO/PPy-SC-25, -50, and -100 over 2000 cycles. CV curves of the CNFs/rGO/PPy-SC-50 (c) at different bending angles, and (d) after different bending cycles (90°) at 50 mV s−1.

cm−2) with a nearly symmetrical triangular shape and a small iR-drop, reveal good electrochemical capacitive behavior. The Cs data of the different supercapacitors are also calculated from the discharge curves without iR-drop, and the results are shown in Figures 5f and S6b. The maximum CA of CNFs/rGO/PPy-SC-50, -25, and -100 is 720, 400, and 590 mF cm−2 at 0.25 mA cm−2, respectively. The corresponding Cg of CNFs/rGO/PPy-SC-50, -25, and -100 for single electrode is 405, 317, and 274 F g−1, respectively. The Cs of CNFs/rGO/ PPy-SC-50 (either CA or Cg) is also much higher than that of some other CNFs-based all-solid-state supercapacitor (Table S1), such as CNFs/RGO aerogel film (207 mF cm−2; 158 F g−1),32 cotton/PPy/MWCNT (206 F g−1),57 CNFs/RGO/ CNT aerogel film (216 mF cm−2; 252 F g−1),59 cellulose/ PEDOT:PSS/MWCNTs (380 F g−1),60 and CNF/RGO/ MoOxNy aerogel film (555 mF cm−2).58 The CA decreases to 527, 288, and 385 mF cm−2 when the current density increases to 2 mA cm−2. As shown in Figure 5f, at 2 mA cm−2, the retention of the initial capacitance is 73%, 72%, and 67%, respectively for the three samples studied, displaying a good rate capability. Furthermore, the Cs of CNFs/rGO/PPy-SC-50 is always higher than that of -25, and -100. The possible reason is, with the increase of Py ratio to 50%, a more uniformly uninterrupted PPy coating was formed and CNFs/rGO/PPy50 shows high conductivity. Further increasing the Py content, the PPy coating becomes too thick and develops aggregates. This leads to increasingly difficult reversible diffusion of counter electrolyte ions, and the reduced effective utilization of electroactive PPy components. Therefore, a thinner uninterrupted PPy coating is useful to realize the full energy storage capability of PPy. Ragone plots of supercapacitors (Figure 6a)

further reveal the better performance of the CNFs/rGO/PPySC-50 than -25 and -100 under the same power density. The CNFs/rGO/PPy-SC-50 shows the highest energy density of 60.5 μWh cm−2 at a power density of 0.1 mW cm−2, which is higher than that of CNFs/rGO/PPy-SC-25 (32 μWh cm−2), and -100 (50 μWh cm−2). The cycle stability of CNFs/rGO/PPy-SC-50, CNFs/rGO/ PPy-SC-25, and CNFs/PPy-SC were carried out using a GCD at corresponding current density of 3, 2.5, and 2 mA cm−2 for 2000 cycles (Figure 6b). The specific capacitance increases in the first 1500, 700, and 500 cycles for CNFs/rGO/PPy-SC-50, CNFs/rGO/PPy-SC-25, and CNFs/PPy-SC, respectively, which may be caused by the self-activation process of PPy and increase of ion accessibility in the porous CNFs/rGO/PPy aerogel film after repetitive charge−discharge cycles. Thereafter, the specific capacitance slightly decreases during the following cycles. Generally, the PPy inside of the thick PPy coating is insert at first. The excellent hydrophilic CNFs enclosed in the PPy coating can act as internal electrolyte reservoirs. These nanoreservoirs not only greatly increase the interfacial connections between electrolyte ions and the PPy coating, but also provide enough ion transmission pathways to the interior of the PPy coating. The long charging/discharging time can make the electrolyte ions gradually diffuse deeply into the PPy coating, which ultimately increases the effective utilization of electroactive PPy. Therefore, the cycling stability increases at first by increasing the cycle number. In addition, 95%, 80%, and 64% retention of Cs for CNFs/rGO/PPy-SC50, CNFs/rGO/PPy-SC-25, and CNFs/PPy-SC are observed after 2000 cycles, respectively. These results confirm that the cyclic stability of PPy can be significantly enhanced when combining PPy with CNFs/rGO aerogel. 11182


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ACS Sustainable Chemistry & Engineering *E-mail: [email protected].

Moreover, the all-solid-state supercapacitors are flexible and can be readily bent (Figure 6c). Figure 6c exhibits CV curves of the supercapacitor at 50 mV s−1 under different bending angles (0° to 180°). The little changed CV curves suggest that the electrochemical performance is almost unaffected by bending. Furthermore, the CV curve does not change significantly even after repeatedly bending 200 cycles at an angle of 90° (Figure 6d), which confirms that the all-solid-state supercapacitors assembled from CNFs/rGO/PPy aerogel film electrodes have good flexibility.

ORCID

Zhen Shang: 0000-0002-5471-7862 Anna Ignaszak: 0000-0002-9089-2828 Shuhui Sun: 0000-0002-0508-2944 Yonghao Ni: 0000-0001-6107-6672 Notes

The authors declare no competing financial interest.

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CONCLUSIONS In summary, CNFs/rGO/PPy aerogel electrodes with thin and uniform uninterrupted PPy were prepared based on the gradual releasing of Fe3+ oxidant in situ for the PPy formation, which was achieved by using citric acid-Fe3+ (CA-Fe3+) complexes as the Fe3+ precursors and the hydrochloric acid vapor treatment. These unique structures associated with the CNFs/rGO/PPy composites lead to a high conductivity and effective utilization of electro-active PPy. Due to the porous structure, high conductivity and remarkable wettability, the CNFs/rGO/PPy-50 aerogel electrode based flexible supercapacitors exhibit very high performance, with the maximum areal capacitance of 720 mF cm−2 (405 F g−1) at 0.25 mA cm−2, good cycle stability (95% capacitance retention after 2000 cycles), and an outstanding energy density of 60.4 μW h cm−2 (0.1 mW cm−2). Moreover, the supercapacitors are flexible and are bendable without notable capacitance loss. Overall, the resultant CNFs/rGO/ PPy composites from this study offer great potential as electrode materials for flexible energy storage devices.

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ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge the financial support from the Canada Research Chairs program of the Government of Canada.

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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/acssuschemeng.9b00321. Preparation of CNFs suspension, preparation of GO suspension, electrochemical characterization; Figure S1, schematic diagram and photograph for the preparation of all solid-state supercapacitors; Figure S2, nitrogen adsorption/desorption isotherms and BJH pore size distribution (inset) for CNFs/rGO/PPy-50 aerogel; Figure S3, dynamic water contact angle measurements for CNFs/rGO/PPy-50 aerogel film; Figure S4, (a) CV curves of CNFs/rGO-SC at different sweep rates, (b) the corresponding GCD curves at different current densities; Figure S5, (a) CV curves of CNFs/PPy-SC at different sweep rates, (b) the corresponding GCD curves at different current densities; Figure S6, gravimetric capacitance of (a) the CNFs/rGO-SC, CNFs/PPy-SC, and CNFs/rGO/PPy-SC-25 and (b) CNFs/rGO/PPy-SC-50, -25, and -100 at different current densities; Figure S7, (a) CV curves of CNFs/ rGO/PPySC-100 at different sweep rates, (b) the corresponding GCD curves at different current densities; and Table S1, electrochemical performance of some graphene and CNFs-based solid-state (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 11183


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