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Interface Confined High Crystalline Growth of Semiconducting Polymers at Graphene Fibers for High Performance Wearable Supercapacitors Suchithra Padmajan Sasikala, Kyung Eun Lee, Joonwon Lim, Ho Jin Lee, SungHwan Koo, In Ho Kim, Hong Ju Jung, and Sang Ouk Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05029 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017
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Interface Confined High Crystalline Growth of Semiconducting Polymers at Graphene Fibers for High Performance Wearable Supercapacitors Suchithra Padmajan Sasikala, Kyung Eun Lee, Joonwon Lim, Ho Jin Lee, Sung Hwan Koo, In Ho Kim, Hong Ju Jung, Sang Ouk Kim*
National Creative Research Initiative Centre for Multi-Dimensional Directed Nanoscale Assembly, Department of Materials Science & Engineering, KAIST, Daejeon 34141, Republic of Korea
KEYWORDS: graphene, fiber, supercapacitor, conducting polymer, interface
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ABSTRACT We report graphene@polymer core-shell fibers (G@PFs), composed of N and Cu co-doped porous graphene fiber cores uniformly coated with semiconducting polymer shell layers with superb electrochemical characteristics. Aqueous/organic interface confined polymerization method produced robust highly crystalline uniform semiconducting polymer shells with high electrical conductivity and redox activity. When the resultant core-shell fibers are utilized for fiber supercapacitor application, high areal/volume capacitance and energy densities are attained along with long term cycle stability. Desirable combination of mechanical flexibility, electrochemical properties and facile process scalability makes our G@PFs particularly promising for portable and wearable electronics.
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Fiber supercapacitors hold a great promise for wearable energy devices owing to its inherent one-dimensional form factor that can effectively adapt to complex deformation modes where the rigid bulk or planar counterparts may fail.1 Graphene fiber (GF) is a promising component for such purposes along with the synergistic ideal combination of exceptional graphene properties such as large surface area, high electrical and thermal conductivity, low density and high mechanical stability.2-5 While pristine GFs are electrically conductive without high electrocatalytic activity, redox capability can be imparted at GF surfaces by incorporating pseudo capacitive inorganic materials or semiconducting organic polymers.6-11 In particular, organic semiconducting polymers with inherent low density, mechanical flexibility, and high electronic and optoelectronic properties are anticipated to offer versatile benefits for flexible electronics when combined with GFs.12-14 GFs can be obtained by facile wet-spinning from graphene oxide liquid crystal (GOLC) followed by chemical/thermal reduction.15-18 Notably, incorporation of organic semiconductors into GFs is very difficult due to the unavoidable damage of organic materials during the GO reduction process. Accordingly, there have been only a few attempts for graphene-polymer composite fibers thus far, which principally rely on the solution phase mixing of GOLC with polymers.8-11, 19
Either direct wet-spinning from this solution mixture and subsequent chemical reduction, or
hydrothermal treatment of this solution mixture in closed tube vessel produced fibers composed of reduced GO sheets (rGO) and organic semiconductors. In these approaches, nonetheless, inhomogeneous physical mixing of GO and polymer, inefficient reduction of GO, and low crystallinity of polymers encapsulated by dielectric surfactants (eg. PEDOT-PSS) commonly led to the final fiber properties far lesser than the expected performance by such a combination. Moreover, how to avoid the chemical damage of organic semiconductors during the harsh 3 ACS Paragon Plus Environment
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chemical/thermal reduction conditions and how to ensure the overall process scalability remain formidable challenges.8-11,19 The limited porosity and low surface area of these fibers also lead to the inherently limited electrochemical performance.
In this work, we report idealized graphene/organic semiconducting polymer hybrid core/shell fibers (G@PFs) with high electrical conductivity and good electrochemical performance based on a straightforward and reliable synthetic route. Our protocol exploits highly confined aqueousorganic interface restricted polymerization of semiconducting polymers at Cu and N co-doped GF surfaces. The heteroatom dopant sites at GFs not only allow the manipulation of physical properties but also facilitate good wettability towards electrolytes.20,21 The GF core has high surface area and contains macropores that act as electrolyte reservoir for supercapacitor electrode applications. The restricted polymerization of organic semiconductors at aqueous/organic interface confines a slow polymer chain growth at GF surface and thereby yields highly crystalline semiconducting polymer shells. These features enable our G@PFs to accomplish high electrical conductivity (387.1 S/cm), specific capacitance (417.9 F/cm3) and energy density (7.0 mWh/cm3) among carbon based fiber devices ever reported thus far (Table S1).
RESULTS AND DISCUSSION
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Figure 1. Schematic illustration displaying the fabrication steps for G@PFs
In our synthetic strategy (Figure 1), GO fibers (GOFs) are wet-spun from GOLC into a coagulation bath containing 5 wt.% Cu(II) ions in ethanol/water mixture (Figure S1a). The airdried GOF shows a solid ribbon-like morphology with highly aligned wrinkles, which is typical of GOFs (Figure S1b). In order to generate porous heteroatom-doped GFs, a chemo-thermal reduction strategy is employed. Firstly, we have compared the chemical reduction effects from hydroiodic acid (HI) and hydrazine vapour on the GOFs, both of which are then followed by a rapid thermal reduction at 700 ◦C under H2/NH3 mixture gas (6:4 sccm). The resulting GFs exhibited a rapid increase of fiber diameter. Porous morphology is generated while reductive byproduct gas is released (Figures S1c-g).22,23 The macroporous morphology will be beneficial in fibers intended for supercapacitor electrodes, because the pores provide open channels for the infiltration of electrolytes, and thus whole volume of fibers can be utilized for electrochemical
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reaction. The surface area is also found to increase from 39.1 to 1174.4 m2/g during this reduction step (more details in S1: Surface area measurement and Figure S2a,b). Notably, the HI treated fiber shows a conductivity of 148.6 S/cm after thermal treatment, considerably higher than hydrazine treated counterpart (39.4 S/cm), which indicates more efficient de-oxygenation and П-conjugation restoration by HI treatment (Figure S3a-d). Comparative experiments with GFs obtained from Ca(II) coagulating ion and HI and hydrazine reduction (both followed by thermal treatment) show the conductivity of 98.7 and 15.7 S/cm, respectively. Apart from the improved electrical conductivity, the emerging 2D peak and increased ID/IG value compared to GOF in the Raman Spectrum further validated the reduction process (Figure S4a-d). The increase of ID/IG is usually explained as an increase in the number of sp2 domains upon reduction.24 While NH3 in the reduction atmosphere induced 3.8 at.% of Ndoping, the Cu(II) ions integrated into the GO fiber during wet spinning process concurrently get doped into GF in H2 atmosphere, which is evident from X-ray photoelectron spectroscopy (XPS) that detects 2.1 at.% of Cu at 932.62 eV (Figures S3a-c). Notably, presence of Cu(0) can also be detected in X-ray diffraction (XRD) analysis of GFs originally spun from a higher concentration of Cu(II) coagulating ions (>10 wt.%) after the thermal reduction step under H2 (Figure S3d). Nevertheless, a possibility of concurrent doping of adsorbed Cu particles at the graphene crystal lattice cannot be ignored. The Cu, and N co-doping may be the reason for higher ID/IG value in the Raman analysis of GFs after thermal reduction (Figure S4c,d).25
The noticeable conductivity difference between fibers made from Cu(II) and Ca(II) as coagulating ions proposes that Cu-doping effectively enhances the electrical conductivity. Cu is known to offer an n-type doping and causes a shift in the Fermi level of graphene from Dirac 6 ACS Paragon Plus Environment
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point to conduction band.26 Importantly, the Cu and N-doping increase the electrolyte wettability of graphene sheets. As a consequence, the electrochemical surface area and electric double layer capacitance are greatly enhanced. In addition, these dopant sites provide pseudocapacitance and thus the electrochemical performance is effectively improved for supercapacitor application. Noteworthy that the electrical conductivity of our GFs is comparable to the previously reported GFs (Table S2), but inferior to the high temperature (3000 °C) treated GFs and fibers consisting of compactly stacked giant graphene sheets. Such compact stack GFs may detrimental to capacitive performance due to limited electrolyte accessibility inside the fibers. The macroporous structure of our GF is beneficial in this respect. In our further experiments we have used Cu and N co-doped macroporous GFs, obtained after the HI and thermal reduction, as a standard sample.
As mentioned above, previous solution mixing methods for polymer-graphene hybrid fiber systems had suffered from inhomogeneous mixing and insulative surfactant coverage (eg. insulating PSS template for PEDOT). Electrochemical polymerization at GF surface could be an alternative route, but also reveals limitations in terms of process scalability and compositional uniformity along the long fiber structure.27,28 At the surface of our Cu and N –codoped GFs, conventional in situ polymerization of semiconducting polymers (routinely practiced for fiber/film synthesis), such as poly[3,4-ethylenedioxythiophene] (PEDOT), polypyrole (PPy) and polyaniline (PAni) results in inhomogeneous and rapid growth of low crystalline polymers (Figure S5a-f).29 Accordingly, the electrical conductivity of the resultant fiber is decreased to 62 wt% in the hybrid fiber) leads to a minor decrease of the surface area from 1174.4 m2/g (of GF) down to 1111.8, 1096.1, 1088.3 m2/g for G@PEDOT, G@PPy and G@PAni, respectively (Figure S2). The slight decrease in surface area can be accounted as polymer growth on the surface pores of graphene fiber.
High resolution transmission electron microscopy (HRTEM) coupled with energy dispersive Xray spectroscopy (EDS) is used to characterize the surface polymer layer (Figure 3). Typical (100) lattice plane of PEDOT (d-spacing: 0.584) has been resolved for G@PEDOT shell in HRTEM and selected area electron diffraction (SAED), verifying a highly crystalline nature (Figure 3a-c). 9 ACS Paragon Plus Environment
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Figure 3 (a) TEM image of G@PEDOT. (b) TEM image of localized PEDOT platelets as identified by EDS spectra (inset). (c) HRTEM of PEDOT platelet displaying its lattice structure. Inset is the electron diffraction pattern (SAED) of the same area, (d,f) TEM and (e,g) HRTEM images displaying lattice structure of PPy and PAni, respectively. SAED patterns of PPy and Pani are on the insets of (e) and (g), respectively. The samples are prepared by cryo-cutting the fractured surface of G@P fiber and depositing in the holey carbon grid. (h) XPS survey of G@PEDOT, G@PPy and G@PAni fibers.
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SAED patterns of G@PPy and G@PAni also exhibit well-defined crystalline patterns of PPy and PAni, respectively (Figures 3d-g). The XPS analysis of G@PEDOT fiber displayed Cl2P and S2P spectra (more details with Figure S7). The deconvoluted high resolution S2P spectra displayed S+2P peak which represent partially oxidized S, which is expected to be charge balanced by ClO4- ions. The doping level was calculated as 31.7% from the areal ratio of Cl2P to S2P peaks. Such a high doping level is in agreement with the crystallinity and high conductivity behaviour of PEDOT.33,34 However similar calculation of doping level for G@PPy or G@PAni fibers were not possible in this study since the N contribution from N doped graphene sheets overlap with the N contribution of PPy and PAni. Nevertheless, EDS (inset in Figure 3b) and XPS survey (Figure 3h) detected insignificant level of Fe contamination, and significant Cl/ClO4-doping for all samples, which reflects positive charge accommodation in the polymer layers. The doping of semiconducting polymer layer may additionally improve the charge mobility, which may contribute to the exceptionally high electrical conductivities (387.1, 282.7 and 259.4 S/cm for G@PEDOT, G@PPy and G@PAni fiber, respectively).33,34Figure 4a presents the utilization of G@PEDOT fiber as a circuit wire for light emitting diode (LED), illustrating the high electrical conductivity, which also makes it possible to use G@PFs as electrochemical capacitor (EC) electrodes without additional conductive additives. For the electrochemical characterization of G@PF electrodes, micro fiber devices are assembled by aligning two identical G@PFs (length~ 1.5 cm, diameter ~70-80 µm, mass ~0.144 mg, density ~1.5 g/cm3) in parallel and coating with PVA/H2SO4 gel electrolyte to achieve a symmetrical solid state supercapacitor structure (Scheme S1). The devices are super-thin with a maximum
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thickness of 95% at various current densities from 0.2 to 0.5 A/cm3. According to the band theory, the origin of pseudocapacitance in semiconducting materials is correlated to the electron transfer from their conduction band. Because, in electrically conductive polymers with conjugated double bonds, the redox active sites interact with each other resulting in the merging of their energy states into a broad band with negligibly small differences between the neighbouring states. As a result, continuous electrons transfer to and from energy states in this broad band can be realized over a wide range of potentials. Thus constant and reversible charge propagation is ensued in the electrode and hence a rectangular CV and triangle shaped galvanostatic charge discharge profiles, typical of ideal double layer capacitors can be observed for high performing hybrid materials with electrically conductive pseudocapacitive polymers.7,36 The area of rectangular curve in CV and discharge time in the CCD profiles followed the order of G@PEDOT > G@PPy > G@PAni. The specific capacitance of GF and G@PFs is calculated from CV and galvanostatic charge discharge (CCD) profiles (Figure 4b-d) and is listed in Table 1.
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Table 1 Eelectrical/electrochemical properties of our GF and G@P core shell fibers Single fiber electrode Without electrolyte
Fibers
Conductivity (S/cm)
G
G@PAni
G@PPy
G@PEDOT
148.6
259.4
282.7
387.1
Cf,V (F/cm3)
(2electrode)
29.1
(3electrode )
32.3
(2electrode)
294.3
(3electrode )
301.5
(2electrode)
328.7
(3electrode )
349.7
(2electrode)
417.9
(3electrode )
442.3
Cf,g (F/g)
Supercapacitor (including two fibers, gel electrolyte and space between the fibers) With PET substrate is not included in the electrolyte calculation ES,A Cf,V CS,V ES,V CS,A (µWh/ (F/cm3) (F/cm3) (F/cm2) (mWh/ cm2) cm3)
39.9
21.2
5.6
0.011
0.5
0.98
396.8
155.2
39.2
0.109
3.5
9.7
462.9
184.1
47.6
0.132
4.2
11.7
694.4
263.1
78.8
0.194
7.0
17.2
Note that, capacitance calculation involving a low mass loading as in the case of low density graphene fibers leads to an exceptionally high gravimetric capacitance which is not representative of actual capacity of the material in real practice. Instead volumetric/areal capacitance values are preferred to mass specific capacitance in order to assess the actual feasibility of fiber supercapacitors.37 The G@PEDOT fiber based supercapacitor displays an ultrahigh volumetric capacitance (considering the entire device volume, Cs,v) of 78.8 F/cm3 at a current density of 0.5 A/cm3. The volumetric capacitance of single fiber electrode (Cf,v) is measured to be either 263.1 F/cm3 (based on the entire volume of single fiber including the surrounding electrolyte) or 417.9 F/cm3 (based on the volume of single fiber only). The volumetric capacitance of our G@PEDOT fiber is either comparable to or better than previous literature values for fiber supercapacitors (Table S1). Similarly, G@PPy and G@PAni fiber electrodes also reveal reasonably high Cf,v (Table 1, Figure 4d). The Cf,v of our polymer-free 14 ACS Paragon Plus Environment
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GFs (21.2 F/cm3 with electrolyte and 29.1 F/cm3 without electrolyte) is also comparable to the previously reported graphene only fibers (Table S1). The Cu and N-dopant sites at GF may incorporate polar character and thereby enhance the wettability with electrolytes. Additionally, pseudocapacitance can be also incorporated by such a heteroatom doping.26 Nonetheless, the measured capacitance value for GF is still low considering our large surface area macroporous structure, presumably due to the low accessibility of electrolyte caused by hydrophobicity of graphene sheets (Figure 5a).38 By contrast, G@PEDOT fiber demonstrated one order of magnitude (14 times) increase in the specific capacity from GF. It can be elucidated that the hydrophilic conductive polymer shell (Figure 5b) not only induces reverible redox reaction for pseudocapacitance, but also facilitates the electrolyte accessibility to the interior volume of fiber (Figures 5c-f). The polymers grown on the surface pores of graphene fiber provide an open channel for the electrolyte to infiltrate into the internal macropores of the fiber. The electrolyte filled macropores of G@PEDOT can act as an electrolyte reservoir and optimizes the ionic diffusion in the interior graphene sheets and maximize the electrochemical surface area of graphene.39 Further to scrutinise the effect from the porosity of fibers on the electrochemical performance, we prepared G@PEDOT reference fibers without macropores (Figure S10a,b). A direct comparison of CV profiles (at 20 mV/s) confirmed the significantly high capacitance of porous G@PEDOT compared to nonporous G@PEDOT fiber (Figure S10c). From these results, we can elucidate that the high capacity of G@PEDOT fiber is attributed to the synergistic effects from redox reaction of PEDOT and electrical double layer capacitance from inner graphene core afforded by macroporous structure. In order to display an ideal supercapacitor behaviour, a faster doping/dedoping rate of conductive polymer is essential to provide a constant current during the redox reaction. While the 15 ACS Paragon Plus Environment
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doping/dedoping process involves both charge (electrones/holes) and ion transport, the ion transport is generally slow to be the rate limiting step.40 Since the polymer shell is very thin (2±1µm) at G@PF, the diffusion pathway is very short for counter ions and thus doping/dedoping process is accelerated for minimal kinetic limitation.40 In addition, such a thin layer of conducting polymers may assist the electrolyte ions to quickly reach the internal area of graphene core, and thus result in the increase of double layer capacitance contribution from graphene core.41
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Figure 5. Water contact angle for thin film made of (a) G and (b) G@PEDOT. SEM images displaying (c-d) cross sectional view of GFs with increasing magnification, displaying the air filled pores. The surface is seen covered by the PVA/H2SO4 aqueous electrolyte, but the electrolyte could not diffuse effectively in to the internal macropores because of its hydrophobic nature. In contrast (e-f) the cross section of G@PEDOT fiber with increasing magnification showed PVA/H2SO4 aqueous electrolyte infiltration into the pores. (g) The side view of G@PEDOT fiber covered in electrolyte. All the SEM imagings are carried out (without any gold/platinum sputtering) immediately after the electrochemical characterization of fibers. The fibers are recovered from solid state supercapacitor device and cross sectioned.
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We also tested GF and G@PFs in a three electrode cell using H2SO4 electrolytes for the determination of electrochemical-specific material characteristics (More details in S2: Three electrode Test and Figure S11). While the presence of redox activity in GFs can be attributed to the residual oxygen functionalities and Cu, N-dopant sites, the redox peaks of G@PFs principally results from the pseudocapacitive polymer shells as well as doped graphene cores. The Cf,v of G@PEDOT in the three electrode cell is increased from 417.9 F/cm3 (of two electrode test) to 442.3 F/cm3. Similar levels of minor increment (3.5 to 7.5 %) are also observed for G@PPy, G@PAni and GF (Table 1), which are in accordance with the previous literature.37,42 Despite the predominant faradic capacitance, the G@PFs maintain nearly rectangular CV curves. The comparable per-electrode specific capcitance in both two and three electrode tests signifies their redox bipolar activities as both cathodes and anodes.
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Figure 6. (a) Electrochemical impedance spectra with increasing frequencies from 50 kHz to 100 mhz for fiber supercapacitors. (b) Ragone plots for G@PEDOT, G@PPy and G@PAni fiber supercapacitors (c) Comparison of energy density of G@PF supercapacitor (denoted by dotted circle) with 4V/500 µAh thin film lithium battery (0.0003-0.01Wh/cm3),commercially available supercapacitors (
