Fabrication of Solid-State Asymmetric Supercapacitors Based on Aniline Oligomers and Graphene Electrodes with Enhanced Electrochemical Performances.

This work describes solid-state asymmetric supercapacitors (ASCs), composed of aniline oligomers as a cathode, nonoxidative graphene sheet as an anode, and polyvinyl alcohol-potassium hydroxide gel as an electrolyte. The synergistic effects resulting from the combination of aniline oligomer and graphene sheet have greatly enhanced the electrical and electrochemical performance of ASCs. The electrical and electrochemical properties of ASCs were highly dependent on the protonation levels of aniline oligomers including aniline tetramer, aniline trimer, and aniline dimer. The aniline tetramer with an appropriate chain length provided higher carrier transport within the anode compared to that of the aniline dimer and trimer. The water-dispersible graphene (WDG) sheets greatly enhanced structural stability and cycle life of aniline tetramers by alleviating swelling, chain scission, and shrinking of the aniline tetramers. The ASC composed of aniline tetramer/WDG sheet exhibited high areal capacitance (62.2 mF/cm2), volumetric capacitance (207.4 F/cm3), and good cycling stability (97.2% after 2000 cycles and 90.4% after 10 000 cycles). The strategy presented in this work is simple and facile, which would give an insight into efficient ways for applying aniline tetramers and graphene sheet for state-of-art electronic applications.

Introduction

With rapidly growing energy needs, clean energy sources including solar, wind, and tidal power have attracted a great deal of interest as next-generation and renewable energy technologies, and diversified technologies for improving the energy efficiency have been greatly developed. However, the technology to efficiently store and reuse clean energy remains a challenge.[1,2] Therefore, it is an urgent requirement to develop and optimize a high-performance energy storage device capable of storing large amounts of electric charges. Among various energy storage devices, supercapacitors are one of the most promising energy storage devices because of their rapid charging process, high power density, excellent low-temperature performance, and long life cycles.[3−9] Such fascinating virtues of supercapacitors have stimulated the future growth of the high-performance supercapacitors with improved energy density, power density, cycle life, and cost-effectiveness. Supercapacitors can be classified as electrical double layer capacitors (EDLCs) and pseudocapacitors depending on the charge storage mechanism.[3−9] In the EDLCs, electric charges are stored through quick adsorption/desorption of electrolyte ions on the surface of carbon materials.[6−8] Pseudocapacitors store electric charges through the reversible oxidation/reduction processes of the metal oxides and conducting polymers (CPs). The EDLCs provide high operation voltage, high power density, and excellent cycle life, while the EDLCs suffer from low energy density because of the limited redox reactions of carbons.[9] Pseudocapacitors provide higher capacitance compared with the carbons, whereas the cycle life and structural stability of pseudocapacitors are inferior to those of the EDLCs.[5−9] During the reversible insertion/desertion of electrolyte ions within the metal oxides and CPs, these materials are usually damaged or abolished. Moreover, the operation voltage of pseudocapacitors is limited to less than 1.23 V; the power density of the pseudocapacitors becomes lower than that of the EDLCs.[3] Asymmetric supercapacitors (ASCs) can take advantage of the synergistic effects from the EDLCs and pseudocapacitors, thereby offering higher output compared to the symmetric supercapacitors. However, the cycle life and potential window of ASC are still inferior to those of the symmetric supercapacitors based on EDLC. For these reasons, it is necessary to create and design ASCs with excellent cycle stability and high operating voltage.[5−9]

Among the numerous CPs, PANI is one of the most promising candidates for pseudocapacitors because of its fascinating virtues, such as facile synthesis, excellent redox sensitivity, and good electrical conductivity up to 103 S/cm.[10,11] Although poly(3,4-ethylenedioxythiophene) (PEDOT) has higher conductivity compared to PANI, PANI offers faster oxidation/reduction reactions and better electrochemical activity. Thus, it is certain that PANI and aniline derivatives are more suitable for energy storage devices than the PEDOT.[9,30,31] Because of such excellent electrochemical and electrical properties, PANI can store large amounts of electric charges. Therefore, various studies on the synthesis of PANI materials for supercapacitors have been carried out through chemical oxidative polymerization and electrochemical polymerization.[10−14] However, PANI usually suffers from volumetric degradation and scission upon exposure to electrolyte ions, resulting in limited cycle life of the supercapacitors. Furthermore, the actual capacitance of PANI is lower than the theoretical capacitance because only PANI near the electrolyte ions is involved in the charging process.[9−11,14] In particular, aniline tetramers provide electrical and electrochemical properties comparable to the PANI, while the aniline tetramer is less susceptible to volumetric degradation, chain scission, and shrinking caused by long chain swelling during the charging and discharging processes.[15,16] Thus, extensive studies have been conducted to synthesize aniline oligomers including dimers, trimers, tetramers, and pentamers to overcome the limited cycle life of PANI.[15−19]

Graphene, a single layer carbon material composed of sp2-hybridized carbons, provides high surface area, good electron mobility, mechanical strength, and chemical stability.[5−8,20] Because of its outstanding properties, graphene is considered as the most attractive candidate to enhance the cycle life and electrical conductivity of PANI and aniline tetramer. In recent years, great efforts have been taken to fabricate supercapacitors based on graphene materials and aniline derivatives including PANI and aniline tetramers.[11,13,14,16] The capacitance of graphene/PANI composite paper was 763 F/g at 1 A/g, and the capacitance of composite paper could remain 82% after 1000 cycles.[21] Lee et al. reported that the aniline tetramer/graphene oxide composite with a specific capacitance of 769 F/g at 1 A/g and capacitance degradation of 2.3% after 2000 cycles.[16] Despite such progress, the cycling stability of aniline/graphene composite-based supercapacitors still requires further improvement compared to the carbon-based EDLCs.[11,13,15,16,21] Furthermore, most aniline/graphene composite-based supercapacitors were demonstrated with three electrode supercapacitors, which cannot be practical in real life.[1,3−11,16,21] Accordingly, appropriate selection of the cell type has become an important issue to ensure the practical application of aniline/graphene composite-based supercapacitors. In addition, the operating voltage of supercapacitors can be enhanced by selecting the asymmetric cells instead of using the symmetric cell.[5−8,22−27] In the ASCs, the graphene sheet as the anode stores electric charges through the EDLC mechanism, while the cathode, which consists of PANI or aniline tetramers, stores electric charges through the oxidation/reduction reaction. Thus, it is necessary to find out effective strategies for constructing the hybrid ASC with high specific capacitance and good reliability.

Herein, this work describes the fabrication of hybrid ASCs based on aniline oligomers as a cathode, water-dispersible graphene (WDG) sheet as an anode, and polyvinyl alcohol–potassium hydroxide (PVA–KOH) gel as an electrolyte. Aniline oligomers including tetramer, trimer, and dimer acted as pseudocapacitors to store electric charges by reversible oxidation/reduction reactions with the PVA–KOH gel electrolyte. The different performances of ASC were mainly due to the level of protonation and the chain length of the aniline oligomer. Therefore, this work mainly focuses on identifying the optimal aniline oligomer to enhance the electrical and electrochemical performances of ASCs. WDG sheets, which were exfoliated by the electrochemical method, were highly dispersible with water without any polymeric binder, resulting in better conductivity compared with the conventional reduced graphene oxides (RGOs).[28,29] These WDG sheets not only act as the EDLCs but also improve the cycle life of ASCs by alleviating the volumetric degradation, scission, and shrinking of aniline oligomers. Because of the synergistic effects arising from the combination of aniline oligomers and graphene sheets, the electrical and electrochemical performances of ASCs were significantly enhanced. The ASC employing aniline tetramers exhibited higher areal capacitance (CA = 62.2 mF/cm2 at 83 mA/cm3), volumetric capacitance (CV = 207.4 F/cm3 at 83 mA/cm3), and cycle life (90.4% after 10 000 cycles) compared to both aniline trimers and aniline dimers.

Results and Discussion

Figure illustrates the structure of an ASC, which is composed of current collectors, electrode materials, and electrolytes. As a current collector, a stainless steel foil was chosen in the ASC. Aniline oligomers, such as tetramer, trimer, and dimer, were prepared by chemical oxidation reaction, and these oligomers were used as cathode materials in the ASC. Protonation of aniline oligomers using hydrochloric acid generates positive charges in the repeating units of the aniline oligomer chain so that the protonated aniline oligomers tend to accept electrons from graphene sheets with high electron density. Therefore, the protonation level of aniline oligomers were a very crucial factor affecting the electron transfers to an anode composed of graphene sheets.[32−34] To ensure good dispersion of the aniline oligomers, the synthesized oligomers were fabricated into the pastes with the aids of Teflon and activated carbons (ACs). The pastes of aniline oligomers were coated onto the stainless steel substrates using the spin coating method; the resulting films were used as cathode materials in the ASC. The aniline oligomers store electric charges through the redox reactions with electrolyte ions. The performances of ASCs are mainly dependent on the type of aniline oligomers. Thus, the following paragraphs primarily focus on the processes of finding an aniline oligomer that can achieve the optimized performance of ASC. WDG pastes were prepared by the nonoxidative and electrochemical exfoliation process, which enables the stable dispersions of graphene sheets without any polymeric binders.[28,29] The WDG sheet plays an important role as the EDLC to store energy through ion adsorption/desorption. The capacitance losses from swelling and breakage of the aniline oligomers are significantly minimized by the WDG sheets. Moreover, the large surface area of WDG allows more electrolyte ions to be adsorbed and desorbed on the electrode surface. The WDG sheets were deposited onto the stainless steel foil, and as-prepared WDG electrode acted as an anode to transfer electrons to a cathode consisting of the protonated aniline oligomers. During the charging process, electrons returned from a cathode to an anode. The PVA membrane improves the reliability of the ASC by preventing evaporation of the KOH electrolyte. Considering these facts, the synergistic effects from the aniline oligomer, WDG sheet, and PVA–KOH electrolyte lead to enhanced electrochemical performances of the ASC.

Figure 1

Illustration for Fabricating ASC based on aniline oligomers, graphenes, and PVA–KOH electrolytes.

Figure represents the field-emission scanning electron microscopy (FE-SEM) images of the electrodes of aniline oligomers. The average particle size of the dimer, trimer, and tetramer was 140–360, 270–628 nm, and 700–890 nm, respectively (Figure a–c). The results imply that size of the oligomer samples increased with increasing molecular weight of the aniline oligomers. In addition, the size distribution of the aniline tetramer was narrower than the size distribution of the dimer and trimer samples. It is assumed that the larger size of tetramers improves the interparticle connectivity, resulting in better electron transfers within the cathodes.[28−31]Figure S1 shows the FE-SEM image of WDG sheets used as anode materials in the ASC. The sizes of WDG sheets ranged from 2 to 5 μm, and these WDG sheets were well-dispersed on the current collector. The successful formation of WDG sheets was proven by Raman spectroscopy (Figure S2). Several distinctive peaks for WDG were observed at 1349, 1575, and 2661 cm–1, corresponding to D band, G band, and 2D band, respectively.[28−31] The D band originates from the breathing mode of the sp2 carbon atoms, which is caused by structural defects. The G band is attributable to the first-order scattering of E2g vibrational mode of sp2 carbon atoms. Moreover, a broad 2D band at around 2661 cm–1 indicates that the WDG is composed of few-layered graphene sheets. As the disordered structure in the graphene sheet increases during the extensive oxidation of graphite and the reduction process of GO, the size of the sp2 domain in the carbon materials decreases. Thus, the intensity ratio of the D to the G band (ID/IG) of RGO (1.23) was higher than that of WDG (0.39). The oxidation of graphite causes the formation of sp3 carbon atoms, resulting in the red shift of the G peak in the spectrum of RGO. Furthermore, the reducing agent changes in the electronic structure of GO, leading to an increased wavenumber of the G peak of RGO. For these reasons, the D band (1355 cm–1) and G band (1580 cm–1) of the RGO are shifted toward higher wavenumbers than that of the WDG sheet. Considering the Raman spectra of WDG and RGO, it was clear that the WDG sheet was different from the conventional RGO sheet.

Figure 2

FE-SEM images of electrode materials: (a) aniline dimer, (b) aniline trimer, and (c) aniline tetramer.

In order to confirm the chemical structures of the aniline oligomers, the hydrogen-1 nuclear magnetic resonance (1H NMR) solution spectra of the samples in the fully reduced state are shown in Figure . In every spectrum of the samples, the peak for DMSO-d6 was found at 4.6–4.8 ppm. The signals of the NH2 group of dimer, trimer, and tetramer appeared at 4.77, 4.66, and 4.63 ppm, respectively.[18,19] In the spectrum of the dimer, the peaks at 6.52–6.55, 6.59, 6.75–6.84, 7.08, and 7.46 were attributed to the H1, H5, H2 + H3, H4, and Hb protons, respectively (Figure a).[18,19] In the spectrum of the trimer, the peaks at 6.48–6.51, 6.62, 6.75–6.79, 6.81–6.84, 6.88–6.91, 7.10, and 7.25 were attributed to the H1, H7, H2 + H3, H4, H5, H6, and Hb protons, respectively (Figure b).[18,19] In the spectrum of the tetramer, the peaks at 6.47–6.50, 6.64, 6.74–6.77, 6.85–6.87, 6.92–6.95, 7.08–7.11, 7.14–7.16, 7.44, and 7.69 were attributed to the H1, H9, H2 + H3, H4+5+6, H7, H8, Hb, Hc, and Hd protons, respectively (Figure c).[18,19] Given these facts, the observed NMR spectra are in good agreement with the predicted formula of the synthesized dimer, trimer, and tetramer.

Figure 3

1H NMR spectra in DMSO-d6 solution of (a) aniline dimer, (b) aniline trimer, and (c) aniline tetramer in the leucoemeraldine oxidation state.

X-ray photoelectron spectroscopy (XPS) was utilized to investigate changes in the elemental compositions and doping states of the aniline oligomers (Figure ). Figure a represents the fully scanned XPS patterns of the dimer, trimer, and tetramer after HCl doping. Every spectrum showed distinctive peaks at 284, 400, and 198 corresponding to C(1s), N(1s), and Cl(2p), respectively.[11,30,31] The peaks for C(1s), N(1s), and Cl(2p) are attributed to the PANI doped by aqueous HCl solutions. The results indicate that the NO2 group of the aniline oligomers was successfully converted into the amine group. Table represents the elemental composition of HCl-doped aniline oligomers obtained from the XPS analyses. The Cl/N ratios of aniline oligomers were almost close to 0.5, suggesting that the oligomers are appropriately doped.[11,30]Figure b–d shows the N(1s) core spectra of the dimer, trimer, and tetramer doped by aqueous HCl solutions. The spectra of aniline oligomers represented three peaks at 399.0–399.6, 400.1–400.6, and 401.3–402.0 eV, corresponding to −NH– (neutral amine nitrogen), −NH•+ (polaron), and =NH+ (bipolaron), respectively.[11,30] The ratio of N+ species (sum of −NH•+ and =NH+) to N species (sum of −NH–, −NH•+ and =NH+) (N+/N ratio) was calculated to evaluate the doping levels of aniline oligomers. The N+/N ratio was 0.32, 0.66, and 0.77 for the dimer, trimer, and tetramer, respectively (Table ). The proportion of positively charged nitrogen of the −NH•+ and =NH+ groups increased with the molecular weight and particle size of the oligomers. The results indicate that the tetramer has less structural defects and more charge carriers than the dimer and trimer.[11,30] In addition, it is assumed that the tetramer with a larger particle size allows the conductive areas in the tetramer to become more connected. Table summarizes the electrical conductivities of WDG and aniline oligomers. Conductivity of the WDG was about 44 S cm–1, indicating that the WDG offers sufficient currents at the anode. Conductivity of aniline oligomers (given in S cm–1) increased in the following order: dimer (7.2 × 10–3) < trimer (3.4 × 10–2) < tetramer (2.5 × 10–1). Although the conductivity of aniline tetramer was lower than that of the WDG, it was clear that the tetramer exhibited better current collection than the trimer and dimer. These conducting values were consistent with the XPS results shown in Figure b–d, indicating that the electrical conductivity of aniline oligomers was directly affected by the level of protonation. Given these facts, the higher doping level of the aniline tetramer enables extended conduction paths for delocalizing more electrons, thereby improving the conductivity of the cathode in the ASC.[10,11,30,31]

Figure 4

(a) Fully scanned XPS spectra of aniline dimer (red), aniline trimer (blue), and aniline tetramer (green) after doping with HCl. XPS core spectra in the N(1s) region of (b) aniline dimer, (c) aniline trimer, and (d) aniline tetramer.

Table 1

Elemental Composition of HCl-Doped Aniline Oligomers Obtained from the XPS Analyses

	atomic ratio (%)
samples	C	N	Cl	Cl/N
dimer	75.80	16.45	7.75	0.47
trimer	75.50	16.34	8.16	0.50
tetramer	74.45	16.83	8.72	0.52

Table 2

Peak Analyses of XPS Core Spectra in the N(1s) Region of HCl-Doped Aniline Oligomers

	peak ratioa
samples	–NH–	–NH•+	=NH+	N+/N ratioa
dimer	0.68	0.26	0.06	0.32
trimer	0.34	0.48	0.18	0.66
tetramer	0.22	0.49	0.29	0.77

Values were calculated using the N(1s) core spectra of the samples.

Table 3

Conductivities of WDG and HCl-Doped Aniline Oligomers

samplesa	conductivityb
WDG	4.4 × 101
dimer	7.2 × 10–3
trimer	3.4 × 10–2
tetramer	2.5 × 10–1

Samples were fabricated as 2 μm thick thin films, which were deposited on the glass substrates.

Values were calculated using the 4-point probe method.

To identify the effects of aniline oligomers on the electrochemical performances of the ASCs, the electrochemical evaluations of ASCs assembled with dimer, trimer, and tetramer are shown in Figures –7. The CV curves of the ASCs were measured in a PVA–KOH electrolyte at scan rates from 10 to 90 mV/s (Figure a–c). Because of the high voltage window of WDG, a wide voltage range of ASCs of 0–2.0 V could be applied. Among the ASCs, the tetramer sample has shown the largest CV area than that of the trimer and dimer at every scan rate. It is considered that both the charge storage by the redox reaction of the tetrameric molecules and the charge storage by the charge adsorption/desorption at the surface of the WDG sheets functioned properly in the ASC containing the aniline tetramer.[13−16] In the case of ASCs containing dimers or trimers, it can be seen that there is not enough redox reaction to store the electric charge. Therefore, the ASC containing the dimers or trimers mainly depend on the charge storage due to the electric double layer reaction at the WDG electrode so that the area of the CV curves become smaller. Figure d shows the Nyquist plots of ASCs assembled with tetramer, trimer, and dimer. Vertical straight lines in the low-frequency region were observed in the Nyquist plots for every sample. This suggests that the effective ion diffusion and proper capacitive behavior can be achieved using the aniline oligomer/graphene cell configuration.[1,3−9,22−27] The equivalent series resistance of the ASCs increased as following orders: tetramer (0.74 Ω/cm2) < trimer (2.73 Ω/cm2) < dimer (7.43 Ω/cm2). This indicates that the aniline tetramers with four electron arms have higher electron density around the molecules, resulting in higher conductivity for the electrolyte and reduced internal resistance (IR) compared to the trimers and dimers.

Figure 5

CV curves of ASCs based on (a) aniline tetramer, (b) aniline trimer, and (c) aniline dimer. (d) Nyquist impedance plots of ASCs based on aniline tetramer, aniline trimer, and aniline dimer in the frequency range of 1 MHz to 10 mHz.

Figure 7

(a) Plots of areal capacitance (mF/cm2) and volumetric capacitance (F/cm3) for ASCs based on aniline tetramer, aniline trimer, and aniline dimer at different currents. (b) Ragone plots of volumetric energy density versus volumetric power density for ASCs based on aniline tetramer, aniline trimer, and aniline dimer. (c) Cycling stability of ASCs based on aniline tetramer, aniline trimer, and aniline dimer.

To evaluate the capacitive performances of ASCs assembled with dimer, trimer, and tetramer, galvanostatic charge–discharge (GCD) curves were acquired at currents of 83, 249, 415, and.830 mA/cm3 with a voltage from 0 to 2.0 V (Figure a–c). The symmetrical shape of the charge and discharge curves for each sample indicates that the charge and discharge currents are stabilized.[3,26] ASCs with tetramers showed longer discharge time at every current. The results indicate that the tetramers have better capacitive behavior compared to the trimers and dimers. As the current increases, the proportion of side reactions increases as a direct result of the Butler–Volmer equation.[3] Thus, the increased side reactions decrease the discharging time of the ASCs. In particular, the ASC assembled with the tetramer shows gradual decreases in the discharge time with increasing current, demonstrating that the tetramer chain has a higher structural stability than the dimer and trimer. In addition, the IR drops observed from the discharging curves are also shown in Figure d. At each current density, the IR drop of ASCs employing tetramers was significantly smaller than that of the trimer and dimer. Furthermore, the IR increase of the cell containing the tetramer was gradually compared to the dimer and trimer. The IR values indicate that the tetramer with the higher protonation level provides higher conductivity compared to that of the trimer and dimer.[10,11,15−19] Because of the high electrical conductivity and structural stability of the tetramer, the applicability of an ASC based on the tetramers is considered to be higher than the ASCs with dimers and trimers.

Figure 6

GCD curves of ASCs based on (a) aniline tetramer, (b) aniline trimer, and (c) aniline dimer at different current densities. (d) IR values of ASCs based on aniline tetramer, aniline trimer, and aniline dimer at different current densities.

To evaluate the practical applicability of supercapacitors for state-of-art electronic applications, areal capacitance (CA) and volumetric capacitance (CV) of the ASCs at different currents are shown in Figure a. As the current increases, both the CA and CV of the samples decreased. This indicates that as the current increases, it is more difficult for the electrolyte ions to diffuse into the electrode materials. The CA (mF/cm2) obtained at a current of 83 mA/cm3 increased in the following order: dimer (26.2) < trimer (28.9) < tetramer (62.2). The same tendency was also observed for the volumetric capacitance (CV, F/cm3) values at a current of 83 mA/cm3 in the following order: dimer (87.2) < trimer (96.2) < tetramer (207.4). These results reconfirm that the improved capacitive behaviors of the ASC with the tetramer are in good agreement with the results of the XPS, CV, and Nyquist plots. In particular, the ASC assembled with tetramers has shown slower decreases in capacitance for currents than the samples containing dimers and trimers. This indicates that the tetramer structure is suitable for promoting the adsorption and desorption of electrolyte ions even at higher currents.[3,16] In contrast to the tetramer, the ASCs with trimers and dimers have shown that the capacitances decrease more rapidly with increasing currents, implying that both the dimer and trimer undergo generally low protonation levels and poor electrolyte adsorption/desorption. Considering these facts, it was clear that the rate capability of the ASC was significantly improved by choosing aniline tetramers and WDG as the cathode and anode materials, respectively. In order to further compare the performance of ASCs employing tetramers, trimers, and dimers, the Ragone plots of the ASCs for volumetric energy density versus volumetric power density are represented in Figure b. It was found that the ASC assembled with tetramers could store more energy per volume than the trimer and dimer samples. The maximum energy density of tetramer sample was 0.115 W h/cm3 with a power density of 0.208 W/cm3 and gradually decreased to 0.088 W h/cm3 with a power density of 2.08 W/cm3. The higher energy density per unit volume implies that the ASC composed of tetramers and WDG electrodes is well suited to achieve miniaturization of advanced electronic devices. The gradual reductions in energy density indicate that the structural stability of the tetramer is superior to both trimers and dimers, which is advantageous to maximize the synergistic effects from both pseudocapacitors and EDLC mechanisms.[1,3−9] By comparison, tetramers store more energy compared to dimers and trimers, indicating that the appropriate chain length of the aniline oligomer provides improved structural stability for interacting with electrolyte ions. To ensure the reliability of the ASCs consisting of aniline oligomers and WDG electrodes, the cycling stabilities of the ASCs containing the tetramer, trimer, and dimer were measured with GCD cycles at a current density of 83 mA/cm3 (Figure c). After 10 000 cycles, the retention rates of the samples (given in %) increased in the following order: dimer (70.8) < trimer (78.0) < tetramer (90.4). The capacitance losses of the samples are mainly ascribed to following reasons.[10,11,14−16] (1) During the adsorption/desorption of electrolyte ions, the volumetric changes and swelling of the aniline oligomers become significant. (2) Evaporation of the electrolyte ions causes the deterioration of the ASC performances. Despite the inevitable losses in the capacitances of the samples, it was found that the tetramer structure provides better structural stability, which prevents the aniline tetramer from swelling and breaking during repeated cycling. In addition, the PVA gel effectively prevented the evaporation of electrolyte ions during a number of charge/discharge cycles.[23] It was also conceivable that the WDG electrode with high mechanical strength and chemical stability contributed to the improved cycling stability of the ASC. Judging from these results, the configuration of ASCs was effective to magnify the synergistic effects from the aniline oligomer and WDG. Table summarizes the overall performances of state-of-art ASCs and our work.[22−27] In comparison to the previous ASCs based on carbon nanomaterials, graphene sheets, transition metals, and polymers, our work has demonstrated higher or comparable capacitive characteristics. This suggests that solid-state ASCs can be successfully constructed by selecting aniline tetramers, WDG, and PVA–KOH as the cathode, anode, and electrolyte, respectively.

Table 4

Electrochemical Performance of ASCs Based on the Two-Electrode Cell

electrode material	voltage window	electrolyte	specific capacitance	cycling stability (cycles)	refs
MnO2–Ni//3D GH	2.0 V	0.5 M Na2SO4	41.7 F/g	83.4% (5000)	(22)
Ni/GF/MnO2//Ni/GF/PPy	1.8 V	1 M KOH–PVA	175.2 F/g, 2.69 F/cm3	90.2% (10 000)	(23)
CuCo2O4/CuO//RGO/Fe2O3	1.6 V	2 M KOH	93 F/g	83.0% (5000)	(24)
MnO2–NPG//PPy–NPG	1.8 V	1 M Na2SO4	193 F/g	85.0% (2000)	(25)
CoNiFe–LDH//CNF	1.6 V	6 M KOH	84.9 F/g	82.7% (2000)	(26)
CNT–MnO2//CNT–VN	1.8 V	0.5 M Na2SO4	160 F/g, 43 F/cm3	80.0% (1000)	(27)
aniline tetramer//graphene	2.0 V	1 M KOH–PVA	207.4 F/cm3
62.2 mF/cm2	97.2% (2000)
90.4% (10 000)	this work

Conclusions

In this work, the hybrid ASCs based on aniline oligomers as a cathode, WDG sheet as an anode, and PVA–KOH gel as an electrolyte were successfully manufactured. The level of protonation of the aniline tetramer was superior to that of the trimer and dimer, thus affecting the resulting performances of the ASCs. The ASC fabricated from the aniline tetramer/WDG configuration exhibited higher areal capacitance (62.2 mF/cm2 at 83 mA/cm3), volumetric capacitance (207.4 F/cm3 at 83 mA/cm3), and energy density (0.115 W h/cm3) than the samples fabricated from the dimer and trimer electrodes. In addition, the capacitive retention rate of the aniline tetramer/graphene hybrid configuration reached up to 90.4% after 10 000 cycles, suggesting that the synergistic effects of the aniline tetramer and graphene lead to excellent reliability of the ASC. Significant improvements in the performance of the tetramer sample were highly related to the improved electrical conductivity and structural stability over the samples containing the trimer and dimer electrodes. Given that both high capacitance and excellent cycling stability are essential to construct the high-performance supercapacitors, the hybrid ASC consisting of aniline tetramer/WDG can provide promise for the real-world applications.

Experimental Section

Materials

Aniline (99%), 1-fluoro-4-nitrobenzene (99%), trimethylamine (TEA, 99.5%), dimethyl sulfoxide (DMSO, 99%), DMSO-d6 (99.9%), ammonium persulfate (98%), iron (iii) chloride hexahydrate (FeCl3·6H2O, 97%), hydrazine monohydrate (N2H4, 98%), potassium hydroxide (KOH, 85%), palladium on carbon (Pd/C), polyvinyl alcohol (PVA, 99%, Mw: 85 000–124 000), polyvinylidene fluoride (PVDF, Mw: 534 000), N-methyl-2-pyrrolidone (NMP), and ethyl acetate (EA) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Stainless steel foil (thickness: 0.1 mm) and AC (5 ± 1 μm) were obtained from MTI Corporation (Richmond, CA, USA.). Hydrochloric acid (HCl, 35–37%), ethanol (95%), and acetone (99%) were purchased from Daejung Chemical & Metals Co., Ltd. (Siheung, Korea). Graphene paste was acquired from MExplorer Co., Ltd. (Ansan, Korea), and the average thickness and lateral size of the graphene sheet are approximately <5 nm and 2–3 μm, respectively.

Synthesis of Aniline Dimer

Aniline (20 mmol), 1-fluoro-4-nitrobenzene (20 mmol), and TEA (24 mmol) were dissolved in DMSO (15 wt % with respect to DMSO), and the solution was stirred at 110 °C for 24 h under argon (Ar) atmosphere. The purification was carried out using chromatography (EA/hexane = 1:1 by volume). The recrystallized solids were obtained from a methanol solvent. As-prepared recrystallized solids were dissolved in ethanol (10 wt % with respect to ethanol) followed by the addition of Pd/C (aniline dimer/Pd/C = 1:0.02 by molar ratio). When the temperature of the reaction medium reached 120 °C, N2H4 (aniline dimer/N2H4 = 1:0.02 by molar ratio) was added into the solution, and then vigorously stirred for 24 h to convert a nitro (NO2) group into a amine group (NH2). Pd/C catalysts were removed from the fully reduced aniline dimer by filtration using celite and acetone. As a result, the precipitates of N-phenyl-1,4-phenylenediamine (fully reduced form of aniline dimer) were obtained. The precipitates of N-phenyl-1,4-phenylenediamine were protonated using 1 M HCl solution. The protonated precipitates were washed several times using water, ethanol, and acetone solvents.

Synthesis of Aniline Trimer

N-Phenyl-1,4-phenylenediamine (20 mmol), 1-fluoro-4-nitrobenzene (20 mmol), and TEA (24 mmol) were dissolved in DMSO (15 wt % with respect to DMSO), and the solution was stirred at 110 °C for 24 h under Ar atmosphere. The product was purified by chromatography (EA/hexane = 1:1 by volume), and the sample was recrystallized from a methanol solvent. The recrystallized solids of aniline trimers having a NO2 group were dissolved in ethanol (10 wt % with respect to ethanol). To reduce a NO2 group of aniline trimers to a NH2 group, N2H4 (aniline trimer/N2H4 = 1:0.02 by molar ratio) was added to the solution followed by vigorous stirring at 120 °C for 24 h. To promote the reduction process of aniline trimer, the Pd/C (aniline trimer/Pd/C = 1:0.02 by molar ratio) catalyst was also added into the solution. Pd/C catalysts were removed from the fully reduced form of aniline trimers through the celite filtering. Precipitates of the aniline trimer were protonated using 1 M HCl solution. The protonated precipitates of aniline trimers were washed several times using water, ethanol, and acetone solvents.

Synthesis of Aniline Tetramer

Aniline trimers in the fully reduced state (20 mmol), 1-fluoro-4-nitrobenzene (20 mmol), and TEA (24 mmol) were dissolved in DMSO (15 wt % with respect to DMSO), and the solution was stirred vigorously at 110 °C for 24 h under Ar atmosphere. The chromatography (EA/hexane = 1:1 by volume) was utilized to purify the product, and the recrystallized sample was obtained from a methanol solvent. The recrystallized solids of aniline tetramers having a NO2 group were dissolved in ethanol (10 wt % with respect to ethanol). To reduce a NO2 group of aniline tetramers to a NH2 group, N2H4 (aniline dimer/N2H4 = 1:0.02 by molar ratio) was added to the solution followed by vigorous stirring at 120 °C for 24 h. To promote the reduction process of aniline tetramers, the Pd/C (aniline tetramer/Pd/C = 1:0.02 by molar ratio) catalyst was also added into the solution. Pd/C catalysts were removed from the fully reduced aniline tetramer through the celite filtering. Precipitates of the aniline tetramer were protonated using 1 M HCl solution. The protonated precipitates of aniline tetramers were washed several times using water, ethanol, and acetone solvents.

Characterizations on Aniline Oligomers and WDG Sheets

Morphological images of the aniline oligomers and WDG sheets were recorded on a field-emission scanning electron microscope (S-4800, Hitachi, LTD, Hitachi, Japan). In order to confirm successful formations of aniline tetramer, aniline trimer, and aniline dimer, NMR analyses were carried out using on a VNS-600 spectrometer (Varian Inc., Palo Alto, CA, USA) operating at 600 MHz. To confirm the chemical compositions and doping states of the aniline oligomers, XPS spectra were measured on a K-Alpha XPS instrument (Thermo K-Alpha XPS, Thermo Fisher Scientific, Waltham, MA, U.S.A.). Raman spectra of RGO and WDG sheets were measured on a T6 (Horiba-Jobin Yvon Co., Tokyo, Japan) spectrometer. Electrical conductivities of the aniline oligomers and WDG were observed using a 4-point probe conductivity meter (Mode Systems Co., Hanam, Korea) equipped with a current source meter (Keithley 2400, Keithley Co., Cleveland, OH, USA). The conductivity values were calculated using the equation σ (S cm–1) = 1/ρ = (ln 2)/(πt)1/R, where ρ, R, and t indicate the static resistivity, sheet resistivity, and thickness of the sample, respectively.[10,11,30,31]

Assembly of ASCs Employing Aniline Oligomers and WDG Sheets

Aniline oligomers (0.16 g), such as dimer, trimer, and tetramer, AC (0.1 g), and PVDF (0.1 g) were dissolved in 9 g of NMP, and the mixture solution was stirred vigorously at 25 °C for 3 h. As-prepared pastes of aniline oligomers were treated sonochemically for an hour. The sonication treatments of the aniline pastes were carried out using an ultrasonic bath (CPX2800H-E, Branson Ultrasonics Co., Danbury, CT, U.S.A.) with 110 W power and 40 kHz frequency. Aniline oligomers (0.1 mL) and WDG (0.1 mL) were deposited on each 2 cm2 of the stainless steel foil, and these samples were dried at 25 °C using a vacuum oven. The thin films obtained from the aniline oligomer and the WDG sheet had thicknesses of 1.8 and 1.2 μm, respectively. The PVA film was immersed in 1 M KOH solution for 3 h. The aniline oligomer (cathode), WDG sheet (anode), and PVA–KOH electrolyte membrane were combined for the fabrication of ASC, which were sealed using a hydraulic pressing machine (HP, Ilshin Autoclave, Co., Ltd., Daejeon, Korea.).

Electrochemical Measurements

Evaluation of the electrochemical characteristics on the ASCs was conducted using a ZIVE SP2 electrochemical workstation (WonAtech, Seoul, Korea). Cyclic voltammograms (CVs) of the samples were measured from 0 and 2.0 V at scan rates 10 from 90 mV s–1. Galvanostatic charge/discharge experiments were performed by cycling the potential from 0 to 2.0 V at currents from 83 to 830 mA/cm3. Areal specific capacitances (CA’s) of the ASCs were calculated using the equation CA (mF/cm2) = IΔt/AΔV.[1,22−27] Volumetric specific capacitances (CV’s) of the ASCs were calculated using the equation CV (mF/cm3) = IΔt/LΔV.[1,22−27] In the equations of CA and CV, the terms I, Δt, ΔV, A, and L and indicate the applied current, discharging time, potential window, electrode area, and electrode volume, respectively. Energy density of the ASC was estimated according to the equation E (W h/cm3) = CVΔV2/2, where CV and ΔV indicate the volumetric capacitance of each ASC and voltage drop upon discharge, respectively.[1,22−27] Power density of the ASC was calculated according to the equation P (W/cm3) = E/t, where E and t indicate the energy density and discharging time of each ASC, respectively.[1,22−27] Electrochemical impedance spectra of the electrochemical cells were analyzed in the frequency range of 1 MHz to 10 mHz.