Polypyrrole (PPy) is a promising electrochemically active material due to its excellent conductivity, low cost, high intrinsic capacitances, and facile synthesis.1–8 However, large volumetric change and counterion drain during charge/discharge processes lead to its poor electrochemical performances and cycling stability, which limit its practical application.9–11 The severe capacity loss is originating from the following aspects: (1) the irreversible insertion/extrusion of counterions, leading to ion channels collapse and less counterions can doped into PPy chains, and hence, reduces the conductivity and capacitance12; (2) the repeated swelling/shrinking of polymer caused by anion exchange between electrolyte and PPy leads to structural pulverization, resulting in capacitance loss13–15; (3) the overoxidation of PPy caused by sustained high potentials/currents resulting in seriously damage of electroactivity.16
A number of strategies have been employed to improve the stability and capacitive performance of PPy, including polymer structure engineering, optimizing doping ions, using protective coatings, etc.17–19 Constructing PPy directly grown on three-dimensional (3D) high conductive carbon materials has been demonstrated to be an effective way to suppress the structural pulverization during cycling. 3D carbon substrates can not only improve the conductive linkages between PPy and current collector but also help to buffer the strain within PPy during repeated swelling and shrinking of pomyler chains, which ensures the long-term stability.20–22 Among the various 3D conductive substrates, functional exfoliated graphite sheets exhibit a higher conductivity, increased surface area and rich porous structure, which benefit the direct growth of polymer chains on the surface of graphene sheets by conventional electrochemical methodes.23–25 Unfortunately, these composites do not address the concentration polarization during electropolymerization and counterion drain from PPy matrix. Pulse-potential polymerization is a promising technology to fabricate hierarchical structures of pseudocapacitive materials on kinds of 3D current collectors for supercapacitor. The pulse-potential deposition process with periodic on/off duration is benefit to create more space between polymers in order for ionic transfer within materials, as well as the insertion/extrusion of counterions.26,27 Moreover, the resting time during deposition allows more pyrrole monomers in the bulk solution to penetrate into the space between 3D substrates before the next deposition pulse, which can effective balance the concentration polarization.28 Therefore, the combination of pulse-potential technology and 3D high conductive substrates should be a promising way to achieve controlled electrodeposition of PPy with both high overall capacitance and cycling stability.
Since the pioneering work on pulse polymerized PPy by Sharma et al.26 in 2008, considerable achievements have been achieved on pulse polymerization technology for polymer electrodes with enhanced electrochemical performance. Qi et al.29 demonstrated a PPy/GO composite by using a pulse polymerization technology, which delivered an excellent capacitance of 660 F/g at 0.5 mA/cm2. A high rate PPy film electrode was also developed by pulse current polymerization, exhibiting enhanced stability with 14% capacitive decay after 50,000 cycles.30 However, although achieving good cycling stability, the aforementioned PPy materials exhibited relatively small material loadings of ≤0.5 mg/cm2. The amount of energy stored in such low loading materials is, in fact, small, which seriously limits their practical application for commercial devices. Therefore, realizing the controlled electro-polymerization of PPy and practical application for high energy system are still a scientific challenge.
Besides, the commercial energy storage devices, such as lead-acid batteries, Ni–Fe batteries, Ni–Cr batteries, and the mainstream lithium-ion batteries, play a leading role in consumer electronics markets and electric vehicle fields because of the high energy density.31,32 However, the safety accidents that occurred frequently in recent years make researchers realize the security and favorable stability is a necessary prerequisite for any energy storage systems, which points to bright future of aqueous power storage devices. Thereby, it is now an urgent requirement for researchers to promote the high-performance electrode materials in the lab toward the real-life application. In this study, we report a new pulse-potential polymerized PPy grown on 3D functionalized partial-exfoliated graphite (FEG) sheets with high material loading of 24.7 mg/cm2 for supercapacitor. This study demonstrates the role of pulse polymerization technology in boosting the kinetics and energy storage capability of pseudocapacitive PPy. The as-synthesized electrode (PP-PPy/FEG) achieves a outstanding areal capacitance of 7250 mF/cm2 (or a gravimetric capacitance of 293 F/g) at 3 mA/cm2, which is considerable higher than the 4666 mF/cm2 of PPy with conventional potentiostatic polymerization method (CP-PPy/FEG). Even at a fast charge rate of 200 mA/cm2, the PP-PPy/FEG can also exhibits large capacitance of 3073 mF/cm2. The assembled symmetric and asymmetric supercapacitors exhibit good energy densities (0.8 mWh/cm2 for asymmetric supercapacitors (ASC) and 0.5 mWh/cm2 for symmetric supercapacitors (SSC)). Furthermore, the fabricated pouch-type device using PP-PPy/FEG electrodes can effortlessly provide power source for 3 C products such as smartphone and tablet, and can remain stable during the test of strong external stimulus. This study sheds light on the effective pulse-potential deposition strategy to design high-performace electrode materials for commercialized aqueous energy storage devices.
RESULTS AND DISCUSSION Preparation and structural characterizationThe FEG substrate was prepared via a facile electrochemical exfoliation method reported in our previous work.33–35 The FEG consists of partial-exfoliated graphene nanosheets that seamless interconnected with expanded graphite layers, which provide large amount of porous structure and vacancies for efficient ionic and electric transport within electrodes (Figure 1B and Figure S1A–C). The transmission electron microscopy (TEM) images shown in Figure S1D–F suggest that the graphene nanosheets have well crystallinity with thickness at nanometer scale. The symmetric and linear profiles of FEG, even at an ultrahigh charge rate of 200 mA/cm2, further demonstrate the highly efficient charge transfer in scaffold (Figure S2). Despite its energy storage capacity is not adequate for practical application (3.1 F/cm3 at 3 mA/cm3 or 123 F/g at 0.4 A/g), the features of excellent electrical conductivity, porous structure and proper specific surface area (6.23 m2/g) make FEG a desired 3D support for electrode materials (Figures S3 and S4).
Enlarge this image.Figure 1. (A) Schematic illustration of the syntheses of PP-PPy/FEG and CP-PPy/FEG. (B) SEM image of FEG. (C) and (D) HRTEM images of the graphene in PP-PPy/FEG. (E) TEM image of PP-PPy/FEG and corresponding EDS elemental mappings of C, S, O, and N. (F) FT-IR spectra of PP-PPy/FEG and CP-PPy/FEG. (G) XPS N 1 s spectra of PP-PPy/FEG and CP-PPy/FEG. CAs of (H) CP-PPy/FEG and (I) PP-PPy/FEG with water. EDS, energy dispersive X-ray spectroscopy; FEG, functionalized partial-exfoliated graphite; FT-IR, Fourier-transform infrared spectroscopy; PPy, polypyrrole; SEM, scanning electron microscope; XPS, X-ray photoelectron spectroscopy
The PPy was polymerized on FEG in a three-electrode cell by using a pulse-potential polymerization method. As illustrated in Figure 1A, during pulse polymerization, two potentials of the higher and lower potential limit (EH and EL) are applied for different times which are denoted as ton and toff, respectively. For the pulse polymerization process, the EH was 0.85 V and lasted 2 ms while EL was 0 V and lasted 2 ms with pulse cycle number of 1,800,000. The as prepared sample is denoted as PP-PPy/FEG. Figure 1C–E shows the high-resolution TEM images of PP-PPy/FEG. The few-layer graphene nanosheets exfoliated from graphite layers are wrapped with pulse polymerized PPy, which will facilitate the charge transfer within electrode. The energy dispersive X-ray spectroscopy (EDS) elemental mappings of C, S, O, and N suggest that the uniform polymerization of sulfate doped PPy on the FEG (Figure 1E). For comparison, PPy was also electro-polymerized by a conventional potentiostatic method with a constant potential of 0.85 V (denoted as CP-PPy/FEG). Figure 1F compares the FT-IR spectra of PP-PPy/FEG and CP-PPy/FEG. Typical characteristic vibrations of PPy are observed in both electrodes, including =C–H out-of-plane vibration at 895 cm−1,36 N–H in-plane deformation vibration at 1065 cm−1,37 C-H in-plane vibration at 1250 and 1340 cm−1,38,39 C–N stretching in pyrrole ring at 1250 and 1340 cm−1,40–43 and C═C of PPy rings at 1636 cm−1.44,45 It verifies the successful preparation of PPy through both pulsed/conventional potentiostatic polymerization method. In addition, Raman spectroscopy of PP-PPy/FEG exhibits two characteristic peaks at 1345 and 1575 cm−1, attributed to the π-conjugated planar structure and ring stretching of the pyrrole backbone respectively (Figure S5).46 The tiny peak at 1089 cm−1 is of C–H in-plane deformation, and peak at 973 cm−1 can be assigned to the quinoid polaronic and bipolaronic structure.47 Figure 1G shows the N 1 s X-ray photoelectron spectroscopy (XPS) spectra of PP-PPy/FEG and CP-PPy/FEG. The amount of quinonoid imine structure (=N–, reflecting the degree of defect in the PPy chain48) with lower binding energy in PP-PPy/FEG and CP-PPy/FEG were 5.6% and 18.8%, respectively, indicating that the PPy chains in PP-PPy/FEG have fewer defects than those in CP-PPy/FEG (Table S1). The polaron ratio (the area percentages of N+ cation of all N-components25) for PP-PPy/FEG is 29.1%, which is much higher than that for CP-PPy/FEG (20.8%). The higher protonation level contributes to better electrical conductivity for PP-PPy/FEG, which ensure fast charge transfer within electrode.12 Table S2 lists the electrical conductivity of FEG, PP-PPy/FEG and CP-PPy/FEG. The PP-PPy/FEG shows significantly higher electrical conductivity than FEG and CP-PPy/FEG, which is consistent with the analysis in X-ray photoelectron spectroscopy. To further study the surface area, nitrogen physisorption tests were conducted on PP-PPy/FEG and CP-PPy/FEG electrodes (Figure S7 and Table S3). The pulse polymerized PPy displayed a large accessible surface area than that of the conventional potentiostatic method, which promoting effective contact with aqueous electrolyte. PP-PPy/FEG also exhibits the typical type-IV isotherm, indicating the presence of mesoporous structure.49 Wetting properties of electrode materials significantly influence the electrochemical performance for energy storage. Figure 1H, I show the contact angles (CAs) of the electrodes with water. The smaller CA of PP-PPy/FEG film (≈0°) shows that it has a superhigh hydrophilicity than CP-PPy/FEG (78.6°), indicating its suitability for aqueous electrolyte. Further experiment suggests that PP-PPy/FEG still has a very small CA that is almost 0° in the 3 mol/L KCl electrolyte, which proves there is only a very small contact resistance between polymer materials and electrolyte again (Figure S8). The above results reveal that the pulse-potential polymerized PPy exhibits enlarged accessible surface area, higher protonation level and enhanced affinity with the electrolyte, which is beneficial to high-efficiency energy storage. The detailed mechanism analysis of the advantages of pulse polymerized PPy are described in the Mechanism section (Figure 3).
Improved capacitive performance and cycling stability of PP-PPy/FEGTo understand the effect of electropolymerization strategy on electrochemical behavior, the electrochemical properities of PP-PPy/FEG and CP-PPy/FEG electrodes were studied in three electrode cells containing 3 mol/L KCl electrolyte. The CV curve of PP-PPy/FEG presents significantly increased current compared with CP-PPy/FEG in the potential range of −0.6 to 0.55 V, suggesting the enhanced capacitance of PPy with pulsed technology (Figure 2A). Figures S9 and S10A show the galvanostatic charge-discharge (GCD) profiles of the two electrodes. The extended discharge time of PP-PPy/FEG electrode under all current densities further demonstrates its superior capacitive performance. PP-PPy/FEG electrode delivers an areal capacitance of 7.25 F/cm2 at 3 mA/cm2 (equivalent to a specific capacitance of 293 F/g normalized to the mass loading of 24.7 mg/cm2), superior to CP-PPy/FEG electrode (4.66 F/cm2, 194 F/g). Similar results for the PP-PPy/FEG were also obtained by CV measurements (Figure S11). For example, the capacitance is calculated to be 7.39 F/cm2 (equivalent to a gravimetric capacitance of 299 F/g) based on the data measured at 0.4 mV/s scan rate, close to the 7.25 F/cm2 (293 F/g) capacitance measured by GCD at the current density of 3 mA/cm2. From the view of volume, despite FEG substrate having an intrinsic thickness after electrochemical exfoliation, the more space within substrate can bear more PPy. Such a free-standing high mass loading PP-PPy/FEG electrode has a volume of 0.28 cm−3 and density of 0.21 g/cm3, still exhibits a high volumetric capacitance of 25.9 F/cm3 at 3 mA/cm2 (Figure S12). And beyond that, the PP-PPy/FEG also exhibits good rate capability (Figure 2B), with 76.3% capacitance retained (5.53 F/cm2) from 3 to 30 mA/cm2, which is higher than CP-PPy/FEG (66.7%, 3.11 mF/cm2 @ 30 mA/cm2) and many PPy-based electrodes that were reported in recent years (Table S4). Electrochemical impedance spectroscopy (EIS) was further utilized to explore the electrochemical performance of these electrodes (Figure S13). The higher slope indicates the ionic transport is more efficient in the PP-PPy/FEG electrode. As for the high-frequency region, the small intercept of the Nyquist plot (related with the solution and electrolyte contact resistance, Re) and the smaller radius of the semicircle (related with the charge transfer resistance, Rct) on the real axis reflect the better conductivity of PP-PPy/FEG, which agree well with its outstanding electrochemical performance. Figure 2C compares the cycling stabilities of PP-PPy/FEG and CP-PPy/FEG electrodes tested by CV cycling for 20,000 cycles at a scan rate of 100 mV/s. The PP-PPy/FEG exhibits outstanding capacitance retention of 94% after 20,000 cycles. The initial increase of capacitance is observed to be due to gradual activation process of electrode.12 In the electro-activation process, aqueous electrolyte gradually diffused into the entire electrode, which allows a more thorough interaction. On the contrary, the CP-PPy/FEG electrode only retained 79.8% of its capacitance after 20,000 cycles. All results clearly show the substantially enhanced capacitive performance and cycling stability of the PP-PPy/FEG electrode constructed by pulse polymerization technology. The mechanism of the enhanced capacitive and cyclic performance in pulse-potential polymerized PPy is discussed in detail in the next section.
Enlarge this image.Figure 2. (A) CV curves of PP-PPy/FEG and CP-PPy/FEG collected at 10 mV/s. (B) Areal capacitances of PP-PPy/FEG and CP-PPy/FEG versus current densities. (C) Cycling stability of PP-PPy/FEG and CP-PPy/FEG electrodes tested at 100 mV/s. FEG, functionalized partial-exfoliated graphite; PPy, polypyrrole
The results reveal that the pulse electropolymerization strategy plays a critical role in determining the final charge storage ability of PPy materials, leading to high capacitance, faster kinetics, and excellent cycling stability comparing with the CP-PPy/FEG by conventional potentiostatic method. In this section, we study the mechanism of the enhanced capacitive performance and cycling stability in pulse polymerized PPy materials.
To study the energy storage mechanism of pulse polymerized PPy, EDS spectra of PP-PPy/FEG at various charge/discharge potentials were collected to study the anion doping behavior. Figure 3A shows the EDS spectra collected at different charge/discharge potentials corresponding to labelled A-F in the GCD curve of PP-PPy/FEG (Figure 3B), and the calculated contents of doped ion are illustrated in Figure 3C. During charging process from A to C, the Cl signal substantial increases from 0.47% to 1.42%, indicating the anion (Cl−) from the electrolyte is inserted into the PPy. At the discharging process from point D to F, the amount of Cl gradually decreases and remains at 0.89%. This suggests a reversible anion doping process during charging/discharging process. Above results suggest a dual doping behavior of Cl− and sulfate (Figure 1E) in PP-PPy/FEG.
Enlarge this image.Figure 3. (A) EDS spectra of the PP-PPy/FEG at different charge/discharge potentials corresponding to labelled (A–F) in (B). (B) GCD curve of PP-PPy/FEG measured at 10 mA/cm2. (C) The element concentration ratio of Cl at different charge/discharge states. SEM images of (D–G) PP-PPy/FEG and (H–K) CP-PPy/FEG afforded from different polymerization periods. (L) Schematic diagram of the growing mechanism of PP-PPy/FEG. (M) Areal capacitances of PP-PPy/FEG with different mass loadings versus current densities. EDS, energy dispersive X-ray spectroscopy; FEG, functionalized partial-exfoliated graphite; GCD, galvanostatic charge-discharge; PPy, polypyrrole; SEM, scanning electron microscope
The pulse electropolymerization process plays an important role in the charge storage ability of PPy. It is critical to understand the scientific basis for the enhanced capacitive performance of PP-PPy/FEG. The morphology and growth process of PPy during pulsed and conventional potentiostatic method provide important insights to understand the possible mechanism. Figure 3D–K shows the SEM images of PP-PPy/FEG and CP-PPy/FEG obtained from different electropolymerization periods (25, 50, 100, and 200 s, see the Experimental section for details), which discloses the detailed growing process of PPy. For pulse polymerization process, small and sparse PPy nanoparticles were grown on the surface of the exfoliated graphite sheets continuously and started to grow larger gradually up to 100 s (Figure 3D–F). From 100 to 200 s, one-dimensional (1D) growth of PPy was promoted by polymerization on the tips of the nanoparticle due to the highest electric field, resulting in the interlaced structure (Figure 3G). The porous structures ensure efficient ionic transport within electrode and maintain the structure stability during repeated swelling and shrinking of pomyler chains. For conventional potentiostatic polymerization process, on the contrary, the formation of PPy nanoparticles was at a much earlier stage of 25 s (Figure 3H). Upon further polymerization (50 and 100 s), the nanoparticles started to 1D growth and resulted in the PPy film rapidly. Finally, at 200 s, PPy film aggregated to form dense cauliflower-like morphologies. The process of morphological evolution accelerates for conventional potentiostatic polymerization of PPy, due to the sustained high potential output and resulting in seriously aggregation of PPy. The SEM images of CP-PPy/FEG with finished polymerization display that there are a large number of bigger PPy nanoparticles on the outer surface, but the inner surface is still a thin film that is like the earlier stage of 400 s (Figure S15A,B). On the contrary, the PPy film with the mass loading close to CP-PPy/FEG was uniformly deposited on the whole graphene sheets forest by pulse polymerization. The thickness of PPy layer on the inner surface is obviously thicker than that of CP-PPy/FEG, and the outer surface still retains the original intercorrected porous structure (Figure S15C,D).
Based on the aforementioned results, the growing mechanism of PP-PPy/FEG can be summarized in Figure 3L. The whole pulse polymerization process compared with the conventional polymerization under the waveform shown in Figure 1A can be divided into four phases. First, the pyrrole monomers are absorbed on surfaces of graphene sheet due to the π − π interactions between pyrrole ring and graphene (Phase Ⅰ). During the deposition pulse potential, pyrrole monomers are polymerized and precipitated as PPy nanoparticles on the surface of graphene sheets. At the same time, all pyrrole monomers suspended in the macroporous structure between graphene films are consumed, leading to significantly reduced pyrrole monomers within the graphene sheet and concentration polarization (phase II). During continuous polymerization (phase III, conventional polymerization process), pyrrole monomers were continually consumed and restored by diffusion from the bulk solution. In this case, the electropolymerization of pyrrole preferred to continue on existed polymer nanoparticles instead of nucleating new chains. These results in larger-sized and continuous polymer particles on the surface of graphene but with very little penetration into the macroporous structure of graphene sheets, led to seriously blocking and significantly reduced surface area for redox reactions. For pulse polymerization process, the pulse resting phases allows pyrrole monomers to diffuse from the bulk solution penetrating into the graphene sheets forest deeply, resulting in uniform deposition of small PPy nanoparticles throughout the macroporous graphene sheets and balancing the concentration polarization (phase IV). Therefore, the repeated pulse period constructed the PPy with less blocked pores than the corresponding conventional polymerized one (Figure 3K). The interconnected porous structures not only ensure facile ionic transport within electrodes but also improve the charge transfer kinetics of PPy. The areal capacitance of as-synthesized PP-PPy/FEG electrodes increases almost linearly along with the PPy loading from 4.6 to 24.7 mg/cm2 at different current densities (Figure 3M and S16-S18). Significantly, the PP-PPy/FEG electrode with a mass loading of 4.6 mg/cm2 delivers excellent capacitance of 531 F/g. Even under an ultrahigh loading of 24.7 mg/cm2, the PP-PPy/FEG electrode also retains an extraordinarily high rate capability of 76.3% (3–30 mA/cm2). These results unambiguously show that ion diffusion and charge transfer in thick pulse polymerized PPy materials are highly efficient. The mass loading is further increased to 39 mg/cm2 though prolonging pulse polymerization time to 7200 s. Though higher areal capacitance can be obtained, the capacitance retention would be severely weakened, only 58.9% from 3 to 30 mA/cm2 (Figure S19). The corresponding SEM image shows that the PPy film aggregated to form dense cauliflower-like morphologies on outer surface of FEG due to more PPy and limited space of substrate, which is not benefit for ionic transport in the entire electrode (Figure S20).
To study the possible mechanism leading to the outstanding cycling durability of PP-PPy/FEG electrode, we detached the cycled electrodes and collected corresponding SEM images, EDS and XPS spectra after the cycling test. The CP-PPy/FEG sample shows apparent peeling off of the PPy layer from the graphite substrate during the repeated swelling and shrinking in cycling test (Figure 4A). It correlates with the fast capacitance decay of CP-PPy/FEG. By contrast, the structure of PP-PPy/FEG was well-retained after testing for 20,000 cycles (Figure 4B). EDS results show the S signal in both samples decreases after cycling test, while the amount of Cl substantial increases for PP-PPy/FEG after 20,000 cycles (Figure 4C). The calculated total contents of doped S and Cl before and after cycling test are summarized in Figure 4D. Although SO42− de-doped from the polymer chain during repeated CV cycles, the Cl− from KCl electrolyte is doped into PP-PPy/FEG gradually. The total contents of S and Cl in PP-PPy/FEG were no obvious change after 20,000 cycles, and hence, retained its electrical conductivity and capacitance performance during cycling test. For CP-PPy/FEG sample, the total contents of S and Cl were seriously decreases after 20,000 cycles. It could be a result of the ion channels collapse of PPy that caused by structural pulverization and activity loss during cycling test (Figure 4A), resulting in less Cl− to be doped into PPy matrix. The effect of doping behavior on the improved cycling stability is furthermore studied. We carried out XPS spectra of PP-PPy/FEG and CP-PPy/FEG after the cycling test (Figure 4E,F). After cycling, the polaron ratio (the area percentages of N+ cation of all N-components, N+/N) of CP-PPy/FEG decreased drastically from 20.86% to 11.9%, while PP-PPy/FEG only shows a small decrease from 29.1% to 25.1% as less of the N+ cations were converted to –N=(Table S5).50 The results clearly showed that the CP-PPy/FEG suffers from the substantial loss of polarons caused by structural pulverization during cycling, while PP-PPy/FEG with good flexibility can effectively retain the amount of polarons. Overall, all results reveal that the uniform porous structures of PP-PPy/FEG resulted from repeated pulse/resting phases ensure the good flexibility, which helps to buffer the internal stress created during cycling and maintain the structure stability.
Enlarge this image.Figure 4. SEM images of (A) CP-PPy/FEG and (B) PP-PPy/FEG before and after the cycling test. (C) EDS spectra and (D) element concentration ratios of S and Cl for CP-PPy/FEG and PP-PPy/FEG before and after the cycling test. XPS N 1 s spectra of (E) PP-PPy/FEG and (F) CP-PPy/FEG before and after the cycling test. EDS, energy dispersive X-ray spectroscopy; FEG, functionalized partial-exfoliated graphite; PPy, polypyrrole; SEM, scanning electron microscope; XPS, X-ray photoelectron spectroscopy
A SSC was assembled with the as-prepared PP-PPy/FEG as electrodes and 3 mol/L KCl as the electrolyte. The SSC exhibits an excellent areal capacitance of 2.7 F/cm2 at the current density of 3 mA/cm2. More importantly, as the charge rate increases to 40 mA/cm2, the device still retains outstanding rate performance of 70.4% (Figure S21C). The energy density and power density of our SSC device are shown in the Ragone plot in Figure S21D. The SSC delivers high areal energy density of 0.5 mWh/cm2 at 1.7 mW/cm2 and 0.35 mWh/cm2 at 23.1 mW/cm2 (10.1 Wh/kg at 34.4 W/kg and 7.1 Wh/kg at 467.6 W/kg, Figure S23). The results are superior to many other devices that were summarized in Table S6. Significantly, the PP-PPy/FEG-based SSC also exhibits superior cycling behavior with zero capacitance loss after 35,000 cycles, suggesting the good stability (Figure S21E). The curves of the first and last 20 cycles were shown in Figure S27B. The slight increased capacitance of SSC could be ascribed to a self-activation process that was caused by the continuous diffusion of polarized electrolyte ion into the dense PPy film induced by electric field.51 In addition, an ASC was also fabricated by assembling a PP-PPy/FEG as cathode, and FEG1 as anode, respectively. The fabrication details and charge balance of the device are described in the Experimental section. Detailed electrochemical characterization of FEG1 and PP-PPy/FEG are shown in Figures S24 and S25. The linear and symmetric charge/discharge profiles of the device demonstrate the typical capacitive behavior (Figure S26B). The ASC device shows a high capacitance performance of 1.7 F/cm2 (3 mA/cm2) and excellent energy densities of 0.8 mWh/cm2 at 2.8 mW/cm2 and 0.6 mWh/cm2 at 27.8 mW/cm2 (Figure S26D). The good stability of ASC was also demonstrated with only 0.2% capacitance decay per cycle (Figure S27).
To explore the suitability of supercapacitors based on PP-PPy/FEG electrode in application, the SSC was further processed into a handy pouch-type device welded by aluminum plastic film packaging and vacuum-sealed, as schematically illustrated in Figure 5A. The rectangular-like CV curves at different scan rates of 10–80 mV/s indicate the ideal capacitive behavior and fast ionic/electronic transmission in this pouch-type device (Figure 5B). All GCD curves show the typical triangular shapes (Figure 5C), corresponding areal capacitance values are summarized in Figure 5D. The single-cell exhibits a high areal capacitance of 2.2 F/cm2 at 30 mA/cm2, and retains 1.8 F/cm2 even at a high current density of 150 mA/cm2, which demonstrates an outstanding rate capability. The energy density of the pouch-type device can achieve 8.4 Wh/kg at 86.6 W/kg (Figure 5E). It is available by multiple cells in series as a standard power supply for 3 C products such as smartphone and tablet (Figures 5F,G). More importantly, this pouch device can withstand some extreme stress tests well such as heavy blows, punctures, fires, and high-frequency vibrations during operation (Video S1), which proves its high security and excellent stability in the complex and changeable reality.
Enlarge this image.Figure 5. (A) Schematic diagram of the pouch-type symmetric supercapacitor. (B) CV curves and (C) GCD curves of the pouch device at different scan rates and current densities. (D) Areal capacitance and capacitance retention at different current densities. (E) Ragone plots of the pouch-type device. Photographic images of the five cells in series to run a smartphone (F) and tablet (G)
In this study, we demonstrate a novel pulse-potential polymerization strategy to significantly improve the electrochemical performance and cycling durability of PPy. Compared to conventional potentiostatic strategy, periodic pulse-potential polymerization of PPy has the following key advantages. (1) The repeated pulse/pauses period allows pyrrole monomers penetrating into the 3D graphite forest deeply and ensures uniform deposition of small PPy monomers with less blocked pores, balancing the concentration polarization. (2) The short pulse on duration facilitates the formation of ordered and shorter molecular structure of PPy during polymerization. It therefore ensures more homogeneous stress distribution and improves the cycle stability during ultralong charging/discharging tests. (3) The pulse-potential polymerized PPy with dual anion doping behavior induces enhanced protonation level and improves the charge transfer kinetics. Therefore, the as-synthesized PPy electrode exhibits an ultrahigh areal capacitance of 7250 mF/cm2 at 3 mA/cm2 and excellent rate capability. More importantly, the assembled supercapacitors exhibit high energy densities of 0.8 mWh/cm2 for ASC and 0.5 mWh/cm2 for SSC, and a high stability with zero capacitive decay in 35,000 cycles. More importantly, the practical application of this electrode was well proved by a pouch-type device. Our work demonstrates the promising capability of pulse-potential deposition strategy for high-efficiency and commercialized electrochemical energy storage systems.
ACKNOWLEDGMENTSThis study was supported by the National Natural Science Foundation of China (No. 52071171), Liaoning Revitalization Talents Program-Pan Deng Scholars (No. XLYC1802005), Liaoning BaiQianWan Talents Program (No. LNBQW2018B0048), Natural Science Fund of Liaoning Province for Excellent Young Scholars (No. 2019-YQ-04), Key Project of Scientific Research of the Education Department of Liaoning Province (No. LZD201902), Shenyang Science and Technology Project (No. 21-108-9-04), Research Fund for the Doctoral Program of Liaoning Province (No. 2019-BS-112), Australian Research Council (ARC) Future Fellowships (Nos. FT210100298, FT210100806), Discovery Project (No. DP190103186), Linkage Projects (Nos. LP210100467, LP210200504, LP210200345), Industrial Transformation Training Centres scheme (No. IC180100005), CSIRO Energy Centre and Kick-Start Project. The Study Melbourne Research Partnerships program has been made possible by funding from the Victorian Government through Study Melbourne. The authors would like to thank Shiyanjia Lab (
The authors declare no conflicts of interest.
DATA AVAILABILITY STATEMENTThe data that support the findings of this study are available in the supplementary material of this article.
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Details
1 Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, Institute of Clean Energy Chemistry, College of Chemistry, Liaoning University, Shenyang, China
2 School of Science, RMIT University, Melbourne, Victoria, Australia
3 School of Materials Science and Engineering, State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, China
4 Shandong Institute of Advanced Technology, Jinan, China
5 Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, Australia