A Hydrogel of Ultrathin Pure Polyaniline Nanofibers: Oxidant

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A Hydrogel of Ultrathin Pure Polyaniline Nanofibers: OxidantTemplating Preparation and Supercapacitor Application Kun Zhou, Yuan He, Qingchi Xu, Qin'e Zhang, An'an Zhou, Zihao Lu, Li-Kun Yang, Yuan Jiang, Dongtao Ge, Xiang Yang Liu, and Hua Bai ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02055 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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A Hydrogel of Ultrathin Pure Polyaniline Nanofibers: Oxidant-Templating Preparation and Supercapacitor Application Kun Zhou#,†, Yuan He#,†, Qingchi Xu‡, Qin’e Zhang†, An’an Zhou†, Zihao Lu†, Li-Kun Yang†,‡, Yuan Jiang*,†,‡, Dongtao Ge*,†, Xiang Yang Liu‡,, and Hua Bai*,†,§

†

College of Materials, Xiamen University, Xiamen, 361005, P. R. China

‡

Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key

Laboratory for Soft Functional Materials Research, Xiamen University, Xiamen, 361005, P. R. China



Department of Physics and the Department of Chemistry National University of Singapore

§

Graphene Industry and Engineering Research Institute, Xiamen University, Xiamen,

361005, P. R. China

# These authors contributed equally to this work.

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KEYWORDS: vanadium pentoxide, supercapacitor, polyaniline, hydrogel, conducting polymer

ABSTRACT:

Although challenging, fabrication of porous conducting polymeric materials with excellent electronic properties is crucial for many applications. We developed a fast in-situ polymerization approach to pure polyaniline (PANI) hydrogels, with vanadium pentoxide hydrate nanowires as both the oxidant and sacrifice template. A network comprised of ultrathin PANI nanofibers was generated during the in-situ polymerization, and the large aspect ratio of these PANI nanofibers allowed the formation of hydrogels at a low solid content of 1.03 wt%. Owing to the ultrathin fibril structure, PANI hydrogels functioning as a supercapacitor electrode display a high specific capacitance of 636 F g−1, the rate capability, and good cycling stability (~83% capacitance retention after 10,000 cycles). This method was also extended to the preparation of polypyrrole and poly(3,4-ethylenedioxythiophene) hydrogels. This template polymerization method represents a rational strategy for design of conducing polymer


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networks, which can be readily integrated in high-performance devices or further platform for functional composites.

Conducting polymers (CPs), exemplified by polypyrrole (PPY),1 polyaniline (PANI),2 and poly(3,4-ethylenedioxythiophene) (PEDOT),3 have attracted considerable attention because of

their highly-tunable electronic properties and electrochemical properties. Fabrication of CPs

into hydrogels can theoretically provide a rational approach to light-weighted materials

carrying properties such as large surface area, soft and biocompatible surface, and high

permeability to electrolyte. Therefore, CP hydrogels can be excellent candidates for fabrication

of high-performance devices, where electric conductivity and permeability are simultaneously demanded. Application opportunities include supercapacitors,4 electrochemical sensors,5 and controlled drug delivery systems.6 Nevertheless, the absence of reliable methods of obtaining

CP hydrogels strongly hinders their widespread applications. As pristine CPs like PPY and

PANI are practically insoluble in almost all organic solvents and water owing to their rigid

molecular chains, the dissolution-gelation route is in principle impractical for obtaining CP gels unless harsh reagents are involved.7 In-situ polymerization proceeding in solution phase 3 ACS Paragon Plus Environment

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turns out the only main stream for fabrication of CP hydrogels.8 Generally speaking, in-situ

polymerization typically leads to precipitates rather than a hydrogel, because the former is the

thermodynamically stable form for insoluble CPs. To promote the gelation, the polymerization was carried out at very high concentration levels (> 5 wt%),9 or other templates like hydrogels,

macromolecules, or nanomaterials were added in the polymerization system for kinetic stabilization of the CP hydrogels.10-13 The high-concentration polymerization usually generate

large CP aggregates, which decrease the specific surface area and accessibility of the hydrogel

obtained, and the additional templates will inevitably deteriorate the electronic properties of

the hydrogels. Though challenging, it is highly desirable to explore reliable methods for

fabrication of pure CP hydrogels exhibiting excellent microstructure and electronic properties. Here, we report an oxidant-templating method for the fabrication of pure PANI hydrogels comprising interwoven PANI nanofibers. V2O5·nH2O nanowires in their colloid dispersion are employed as the oxidant agents for the polymerization of aniline and meanwhile, as the sacrifice template to guide the formation of three-dimensional (3D) network. Fast oxidative polymerization of aniline occurring on V2O5·nH2O nanowires leads to immediate formation of ultrathin PANI nanofibers, which then undergo a 3D assembly process for quick gelation.


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V2O5·nH2O is removed spontaneously by being converted into soluble salts, so the hydrogel is comprised of pure PANI nanofibers. A low critical gelation concentration of ~ 1 wt% is realized because of the extreme aspect ratios of PANI nanofibers. Both continuous conductive frameworks and the ultrathin nanofiber structure render the PANI hydrogels excellent electrode materials for supercapacitors. A high specific capacitance (636 F g−1 at a current density of 2 A g−1), excellent rate performance (98% capacitance retention when current density increases from 2 A g−1 to 25 A g−1), and long cycling life (83.3% capacitance retention after 10000 cycles) are achieved simultaneously.

RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the fabrication route to a pure PANI hydrogel.


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The PANI hydrogels were prepared following the process shown in Figure 1. A V2O5·nH2O nanowire dispersion (Figure 2A left) was first prepared following a reference.14 Both transmission electron microscopy (TEM, Figure 2C) and atomic force microscopy (AFM, Figure S1) images show that V2O5·nH2O are ultrathin nanowires with the average diameter of 3.8 ± 0.8 nm (N = 6). The viscous V2O5·nH2O nanowire dispersion was filled in a mold and immerged in an acidic aniline solution for the quick mixing of both chemicals. The in-situ oxidative polymerization occurred rapidly, as the color of V2O5·nH2O nanowire in the dispersion (Figure 2A right) switched from dark red to dark green in seconds. Meanwhile, the gelation process proceeded simultaneously, and the tube-inversion test verified that a hydrogel was formed within 10 seconds. PANI hydrogels different in shape could be prepared directly in different molds, or by carving a large monolith (Figure 2B), demonstrating good processability.

The mechanical properties of these PANI hydrogels are largely dependent on the volume fractions of the V2O5·nH2O dispersions used.15 Three V2O5·nH2O dispersions with different volume factions of 0.43 vol.%, 0.72 vol.%, and 0.90 vol.% were employed to fabricate PANI hydrogels, which were named as PHG-1, PHG-2, and PHG-3, respectively. Their solid contents 6 ACS Paragon Plus Environment

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are 1.03 wt%, 1.13 wt%, and 1.35 wt% in sequence. All PANI hydrogels are free-standing monoliths with good mechanical strength. Figure 2D shows the rheological profiles of the sample PHG-2. The storage modulus is about one order of magnitude higher than the loss one at the angular frequencies of 1 ~ 100 rad s−1, which implies that the elastic response is predominant in the permanent network. The high storage modulus (3.1 ×105 Pa) also confirms that the sample PHG-2 is strong in mechanical performance. Our study shows that the mechanical strength of PANI hydrogels becomes stronger with the increased volume fractions of V2O5·nH2O dispersions (Figure S2 for the rheological profiles of PHG-1 and -3). The critical volume fraction of the V2O5·nH2O dispersion used to form PANI hydrogels is approximately 0.43 vol.%, which is equal to the solid content of 1.0 wt% of the PANI hydrogel. As a comparison, existing approaches to PANI hydrogels require higher solid content to reach the percolation value (4 wt% for PEDOT and 5 wt% for PANI).7, 9


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Figure 2. (A) Digital photos of V2O5·nH2O nanowire dispersions (left) and PANI hydrogels (right). (B) Photos of PANI hydrogels different in shapes. (C) TEM image of the V2O5·nH2O nanowires. (D) Rheological behavior of the sample PHG-2. (E) Raman spectra of the lyophilized PHG-2 and V2O5·nH2O nanowires. (F) FT-IR spectrum of the sample PHG-2. (G) EDX spectrum of lyophilized PANI hydrogels. (H) SEM image of lyophilized PANI hydrogels (PHG-2). (I-J) TEM images of PANI hydrogels.

The formation of PANI was confirmed by both Raman and FT-IR spectroscopy (Figure 2E and 2F). The Raman spectrum of the sample PHG-2 shows characteristic bands of doped PANI. 8 ACS Paragon Plus Environment

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The bands at 1588 cm−1 and 1470 cm−1 are attributed to C=C and C=N stretching of the quiniod units in doped PANI, respectively.16 The band at 1225 cm−1 is assigned to in-plane ring deformation of quiniod units. The bands related to C−N stretching and C−H bending of phenyl units are found at 1260 and 1167 cm−1. The bands belonging to the V2O5·nH2O constituents are not observed. The FT-IR spectrum of PHG-2 indicates that the sample is the emeraldine salt form of PANI. The two characteristic bands at 1592 and 1456 cm−1 are ascribed to the stretching vibration of the quinoid and phenyl units, respectively.17,18 The band at 1310 can be assigned to C–H stretching vibration with aromatic conjugation, and the band near 1159 cm–1 is resulted from the N=Q=N (Q denotes quinoid ring) stretching mode.18 The results confirm the in-situ oxidative polymerization of aniline leads to PANI. Both energy dispersive X-ray (EDX) spectrum and thermogravimetric analysis (TGA) were performed to examine whether the element vanadium existed in the PANI hydrogel (Figure 2G and S3). The residual weight of the sample PHG-2 at 640°C is about 0.074%, indicating that there exists no non-volatile inorganic component in the lyophilized sample. The EDX spectrum also shows that no vanadium element is observed in the same sample. The above results clearly state that the hydrogels are composed of pure PANI.


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SEM images of the lyophilized samples show that all three PANI hydrogels have similar 3D networks composed of interconnected nanofibers with extreme aspect ratios (Figure 2H and Figure S4). A close observation by TEM (Figure 2I and J) reveals that the structural units of these networks are ultrathin PANI nanofibers, which is consistent with AFM measurement results (Figure S5). The diameter statistics from the AFM image confirms the average diameter of 3.4 ± 0.8 nm (N = 6) (Figure S5). The morphological similarity between PANI nanofibers and V2O5 nanowires reveals that the skeleton of the former was well duplicated by the latter. Such ultrathin PANI nanofibers are obviously different from the short rods frequently found in conventional PANI hydrogels.2, 9, 19-22 For example, PANI nanofibers with the large diameter of ~100 nm and moderate length smaller than 1 µm were observed in a PANI hydrogel with solid content of 11 wt%.9 The Brunauer–Emmett–Teller (BET) specific surface area of lyophilized PHG-2 was determined to be 55.03 m2·g−1. The small gas uptake at low pressure (p/p0 < 0.05) in the N2 adsorption/desorption curve indicates that there exist few micropores in the sample, and the hysteresis loop reveals the existence of both mesopores and macropores (Figure S6). The N2 adsorption/desorption results are in agreement with the network structure of PHG-2. The ultrathin structural units and large porosity of the PANI hydrogels obtained in


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this study ensure fast diffusion of electrolytes both between and inside nanofibers, and consequently PANI hydrogels are expected to be promising electrode materials.22

Figure 3. (A) Photo of V2O5·nH2O dispersions and the corresponding lyophilized monolith. (B) SEM image of the lyophilized V2O5 dispersions. (C) Photo of PANI suspensions prepared by mixing an aniline acidic solution with an ammonium persulfate one. (D) SEM image of the lyophilized PANI suspension in (C).

Next, the formation mechanism of PANI hydrogels is explained in detail. It is worth noting that the V2O5 nanowires play a key role in the formation of the hydrogels. Firstly, the V2O5·nH2O nanowires in a nematic dispersion are characteristic of extreme aspect ratios (Figure S1),21 allowing for formation of dynamic 3D networks at relatively low volume


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fractions. According to colloidal theories, the critical volume fraction of shape-anisotropic colloids for formation of 3D networks decreases with the increase of their aspect ratios.23 This network can be fixed and permanently preserved by lyophilization and the obtained lyophilized monoliths maintain both the original shape and apparent volume of the dispersion (Figure 3A). Microscopic imaging verifies that the 3D porous network of the monolith is composed of massive nanofibers (Figure 3B). As the V2O5·nH2O nanowires are highly hydrophilic, the solvation effect will prevent the massive aggregation of nanowires. Therefore, the V2O5·nH2O nanowire network is dynamic and weak, and the dispersion is a viscous liquid rather than a hydrogel. Secondly, V2O5·nH2O nanowires are the excellent oxidant and template for aniline polymerization. This polymerization occurs selectively on the surface of V2O5·nH2O nanowires. Meanwhile, reduction products like V(IV) salts are soluble and hence, diffuse continuously into the bulk phase.24-26 After the completion of the reaction, the PANI nicely replicates the skeleton of the V2O5 nH2O nanowires to form nanofibrillar porous networks.24 We note that the selectivity of the oxidative polymerization occurring on the V2O5·nH2O nanowires is so high that no precipitation forms in the bulk solution. Thirdly, the dynamic V2O5·nH2O 3D network can be permanently immobilized by the fast polymerization of aniline.


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PANI nanofibers are hydrophobic and thus will form an irreversible 3D self-assembly network via multiple junctions with adjacent ones, leading to a PANI hydrogel. Therefore, the templating effect of V2O5·nH2O nanowires is a crucial decisive factor for the formation of a PANI hydrogel. In comparison, when ammonium persulfate is used as the oxidant, PANI particles tend to aggregate and then precipitate as powders instead (Figure 3C). The SEM image reveals that the precipitates are composed of irregularly-shaped particles ranging from hundreds of nanometers to several micrometers (Figure 3D). These irregularly-shaped particles have low aspect ratios for the formation of the PANI network. Obviously, the template effect of V2O5·nH2O nanowires leads to the PANI nanofibers and hence, promotes the formation of a hydrogel.

From the above mechanism discussion, we can find out that the preparation of PANI hydrogel is not dependent on the particular chemical property of PANI. Therefore, we readily extend the method to fabricate other CP hydrogels like PPY and PEDOT. For instance, an acidic pyrrole or 3,4-ethylenedioxythiophene aqueous solution was mixed with V2O5·nH2O dispersions to fabricate CPs hydrogels (Figure S7). Both hydrogels obtained are characteristic of nanofibrillar networks (Figure S8). The chemical structure of PPY and PEDOT hydrogels 13 ACS Paragon Plus Environment

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is analyzed by FT-IR and Raman spectrum (Figure S9 and S10). These results show that the oxidant-templating method using V2O5·nH2O nanowires can provide a practical, fast, general

Potential / V vs. SCE

80 (A) 1 60 Scan rate / mV s 10 40 25 20 50 0 -20 -40 75 -60 100 -80 -0.2 0.0 0.2 0.4 0.6 Potential / V vs. SCE

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approach to fabricate CPs hydrogels in aqueous phase.

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0.8 0.6 0.4

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Figure 4. Capacitive performance of PHG-2. (A) The CV curves of PHG-2 at different scan rate. (B) GCD curves of PHG-2 at different current density. (C) Specific capacitance of PHG2 and PANI film at different current density. (D) Nyquist plot of PHG-2 electrode in the frequency range of 10−1 ~ 105 Hz. (E) Capacitance retention of PHG-2 and PANI film over 10000 cycles. (F) IR drop change of PHG-2 and PANI film over 10000 cycles. Current density in (E) and (F): 32.9 A g−1 for PANI film and 35 A g−1 for PHG-2.

In combination with high conductivity of 0.12 S cm-1 and porous structure, PANI hydrogels are exploited as a supercapacitor electrode. We used PHG-2 to investigate the electrochemical property because of its moderate mechanical strength and loose network structure. The capacitive performance of PHG-2 is measured in a conventional three-electrode system. Because the PHG-2 has good mechanical strength, it is cut into a desired shape and compressed onto the Pt current collector directly, without the help of any conductive additive or binder. Figure 4(A) shows the CV curves of PHG-2 in the potential window of −0.2 to 0.8 V vs. saturated calomel electrode (SCE, all the electrode potentials are referenced to SCE except as otherwise noted) reference electrode at scan rates of 10, 25, 50 and 100 mV s−1. The waves at 0.1 and 0.5 V are attributed to the transition between luecoemeraldine/emeraldine and redox of 15 ACS Paragon Plus Environment

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hydroxide-/amino- terminated oligoanilines, which form in-situ via the degradation of PANI during the electrochemical test.27-29 The CV curves retain their shape even at high scan rate of 100 mV s−1, implying that PHG-2 electrode has fast redox rate and low internal resistance. The galvanostatic charge-discharge (GCD) curves in Figure 4B demonstrate that a plateau at 0.6 V made a large contribution to the specific capacitance, and this plateau is corresponding to the oligoanilines. The specific capacitance values at different scan rates are calculated from GCD curves and summarized in Figure 4C. A high specific capacitance of 636 F g−1 (1.54 F cm−2) is achieved at current density of 2.0 A g−1 (5.0 mA cm−2). These values are significantly higher than those obtained with existing pure PANI electrodes in literature.2, 30-33 For a comparison, a compact PANI film electrode prepared by drop-casting a PANI solution only has a specific capacitance of 465 F g−1 at current density of 2.0 A g−1. The performance of PHG-2 is also measured in an asymmetric two-electrode device, with reduced graphene oxide as cathode34 and the specific capacitance of the device is 76 F g−1 at current density of 0.58 A g−1 (Figure S11). The performance of our devices is superior to that of many other PANI-based asymmetric supercapacitors (Table S1). One reasons for the high specific capacitance of the PANI hydrogel


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is that all PANI nanofibers within the 3D porous network are accessible to the electrolyte, which could efficiently reduce the dead mass in the electrode.

In addition, the electrode based on the PANI hydrogel shows excellent rate performance with only ~1.6% capacitance loss when current density is increased from 2.0 A g−1 to 25 A g−1 (from 636 F g−1 to 626 F g−1). As a comparison, a typical 10 ~ 40% capacitance loss is detected in existing PANI-based electrodes at high current density.2, 13, 30, 35-37 The EIS data also shows the high rate performance and good conductivity of PHG-2 electrode (Figure 4D). At frequency lower than 1.0 Hz, the Nyquist plot becomes nearly vertical to the x axis, showing an ideal capacitive behavior caused by finite diffusion length. Obviously, the highly porous network together with ultrathin nanofibrillar structural subunits facilitates rapid ion transport in the hydrogel. According to the intercept of EIS with real axis at high frequency region, the equivalent series resistance of the electrode is estimated to be ~1.25 Ω, which is a small value for a PANI mass loading of 2 mg cm−2. The low resistance is ascribed to the interconnected PANI nanofibrils with good electric conductivity (0.12 S cm−1), which can provide a 3D continuous pathway for electron transport.


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Interestingly, PHG-2 electrode shows good cycling stability (Figure 4E). The capacitance retention is as high as 90% over 4000 cycles and 83.3% retention over 10,000 cycles, far superior to the PANI film electrode (50% retention after 10,000 cycles). The excellent cycling performance of our PANI hydrogel electrodes is partially attributed to nanofibrillar networks. According to a previous research, one of the reasons for the loss in specific capacitance of a PANI electrode is the mechanical damage of the electrode cause by the repeated volume change of PANI during the doping/dedoping process.38 The porous network can efficiently release the stress of cyclic swelling and shrinking of the PANI framework during the long-term charging and discharging process,22 and thus promote the stability of the PANI electrode. Electrochemical degradation of PANI is another reason for the bad cycling life of conventional PANI electrode. Figure 4F shows that the ohmic drop of PANI film dramatically increased from 0.11 V to 0.65 V in 10000 GCD cycles, caused by electrochemical degradation of PANI. However, the ohmic drop of PHG-2 only slightly increases from 0.16 V to 0.35 V, reflecting a much slower electrochemical degradation of PANI in PHG-2. We also notice that the PANI hydrogel studied herein exhibited high solvent resistance, and about 40% PANI hydrogel is insoluble in NMP. As a comparison, the PANI powder oxidized by ammonium persulfate can


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be dissolved in NMP at the same concentration. Therefore, it is highly possible that molecular chains of PANI in a nanofiber are partially crosslinked, during in-situ oxidative polymerization occurring on V2O5·nH2O nanowires. This chemical structure can effectively prevent the continual degradation of PANI and meanwhile, and lock the degradation products, like hydroxide-/amino- terminated oligoanilines, within PANI nanofibers. In turn, the presence of degradation products makes a significant contribution to the capacitance of the electrode. These above mentioned factors function synergistically in causing the PANI hydrogel with good cyclic stability.

CONCLUSIONS

In summary, we report a facile oxidant-templating method to prepare pure PANI and other

CP hydrogels by using V2O5·nH2O nanowires as the oxidant and the sacrificial template. The

extreme aspect ratios of the V2O5·nH2O nanowires allows us to prepare PANI hydrogels with

lower solid contents (1 wt%), and the highly porous network composed of ultrathin PANI

nanofibers endows the hydrogels with high capacitive performance. A PANI hydrogel can exhibit high specific capacitances of 636 F g−1 and 626 F g−1at 2.0 A g−1and 25.0 A g−1,

respectively. Meanwhile, it can maintain 83.3% capacitance retention up to 10,000 cycles. 19 ACS Paragon Plus Environment

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This method provides an avenue to fabricate conducting polymer hydrogels for many

applications, such as energy, sensors, and bio-medicine, and may also establish a technique to

process conducting polymers.

EXPERIMENTAL SECTION

Preparation of V2O5· nH2O nanowire dispersions The synthesis of V2O5·nH2O nanowire dispersion was following a reported procedure.14 1 g of NH4VO3 was grinded with few drops of DI water, and the resulted fluid was then mixed with 10 mL of 1 M HCl with continuous stirring. Afterward, as the suspension became red, DI water was added to produce a total volume of 20 mL. After sedimentation of the red precipitate, the supernatant was removed. The resulting red precipitate was dispersed in hot water (80 ~ 90°C) with a total volume was 20 mL. After vigorous stirring, the supernatant was removed. This process was repeated three times. Then the red dispersion was refilled with hot water to a total volume of 40 mL. V2O5·nH2O dispersions with nanowires different in aspect ratios were produced by changing the storage time from 5 d to 3 m.


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Preparation of Polyaniline Hydrogels. V2O5 nanowire dispersions different in volume fractions at 0.43 vol.%, 0.72 vol.%, and 0.9 vol.%, respectively, were denoted as solution A. The solution B was prepared by mixing 0.4 mL aniline with 39.6 mL DI water. To prepared PANI hydrogel, solution A was mixed with solution B in a certain volume ratio, and the mixture was shaken violently for 5 s and left still for 3 h. PANI hydrogel formed in 10 s. To remove excess acid and by-products from the hydrogel, the PANI hydrogel was dialyzed in 0.1 M HCl and a large volume of DI water.

Electrochemical tests. The electrochemical properties of PANI film and PANI hydrogels were measured by using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and A.C. electric impedance (EIS) techniques in a three-electrode system. The current collector for PANI hydrogel is Pt foil, and the PANI hydrogel was cut into a desired shape and compressed onto the Pt foil. A reduced graphene hydrogel34 pressed on Pt foil was used as the counter electrode. For PANI film, Pt foil was used as the counter electrode, and the PANI was coated onto the glass carbon electrode by drop-casting PANI solution in NMP. Saturated calomel electrode (SCE) was employed as the reference electrode and 1 M H2SO4 was used


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as electrolyte for all the electrochemical measurements. The mass loadings for PANI film and PANI hydrogel were 1.7 ~ 2.5 mg cm-2.

The electrical impendence spectra (EIS) of PANI hydrogels were obtained under their open circuit potential by A.C. impedance tests in the frequency range of 0.01 Hz ~ 1 MHz at an amplitude of 5 mV. The specific capacitance was calculated by galvanostatic charge/discharge tests using the following equation.

Cs 

It m  V  IR 

(1)

where Cs is the specific capacitance, I is the charge-discharge current, t is the discharging time, m is the mass of the dried hydrogel, V represents the potential range of the working electrode, and IR represents the voltage drop upon discharging caused by internal resistance of the system.

The two-electrode device was fabricated with a PHG-2 as the anode and a RGO hydrogel as the cathode. The specific capacitance was calculated according to the following equation:


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ACS Nano

CD 

It m U  IR 

(2)

where CD is specific capacitance of the device, m is total mass of two electrodes, I is the current applied on the device, t is the discharge time, U is the highest voltage in the GCD curves, and IR represents the voltage drop at the beginning of the discharge process, caused by internal resistance of the device.

Characterization. The morphologies of the PANI hydrogels and PANI powders were observed by a SU-70 scanning electron microscopy (Hitachi, Japan) at an accelerating voltage of 20 kV equipped with a energy dispersive X-ray analyzer. The morphology of V2O5·nH2O nanowire was observed on JEM-2100 transmission electron microscopy (JEOL, Japan) at 200 kV. The conductivity of PANI hydrogel was measured by a standard four-point-probe method at room temperature. The atomic force microscope (DI Multimode, Veeco, USA) was used to image topographic information of V2O5·nH2O nanowires and PANI hydrogels. A FEI Talos F200 environmental transmission electron microscope was employed to examine the morphology and structure of V2O5·nH2O and PANI hydrogel. The samples for TEM observation were prepared by dispersing hydrogel or V2O5·nH2O nanowire dispersion into 23 ACS Paragon Plus Environment

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ethanol by ultrasonic treatment, and dropping the obtained ethanol dispersion onto copper mesh. Rheology measurements were carried out by using a MCR 302 Rhometer (Anton Paar, Austria). Raman spectra were recorded at room temperature with a MicroRaman System RM3000 spectrometer and an argon ion laser operating at a wavelength of 633 nm as the excitation. The FT-IR spectroscopic measurements were carried out using a Nicolet IS10 Fourier transform spectrometer (Thermo-Fisher, US). Thermogravimetric analysis (TGA) was carried out on a STA 449 F3 Jupiter simultaneous thermal analyzer with a heating rate 20°C min−1 in the air condition. All the electrochemical tests were performed on a 660D electrochemical work station (CHI, USA) at room temperature. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org.

AFM image of V2O5·nH2O nanowires and PANI nanofibers, Rheological properties, TGA curve, SEM images, and nitrogen adsorption–desorption isotherm of PANI hydrogels, SEM


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ACS Nano

images and spectra of PPY and PEDOT hydrogels, capacitive performance of PANI-hydrogelbased asymmetric device, and comparison of the performance with literature data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]



Financial interest statements

The authors declare no competing financial interest.

ACKNOWLEDGMENT

We acknowledge financial support from the National Natural Science Foundation of China (21303144, 21705135, 21774104, and 21503175) and the Fundamental Research Funds for


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Xiamen University. Dr. Chuan Liu, Jinming Wang, Bin-Bin Xu, Xinyu Liu, Xiuming Zhang, Xiaoqing Qi, and Litao Yan are acknowledged for characterization and fabrication assistance.

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Graphical Table of Contents


A Hydrogel of Ultrathin Pure Polyaniline Nanofibers: Oxidant

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