Tunable Three-Dimensional Nanostructured Conductive Polymer

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Functional Nanostructured Materials (including low-D carbon)

Tunable 3D Nanostructured Conductive Polymer Hydrogels for Energy Storage Application Chunying Yang, Pengfei Zhang, Amit Nautiyal, Shihua Li, Na Liu, Jialin Yin, Kuilin Deng, and Xinyu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19180 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Materials & Interfaces

Tunable 3D Nanostructured Conductive Polymer Hydrogels for Energy Storage Application Chunying Yang†, Pengfei Zhang†, Amit Nautiyal‡, Shihua Li†, Na Liu†, Jialin Yin†, Kuilin Deng†* and Xinyu Zhang†‡* †Analytical

Science and Technology Laboratory of Hebei Province, College of Chemistry

& Environmental Science, Hebei University, Baoding, Hebei, China ‡Department

of Chemical Engineering, Auburn University, Auburn, US

*Corresponding authors: [email protected]; [email protected] Abstract: Three dimensional (3D) nanostructured conducting polymer hydrogels represents a group of high performance electrochemical energy storage materials. Here, we demonstrate a molecular self-assembly approach toward controlled synthesis of nanostructured polypyrrole (PPy) conducting hydrogels, which was “cross-linked” by a conjugated dopant molecule trypan blue (TB) to form a 3D network with controlled morphology. The protonated TB by ion bonding aligns the free sulfonic acid groups into a certain spatial structure. The sulfonic acid group and the PPy chain are arranged by a self-sorting mechanism to form PPy nanofiber structure by electrostatic interaction and hydrogen bonding. It is found that PPy hydrogels doped with varying dopant concentrations and changing dopant molecules exhibited controllable morphology and tunable electrochemical properties. In addition, the conjugated TB dopants promoted interchain charge transport, resulting in higher electrical conductivity (3.3 S/cm) and pseudocapacitance for the TB doped PPy, compared with PPy synthesized without TB. When used as supercapacitor electrodes, the TB doped PPy hydrogel reaches maximal specific capacitance of 649 F/g at the current density 1 A/g. The result shows that PPy nanostructured hydrogel can be tuned for potential applications in next generation energy storage materials. Keywords: conducting polymer hydrogel; self-assembly; 3D nanostructure; trypan blue; energy storage

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Introduction Hydrogels is polymer network where polymer chains are held together by crosslinks. They have high degree of hydration and have solid 3D network resulted from cross linked chains. They are highly flexible due to their high-water content. Conducting polymer hydrogels (CPH) is a group of materials that possess both electrical conductivity and flexibility, which overcomes the intrinsic high rigidity of the conjugated system in conducting polymers, and the hydrophobicity caused by the π-π stacking of the chain.1 2 3 The synergy between conducting polymer and hydrogel combines the properties of the organic conductor and 3D network hydrogel.4 5 6 The 3D conducting framework of CPH promotes the charge carrier transportation through the network.7

8 9

Additionally, CPHs provide excellent interface between various

systems such as electrode and the electrolyte, biological and synthetic systems, which offers them great potential for various application areas, such as electrode materials for energy storage devices, biofuel cells and bioelectronics.10 11 12 13 Previous reported conducting hydrogels were interconnected nanofibers of conducting polymers that exhibit poor mechanical properties, limiting their applications in flexible electronics. On another aspect, conductive networks composed of conducting polymer and polyacrylic acid (PAA) hydrogel showed high mechanical strength.14 However, using PAA their limited mass loading of active materials (conducting polymer) results in low electrochemical performance. This route to manufacture conductive hydrogel is known as hybrid systems, where, prefabricated hydrogels are used as scaffold for conducting polymer to grow.15

16 17 18

Another way to synthesize hydrogel is

continuous phase where only conducting polymer is used as 3D network using some crosslinking agent or gelator.15 19 The hydrogel is formed by self-assembly of conducting polymer chains. In previous reported work, researchers typically used phytic acid as dopant and gelator to form conducting hydrogel. However, their electrochemical performance is not as high as expected due to the insulating property of phytic acid.16 20 This self-assembly of polymer chains can provide an effective method for the controlled synthesis of polymer hydrogels through noncovalent interactions. It provides the smallest one-dimensional (1D) nanofibrous structure that provides efficient charge and energy transfer.21 22 23 Nanomaterials with unique structural stability have important research implications for


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enhancing the capacitive properties of conductive hydrogels,24 25 26 27 and the application of structurally related properties involving such materials requires scalable and efficient morphologically tunable synthesis methods.28 29 30 In recent years, disc-shaped phthalocyanine molecules have been coupled to various metal elements that can be used as dopants and can self-assemble into highly oriented and predictable features.31

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They showed the

morphological control of polypyrrole hydrogels,36 and promoted synergistic effects to enhance both the mechanical and electrical properties. Li and co-workers used copper (II) phthalocyanine tetrasulfonate salts as a crosslinking agent to prepare a tunable 3D nanostructured polypyrrole hydrogel framework, that was used for Li-ion battery electrodes.37 Metal-ligand and conducting polymer network were used to assemble self-healing conducting hydrogel.38 Yang and co-workers used iron-based tetrakis (4-carboxyphenyl) porphyrin selfassembled polypyrrole hydrogel for water splitting.39 However, these molecules have limited application due to high cost and tedious synthesis process.34 40 Therefore, discovery of new dopants is very essential in terms of enhancement of electrical conductivity, and bridging the conducting polymer networks. At the same time, the functionalization of CPH backbones will be able to induce unexpected properties.41 In this work, we introduced a liquid crystal conjugated molecule, trypan blue contains numerous hydrophilic groups that can be used as a dopant and a gelling agent, through a static in-situ polymerization of pyrrole monomers. This leads to self-assembled nanostructured polypyrrole hydrogel with controlled morphology. The gelation process occurs simultaneously with the polymerization process. The amino group in the TB molecule is protonated in the acidic ammonium persulfate (APS) aqueous solution, and the partially protonated TB molecule is ionically bonded to the sulfonic acid groups on other TB molecules. The free sulfonate ion forms a certain spatial structure through a self-sequencing mechanism, which interacts with the PPy chain through ionic and hydrogen bonding, and the PPy chains were oriented through a self-sorting mechanism. As shown in figure 1, PPy nanofibers are formed through the proposed mechanism. Changing the dopant concentration can form nanofiber structures with different morphological sizes. Compared with polypyrrole which tends to grow into nanoparticles,42 polyaniline (PANI) is beneficial to its growth into a fibrous structure.17 In a series of



experiments, polyaniline nanofibers with controlled morphology are also obtained. Moreover, since the TB molecule is a good organic semiconductor, the dopant TB enhances the interchain charge transfer of PPy, resulting in great improvement of the conductivity and electrochemical capacitance. In addition, the hydrogel is grown on carbon cloth without the need for additional binders, which leads to good mechanical adhesion and ductility, and establishes strong electronic transmission pathway between PPy-TB and carbon cloth, the current collector.43 with a specific capacitance of 649 F/g, PPy-TB has a great potential for energy storage applications.

Figure 1. Mechanism for synthesis of polypyrrole hydrogel-TB (PPy-TB). The TB molecule selfassembles into an ordered 1D nanostructure by self-sorting mechanism and spatial effect

Experimental Section Synthesis: According to the previous report,36 1.2 mmol of pyrrole (99%, Aladdin) was dissolved in 1 ml of isopropanol, to prepare solution A. 0.03 mmol of trypan blue (60%, J&K) was dissolved in 2ml of deionized water to prepare a dopant solution, and then APS is dissolved in the dopant solution to obtain solution B. The two solutions were cooled to 4°C and then mixed together, resulting in a formation of black gel, which indicates the formation of polypyrrole hydrogel. Further purification of the polypyrrole hydrogel was carried out after 12 h reaction, using deionized water dialysis for 24 h, in order to remove impurities from the reaction system. Other dopants were used for comparison, includes direct blue 2 (0.04 mmol, DB2) (TCI) and direct violet 1 (0.06mmol, DV1) (TCI), which have three and two sulfonic acid functional groups, respectively. The same method was used to synthesize other conductive


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polymers, including polyaniline and poly 3,4-ethylene dioxythiophene (using aniline (99%, J&K) and 3,4-ethylene dioxythiophene (99%, Aladdin), respectively). In addition, PPy was synthesized in water as a blank control, using APS as oxidant. Electrode preparation: The carbon cloth (1 cm × 2 cm), used as a current collector, was washed with deionized water and ethanol to remove impurities. This was followed by treatment with concentrated HNO3 for 8 h at room temperature. The treated carbon cloth was then washed with deionized water and ethanol again, dried in a vacuum oven at 60°C for 30 minutes. The carbon cloth (1 cm × 1 cm) was dipped into the reaction mixture, immediately after mixing solutions A and B. After 12 h of reaction, the PPy hydrogel coated carbon cloth was purified by deionized water for 24 hours for electrochemical test. The galvanostatic current charge and discharge tests were carried out after vacuum drying at 50°C. Using the following equation, specific capacitance was calculated according to the galvanostatic charge/discharge curve:

Cs 

I  t m  V

（1）

where Cs is the specific capacitance, I is the current, ∆t is the discharging time, ∆V is the potential window, and m is mass of the electroactive material. Similarly, according to the equation 2, the area of the cyclic voltammetry (CV) curve is used to calculate the Cs value: Cs 

 IdV

2m  v  V

（2)

where I is the current density, m is mass of the electroactive material, v is the scan rate, ∆V represents scanned potential window. Characterization: Scanning Electron Microscopy (SEM) (JSM-7500, Hitachi) is used to observe microscopic characteristics of samples. Powder X-ray diffraction (XRD) from Bruker’s D8 ADVANCE was used to analyze crystal morphology. The Quantachrome Autosorb-IQ apparatus was used for N2 adsorption/desorption experiments. Elemental Analyzer (ce-440, Exeter Analytical, Inc, USA) was used to determine element ratio. The



Fourier Transform Infrared Spectrometer (FT-IR), used for determining the chemical structure of the sample, was Nicolet iS10 from Thermo Fisher. The pyrolysis behavior of different samples was studied using a thermogravimetric analyzer (TGA 4000, Perkin Elmer) in N2 at a heating rate of 10°C min−1 from 40 to 800°C. Electrochemical testing including CV and electrochemical impedance spectroscopy (EIS) were carried out with electrochemical workstation (CHI660E, Shanghai) in three-electrode system. The reference electrode was Ag/AgCl, the counter electrode was platinum wire, and the electrolyte was 1 M H2SO4 aqueous solution. Conductivity is measured using a semiconductor powder resistivity tester (ST-2722, Suzhou Jingge). Results and discussion

Figure 2. FT-IR spectra of PPy-TB doped hydrogels and PPy without TB.

Fourier Transform-IR (FT-IR): The structural properties of the CPHs were investigated by FTIR, as shown in figure 2. The PPy control (without TB) shows characteristic peaks as listed below: in-ring stretching of C=C bonds (1552 cm-1), ring stretching mode (1340 cm-1),44 inplane deformation of N−H bond (1045 cm-1),38 C−C stretching (1297 cm−1), in-plane deformation of the C−H bond and N−H bond (1042 cm−1),45 in-plane bending absorption peak of the C=N bond (1548 cm-1), and stretching vibrations of C=N+ bonds (1174 cm−1) and CN+-C bonds (903 cm−1) respectively.39 All the peaks mentioned above can be found in TBdoped PPy, which indicates the formation of PPy-TB nanocomposites. Compared to the PPy control sample, the characteristic peaks of TB-doped PPy slightly shifted to lower wavenumber (red shift), which indicates the conjugated TB structures promoted delocalization of the πelectrons from PPy backbones.38 In addition, we also found that there are significant red shifts in the stretching vibration peaks of C=N+ and C-N+-C functional groups in different


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concentrations of TB-doped PPy and PPy with different dopant molecules（DB2, DV1)(figure S1).

Figure 3. Comparison of XRD patterns of PPy-TB, PPy and TB

X-ray diffraction (XRD): The crystalline structure of PPy and PPy-TB hydrogel were investigated by XRD as shown in figure 3. Sharp peaks can be observed for TB at 10°, 15°, 20°, 21°, 30° and 47°. The peaks associated with crystalline TB disappeared in PPy-TB, indicating that the crystal structures of TB undergo an oxidation reaction after in-situ polymerization and a recombination of TB molecule occurred in the PPy matrix.37 The broad peak around 22° for PPy, and the peak between 20° to 30° for PPy-TB nanocomposite could be assigned to the π-π stacking of the aromatic rings of PPy. This could serve as an indication of well packed polypyrrole along the polymer backbone.46



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Figure 4. SEM images of PPy with varied concentrations of TB: (a) 0.015 M TB (PPy-TB1); (b) 0.0075 M TB (PPy-TB2); (c) 0.00325 M TB (PPy-TB3); (d) 0.001875 M TB (PPy-TB4); (e) 0.0009375 M TB (PPy-TB5); (f) no TB (PPy).（Inset：Average diameters of nanofibers of PPy hydrogels）

Morphology: Through scanning electron microscopy (SEM), TB-doped PPy tends to form interconnected nanoscale conductive matrix nanofibers rather than agglomerated bundles, which greatly increases the specific surface area and enhances the charge carriers transport properties. The morphology of PPy hydrogels reveals that concentration of TB plays an integral role in PPy self-assembly. As TB concentration decreases, the diameter of PPy nanofibers gradually decreases, i.e. the lowest average diameter (57 nm) was obtained at 0.001875 M TB. However, further reducing the TB concentration to 0.0009375 M, much higher nanofiber diameter (~110 nm) was obtained with some granular particles on the nanoﬁber surface, indicating the TB concentration affects the interaction between the TB molecules (figure 4 inset). Due to the strong molecular interaction between CPH and TB, the PPy-TB hydrogel establishes a uniform and interconnected 3D network, with possibly better mechanical properties. As a result, it reduces the agglomeration of fiber significantly and made PPy nanofibers more regularly oriented. This is due to TB facilitates more spatially tight alignment between the molecules. In contrast, the morphology of the PPy without TB is not nanofiber due


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to possible side chain formation and cross-linking along PPy chains, which interferes ordered packing for PPy nanofiber formation.47 48 In addition, spatial structure between TB molecules may prevent cross-linking among PPy backbones, and promote nanofiber growth. Therefore, TB molecules play significant role in polypyrrole self-assembly and its morphological regulation. In fact, the self-sorting mechanism of TB molecules allows free sulfonic acid groups to be spatially ordered and arranged to make PPy chains to grow to one dimensional nanostructures. This general synthetic route can be extended to the synthesis of PANI hydrogels (figure S2) with interconnected nanofiber morphology and with morphological adjustability, which indicates that the formation of nanofibers between TB molecules and polymers is mainly due to hydrogen bonding effect. However, when applying this synthetic method to polymerize 3,4ethylene dioxythiophene (EDOT), it didn’t give homogeneous nanofiber structures. It probably due to the difficulty to form hydrogen bonds between TB molecules and PEDOT due to the limited water solubility of EDOT monomers. To better understand dopant effects on PPy self-assembly, we selected direct blue 2 (DB2) and direct violet 1 (DV1) as control dopants, with tribasic sulfonic acid and two sulfonic acid groups (figure 5 (e) and (f)). DB2 and DV1 have similar molecular formulas to TB (figure 5 (d)) but exhibit completely different spatial structures, as shown in figure 5 (g), (h) and (i). The structural morphology of the PPy hydrogel is determined by both the sulfonic acid groups they have, and the structure of TB, DB2 and DV1. In figure 5 (a), (b) and (c), the morphology of PPy hydrogels synthesized by TB, DB2 and DV1 doping is nanofibers, short particle necklaces and particles, respectively. When using TB with tetrasulfonate group as dopants, smooth PPy nanofibers were obtained, while DB2 with trisulfonate group yielded agglomerated nanofibers with granules on the nanofiber surface. It was observed from SEM images that DB2 is still conducive to the arrangement of PPy chains, which indicates that it can arrange the free sulfonic acid groups in space through the self-sorting mechanism and form PPy 1D nanostructures through spatial effects. On the other hand, when PPy was doped using the DV1, it formed granular nanostructure similar to that of typical PPy morphology, which implies that DV1 behave very differently in terms of templating PPy self-assembly, compared to TB.



Figure 5. SEM images of PPy doped with different dopants: (a)PPy-TB4; (b)PPy-DB2; (c)PPy-DV1 with their corresponding structured. Molecular structure (d) TB; (e) DB2 and (f) DV1, respectively, 3D model of molecular structure ; (g) TB; (h) DB2 and (i) DV1, respectively

Porosity and specific surface area are the two key factors determining the energy storage properties of nanomaterials. To further understand the regularity and the orientation of PPyTB nanofibers, N2 adsorption/desorption isotherms were measured, and a type IV isotherm was obtained, as shown in figure 6 (a).49 50 51 52 53 The adsorption characteristics are fairly low at P/P0 lower than 0.9, there is no obvious change related to the adsorption of N2 in micropores (

Tunable Three-Dimensional Nanostructured Conductive Polymer

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