Enhanced Electrochemical Performance by Strongly Anchoring Highly

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Enhanced Electrochemical Performances by Strongly Anchoring Highly Crystalline Polyaniline on Multi-Walled Carbon Nanotube Su-Min Wang, Jiayin Shang, Qiguan Wang, Wenzhi Zhang, Xinming Wu, Jian Chen, Wenhui Zhang, Shenbao Qiu, Yan Wang, and Xinhai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11567 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrochemical Performances by Strongly Anchoring Highly Crystalline Polyaniline on Multi-Walled Carbon Nanotube Sumin Wang†, Jiayin Shang†, Qiguan Wang†∗, Wenzhi Zhang†, Xinming Wu†, Jian Chen†, Wenhui Zhang†, Shenbao Qiu†, Yan Wang†, Xinhai Wang‡ †Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China ‡ School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China KEYWORDS: Crystalline polyaniline, Carbon nanotube, Electrochemical performance, Isothermal crystallization method, Nanohybrid

ABSTRACT: The multi-walled carbon nanotube (MWNT) surface was strongly anchored by highly crystalline polyaniline (PANI), slowly grown from a controlled isothermal crystallization method under π–π interactions. The crystalline PANI particles are approximately 10–38 nm thick and the space between them varies from near zero to 55 nm as reaction conditions vary. The highly crystalline nanohybrid (CNH) showed electrochemical performance enhancement compared with neat MWNT, PANI and the referenced hybrid synthesized from chemical polymerization. The specific capacitance (SC) of CNHs can be 726 F g–1 coupled with an excellent rate capability. Moreover, the strong combination between PANI and MWNT as well as the crystalline structures in PANI improved the bulk conductivity, the interfacial charge transportation and the cycling stability of CNHs. The SC value of CNHs can keep almost unchanged upon 1000 charge-discharge cycles, followed by just slight decline of 2.6% after 10000 cycle tests. XRD data shows the SC decline was mainly resulted from the structural variation of crystalline PANI. Besides, the resulting CNHs showed significant electrocatalytic behaviours towards H2O2 and exhibits a low detection limit of 4.4 µM due to the enhanced electron transportation between MWNT and 1 ACS Paragon Plus Environment


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PANI. The reported method opens a gateway to design high-performance MWNT/PANI hybrids used in electrochemical sensor, fuel cells and energy-storage related devices. 1. Introduction

The increasing requirement for green energy devices has stimulated recent research efforts into the synthesis of advanced electrode materials, in order to fabricate various electrochemical active systems with high electrochemical performances.1,2 Because of the high power density, rapid charge-discharge rate, and the long term operation stability, the capacitive materials have been significantly studied for use in sustainable energy supply.3,4 Polyaniline (PANI) is one of the most promising pseudocapacitive electrode materials, due to its low cost, facile synthesis,5 high electrical conductivity6 and good processability for large-scale devices.7,8 Based on the synthesis of nanostructured PANI with various forms such as nanowires, nanofibers, and nanotubes, the intrinsic conductive PANI has showed electrochemical capacitance higher than 400 F g– 1 9,10

.

However, its poor stability limits its actual application owing to the large volumetric swelling and

shrinkage as well as irreversible degradation of PANI chains during the charging-discharging process,11 which leads to structural breakdown and significant capacitance decay. At present, the specific capacitance (SC) of most PANI electrodes is reported no more than 80% retention after 1000 cycles.12–14 The critical reasons lie in two aspects: 1) The strong hyperconjugation effect makes PANI chain significant brittle, which tends to be cracked during the charge-discharge process owing to the large volumetric variation. 2) The intrinsic conductivity of PANI is still relatively poor, which provides high internal resistance and generates great heat during the charge-discharge cycles, resulting in the irreversible degradation of PANI chains. Carbon nanotube (CNT) is considered as a kind of promising nanomaterials, capable of being used as the support additive in many conductive polymers, because of its large accessible surface area, high mechanical strength, good electrical conductivity, and remarkable structural stability.15 To enhance the mechanical strength of PANI and its conductivity, in many studies PANI was incorporated with CNTs to

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produce hybrid electrodes, which illustrated the synergistic effect of individual constituents on the supercapacitance.16–18 In all those works19–23 chemical or electrochemical polymerization of aniline was generally used to prepare CNT/PANI composites via an in-situ approach. Because CNT can adapt to volumetric changes and protect the electrode from mechanical destruction, a good electrochemical performance for CNT/PANI composite is observed during charge-discharge cycling. The SC values for CNT/PANI composites prepared by chemical polymerization methods24–26 were 163–485 F g–1. The capacitance retention after 1000 cycles can be improved to 90–93%.27,28 On the other hand the distribution state of PANI along CNT also strongly affects the capacitive properties.29,30 It is reported the capacitive value of the core–shell MWNT/PANI composites can be as high as 619 F g–1 at a current density of 0.5 A g–1.31 Because the MWNTs core possesses excellent mechanical performances, which can effectively overcome the cycle degradation problems, the resulting core-shell composite shows 96.7% of capacitance retention after 1000 charge-discharge cycles. Aiming at the practical application, preparation of electrochemical hybrids which can endure more than 1000 (for example: 5000-10000) charge-discharge cycles at large current density (for example: 1.0 A g–1) is considerably required. However, the performances of all above CNT/PANI composites haven’t been reported to meet this requirement. The probable reasons are as follows: 1) In the chemical or electrochemical polymerization process, the PANI granules formed in the liquid phase near CNT surface have been quickly hardened32 prior to deposition on CNTs (Figure 1). This makes the adhesive strength between PANI and CNT in the obtained hybrids quite weak. Thus, CNTs cannot make well contributions to the mechanical strength of PANI due to the poor combination between them. 2) By using the chemical or electrochemical polymerization method, the prepared PANI particles were factually formed by loosely and randomly stacked PANI granules, in which masses of steric hindrance for electron transportation exist. Thus, the intrinsic conductivity of PANI doesn’t been evidently improved.



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In order to enhance the adhesive strength between PANI and CNT, maintaining PANI at monodispersed state instead of solid granules during fabricating the hybrid would be an appropriate approach. An isothermal crystallization technique was herein used above the clear temperature of PANI in a polar solvent, where PANI can be maintained at a high crystalline state during anchoring on MWNT (Figure 1), and the anchoring speed can be adjusted so slow as to allow tightly and orderly stacking of PANI chains. Finally, a three-dimensional mesoporous structure consisting of highly crystalline PANI decorated MWNTs was synthesized, according to the surface induced crystallization mechanism, in which the interactions between PANI and MWNT can be greatly enhanced compared with the traditional physical mixtures.33 The resulting crystalline nanohybrid (CNH) displayed good structural stability because of the strong combination between PANI and MWNT, as well as the high crystallization of PANI, which makes CNH show enhanced electron transportation. As a capacitive material, the CNH showed great capacitance value and high cycling stability. In addition, the CNHs illustrate significant catalytic behaviours towards H2O2 because of the enhanced electron communication.

Figure 1. Schematic representation of the formation steps for the chemically polymerized MWNT/PANI composites and the CNHs by surface induced crystallization. 2. Experimental section


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2.1 Materials Aniline was distilled under vacuum before using. MWNT samples were purchased from Chengdu Institute of Organic Chemistry, CAS, with purity over 99% and length 10–20 µm, which were used without any treatment. Ammonium persulfate (APS), hydrochloric acid (37%), sulfuric acid (98%), ammonia (28%), methanol, N-methyl pyrrolidone (NMP) and N,N-dimethylformamide (DMF) were of analytical purity and used as purchased without any further purification. Ultrapure water (18.25 MΩ · cm) was prepared by using a water purification machine from Millipore. 2.2 PANI Synthesis The amorphous PANI powder was synthesized according to a modified method reported previously.34 Briefly, by using APS (5 mmol) as the chemical oxidant, the polymerization of aniline monomer (0.4 ml, 4 mmol) in the aqueous hydrochloric acid solution (1.0 M) proceeded for 12 h in an ice-water bath. After rinsed by aqueous hydrochloric acid solution (1.0 M), methanol and water in sequence, the PANI products from the chemical oxidative polymerization of aniline were immersed in 0.1 M ammonium hydroxide solution for 2 h. Then the resulting PANI products in emeraldine base state were dehydrated by vaccum drying at 50 oC for 24 h. 2.3 CNH Synthesis CNHs of MWNT/PANI were synthesized following an isothermal crystallization procedure35 by using MWNT as seeds. In brief, 18.5 mg of emeraldine base was ultrasonicated for 10 min in 5 ml of DMF to give a saturated solution at 55 oC. Afterwards, the dark blue solution was heated to 60 oC. At this temperature, various weight ratios of MWNT were then added and the mixture was ultrasonicated for 10 min, forming a stable solution. After sonication, the solution was free cooled in air to 57 oC to induce heterogeneous crystallization and epitaxial growth of PANI on the surface of MWNT. After crystallization performed under vigorous agitation for 4 h, the final solution was then isothermally filtered to remove any remaining uncrystallized PANI, which leaves CNHs on the polycarbonates membrane 5 ACS Paragon Plus Environment


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filter (0.2 µm). To thoroughly move the amorphous PANI, the CNHs were ultrasonicated for 30 min in DMF, and then dried in a vacuum desiccator at 80 oC. According to the feed mass ratios of the MWNT to PANI (0.05:1, 0.1:1 and 0.25:1), the obtained products are designed as CNH-1, CNH-2 and CNH-3, respectively. 2.4 Capacitance measurements For analyzing the capacitance of CNHs, the powder of CNH samples was firstly mixed with acetylene black and polytetrafluoroethylene in the 85:10:5 weight ratios in NMP solvent. The formed dispersion was coated on a stainless steel mesh followed by drying at 80 oC for 2 h, and then pressed to fabricate electrochemical active electrodes. After immersed in 1.0 M H2SO4 solution for 12 h, two plates of above electrodes were symmetrically assembled with one piece of microporous film composed of polyethylenepolypropylene as the separator, which formed a sandwich structured cell. Such a cell was placed in a CR2032 coin-type battery case and sealed. The galvanostatic charge discharge test was carried out on a CT3008W Neware battery test workstation from 0.1 to 0.8 V. 2.5 Electrochemistry measurements All the electrochemical tests were carried out on a CHI 660 electrochemical workstation, using a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The working electrode was fabricated by coating MWNT, PANI, or CNH samples on the bare ITO glasses (2×2 cm). The mass loading of all samples on the modified ITO electrode is the same. For example, 20 milligram sample was firstly homogeneously dispersed in 10 milliliter DMF. Then, take about 50 µl of the dispersion and cast it on a bare ITO glass. After drying at 60 oC for 4 h, the working electrode is available. The electrochemical impedance spectra (EIS) were conducted at the open circuit potential from 105 Hz to 10‒1 Hz with 5 mV ac perturbation. 2.6 Characterization


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The CNHs were characterized by a Nicolet Magna–IR 750 Fourier transform infrared (FT–IR) spectrometer using the KBr pressed-disk technology. The UV−visible spectra were detected by using a Shimadzu 1901 spectrophotometer. UV−visible spectroscopy analysis was carried out by dispersing CNH samples in NMP solvent. The x–ray photoelectron spectra (XPS) of CNHs were performed on a Kratos AXIS 165 machine. A field emission scanning electron microscope (FE–SEM, Hitachi S–4800) and a transmission electron microscope (TEM, JEOL JEM–2010) were used to investigate the CNHs morphologies. By using a Shimadzu XRD–6000 x–ray diffractometer, the x-ray diffraction (XRD) patterns of the CNH samples were recorded. The porosity and surface area of CNH samples were analyzed using a JW-BK132F V2.07 surface area analyzer. 3. Results and discussion 3.1 Structure and morphology of CNH samples

The obtained CNHs were characterized by the FT–IR spectra, as shown in Figure 2a. For the asreceived MWNTs, the band near 1633 cm–1 is resulted from C=C graphitic stretching, and the broad peak at 3430 cm–1 together with the peak at 1636 cm–1 could be from trace amount of water in pressing KBr pellets.36 The spectrum of pristine PANI shows characteristic peaks near 1590 cm–1, attributed to the quinoid diimine unit C=C and C=N stretching vibration mode. The band for benzenoid unit C=C aromatic ring stretching is located at 1491 cm–1.37 The characteristic peak related to the secondary aromatic amine C–N stretching is near 1280 cm–1.38 It should be noted that peaks located at 1590 cm–1 related to the quinoid diimine unit stretching in the CNHs were shifted to relatively low wavenumbers (for example CNH-1 at 1587 cm–1, CNH-2 at 1577 cm–1 and CNH-3 at 1574 cm–1), compared with pure PANI, which suggests that strong interactions between carbon nanotubes and the π-electrons in the PANI chains occurred.



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Figure 2. FT–IR (a) and UV−visible (b) spectra of PANI, MWNTs and CNHs, and N1s core-level XPS spectra: PANI (c), CNH-1 (d), CNH-2 (e), CNH-3 (f). The electron interactions at the MWNT/PANI interface were analyzed by UV-vis absorption spectra. The concentration of sample solutions is 10.0 mg L–1 with NMP as the solvent. In the UV-vis spectrum for PANI (Figure 2b), two absorption peaks located at 332 and 631 nm were observed, which are attributed to π–π* transition of benzenoid ring and quinoid ring respectively. They are identical to those for dedoped PANI.39 However, in the case of CNHs, the characteristic peaks were red-shifted differently with the variation of MWNT weight ratios compared to that of pure PANI. For that of quinoid ring at 631 nm, it is red-shifted to 635 nm in the case of CNH-1, and in the case of CNH-2 and CNH-3, it is redshifted to 644 nm and 648 nm respectively. Meanwhile, the peaks related to the π–π* transition of benzenoid ring also show the similar red-shift. This phenomenon should be mainly resulted from the π–π interaction between PANI and MWNT, whose strength was enhanced from CNH-1 to CNH-2 and CNH-3 in sequence. Because the weight ratio of MWNT added during CNH synthesis is gradually increased from CNH-1 to CNH-2 and CNH-3, this results in the strength increase of π–π interactions accordingly. Therefore, it leads to larger red-shift in the hybrid synthesized from higher weight ratios of MWNT. It should be noted that the increase of π–π interaction strength in turn significantly promoted the PANI 8

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anchoring on MWNT, which enhanced PANI content in the obtained hybrids. From TGA curves illustrated in Figure S1 in the supporting information, it can be found that as more MWNTs were added in the CNH synthesis, higher PANI content was obtained in CNH samples. The actual weight content of PANI is around 21.2 wt%, 40.5 wt% and 60.1 wt% in CNH-1, CNH-2 and CNH-3 respectively. The π–π interactions between PANI and MWNT can also be understood by the analysis of the XPS spectra. N1s core-level XPS spectra with quantitative analysis for the amorphous PANI and CNHs are shown in Figure 2c–2f. As shown in Figure 2c, the N1s spectrum of the PANI could be deconvoluted to two major peaks40 centered at 398.29 eV (originating from =N–), 399.39 eV (originating from –NH–) with area fractions of 46% and 54% respectively. It indicates that in the emeraldine base approximately equal amounts of imine and amine units exit. However, the N1s XPS spectra of CNH samples showed two different peaks besides the imine and amine peaks. They are centered at 400.3 eV and 401.7 eV,41 corresponding to the radical cation nitrogen and the generated iminium ions (–N+=), respectively. The ionic nitrogen units are mainly attributed to the doping effect of MWNT on PANI through the π–π∗ conjugation. Moreover, the peak area is correspondingly increased with the PANI content increase in CNHs. In the case of CNH-1 (Figure 2d), the two peaks for ionic nitrogen units centered at 400.3 eV and 401.7 eV are about 7.8% of the total nitrogen atoms, which were increased to 11.3% and 28.5% respectively, in the case of CNH-2 (Figure 2e) and CNH-3 (Figure 2f). The reason is the π–π interactions between PANI and MWNT are enhanced in those samples, which results in the improved doping level of nitrogen atoms in PANI.



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Figure 3. TEM images of CNH-1 (a), CNH-2 (b), CNH-3 (c) and MWNT (d). Insert to (b): SAED patterns of CNH-2. Insert to (c): TEM image of single tube of CNH-3. Figure 3 shows the TEM images of as-received MWNTs and CNHs. For as-received MWNTs (Figure 3d), they showed a tube-like morphology with smooth surfaces. The tubular diameter is around 20–40nm. In the case of CNH-1 (Figure 3a), after anchoring by PANI, the surfaces of MWNTs changed relatively coarse. Besides few protrusion-shaped particles on MWNTs, large PANI aggregates were found in the system. In contrast, for the CNH-2 samples, it is evident from the Figure 3b that the central MWNT stems are anchored by many protrusion-shaped objects. The inset to Figure 3b is the selected area electron diffraction (SAED) pattern taken from the PANI on MWNTs. The well-defined rings indicate the polycrystalline characteristic of PANI. In addition, it can be seen the crystalline PANI thickness was in the range of 10–18 nm and the space between them were in the range of 20–55 nm. Interestingly, the size of crystalline PANI particles and the space between them are significantly varied with the further increase of PANI content in CNH samples. As shown from TEM images for CNH-3 (Figure 3c), the central 10

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MWNT stems are thoroughly wrapped by large PANI particles, whose size is increased to 13–38 nm, and the space between them was decreased to near zero. This is because the CNH-3 sample possesses the highest PANI loading on MWNT, as analyzed by TGA results.

Figure 4. FE–SEM images of CNH-1 (a), CNH-2 (b), CNH-3 (c) and MWNT (d).

SEM images showed the similar morphology variations. As shown in Figure 4a, CNH-1 showed tubular structures with the similar diameter as the neat MWNTs (Figure 4d). Moreover, large-sized aggregates of PANI are found besides few protrusion-shaped particles anchored on MWNTs. As the content of PANI was increased such as in CNH-2 (Figure 4b), more crystalline PANI particles were grown on MWNT. The diameter of crystalline PANI is around 16 nm, consistent with the results from TEM (Figure 3b). When the PANI content was further increased, the central MWNT stems cannot be clearly observed due to the thoroughly wrapping by the crystalline PANI (CNH-3, Figure 4c). It can be found from Figure 4c the average thickness of the crystalline PANI particles is around 20 nm, which makes the diameter of CNH increased to 55–75 nm. Figure 5a shows the XRD patterns of MWNT, CNH and PANI. The diffractogram of the pure MWNTs exhibits the typical peaks at 2θ angles 26o from inter-layer spacing and 43o from in-plane regularity, corresponding to the graphite (002) and (101) reflections, respectively.42 The amorphous PANI powders exhibit four broad peaks at 2θ angles 15.2 o, 17.8 o, 19.5 o and 23.3 o resulted from regular repetition of 11 ACS Paragon Plus Environment


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monomer unit aniline, corresponding to the (010), (012), (020) and (200) reflections of polyaniline in its emeraldine base form, respectively.43,44

Figure 5. (a) XRD patterns of PANI, CNHs and MWNT, (b) Comparative galvanostatic charging-discharging curves of MWNT, PANI, and CNHs at a constant charge/discharge current density of 1.0 A g–1, (c) Plots of capacitance of PANI and CNHs at different constant current densities, (d) Plots of the variation of capacitance retention as a function of the cycle number of PANI and CNHs at a constant current density of 1.0 A g–1, (e) X-ray diffraction pattern for original CNH sample, CNH sample after 1000 and 10000 charge-discharge cycles, (f) Nyquist plots of PANI and CNHs. Inset: Equivalent circuit used for fitting the Nyquist plots.

However, in the diffractogram of CNH sample some additional sharp narrow peaks are clearly observed besides MWNT and amorphous PANI peaks. This shows quantities of highly crystalline PANI were formed induced by the surface of MWNT during the isothermal process. Based on the FT–IR and UV–vis analysis aforementioned, it is reasonable the crystalline PANI was mainly induced by the π–π interactions at the MWNT/PANI interface. As a control experiment, no solids can be observed under the same synthesis condition as CNHs, without the presence of MWNT. In addition, the XRD peak intensity of MWNT without changing the peak position was significantly reduced, due to the anchoring by crystalline PANI on its surface. The significant increase of PANI crystallinity in CNHs can also be shown by DSC analysis. In Figure S2 from the supporting information, it can be found from the DSC curves that the crystallinity increase of PANI in CNHs gave rise to a remarkable increase of the crystalline temperature (Tc) and a large decrease of the area corresponding to exothermic peaks compared with 12 ACS Paragon Plus Environment

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amorphous PANI. The exothermic peak area decreasing was corresponding to the decrease of amorphous components and the increase of crystalline PANI ratio in CNH samples. Moreover, for the CNH-1 sample with low content of PANI, the sharp peaks corresponding to crystalline PANI cannot be found. This shows the surface induced crystallization is difficult to occur due to the weak π–π interactions between MWNT and PANI in this case. However, with the increase of PANI content in CNH-2 and CNH-3, the obtained CNHs showed narrow XRD peaks at 2θ angles 29.3o, 35.9o, 39.5o, 43.2o, 47.6o, and 48.6o, which are corresponding to the (211), (121), (221), (213), (034) and (313) reflections of PANI,45 respectively, accompanied by the disappearing of the peak at 2θ angle 15.2o and 17.6o. This showed as the π–π interactions at the MWNT/PANI interface change stronger the PANI crystallinity was more enhanced in the system. The specific surface area (SSA) and porosity of the CNHs have been studied by means of Brunauer−Emmett−Teller (BET) and Barrett-Joyner-Halenda (BJH) method, based on nitrogen adsorption−desorption isotherm.46 Figure S3a in the supporting information displays the BET isothermal plots of CNH samples, measured under the standard conditions. Compared with as-received MWNTs (143.74 m2 g–1), the SSA of CNHs (84.47 m2 g–1for CNH-1, 135.37 m2 g–1 for CNH-2, and 127.95 m2 g–1 for CNH-3) was slightly decreased, due to the loss of some microspores in MWNTs after anchored by crystalline PANI particles. Figure S3b in the supporting information illustrates the relationship between pore diameter and desorption dV/dlog(D) pore volume, according to the BJH calculation method while Table S1 in the supporting information lists the pore structure parameters of the as received MWNTs and the CNHs in detail. As shown from Figure S3b and Table S1 in the supporting information, it clearly exhibits the mean pore diameters of CNHs are larger, corresponding to higher average mesopore volumes than the neat MWNTs. In the case of CNH-2, the mean pore diameter (19.65 nm) and the average mesopore volume (0.804 cm3 g–1) showed the maximum values. The large mesopore diameter and high mesopore volume



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enable ions to rapidly migrate inside and outside the pores, which causes CNHs to be performanceenhanced electrodes. To study the potential applications of CNHs in electrodes, their conductivity was measured using the four-probe method at room temperature. For references, the hybrids from chemical polymerization (HCP) were also prepared according to the same method as PANI. The HCP samples were synthesized from the mixture of MWNT and aniline, where the weight content of aniline was adjusted as that of PANI in CNH. The products were denoted as HCP-1, HCP-2 and HCP-3 respectively, referenced by CNH-1, CNH-2 and CNH-3. Because the crystalline structure is beneficial to the electron transportation, the CNHs show higher conductivity compared with HCPs. As shown in Figure S4 from the supporting information, the conductivity of CNH samples is around twice times that of HCP samples. The CNH-2 (46.7 S cm-1) shows the conductivity almost as high as MWNT (50.3 S cm-1). 3.2 Supercapacitance of CNHs Galvanostatic charge-discharge properties of the CNHs were performed at a constant current density of 1.0 A g–1. From the charge-discharge profiles shown in Figure 5b, the discharge time (∆t) related to the voltage difference (∆V) can be found, which generally shows the capacitive performances of the electrodes. Moreover, according to the mass loading of active materials on the electrodes (m) and the discharging current (I), the specific capacitance (SC) of the two electrodes can be calculated based on the equation of CSC = I∆t/(m∆V). Thus, the SC value of MWNT and PANI is 11 and 229 F g–1, respectively. However, for CNHs, the SC value is remarkably enhanced. It reaches 309 F g–1 for CNH-1, and is increased to 726 F g–1 in the sample of CNH-2, higher than the reported value.31 This is because the relatively ordered stacked chains in the highly crystallized PANI in CNHs are easier to be found and reacted by the inserted ions compared with the entrapping effect of random coils in the amorphous PANI, which enhanced the effective electroactive sites in CNH samples. In addition, the capacitive value of CNH-2 (726 F g–1) is higher than many typical composites based on PANI/CNT, PANI/graphene and PANI/graphene oxide synthesized from various methods (Table 1). Surprisingly, the SC value was 14 ACS Paragon Plus Environment

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decreased to 386 F g–1 for CNH-3. The reason is that the MWNTs were thoroughly anchored by crystalline PANI in CNH-3 (shown as the TEM and SEM images in Figure 3c and Figure 4c), which hinders good penetration and fast motion of electrolyte ions. At the highest SC value (726 F g–1), the energy density and the power density can be calculated as 64.53 Wh kg–1 and 932 W kg–1 respectively according to the reported method.60 Notably, compared with HCP samples (Figure S5: 180 F g–1 for HCP1, 337 F g–1 for HCP-2 and 280 F g–1 for HCP-3, see the supporting information), the capacitance of CNH is also much better.

Table 1. Electrochemical performance comparison of various supercapacitors fabricated based on nanocarbon/polyaniline electroactive materials. Material

Preparative Method

Discharge Rate

Specific Capacitanc e/F g‒1

Capacity Retention (Cycling Numbers)

Reference

PANI/SWNT

Electropolymerizat ion

5 mA cm‒2

485

94% (1500)

47

PANI/MWNT

Chemical oxidative polymerization

5 mA cm‒2

328

94% (1000)

48

PANI/MWNT


1 mV s‒1

560

70.9% (700)

49

PANI/CNT

Enzymatic polymerization

5 mV s‒1

440

93% (1000)

28

PANI/MWNT film

Doping by polymeric acid

0.5A g‒1

371

60% (1000)

50

Graphene/PANI


0.1 A g‒1

1126

84% (1000)

51

Graphene-PANI NF

Solution mixing

0.3 A g‒1

579.8

96% (200)

52



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Graphene/PANI

Solution mixing

0.1 A g‒1

257

98% (1000)

13

PANI nanowire arrays/GO


0.2 A g‒1

555

92% (2000)

53

NH2-rGO/PANI


2 mV s‒1

500

‒

54

CFGO-PANI


0.3 A g‒1

525

91% (200)

55

ERGO-PANI

Electropolymerizat ion

0.47 A g‒1

716

‒

56

PANI NFs/GO

Interfacial polymerization

0.2 A g‒1

79.5

80.6% (1000) 57

PANI/GO

Pickering emulsion 0.1 A g‒1 polymerization

487.2

60% (100)

58

GO/PANI nanotube


1 A g‒1

277

94% (1000)

59

Crystalline PANI/MWNT

Isothermal crystallization technique

1 A g‒1

726

97.4% (10000)

this work

It should be noted that the columbic efficiency of CNHs (~63%) calculated from Figure 5b is worse than the PANI samples (~85%). For the crystalline PANI chains in CNHs during charging process, the volumetric swelling for charges intercalation is relatively slower than the amorphous PANI because of the stronger chain-chain interactions in the crystalline PANI, which may lead to the localized overoxidation61 on the crystalline PANI chains. Then during discharging process a part of the stored energy was used to reduce the over-oxidized polyaniline in CNHs and thus results in the decreased columbic efficiency62 compared with the amorphous PANI samples. However, the abundant mesopores support 16 ACS Paragon Plus Environment

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CNH-2 samples excellent rate capability. As shown in Figure 5c, CNH-2 samples maintain 90.5% of the initial capacitance with growing current densities from 1.0 to 10.0 A g−1, higher than those for CNH-1 (58.3%), CNH-3 (66.8%) and pure PANI (41.0%). As far as electrode materials concerned, the cycling stability is a critical parameter influencing their applications. For this reason, the cycle properties of the CNH samples were tested at 1.0 A g−1. From the comparison between TGA and DSC curves in Figure S1 and Figure S2 in the supporting information, the initial degradation temperature of crystalline PANI in CNHs is very higher than that of amorphous PANI, suggesting crystalline PANI possesses a good structural stability. Therefore, the CNH electrodes showed higher cycling stability than the pure PANI electrode (Figure 5d). In addition, it can be seen from Figure 5d that the CNH-2 electrode has the highest stability among the three CNH samples, which maintained 99.4% of the initial capacitance during the stage of 1 to 1000 cycles, higher than many composites based on PANI/CNT, PANI/graphene and PANI/graphene oxide synthesized from various methods (Table 1). This is probably because the considerable space between crystalline PANI supports CNH-2 materials enough freedom for the large volume changes during charge–discharge without the initiation of fracture in bulk.63 For CNH-3, the absence of free space between crystalline PANI limited the large volume changes of PANI during charge–discharge cycles, which cannot rapidly release the internal stress. So the structural destruction and capacitance decay occur. The cycling stability can also be seen from the evolution of crystalline PANI during charge–discharge process. As shown from Figure 5e, the XRD patterns of CNH-2 electrode were kept intact at 1000 cycles of charge-discharge compared with the original sample. Unfortunately, the typical sharp peaks attributed to crystalline PANI disappeared after 10000 cycles of charge-discharge. Meanwhile, the SC value of the CNH-2 electrode was decreased to 97.4% of its original SC. Therefore, the capacitance decay was obviously derived from the variation of the crystalline structures of PANI. In addition, accompanying with the structural variation, the columbic efficiency of CNH-2 was increased from 63% to 68.3% during the stage of 1000~10000 cycles (as shown in Figure S6 in the supporting information), which was resulted from suppression of the localized over-



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oxidation effect due to the decrease of crystalline PANI content in this stage. While the amorphous PANI showed a gradual decrease of columbic efficiency from 85% to 64% because of its structure decomposition during charge-discharge cycles. It is noted that the CNH samples also illustrate significantly enhanced stability compared with HCP samples, due to the structural stability from the strong combination between PANI and MWNT as well as the crystalline geometry of PANI. For HCP-1, HCP-2 and HCP-3, the capacitance retention is 73%, 87% and 80% after 1000 cycles, and 45%, 61% and 51% after 10000 cycles (Figure S7 in the supporting information). For the sake of understanding the electron transfer property of CNH electrodes, electrochemical impedance spectroscopy (EIS) was measured. From the semicircle diameter in the Nyquist plots, the critical parameter of charge transfer resistance (Rct) can be obtained, which manifests the electrochemical reaction facility on the electrode.59 Figure 5f shows the Nyquist plots, and the inset demonstrates the equivalent circuit for calculating Rct. As shown in the Figure 5f, the Rct value of the CNH electrodes resulted from the semicircle diameter in the Nyquist plots is lower than that of the PANI electrode (1.37 Ω). It should be attributed to the high conductivity from the strong combination between PANI and MWNT as well as the crystalline geometry of PANI, which makes the interfacial electron transportation improved and the internal resistance decreased. Interestingly, it can be found that the Rct value of CNHs showed variation with the change of PANI content. It is 1.23 Ω for CNH-1, while deceased to 1.16 Ω for the CNH-2 sample, because of the presence of more pathways for electron transportation from the high content of crystalline PANI in CNH-2 sample. However, the Rct value was increased to 1.19 Ω for CNH3 sample. The reason is the thoroughly anchoring of MWNT by crystalline PANI particles in CNH-3 hinders the facile diffusion and fast motion of electrolyte ions. Even so, the CNH samples still showed decreased resistance compared with the HCP samples with the same PANI content, because of the fast ion motion from the strong combination between PANI and MWNT as well as the crystalline geometry of PANI. As shown in Figure S8 from the supporting information, the Rct value was 3.1, 3.3 and 5.2 Ω for HCP-1, HCP-2 and HCP-3 respectively.


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Figure 6. CV curves of pure PANI (a) and CNH-2 electrode (b) in dilute aqueous sulfuric acid solution (1.0 M).

The CV curves of CNH samples at various sweep rates in Figure 6 also showed the better electrochemical performance than pure PANI. As illustrated from Figure 6, a pair of redox peaks was observed for each voltammogram, implying the electrochemical capacitance was mainly arisen from the Faradaic redox reaction. As the scan rate varied from 8 mV s‒1 to 300 mV s‒1, the CV curve shape of pure PANI (Figure 6a) was significantly changed, while in the case of NCH-2 sample (Figure 6b), both oxidative and reductive curves kept their typical shape. This showed fast electrochemical process can be more favourable in CNH-2 sample than in pure PANI, due to the fast electron motion at the presence of highly crystalline PANI in CNH-2. 3.3 Electrochemistry of CNHs

0.0012

d 0.0008

e

0.0004

I (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

c

0.0000

b

-0.0004 MWNT PANI CNH-1 CNH-2 CNH-3

-0.0008 -0.0012 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

E (V)

Figure 7. Cyclic voltammograms of the modified ITO electrodes with the pure MWNT (a, MWNT/ITO), PANI (b, PANI/ITO) and CNH composite (c, d and e, CNH/ITO) in 0.1 M PBS at pH=7.0 in the presence of 1.0 mM H2O2 at scan rate of 500 mV/s. The good electrochemical effect of CNH was exploited in a sensor that responds to H2O2. H2O2 is a product of classic reaction catalyzed by oxidase enzymes, so H2O2 detection is very important in many 19 ACS Paragon Plus Environment


fields including food, medicine and environment.59,64,65 Figure 7 shows the cyclic voltammograms of 1 mM H2O2 at the ITO electrodes modified with pure MWNT (MWNT/ITO), pure PANI (PANI/ITO) and CNH (CNH/ITO) in a 0.1 M phosphate buffer solution (PBS) at pH=7.0. Smooth curves are obtained in the case of MWNT/ITO (curve a) and PANI/ITO (curve b), with no well-defined anodic and cathodic peaks. For CNH-1/ITO electrode (curve c) with 21.2 wt % PANI, there appears a weak cathodic peak at 0.13 V. This suggests that CNH-1/ITO electrode showed electrocatalytic behaviours towards H2O2, because of the synergetic effect from the binary components. With the increase of PANI content to 40.5 wt % in the hybrids, the CNH-2/ITO (curve d) showed a pair of well-defined and quasi-reversible redox responses at 0.25 and -0.18 V, accompanied by the significant increase of redox peak currents. The net value of the reduction peak current of H2O2 obtained at CNH-2/ITO (1.1 µA) was approximately 5.8 times higher than that of CNH-1/ITO (0.19 µA). However, in the case of CNH-3/ITO (curve e) with highest PANI content (60.1 wt %), the redox peak current of H2O2 was significantly decreased, compared with the CNH-2/ITO. This was mainly resulted from the thoroughly anchoring of MWNT by crystalline PANI in CNH-3, which blocks the ionic movement and electron transportation from electrolyte/CNH interface to ITO.

0.000045

0.000027

Current (µA)

0.000036

PANI CNH-1 CNH-2 CNH-3 MWNT 20mM

CNH-2

16

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12

8

4 CNH-3 CNH-1

0

0.000018

0

4

8

12

16

20

24

28

Concentration (mM)

1mM

0.000009 0.1mM

2mM

20mM

2mM

2mM

0.000000 200

400

600

800

1000

Time (s)

Figure 8. Current–time curve for H2O2 oxidation at MWNT/ITO, PANI/ITO and CNH/ITO in 0.1 M phosphate buffer (pH=7) for the successively increasing the concentration of H2O2 at the anodic peak potential. Inset: the linear relationship between the current response and H2O2 concentration.


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The current response of H2O2 at the modified ITO electrodes was investigated under the anodic peak potential. Figure 8 compares the amperometric responses at the MWNT/ITO, PANI/ITO and CNH/ITO recorded against successive additions of H2O2. In agreement with the voltammetric data, the MWNT/ITO and PANI/ITO exhibit low amperometric response for H2O2. In the case of CNH/ITO electrodes, the CNH-2/ITO gives highest amperometric response for H2O2. The inset of Figure 8 was the linear relationship between current and H2O2 concentration, which clearly show that the CNH-2/ITO displays 101-fold and 20-fold enhanced sensitivity compared with those of CNH-1/ITO and CNH-3/ITO, respectively. From the inset of Figure 8, the CNH-2/ITO electrode gives a good linearity range from 0.1 to 20 mM and exhibits a low detection limit of 4.4 µM at a signal-to-noise ratio of 3, while it is 50.8 µM and 41.3 µM for CNH-1/ITO and CNH-3/ITO, respectively. This shows that the anchoring crystalline PANI on MWNT enhanced considerably the electrochemical activity of the CNH-2/ITO and its detection of H2O2. The detection limit of 4.4 µM is comparable with those enzyme-free electrochemical sensors from screen-printed film and graphitic carbon nitride nanosheets.66,67 Therefore, this hybrids material can be applied as a promising platform for the fabrication of electrochemical sensor, fuel cells besides electrochemical energy-storage devices. Conclusion Highly crystalline nanohybrids (CNH) were successfully prepared by strongly anchoring highly crystalline polyaniline (PANI) on multi-walled carbon nanotube (MWNT) from a controlled isothermal crystallization method. The thickness of crystalline PANI particles is around 10–38 nm and the space between them is from zero to 55 nm. The CNH showed electrochemical performance enhancement compared with neat MWNT, PANI and the referenced hybrid synthesized from chemical polymerization. The CNHs showed a high specific capacitance (SC) of 726 F g–1. Because of the high structural stability from the strong combination between PANI and MWNT as well as the crystalline structures in PANI, the SC value keeps almost unchanged upon 1000 charge-discharge cycles, and followed by just slight decline of 2.6% at the end of 10000 cycles. XRD data shows the SC decline was mainly resulted from the 21 ACS Paragon Plus Environment


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structural variation of crystalline PANI. In addition, the resulting CNHs showed significant electrocatalytic behaviours towards H2O2 and exhibits a low detection limit of 4.4 µM due to the enhanced electron transportation between MWNT and PANI, which showed the promising applications in electrochemical sensor, fuel cells and energy-storage devices. Supporting Information. TGA and DSC thermograms of as-received MWNT, amorphous PANI, CNH1, CNH-2 and CNH-3; nitrogen adsorption−desorption isotherm and pore volume vs pore radius of MWNTs and CNHs; table showing pore structure properties of MWNT and CNHs; conductivity of MWNT, CNH and HCP; galvanostatic charge/discharge plots of HCP; coulombic efficiency variation of amorphous PANI and CNH-2 samples during charge-discharge cycles; plots of the variation of specific capacitance retention as a function of the cycle number of HCPs at 1.0 A g–1 constant current density; Nyquist plots of HCPs.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: +86 29 86173324. ACKNOWLEDGMENT The program of the National Natural Science Foundation of China (Grant No. 21772152), the program of the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the program of the Natural Science Foundation of Shaanxi Province (Grant No. 2015JM5224) provided financial support for this work.

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Enhanced Electrochemical Performance by Strongly Anchoring Highly

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