Polyaniline Hybrid Material for High

Sep 7, 2017 - Black Phosphorus Nanoflakes/Polyaniline Hybrid Material for High-Performance Pseudocapacitors. Ali Sajedi-Moghaddam†∥, Carmen C...
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Black Phosphorus Nanoflakes/Polyaniline Hybrid Material for HighPerformance Pseudocapacitors Ali Sajedi-Moghaddam,†,∥ Carmen C. Mayorga-Martinez,† Zdeněk Sofer,‡ Daniel Bouša,‡ Esmaiel Saievar-Iranizad,§ and Martin Pumera*,†,‡ †

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic § Physics Department, Faculty of Basic Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, I.R. Iran S Supporting Information *

ABSTRACT: Electrochemical supercapacitors based on conducting polymers are promising energy storage devices which can combine the merits of both rechargeable batteries and conventional electrostatic capacitors. Herein, we demonstrate that the novel hybrid electrode material made of few-layered black phosphorus (BP) and polyaniline (PANI) can outperform the constructing components. BP/PANI nanocomposite demonstrates high specific capacitance of 354 F g−1 at a current density of 0.3 A g−1. The 2D morphology of BP nanosheets provides large-surface-area support for nucleation of PANI, thus offering more ion-accessible sites to the active material.



INTRODUCTION Electrochemical supercapacitors have received considerable attention in the quest for advanced energy storage systems to power hybrid electric vehicles, portable electronic equipment, and energy back-up systems.1−3 However, the unsatisfactory energy density of supercapacitors greatly restricts their practical applications. In an attempt to overcome this challenge, the rational design and fabrication of electrode materials that provide high specific capacitance and hence large energy density is of significant importance. Among various types of electrode materials, conducting polymers such as PANI with pseudocapacitive behavior are at the center of the attention owing to their desirable features such as low cost, environmental friendliness, high conductivity in a doped state, large voltage window, and mechanical flexibility.4 In spite of these outstanding features, it should be noted that conducting polymers suffer from agglomeration which limits easy access of electrolyte ions to the active surface of the material, thus resulting in unsatisfactory specific capacitance. In addition, conducting polymers experience expansion and contraction during the doping/undoping process that leads to gradual deterioration in the electrochemical performance of the supercapacitor.5,6 An attractive strategy to overcome the aforementioned drawbacks is to incorporate 2D nanosheets into a conducting polymer matrix. Large-surface-area 2D materials provide a preferred template for nucleation and growth of conducting © XXXX American Chemical Society

polymers. Thus far, most studies have focused on compositing conducting polymers with 2D nanomaterials such as transition metal dichalcogenides and graphene-based materials for supercapacitor applications.7−13 BP is a member of a family of 2D materials that have received increasing interest since its successful demonstration for fieldeffect transistors.14,15 From the structural point of view, it shows a puckered honeycomb lattice in which each phosphorus atom is covalently bonded with three adjacent phosphorus atoms, and the layers are stacked together by van der Waals interactions.16 Featuring outstanding properties such as tunable bandgap,17 good charge carrier mobility,18 anisotropic magnetic and electronic properties,19,20 high optical transparency, intrinsic hydrophilicity,21 inherent electrochemistry,22 and biocompatibility,23 few-layered BP has been utilized in diverse applications such as optoelectronic devices,24 photovoltaics,25 biomedicine,26,27 fiber lasers,28 and sensors.29−31 In addition, BP has been demonstrated to be an attractive electrode material for electrochemical energy storage applications such as rechargeable batteries and supercapacitors.32−36 The interlayer spacing in BP is about 5.5 Å which can provide a good reservoir for intercalation and deintercalation of small ions.37 As a low cost and abundant material, the theoretical specific capacity of Received: July 14, 2017 Revised: August 18, 2017

A

DOI: 10.1021/acs.jpcc.7b06958 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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BP can reach as high as 2596 mAh g−1 with most of its capacity at the discharge potential window of 0.4−1.2 V.36 Very recently, liquid-exfoliated BP nanoflakes have been utilized in constructing all-solid-state supercapacitors. The fabricated device delivers a capacitance of 45.8 F g−1 at the scan rate of 10 mV s−1 and demonstrates excellent flexibility.35 Herein, we report the electrochemical performance of a BP/ PANI hybrid material with very high capacitance for the first time. The obtained nanocomposite demonstrated superior capacitive performance when compared with pure PANI and pure BP nanosheets. The improvement in the capacitive performance of BP/PANI hybrid is due to the synergistic effect between BP nanosheets and PANI. BP not only functions as a support material, but also offers a large surface for the growth of PANI and hence results in improved accessibility of electrolyte ions to the active material.

1:1.5). Polymerization was started by rapid mixing of precooled solutions, and the final solution was kept at −15 °C for 24 h. Reaction products were collected by centrifugation and dried under vacuum. For comparison, PANI was synthesized at 100 mM aniline in the absence of BP nanosheets via a similar procedure. In order to investigate the effect of BP concentration, BP/PANI nanocomposites were also synthesized by different concentrations of BP nanosheets (0.12 mg mL−1 and 0.60 mg mL−1). In addition, BP/PANI nanocomposites were also synthesized by varying the initial concentration of aniline monomer. For this purpose, aniline monomer with different concentrations of 5 mM, 10 mM, and 50 mM was used at a fixed concentration of BP (0.36 mg mL−1) with the same conditions as those described above. Apparatus. X-ray photoelectron spectroscopy (XPS) was carried out with a Phoibos 100 spectrometer with a monochromatic Mg X-ray radiation source (SPECS, Germany). Contact angle measurement was performed using an Theta Lite optical tensiometer (Biolin Scientific, Sweden). An inVia Raman microscope (Renishaw, England) with a CCD detector was used for Raman spectroscopy in backscattering geometry. An Nd:YAG laser (532 nm, 50 mW) with a 50× magnification objective was used for measurements. X-ray diffraction was done with a Bruker D8 Discoverer diffractometer in Bragg− Brentano parafocusing geometry. Transmission electron microscopy images were obtained using an EFTEM Jeol 2200 FS microscope. The sample was prepared by drop casting of sample suspension (1 mg/mL) on a 200 mesh TEM grid. Elemental maps and EDS spectra were enquired with an SDD detector X-MaxN 80 T S from Oxford Instruments (England). Electrochemical Measurements. Cyclic voltammetry (CV), Galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS) experiments were conducted Autolab III electrochemical analyzer (Eco Chemie B.V., Utrecht, The Netherlands), controlled by Nova Version 1.11 software in a three electrode electrochemical cell. A glassy carbon (GC), platinum, saturated AgCl/Ag, and 1 M H2SO4 aqueous solution were used as the working electrode, counter electrode, reference electrode, and electrolyte, respectively. Cyclic voltammograms were collected between −0.4−0.6 window potential versus AgCl/Ag with scan rates ranging from 25 to 400 mV/s. The polished GC electrode was modified with 6 μL of active material dispersed in deionized water at a concentration of 1 mg/mL.



EXPERIMENTAL SECTION Chemicals and Reagents. Red phosphorus (99.999%), aniline monomer, sulfuric acid, hydrochloric acid, and ammonium persulfate (≥98%) was purchased from SigmaAldrich. Ethanol (≥99.5%) and N,N-Dimethylformamide (DMF) were obtained from Fisher Scientific and PENTA, respectively. Deionized water was used in all experiments. Synthesis of BP Nanosheets. A 10-g portion of red phosphorus was covered with graphite foil and put in a high pressure and high temperature uniaxial pressing apparatus of 1″ size. The sample was pressed under a pressure of 6 GPa and heated (100 °C min−1) to 600 °C for 30 min and then cooled back to room temperature at a rate of 100 °C min−1. Then, the graphite foil was removed by polishing. The obtained BP crystals was ground in an agate mortar followed by sieving to obtain particles with sizes smaller than 0.5 mm. Afterward, BP powder was dispersed in DMF (9 mg mL−1) and sonicated for 15 min. Afterward, milling using shear force milling apparatus at 17 000 rpm was carried out in a glass jacketed vessel under an argon atmosphere at 15 °C for 1 h. Synthesis of BP/PANI Nanocomposites. BP/PANI nanocomposites were synthesized by in situ oxidative polymerization in the presence of BP nanosheets and aniline monomer. In a typical procedure, 10 mL of BP dispersion with concentration of 0.36 mg mL−1 in 1 M HCl aqueous solution was obtained by ultrasonication. Then, 10 mL of ethanol was added into the reaction solution to prevent it from freezing. Afterward, 100 mM (228.25 μL) aniline monomer was added into the above solution and sonicated for half an hour to form a uniform mixture, followed by cooling to −15 °C. The Ammonium Persulfate (APS) as oxidant was dissolved in 5 mL of aqueous HCl solution and cooled to −15 °C. (The mole ratio between APS and aniline monomers was controlled to be



RESULTS AND DISCUSSION The synthesis of BP/PANI nanocomposite was conducted through in situ oxidative polymerization of aniline monomers in the presence of few-layered BP suspension. Bulk crystals of BP were first synthesized by a previously reported high-pressureB

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Figure 1. (A) TEM and HRTEM images of BP nanosheet; inset in (A) is the corresponding SAED pattern (B) TEM image of BP/PANI nanocomposite and corresponding elemental mapping images of phosphorus (P), nitrogen (N), and carbon (C).

conversion method, using red phosphorus as precursor.38 Then, BP nanosheets were obtained by shear-force milling. Figure S1 of the Supporting Information (SI) shows the Raman spectroscopy and X-ray powder diffraction (XRD) of the BP nanosheets, data in good agreement with the literature.31 Afterward, the polymerization process was carried out by in situ polymerization of aniline monomer in the presence of BP nanosheets (see Scheme 1). The morphology and crystal structure of BP nanosheets and BP/PANI nanocomposite were investigated using transmission electron microscopy (TEM). A typical TEM image of fewlayered BP nanosheets with lateral dimensions of approximately 200 nm can be observed in Figure 1A. The high-resolution TEM (HRTEM) image illustrates the orthorhombic crystalline structure of the BP nanosheets, which is further confirmed by the selected area electron diffraction (SAED) pattern shown in the inset of Figure 1A. The few-layered BP nanosheets with large surface area serves as a 2D substrates for the

polymerization and uniform growth of PANI. As can be seen from Figure 1B, PANI has successfully grown on the surface of BP nanosheets. The successful growth of PANI on BP nanosheets was further verified by elemental mapping images of phosphorus (P), nitrogen (N), and carbon (C). X-ray photoelectron spectroscopy (XPS) was utilized to probe the chemical composition and structure of BP nanosheets and BP/PANI nanocomposite. For exfoliated BP nanosheets, three peaks located at 129.8, 130.6 (shoulder peak), and 134.6 eV are observed (Figure 2A). The peaks at 129.8 and 130.7 eV correspond to the P 2p3/2 and P 2p1/2 of phosphorus, respectively.39 The higher binding energy peaks at 134.4 and 135.5 eV can be assigned to the partial surface oxidation of BP nanosheets.40 Figure 2B indicates the deconvolution of BP P 2p core-level spectra in the BP/PANI nanocomposites. Three peaks located at 129, 130.1, and 133.5 eV are observed. The lower intensity of P−P bonding in comparison to P−O bonding can be assigned to the C

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(CC stretching) at 1614 and 1469 cm−1, respectively, indicates the presence of PANI in the composite. The surface wettabilities of BP nanosheets, PANI, and BP/ PANI nanocomposite were investigated by water contact angle measurements (see Figure S3). For BP nanosheets, the average contact angle is less than 5°, indicating its high hydrophilicity. This hydrophilic nature of BP can be assigned to its strong outof-plane dipolar moment.21,41 Interestingly, PANI thin films display a strong hydrophilic property with an average contact angle of around 5° which is in agreement with previous reports.42 The BP/PANI nanocomposite exhibits a superhydrophilic nature which originates from the hydrophilic nature of both oxidized BP nanoflakes43 and PANI. This interesting feature is highly desirable in ion adsorption for the development of high-rate supercapacitors.44,45 In an attempt to optimize the BP/PANI nanocomposite, a polymerization process was initiated with four different aniline concentrations of 5 mM, 10 mM, 50 mM, and 100 mM. The electrochemical performance of these nanocomposites was evaluated by cyclic voltammetry (CV) using a three-electrode system over scan rates of 25−400 mV s−1 in 1 M H2SO4 electrolyte, and the potential window between −0.4 and 0.6 V. For the samples prepared with initial concentrations of 5 mM and 10 mM, no peaks related to PANI were observed which suggests unsuccessful polymerization of PANI (Figure 3A inset). In the sample with an initial concentration of 50 mM, very small peaks around 0.3 and 0.1 V can be observed which are related to redox peaks of PANI (Figure 3A inset). However, for BP/PANI nanocomposite with an initial aniline concentration of 100 mM, a large integration area of CV curve with a pair of distinct redox peaks around 0.2 and 0.05 V related to the typical pseudocapacitive behavior of the PANI can be seen (Figure 3A), which arises from the transitions between a semiconducting state (the leucoemeraldine form) and a conducting state (the emeraldine form).46 In addition, cyclic performance BP/PANI nanocomposites with different concentrations of BP at a fixed aniline concentration of 100 mM were investigated. No significant changes in the CV curves were observed (Figure 3B). On the basis of the obtained results, we selected a BP/PANI nanocomposite with an aniline concentration of 100 mM for detailed investigation. Figure 4A depicts the cyclic voltammetry (CV) profiles of BP/PANI nanocomposite over scan rates of 25−400 mV s−1. With increasing scan rate, the redox current increases clearly which indicates good rate capability of the electrode material. Furthermore, by increasing the scan rate, a positive shift of

Figure 2. High-resolution XPS spectra of (A) P 2p region of BP nanosheets before polymerization, (B) P 2p, and (C) N 1s regions of BP/PANI nanocomposite.

considerable degree of oxidation due to the presence of oxidant in the polymerization process. In addition, the presence of N 1s in the BP/PANI nanocomposites indicates that aniline was successfully polymerized onto the BP surface (Figure 2C). Further, Fourier-transform infrared spectroscopy (FTIR) was performed on BP/PANI nanocomposite (see Figure S2). The appearance of the quinonoid and benzenoid ring vibrations

Figure 3. (A) Influence of aniline monomer concentration upon cyclic voltammetry performance of BP/PANI nanocomposite (red: 5 mM, blue: 10 mM, green: 50 mM, and black: 100 mM); Inset is the enlarged CV curves of BP/PANI nanocomposite at lower initial concentrations of aniline. (B) Influence of concentration of BP nanosheets on the cyclic voltammetry performance of BP/PANI nanocomposite (black: 0.12 mg mL−1, red: 0.36 mg mL−1, and blue: 0.60 mg mL−1). D

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charge storage capacity of the supercapacitor is the largest for the BP/PANI nanocomposite when compared to bare BP or bare PANI (Figure 5A), implying the synergistic effect between BP nanosheets and PANI. For BP/PANI and pristine PANI, two pairs of redox peaks can be seen which correspond to the typical pseudocapacitive characteristic of PANI. Thus, the capacitance of BP/PANI originates from surface redox reaction between PANI and electrolyte. Galvanostatic charge/discharge measurements of BP, PANI, and BP/PANI nanocomposite performed at 0.3 A g−1 are shown in Figure 5B. The curves related to PANI and BP/PANI nanocomposite show capacitive behavior with almost symmetric charge−discharge curves. Moreover, the slight deviation from linearity is characteristic of a pseudocapacitive contribution, which indicates that the capacitances of PANI and BP/PANI nanocomposite originate from pseudocapacitance. Furthermore, Figures S4 and S5 show the CVs at various scan rates (25−400 mV s−1) and galvanostatic charge/discharge curves at various current densities (0.3−1.7 A g−1) for pure BP and PANI, respectively, that demonstrate their poor performance against the BP/PANI nanocomposite. The rate performance of BP/PANI composite and pure PANI was evaluated and compared by charging/discharging at different current densities, as shown in Figure 5C. As can be seen, the specific capacitance of both materials decreased with increasing discharge current densities. However, the BP/PANI composite demonstrated higher capacitance in comparison to PANI at all current densities. The specific capacitance of BP/ PANI was 354 F g−1 at a discharge current of 0.3 A g−1 and was still as high as 308 F g−1 (87% of initial value) even at a discharge current density of 1.7 A g−1. Electrochemical impedance spectroscopy (EIS) measurements were carried out to understand the electrochemical process at the interface of the electrode and the electrolyte. The Nyquist plots of BP, PANI, and BP/PANI electrodes were obtained by EIS measurements over a frequency range of 10 000 to 0.1 Hz (Figure 5D). There are no semicircles present in all the electrodes in the high-frequency regions, suggesting very small electron transfer resistances. In the low frequency region, the straight line of the BP/PANI composite is more vertical in comparison with constructing components, more closely to an ideal capacitor. All the above-mentioned results demonstrate that the BP/PANI composite showed much better supercapacitive performance compared to other electrodes.

Figure 4. BP/PANI nanocomposite pseudocapacitor performance. (A) Cyclic voltammograms of BP/PANI obtained at various scan rates (25−400 mV s−1). (B) Galvanostatic charge/discharge curves at various current densities (0.3−1.7 A g−1). (C) Cyclic stability of BP nanosheet/PANI for 175 cycles at current density of 0.3 A g−1.

oxidation peak and a negative shift of reduction peak are observed, which can be assigned to the resistance of the electrode material.47 Galvanostatic charge/discharge curves of BP/PANI were also recorded at different current densities in a voltage window from −0.1 to 0.6 V (Figure 4B). The specific capacitances were calculated using the equation C = IΔt/mΔV, where C, I, Δt, m, and ΔV are the specific capacitance (F g−1), current density (A), discharging time (s), mass (g), and potential window (V), respectively. The specific capacitance of BP/PANI reaches 354 F·g−1 at 0.3 A·g−1 and 308 F·g−1 at 1.7 A·g−1. Moreover, no obvious IR drop was observed in the charge/discharge curves due to low internal resistance. The cycle life stability of the capacitive behavior of BP/PANI was also evaluated without the use of any organic binder (Figure 4C). The nanocomposites exhibit an excellent long cycles life by showing 96% of the initial capacitance retention after 175 cycles. The compact interfacial contact between BP and PANI should be responsible for the long-term cycling stability. Figure 5 compares the electrochemical performance of pure PANI, BP nanoflakes, and BP/PANI nanocomposite at the scan rate of 400 mV s−1. Cyclic voltammetry indicates that the



CONCLUSIONS In summary, we reported a facile strategy for large-scale synthesis of a novel BP/PANI hybrid nanostructure, which was used as a high-performance pseudocapacitor electrode material. The BP layers provide a large surface area for the growth of PANI, allowing for efficient charge storage with enhanced ion transport. In addition, the intrinsic hydrophilicity of the nanocomposite results in the effective diffusion of electrolyte ions during the charge/discharge process. As a result of these unique features, the BP/PANI nanocomposites exhibit outstanding capacitive performance, excellent rate capability, and long-term cycling stability when compared to pristine BP and PANI. We believe that such low-cost, high-performance, and lightweight electrode material, which can be fabricated using facile and scalable methods using abundant sources, may guide the way for developing the next generation of high-performance energy storage devices. E

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Figure 5. (A) Comparison of cyclic voltammetry curves of BP nanosheets, PANI, and BP/PANI at a scan rate of 400 mV s−1. (B) Comparison of galvanostatic charge/discharge curves of BP nanosheets, PANI, and BP/PANI at a current density of 0.3 A g−1 in the voltage range of −0.1−0.6 V. (C) Specific capacitance of PANI and BP/PANI at different current densities. (D) Nyquist plots of BP nanosheets, PANI, and BP/PANI obtained by applying 5 mV signal amplitude with a frequency range from 10 kHz to 0.1 Hz. Inset is an enlarged view of the Nyquist curves.



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06958. Raman spectra of BP nanosheets; XRD pattern of BP nanosheets; hydrophilicity/hydrophobicity investigation of BP, BP/PANI, and PANI thin films; cyclic voltammetry and Galvanostatic charge/discharge curves of BP nanosheets; cyclic voltammetry and Galvanostatic charge/discharge curves of PANI; and FTIR spectrum of BP/PANI nanocomposite (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.P.). ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest. ∥ On leave from Tarbiat Modares University.



ACKNOWLEDGMENTS Z.S. and D.B. were supported by Czech Science Foundation (GACR No. 15-09001S) and by specific university research (MSMT No. 20-SVV/2017). This work was created with the financial support of the Neuron Foundation for science support and Iran Science Elites Federation. This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR).



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DOI: 10.1021/acs.jpcc.7b06958 J. Phys. Chem. C XXXX, XXX, XXX−XXX