Synthesis of Ag/PANI@MnO₂ core–shell nanowires and their capacitance behavior

Abstract

Polyaniline (PANI) nanowires coated with Ag nanoparticles and MnO₂ nanoflakes were fabricated via a facile two-step in situ oxidative polymerization process, and their applications for electrochemical energy storage were explored. A morphological study confirmed the formation of Ag nanoparticles on the PANI surface, which was uniformly coated by MnO₂. Electrochemical tests showed that the ternary Ag/PANI@MnO₂ nanoscale architectures displayed a high specific capacitance of 518.0 F g⁻¹ at a current density of 0.1 A g⁻¹, with a retention ratio of 88.4% after 1600 cycles at a current density of 1.0 A g⁻¹. The good performance is attributed to the synergistic effects of the different components of the Ag/PANI@MnO₂ nanocomposite system.

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Introduction

Supercapacitors have attracted intense interest because of their higher power density (compared to batteries) and higher energy density (compared to conventional dielectric capacitors).^1–3 In recent years, the ever-increasing demands for energy storage applications have attracted significant research efforts in developing novel electrode materials with higher storage capacity, better rate capability and cycling stability.^4,5 Transition-metal oxides, due to their pseudo-capacitance behavior render much higher energy density than those based on carbon materials and conducting polymers.^6,7 Among the transition metal oxides, MnO₂ is the most competitive candidate for supercapacitors due to its low cost, natural abundance, environmental safety, a very high theoretical specific capacitance (SC) of 1370 F g⁻¹,⁸ and wide operating potential range in mild electrolytes.^9,10 However, its intrinsically poor electronic conductivity limits the practical capacitance to only about one-sixth to one-fifth of the theoretical value. To further increase the electrical conductivity, composites of conducting polymers and MnO₂ have been investigated.^11,12 The conductive polymers act not only as 3-dimensional (3-D) scaffolds but also provide better conductive channels, resulting in enhanced specific capacitance and cycle stability. PANI has been recognized as one of the most promising pseudo-capacitive materials (ESI Fig. S1†) suitable for the next generation of supercapacitors since it has the merits of high pseudo-capacitance, light weight, low cost, controllable electrical conductivity, high energy density, environmental friendliness, and facile synthesis^13–19 Moreover, it exhibits excellent SC in the range typically between 500 to 3400 F g⁻¹ depending on preparation conditions,²⁰ which is substantially larger than that of conventional carbon-based electrodes (∼100–200 F g⁻¹).²¹ Among various desirable morphologies, one-dimensional (1D) nanostructured materials as building components in electrochemical energy storage are more attractive because they provide short diffusion path lengths to ions, leading to high charge/discharge rates.²² Most of the present strategies involve the formation of a 1D conducting polymer–MnO₂ composite in confined nanochannels through electrochemical or chemical reduction of KMnO₄.²³ Jiang et al.²⁴ reported the synthesis of MnO₂–PANI core–shell nanofibers by simply soaking polyaniline nanofibers in a KMnO₄ solution. Han et al.²³ reported that the combination of MnO₂ nanorods and conducting polymer nanofibers into 1D hierarchical structure had excellent electrochemical properties.

To further improve the SC of MnO₂–PANI nanocomposites, extensive research work has been focused on ternary composite films as electrodes. In these uniquely structured systems, each component contributes a specific functionality to achieve the goal of enhancing capacitance and energy density. Ternary composites have demonstrated high specific capacitance, good rate capability, and good cycling stability.²⁵ It has been reported that Ag/MnO₂ nanocomposite system exhibited higher SC than pure MnO₂.^26–29 It was proposed that the addition of Ag nanoparticles could reduce the electrical resistance of oxide pseudo-capacitor materials. Furthermore, the incorporation of Ag into PANI is thought to increase the system's electrical conductivity and thus electrochemical storage capacity. For example, Xie et al.³⁰ synthesized PANI–Ag nanocable arrays and obtained a specific capacitance as high as 850 F g⁻¹ at 10 mV s⁻¹ sweep rate. A PANI–Ag system with a Ag loading of 0.9 wt% exhibited almost a 2-fold higher SC than pure PANI.³¹ Therefore, rational design of a low cost and facile ternary nanoarchitecture system with better electrochemical performance, although challenging, is desirable. We selected to develop the Ag/PANI@MnO₂ core–shell ternary nanocomposite to achieve this goal.

In the present work, we fabricated new ternary Ag/PANI@MnO₂ core–shell nanowires. The Ag/PANI@MnO₂ system exhibited high specific capacitance and retention rate. At an applied current density of 0.1 A g⁻¹, the maximum specific capacitance of Ag/PANI@MnO₂ reached 518.0 F g⁻¹, which is about 4-, 2-, and 2-fold higher than the capacitance of using PANI, Ag/PANI, and PANI@MnO₂, respectively.

Experimental section

Chemical reagents

All chemicals used in these experiments were of analytical grade and were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). The aniline was distilled under reduced pressure before use and all the other chemicals were used as received. The deionized water were prepared by a Milli-Q Plus system (Millipore, France) with a measured electrical resistivity of 18.2 MΩ.

Synthesis of PANI nanowires

The aniline monomer was distilled under reduced pressure and H₂SO₄ and ammonium persulfate ((NH₄)₂S₂O₈, APS) were used as the dopant and oxidant, respectively. A typical synthesis procedure for PANI nanowires was as follows: 0.2 mol sulfuric acid and 0.03 mol aniline were added to a 0.2 L aqueous solution, under stirring in ice bath for about 30 min. Then, another 0.2 L aqueous solution containing APS was added dropwise to the mixture and stirred for 10 min. The molar ratio of ANi : APS was 4 : 1. The polymerization was carried out at 0 °C for 6 h. The obtained powders were filtered and washed several times with distilled water to remove residual APS oxidant, and then were dried at 80 °C for 12 h.

Synthesis of Ag/PANI nanowires

The Ag/PANI were prepared according to a previously described method.^32,33 In a typical procedure, 1 M AgNO₃ solution was made with deionized (DI) water and stirred for 10 min using a magnetic stirrer at room temperature. Then 0.05 g PANI nanowires were added to the above solution followed by 5 min ultrasonic mixing. The mixture was then subjected for 1 h stirring. Ag/PANI nanowires were obtained by drying the mixture in an oven at 80 °C for 12 h.

The incorporation of Ag nanoparticles was carried out by redox reaction between PANI and Ag⁺ ions. The emeraldine form of PANI is oxidized with silver nitrate to pernigraniline while the Ag⁺ ions are reduced to metallic silver.³⁴

Synthesis of Ag/PANI nanowires@MnO₂ nanocomposite

In a typical procedure, 0.05 g of as-synthesized Ag/PANI nanowires were added into 20 mL 0.015 M KMnO₄ aqueous solution under stirring. After the addition of KMnO₄ to aqueous solution containing conducting polymer nanowires, the redox reactions between conducting polymers and KMnO₄ should occur near the surfaces of the conducting polymer nanowires. The MnO₂ clusters nucleated and grew on the conducting polymer nanowires.²³ The mixed solution was sealed in a 30 mL Teflon-lined stainless steel autoclave, and heated in an oven at 120 °C for 2 h, and then cooled down to room temperature in air. The Ag/PANI nanowires@MnO₂ powders were filtered, washed with DI water, and dried at 80 °C for 12 h. The content of MnO₂ is 32.4% by inductively coupled plasma atomic emission spectroscopy (ICP, 7300DV, Perkinelmer) analysis.

Synthesis of PANI@MnO₂ nanowires

MnO₂@PANI composites were synthesized by following the same procedure as synthesis of Ag/PANI nanowires@MnO₂, except that 0.05 g of the as-synthesized PANI nanowires, instead of the Ag/PANI nanowires, were used.

Characterization

The morphology and crystal structure of the products were examined by a field-emission scanning electron microscope (FESEM; JEOL, JSM-7800F) equipped with an energy dispersive X-ray (EDX) system (Oxford Instruments X-Max), transmission electron microscope (TEM; JEOL, JEM-2100, 200 kV), and X-ray photoelectron spectroscopy (XPS, Escalab 250, Al Ka). XRD measurements were performed on a D8 X-ray diffractometer (X'pert Pro-1), employing monochromatized Cu K_α incident radiation.

The electrochemical properties of the as-synthesized Ag/PANI nanowires@MnO₂ nanocomposites were investigated using a three-electrode cell configuration at room temperature. The working electrodes were fabricated by mixing the prepared powders with 10 wt% acetylene black and 5 wt% polyvinylidenefluoride (PVDF) binder. A small amount of N-methylpyrrolidinone (NMP) was added to the mixture to produce a homogeneous paste. The mixture was pressed onto nickel foam current-collectors (1.0 cm²) to make electrodes (the Ni foam has good stability in Na₂SO₄ electrolyte, as shown in ESI Fig. S2†). The coated active material electrodes were dried at 80 °C overnight in a vacuum oven. The loading density of the active material is 4.0 mg cm⁻². Before the electrochemical test, the as-prepared electrode was soaked in 1 M Na₂SO₄ solution overnight.

Electrochemical tests of cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) were performed on a CHI660D electrochemical workstation (Shanghai Chenhua Instruments Co., China). All measurements were carried out in a three-electrode cell with a working electrode, a platinum plate counter electrode and an Ag/AgCl reference electrode (SCE). The electrolyte was 1 M Na₂SO₄ aqueous solution. The potential was swept between 0 and 0.9 V at a scanning rate of 5–100 mV s⁻¹. EIS tests were carried out with a frequency loop from 10⁵ Hz to 0.01 Hz using perturbation amplitude of 5 mV. The SC of the electrode is calculated from the GCD curves based on the following equation:

where C_m (F g⁻¹), I (A), m (g), Δt (s) and ΔV (V) are the SC of the electrodes, the discharge current, the mass of the active materials, the discharging time and the discharging potential range, respectively.

Results and discussion

Characterization of Ag/PANI nanowires@MnO₂ nanocomposite

The XRD patterns of PANI nanowires, Ag/PANI, PANI@MnO₂, and Ag/PANI nanowires@MnO₂ nanocomposite are shown in Fig. 1. For the Ag/PANI, the sharp crystalline peaks at 2θ values of 38.09°, 44.30°, 64.38°, and 77.33° correspond to the face-centered cubic (fcc) phase of Ag (111), (200), (220), and (311), respectively.^35,36 The same crystalline peaks were observed for the Ag/PANI nanowires@MnO₂ composite. The existence of sharp peaks in both Ag/PANI and Ag/PANI nanowires@MnO₂ clearly indicates the presence of large Ag nanoparticles with a crystalline nature in the composite. For MnO₂, two peaks at 36.38° and 65.35° were observed, and can be indexed to the monoclinic K-birnessite MnO₂ (JCPDS no. 80-1098). For pure PANI, two peaks at 20.39° and 25.25° were observed, representing the periodicity parallel and perpendicular to the polymer chain of PANI.³⁷ It was observed that all of the XRD patterns show a broad peak at a 2θ value of 15–27°. This is mainly because of the amorphous nature of PANI.

Fig. 1 XRD patterns of (a) PANI, (b) Ag/PANI, (c) PANI@MnO₂, and (d) Ag/PANI nanowires@MnO₂ composite.

Surface measurements by XPS were carried out for PANI nanowires, Ag/PANI and Ag/PANI nanowires@MnO₂. XPS N 1s spectra of PANI and Ag/PANI samples are presented in Fig. 2a and b. Usually, the N peak in XPS spectra of PANI can be deconvoluted into three distinct peaks, related to the quinoid imine, the benzenoid amine and the positively charged nitrogen. Fig. 2a and b show the two major peaks related to quinoid imine and benzenoid amine, which are observed at 399.33 eV and 400.43 eV for PANI nanowires, whereas they are observed at 398.55 eV and 399.68 eV for Ag/PANI. There is a small variation observed for the binding energies between PANI nanowires and Ag/PANI due to the presence of Ag in PANI. The Ag (3d) spectrum of the Ag/PANI sample is shown in Fig. 2c. The Ag (3d_5/2), and Ag (3d_3/2) peaks are located at the binding energies of 368.55 (0.72) and 374.52 eV (0.01), respectively. The Mn 2p spectrum is analyzed in Fig. 2d. Both the Mn 2p_3/2 peak at 642.05 eV and the Mn 2p_1/2 peak at 653.71 eV are clearly observed, in good agreement with the energy splitting of the standard spectrum of MnO₂.³⁸ The peak-to-peak separation between Mn 2p_1/2 and Mn 2p_3/2 level is about 11.7 eV, suggesting the 4⁺ oxidation state for Mn, in agreement with those reported in literature.³⁹ Fig. 2e shows the deconvoluted O 1s spectrum, where a sharp peak located at 529.69 eV and two broad peaks located at 531.20 and 532.68 eV can be observed. This spectrum is in good agreement with literature reports of 529.3–530.3 eV for oxide, 530.5–531.5 eV for hydroxide, and 531.8–532.8 eV for water.⁴⁰

Fig. 2 N 1s XPS core level spectra of (a) PANI, (b) Ag/PANI; (c) Ag (3d) XPS spectrum of Ag/PANI; (d) XPS spectrum of Mn 2p of Ag/PANI nanowires@MnO₂; (e) XPS spectrum of O 1s of Ag/PANI nanowires@MnO₂.

Fig. 3a shows the pure PANI nanowires with a diameter of 60–80 nm and a length around 500 nm. Fig. 3c and d represent the secondary electron (SE) and backscattered electron (BSE) images of the as-synthesized Ag/PANI composites, respectively. The BSE image clearly shows that the PANI nanowires were covered by small Ag nanoparticles. The Ag tends to bind with the nitrogen sites of PANI leading to interchain linkage between many adjacent PANI chains by coordination.⁴¹ The electron density on the nitrogen atoms of PANI decreases upon the formation of Ag complex, making the nitrogen atoms electron deficient. Such a configuration is supposed to enhance the conductivity of the composite system. Hence the increase in the conductivity of the PANI electrodes by Ag coating may help to enhance the specific capacitance.³⁶ After the loading of MnO₂, a layer of uniform nanoscale floccules on the Ag/PANI nanowires (vermicular structures) is observed in the ternary composite material (Fig. 3e and f), suggesting a core–shell nanostructure. EDS analysis of the ternary Ag/PANI@MnO₂ composite sample confirms the existence of Ag and Mn.

Fig. 3 FE-SEM image of (a) PANI nanowires; (b) PANI@MnO₂ nanowires; (c) Ag/PANI nanowires; (e) Ag/PANI@MnO₂ composite; BSE image of (d) Ag/PANI nanowires and (f) Ag/PANI@MnO₂ composite, (g) EDS spectrum of Ag/PANI@MnO₂ composite.

Fig. 4a shows the TEM image of PNAI nanowires coated with porous MnO₂. It can be seen that the MnO₂ nanoflakes directly grow on the PNAI nanowires. TEM images of Ag/PANI (Fig. 4b) show the attachment of Ag nanoparticles on the PANI surfaces. The Ag nanoparticles are uniformly distributed with a spherical shape and an average diameter of 2–5 nm. The agglomerated Ag nanoparticles (indicated by yellow circles) are also observed to have a diameter of 10–30 nm. Fig. 4c and d show the TEM images of Ag/PANI nanowires coated with porous MnO₂. It can be seen that the porous MnO₂ layer is composed of numerous tiny nanoflakes, these MnO₂ nanoflakes are interconnected and uniformly distributed on the Ag/PANI nanowires.

Fig. 4 (a) TEM image of PANI@MnO₂, (b) TEM image of Ag/PANI composites. (c and d) TEM images of Ag/PANI@MnO₂ composites.

Evaluation of capacitive performance

Fig. 5a shows the CV (cyclic voltammograms) curves of PANI, Ag/PANI, PANI@MnO₂, and Ag/PANI@MnO₂ samples tested in 1 M Na₂SO₄ at potential intervals from 0 V to 0.9 V vs. Ag/AgCl reference electrode at a scan rate of 5 mV s⁻¹. The CV curves clearly deviate from a rectangle, the typical shape for electric double-layer capacitors, indicating that redox processes occurred. All the samples were reversibly reduced and oxidized in the Na₂SO₄ electrolyte, which suggests that pseudo-capacitive processes occurred. For the CV curve of PANI, two pairs of redox peaks at 0.32, 0.52 V and 0.40, 0.79 V appear. These are due to the redox transition of PANI between the semiconducting state (leucoemeraldine form) and the conducting state (polaronicemeraldine form). For Ag/PANI and Ag/MnO₂/PANI composites, an additional anodic peak was observed at 0.20 V, and 0.18 V, respectively, due to the oxidation of Ag. The slight shift of this anodic peak is caused by electrode resistance. For PANI@MnO₂, the sample exhibits rectangular and symmetric CV curves, proving their ideal pseudo-capacitive nature with a fast charge–discharge process.⁴² Among all of the electrode materials, Ag/MnO₂/PANI nanocomposite showed the highest specific capacitance value of 440 F g⁻¹ at the 5 mV s⁻¹ scan rate, whereas PANI, PANI@MnO₂ and Ag/PANI exhibited specific capacitance of 96.8, 228.2 and 169.8 F g⁻¹, respectively. The enhanced electrochemical performance of the ternary Ag/PANI@MnO₂ composite electrode can be understood as a combined effect of several factors. First, the uniform coating of MnO₂ on the PANI nanowires with Ag nanoparticles yields a superior nanoarchitecture for fast diffusion of ions which may enhance the specific capacitance. Second, the Ag nanoparticles in the nanocomposite greatly increase the electrical conductivity.⁴³ There is good interaction between PANI and MnO₂ for the enhancement of the specific capacitance. And third, the protective MnO₂ layers prevent structural damage to the PANI during charging and discharging processes. Considering the fact that PANI and Ag contribute small capacitance to the electrode the major capacitance contributor is MnO₂ (ESI Fig. S3, Table S1†). The MnO₂ layers enable fast and reversible redox reactions to improve the specific capacitance, thus contributing to a high overall specific capacitance.^24,44 The CV curves of Ag/PANI@MnO₂ in 1 M Na₂SO₄ at different scan rates are shown in Fig. 5b. For all samples, the SCs decrease with increasing scan rates, revealing a charge storage process. Electrolytes (i.e., Na⁺ and H⁺) can only reach the outer surface of the electrodes at high scan rates. At low scan rates, the electrolytes have enough time to diffuse into the deep pores leading to high SC.

Fig. 5 (a) Cyclic voltammograms of the samples recorded at a scan rate of 5 mV s⁻¹ for PANI, Ag/PANI, PANI@MnO₂, and Ag/PANI@MnO₂ composite. (b) Cyclic voltammograms at different scan rates for Ag/PANI@MnO₂.

For all samples, the SCs of all as-prepared electrodes were also determined by galvanostatic charge–discharge (GCD) measurements. The GCD curves of PANI, PANI@MnO₂, Ag/PANI, Ag/PANI@MnO₂ at 0.1 A g⁻¹ current density are compared in Fig. 6. The curvature of the GCD curves indicates the pseudo-capacitive behavior of the electrodes. The IR drops in Fig. 6a (0.22, 0.17, 0.04 and 0.02 V for PANI, Ag/PANI, PANI@MnO₂ and Ag/PANI@MnO₂, respectively) indicate that the thin layer of Ag nanoparticles and MnO₂ can greatly reduce the internal resistance, and thus facilitates the transport and collection of electrons. The Ag/PANI@MnO₂ nanocomposite has the longest discharge time, which indicates that this electrode has the best electrochemical performance. The SC of the electrode can be calculated according to eqn (1). The specific capacitance of Ag/PANI@MnO₂ (518.0 F g⁻¹) is much higher than those of PANI@MnO₂ (273.1 F g⁻¹), Ag/PANI (218.3 F g⁻¹) and PANI (119.7 F g⁻¹) at the same current density of 0.1 A g⁻¹. Fig. 6b depicts the galvanostatic charge/discharge plot of Ag/PANI@MnO₂ composite at different current densities of 0.1, 0.2, 0.3, 0.5 and 1 A g⁻¹. The charge curves are symmetrical to their discharge counterparts. The ​same trend is observed for Ag/PANI@MnO₂ when further increasing current density (Fig. 6c), indicating the robustness of the as-prepared hybrid Ag/PANI@MnO₂ as electrode materials. Fig. 6d shows the specific capacitance of Ag/PANI@MnO₂ as a function of cycling number at a current density of 1.0 A g⁻¹ in the voltage range 0–0.9 V. The SC shows a slight increase for the first 200 cycles before decreases monotonically, probably due to the insufficient contact of composites with Na₂SO₄ aqueous solution at the beginning of electrochemical measurement.^45,46 A retention of 88.4% of the highest capacitance was obtained after 1600 continuous galvanostatic charge/discharge cycles. The higher cyclic stability of the Ag/PANI@MnO₂ composite can be attributed to the presence of MnO₂ over Ag nanoparticles which coated the PANI surfaces.

Fig. 6 GCD curves of (a) specific capacitance of PANI, PANI@MnO₂, Ag/PANI and Ag/PANI@MnO₂ at 0.1 A g⁻¹ current density; (b) Ag/PANI@MnO₂ at different current density; (c) specific capacitance curves of PANI, PANI@MnO₂, Ag/PANI and Ag/PANI@MnO₂ composites at different current densities; (d) specific capacitance as a function of cycle number at 1.0 A g⁻¹ of the Ag/PANI@MnO₂.

The electrochemical properties of Ag/PANI@MnO₂ were further evaluated with electrochemical impedance spectroscopy (EIS). The impedance spectra of the composite electrodes were measured in the frequency range of 100 kHz to 0.01 Hz (Fig. 7). The equivalent series resistance (ESR) of pristine PANI, Ag/PANI and Ag/PANI@MnO₂ obtained from the intersection of the Nyquist plot at the x-axis is 1.22, 1.17, and 1.05 V, respectively. Considering the similar morphology of Ag/PANI and Ag/PANI@MnO₂, the difference in ESR of electrodes can be attributed to the different conductivities of the electrode materials. The smaller ESR of Ag/PANI@MnO₂ than Ag/PANI suggests the decrease in the charge transfer resistance in the presence of MnO₂ nanoflakes. The high resistance of ion transfer in Ag/PANI would be attributed to high charge density, resulting in low capacitance.

Fig. 7 Nyquist impedance plots of PANI, Ag/PANI and Ag/PANI@MnO₂ composites.

Conclusions

In conclusion, we have prepared a high-performance electrode material by decorating PANI nanowires with MnO₂ coated on Ag nanoparticles, via a facile two-step in situ oxidative polymerization process. The as-prepared ternary composite exhibits a high specific capacitance of 518.0 F g⁻¹ with good rate and cycling stabilities in 1 M Na₂SO₄ aqueous solution. The presence of Ag nanoparticles on PANI nanowires provides a least resistance path to electrons. The synergistic effects of the combined pseudo-capacitive contributions from both the PANI and MnO₂ enhance the overall capacitance. The synthetic approach, discussed in this paper, to nanoarchitectured systems with desired properties is general and can be extended to many other similar systems for developing supercapacitors and batteries with enhanced performances.

Acknowledgements

We acknowledge the financial support of the National Natural Science Foundation (NO. 21102033), the China Postdoctoral Science Foundation (2014M551138). JL acknowledges the funding of the College of Liberal Arts and Sciences of Arizona State University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18403g

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