Hierarchical ZnO@MnO₂@PPy ternary core–shell nanorod arrays: an efficient integration of active materials for energy storage

Abstract

In this paper, ZnO@MnO₂@PPy ternary core–shell nanorod arrays (NRAs) were fabricated through the layer-by-layer process. In this process, the incorporation of polypyrrole, a highly conductive material, on the surface of a binary ZnO@MnO₂ core–shell structured composite is adopted to optimize the charge transfer process to further improve the electrochemical performance. Because of enhanced electron transfer capability, charge transfer resistances of the ZnO@MnO₂@PPy ternary core–shell nanorod arrays are reduced and the electrochemical performances are improved. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (1281 F g⁻¹ at a current density of 2.5 A g⁻¹), together with a considerable areal capacitance (1.793 F cm⁻² at a current density of 3.5 mA cm⁻²). Furthermore, a capacitance retention of 90% after 5000 charge–discharge cycles at 5 A g⁻¹ is obtained, indicating the excellent cycling stability of the ZnO@MnO₂@PPy ternary core–shell electrode. The superior electrochemical capacity demonstrates the potential of ZnO@MnO₂@PPy ternary core–shell NRAs to further improve the performance in supercapacitor electrodes.

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1. Introduction

The rapid increase in the demand for renewable energy has driven the development of electrochemical energy storage (EES) and conversion devices. Among the effective and practical technologies for energy storage and conversion, electrochemical capacitors (ECs), also known as supercapacitors, are considered to be one of the key technological systems, which have a wide range of applications because of their high power density, long cycling life and environmental friendliness.^1–5 In general, the electrode materials of supercapacitors can be classified into three groups: (i) carbon materials,^6,7 (ii) conducting polymers^8,9 and (iii) metal oxides.^10–13 In addition to a high specific surface area, the rational design of nanoarchitectures for EC electrodes is an effective way to improve electrochemical performance because the nanoarchitecture gives a broader real reaction area and enhances efficient charge and mass exchange.^14,15 Thus, attempts for developing a high-performance electrode material are becoming increasingly common.

Among the various types of reported pseudocapacitive materials, transition metal oxides have been considered to be the widest range of electrode materials with novel pseudo-capacitive features.^16–18 As an ideal example of these so-called transition metal oxides electrode materials, MnO₂ has attracted considerable attention due to its high theoretical specific capacitance (1475 F g⁻¹), low-cost, and environmental friendliness.^19,20 However, for a relatively high number of the previously reported MnO₂-based pseudo-capacitors, poor electrical conductivity still restrains the realization of their high theoretical capacitance. To overcome this limitation, tremendous efforts have been devoted to the following structural constructions. Among these strategy, on the one hand, optimizing the core material becomes a feasible option. Such as Wang and co-workers incorporate more highly conductive material into the core structure to solve the problem of high charge transfer resistance. Their contrastive electrochemical results demonstrate that higher conductivity core material improves the charge transfer process.²¹ Moreover, porous core material also accomplishes this effect.^22,23

On the other hand, according to above strategy, optimizing the shell material also becomes a feasible technique. However, to date, to the best of our knowledge, there are no reports on the synthesis of the ZnO@MnO₂@PPy ternary core–shell nanorod arrays by combining the merits of these two methods via a simple hydrothermal process and electrochemistry polymerization process.

In this paper, our main aim is to incorporate more highly conductive material into the binary core–shell structure to solve the problem of high charge transfer resistance. The ZnO@MnO₂@PPy ternary core–shell NRAs is fabricated through the layer-by-layer process, which involves a simple hydrothermal process and electrochemistry polymerization process. In this process, the ZnO nanorod is used as a scaffold and support the capacitance materials, the main reason is that the ZnO nanomaterials have a high chemical stability and high specific surface area. Furthermore, our as-prepared ZnO nanorod regularly and vertically grow on the surface of the Zn foil, which is conducive to depositing large numbers of MnO₂ nanofilm. As is well-known, the MnO₂ nanomaterials serve as one of the pseudocapacitance material, which occupy a high theoretical specific capacitance (1475 F g⁻¹). Meanwhile, the PPy layer are deposited on the surface of the ZnO@MnO₂ core–shell nanomaterial to enhance electrical conductivity and make full use of the high theoretical capacitance. Therefore, our as-prepared ZnO@MnO₂@PPy ternary core–shell NRAs occupy a high theoretical specific capacitance. As a proof, the ZnO@MnO₂@PPy ternary core–shell NRAs are described in detail. Compared with the binary core–shell composite, electrochemical analysis results show that the charge transfer process of the ZnO@MnO₂@PPy ternary core–shell NRAs composite is improved. In addition, with the advantage of high electrical conductivity of the PPy layer, the hybrid ZnO@MnO₂@PPy ternary core–shell NRAs exhibited a high specific capacitance of 1281 F g⁻¹ (1.793 F cm⁻²) at a current density of 2.5 A g⁻¹. These results indicate that the incorporation of a highly conductive material is an effective technique to improve the electrochemical performance of binary core–shell structured composites. Furthermore, a capacitance retention of 90% after 5000 charge–discharge cycles at 5 A g⁻¹ is obtained, indicating the excellent cycling stability of ZnO@MnO₂@PPy ternary core–shell NRAs electrode. The superior electrochemistry capacity demonstrates that the ZnO@MnO₂@PPy ternary core–shell NRAs are promising electrode materials for supercapacitor applications.

2. Experimental sections

2.1 Synthesis of ZnO nanorod arrays

The ZnO nanorod arrays (ZnO NRAs) was synthesized by a simple hydrothermal method. Typically, zinc foil (1 cm × 2 cm) was cleaned from consecutive ultrasonication in acetone, ethanol and distilled water. Then the Zn foil was immersed in a vertical position into 60 mL Teflon lined stainless steel autoclave which contains homogeneous 20 mL distilled water, 2.5 M ammonium hydroxide. Finally the autoclave was sealed and maintained at 90 °C for 8 h, and cooled naturally to room temperature. The products were washed with ethanol and distilled water for several times. Finally, the prepared the ZnO nanorod materials were dried at 60 °C for 2 h.

2.2 Construction of hybrid ZnO@MnO₂ core–shell nanorod arrays

The as-prepared ZnO NRAs samples were used as the scaffold for MnO₂ nanofilm growth by a facile hydrothermal process. 0.5 M KMnO₄ were mixed with 20 mL deionized water and stirred for 10 min to form a precursor solution, which was then transferred into 60 mL Teflon-lined stainless steel autoclave. Subsequently, as-prepared ZnO NRAs samples on Zn foil were immersed into the solution. The autoclave was sealed and hydrothermally treated at 120 °C for 8 h. After the product was cooled down to room temperature, the as-prepared ZnO@MnO₂ sample on Zn foil were washed using ethanol and deionized water. Finally, the prepared the ZnO@MnO₂ NRAs materials were dried at 60 °C for 2 h. The loading mass of the ZnO@MnO₂ sample on the Zn foil was about 1.2 mg cm⁻² exclude the weight of the Zn foil.

2.3 Construction of hybrid ZnO@MnO₂@PPy ternary core–shell nanorod arrays

For synthesis of ZnO@MnO₂@PPy ternary core–shell nanowire arrays, the self-supported ZnO@MnO₂ NRAs was directly used as backbone for the growth of PPy layer by electro-deposition process.^24,25 Briefly, the electro-deposition was conducted by a CHI760D electrochemical workstation with a standard three-electrode cell at room temperature. The reaction solution for electro-polymerization was obtained by mixing pyrrole (50 mM), LiClO₄ (40 mM) and SDS (56 mM) in 100 mL ultrapure water. The deposition of PPy layer was carried out at 0.8 V (vs. Ag/AgCl) for 90 s. The loading mass of the ZnO@MnO₂@PPy sample on the Zn foil was about 1.4 mg cm⁻² exclude the weight of the Zn foil.

2.4 Material characterization

The morphologies and structures of samples were characterized by field-emission scanning electron microscopy (SEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectroscopy detector (EDS), transmission electron microscopy (TEM Tecnai G2 F20 U-TWIN), X-ray diffraction (XRD, Rigaku, Max-2200 Cu Kα radiation, λ = 0.15406 nm) and X-ray photoelectron spectroscopy (XPS; ESCALAB250).

2.5 Electrochemical measurements

All the three-electrode electrochemical measurements were carried out at ambient temperature in 1 M Na₂SO₄ aqueous solution as the electrolyte with a Pt wire and Ag/AgCl (saturated KCl) electrode used as the counter electrode and reference electrode, respectively. For comparison, Zn foil under the same pretreatment and the prepared ZnO@MnO₂ and ZnO@MnO₂@PPy NRAs grown on Zn foil (1 cm × 2 cm) were directly used as the working electrode independently. All potentials were referred to the reference electrode. The electrochemical performances of the samples were performed on a CHI760D (Chenhua, Shanghai) workstation for cyclic voltammetry (CV), galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy (EIS) tests. The specific capacitance (C_sp) or areal capacitance (C_a) in the three-electrode was calculated from the discharge curves using the following eqn (1) and (2):^26,27

3. Results and discussion

3.1 Morphology and structure characterization

The ZnO@MnO₂@PPy ternary core–shell NARs was synthesized by a layer-by-layer process. Fig. 1 illustrates the formation mechanism of the ZnO@MnO₂@PPy NRAs on Zn foil through a three-step method. First, the ZnO nanorod was vertically grown on the Zn foil by a simple hydrothermal method. Second, MnO₂ nanofilms were grown onto ZnO nanorods through the facile and popular direct hydrolysis of KMnO₄ method. Finally, the PPy layer was coated onto the ZnO@MnO₂ NARs via monomer polymerization.

Fig. 1 Schematic illustration of the formation process for ZnO@MnO₂@PPy ternary core–shell nanorod arrays.

Typical SEM images of the ZnO@MnO₂ core–shell NRAs and ZnO@MnO₂@PPy ternary core–shell NRAs on Zn foil are shown in Fig. 2a–d, respectively. As shown in Fig. 2a and b, ZnO@MnO₂ core–shell NRAs are uniformly grown and roughly vertically aligned on the Zn foil. Close observation of the nanorod shows that the individual ZnO@MnO₂ nanorod with average diameter is 200–400 nm and length up to several microns. Furthermore, close observation shows that the surfaces of the ZnO nanorod backbone are covered by highly dense MnO₂ nanofilm. These MnO₂ nanofilms are closely connected with each other and form a networked structure (see Fig. 2a). Meanwhile, in Fig. 2c and d, we can see that ZnO@MnO₂@PPy ternary core–shell NRAs have faithfully preserved the morphology of the ZnO@MnO₂ core–shell NRAs. Close observation of ZnO@MnO₂@PPy ternary core–shell NRAs shows that a layer PPy is covered on the surface of the ZnO@MnO₂ core–shell NRAs. This results suggest that the ZnO@MnO₂@PPy ternary core–shell NRAs were successfully formed (see Fig. S1†). The successful preparation of ZnO@MnO₂@PPy ternary core–shell NRAs on Zn foil was also suggested by XRD analysis as shown in Fig. 3a.

Fig. 2 Typical FESEM images at different magnifications of (a and b) ZnO@MnO₂ nanorod arrays; (c and d) ZnO@MnO₂@PPy nanorod arrays supported on Zn foil.

Fig. 3 (a) XRD patterns of ZnO@MnO₂ nanorod arrays on Zn foil; (b) typical TEM images of ZnO@MnO₂ nanorod arrays; (c) HRTEM image of ZnO@MnO₂ nanorod arrays; (d) corresponding SAED pattern of the ZnO@MnO₂ nanorod arrays.

The four typical peaks, marked with black five-pointed star, originate from Zn foil. The peaks of the ZnO nanorod arrays are relatively strong and broad, the XRD peaks at 31.6°, 34°, 36°, 47° and 67.9°, corresponding to the (100), (002), (101), (102) and (112) planes, respectively. This result is consistent with the previous report.^28,29 The diffraction peaks of MnO₂ nanofilm are relatively weak, indicating the low crystallinity. Seven diffraction peaks at 12.8°, 25.7°, 54.4°, 56.8°, 63.1°, 70.5° and 77.7° corresponding to the (001), (002), (102), (240), (151), (110) and (004) diffraction planes, respectively, can be indexed to the cubic phase of MnO₂ (JCPDF 11-0055). To better illustrate the structure, the ZnO@MnO₂ core–shell NRAs are further investigated by TEM characterization. As shown in Fig. 3b, the surface of the ZnO nanorod arrays are consist of numerous interconnected MnO₂ nanofilm and the planar size is about 50–150 nm. Fig. 3c is the corresponding high-resolution TEM (HRTEM) of the Fig. 3b, it shows that the interplanar spacing is about 0.44 nm, corresponding to the (100) planes of MnO₂. Fig. 3d is the corresponding SAED pattern of the ZnO@MnO₂ NARs. The EDS of the ZnO@MnO₂ nanorod arrays is showed in Fig. S2 in ESI.†

The elemental composition and chemical state of ZnO@MnO₂ core–shell NRAs have been investigated by XPS measurements (see Fig. 4a–d). The XPS survey spectrum shown in Fig. 4a indicates the presence of Mn, Zn and O elements. The C element presence is due to exposure to the air. Fig. 4b shows the core level spectrum of the Zn 2p region. The strong peaks at 1045.7 and 1022.5 eV correspond to Zn 2p_3/2 and Zn 2p_1/2 of Zn(II), respectively.³⁰ As shown in Fig. 4c, we can observe two main peaks located at about 642.3 and 653.9 eV that are, respectively, characteristic of Mn 2p_1/2 and Mn 2p_3/2 in MnO₂.³¹ The peaks appearing at 529 eV and 532 eV in Fig. 4d can be assigned to the O 1s arising from Mn–O and Mn–OH, respectively.³² Together, the XPS results have shown that a MnO₂ shell terminated with –OH groups was formed on the surface of the ZnO nanorod core successfully. Fig. S3† shows the adsorption–desorption isotherm of the ZnO@MnO₂@PPy nanocomposites. The calculated specific surface area of the ZnO@MnO₂@PPy nanocomposites is 251.5 m² g⁻¹. The large specific area could greatly increase the utilization of ZnO@MnO₂@PPy as an electrochemically active material in electrodes.

Fig. 4 XPS spectra of the ZnO@MnO₂ nanorod arrays. (a) Survey spectrum (b) Zn 2p, (c) Mn 2p, and (d) O 1s.

3.2 Electrochemistry measurements

The electrochemical performance of the ZnO@MnO₂ and ZnO@MnO₂@PPy core–shell NRAs were also investigated. Fig. 5a shows the typical cyclic voltammetry (CV) curves of ZnO@MnO₂ core–shell NRAs with the potential window from −0.2 to 0.6 V (vs. Ag/AgCl) at various sweep rates ranging from 5 to 100 mV s⁻¹. It is clear that no obvious redox peaks were observed, which is characteristic of electroactive MnO₂ in this range.³³ These CV curves exhibit approximately rectangular shape, especially at slow scan rates. The observation indicates excellent capacitive behaviour and a relatively low contact resistance of the electrodes made from our synthesized ZnO@MnO₂ core–shell NRAs. Meanwhile, as shown in Fig. 5b, we can see that cyclic voltammetry curves of the ZnO@MnO₂@PPy ternary core–shell NRAs still keep a similar shape compared with the cyclic voltammetry curves of ZnO@MnO₂ core–shell NRAs. However, as commonly observed, the cyclic voltammetry curves of the ZnO@MnO₂@PPy ternary core–shell NRAs possess a larger area than the ZnO@MnO₂ core–shell NRAs. It suggests that ZnO@MnO₂@PPy ternary core–shell NRAs posses a higher capacitance compared with the ZnO@MnO₂ core–shell NRAs.

Fig. 5 Typical CV curves of the ZnO@MnO₂ (a) and ZnO@MnO₂@PPy nanorod arrays (b) at various scan rates; the GCD plots at various current densities of E-mail: ZnO@MnO₂ (c) and ZnO@MnO₂@PPy (d) nanorod arrays supported on Zn foil; (e) specific capacitance and areal capacitance of ZnO@MnO₂ nanorod arrays, and ZnO@MnO₂@PPy nanorod arrays; (f) cycling performance and Coulombic efficiency of ZnO@MnO₂@PPy nanorod arrays at a current density of 5 A g⁻¹.

Fig. 5c and d shows the galvanostatic charge–discharge (GCD) curves of ZnO@MnO₂ core–shell NRAs and ZnO@MnO₂@PPy ternary core–shell NRAs with a potential window of −0.2–0.6 V (vs. Ag/AgCl) at various current densities ranging from 2.5 to 30 A g⁻¹, respectively. As shown in Fig. 5c and d, the respective charging times and discharging times are nearly the same, indicating superior reversibility of charging and discharging reactions of the electrodes. Furthermore, we can see that the specific capacitance of the ZnO@MnO₂@PPy ternary core–shell NRAs obviously exceed the specific capacitance of the ZnO@MnO₂ core–shell NRAs, the result may be attribute to the PPy. It not only enhances the conductivity of the ZnO@MnO₂ core–shell NRAs, but also provides extra capacitance.

The specific capacitance (F g⁻¹) and areal capacitance (F cm⁻²) of the electrode in a three-electrode system can be calculated from the galvanostatic discharge curves according to the eqn (1). The calculated specific and areal capacitance values as a function of the applied current density for ZnO@MnO₂ core–shell NRAs, and ZnO@MnO₂@PPy ternary core–shell NRAs are shown in Fig. 5e. Impressively, the ZnO @MnO₂@ PPy ternary core–shell NRAs electrode delivers high specific capacitances are 1281, 1250, 1187, 1125, and 1012 F g⁻¹ at current densities of 2.5, 5, 10, 20 and 30 A g⁻¹, respectively. And the areal capacitance of 1.793, 1.75, 1.66, 1.58 and 1.42 F cm⁻² at the corresponding current densities above mentioned, respectively. However, the ZnO@MnO₂ core–shell NRAs electrode delivers low the specific capacitances are 937, 875, 850, 770 and 679 F g⁻¹ at current densities of 2.5, 5, 10, 20 and 30 A g⁻¹, respectively. And the areal capacitances of 1.12, 1.05, 1.02, 0.92 and 0.81 F cm⁻² at current densities above mentioned, respectively. The C_area of the ZnO@MnO₂@PPy ternary core–shell NRAs electrode is estimated to be 1.793 F cm⁻², which is nearly 1.6 times that of ZnO@MnO₂ nanorod arrays electrode (1.12 F cm⁻²). To evaluate the durability of ZnO@MnO₂@PPy ternary core–shell NRAs electrode, charge–discharge cycling tests at a constant current density of 5 A g⁻¹ were employed to characterize their cycling performance. As shown in Fig. 5f, the ZnO@MnO₂@PPy ternary core–shell NRAs electrode retains a capacitance of 1125 F g⁻¹ with 90% of the initial values after 5000 cycles, indicating the excellent cycling stability of ZnO@MnO₂@PPy ternary core–shell NRAs electrode.

Nyquist impedance spectral measurements were also carried out to evaluate the charge transfer and electrolyte diffusion in the electrode/electrolyte interface, as shown in Fig. 6. Obviously, the Nyquist impedance plots of ZnO@MnO₂ and ZnO@MnO₂@PPy core–shell NRAs electrodes are composed of a semicircle at the high-frequency region and a straight line at the low-frequency region. At a high frequency, the X-intercept of the Nyquist plot represents the resistance of the electrochemical system (R_s).

Fig. 6 Impedance Nyquist plots of the ZnO@MnO₂@PPy, ZnO@MnO₂ nanorod arrays and Zn foil at open circuit potential. Inset shows a amplification graph.

Usually, this resistance includes the ionic resistance of the electrolyte, the inherent resistance of active material and contact resistance between the electrolyte and electrode.^34,35 As expected, the ZnO@MnO₂@PPy ternary core–shell NRAs electrode have a smaller R_s, demonstrating that this ZnO@MnO₂@PPy NRAs electrode possesses higher electrical conductivity. The equivalent circuit diagram and the EIS fitting data of the ZnO@MnO₂@PPy ternary core–shell nanorod arrays is shown in Fig. S4.†

The above mentioned result shows that because of the high conductivity of PPy as layer, the electrochemical performance of the ZnO@MnO₂@PPy ternary core–shell structured composite is superior to the binary core–shell structured composites. The reasons may be as follows: (1) the unique 3D hierarchical core–shell heterostructure has a larger specific area and is beneficial for the ionic diffusion of the electrolyte. (2) Taking full use of the high conductivity of PPy to further reduce R_s. Thus, the pseudocapacitor behaviors of MnO₂ are optimized. (3) Providing a larger real reaction area. The surface area of the nanorod after coating with the PPy layer is enlarged (because the diameter of the nanorod is increased). In addition, the porous structures of MnO₂ on different nanorod also play an important role in electrochemical performance.

4. Conclusions

In summary, the ZnO@MnO₂@PPy ternary core–shell nanorod arrays (NRAs) is fabricated through the layer-by-layer process, which involves a simple hydrothermal process and electrochemis-polymerization process. In this process, the incorporation of a highly conductive material (polypyrrole) on the surface of a binary ZnO@MnO₂ core–shell structured composite is adopted to optimize the charge transfer process to further improve the electrochemical performance. Because of enhanced electron transfer capability, charge transfer resistances of the ZnO@MnO₂@PPy ternary core–shell nanorod arrays are reduced and the electrochemical performances are improved. The electrochemistry tests show that these self-supported electrodes are able to deliver ultrahigh specific capacitance (1281 F g⁻¹ at a current density of 2.5 A g⁻¹), together with a considerable areal capacitance (1.793 F cm⁻² at a current density of 3.5 mA cm⁻²). Furthermore, a capacitance retention of 90% after 5000 charge–discharge cycles at 5 A g⁻¹ is obtained, indicating the excellent cycling stability of the ZnO@MnO₂@PPy ternary core–shell electrode. The superior electrochemistry capacity demonstrates the potential of ZnO@MnO₂@PPy ternary core–shell NRAs to further improve the performance in supercapacitor electrodes.

Acknowledgements

This work was financially supported by the projects (21371007) from National Natural Science Foundation of China, Anhui Provincial Natural Science Foundation (1208085QB28), Anhui Provincial Natural Science Foundation for Distinguished Youth (1408085J03), Natural Science Foundation of Anhui (KJ2012A139) and the Program for Innovative Research Team at Anhui Normal University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06765k

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