Hybrid core–shell nanostructured electrodes made of polypyrrole nanotubes coated with Ni(OH)₂ nanoflakes for high energy-density supercapacitors

Abstract

This work describes the design of Ni(OH)₂@PPy-NTs core–shell nanostructures with potential application as an electrode material for supercapacitors. Initially, one dimensional (1D) polypyrrole nanotubes (PPy-NTs) were synthesized through a chemical oxidation mediated soft template-directed route using as the anion the azo dye methyl orange (MO). Subsequently, three dimensional (3D) Ni(OH)₂ nanoflakes were grown onto PPy-NTs by a simple hydrothermal route. This exclusive Ni(OH)₂@PPy-NTs nano-architecture helps to improve the overall electrochemical performance of the electrode, due to the high surface area provided by 3D nanoflakes and excellent electronic/ionic conductivity of 1D nanotubes. The maximum specific capacitance obtained for Ni(OH)₂@PPy-NTs was 536 F g⁻¹ with good capacity retention after 1000 charge/discharge cycles. Last but not least, EIS technique showed a low electrochemical series resistance for Ni(OH)₂@PPy-NTs confirming their promise as a high-performance energy storage material.

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Introduction

The continuous increase in the use of portable electronic devices and the emergence of new markets like the one of electric vehicles strongly calls for ever more reliable electric energy storage systems. Among different existing storage systems, supercapacitors have attracted widespread research interest because of their high power density, fast recharge capability and long cycle life.¹ When reversible faradaic redox reactions are introduced at the electrode/electrolyte interfaces, a pseudocapacitor electrode is formed, which has capacitance considerably exceeding that of carbon as a supercapacitor based on electrical double-layer charge storage.² That is why transition metal oxides such as MnO₂, RuO₂, Fe₂O₃ etc. have been extensively investigated for use as supercapacitors in view of their multiple oxidation states for pseudocapacitance contribution.^3–6 However, due to the intrinsically poor electrical conductivity of metal oxides/hydroxides and the short diffusion distance of electrolytes into electrodes, only the exposed surface of electroactive materials can effectively contribute to the total capacitance leading to lower satisfactory parameters.⁷ Hence, it is a great challenge to boost the electrochemical utilization of active materials by rationally designing electrodes with novel nanostructures.

An emerging attractive concept is to fabricate core–shell nanostructure with the combination of two types of materials and/or nanostructures, to meet the individual disadvantages.^4,8 Core–shell type materials often demonstrate distinct characteristic from the original materials,⁹ such as, stability, chemical composition, surface area, magnetic and optical properties, as a result of, structure, chemical and physical changes that occur due to this specific type of combination of precursors. Therefore the easiest way to enhance accessible electroactive sites, shorten ion transport pathways, increase the electron collection efficiency, and even attractive synergetic properties of components simultaneously achieved to deliver high capacitance, excellent cycling life and rate performance.^8,10

In fact, the development of core–shell nanostructures with two different materials and two different nanostructures is a great design approach which can combine physical/chemical properties of materials as well as nanostructures.^10,11 The materials used can be organic or inorganic both core and shell, or mixed combinations.^11,12 Controlling the shell-thickness and the core-size is possible to modify the properties, generating a versatility of applications of these systems⁹ such as energy storage devices, solar cells, photochromic devices, biomedical applications, including, drug carriers for controlled drug release.^13,14

Aiming towards the fabrication of high performance supercapacitive electrodes, we chose polypyrrole (PPy) nanotubes as core material whereas Ni(OH)₂ nanoflakes was targeted as shell material in order to form Ni(OH)₂@PPy-NTs core–shell nanostructures.^15,16 Both materials are from different families and chemistries but they are both strong contenders as supercapacitive electrode materials.³ Among different conducting polymers, PPy is one of the most widely investigated.^17,18 It possesses high electrical conductivity and good environmental stability compared to other conductive organic polymers due to its unique π-stacked co-planar structure in solid state assisted by π–π conjugation interaction.¹⁹ Whereas, Ni(OH)₂ is a classical material to modify electrodes, the Ni(OH)₂/NiOOH redox pair has remarkable properties that have been successfully applied in secondary batteries, electrocatalysis and electrochromism.^20–23 Two different polymorphs are possible to begin with (β-Ni(OH)₂ and α-Ni(OH)₂) characterized by the orientation along c axis.²³ The capability to energy storage and stability of the electrode is associated to the polymorph structure.

Herein, we present the synthesis of unique (Ni(OH)₂@PPy) core–shell nanostructures in which 3D Ni(OH)₂ nanoflakes grow as a ‘shell’ onto polypyrrole nanotubes which constitute the ‘core’ material in order to fabricate high performance supercapacitor electrode. To begin with, polypyrrole nanotubes (PPy-NTs) were synthesized through chemical oxidation mediated soft template-directed route. Subsequently, Ni(OH)₂ nanoflakes were grown onto PPy-NTs by a simple and cost-effective hydrothermal route. These nanostructures were characterized by different techniques in order to get clear insights about structure, morphology, surface area etc. The electrochemical properties were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance techniques in 6 mol L⁻¹ KOH electrolyte.

Experimental section

Synthesis of PPy-NTs

PPy-NTs were prepared using the procedure described in the literature.²⁴ Briefly, 0.15 mmol MO (sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate) and 1.5 mmol FeCl₃ were dissolved in 30 mL deionized water (Milli Q, 18 MΩ resistance), which will turn in a flocculent precipitate immediately. Then, 1.5 mmol pyrrole (Py) freshly distilled was added to the above solution, followed by stirring at room temperature for 24 h. The formed precipitate was filtered-off and repeatedly washed with mixture of deionized water and ethanol and dried overnight under vacuum at 70 °C.

Synthesis of Ni(OH)₂

Nickel hydroxide (Ni(OH)₂) was prepared by hydrothermal synthesis. To do so, ammonium hydroxide (NH₄OH) 35% (from Fluka) was dropped slowly at 50 mL of 0.1 mol L⁻¹ nickel nitrate solution to maintain pH around 12. Then, the reaction mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 80 °C for 7 h. The precipitate formed was collected by filtration, washed with deionized water and ethanol several times and finally dried overnight under vacuum at 70 °C.

Synthesis of nanocomposite Ni(OH)₂@PPy-NTs

Important steps involved in the synthesis of Ni(OH)₂@PPy-NTs core–shell nanostructure are schematically shown in Fig. 1. First, 0.1 mg mL⁻¹ PPy-NTs were dispersed in 25 mL of deionized water by ultra-sonication. Then, 25 mL of 0.1 mol L⁻¹ Ni(OH)₂ precursor solution was added to the above solution. The mixture was sonicated for 30 min and stirred for 1 h in order to get a uniform dispersion. Finally, this whole solution was transferred to a Teflon-lined stainless steel autoclave and maintained at 80 °C for 7 h. At the end, the product was collected by filtration, washed with deionized water and ethanol several times and dried in vacuum oven at 70 °C overnight.

Fig. 1 Scheme of the 1D core–shell nanostructures synthesis.

Material characterization

Surface morphological analyses were carried out by scanning electron microscopy (FEI Quanta 650F Environmental SEM) and transmission electron microscopy (Tecnai G2 F20 S-TWIN HR(S) TEM, FEI). Structural properties were examined by powder X-ray diffraction (XRD) using Panalytical X'Pert Pro-MRD instrument (Cu Kα source (λ = 0.154)). X-ray photoelectron (XPS) analysis was carried out by X-ray photoelectron spectroscopy (XPS, SPECS Germany, PHOIBOS 150). Fourier transform infrared (FTIR) spectra in a range of 4000–400 cm⁻¹ were carried out with a Tensor 27/PMA 50 FTIR Spectrometer. N₂ adsorption/desorption was determined by Brunauer–Emmett–Teller (BET) measurements using Micromeritics instrument (Data Master V4.00Q, Serial#:2000/2400).

Electrode preparation and electrochemical testing

The working electrode was prepared by doctor blade method, by spreading through a moving blade onto a stationary substrate.²⁵ The pastes were prepared using 80% of active material, 10% of binder (PVDF) and 10% of conducting material (carbon powder) in mass, dissolved in n-methylpyrrolidone (NMP). The supercapacitive performance of the materials was tested in 6 mol L⁻¹ KOH using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance techniques with conventional three electrode cell comprising a PPy-NTs, Ni(OH)₂ or Ni(OH)₂@PPy-NTs as a working electrode, platinum as a counter and Ag/AgCl as reference electrode, respectively. All electrochemical measurements were carried out with a Bio-Logic VMP3 Potentiostat.

Results and discussion

Fig. 2(a) and (b) show the top-view field-emission scanning electron microscopy (FESEM) images of the PPy-NTs and Ni(OH)₂ samples, respectively. The PPy nanotubes are randomly spread with well-developed, highly open and porous structure on a large scale. No aggregation and alignment of bundles of PPy-NTs has been observed (Fig. 2(a)). Whereas Ni(OH)₂ exhibits irregular shaped micro-belts of random size (see Fig. 2(b)). In addition to this, we found the voids and pores between interconnected micro-belts and approximate length of the micro-belts is about 1–2 μm in range. Fig. 2(c) and (d) show the typical FESEM images of final Ni(OH)₂@PPy-NTs core–shell nanostructures at two different magnifications. Clearly, no Ni(OH)₂ is packed in the interspace of the nanotubes, suggesting that Ni(OH)₂ nanoflakes are preferentially deposited onto the PPy nanotubes surface. Thus, the uniform coverage of Ni(OH)₂ nanoflakes on each PPy nanotube surface can be seen. Remarkably, even with two components integrated, the uniform 3D nanoflakes with 1D nanotubular structure are still well retained. Thus, nearly all the core–shell nanostructures are highly accessible to electrolytes for energy storage due to the presence of convenient diffusion channels.

Fig. 2 SEM image of PPy-NTs (a), Ni(OH)₂ (b) and Ni(OH)₂@PPy-NTs (c and d). TEM images of PPy-NTs (e), Ni(OH)₂@PPy-NTs (f), inset shows SAED pattern.

In order to get more insights about the core–shell nanostructure, TEM analysis was carried out and is presented in Fig. 2(e) and (f). As can be seen in the TEM image of the PPy-NTs (Fig. 2(e)), these nanotubes are highly porous and separated from each other. From Fig. 2(f), it is evidently observed that the PPy nanotubes (core material) is tightly bonded and totally covered with leaf-like thin Ni(OH)₂ nanoflakes, forming a typical core–shell hetero-structured architecture. Also, the TEM analysis further confirmed that the final nanostructure Ni(OH)₂@PPy-NTs exhibits diameter of less than 200 nm (Fig. 2(f)). Inset of Fig. 2(f) shows selected area electron diffraction pattern of Ni(OH)₂@PPy-NTs composite. Diffused circles are clearly observed in SAED pattern which are show by dotted white circles confirming ultrathin nature of Ni(OH)₂ nanoflakes. In addition, the planes correspond to ‘α’-type Ni(OH)₂ polymorph. There is no plane corresponding to PPy nanotubes has been observed suggesting amorphous nature of the PPy nanotubes.

Fig. 3(a) shows the XRD patterns of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs samples. It is interesting to note that, for PPy-NTs, a small hump is observed, corresponding to the repeat unit of pyrrole ring in PPy, implying the polymer chain is highly oriented. Whereas, XRD pattern for Ni(OH)₂ shows the polycrystalline nature with brucite-like hexagonal crystal phase as described in the literature^23,26 and the pattern PDF number available in the database (00-039-1519 and 00-001-1047). Moreover, the characteristic peak of ‘β’-type Ni(OH)₂ at 19.35° corresponding to (001) plane along crystallographic c axis is also observed.^27,28 Strikingly, Ni(OH)₂@PPy-NTs exhibits the ‘α’-type Ni(OH)₂ polymorph confirmed by the characteristic (001) peak at 12.39°.^28,29 It is well-known that, β-Ni(OH)₂ possesses more ordered structure along c axis, and it is the classic material used in battery system while α-Ni(OH)₂ exhibits more disordered structure as well as a larger separation distance between the layers than that of β-Ni(OH)₂, which can improve the electrochemical properties of the electrode and facilitate ion diffusion mechanism.^28,30 Moreover, the peak at 59° is characteristic of the hexagonal symmetry for both α and β polymorphs of Ni(OH)₂. The peak in the region between 30 to 45° is characteristic of the turbostratic disorder of the material.²⁷ The broadening of the peaks observed in the Ni(OH)₂@PPy-NTs composites may be attributed to the decrease of the hydroxide particles size and thus to the decrease of crystallite size upon PPy coating.

Fig. 3 (a) XRD patterns (b) FTIR spectra of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs samples.

Fig. 3(b) shows FTIR spectra of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs samples. The narrow and strong peak at 3632 cm⁻¹ is assigned to hydroxyl groups stretching vibrations, which indicates the presence of free OH groups.^31,32 In addition, the peaks at 3578 cm⁻¹ and 1630 cm⁻¹ correspond to –OH stretching vibrations and water molecules intercalated in the lamellar Ni(OH)₂ structure, respectively. Two peaks at high frequencies 498 and 416 cm⁻¹ are associated to (Ni–OH) bending vibration and the (Ni–O) stretching, respectively.^31–33 Furthermore, the characteristic peaks at 1540 and 1450 cm⁻¹ are ascribed to the antisymmetric (C–N) and symmetric (C–C) stretching mode of the pyrrole ring. Additional peaks at 1296, 1031 and 963 cm⁻¹ can be assigned to (=C–H) in and out plane and (C–C) in plane vibrations, respectively.^34,35 It is interesting to note that, a blue-shift is observed for (Ni–OH) bending vibration (630 cm⁻¹) and the peak around 3400 cm⁻¹ (hydrogen-bonded) in case of Ni(OH)₂@PPy-NTs hybrid which are characteristic of the α-Ni(OH)₂ phase, in good agreement with XRD analysis. Moreover, a slight shift from 1175 to 915 cm⁻¹ and 1550 to 1430 cm⁻¹ is observed for PPy-NTs.³⁶

Fig. 4(a) shows the core-level XPS spectrum of Ni 2p for Ni(OH)₂@PPy-NTs sample. The doublet of both transitions Ni 2p_1/2 and Ni 2p_3/2 between 850 to 890 eV are attributed to the presence of different oxidation states of Ni.^37–39 The characteristic peak at 857.68 is attributed to the presence of Ni(OH)₂. Biesinger et al.,³⁷ correlates peak positions with the presence of β-Ni(OH)₂/NiOOH form, in higher ratio of NiOOH. In addition, two satellite peaks at 863.2 and 881.4 eV are also observed which are attributed to a sudden change in coulombic potential when the photo-ejected electron passes through the valence band.⁴⁰ The obtained N 1s peak in Ni(OH)₂@PPy-NTs sample corresponds to the characteristic nitrogen from PPy-NTs. The deconvolution of N 1s peak gives three components as shown in Fig. 4(b). The more intense component peak at 399.9 eV is attributed to the neutral N in the pyrrole ring (–NH) while the signals at high binding energies (401.1 eV) are attributed to the presence of oxidized states of the polypyrrole, known as polaron and bipolaron structures.^41,42 Also, the component peak at 398.2 eV could be ascribed to the π* character bonding from C=N⁺=C commonly found in “pyridinic” form.⁴² The oxygen contributions (Fig. 4(c)) are attributed to Ni-hydroxide (532.0 eV),³⁸ polymer oxidized (534.8 eV), water molecules intercalated in the material structure (533.7 eV),¹⁴ and oxides (530.8).⁴⁰

Fig. 4 XPS spectra of Ni(OH)₂@PPy-NTs sample: (a) Ni 2p, (b) N 1s and (c) O 1s, respectively.

BET analysis was carried out in order to investigate the specific surface area of materials using N₂ adsorption/desorption analysis (see Fig. 5). The shape of the isotherms acquired for all materials show the hysteresis loop in relative pressure (P/P₀) between 0.6 to 1.0, showing characteristic of mesoporous materials.⁴³ The BET specific surface area obtained were 27.14, 43.18 and 77 m² g⁻¹ for Ni(OH)₂, PPy-NTs and Ni(OH)₂@PPy-NTs, respectively. Thus, the surface area of the PPy-NTs is higher than that of Ni(OH)₂, as expected for 1D nano-structures⁴⁴ while Ni(OH)₂@PPy-NTs exhibits remarkably larger specific surface area due to the synergetic combination of 1D nanotubes coated with 3D nanoflakes. Interestingly, Ni(OH)₂@PPy-NTs core–shell structure exhibits higher specific surface area than other core–shell structures. For example, Ni(OH)₂/CNT core–shell nanostructures deposited onto nickel foams (0.819 m² g⁻¹),⁴⁵ TiO₂ nanotubes composite with PPy film (39 m² g⁻¹)⁴⁶ and comparable with PPy nanowires synthesized by alumina membrane template (75.66–172.90 m² g⁻¹).⁴⁷ The increase of the specific surface area is directly associated to improvement of the supercapacitive properties of the materials, due to the mesoporous structures which enable the soaking of electrolyte, facilitating the ion diffusion and provide more electroactive sites for energy storage.^43,48

Fig. 5 Nitrogen adsorption/desorption isotherm of PPy-NTs (a), Ni(OH)₂ (b) and Ni(OH)₂@PPy-NTs (c).

Electrochemical performance

Fig. 6(a) displays the typical cyclic voltammetry (CV) curves of Ni(OH)₂, Ni(OH)₂@PPy-NTs and PPy-NTs at constant scan rate of 20 mV s⁻¹ in KOH (6 mol L⁻¹). The shapes of CV curves for all three samples are different. In the case of PPy-NTs, the shape is quasi-rectangular reflecting the existence of purely capacitive (pseudo-capacitive) charge storing behavior.^10,49 Whereas, the CV curve for Ni(OH)₂ exhibits well-defined redox peaks which indicate that the capacity results mainly from a pseudo-capacitive behavior based on a redox (faradaic) mechanism. The appearance of anodic and cathodic peaks correspond to the Ni(OH)₂/NiOOH redox reaction.^49–51 The absence of a well-defined oxidation peak at the Ni(OH)₂ modified electrode is a clear evidence of the β-Ni(OH)₂ phase with lower electroactivity, as commented earlier. On the other hand, the Ni(OH)₂@PPy-NTs shows well-defined redox behavior with remarkable increase in current density, as denoted by the α-Ni(OH)₂ phase, corroborating the XRD and FTIR results. This considerable enhancement corresponds to Ni(OH)₂@PPy-NTs core–shell structures providing a substantially enhanced electrochemically active surface area as well as easy access to electrolyte ions. Moreover, for Ni(OH)₂@PPy-NTs composite, the redox peaks are more pronounced, suggesting an enhanced interaction at electrode/electrolyte interface which consequently improves their electrochemical properties.^49,52

Fig. 6 (a) CV curves of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs electrodes at 20 mV s⁻¹ scan rate. CV curves at various scan rate from 2–50 mV s⁻¹ of (b) PPy-NTs, (c) Ni(OH)₂ and (d) Ni(OH)₂@PPy-NTs in 6 mol L⁻¹ KOH aqueous solution.

In order to check the rate capability of electrode materials, CV curves at different scan rates were investigated and are presented in Fig. 6(b)–(d). It is seen for all samples that the area under each curve increases with scan rate suggesting that the current density is directly proportional to scan rate. It is interesting to note that the shapes of CV curves of PPy-NTs even at high scan rates show good rate capability for this material (see Fig. 6(b)). Further, the shapes of CV curves of Ni(OH)₂@PPy-NTs (Fig. 6(d)) and Ni(OH)₂ (Fig. 6(c)) are similar except that the redox process is more evidenced and the current density is higher for Ni(OH)₂@PPy-NTs (Fig. 6(d)), due to the synergy between pseudo-capacitive Ni(OH)₂ and the highly conductive polymer nanotubes.^10,18 This excellent pseudo-capacitive behavior manifested in Ni(OH)₂@PPy-NTs is due to the conversion from Ni^II(OH)₂/Ni^IIIOOH present on the shell of the hybrid material. Moreover, it should also be noted that as the scan rate increases, the shape of the CV changes, the potential of the anodic and cathodic peaks shift to more positive and negative directions, respectively, and the capacitance inevitably decreases. It is worth noting that the performance and reversibility of Ni(OH)₂@PPy-NTs electrode at high scan rate is preserved, suggesting the effective utilization of active materials.^49,53

The electrochemical performance of the materials were also investigated by galvanostatic charge/discharge (GCD) tests at different current densities (2–20 mA cm⁻²) and are displayed in Fig. 7. As seen in Fig. 7(a), the shapes of CD curves are different for all three samples. It can be seen that all of the curves are not ideal straight lines, indicating the involvement of a faradaic reaction process. Furthermore, charging curves are not symmetric to its corresponding discharging counterpart. In addition, there is an initial drop in potential, which may be caused by the internal resistance as well as different rates of oxidation and reduction reactions. It is also noted that the discharging time of Ni(OH)₂@PPy-NTs sample is much longer than that of PPy-NTs and Ni(OH)₂ samples at identical current density conditions (Fig. 7(a)).

Fig. 7 (a) Galvanostatic charge–discharge curves of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs and (b) GCD curves of Ni(OH)₂@PPy-NTs at different current densities in 6 mol L⁻¹ KOH aqueous solution. (c) Variation of specific capacitance with current density and (d) plot of capacity retention versus number of cycles for Ni(OH)₂@PPy-NTs sample over 1000 cycles.

Fig. 7(b) shows the charge/discharge curves of Ni(OH)₂@PPy-NTs at different current densities. A low current density is used to allow for the complete reaction between the electrolyte and the electrode. The discharging time decreased with increasing current density, showing the redox reaction is a diffusion-controlled process. The nonlinearity and relatively slow charge/discharge process could be explained from contributions of faradaic reaction occurring at the material surface.⁴⁹ Monitoring discharge process is possible to assess the capacitance properties of the material. Specific capacitance values of Ni(OH)₂, PPy-NTs and Ni(OH)₂@PPy-NTs samples have been calculated from the voltage–time measurements (excluding internal resistance drop) at different applied current densities using the following equation:

where, C_s is specific capacitance, i_D is applied discharge current, Δt_D is discharge time, m is mass of the active material and ΔV is potential range. The masses of active material were 2.3, 1.06 and 0.35 mg cm⁻² for Ni(OH)₂, Ni(OH)₂@PPy-NTs and PPy-NTs, respectively. The maximum specific capacitance obtained were 536, 201 and 115 F g⁻¹ for Ni(OH)₂@PPy-NTs, Ni(OH)₂, and PPy-NTs, respectively. The specific capacitance decreases with the increase in current density, as expected which is attributed to delayed ion diffusion taking place when faster discharge is imposed to the material. Also the electrochemical kinetics is limited, which reduce the electrode efficiency at high current densities.⁴⁹

The long-term cycling stability is another crucial parameter for deciding the practical applicability of prepared Ni(OH)₂@PPy-NTs sample; therefore the cycling stability of Ni(OH)₂@PPy-NTs symmetric cell was executed using charge/discharge cycles for 1000 cycles. The plot of capacity retention versus number of cycles for Ni(OH)₂@PPy-NTs symmetric cell is shown in Fig. 7(d). It is observed that cell demonstrates the high capacity retention of 90.1% over 1000 charge/discharge cycles suggesting good cycling stability which is highly desirable in high performance energy storage applications.

Electrochemical impedance analysis was carried out to investigate the resistances involved in the overall system at room temperature and is presented in Fig. 8. The electrodes exhibit small equivalent series resistance (ESR) 0.06, 0.14 and 0.12 Ω for Ni(OH)₂, Ni(OH)₂@PPy-NTs and PPy-NTs, respectively. Moreover, charge transfer resistances were found to be 0.06, 0.03 and 0.13 Ω, for Ni(OH)₂, Ni(OH)₂@PPy-NTs and PPy-NTs, respectively. Low ESR and charge transfer resistance reveals high conductivity for all samples. The frequency at which there is deviation from the semicircle is called as “knee frequency”, which reflects the maximum frequency at which capacitive behavior is dominant, in other words, the capability of energy storage.⁴⁹ The “knee frequency” found for Ni(OH)₂, PPy-NTs and Ni(OH)₂@PPy-NTs were 1313, 602 and 2869 Hz, respectively.

Fig. 8 Nyquist plots of PPy-NTs, Ni(OH)₂ and Ni(OH)₂@PPy-NTs.

The excellent electrochemical supercapacitive properties of the Ni(OH)₂@PPy-NTs core–shell structure may be attributed to the following factors: (1) PPy-NTs have a remarkable capability to improve their electroactive properties in a 1D nanostructure, with the quantum confinement effect in one dimension facilitating charge transfer by high electronic conductivity;⁵⁴ (2) in addition, the anisotropic morphology can provide fast ion exchange;⁵⁵ (3) the core–shell nanostructures result in higher improvement by the combination of Ni(OH)₂ redox characteristic with the polymer capacitive properties, especially in supercapacitors application;^53,56,57 (4) thus, the charge storage at the electrode/electrolyte interface combines the physical process (electrical double layer) and the reversible oxidation/reduction reaction, resulting in the so called hybrid supercapacitors;⁵³ (5) last but not least, 1D nanotubular PPy core material and 3D Ni(OH)₂ nanoflakes shell material provide high surface area and orientation of the active materials, small size and porous nature^15,49,55 which increase active sites for energy store and facilitates ion diffusion.

Conclusion

In conclusion, a facile and scalable strategy has been developed to construct unique Ni(OH)₂@PPy-NTs core–shell hetero-structured nano-architectures with high electrochemical performance for supercapacitors. As fabricated Ni(OH)₂@PPy-NTs electrode shows maximum specific capacitance of 536 F g⁻¹ at 2 mA cm⁻² current density which is almost five times larger than the corresponding values for the individual component materials. This enhanced capacitive behavior is attributed to the unique hierarchical core–shell heterostructured nanoflakes@nanotubes configuration and the synergistic effects of the combined pseudocapacitive contributions from the mesoporous PPy-NTs core and faradaic Ni(OH)₂ nanoflakes shell layer. This work confirms the feasibility of rational design of advanced integrated electrode materials for high-performance supercapacitors.

Acknowledgements

The authors acknowledge the financial support of the CAPES Foundation, Ministry of Education of Brazil: Process BEX 3196/14-3. Authors appreciate the award to DPD with the support of the Secretary for Universities and Research of the Ministry of Economy and Knowledge of the Government of Catalonia and the Co-fund programme of the Marie Curie Actions of the 7^th R&D Framework Programme of the European Union. Partial funding from Spanish Grant MAT2012-39199-C02-01 is acknowledged. ICN2 acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295.

References

1. P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845–854.
2. C. Guan, X. Li, Z. Wang, X. Cao, C. Soci, H. Zhang and H. J. Fan, Adv. Mater., 2012, 24, 4186–4190.
3. F. Shi, L. Li, X.-l. Wang, C.-d. Gu and J.-p. Tu, RSC Adv., 2014, 4, 41910–41921.
4. X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X.-b. Zhao and H. J. Fan, ACS Nano, 2012, 6, 5531–5538.
5. T. Cottineau, M. Toupin, T. Delahaye, T. Brousse and D. Belanger, Appl. Phys. A: Mater. Sci. Process., 2006, 82, 599–606.
6. F. Li, G. Li, H. Chen, J. Q. Jia, F. Dong, Y. B. Hu, Z. G. Shang and Y. X. Zhang, J. Power Sources, 2015, 296, 86–91.
7. Z. Lu, Q. Yang, W. Zhu, Z. Chang, J. Liu, X. Sun, D. G. Evans and X. Duan, Nano Res., 2012, 5, 369–378.
8. X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang and H. J. Fan, Nano Lett., 2014, 14, 1651–1658.
9. F. Caruso, R. A. Caruso and H. Mohwald, Science, 1998, 282, 1111–1114.
10. P. Li, Y. Yang, E. Shi, Q. Shen, Y. Shang, S. Wu, J. Wei, K. Wang, H. Zhu, Q. Yuan, A. Cao and D. Wu, ACS Appl. Mater. Interfaces, 2014, 6, 5228–5234.
11. F. Caruso, Adv. Mater., 2001, 13, 11–22.
12. J. Jang and J. H. Oh, Adv. Mater., 2003, 15, 977–980.
13. S. M. Marinakos, L. C. Brousseau, A. Jones and D. L. Feldheim, Chem. Mater., 1998, 10, 1214–1219.
14. C. Yuan, L. Zhang, S. Zhu, H. Cao, J. Lin and L. Hou, Nanotechnology, 2015, 26, 145401–145412.
15. Y.-Q. Zheng, J.-Y. Wang and J. Pei, Sci. China: Chem., 2015, 58, 937–946.
16. J. Zhang, L. Shi, H. Liu, Z. Deng, L. Huang, W. Mai, S. Tan and X. Cai, J. Alloys Compd., 2015, 651, 126–134.
17. T. V. Vernitskaya and O. N. Efimov, Usp. Khim., 1997, 66, 489–505.
18. L. Pan, H. Qiu, C. Dou, Y. Li, L. Pu, J. Xu and Y. Shi, Int. J. Mol. Sci., 2010, 11, 2636–2657.
19. X. Wang, C. Yang, T. Wang and P. Liu, Electrochim. Acta, 2011, 58, 193–202.
20. S. Cabanas-Polo, Z. Gonzalez, A. J. Sanchez-Herencia, B. Ferrari, A. Caballero, L. Hernan and J. Morales, J. Eur. Ceram. Soc., 2015, 35, 573–584.
21. Z. Luo, S. Yin, K. Wang, H. Li, L. Wang, H. Xu and J. Xia, Mater. Chem. Phys., 2012, 132, 387–394.
22. F. Wolfart, A. L. Lorenzen, N. Nagata and M. Vidotti, Sens. Actuators, B, 2013, 186, 528–535.
23. K. Provazi, M. J. Giz, L. H. Dall'Antonia and S. I. C. de Torresi, J. Power Sources, 2001, 102, 224–232.
24. X. M. Yang, Z. X. Zhu, T. Y. Dai and Y. Lu, Macromol. Rapid Commun., 2005, 26, 1736–1740.
25. A. Berni, M. Mennig and H. Schmidt, in Sol-Gel Technologies for Glass Producers and Users, ed. M. Aegerter and M. Mennig, Springer, US, 2004, ch. 10, pp. 89–92,  DOI:10.1007/978-0-387-88953-5_10.
26. M. Vidotti, R. Torresi and S. I. Cordoba de Torresi, Quim. Nova, 2010, 33, 2176–2186.
27. C. Faure, C. Delmas and M. Fouassier, J. Power Sources, 1991, 35, 279–290.
28. M. Freitas, J. Power Sources, 2001, 93, 163–173.
29. S. N. Begum, V. S. Muralidharan and C. A. Basha, Int. J. Hydrogen Energy, 2009, 34, 1548–1555.
30. D. S. Hall, D. J. Lockwood, C. Bock and B. R. MacDougall, Nickel hydroxides and related materials: a review of their structures, synthesis and properties, 2015.
31. T. N. Ramesh and P. V. Kamath, J. Power Sources, 2006, 156, 655–661.
32. L. Xu, Y.-S. Ding, C.-H. Chen, L. Zhao, C. Rimkus, R. Joesten and S. L. Suib, Chem. Mater., 2008, 20, 308–316.
33. Q. Song, Z. Tang, H. Guo and S. L. I. Chan, J. Power Sources, 2002, 112, 428–434.
34. S. Ravichandran, S. Nagarajan, A. Kokil, T. Ponrathnam, R. M. Bouldin, F. F. Bruno, L. Samuelson, J. Kumar and R. Nagarajan, Langmuir, 2012, 28, 13380–13386.
35. Q. Wen, X. Pan, Q.-x. Hu, S.-j. Zhao, Z.-f. Hou and Q.-z. Yu, Synth. Met., 2013, 164, 27–31.
36. S. Jing, S. Xing, L. Yu and C. Zhao, Mater. Lett., 2007, 61, 4528–4530.
37. M. C. Biesinger, B. P. Payne, L. W. M. Lau, A. Gerson and R. S. C. Smart, Surf. Interface Anal., 2009, 41, 324–332.
38. L. Marchetti, F. Miserque, S. Perrin and M. Pijolat, Surf. Interface Anal., 2015, 47, 632–642.
39. S. J. Lee, J. H. Jun, S. H. Lee, K. J. Yoon, T. H. Lim, S. W. Nam and S. A. Hong, Appl. Catal., A, 2002, 230, 61–71.
40. D. S. Hall, D. J. Lockwood, S. Poirier, C. Bock and B. R. MacDougall, J. Phys. Chem. A, 2012, 116, 6771–6784.
41. M. Sevilla, P. Valle-Vigon and A. B. Fuertes, Adv. Funct. Mater., 2011, 21, 2781–2787.
42. C. Morant, R. Torres, I. Jimenez, J. M. Sanz and E. Elizalde, J. Nanosci. Nanotechnol., 2009, 9, 3633–3638.
43. D. P. Dubal, S. H. Lee, J. G. Kim, W. B. Kim and C. D. Lokhande, J. Mater. Chem., 2012, 22, 3044–3052.
44. J. Lu, S. X. Lim and C. H. Sow, J. Mater. Sci. Technol., 2015, 31, 616–629.
45. Z. Tang, C.-h. Tang and H. Gong, Adv. Funct. Mater., 2012, 22, 1272–1278.
46. Y. Gao, K. Ding, X. Xu, Y. Wang and D. Yu, RSC Adv., 2014, 4, 27130–27134.
47. N. Hassanzadeh, H. Omidvar and S. H. Tabaian, Superlattices Microstruct., 2012, 51, 314–323.
48. Y. Kim, J. G. Kim, Y. Noh and W. B. Kim, Catal. Surv. Asia, 2015, 19, 88–121.
49. B. K. Kim, S. Sy, A. Yu and J. Zhang, in Handbook of Clean Energy Systems, John Wiley & Sons, Ltd, 2015,  DOI:10.1002/9781118991978.hces112.
50. L. G. Bettini, G. Divitini, C. Ducati, P. Milani and P. Piseri, Nanotechnology, 2014, 25, 435401–435409.
51. D. P. Dubal, S. H. Lee and W. B. Kim, J. Mater. Sci., 2012, 47, 3817–3821.
52. J. W. Lee, T. Ahn, D. Soundararajan, J. M. Ko and J.-D. Kim, Chem. Commun., 2011, 47, 6305–6307.
53. D. P. Dubal, O. Ayyad, V. Ruiz and P. Gomez-Romero, Chem. Soc. Rev., 2015, 44, 1777–1790.
54. Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates, Y. D. Yin, F. Kim and Y. Q. Yan, Adv. Mater., 2003, 15, 353–389.
55. A. K. Singh, D. Sarkar, G. G. Khan and K. Mandal, J. Mater. Chem. A, 2013, 1, 12759–12767.
56. H. Wang, H. Yi, X. Chen and X. Wang, J. Mater. Chem. A, 2014, 2, 3223–3230.
57. D. P. Dubal, R. Holze and P. Gomez-Romero, Sci. Rep., 2014, 4, 7349.

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