TiO₂ fibre/particle nanohybrids as efficient anodes for lithium-ion batteries

Abstract

We report the synthesis of a TiO₂ nanohybrid with a unique morphology consisting of TiO₂ nanoparticles decorating the surface of TiO₂ nanofibers, obtained by a simultaneous electrospinning and electrospraying technique, and its electrochemical studies as efficient anodes for the Li-ion battery. The TiO₂-fiber/particle composite electrode exhibited a very high discharge capacity (190 mA h g⁻¹ after 50 cycles, at C/10), excellent rate capability with remarkable capacity retention of 77% of initial capacity at 5C rate, and good cyclic stability, compared to TiO₂ nanofibers and nanoparticles. The capacitive contribution from these electrodes is studied in detail by using cyclic voltammetry measurements, and the results are correlated with the overall electrochemical performance of the electrodes. The exceptional electrochemical characteristics exhibited by the TiO₂-fiber/particle composite electrode, synthesized through a low-cost and scalable electrospinning technique, make it an ideal anode material for large-scale Li-ion battery applications.

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Introduction

With the advances in technology over the years, there has been a tremendous increase in battery-powered consumer electronic devices, which are dominated by Li-ion batteries. The recent expansion of the application domain of lithium-ion batteries to the electric vehicle sector has led to rapid growth of the global Li-ion battery market, which in turn has resulted in a great need for the development of electrode materials with high capacity, long cyclability, improved safety and low cost.^1,2 Nanoscale approaches to the design and fabrication of electrode materials offer unique properties, such as high surface-to-volume ratio, enhanced Li-ion diffusion, improved electrode–electrolyte interfaces, better electrical conductivity for the development of high-performance Li-ion batteries, etc.^3–6 Several nanomaterials with varying morphologies have been thoroughly studied as anodes and cathodes for Li-ion batteries, thanks to the rich and versatile chemistry available.^5,7,8 Over the past decades, nanostructured metal oxides such as TiO₂, Co₃O₄, CoO, NiO, and Fe₃O₄ have shown great promises as anodes for Li-ion batteries.^8–12 TiO₂, in particular, has attracted great interest, owing to its improved safety, structural stability, low cost and abundance in nature.^11,13–17 Though nanostructured TiO₂ in anatase, rutile and TiO₂-B forms have been studied as potential Li-ion battery anodes, the high-current performance of these electrodes are generally poor due to their low electrical conductivity. Several efforts have been undertaken to improve the electrical conductivity of TiO₂ nanostructures, including carbon coating,¹³ metal doping,¹⁸ making composites with graphene and carbon nanotubes, etc.¹⁹ Another strategy followed in this direction is to fabricate 1-dimensional (1-D) TiO₂ such as nanorods, nanotubes and nanofibers by using different techniques such as sol–gel, template-assisted synthesis, electrochemical anodization, electrospinning, etc.^20–22 Electrospun materials have recently been explored as electrodes and separators for Li-ion battery applications and are attracting wide interest due to the tunability and scalability of their fabrication technique.^23–25 The versatility of the technique allows the fabrication of 1-D nanostructures such as nanofibers, nanotubes, nanorods, etc.²³ Recently, Arun et al. successfully demonstrated a full-cell Li-ion battery wherein 1-D electrospun TiO₂ nanofibers were used as anode, LiMn₂O₄ as cathode and PVDF-HFP as separator.²⁵ However, the TiO₂ nanofibers appear to suffer from limited surface area, as reflected from the reduced capacity achieved when studied as anodes for Li-ion battery applications. In one of our recent reports, we demonstrated much improved surface area for tailored TiO₂ nanoarchitectures with hybrid morphology, combining long 1-D nanofibers with 0-D nanoparticles, and studied its performance as photoanode for dye-sensitized solar cells (DSSCs).²⁶ Such nanoarchitectures with unique morphology offer improved surface area and additional channels leading to enhanced Li-ion storage capacity and faster kinetics for Li-ion diffusion. Here, such TiO₂ nanoarchitectures with TiO₂ nanoparticles uniformly decorated on the surface of TiO₂ nanofibers (TiO₂-fiber/particle composite) were fabricated by a simultaneous electrospinning and electrospraying technique and are studied as high-performance anodes for Li-ion battery applications. TiO₂-based electrodes with unique fiber/particle morphology show improved electrochemical characteristics, with exceptionally high Li-ion storage capacity and rate capability, compared to 1-D nanofibers and 0-D nanoparticles of TiO₂.

Experimental methods

Materials synthesis

Titanium isopropoxide (99.99%, Aldrich, Germany), polyvinyl pyrrolidone (PVP, 1.3 × 10⁶, mp > 300 °C, Aldrich, Germany), acetic acid (99.9%, Alfa Aesar, USA), methanol (absolute, Aldrich, Germany), isopropyl alcohol (absolute, Alfa Aesar, USA), and Triton X-100 (molecular biology grade, Aldrich, Germany) were used as received.

a. Preparation of the colloidal solution of TiO₂ nanoparticles for electrospraying. The colloidal solution was prepared as per our previous report.²⁷ Twenty millilitres of titanium isopropoxide (TiP) and 2.5 mL of glacial acetic acid were added to 25 mL isopropyl alcohol (IPA). A continuous stream of steam was applied to this solution, which resulted in the rapid hydrolysis of TiP along with the expulsion of the organic solvents, leading to the formation of a thick TiO₂ colloid (white slurry). The TiO₂ colloid was ground with 50 mL of distilled water by using a mortar and subsequently autoclaved at 180 °C for 3 h. From the autoclaved solution, 20 mL was taken and added to a mixture of 10 mL of isopropyl alcohol, 2.5 mL acetic acid and 5 drops of Triton X-100 and used for electrospraying as detailed below.

b. Preparation of TiO₂ nanofibers for electrospinning. About 1 g polyvinyl pyrrolidone (PVP) solution was made in 14 mL of methanol. To this, 4 mL of glacial acetic acid was added and stirred well followed by the addition of 2 mL of TiP. The prepared solution was stirred for 2 h before electrospinning.

c. Preparation of fiber–particle composite by co-electrospinning and electrospraying. The TiO₂ nanofiber–nanoparticle composite was fabricated as per our recently reported method.²⁶ The polymer solution containing TiP was taken in a 20 mL syringe and fed into the electrospinning set-up (Zeonics, India) for electrospinning. The electrospinning was done at a voltage of 15 kV with a tip-to-target distance of 10 cm. The TiO₂ colloidal solution for electrospraying was also fed into a 20 mL syringe connected to another pump vertically opposite that of the electrospinning one as shown in the schematic in Fig. 1. The colloidal solution was sprayed at a voltage of 8 kV with a needle-to-collector distance of 8 cm. The fibers were spun at a flow rate of 1.2 mL h⁻¹, and the particles were sprayed at 1 mL h⁻¹, respectively. Al foil wrapped on a grounded rotating drum (angular velocity of 4000 rpm) was used as the collector to collect the co-electrospun–electrosprayed composite. The thick white sheet of the TiO₂ composite was peeled off from the Al foil and kept for annealing at 450 °C for 3 h for the degradation of polymer and crystallization of TiO₂. It must be noted that all the above processing parameters have been arrived at after several rounds of optimization, and they form the key parameters for obtaining the desired fiber–particle composite morphology as revealed later in this manuscript.

Fig. 1 A schematic representation of the fabrication route for TiO₂ nanofiber–TiO₂ nanoparticle composite by co-electrospinning–electrospraying.

Characterization

The phase and structural characterization of the fiber–particle composite was carried out using X-ray powder diffractometry (X'Pert PRO Analytical) using Cu Kα radiation with a current of 30 mA and a voltage of 40 kV. The standard JCPDS database was used for phase identification. The Raman spectra were measured with a confocal Raman spectrometer (WITEC ALPHA 300 RA) using an excitation laser of 488 nm wavelength and power of 0.6 mW (the spot size was > 2 mm). X-ray photoelectron spectroscopy (XPS, Kratos Analytical, Axis Ultra) was used to obtain the elemental composition of the TiO₂ composite and oxidation states of the elements. The morphological analysis was carried out using scanning electron microscopy (SEM, JEOL JSM 6490 LA, at an operating voltage of 15 kV). A thin film of gold was sputtered on the sample by using a sputter-coating machine (JEOL-Tokyo, Japan). For transmission electron microscopy (TEM, SEI Tecnai G230, operating voltage at 200 kV), the prepared composite was dispersed in methanol and sonicated well before drop-casting onto the carbon-coated copper grid. Electrochemical measurements were carried out in standard CR2032 coin cells, assembled inside an mBraun (Unilab, Germany) glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm), with Li foil used as both counter and reference electrodes. The working electrodes were prepared by mixing the active material with acetylene black and polyvinylidenedifluoride (PVDF) binder in the weight ratio 80 : 10 : 10. The mixture was then thoroughly ground to attain homogeneous mixing, made into a slurry by adding two or three drops of N-methyl-2-pyrrolidone (NMP), and coated uniformly onto a stainless steel current collector. The electrolyte consisting of a solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC), obtained from Solvionic (EC : DMC, 1 : 1 by volume), was used as received. Whatman glass fibre filter was used as the separator. All the electrochemical measurements, viz. galvanostatic charge–discharge, cyclic voltammetry and electrochemical impedance spectroscopy, were performed on a Biologic SAS VMP3 electrochemical workstation in the potential range of 3.0–1.0 V vs. Li⁺/Li, at different current densities and scan rates.

Results and discussion

Fig. 2A shows a large-area SEM image of the sintered particle–fiber composite showing lumps of spherical particles decorating the TiO₂ fiber surfaces. The TiO₂ fibers had an average diameter of about 200 nm, and the TiO₂ particle aggregates on fibers had sizes in the range of 50–200 nm. Fig. 2B and C, respectively, are TEM images of the composites, showing the distribution of spherical aggregates over the fiber surfaces. Fig. 2C is a high-resolution image of a single fiber of the composite showing that the aggregated particles are made up of small, spherical nanoparticles with an average size of ∼15 nm. Fig. 2D is the lattice-resolved TEM image of the composite showing an interplanar distance of 0.35 nm, which corresponds to the (101) lattice orientation of anatase-phase TiO₂. The SAED pattern (inset of Fig. 2D) clearly depicts the high crystallinity of the prepared nanocomposite. Fig. 3A–D shows the morphology of the TiO₂ nanofibers and the nanoparticles used as the controls. The SEM image of the TiO₂ nanofibers is shown in Fig. 3A, which shows that the nanofibers have diameters ranging from 100–500 nm. Fig. 3B shows the TEM image of a single nanofiber with a diameter of about 175 nm, clearly indicating the morphology of nanofibers is made up of small, spherical TiO₂ nanoparticles of 15–20 nm diameter. The inset of Fig. 3B shows the SAED pattern, which confirms the crystallinity of the TiO₂ nanofibers. The TEM image of the nanoparticles used for electrospraying is given in Fig. 3C. The nanoparticles were found to be in the size range of 15–20 nm. Fig. 3D shows the lattice-resolved HR-TEM image of a TiO₂ nanoparticle, which shows an interplanar distance of 0.35 nm, corresponding to the (101) lattice plane of anatase TiO₂. The inset shows the SAED pattern, which depicts the crystallinity of the TiO₂ nanoparticles.

Fig. 2 (A) Large-area SEM image of the TiO₂ fiber–particle composite, (B) large-area TEM image of the composite, (C) enlarged TEM view of a single composite architecture, and (D) a lattice-resolved TEM image of a particle in the fiber backbone. Inset of (D) is an SAED pattern showing the crystallinity of the composite.

Fig. 3 Microscopy of the TiO₂ nanostructures used as controls. (A) SEM image of the electrospun TiO₂ nanofibers; (B) TEM image of a single fiber showing the morphology (fibers are made of fusion of small spherical particles), inset of (B) shows the SAED pattern from the fibers; (C) TEM image of the TiO₂ nanoparticles; (D) lattice-resolved image of a single TiO₂ particle showing the 0.35 nm lattice spacing corresponding to the (101) lattice orientation of anatase TiO₂. Inset of (D) shows an SAED pattern of the nanoparticles.

The prepared nanocomposite was further characterized by XRD, Raman and XPS techniques. Fig. 4A shows the phase-pure XRD pattern of the composite, and all the peaks are indexed corresponding to anatase-phase TiO₂ (JCPDS file No. 21-1272). The Raman spectrum of the nanocomposite is shown in Fig. 4B. The six Raman-active modes (A1_g + 2B1_g + 3E_g) of a pure anatase single-crystal TiO₂ are at 144 cm⁻¹ (E_g), 197 cm⁻¹ (E_g), 399 cm⁻¹ (B1_g), 513 cm⁻¹ (A1_g), 519 cm⁻¹ (B1_g) and 639 cm⁻¹ (E_g), respectively. For the prepared nanocomposite, the respective peaks were at 157.7, 205.5, 404, 524.7 and 646.1 cm⁻¹, respectively. The peaks for the composite were red-shifted compared to those for the single crystals. The peak at 524.7 cm⁻¹ could be the combination of (A1_g + B1_g), which is indicative of the Ti–O stretching vibration in the TiO₂. The shift in the peaks towards higher wavenumbers could be attributed to the small sizes of the particles in the composite leading to volume contraction, which results in an increase in force constant. Since force constant is directly proportional to the wavenumber, the wavenumber increases as the particle size decreases.^28,29 Fig. 5A shows the XPS survey spectrum (wide spectrum) showing the presence of Ti and O, which confirms that the prepared sample is pure TiO₂ and is devoid of any impurities. The high-resolution spectrum of Ti (Fig. 5B) showed two peaks at 456 eV and 461 eV that correspond to Ti⁴⁺ (2p3/2) and Ti⁴⁺ (2p1/2), respectively.³⁰ The O 1s peak (Fig. 5C) could be de-convoluted to two peaks at 530.9 eV and 532.0 eV, respectively, which correspond to the O of the Ti–O–Ti bond and the surface hydroxyl groups (–OH) of TiO₂.³¹

Fig. 4 Powder XRD pattern (A) and Raman spectrum (B) of the TiO₂ nanofiber/nanoparticle composite.

Fig. 5 Wide-spectrum XPS (A) and high-resolution spectra of the Ti and O ((B and C), respectively).

Electrochemical properties of TiO₂ nanostructures are studied by using galvanostatic charge/discharge and cyclic voltammetry measurements in half-cell configuration between 1 and 3 V vs. Li⁺/Li, at 20 °C. All the materials showed an open-circuit voltage (OCV) of ∼3.0 V vs. Li. Fig. 6A shows the cyclic voltammograms (CV) of various TiO₂ samples, obtained between OCV and 1 V vs. Li, at a scan rate of 0.1 mV s⁻¹. All the three samples exhibit sharp and distinct cathodic and anodic peaks around 1.5 and 2.1 V vs. Li, which corresponds to the reduction of Ti⁴⁺ to Ti³⁺ followed by the oxidation of Ti³⁺ to Ti⁴⁺ for anatase phase.³² Electrochemistry of Li insertion is a powerful tool to confirm the phase purity of TiO₂-based samples. It is interesting to note the variation in the peak currents, with TiO₂ fibres exhibiting the least peak current, and TiO₂ particles and TiO₂-fiber/particle composites show higher and similar peak currents. Such variation in peak currents could easily be correlated with the electrical conductivity and the faradaic or non-faradaic processes associated with the respective materials. The electrical conductivity measurements performed on TiO₂-fiber/particle composite, TiO₂ fibers and TiO₂ particles showed a conductivity of 8.0 × 10⁻⁸ S cm⁻¹, 1.8 × 10⁻⁷ S cm⁻¹ and 8.1 × 10⁻⁸ S cm⁻¹, respectively. There is not much difference in the electrical conductivity between the samples. However, peak current is minimum for the fibre-based electrodes because the total current is divided into the faradaic contribution and the capacitive contribution resulting from the prevailing surface charge storage mechanism. The CV profiles showed good electrochemical reversibility for all the samples, with no peak shift observed after the first cycle. The overall electrochemical lithiation/de-lithiation reaction in anatase TiO₂ can be represented by TiO₂ + xLi⁺ + xe⁻ → Li_xTiO₂.²⁰ Though certain nanostructures of TiO₂ exhibit x ∼ 0.8, generally, TiO₂ in the anatase phase shows x = 0.5, corresponding to a capacity of 167.5 mA h g⁻¹.³³ However, TiO₂ (B), a metastable phase of TiO₂ has been reported to show x = 0.98 in the first cycle of galvanostatic charge–discharge.³⁴ Irrespective of the phase, all forms of TiO₂ show a theoretical capacity of 336 mA h g⁻¹. Following the CV studies, galvanostatic charge/discharge measurements were carried out at C/10 rate for all the samples. Fig. 6B shows the initial charge/discharge profiles of TiO₂-fiber/particle composites, TiO₂ fibers and TiO₂ particles, which concur very well with the obtained CVs. The TiO₂-fiber/particle composite electrode showed an exceptionally high initial discharge capacity of 263 mA h g⁻¹, while TiO₂ fibers and particles delivered initial discharge capacity of 219 and 210 mA h g⁻¹, respectively. While TiO₂-fiber/particle composite electrode delivered an initial charge capacity of ∼235 mA h g⁻¹ with a coulombic efficiency of 87.2%, the TiO₂ particle and TiO₂ fiber electrodes yielded coulombic efficiencies of 82.1% and 77.4%, respectively, for the first cycle. Voltage profiles of all the three electrodes showed almost similar features, including a flat plateau (∼1.7 V), corresponding to the two-phase formation of Li-poor and Li-rich phases³⁵ and a sloppy region (between 1.7 V and 1.0 V), representative of surface storage of Li⁺ ions, which is capacitive in nature. The flat plateau region contributed to about 60% of the total capacity for the TiO₂-fiber/particle composite electrode, while for the TiO₂ fiber electrode, only ∼32.6% of the total capacity was delivered from diffusion-limited insertion of Li⁺ ions into the octahedral sites of anatase TiO₂. TiO₂ particles showed larger contribution from the flat plateau region (82% of the total capacity). This clearly points out the higher Li⁺ ion mobility and diffusion kinetics for the composite electrode compared to the sluggish Li⁺ ion diffusion in TiO₂ fibers and particles, which is in agreement with the results obtained from the detailed CV studies (Fig. S1 and S2†).

Fig. 6 Cyclic voltammograms of (A) TiO₂ particles, (B) TiO₂ fibers and (C) TiO₂ particles/fiber composites within a potential range of 1.0–3.0 V vs. Li⁺/Li at a scan rate of 0.1 mV s⁻¹; (D) first-cycle voltage profiles for each of the TiO₂ samples in 1 M LiPF₆ in EC : DMC (1 : 1 vol%) when galvanostatically cycled at a current rate of C/10.

Capacitive current contribution was found to be 45, 54 and 51% for TiO₂ particle, fiber and fiber/particle-based electrodes, respectively. Though the composite material shows maximum surface area, fiber-based electrodes show higher capacitive current than the particle-based electrodes by 9%. The TiO₂-fiber/particle composite material exhibited a high surface area of 100 m² g⁻¹, compared to the obtained specific surface areas of 87 and 89 m² g⁻¹ for TiO₂ fiber and particle, respectively. The composite shows an averaged capacitive contribution because the capacitive contribution is not exclusively dependent on the surface area; instead, electronic conductivity of the material and ionic conductivity in the pores of the electrode also play a very critical role. Hence, the direct correlation of the surface area to the capacitive current could be misleading. With larger surface area resulting from the interstitial spaces and exposed surfaces, the TiO₂-fiber/particle composite is expected to display a greater ability to intercalate Li⁺ ions and, consequently, higher reversible storage capacity.^4,36 Further, the capacity retention upon repeated cycling is tested using galvanostatic charge–discharge experiments at a current rate of C/10 (Fig. 6C). The TiO₂-fiber/particle composite electrode showed a very high capacity retention of ∼72% in 50 cycles. The TiO₂ fibers and particles also showed better capacity retention, viz. 70% and 67%, respectively. This shows good structural stability of the electrodes, which is very typical of anatase TiO₂. In order to evaluate the rate capability, the electrodes were galvanostatically cycled at different current rates from C/10 to 5C, with 7 cycles for each rate (Fig. 6D). Electrodes fabricated by either fibres or particles suffered a severe progressive capacity loss, and at 5C, the capacity was as low as 33% of the initial capacity, whereas the TiO₂-fiber/particle composite electrode exhibited a remarkable capacity retention of 150 mA h g⁻¹ (60% of the initial capacity) at 5C and retained a capacity of 200 mA g⁻¹ at C/10. EIS measurements were recorded in the frequency range of 40 kHz to 10 mHz for the three samples in the coin cell configuration by applying an AC potential of 10 mV, as shown in the complex plane impedance Nyquist plot (Fig. 7). The impedance spectra were deciphered by fitting to an equivalent electrical circuit as shown in the inset of the Nyquist plot for each of the samples (Fig. 7A–C). All the electrical equivalent circuits showed a common value for R₁ (∼4 ohms), which corresponds to the electrolyte resistance and the resistance of the cell components. The electrode fabricated using TiO₂ nanoparticles shows a semi-circle at the high-frequency region, arising from the parallel combination of R₂ and Q₁. Usually, only one semi-circle is seen in the high- to medium-frequency range; hence, R₂ has non-separable contributions from surface film and charge transfer. Similarly, Q₁ is associated with the constant phase element (CPE) associated with the surface film and the electrical double layer. R₃ and Q₂ are associated with the bulk resistance and the CPE, which gives rise to the second semi-circle in the low-frequency region. However, the electrolyte trapped in the pores of the electrode coating also contributes along the bulk impedance of the active material. R₂ and R₃ were found to be 159.2 and 95.2 Ω, respectively. At the low-frequency range, all the spectra had a typical straight line attributed to the Warburg region (W), which is followed by a capacitor element originating from the intercalation capacitance. Nyquist plots for the electrodes fabricated using TiO₂ fibers and particle/fibre composite showed only one semi-circle in the high-frequency range and fitted using a simpler equivalent electrical circuit, as shown in the inset of Fig. 7B and C, composed of R₁, R₂‖Q₁, W and C. R₂ is minimum (49.2 Ω) for the fibre based electrode, whereas it is 69.3 Ω for the composite-based electrode. The Warburg impedance is found to be maximum in TiO₂ particles and minimum in particle/fibre composite electrodes. Analysis of the phase angle gives valuable information about the Li⁺ ion diffusion and kinetics. This information is critical for better understanding of the variation of electrochemical properties of three different electrodes. The TiO₂ particle-based electrode shows a very low phase angle indicating the diffusion-limited kinetics. Hence, the poor Li⁺ ion diffusion and high resistive effects collectively affect the electrochemical performance of the particle-based electrode. Excellent electrochemical properties exhibited by the composite electrode could be attributed to its improved Li⁺ ion diffusion as indicated by high phase angle at the low-frequency range and low Warburg impedance (2 mΩ as opposed to 20 and 10 mΩ for particles and fibers, respectively), despite the slightly higher resistive effects compared to the fiber-based electrode.

Fig. 7 (A–C) Represent the Nyquist plots of the half cells fabricated using TiO₂ particles, fibres and particle/fibre composites, respectively. The equivalent electrical circuits corresponding to each electrode are shown in the inset. Insets of (B and C) also have the zoomed-in spectra of the respective electrodes in the high-frequency region.

Conclusions

In summary, we have demonstrated the superior electrochemical performance of a TiO₂ nanoarchitecture with a unique morphology consisting of TiO₂ nanoparticles decorating the surface of TiO₂ nanofibers, obtained by a simultaneous electrospinning and electrospraying technique, and acting as efficient anodes for the Li-ion battery. The TiO₂-fiber/particle nanohybrid electrode exhibited excellent rate capability, remarkable cyclic stability and very high capacity compared to TiO₂ nanofibers and nanoparticles. The enhanced electrochemical performance is attributed to the unique morphology of the composite electrode, which provides improved electrical conductivity and higher Li⁺ mobility in the material along with greater surface area, which facilitates high lithium intercalation into the material. The excellent electrochemical characteristics of the TiO₂ composite electrode material, along with its intrinsic safety features and structural stability, prepared by using a low-cost and scalable electrospinning technique, make it suitable for high-rate and large-scale Li-ion battery applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs).

Acknowledgements

DD acknowledges CSIR, Govt. of India, for the research fellowship. GSA and ASN acknowledge financial support from the Ministry of New and Renewable Energy (MNRE) and Solar Energy Research Initiative (SERI of DST), respectively, of the Govt. of India. MMS acknowledges the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Govt. of India, for the financial support through a DAE Young Scientist Research Award (No. 2012/20/34/5/BRNS). Authors acknowledge Ms Lashmi P. G. and Reshmi Varma P. C. for the help extended with the impedance analysis and electrical conductivity measurements, respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Calculation of capacitive current contribution to the total current. See DOI: 10.1039/c6ra04889g

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