DOI:10.30919/esee8c221

Received: 02 Jan 2019
Revised: 02 Apr 2019
Accepted: 03 Apr 2019
Published online: 04 Apr 2019

Hydrothermal Synthesis of CuCo2S4 Nano-structure and N-Doped Graphene for High-Performance Aqueous Asymmetric Supercapacitors

Hong Dong, Yuanyuan Li, Hui Chai, Yali Cao and Xin Chen

Key laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, China

*E-mail: [email protected]

 

Abstract

Recently, transition metal sulfides have drawn a lot of attention due to their potential application in energy and environmental fields. In this paper, we present a simple and facile method for preparing CuCo2S4 nanoparticles as high performance supercapacitor electrode materials. Electrochemical measurements exhibit that the CuCo2S4 nanoparticles electrode has a high specific capacitance of (190 C g-1 at 1A g-1), relatively high rate capability (46 % capacitance retention at 10A g-1) and good cycling stability (77 % of the initial specific capacitance can be maintained after 3000 cycles at 4A g-1). When fabricated as aqueous asymmetric supercapacitor by using CuCo2S4 nanoparticles and N-doped graphene (NG) composites as the positive electrode and the negative electrode, respectively. The assembled device exhibits a high energy density of 32.7 Wh kg-1 at a power density of 794 W kg-1. And it still operates at a high power density of 6.4 kW kg-1 with an energy density of 19.5 Wh kg-1. Moreover, after 6000 consecutive GCD cycles at a current density of 4 A g-1, 78.0 % of the initial capacitance value can be maintained. These attractive results manifest that CuCo2S4 //NG hold a great potential for practical applications in the field of energy storage system.

Table of Content

The copper and cobalt sulfides nanomaterials are achieved and the obtained electrode exhibits high capacitance and excellent retention.

 

 

 

 

Keywords: Transition metal sulfides; Electrode material; Aqueous asymmetric supercapacitors; Assembled device


1. Introduction

With continuous increase in the demand for fossil fuelsand theconsumption of non-renewable energyand global warming challenge,there is an urgent demand for developing cleaner and moreefficient energy storage systems, such asfuel cells, lithium-ion batteriesand supercapacitors (SCs).1-3 Amongthem,supercapacitors have been considered as one of themost promising energy storage devices due to theirrelatively higher power density,quicker charge/discharge capability andlonger recyclability compared withconventional rechargeable such as lithium-ion batteries.4,5 SCscan be dividedinto two types bydifferentenergy storagemechanisms: electrical double-layer capacitors and faradaic redox reaction pseudocapacitors.6 However, supercapacitor has low potential window andan order of magnitude lower energy densitycompared with batteriesin practical application. Recently, battery-type faradaic electrodes are emerging as possible solution to overcomethe lagging status with respect to energy density by takingadvantage of enhanced charge storage originating from deep surface faradaic processes. In order to improve the performance ofbattery-type capacitor, a sequence of transition metal oxides have been investigated as electrodes materials,such asmixed or ternary metal oxides with two differentmetal cations. For example, NiCo2O4,7 CuCo2O4,8 and MnCo2O4,9 and have attractednumerousattentions inenergy storage fields due to their higherelectroactivity and charge capacity.Unfortunately, the applications of these materials are mainly restricted by various aspects as low energy capacity and poor cycle life. Meanwhile, a number of researchers have been engaged in developingtransitionmetal sulfides, which have been considered as notable electrodematerials for battery-type faradaic electrodessince they exhibitlower electronegativity,higher specific capacitance and richer redox reactions compared with corresponding oxides.10 As a kind of the mostsignificanttransition metal sulfides, CuCo2S4 sparked worldwide interest as an attractive battery-type faradaic electrodeto fabricateadvanced capacitor devices for the presence of Cu+/Cu2+ and Co2+/Co3+ redox couples in the spinel structure.11 What are markable that the additionof Co can largely facilitateelectron transport. Significantly, CuCo2S4 has also been used in many other fieldssuch as water oxidationand Li ion batteries.12 The excellent performance in the fields of lithium and oxygen reductionconfirms that CuCo2S4 has enormous potential in the application of electrochemical reactions.Graphene has received significant attention because of its extraordinary electrical conductivity, high surface area, and good mechanical properties. Compared with other carbonaceous materials, including carbon nanotubes, activated carbons, and carbide derived carbons, graphene with hierarchicalnanosheets structures can achieve superior electrochemical properties due to their large specific surface area, superior elasticity, chemical stability, and excellent electrical conductivity. But the capacitance of pure graphene is fundamentally limited by its electrical double layer capacitance mechanism. Recently, researchers reported that the introducing of N-element could improve the electrochemical performance of graphene flakes, such as specific capacitance, cycle stability and son on.13 S. Suresh Balaji14 reported that two-dimensional N-doped graphene flakes possessedhigh specific capacitance and good cycle stability. Sun15 fabricated nitrogen-doped graphene,which facilitated the rapid transport of the electrolyte.Herein, we demonstrate a strategy withaspartic acid as a nitrogen sourcetoprepared coiled flake nitrogen-doped graphene.Coiled flakeare important for enhancing exposed surface area and facilitating electrolyte ion transport for energy storage.

How todesign and synthesize advancedelectrode materialsin inexpensive raw material and easy fabrication processes for battery-type supercapacitor, whichcan enhance power density due to its highelectrochemical reactivity caused by fast redox reactionremains challenges.In the past year,various studies have been carried out to address these challenges. Different nanostructures have been developed such as nanorods,16 nanowires,12 nanocubes,17 microspheres,18 nanosheets19 and core-shell hybrid nanostructures.20 Despite these developments, there are still more space to improve the morphological structure through easier process and use of inexpensive materials.

Present work aims to enchance the electrochemical performance of battery-type capacitor to further improve the morphological structure of electrode material. Polyvinyl pyrrolidone(PVP),as a growth modifier and surface stabilizer,was used to synthesize CuCo2S4 nanoparticles by hydrothermal method.And a novel asymmetric supercapacitor was fabricated using CuCo2S4 and NG asthe anode electrode and thecathode electrode, respecitively. The rational design of the asymmetric device afforded outstanding electrochemical performance witha high energydensity of 32.7 Wh kg-1 at a power density of 794 W kg-1and a high power density of 6.4 kW kg-1 with an energy density of 19.5 W h kg-1. Furthermore, the CuCo2S4//NG maintains 78.0 % initial capacitanceafter 6000 cycles at current density of 4 A g-1.These attractive results show thatCuCo2S4//NGis inspired in the energy storage system forpractical applications.

2. Experimentalsection

All the chemicals were of analytical grade and usedwithout further purification.

2.1 Synthesis of CuCo2S4Nanoparticles

In a typical synthesis procedure, 2.5 mmol Cu(NO3)2·6H2O, 5mmol CoCl2·2H2O, 15 mmol HMT(C6H12N4)and 1g PVP were dissolved in 70 mL distilled water. Then the mixedsuspension was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and subjected to heat treatment at 95 oC for 10 h. After being cooled to room temperature naturally, the product was carefullycleaned with deionized water and absolute ethanol.Thenthe obtained hydroxides were annealed in air at 350 oC for 3 h with aheating rate of 1 oC min-1. Finally, the CuCo2S4 were fabricated by placinghydroxide nanoparticles in a Teflon-lined stainless steel autoclavekept at160 oC for 8h containing 25 mmol Na2S·9H2O aqueous solution and thefinal product was cleaned with distilled water and dried in vacuum then dried at 60 oC overnight.

2.2 Synthesis ofN-doped Graphene (NG) Aerogel

Graphene oxide (GO) was prepared by a modified Hummers method inour previous paper.21 N-doped graphene aerogel (NG) was synthesized as following: 150 mg aspartic acid was suspended in 30 mL of GO aqueous dispersion (2mgmL-1) andthoroughly dispersed with ultrasonic treatment for 15 min.The solution was transferred to a 50 ml Teflon-lined autoclave and kept at 160 oC for 3 h. Then the resulting products were collected, washed with deionized water for several times and freeze-dried.

2.3 Materials Characterization

The phase compositionand chemical valence states of the obtained materials were investigated by X-ray diffraction (XRD, Bruker D8, Cu-Kα radiation λ=1.5406 Å) and X-ray photoelectron spectroscopy (XPS) were performed on an ESCALAB 250Xi electron spectrometer (Thermo Fisher Scientific) using an Al-Kα radiation. The morphology and microstructure of synthesized materials were characterized by scanning electron microscopy (SEM, S-4800, Japan) equipped with an electron dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, JEM-2100F, Japan).

2.4 Electrochemical Characterization

All the electrochemicalperformance was conducted using a three-electrode system in 3M aqueous KOH electrolyte with Pt as thecounter-electrode and Hg/HgO as the reference electrode, respectively. For preparation of the working electrode, the as-synthesized samples were mixed with acetylene black and poly-(tetrauoroethylene) in a mass ratio of 8:1:1 by adding a small quantity of ethanol.The mixture was stirred adequately to form a homogeneous slurry, and then coated and pressed onto a nickel foam (1×1cm2) current collector and dried at 60 oC under vacuum. The mass loading of the active material was about 3.0 mg. All of the electrochemical performance of electrodeswere carried out by a CHI 760E electrochemical workstation. Thespecific capacitance of the battery-type electrode was calculated from the charge-discharge curve based on the following equation:22

Where Cs, i, t, m represents the specific capacity (C g-1), the current applied (A), the discharge time (s), and the mass of active materials, respectively. Energy density (E, Wh kg-1) and power density (P, W kg-1) of the device can be evaluatedaccording to the following equations23:

where I (A g-1) is the current density, V(t) (V) is the cell voltage, dt (s) is the timedifferential and ∆t (s) is the discharge time.

 

3. Resultand Discussion

3.1 Synthesis and Characterization

Fig. 1 Crystallography and chemical states of copper, cobalt-based examples:(a) XRD patterns of CuCo2S4 and CuCo2O4,(b) XPS survey spectrum and high resolution XPS spectra of (c) Cu 2p, (d) Co2p, (e) S 2p, and (f) O 1s for Cu-Co oxysulfide.

X-ray diffraction(XRD) patterns of CuCo2S4 and CuCo2O4 wereshown in Fig.1a.The diffraction peaks located at 16.1º, 26.6º, 31.2º, 37.9º, 46.9º, 49.9º, 54.8º, 57.6º, 64.4º, 68.5ºcorrespond to the respective (111),(220), (311), (400), (422),(511),(440), (531), (533) and (444) planes of CuCo2S4(JCPDS card no.09-0425). All reflection peaksof the CuCo2O4 (Fig.1a) also can beindexed to the cubic phase CuCo2O4, in good accordance with thereported values from the JCPDS card (no. 01-1155). Note that, there are no extra peaks detected afterwards, implyingthe purity of all synthesized samples.

To identify elemental compositionandelectronic structureof the samples, thecharacterization of X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 1 (b) is the survey spectrum of the sample,wherein the attributions of all the peaks have been marked.Evidently, all the peaks can be ascribed to the elements Cu, Co, O, C, and S. The C (as reference) elements can beattributed to the exposure of air. So, the chemicalcomposition in the near-surface range of the product is Cu, Co, S and O, which is in good agreement with our experiment. The Cu 2p, Co 2p, and S 2p high-resolution XPS spectra were fitted by Gaussian fitting method considering two spin–orbit doublets and shakeup satellites (marked as “Sat.”). As regards the Cu 2p XPS spectrum, the binding energies at 932.2 eV in Cu 2p3/2 and 952.1 eV in Cu 2p1/2 are characteristic of Cu+24.In Fig.1(c), the binding energies of 778.76 eV and 796.35 eV of the Co 2p peaks are assigned to Co3+ and the binding energies25of 782.32 eV and 797.70 eV to Co2+. For the Co 2p spectrum, two distinguished doublets are shown at low-energy (Co 2p3/2) and high energy bands (Co 2p1/2). Moreover, the spin-energy separation value of Co 2p1/2 and Co 2p3/2is over 15 eV, indicating the coexistence26of Co2+ and Co3+. In the Fig. 1(d), the core-level spectrum of S 2p region and the binding energy of 161.8 eV are corresponding to S 2p3/2. The peak centered at about 162.98 eV is consistent with the binding energies of metal-sulfur bonding (Ni−S and Co−S bonding)27.The binding energy of 169.06 eV is attributed to shakeup satellites (indicated as “Sat.”). In addition,the detailed O 1s spectrum was presented in Fig. 1(f), exhibits three oxygenbonding features.The peak of O 1 at 529.53 eV is a typicalmetal-oxygen bond,28 while the peak of O2 at 531.13 eV are corresponding toahigh number of defect sites with low oxygen coordination inthe material with a small particle size.29 And the peak of O3 at 532.93 eV can be ascribed to physically and chemically adsorbed water on the surface.13 According to the XPS analysis, the near-surface composition ofthe CuCo2S4 sample is composed of Co2+, Co3+, Cu2+and S2- ,12, 15, 30 which match well with the molecular formula of CuCo2S4.

Fig. 2 SEM images of CuCo2S4(a,b) and CuCo2O4(c,d) at different magnifications.

 

The morphologiesof the as-synthesized CuCo2S4 and CuCo2O4 were characterized by SEM(Fig. 2). It is obvious thatthe as-synthesized CuCo2S4 is composed of nanoparticles.In the first step of the hydrothermal process,the OH- ions gradually were released by the hydrolysis of hexamethylenetetramine (HMT) at high temperature, facilitating the formation and uniform assembly CuCo(OH)X nanoparticles. Thenthe CuCo(OH)X releases OH-viahigh temperature calcinationto obtain CuCo2O4 nanoparticles.31 After sulfidation reaction, it can be clearly seen (Fig. 2a and b) that the CuCo2S4 emerge a relatively uniformnanoparticles morphology and are homogeneously distributed. And the surfacesof nanoparticles are smooth and there are no obvious defects, which are benefitting for Faradaic reactions.Instead, for CuCo2O4(Fig. 2c and d), the edges of nanoparticlesare dimmed, and the top surface is not smooth any more instead of being full with desultory nanoparticles after calcination.Bulk materials usually possess limited electrochemical active sites and will increase the conduction resistanceof ion/electrons, hence resulting in unsatisfactory electrochemical performance.

Fig. 3 TEM and HRTEM images of (a-c)CuCo2S4, (d-f) CuCo2O4.

The detailed microstructures of CuCo2S4 and CuCo2O4 were characterized by TEM and HRTEM. TEM images of CuCo2S4with different magnifications are shown in Fig. 3a-b. It can be observed that the CuCo2S4 sample containsnumerous nanoparticles with the diameter about 50-60 nm, confirmed tothe SEM tests.The nanoparticles structure effectively pushes up the amount of electroactive sites and can greatly increase the electrode/electrolyte contact interface and enhance ion and electron diffusion. Fig.3d-eare typical TEM images of CuCo2O4 with different magnifications. The Fig.3d clearly illustrates a large number of disordered CuCo2S4 nanoparticles which are interconnected with each other in the range of 3-6 nm. And these nanoparticles with very small gap could facilitaterapid redox reactions and increase the contact area between the electrode andelectrolyte. From the HRTEM of Fig. 3f, the crystal latticeswith d-spacing of ~0.32 and ~0.405 nm correspondto the (511) and (731) planes of CuCo2S4, while the d-spacings of 0.305 and 0.409 nm correspond to the (422) and (533) planes of the CuCo2O4 crystal (Fig. 3c), respectively, which arewell consistent with the XRDresult. Furthermore, The SAED pattern of image (Fig. S1†)furtherdemonstrates that CuCo2S4nanoparticlesare single-crystal structure.

Fig. 4 Dark-field STEM elemental mapping analysis of the CuCo2S4 nanoparticles (a) selected area and corresponding elemental mapping of (b) cobalt, (c) copper, (d) sulfur, and (e) STEM-EDS elemental line mapping images of CuCo2S4 nanoparticles.

The elemental distribution and composition of CuCo2S4 nanoparticles were further investigated by STEM with energy dispersive X-ray spectroscopy (EDS) analysis. The dark-field STEM images as well as the corresponding cobalt, copper and sulfur mappings are shown in Fig. 4. The EDS mapping showed that cobalt, copper, and sulfur were welldispersed,which is consistent with the TEM results. The STEM-EDS line mapping (Fig. 4g) clearly revealed that the synthesized CuCo2S4 nanoparticles consist of Co, S, and Cu. Meanwhile,the elemental distribution and composition of CuCo2O4 nanoparticles were also investigated by STEM with energy dispersive X-ray spectroscopy (EDS) in the Fig.S2†.

Fig.5 Electrochemical performances using a three-electrode mode in 3 KOH aqueous electrolyte. CV curves of (a) CuCo2S4 (b) CuCo2O4 electrode at various scanrates ranging from 1-20 mV s-1. GCDcurves of (c)CuCo2S4 (d) CuCo2O4 electrode at various current densities in the voltage range of 0-0.45 V. (e) Comparative of specific capacitance of CuCo2S4 and CuCo2O4 at various current densities in 3 M KOH aqueous solution. (f) Cycling stability of CuCo2S4 and CuCo2O4ata current density of 4A g-1.

To verify the superiority of the CuCo2S4 after sulfurization, the electrochemical performance of CuCo2S4 and CuCo2O4 were investigated, respectively. As shown in Fig.5a-b,the CV curves of CuCo2S4 and CuCo2O4 reveal the Faradic nature attributed to M-S/M-S-OH of the battery-type electrodes, where M represents both Cu and Co ions12.Notably, the CV curves of samples show adistinct pair of redox peaks. Moreover, the redox peaks of CuCo2O4 represent moreobvious than CuCo2S4 indicating more faradic reaction occurred. Fig. 5c-d shows the GCD tests thatwere performed within the voltage range of 0-0.45 V. From the GCD plots, the CuCo2S4 exhibited specific capacities of ~190, 167, 152, 129, 103, and 88.3 C g-1, at 1, 2, 3, 5, 8, and 10 A g-1.In contrast, the CuCo2O4 delivers good capacitiesof ~82, 75, 67, 57, 43 and 35 C g-1 under the same series of current densities (seen from Fig. 5d).Compared with CuCo2O4, the straight line of CuCo2S4is nearly perpendicular to the real axis, indicating higher capacitive behavior. In addition, cycling stability of CuCo2S4 and CuCo2O4were also evaluated by the long-term cycling performance at the constant current density of 4A g-1 and the results were shown in Fig. 5f. 71% of the initial specificcapacitycan be maintained after 3000 cycles. However, specific capacityof the CuCo2S4 still remains 77 % of the initial specific capacityretention. The comparison of the electrochemical performance of the CuCo2S4 between this work and other studies reported in the literature were listed in the form of a table (Table S1†in Supporting Information). These results suggest that the CuCo2S4 with unique structure exhibitsa good long-term stability than that of CuCo2O4.

Fig. 6 (a) CV curves of CuCo2S4//NG cell at different voltage windows in a 3 M KOH electrolyte at a current density of 5mV s−1. (b) CV curves measured between 0 and 1.6 V with different scan rates from 2 to 50 mV s-1. (c) GCD curves of CuCo2S4//NG at various current densities in the voltage range of 0-0.45 V. (d) Specific capacitance versus current density curves. (e) Ragone plot of the ASC device for comparison. (f)Cycling performance of the CuCo2S4//NG at a current density of 4A g-1.

To further evaluate the CuCo2S4 electrode for practical applications, a novel ASC as developed using CuCo2S4 and NG as thecathode and anodeelectrodes, respectively. A series of CV and the GCD curves of the NG at various scan rates ranging from 2 to 50 mV s-1 in a 3 M KOH aqueous electrolyte were shown in Fig. S2†of SI†. Fig. 6a shows the CV curves of the asymmetric supercapacitor cell at different voltage windows in a 3 M KOH electrolyte at a scan rate of 5 mV s−1. It is noted that the stable electrochemical window of the asymmetric supercapacitor cellvoltage can be extended to 1.6 V. Fig. 6b presents the CV curves of CuCo2S4//NG ASC at different scan rates from 2 to 50 mV s-1 within a 0-1.6 V potential window. The ASC device shows a relatively quasi-rectangular CV shape with weak redox peaks, which indicates CuCo2S4//NG exhibiting good capacitive behavior with the combination of EDLC and pseudocapacitance. With the increase of scan rate from 2 to 50 mV s-1, the shapes of the CV curves of the device do not change, implying the desirable rapid charge/discharge characteristic for supercapacitors. Fig. 6c shows the GCD curves of CuCo2S4//NG ASC with cell voltage as high as 1.6 V at various current densities from 1 to 8 A g-1. During the charge/discharge procedure, the charge curves of CuCo2S4//NG ASC are almost symmetrical to its corresponding discharge counterpart, confirming the excellent electrochemical reversibility. The Cs of the CuCo2S4//NG ASC based on the total mass of the device was calculated from the charge/discharge curves according to eq. (1). The values of Cs are 148, 131, 117, 109, and 88 C g-1 at current densities of 1, 2, 3, 5, 8 A g-1, as shown in Fig. 6c. The energy and power densities are calculated according to eq. (2) and (3) to further demonstrate the electrochemical performance of the supercapacitors, as shown in Fig. 6e. Remarkably, the present device displays a high energy density about 32.7 Wh kg-1 at a power density of 794 W kg-1, which confirms thesuperiority of our device compared to the other reportedASC. For example, NiMoO4//NGO (30.3 W h kg-1 at 187 W kg-1)32, NiCo2S4//RGO (31.5 W h kg-1 at 156.6 W kg-1)33, CuCo2O4//AC (18 W h kg-1 at 259 W kg-1)34, and CuS//AC (15.06 Whkg-1 at 392.9 W kg-1)35.The cycling performance of the CuCo2S4//NG ASC is also tested (Fig. 6f). After 6000 consecutive GCD cycles at acurrent density of 4 A g-1, 78 % of the initial specific capacitance can be maintained. This reveals that CuCo2S4 nanoparticlesare a class of promising electrode materials for supercapacitors.

4. Conclusions

In conclusion, CuCo2S4 nano-structure was successfully synthesizedthrough a facile hydrothermal method for electrochemical energy storage. The efficient and low-cost fabrication method involves the hydrothermal method and anion-exchange reaction. CuCo2S4 delivers ahigh specific capacityof 190 C g−1 at 1 A g-1, as well as good rate capability and long cycle stability, which are mainly owing togood mechanical and electrical contact with electrolyte ions and electrons, low crystallinity and good wettability without an annealing process, rich redox reactions, as well as high conductivity and transport rate. An asymmetric supercapacitor cell was fabricated with CuCo2S4 nanoparticles and NG as anode and cathode electrode, respectively,which exhibits a high energy density of 32.7 Wh kg−1 at a power density of 794 W kg−1. Our results suggest that CuCo2S4 nanoparticles canhold great promise for application in energy storage devices.

Acknowledgment

This work was supported byNational Natural Science Foundation of China (No. 21461024). We also gratefully acknowledge financial support byNational Natural Science Foundation of Xinjiang Uygur Autonomous Region (No. 2015211c250) andScientific Research Program of the Higher Education Institution of Xinjiang (No. XJEDU2014I008).


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