Highly rate capable nanoflower-like NiSe and WO3@PPy composite

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Highly rate capable nanoflower-like NiSe and WO3@PPy composite electrode materials toward high energy density flexible all-solid-state asymmetric supercapacitor Amit Kumar Das, Sarbaranjan Paria, Anirban Maitra, Lopamudra Halder, Aswini Bera, Ranadip Bera, Suman Kumar Si, Anurima De, Suparna Ojha, Sumanta Bera, Sumanta Kumar Karan, and Bhanu Bhusan Khatua ACS Appl. Electron. Mater., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Electronic Materials

Highly rate capable nanoflower-like NiSe and WO3@PPy composite electrode materials toward high energy density flexible all-solid-state asymmetric supercapacitor Amit Kumar Das, Sarbaranjan Paria, Anirban Maitra, Lopamudra Halder, Aswini Bera, Ranadip Bera, Suman Kumar Si, Anurima De, Suparna Ojha, Sumanta Bera, Sumanta Kumar Karan, Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India

*Corresponding Author Dr. B.B. Khatua (Email: [email protected]). Materials Science Centre, Indian Institute of Technology, Kharagpur – 721302, India. Tel.:91-3222-283982 1

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ABSTRACT: In this study, an advanced novel all-solid-state asymmetric supercapacitor (ASC) device of high energy and power densities is designed based on the nanoflower-like NiSe as positive electrode and WO3@PPy composite as the negative one. The porous NiSe was prepared by facile selenization of predesigned NiO nanoflowers and the WO3@PPy composite was synthesized through in-situ oxidative polymerization of pyrrole in presence of dispersed WO3 nanosticks which, in turn, was produced by a simple sulphate-assisted hydrothermal method. When tested as a supercapacitor positive electrode in three-electrode system, the NiSe exhibits an appreciably higher specific capacitance of 1274 F g-1 at 2 A g-1 than that of NiO nanoflower (774 F g-1). Again, as the negative electrode, the WO3@PPy composite also shows an excellent electrochemical properties with higher specific capacitance (586 F g-1 at 2 A g-1) than those of either of its components (WO3: 402 F g-1 and PPy: 224F g-1). Based on these properties of the respective electrodes, a flexible ASC device was designed and fabricated by assembling the NiSe as positive electrode with WO3@PPy composite as negative electrode. The NiSe//WO3@PPy solid-state ASC with an extended potential window of 1.25 V demonstrates an admirable energy density of 37.3 Wh kg-1 at a power density of 1249 W kg-1 at 2 A g–1 along with outstanding long-term cycling stability that retains 91 % of the initial capacitance even after 5000 charging and discharging cycles. All these results reveal the proficiency of the ASC device as a high-performance energy storage system for next-generation portable electronics.

Keywords: Supercapacitor, selenization, oxidative polymerization, specific capacitance, energy density. 2


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1. INTRODUCTION With continuous rising demand for energy globally along with the environmental protection, researchers are nowadays devoted to investigate high-performance energy conversion systems and storage devices from alternative sustainable energy sources. Currently, supercapacitors have emerged to be environmentally viable, durable energy storage devices with high energy and power densities, which can be used in advanced portable electronic devices, sensor networks and electric vehicles (EVs).1−5 Supercapacitors can also interestingly fill the gap between batteries and conventional dielectric capacitors exhibiting higher energy density than ordinary capacitors and higher power density than batteries.6 According to the charge storage mechanism, supercapacitors are mainly categorized into three types: electrical double layer capacitors (EDLCs), the one accumulates the charges electrostatically via the formation of Helmholtz double layer at the electrode/electrolyte interface and mainly based on carbonaceous nanomaterials, whereas the other kind named as pseudocapacitors storing the charge by fast, reversible faradaic processes occurring at the electrode-electrolyte interface and these are mostly based on transition metal compounds and conducting polymers,7,8 and finally the third type is the hybrid asymmetric supercapacitors (ASC) combining both the EDLCs and faradic materials in the same cell for the synergistic improvement of electrochemical performance with superb stability.2,9,10 Since this hybrid ASC can utilize respective potential windows of the positive faradaic-type electrode and negative capacitive-type electrode in the same aqueous electrolyte, designing and development of this kind of ASC is a very useful approach to boost the working voltage of supercapacitors and hence, finally results in appreciable enhancement in energy density (E) as expressed by the equation, E = (1 ∕ 2) CspV2, where Csp is the specific capacitance of the ASC device and V is the operating voltage window. Now, Csp of any supercapacitor primarily depends on the fundamental properties of the electrode materials, such as good 3



electrical conductivity, unique nanostructured morphology with large specific surface area, excellent chemical stability and multiple oxidation states as possessed by the transition metals.4,11,12 Thus, designing and synthesis of novel positive and negative electrode materials for supercapacitor with significant electrochemical characteristics is of great concern for considerable improvement in overall electrochemical performance of the fabricated ASC.13 In recent times, pronounced research activities have been focussed on charge storage mechanism and electrochemical characteristics of different nickel chalcogenides based positive electrode materials owing to their fast surface redox processes for charge storage in alkaline electrolytes and hence, these reveal higher specific capacitances as well as energy densities compared to the normal capacitors.

6,14−17

Among various nickel chalcogenides,

nickel selenide (NiSe) is especially propitious faradic electrode material due to its tunable electronic configuration with multiple oxidation states and much improved electrical conductivity which enable it to display noticeably fast transport kinetics of charges and ions lessening the diffusion paths, less charge transfer resistance and excellent electrochemical activity with regard to high capacitance, rate capability and cycling stability etc. and all these useful properties are mainly attributed to the better metallic property of Se compared to O and S.2,4,9,14,18,19 The higher conductivity of NiSe than its oxide can be realized from their respective band gap values (NiSe:2.0 eV and NiO:4.0 eV) when applied as a p-type semiconductor.2 Thus, NiSe based electrodes can diminish the extra power loss during their charging-discharging and effectively be applied in electrocatalyst.2 Owing to these amazing physical and electrochemical characteristics, several delightful results have been reported till date for the faradic NiSe based electrode materials for efficient supercapacitors. For example, Guo et al. reported a facile one-step method for the preparation of hierarchical nanosheetbased NiSe microspheres for which a specific capacitance of 492 F g-1 was obtained at a current density of 0.5 A g-1 in 2 M KOH.18 Tang et al. hydrothermally prepared a NiSe 4


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nanowire film on nickel foam (NiSe/NF) as the novel binder-free electrode for supercapacitors and obtained a high specific capacitance of 1790 F g-1 and an areal capacitance of 5.01 F cm-2 at 5 A g-1 current density.9 Ye et al. fabricated NiTe/NiSe composites by the electrodeposition of NiSe on NiTe thin films grown in-situ on Ni foam and reported the specific capacitance of the electrode as 1868 F g-1 at 1 A g-1 current density.19 Therefore, construction of NiSe based positive electrode material can be a promising approach for the fabrication of high-performance supercapacitor. Also, further research on the electrochemical characteristics of NiSe is deemed necessary as less attention has been paid to this particular faradic electrode material as compared to its oxide and sulphide counterparts. Since it is a well known and proved fact that a porous nanostructured material exhibits enhanced electrochemical and physical properties,2 therefore, in this work, porous nanoflower structured NiSe has been synthesized and investigated as the positive electrode material. Now, in most of the ASC devices, various porous carbonaceous materials are commonly employed as the negative electrodes by virtue of their large surface area, superior electrical conductivity and high electrochemical stability.4,20 However, a serious problem of using any carbonaceous negative electrode material is its low specific capacitance, which is the major barrier to the high energy density of the ASC device. Therefore, a new class of conducting negative electrode materials enriched with high specific capacitance is really necessary for developing an advanced high-performance ASC device. In this regard, conducting polymer based composite, such as WO3/polypyrrole composite (WO3@PPy) can be a capable candidate as the negative electrode. This typical stable conjugated polymer, PPy, can serve as the low cost active electrode material for supercapacitor owing to its high conductivity, excellent energy storage ability with notable electrochemical oxidationreduction reversibility and easy fabrication.21,22 On the other hand, WO3, as a prospective charge storage electrode material for supercapacitor, has recently drawn a considerable 5



attention because of its low cost, easy synthesis, various morphology, significant chemical stability, reversible redox transition WVI/WV and high theoretical specific capacitance.3,23,24 Now the reversible charge transfer causing the redox transition in WO3 greatly relies on its microstructure which is actually dependent on the synthesis conditions. In this work, 1D nanostick structure of WO3 has been synthesized, which is anticipated to exhibit enhanced electrochemical properties as this kind of microstructure furnishes appreciable specific surface area and shortens the diffusion paths for ions.9 However, the major demerits of using WO3 are poor electrical conductivity and rate performance limiting the electrochemical performance of the supercapacitor. In order to combat these shortcomings, several feasible efforts have been made such as optimization of morphology and structure and combination of WO3 with other conducting materials like conducting polymers (like PPy or polyaniline (PANI)) or various carbonaceous nanomaterials fabricating a composite and in doing this, the charge transfer resistance of the electrode material can be lowered resulting in better specific capacitance and long cycle life.24,25 Compared to conducting carbonaceous nanomaterials, PPy can be grown into 3D structure through in-situ polymerization encapsulating the outer surface of the WO3 and this continuous 3D encapsulation of conducting, flexible and porous PPy in WO3@PPy composite not only offers easy accessibility of the charge on the electrode surface forming a high electronic path but also shortens the diffusion path length for ions facilitating fast electron transport.25−28 Moreover, in this kind of composite, the stability of WO3 also increases via its immobilization into the polymer matrix. Therefore, such a reasonable architecture with PPy coated WO3 nanosticks in the WO3@PPy composite, by the virtue of the synergistic effect between these two components, can remarkably improve the electrochemical performance of the ASC device. However, till date, almost all PPy and WO 3 based electrode materials have been investigated as the positive electrodes in ASC devices

6


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and this motivated us to develop the WO3@PPy composite and explore its electrochemical characteristics as the negative electrode. In this paper, we report designing and fabrication of an all new flexible all-solid-state ASC device with high energy storage ability and capability to provide high energy density using nanoflower-like NiSe as a positive electrode and WO3@PPy composite as a negative electrode in polyvinyl alcohol (PVA)/KOH gel as the electrolyte. The significance of this work is the employment of WO3@PPy composite as the negative electrode material in an ASC device since PPy or WO3 based negative electrodes in ASCs are rarely reported.20 In addition, assembly of WO3@PPy composite negative electrode material with faradaic nanoflower-like NiSe positive electrode in basic gel electrolyte introduces a novel all-solidstate flexible NiSe//WO3@PPy ASC device. As the positive electrode, interconnected nanoflower structured NiSe can furnish a huge redox active sites and incessant transport path to promote the diffusion of electrolytic ions and hence, a high capacitance value with enhanced rate capability is obtained for this electrode. In the WO3@PPy composite employed as the negative electrode, the porous conducting PPy three-dimensionally interconnected with and grown on pseuodocapacitive WO3 nanosticks facilitates the transport kinetics and ionic conductivity ultimately enabling the composite electrode to show appreciable electrochemical energy storage features as well as increased power density.29 Figure 1 schematically demonstrates the strategy for synthesizing different electrode materials. Now, the asfabricated all-solid-state ASC (NiSe//WO3@PPy) (as schematically illustrated in Figure 1) can work at the potential range of 1.25 V based on the combined working potential window of NiSe and WO3@PPy composite and delivers a maximum energy density of 37.3 W h kg-1 at a power density of 1249 W kg-1 at 2 A g-1 current density. Furthermore, this NiSe//WO3@PPy ASC device has got excellent cycling stability as it retained 91% of its initial capacitance even after 5000 cycles of charging and discharging. The workability of the 7



device in bent mode has also been investigated to test its suitability for the flexible electronic devices. This work might be extended to different transition metal selenides to integrate high power next-generation portable devices.

Figure 1. Schematic illustration for the synthesis of different electrode materials: (1) synthesis of NiSe and construction of positive electrode and (2) synthesis of WO3@PPy composite and construction of negative electrode and final fabrication of flexible all-solidstate NiSe//WO3@PPy ASC device assembling these two electrodes. 2. EXPERIMENTAL 2.1. Preparation of the electrode materials 2.1.1. Synthesis of NiO For the preparation of NiO nanoflower, firstly flower-like Ni(OH)2 was prepared by adding 2.10 g hexamethylenetetramine (HMT) to the Ni(NO3)2.6H2O solution (0.87 g 8


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dissolved in 40 ml distilled water) followed by magnetically stirring. Then, the solution was filled in a 50 ml Teflon-lined stainless steel (SS) autoclave and heated at 85 ºC for 12 h. Next, the cooled precipitated product was washed with distilled water and ethanol and subsequently collected after drying it at 60 ºC for overnight. Finally, as-prepared Ni(OH)2 was annealed at 300 °C for 2 h to obtain nanoflower structured NiO powder. After calcination, a change in colour from light bluish green to gray was observed, which confirms the conversion of Ni(OH)2 to NiO. 2.1.2. Synthesis of flower-like NiSe Flower-like nanostructured NiSe was prepared by the selenization of as-prepared flower-like nanostructured NiO via a facile hydrothermal process. In a typical synthesis approach, 0.026 g (0.35 mmol) of NiO was dispersed in 35 ml distilled water by ultrasonic sound by treatment. Next, calculated amount of Na2SeO3 was added to the above dispersed suspension followed by magnetically stirring for about 30 min. Finally, this mixture was poured and sealed into a 50 ml Teflon-lined SS autoclave and subsequently heated at 180 °C for 12 h. After getting cooled naturally, the resulting product was washed with distilled water and absolute ethanol subsequently and then, dried at 60 ºC for overnight. 2.1.3. Synthesis of WO3 nanosticks A simple sulphate-assisted hydrothermal method was followed to synthesize WO3 nanosticks, as reported elsewhere.3 In details, 1.6 g of Na2WO4. 2H2O was first dissolved in 30 ml distilled water and to it, 3 M HCL solution was added till the solution attains a pH of ~2. Subsequently, 1.4 g of (NH4)2SO4 as the morphology controlling agent was added to the above solution with magnetic stirring for about 1 h. The obtained clear solution was then transferred into a Teflon-lined SS autoclave of 50 ml volume capacity and heated at 180 °C for 24 h. After cooled to room temperature, the obtained precipitate of WO3 was collected, washed with distilled water and absolute ethanol several times and eventually dried at 60 ºC 9



overnight. Lastly, the resulting WO3 powder was annealed at 300 °C for 3 h to obtain WO3 nanosticks. 2.1.4. Synthesis of WO3@PPy composite The WO3@PPy composite was prepared by covering WO3 nanosticks by PPy through in-situ oxidative polymerisation process. Typically, 0.1 g of WO3 nanosticks and 4 mg of SDS were taken in 40 ml of distilled water and the resulting mixture was first sonicated for 30 min and then magnetically stirred for 3 h. Afterwards, 20 μL pyrrole monomer was added to the above mixture with vigorous stirring for another 1 h. Finally, dropwise addition of 0.1 M ammonium persulphate solution led to gradual change in colour of the mixture from light greenish to black specifying the formation of the polypyrrole (PPy) and the polymerization process was continued for 4 h at normal temperature. The obtained black WO3@PPy composite was then washed with distilled water and absolute ethanol a few times, and dried at 60 ºC in a hot air oven. Pristine PPy was also synthesized for the comparative studies following the same procedure without adding the WO3 nanosticks and SDS. 2.2. Electrochemical characterizations and assembly of the asymmetric supercapacitor (ASC) device Electrochemical characteristics of all the electrode materials were first evaluated in a three-electrode electrochemical cell (using as-synthesized materials as working electrodes, Pt counter electrode and a saturated calomel electrode (SCE) as the reference one) using BioLogicSP-150 electrochemical workstation at room temperature. The electrochemical measurements, i.e., cyclic voltammetry (CV), cyclic charge–discharge (CCD) and electrochemical impedance spectroscopy (EIS), were conducted in 2 M KOH aqueous solution, taken as electrolyte. All the working electrodes were constructed by uniformly coating the homogeneous slurry containing the electroactive material, polyvinylidenefluoride 10


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(PVDF) as a binder and acetylene black in 8:1:1 weight ratio in N-methyl-2-pyrrolidone (NMP) solvent on a 1 cm2 area of a polished SS foil followed by drying at 60 oC for a day. Furthermore, the flexible solid-state ASC devices of two electrode configuration were constructed (schematically illustrated in Figure 1) using nanoflower-like NiSe positive electrode and WO3@PPy composite negative electrode and PVA/KOH gel as the electrolyte. To prepare the gel electrolyte, firstly 6.0 g poly(vinyl alcohol) (PVA) powder was taken in 60 ml distilled water and the mixture was heated at 90 °C for 1 h with continuous stirring. When the solution became clear, it was kept for cooling naturally and then, 3.0 g KOH was added to the cold clear solution under constant strong stirring for another 30 min. Next, the two electrodes and a cellulose filter paper (Whatman 42) separator were soaked in the PVA/KOH gel electrolyte and then removed and allowed to solidify at room temperature for 24 h. Finally, the electrodes and the separator were assembled into an all-solid-state ASC and kept at 50 °C for 12 h for the removal of surplus water present in the electrolyte. During the construction of the ASC device, the loading mass ratio of the electro-active materials was maintained to be (m+:m– = 0.82:1) as calculated on the basis of their respective specific capacitance values and working potential ranges. Now, the CV measurement for the positive electrode was done between the potential window of 0.0–0.45 V (vs. SCE) and for the negative electrode within –0.8 to 0.0 V (vs. SCE) at various scans rates (2 to 100 mV s-1). CCD was done between the same potential window differing the current densities (2 to 20 A g-1). Hence, for the ASC device, the combined potential window for CV and CCD was 1.25 V. The EIS tests were done in the frequency between 1 MHz–100 mHz. Calculation of specific capacitance (Csp) of the electrode material was done based on the equation as follows: I  t

Csp  m  V

(1) 11



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Where, I (A), Δt (s), m (g) and ΔV (V) stand for the charging-discharging current, discharge time, mass of the active electrode material (for the ASC device, total mass of both positive and negative electrodes was considered) and the potential range of the cell in this CCD process, respectively. Energy density (E, W h Kg-1) and power density (P, W Kg-1) were determined from CCD plots, as per the following equations: 1 C spV 2 2

E

(2)

P  E t

(3)

Where, Csp (F g-1) represents the estimated specific capacitance of the ASC device, V (V) is the working potential window and Δt (s) is the discharging time as obtained from the CCD profiles. The coulombic efficiency specifies the efficiency and reversibility of charge transfer processes during charging-discharging and is usually evaluated to investigate the cycle stability of the device by comparing the first and the end cycle. The coulombic efficiency (η) is defined as the ratio of discharging time and charging time when the charge–discharge current densities are equal. It can be calculated by the following equation:



tD 100% tC

(4)

Where tD and tC are the discharge and charge times in second. 3. RESULTS AND DISCUSSION 3.1. Structural and morphological characterizations of as-prepared positive electrode materials 3.1.1. Wide angle X-ray diffraction (WAXD) study and X–ray photoelectron spectroscopic (XPS) analysis

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Typical WAXD analysis was first done to investigate the crystallographic structures and phases of the synthesized NiO and the obtained diffractograms are depicted in Figure 2a. In the X-Ray diffractogram for NiO, the prominent peaks located at 2θ = 37°, 43°, 62.4° and 75° can be attributed to the (111), (200), (220) and (311) planes of standard cubic NiO, respectively (JCPDS: 47−1049) and this indicates that the precursor is successfully converted to NiO after the calcination process. On the other hand, the XRD pattern of flower-like NiSe contains strong diffraction peaks only at 2θ =33.5°, 45°, 50.6°, 60.3°, 62° and 70°, which can be indexed to (101), (102), (110), (103), (201) and (202) crystal planes of NiSe (JCPDS, card no: 65-6014). Furthermore, the surface compositional analysis and the chemical state of the assynthesized flower-like NiSe sample were also done via XPS measurement and Figure 2b displays the corresponding XPS survey spectrum. Here, the binding energy of an aliphatic carbon (i.e., ≈ 284.04 eV) was used as the reference to record the XPS data of the sample under investigation. The peaks in the wide-range survey scan spectrum [Figure 2b] reveal the coexistence of constituent elements Ni and Se along with C and O. It is noteworthy that the peaks assigned for O and C appear due to interaction of the sample with air and consequent adsorption of it on the surface of the sample.14,30 Now, the chemical states of Ni and Se elements were examined by analysing the respective high resolution emission spectrum after fitting by Gaussian method, as shown in Figure 2c and d. The high-resolution XPS spectrum of Ni 2p [Figure 2c] shows the appearance of two peaks at the binding energies of 855 and 872.75 eV ascribable to Ni 2p3/2 and Ni 2p1/2, respectively, with two associated shake-up satellites (marked as “Sat.”) at 860.62 and 878.62 eV signifying the Ni2+ oxidation state of Ni.9,18 Again, the Se 3d spectrum [Figure 2d] typically features metal-selenium bonds as the peaks sitting at 53.75eV and 54.5eV indicate the binding energies of Se 3d 5/2 and Se 3d3/2,

13



respectively, and the additional peak at 58 eV is ascribed to the surface oxidation of Se.2,18 All these results show a good consistency with those for NiSe, as reported elsewhere.2,9,18 On the basis of above XRD and XPS results, the successful formation of NiSe via facile selenization of NiO can be confirmed.

Figure 2. XRD patterns of NiO and NiSe (a), survey XPS spectrum of NiSe (b) and high resolution XPS spectra of Ni 2p (c) and Se 3d (d) regions. 3.1.2. Morphological Analysis FE-SEM and TEM techniques were employed to characterize the morphologies of NiO and NiSe electrode materials and the obtained corresponding micrographs are displayed in Figure 3. The FE-SEM images (Figure 3a and b) of NiO reveal its open flower-like microstructure, where a number of nanoflake-like porous petals constitute each flower of 14


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submicrometer diameter by getting disorderly interconnected with each other. This flowerlike structure is notably so stable that annealing could not alter the integral appearance of the flower-like Ni(OH)2 precursor nanostructures. As observed in the FE-SEM images of NiSe [Figure 3c and d], the microstructure has been changed a bit after the selenization of NiO as the smooth surface of the nanopetals has become somewhat rough with a small increase in thickness of the same. But, it is fascinating to notice that the flower-like morphology with flake-like nanopetals remains roughly intact in NiSe even after repeated washing and ultrasound treatment during its synthesis indicating high stability of the NiSe flower-like architecture. Now, the rough surface of the nanopetals in NiSe not only facilitates the transport of electrolytic ions but also promotes diffusion kinetics by furnishing increased electrode-electrolyte contact area and thus, the capacitive performance of NiSe is greatly enhanced.31 The elemental mapping of NiSe was done by FE-SEM equipped with energydispersive X-ray spectroscopy (EDX) mapping to verify the distribution of the elements and the obtained results are shown in Figure S2, Electronic Supplementary Information (ESI). TEM was employed to further examine the morphological feature of as-synthesized NiSe in detail. The TEM micrograph [Figure 3e] clearly reveals that randomly oriented highly vented NiSe nanoflakes assemble to form spherical microstructure. This porous 3D structure with high surface area and ample inner spaces smoothes the mass and charge transfer boosting the electrochemical performance.19 The selected area electron diffraction (SAED) pattern of NiSe, as shown in Figure 3f further establishes its crystalline structure (JCPDS, card no: 65−6014).

15



Figure 3. FE-SEM images of NiO (a and b) and NiSe (c and d) at different magnifications, TEM image of as-synthesized of NiSe (e) and the SAED pattern of NiSe (f). 3.2. Structural and morphological characterizations of as-prepared negative electrode materials The detailed structural characterizations (i.e., WAXD, FTIR and Raman spectroscopic analysis) of as-prepared WO3, PPy and WO3@PPy composite have elaborately been 16


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discussed in the ESI. FE-SEM and TEM images (Figure 4) reveal the morphologies of pure WO3, pure PPy and the WO3@PPy composite. The FE-SEM image (Figure 4a) shows randomly arranged nanosticks like microstructure for the as-synthesized WO3. As can be seen from Figure 4b, PPy particles aggregate to form agglutinated small hemispherical granular microstructure with a rough surface though no regular morphology is observed for it as polymerisation of pyrrole follows multi-level growth process.32 As observed from FE-SEM image [Figure 4c] of the WO3@PPy composite, after in-situ oxidative polymerization of pyrrole, the 3D hemispherical PPy particles not only have successfully inserted into the WO3 nanosticks but also covered those making the surface of the nanosticks more coarse as compared to those of pure WO3. This points towards efficient interactions between the components in WO3@PPy composite in which the embedded porous conducting PPy along with WO3 nanosticks synergistically furnish a 3D network microstructure and thus, the electrolyte infiltration into the whole electrode material is facilitated exhibiting enriched electrochemical energy storage property for high-power application. The elemental composition in WO3@PPy composite was investigated through EDX and the obtained result [Figure 4d] shows the signals of the elements constituting the composite. More detailed investigation for microstructural features of the samples was done by TEM and the typical images are demonstrated in Figure 4. It can be clearly perceived from Figure 4e that the WO3 nanosticks are randomly oriented in a diverse direction and this is in good agreement with the FE-SEM result. The SAED pattern (inset in Figure 4e) illustrates the well-defined diffraction spots which confirm the crystalline structure of the WO3. TEM image (Figure 4f) of the WO3@PPy composite shows that the granular PPy particles have covered and inserted into randomly distributed WO3 nanosticks. This ensures efficient connections between the components of the composite, which is certainly very useful for facilitated electron transfer and electrolytic diffusion. Thus, the electrochemical activity of 17



the WO3@PPy composite is finally improved as the charge carriers could be quickly conducted to and fro from WO3 nanosticks to the inside and outside of PPy.33

Figure 4 FE-SEM images of as-synthesized WO3 nanosticks (a), pure PPy (b) and WO3@PPy composite (c), EDX spectrum resulting from the WO3@PPy composite (d), TEM image of WO3 nanosticks with the inset showing the corresponding SAED pattern taken on the nanosticks (e) and TEM image of the WO3@PPy composite (f).

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3.3. Electrochemical characterizations of positive electrode The electrochemical characteristics of NiSe as the positive electrode material deposited on SS fabric were first explored by cyclic voltammetry (CV) and cyclic charge– discharge (CCD) measured in 2 M KOH solution as the electrolyte in a standard threeelectrode electrochemical cell. As a comparison, NiO electrode was also tested under similar conditions to realize the effect of the selenization reaction. Figure 5a displays the typical CV profiles of the as-synthesized NiO and NiSe at a scan rate of 2 mV s-1 within the operating potential range of 0.0−0.45 V (vs. SCE). For NiO, a pair of redox current peaks is clearly noticed revealing its faradaic capacitive characteristics. These peaks are actually the manifestation of redox interconversion between NiO and NiOOH on the surface of the NiO electrode according to the reaction as follows:34,35 NiO  OH   NiOOH  e 

The NiSe electrode also shows similar battery-type faradaic characteristics as represented by the redox peaks in its voltammogram [Figure 5a], specifying its capacity resulting from faradaic redox processes. This feature of NiSe could be attributed to the redox conversion of Ni2+ and Ni3+ in aqueous alkaline solution as expressed by following reactions:2,9

1 NiSe  H 2O  O2  Ni (OH )2  Se 2

Ni (OH )2  OH   NiOOH  H2O  e It is worth mentioning that the CV curve corresponding to NiSe has more area under the curve than that for NiO revealing better electrochemical charge storage property of the former. Figure 5b displays the CV plots for NiSe at various scan rates ranging from 2 mV s-1 to 100 mV s-1. Now, it is observed that both the area under the curve and current response increase accordingly with increase in scan rate but without any considerable distortion in the

19



shapes of the plots. This signifies quick redox processes in as-synthesized NiSe electrode with outstanding rate performance.4 To further explore the electrochemical properties, galvanostatic CCD tests were carried out within the potential window of 0−0.45 V (vs. SCE) and the CCD curves of NiO and NiSe measured at the current density of 2 A g-1 were plotted together for comparison and illustrated in Figure 5c. The CCD measurement was also done at several other current densities and Figure 5d depicts the obtained CCD curves for the NiSe electrode material. Now, the CCD curves of as-synthesized NiSe electrode possess voltage plateaus as a sign of typical redox processes by adsorption and desorption of electrolytic OH– ions at the electrode-electrolyte interface during charging and discharging, consistent with the results obtained from CV measurements, revealing its battery-type charge storage mechanism. The respective specific capacitances (Csp) of both NiO and NiSe electrodes corresponding to various current densities were calculated using equation (1) and illustrated in Figure 5e and summarized in Table 1. Promisingly, the NiSe nanoflower electrode can deliver an excellent Csp of 1274 F g-1 at the current density of 2 A g-1, which is much higher than that of pure NiO (774 F g-1) at the same current density and also, more significantly, NiSe exhibits 71.4% retention of the original capacitance even at ten times higher current density (i.e., 910 F g-1 at 20 A g-1) reflecting admirable rate capability important for its practical application. The improvement in Csp and rate capability of NiSe prepared via selenization of NiO, may be accredited to its improved electrical conductivity and flower-like porous nanostructure with high specific surface area facilitating the redox processes.4,19 Higher current density limits the proper utilization of the electroactive materials for faradaic redox reactions and also increases polarisation with a consequent decrease in Csp of the electrode materials, as can be seen in Figure 5e.6

20


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Table 1. Csp values of NiO and NiSe electrodes obtained from CCD measurements at different current densities Csp (F g-1)

Current Density (A g-1) NiO

NiSe

2

774

1274

3

754

1251

5

709

1206

8

653

1149

10

612

1100

20

418

910

The following Table 2 represents a comparative study of the Csp obtained for assynthesized NiSe with some of reported results on NiSe and other similar electrode materials. Table 2. Comparison of the Csp for as-synthesized NiSe with the literatures

Material

Current density

Working Voltage (V)

Csp (F g-1)

Electrolyte

Ref.

NiSe

0.5 A g-1

0−0.5 V (vs. Hg/HgO)

492

2 M KOH

[18]

NiSe/NF

5 A g-1

0−0.5 V (vs. SCE)

1790

2 M KOH

[9]

NiSe

5 A g-1


1295

3 M KOH

[19]

NiTe

5 A g-1


498

3 M KOH

[19]

1 mA cm−2


742.4

6 M KOH

[36]

NiSe2/CFC

2 A g-1

0−0.55 V (vs. Hg/HgO)

1058

3 M KOH

[37]

CoSe2/C

2 A g-1

0−0.55 V (vs. SCE)

726

2 M KOH

[38]

NiSe2

3 A g-1

0−0.4 V (vs. Ag/AgCl)

1044

4 M KOH

[6]

NiSe

2 A g-1

0−0.45 V (vs. SCE)

1274

2 M KOH

This

NiCo2.1Se3.3 NSs/3D G/NF

Work 21



Page 22 of 45

To better understand the charge transfer process and the kinetics of ion diffusion at the interface of the supercapacitor electrode materials, EIS measurement was performed in the frequency range of 1 MHz−100 mHz and Figure 5f depicts the corresponding Nyquist plots obtained for NiO and NiSe electrode materials along with an equivalent circuit model (inset) fitting the impedance plots. Here, the whole resistive property is the resultant of four different types of resistances as indicated by the fitting circuit: bulk solution resistance or internal resistance (Rs) specifying the inherent resistance of the electrode, electrolytic ion resistance and the contact resistance at the electroactive material-current collector interface, charge transfer resistance (Rct) between the electrode and electrolyte during faradaic processes, the Warburg impedance (W) and a constant phase element (CPE). Now, both the Nyquist plots of NiO and NiSe consist of two sections: an inconspicuous semicircle in the higher frequency region and an inclined linear segment in the lower frequency region. The intercept of the curve on the real axis in high frequency region is the measure of R s and the diameter of the semicircle gives the Rct and the sloped line, specified as W, characterizes the electrolytic diffusion controlled processes.39 After fitting the obtained results, EIS data showed that the Rs of NiSe is 0.76 Ω which is somewhat lower compared to that (1.32 Ω) for NiO and also the Rct of NiSe is substantially lower than that of NiO as revealed by the smaller semicircle in the Nyquist plot of NiSe as compared to that of NiO [Figure 5f]. This low Rct of NiSe actually denotes more facilitated redox processes occurring in it. Furthermore, the low frequency straight line in the Nyquist plot of NiSe has higher slope than that corresponding to NiO demonstrating reduced diffusion resistance of electrolytic ions, i.e., quick movements of ions into the NiSe electrode material. It is noteworthy that the fitting circuit contains a part named CPE as none of the electrode materials show ideal capacitive characteristics. All these above results reflect an excellent electrochemical property and conductivity

of

nanoflower-like

NiSe

electrode

material.

22


This

improvement

in

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electrochemical and electrical properties of as-synthesized NiSe electrode material could be ascribed mostly to the porous structure of NiSe, which acts as “electrolyte reservoir” to boost the transport of electrolytic ions during faradaic processes and also, the Se atom has more metallic property than O atom, and both these together might be a crucial factor for its high rate capability. Hence, the as-synthesized NiSe can be a promising positive electrode material for supercapacitors in basic electrolyte.

Figure 5. (a) Cyclic voltammograms of NiO and NiSe at 2 mV s-1 scan rate, (b) CV plots of NiSe at different scan rates, (c) CCD curves of NiO and NiSe at 2 A g -1 current density, (d) CCD curves of NiSe at several current densities and (e) Csp values of both the electrodes at different current densities, (f) Nyquist plots and the inset is the fitting equivalent electrical circuit model. 23



3.4. Electrochemical characterizations of negative electrode To evaluate the electrochemical properties of WO3, pure PPy and WO3@PPy composite as the negative electrode materials for supercapacitor, CV and galvanostatic CCD experiments in a three electrode system were done in the anodic potential range of −0.8 to 0.0 V (vs. SCE) at different scan rates and current densities, respectively and Figure 6 shows the obtained results. Figure 6a compares the individual voltammograms of all the three electrode materials obtained at a same scan rate of 2 mV s-1 in 2 M KOH electrolyte. The shape of the CV curve for WO3 describes its faradaic characteristics as it shows a redox peak at around −0.32 V ascribable to the intercalation/deintercalation of both the electrolytic ions and electrons within the host WO3 electrode material.3,23 As a result of this process, the electrochemical charge transfer reaction, W6+ ↔ W5+, is established in presence of the electrolyte enabling the as-synthesized WO3 to serve as the suitable faradaic electrode material for pseudocapacitor application.23,40 The nature of the CV curve of pure PPy is distinct from that of a perfect EDLC material for which the CV curve appears perfectly rectangular, as it shows a little pseuodocapacitive characteristics in its quasi-rectangular CV curve due to probable insertion and expulsion of electrolytic ions.32 However, in the CV profile of the WO3@PPy composite, the redox peak observed in case of pristine WO3 is no longer noticeable, which might be due to the coating of PPy over WO3 nanosticks and the nature of the CV curve indicates that the total capacitance is comprised of the pseudocapacitances of both the WO3 and the conducting polymer PPy. Furthermore, the WO3@PPy composite has improved capacitive feature as can be perceived from its CV curve having larger enclosed area than those of either of its components, i.e., WO3 and PPy samples. Figure 6b depicts the CV profiles of the WO3@PPy composite electrode at different scan rates ranging from 2 to 80 mV s-1. It is noteworthy, even when the scan rate is quite

24


Page 24 of 45

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high, the current response is not much distorted illustrating appreciable reversibility of this composite. The galvanostatic CCD is a reliable and complementary method for further evaluation of the electrochemical properties and determines the Csp values of the electrode materials at constant current. Figure 6c represents the comparison of the CCD profiles for pure WO3, PPy and the WO3@PPy composite obtained at the same current density of 2 A g-1. The CCD plot of the composite expectedly displays a longer discharging time than those of its two components, which specifies its superior electrochemical capacitive property. CCD tests were also done at various current densities within the range of 2 to 20 A g-1 to investigate the rate capability for practical application potential of the WO3@PPy composite and Figure 6d depicts the acquired CCD curves at those current densities. As can be seen, the CCD curves are fairly symmetric in nature revealing admirable capacitance behaviour of the composite and a little deviation of those curves from the perfect isosceles triangular shape is mainly accredited to the pseudocapacitive effects present in the composite. Furthermore, low IR drops at the turning point of the CCD lines [Figure 6d] also reflect reduced power loss of the composite. The Csp values of all the aforementioned electrode materials as estimated at different current densities using equation (1) are demonstrated in Figure 6e and tabulated in Table 3. As can be seen, the composite exhibits the highest Csp of 586 F g-1 at 2 A g-1 current density among all the electrode samples (WO3: 402 F g-1 and PPy: 224 F g-1) and delivers an appreciable Csp of 455 F g-1 (i.e., about 78% retention) even at a very high discharge current density of 20 A g-1 while, by contrast, those of pure WO3 and PPy rapidly reduce to only 258 F g-1 (≈ 64% retention) and 98 F g-1 (≈ 44% retention), respectively, at the same current density. For all the electrode materials, the decrease in Csp with increasing current densities can be explained on the basis of the diffusion process of the electrolytic ions into the electrode material. When the current density is low, the electrolytic ions diffuse deep inside 25



the electrode material furnishing high Csp but as the current density gets higher, the diffusion process is limited only at the outer surface of the electrode with a consequent decrease in Csp. For all the current densities, the composite exhibits higher Csp values, i.e., better electrochemical properties than those of its components (WO3 and PPy). The reason behind these experimental results is the synergistic effect between the conducting polymer PPy and the pseudocapacitive WO3 in the WO3@PPy composite. The electrochemical performance of pure WO3 is hampered by its poor electrical conductivity and rate performance. Thus, its combination with conducting polymer PPy in the WO3@PPy composite lowers the charge transfer resistance and makes the whole material conducting enough to furnish better Csp with long cycle life. As the FESEM image [Figure 4c] reveals, a 3D continuous network microstructure developed in the composite offers easy accessibility of the charge on the electrode surface and also promotes the electron transport kinetics by shortening the diffusion path length for ions. Consequently, facilitated infiltration of the electrolyte into the WO3@PPy composite electrode leads to own better electrochemical property than either of its components, WO3 and PPy. Now, the conductivity of conducting polymers can be varied under different chemical environments and PPy tends to lose its conductivity in an alkaline solution/electrolyte system. The conducting polymer (e.g., PPy or PANI) based electrodes are most commonly characterised in acidic electrolyte due to their positive response in that medium. However, degradation of PPy chain, known as 'overoxidation', may occur in neutral or acidic aqueous solutions via the formation of maleimide, as reported elsewhere.41,42 In this work, the WO3@PPy composite is structurally stable and as the electrochemical results reflect, this composite exhibits appreciable electrochemical performance in basic electrolyte too. In true sense, the inherent conductivity loss depends on the morphological features. Formation of high molecular weight polymer and dense packing restrict the breakage of the polymer 26


Page 26 of 45

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backbone and hence do not hamper the hopping mechanism. To investigate the effect of basic electrolyte on the conductivity and morphology of the WO3@PPy composite during electrochemical measurements, EIS and FE-SEM analysis of the as-prepared WO3@PPy composite electrode were performed before and after 5000 CCD cycles and the obtained results, as shown in Figure S6a and Figure S6b, ESI, revealed no significant changes in resistive property as well as morphology supporting the restricted degradation of the PPy backbones in basic electrolyte. All these above results led us to employ the high rate-capable WO3@PPy composite as the promising anode material in assembling a high performance aqueous ASC. Table 3. Values of Csp for pure WO3, PPy and WO3@PPy composite electrodes as estimated from the CCD measurements at different current densities Csp (F g-1) Current Density (A g-1) WO3

PPy

WO3@PPy

2

402

224

586

3

389

194

567

5

363

160

540

8

338

135

513

10

324

125

499

20

258

98

455

The result obtained in this work for WO3@PPy composite negative electrode has been compared with similar type of electrode materials along with their respective working potential ranges in the following Table 4.

27



Page 28 of 45

Table 4. Comparison of the Csp for as-synthesized WO3@PPy composite with the literatures

Material

Current density

Working Voltage (V)

WO3/PANI

1.28 mA

−0.5 to 0.7 V (vs.

composite

cm−2

SCE)

PEDOP– Au@WO3

1 A g-1

hybrid

−0.5 to 0.65 V (vs.

composite

cm−2

SCE)

composite

WO3-RGO

1 A g-1

WO3·H2O/rGO

1 A g-1

WO3@PPy

2 A g-1

Ref.

168

1.0 M H2SO4

[43]

130

Ionic liquid

[44]

201

1.0 M H2SO4

[45]

615

0.5 M H2SO4

[46]

495

0.5 M H2SO4

[24]

244

1.0 M H2SO4

[25]

586

2 M KOH

Ag/AgCl)

1.28 mA

1.4 A g-1

Electrolyte

−0.4 to 0.8 V (vs.

WO3/PANI

PEDOT/WO3

Csp (F g-1)

−0.3 to 0.7 V (vs. Ag/AgCl) −0.4 to 0.3 V (vs. SCE) −0.4 to 0.1 V (vs. SCE) −0.8 to 0.0 V (vs. SCE)

28


This Work

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Figure 6. (a) Cyclic voltammograms of WO3, PPy and WO3@PPy composite at 2 mV s−1 scan rate, (b) CV curves of WO3@PPy composite electrode at various scan rates, (c) CCD profiles of WO3, PPy and WO3@PPy composite at the current density of 1 A g−1, (d) CCD curves of WO3@PPy composite at several current densities and (e) variation of estimated C sp with those current densities.

29



3.5.

Electrochemical

performance

of

the

as-assembled

Page 30 of 45

all-solid-state

flexible

NiSe//WO3@PPy ASC device The brilliant electrochemical features of nanoflower-like NiSe and WO3@PPy composite electrode materials with complementary working potential ranges in the same basic electrolyte inspired us to assemble a flexible gel electrolyte based all-solid-state ASC device (NiSe//WO3@PPy) with these aforementioned electrodes employed as the positive and negative electrodes, respectively. To inspect the stable operating potential window of the ASC device, the CV tests were first done on the individual electrodes, i.e., NiSe and WO3@PPy composite, using a three-electrode configuration in aqueous 2 M KOH solution. As depicted in Figure 7a, the NiSe and WO3@PPy composite electrodes were examined within the voltage range of 0.0 V to 0.45 V and −0.8 V to 0.0 V (vs. SCE), respectively at the scan rate of 2 mV s-1. Thus, it is predicted that a working potential of 1.25 V, which is actually the sum of individual voltage ranges of the electrodes, can be attained for the proposed NiSe//WO3@PPy ASC device. Again, to accomplish the optimum cell potential and maximum performance of the ASC device, proper mass loading of both the electrodes was necessarily done by using charge balance method expressed as q+ = q–, where q+ and q– are the charges of positive and negative electrodes, respectively. Now, the amount of charge stored in each electrode depends on the Csp of individual electrode, the working voltage for the charge-discharge process (ΔV) and the mass of the electrode (m), as represented by the following equation:39 q = Csp × ΔV × m Hence, the mass ratio (m+/m–) was finally evaluated as: m C   V  m C   V

30


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In this work, the optimum mass ratio (m+/m–) in the ASC device was calculated to be 0.82, considering the areas of the positive and negative electrodes in the device are same. The CV curves of the NiSe//WO3@PPy ASC device measured at a range of scan rates starting from 2 to 80 mV s-1 within the working voltage of 1.25 V are depicted in Figure 7b. All the CV curves are found to have distorted rectangular shape owing to the faradaic contribution from the positive electrode material. In addition, the shapes of the curves remain almost invariant even at higher scan rates, which manifests that the device has the ability to show stable electrochemical performance with efficient reversibility. Figure 7c illustrates the comparative CV plots of the as-fabricated all-solid-state ASC device obtained initially and under bending mode (inset in Figure 7c) at the scan rate of 2 mV s-1 and interestingly, the nature of the CV plot is not much altered in bent condition, which clearly demonstrates excellent flexibility of the NiSe//WO3@PPy ASC device. Thus, the SS substrates used in this work not only play the role of current collectors but also flexible electrode supports making the device flexible. Moreover, the PVA/KOH gel electrolyte restricts the leakage of current and can also endure the mechanical stresses.47 The galvanostatic CCD measurement was also done at various current densities (from 2 to 20 A g-1) within the same operating potential window of 1.25 V to further explore the electrochemical characteristics of the as-assembled ASC device and the obtained results are shown in Figure 7d. As observed, the discharge curves of the ASC device have appeared to be roughly symmetric with the corresponding charge curves revealing its reversibility in redox processes. Furthermore, the CCD curve not only preserves its shape but also shows a small I-R drop even at higher current densities, which reflect its ability to sustain against higher current application and deliver a high power during practical applications. From these CCD curves, calculation of the Csp values of the device at different current densities was done based on the total mass of both the electrodes and the results are tabulated in Table 5 and plotted in Figure 7e. The ASC device successfully 31



delivers a maximum Csp of 172 F g-1 at 2 A g-1 current density and still remarkably retains 108 F g-1 at ten times increased current density of 20 A g-1 demonstrating its significant rate performance. Table 5. The Csp values of the NiSe//WO3@PPy ASC device at different current densities Current Density (A g-1)

Csp (F g-1)

2

172

3

165

5

157

8

146

10

141

20

108

The long-term cycling stability is one of the most important characteristics of any supercapacitor device for its practical applicability and hence, the cycling stability of the assembled NiSe//WO3@PPy ASC device was verified through CCD measurement between 0 and 1.25 V at the current density of 2 A g-1, as displayed in Figure 7f. The ASC device attractively exhibits a little decline in the discharge time even after 5000 CCD cycles and retains 91% of the initial Csp value (172 F g−1 to 156.5 F g−1) demonstrating its ultra-long cycling stability. It is also evident from Figure 7f that the Csp of the device enhances initially, which is accredited to suitable activation of the electrode materials caused by their proper wetting by the electrolyte at the initial stage of the CCD process lessening the internal resistances. Furthermore, coulombic efficiency (η) of the NiSe//WO3@PPy ASC device was estimated using equation (4) and expressed as a function of current density [Figure S7a, ESI]. The estimated results show the increase in η of the device from 58% to reasonably high 94% with increase in current density from 2 to 20 A g-1. In addition to this, the η was also 32


Page 32 of 45

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estimated at the constant current density of 2 A g-1 upon prolonged cycling and the obtained result [Figure S7b, ESI] shows that around 93% η is maintained after 5000 continuous galvanostatic CCD cycles. This result once again establishes enhanced cyclic stability and superior electrochemical reversibility in the faradic processes over the long-term cycling test occurring in the NiSe//WO3@PPy ASC device. The leakage current, being very important self-discharge parameter of any supercapacitor device, was recorded [Figure S7c, ESI] by charging the device to a constant voltage of 1.25 V and holding the voltage for 24 h. The obtained result shows a sudden fall in the leakage current initially and then it slowly becomes steady and finally achieves a current of around 0.0023 mA after maintaining a constant voltage for 24 h. This moderate value of leakage current is indicative of good capacitive performance of the device. To further investigate and explore the resistive and capacitive property of the ASC device, EIS measurement was performed within the frequency region from 1 MHz−100 mHz and the obtained result is represented through the typical Nyquist plot, as shown in Figure 7g. The inset in Figure 7g is the corresponding equivalent circuit model fitting the EIS result. The Nyquist plot of the NiSe//WO3@PPy ASC device contains a small semicircle in the high frequency region followed by a linear part in the lower frequency region which specifies diffusion of electrolytic ions into the electrodes in the device. Now, as per the impedance parameter values obtained from circuit fitting, the device owns a low Rs of 2.1 Ω along with small Rct of around 3 Ω and all these values reveal the pronounced capacitive performance of the device with smooth accessibility of electrolytic ions during electrochemical processes. Apart from the high Csp with low resistive property and long cycling life, the other two major properties of any supercapacitor for its practical usability are energy density (E) and power density (P), calculation of which were done using equation (2) and (3) from CCD data at different current densities. Figure 7h describes the relationship between the obtained E 33



Page 34 of 45

and P at different current densities, which is commonly known as the Ragone plot. Usually, the value of P increases with increase in current densities though E shows the reverse behaviour and as desired, an energy storage device like supercapacitor must exhibit significantly large E without much decline in P. As illustrated in Figure 7h, the highest E achieved for the ASC device is 37.3 W h kg-1 at the P of 1249 W kg-1 at 2 A g-1 current density and that still retains 23.4 W h kg-1 at the maximum P of 12480 W kg-1. These outstanding results could be ascribable to the appreciable Csp with high cell voltage of the ASC device, which are, in turn, the combined outcome of excellent electrochemical characteristics of both the component electrodes. Now, the E and P obtained for the NiSe//WO3@PPy ASC device have been compared with some of published research works and summarized in Table 6, from which the superiority of the device can easily be realized. Table 6. Comparative analyses of different ASC devices with the NiSe//WO 3@PPy ASC device Energy density (W h kg-1)

Power density (W kg-1)

Ref.

Device

Csp (F g )

Working Voltage (V)

NiSe2//AC

104 (at 1 A g-1)

0−1.6 V

44.8

969.7

[6]

NiTe/NiSe//AC

94.9 (at 1 A g-1)

0−1.6 V

33.7

4000 (maximum)

[19]

NiSe2/CFC//AC

92 (at 1 A g-1)

0−1.6 V

32.7

800

[37]

CoSe2//N-doped carbon nanowalls

90.5 (at 10 mA cm−2)

0−1.6 V

32.2

1914.7

[1]

-1

34


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PANI//WP0.08M

48.6 (5 mV s−1)

0−1.2 V

9.72

53

[43]

RuO2·xH2O//WO3– WO3.0.5H2O

−

0−1.6 V

23.4

5200 (maximum)

[10]

CAC/PANI//WO3

274 (at 0.5 A g-1)

0−1.5 V

15.4

252

[3]

NiSe//WO3@PPy

172 (at 2 A g-1)

0−1.25 V

37.3

1249

This Work

The demo experiment regarding the practical applicability of the as-fabricated NiSe//WO3@PPy ASC device was performed by lighting up a red light emitting diode (LED) (Figure 7i and j) with two ASC devices connected in series. In this experiment, the connected device was firstly charged to 2.5 V within only 10 s and then while discharging, the current flowed through the red LED bulb from the device with consequent illumination of the bulb with ample intensity. The mesoporous nanoflower-like microstructure of the faradaic NiSe electrode material and the presence of conducting porous polymer in WO3@PPy composite synergistically boost the pseudocapacitance of the same finally showing this superior electrochemical performance of the device. The mesoporous structures of the electrode materials ensure large contact area between the electrode materials and electrolyte enabling improved utilization of the electroactive materials. On the other hand, the conducting nature of both the electrodes facilitated the fast redox processes and conduction of electrons and ions. The combination of all these factors endorses the stupendous electrochemical characteristics of both the component electrodes in the three electrode system as well as in the ASC device. Finally, these results successfully demonstrate the brilliant and promising

35



electrochemical performance of NiSe//WO3@PPy ASC device, by the virtue of which it can be used in high energy and high power storage applications.

Figure 7. (a) CV profiles of NiSe and WO3@PPy composite electrode materials at 2 mV s-1 scan rate, (b) CV plots of the all-solid-state ASC device at various scan rates, (c) comparison 36


Page 36 of 45

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between the CV curves of the ASC device: initially and under bending mode at 2 mV s-1 (inset represents a schematic illustration of the ASC device in different modes), (d) CCD plots of the ASC device at different current densities, (e) variation of the obtained specific capacitances of the device with the current densities, (f) specific capacitance retention after 5000 consecutive CCD cycles at 2 A g-1, (g) Nyquist plot of the ASC device and the inset represents the equivalent fitting circuit diagram, (h) Ragone plot of the ASC device, (i) and (j) photographs of the illumination of a red LED bulb by two NiSe//WO3@PPy ASC devices connected in series. 4. CONCLUSIONS In summary, porous NiSe positive electrode with nanoflower-like architecture and significant specific capacitance was prepared via facile selenization of NiO nanoflower. The as-prepared NiSe electrode material was characterized by XRD, FE-SEM, TEM, BET and a several electrochemical techniques and interestingly, NiSe showed better electrochemical activity than its NiO precursor, as suggested by the corresponding results obtained from the electrochemical characterizations. Also, WO3@PPy composite with considerable specific capacitance was synthesized as the negative electrode by in-situ oxidative polymerization of conducting pyrrole in presence of WO3 nanosticks. Finally, based on the suitable microstructures and electrochemical performances of nanoflower-like NiSe positive electrode and conducting WO3@PPy composite negative electrode, an advanced NiSe//WO3@PPy allsolid-state ASC device was designed, which is functional within a maximum voltage of 1.25 V and delivered the highest energy density of 37.3 W h kg-1 at the power density of 1249 W kg-1 at 2 A g-1 current density and showed commendable energy density of 23.4 W h kg-1 at the maximum power density of 12480 W kg-1 (at 20 A g-1). Moreover, the ASC device exhibited magnificent cyclic stability as it retained 91% of its initial capacitance even after 5000 CCD cycles. These results unambiguously reflect the potential of the as-synthesized 37



materials to serve as the promising electrodes for energy storage applications. This present work is important as it presents an easy approach to synthesize a faradaic transition metal based selenide positive electrode and construct a novel flexible ASC device assembling it with WO3@PPy composite negative electrode, which may satisfy the requirements of high power for next-generation portable electronics.

ASSOCIATED CONTENTS Supporting Information (SI) Materials details, Characterizations of synthesized materials, Measurement of the surface area and pore size distribution of as-synthesized NiSe, Elemental mapping of NiSe, Structural characterizations of as-prepared WO3, PPy and WO3@PPy composite (WAXD study, FTIR study, Raman spectroscopy), Investigation of the effect of basic electrolyte on the conductivity and morphology of the WO3@PPy composite during electrochemical measurements, Estimation of Coulombic efficiency and leakage current of the NiSe//WO3@PPy ASC device. AUTHOR INFORMATION *Corresponding Author Dr. B.B. Khatua Materials Science Centre, Indian Institute of Technology, Kharagpur – 721302, India. Tel.:91-3222-283982, Email: [email protected]. Notes: The authors declare that there is no conflict of interest regarding the publication of this article. ACKNOWLEDGEMENTS We are very much thankful to the Indian Institute of Technology Kharagpur for financial support. 38


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TOC (Graphical Abstract)

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Highly rate capable nanoflower-like NiSe and WO3@PPy composite

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