Synthesis of a 3D free standing crystalline NiSex matrix for electrochemical energy storage applications

Syed Mukulika Dinara a, Aneeya K. Samantara bc, Jiban K. Das bc, J. N. Behera *bc, Saroj K. Nayak a, Dattatray J. Late d and Chandra Sekhar Rout *e
aSchool of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha 751013, India
bNational Institute of Science Education and Research (NISER), Khordha 752050, Odisha, India. E-mail: jnbehera@niser.ac.in
cHomi Bhabha National Institute (HBNI), Mumbai, India
dPhysical & Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pashan, Pune 411008, India
eCentre for Nano and Material Sciences, Jain University, Jain Global Campus, Ramanagaram, Bangalore 562112, India. E-mail: r.chandrasekhar@jainuniversity.ac.in

Received 2nd August 2019 , Accepted 10th October 2019

First published on 10th October 2019


The electrochemical performance for energy storage of three-dimensional (3D) self-supported heterogeneous NiSex cubic-orthorhombic nanocrystals grown by a facile one-step chemical vapour deposition (CVD) approach on Ni foam substrates has been explored. NiSex shows a high specific capacitance of 1333 F g−1 with ultra-high energy (105 W h kg−1) and power (54 kW kg−1) densities. Furthermore, by integrating the as-grown NiSex as the anode and reduced graphene oxide as the cathode, a hybrid supercapacitor (HSC) prototype with a coin cell configuration has been fabricated. The device shows better capacitance (40 F g−1) with high energy (22 W h kg−1) and power (5.8 kW kg−1) densities and robust cycling durability (∼88% capacitance retention after 10[thin space (1/6-em)]000 repeated cycles). For practical reliability of the as-fabricated HSC, a red LED has been illuminated by connecting it with two charged coin cells. These outstanding performances of the HSC prove to be promising for applications in high energy storage systems.


Introduction

Research activities on renewable energy storage systems are gaining significant attention to achieve alternative sources for sustainable development. In this context, charge storage systems have become an important part of renewable energy research aimed at fulfilling the requirements of high-power density and long cycling stability. Notably, both two-dimensional (2D) and three dimensional (3D) materials have shown potential in energy research with exciting electronic, physical, and chemical properties.1–5 In this regard, various processes have been developed to synthesize materials with variable morphologies and explore their electrochemical catalytic performances.6–8

The charge storage performance of a particular material strongly depends on its physicochemical properties like elemental composition, surface morphology, electrical conductivity etc. Among various reported electrode materials, the current research trend is directed towards 2D–3D transition metal dichalcogenides (TMDs) owing to their higher electrical conductivity and charge storage capability compared to other metal oxides.2,9–11 Moreover, sulphides and selenides are widely employed as active electrode materials for high-performance supercapacitors with nearly equal electrochemical properties (as they belong to the same group in the periodic table).7,8,11–18 But the higher conductivity of selenium (1 × 10−3 S m−1) as compared to sulfur (1 × 10−28 S m−1) results in a faster electrochemical reaction and carrier movement.17,19 In comparison with transition metal sulphides, transition metal selenides show better charge storage performances. Recently, there have been several reports on various selenide compounds with different morphologies including layered MoSe2 nanosheets, NiSe@MoSe2 nanosheet arrays, layered VSe2 nanosheets, 3D cuboidal VSe2, WSe2 micro/nanorods, nanowires and nanosheets like WSe2, CuSe2 nanoneedles, vertically oriented interpenetrating CuSe nanosheets, hierarchical 3D CoSe2 and NiCoSe2 nanowires etc.16,17,20–26 The surface morphology of nanostructures also plays a vital role in their electrochemical energy storage performances. Therefore different synthesis protocols are routinely employed to design morphology tuned nanomaterials as active electrodes of energy storage devices. Among various transition metal selenides, NiSex is the favoured electrode material for advanced charge storage applications due to its multiple oxidation states, tuneable electronic configuration, Pauli paramagnetic metallic nature (resistivity <10−3 Ω cm), and high electroconductivity and thermochemical stability.27–31 Furthermore, the nanostructured NiSex based materials possess large active surface areas, very short distances of the ion diffusion path, fast charge transfer rates, negligible interfacial resistance and long-term charge retention.11,32–34 Inspired by these attractive features, NiSex based nanomaterials with different stoichiometries and variable geometries have been explored in recent times as powerful electrode materials for high performance energy storage systems. For example, Du et al. and Wang et al. reported the synthesis of honeycomb-like layered Ni0.85Se nanosheets and Ni0.85Se nanowires respectively by the hydrothermal route.11,32 Moreover, Tian et al.9 developed NiSe nanorod arrays by the one-pot hydrothermal method showing a high value of areal capacitance of 6.81 F cm−2. Also, truncated cube-like single crystal NiSe2 and hexapod-like 2D NiSe2 have been reported with exciting electrochemical properties.31 From the literature, it is evident that the engineering of NiSex nanostructures has been performed mostly by the hydrothermal approach. However, it is still a great challenge to achieve chemical vapor deposition (CVD)-grown nanostructured NiSe with excellent electrochemical properties such as a large active surface area, a very short distance of the ion diffusion path, a fast charge transfer rate, negligible interfacial resistance and long-term charge retention compared with the reported hydrothermally grown NiSe. CVD is an industry favoured and more powerful technique to control the growth of 2D and 3D nanomaterials and to tune their properties as compared to the hydrothermal method.35 The electrical and physical properties of 2D materials can be significantly varied by morphological orientation, stoichiometric variation and interfacial arrangement of the materials. In this context, CVD is assumed to be an appropriate technique to control nanostructural variation upon different orientations by controlling the growth parameters. Moreover, CVD is used as the most powerful technique to grow homogeneous and continuous thin films with very low defect density as well as high electroconductivity.35–38 However, to our knowledge, 3D-crystalline NiSex grown by CVD for energy storage device applications has not been reported yet.

Moreover, to effectively enhance the energy density of supercapacitor materials, asymmetric/hybrid supercapacitors are designed with appropriate electrode materials of opposite polarities (i.e. like a cathode and anode) as well as different potential windows that are observed in the same electrolyte. These hybrid supercapacitor (HSC) operates in widen potential window thereby improving the specific energy under similar electrolyte condition. Among two-dimensional carbon materials, graphene is one of the good electrode materials for energy storage applications due to its large surface area, stable electrochemical performance and high electrical conductivity. In this regard various graphene-based materials like reduced GO or various composites with graphene have been reported for energy storage applications. However, it is still a great challenge to achieve new generation high speed energy storage systems with high energy density and prolonged stability. So far, there have been a few reports on transition metal oxide/rGO-based HSCs like MnO2 nanowires/rGO, CoFe2O4/rGO, rGO-RuO2/rGO-PANi etc., where rGO acts as a negative electrode material.39–41 However, transition metal-based compounds, especially chalcogenides, are now attracting much attention as promising electrode materials for energy research due to their high electrical conductivity, exciting redox potential characteristics with variable oxidation states, and various types of nanostructural geometries.

In this work, 3D NiSex nanostructures with close-packed cubic-rhombohedral lattices are synthesized on a Ni foam by the CVD method. Herein, the nickel foam has been used as a substrate as well as a precursor material to form a self-supported free standing electrode that overcomes the side reactions caused by inactive components (i.e. the binder, current collector etc.) and reduces the cost of the electrode material for device applications.42 In addition, the porous 3D network structure of the Ni foam would be advantageous to achieve a high surface area with a large number of active sites, the penetration of electrolyte ions and the very fast response of the charge/discharge reaction.43 So, the 3D NiSex self-supported electrode material would be easily synthesized on the Ni foam surface.

The structure, stoichiometry and elemental composition have been verified by FESEM, EDAX, XRD, XPS and HRTEM and the growth mechanism has been discussed elaborately. Thereafter, the electrochemical performances of the supercapacitor have been studied using a three-electrode electrochemical set-up. Furthermore, for practical applications, a hybrid supercapacitor has been assembled by integrating reduced graphene oxide and as-grown NiSex in a coin cell configuration. The HSC device shows a specific capacitance of 40 F g−1 with higher energy and power densities along with robust cycling durability.

Experimental section

Synthesis of NiSex

NiSex (NiSe2–NiSe) nanostructures were grown by the CVD route under specific growth conditions by utilizing the two separate temperature zones of the CVD chamber. Prior to the growth, the Ni foam was cleaned by successive rinsing in 1 M HCl, deionized (DI) water, ethanol and again in DI water to remove the surface oxide layer and impurities. After cleaning, the Ni foam was immediately kept in a vacuum to avoid further oxidation. Commercial selenium powder (Alfa Aesar, 200 mesh, purity 99.999%) and the oxide free pure Ni foam were kept in the first zone and at the center zone of the three zone CVD furnace, respectively. Initially before heating the CVD chamber, Ar gas was purged into the reaction chamber at 1000 sccm for 5 min to create an uncontaminated growth atmosphere. After that the CVD chamber was pumped to high vacuum to avoid oxidation reactions. In this preferred growth atmosphere, the heating process was carried out at a maximum of 10 °C min−1 to get the desired temperatures of the respective zones, i.e. the precursor zone of 280 °C and the substrate zone of 300 °C. The temperatures of the respective zones were kept constant at their respective set points for 30 min before carrying out the synthesis process. During synthesis (i.e. for 30 min), the argon and hydrogen gas mixture was used as the carrier gas for NiSe2 growth. After completion of the growth process, the furnace was gradually cooled down to room temperature and the samples were removed for characterization and electrochemical measurements.

Synthesis of graphene oxide and reduced graphene oxide

Graphene oxide (GO) was synthesized by the modified Hummer's method.44,45 In detail, 1 g of graphite powder was added to a conical flask containing 25 ml of concentrated H2SO4 and stirred to mix properly in an ice bath. Under continuous stirring conditions, 3.5 g of KMnO4 was added slowly and left to stir for the next 2 hours in a water bath. After that the mixture contained in the conical flask was diluted by adding deionized water followed by the addition of sufficient amounts of H2O2 (30%) until the effervescence of the gas ceased. The brown colored suspension thus formed was filtered and repeatedly washed with 0.1 M HCl followed by deionized water. The washed graphene oxide was dried by using a rotary evaporator and stored in a cool and dry place for future use.

Reduced graphene oxide (rGO) was synthesized by following a single step hydrothermal reduction method.44 In detail, GO suspension (1 mg ml−1) in deionized water was prepared by ultrasonication for 45 minutes. Then the suspension was transferred to a Teflon lined stainless steel autoclave and placed in a hot air oven at 200 °C for 24 hours. After cooling down to room temperature, the precipitates were collected, repeatedly washed with deionized water/ethanol, dried in an oven and stored in a desiccator for future use.

Structural characterization

Morphological and elemental analysis: morphological analysis was performed by using a field emission scanning electron microscope (FESEM-Merlin Compact, Carl Zeiss Private Limited, Germany; Gemini-I electron column) and an energy dispersive X-ray spectroscope (EDX, Oxford Instruments, UK). The confirmation of the as-synthesized material and its crystallographic information was obtained by high-resolution X-ray diffraction (HR XRD) analysis by using a Bruker D8 Advance Diffractometer (Bruker AXS GmbH: a Cu Kα source with energy 8047.79 eV and wavelength 1.5406 Å, and the amplification gain of the detector: 4–15 keV, 4-0, 8 Å). The chemical states of the respective elements of the sample were also scrutinized by X-ray photoelectron spectroscopy (XPS) using an ultrahigh vacuum system (XPS: VG Microtech, England (Multi Lab, ESCA-3000, sr. no. 8546/1)) with AlKα as the X-ray source. Also, Raman spectroscopy was carried out (Horiba Scientific) at a laser excitation wavelength of 525 nm. Moreover, the crystal structure was examined by high resolution transmission electron microscopy (HR-TEM) (TEM, JEOL-2100, acceleration voltage – 200 kV).

Electrochemical measurements

Electrochemical measurements were conducted in a two compartment three electrode electrochemical cell containing 6 M KOH aqueous solution. Here, NiSex was used as the working electrode, Pt wire as the counter electrode and Ag/AgCl as the reference electrode. All the electrochemical measurements were performed using a PG262A potentiostat/galvanostat (TechnoScience Ltd, Bangalore). Cyclic voltammetry (CV) experiments were conducted at different scan rates within a voltage window of −0.1–0.65 V, while galvanostatic charge–discharge (GCD) measurements were carried out at different applied current densities. Electrochemical impedance spectroscopy (EIS) was performed over the frequency range of 100 kHz to 0.01 Hz with 5 mV sinusoidal superimposed perturbation. After that the hybrid supercapacitor (HSC) prototype was prepared in a coin cell configuration. Here, the as-grown NiSex was taken as the anode and the powdered reduced graphene oxide as the cathode material. Both the electrodes were separated by a 6 M KOH soaked PVDF membrane. After complete arrangement, the cells were crimped at an optimal pressure and the electrochemical measurements (CV, GCD and EIS) were carried out with a Bio-logic instrument (VSP-200). The main parameters to judge a material for use as an energy storage material are its specific capacitance (Csp), specific capacity, energy (ED)/power density (PD), and charge retention capability.

Results and discussion

A schematic representation of the three zone CVD furnace with positions of the Ni substrate and Se powder is shown in Fig. 1. An increasing temperature profile was followed to maintain the growth kinetics from the precursor zone (temperature denoted as T1) to the substrate zone and then the synthesis temperature was kept at a constant temperature point (T2) throughout the substrate zone and the next upstream zone. The morphological changes of the Ni foam have been pictorially presented in 3D view. In this context, the surface roughness and porosity of the Ni foam endow it with some special attributes like the ability to form differently oriented crystals, a large surface area, highly dense porous structures, and a large number of active sites.46–48
image file: c9dt03150b-f1.tif
Fig. 1 The schematic diagram of the CVD system along with the temperature profile and growth conditions of the NiSex crystalline film.

Fig. 2 shows the scanning electron microscopy images of the as-synthesized NiSex grown by the CVD method. The morphological changes of the Ni foam after the growth of the material were observed using the surface focused image (Fig. 2a) on the macroscale, where the white region of the image indicates the growth portion of the Ni foam. The elemental composition of NiSex and the corresponding atomic percentages of the elements were determined by energy dispersive X-ray analysis (EDAX) shown in Fig. 2b. As depicted in Fig. 2c and d, at low and high magnifications, the surface morphology showed a closely packed crystal-like geometry in different forms like cubes and rhombohedra with heterogeneous orientations. The evaluation of actual ratio of atomic weight percentage of corresponding elements from the EDAX analysis is not possible owing to the discontinued surface morphology as well as formation of mixed phase NiSex. Grain orientation or grain alignment is an important part of lattice growth during nucleation from the vapor phase.35,49,50 Moreover the porous structure of the Ni foam might be responsible for a misoriented or heterogeneous morphology due to the discontinuity of the substrate surface.47 The heterogeneous orientation of the crystal lattice might be advantageous for developing a large volumetric area as an active electrode surface.51 In addition, the 3D crystal structure would effectively participate in increasing charge accumulation throughout the surface due to its large number of active sites.51,52 Importantly, due to the misorientation of the adjacent domains, the grain boundary may consistently increase the charge accumulation rate throughout the activated electrode surface.


image file: c9dt03150b-f2.tif
Fig. 2 FESEM images of NiSex nanocrystals grown on the Ni foam with different crystallographic orientations: (a) a low magnification image of the Ni foam after CVD deposition, (b) the EDAX spectrum of NiSex along with the percentage of the corresponding elements present, and (c and d) the morphology of both cubic and orthorhombic crystal like structures of nickel selenide in a single frame at different magnifications.

The crystal structure of the NiSex@Ni sample was examined by X-ray diffraction (XRD) analysis and is presented in Fig. 3a. The highly intense and sharp diffraction peaks reveal the high crystallinity of NiSex. The XRD result was scrutinized by JCPDS files and the confirmation of the mixed phase (NiSe2 and NiSe) was achieved by using the corresponding diffraction peak positions of the respective stoichiometric materials. The diffraction peaks at 44.5°, 51.9° and 76.5° were marked by special symbols and matched with the Ni foam substrate (JCPDS no. 87-0712).53 The other peaks observed at 29.97°, 33.61°, 42.89°, 50.78°, 53.21°, 62.28°, 64.42°, 70.65° and 82.48° were assigned to the corresponding planes (200), (210), (220), (311), (222), (400), (410), (420) and (431) of cubic NiSe2 (JCPDS no. 88-1711).54 Moreover, some peaks were also observed at different diffraction angles of 38.54°, 60.18°, 77.73° and 83.25° and they do not match with the diffraction pattern of NiSe2 and are in good agreement with the diffraction peaks of NiSe (JCPDS no. 89-2058)55 with a rhombohedral crystal structure. In this work, the Ni foam is used as both the Ni precursor and substrate. It has been verified that the Ni foam constitutes closely packed Ni atoms with very short Ni–Ni linkage.56 During the course of the reaction, the Ni ions on the surface of the Ni foam interact with Se atoms forming Ni–Se linkage. Here two distinct forms of selenides are formed—NiSe and NiSe2. According to the phase diagram, both the structures are stable phases of nickel selenide.57 Generally, NiSe grows in all three directions (a, b, and c axes) forming anisotropic morphology. But in this case, it grows along the c-axis forming a thermodynamically stable rhombohedral structure.58 Furthermore, with the passage of time, the surface of nickel precursors gets consumed and NiSe2 (1[thin space (1/6-em)]:[thin space (1/6-em)]2 Ni to Se ratio) formation is initiated at higher reaction temperature. Thus, the final product is a mixture of both the rhombohedral NiSe and cubic NiSe2 phases. Hence, the XRD pattern of the as-deposited NiSex shows a mixture of the two phases of nickel selenide with different crystallographic orientations. Moreover, the diffraction patterns show sharp and high intense characteristic peaks of NiSe2 as compared to the diffraction patterns of NiSe at 33.61° along the (210) plane. This observation signifies the formation of a pure NiSe2 film along with a very low percentage of NiSe. Also, the absence of any other impurity peaks demonstrates the growth of a pure and good quality NiSex crystalline film by the CVD approach.


image file: c9dt03150b-f3.tif
Fig. 3 (a) The XRD pattern of NiSex noted by different symbols with the corresponding JCPDS file, the XPS spectra of (b) Ni 2p and (c) Se 3d of NiSex, (d) the TEM image of CVD-grown polycrystalline NiSex and (e) the SAED pattern of NiSex confirming its polycrystalline nature.

Moreover, Raman spectroscopy was carried out to further confirm the presence of both phases of NiSex (NiSe2 and NiSe) in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]2 stoichiometry ratios. The Raman spectrum (Fig. S1) of NiSex shows three characteristic peaks at around 217, 511, and 1056 cm−1, corresponding to the Tg mode of NiSe2, the longitudinal optical (LO) one-phonon mode and the transverse optical (TO) two-phonon mode of NiSe respectively demonstrating the presence of both NiSe and NiSe2 as confirmed by XRD analysis.47,48,59,60 Furthermore, the elemental composition of the NiSex heterogeneous matrix samples has been verified by X-ray photoelectron spectroscopic analysis and is presented in Fig. 3b and c. The corresponding XPS spectra of Ni and Se are fitted by the Gaussian fitting method. As depicted in Fig. 3b, the Ni 2p spectra show two main peaks at 855.4 and 873.3 eV which can be assigned to Ni 2p3/2 and Ni 2p1/2 with two satellite shake-up peaks at 861.2 and 880.1 eV.11,61,62 Moreover, a low intensity peak at 852.5 eV in Ni 2p spectra shows the presence of a partially unreacted metallic Ni foam. As depicted in Fig. 3c, the Gaussian fitted Se 3d spectra exhibit a prominent peak at 54.6 eV and a low intensity peak at 53.8 eV, which indicate the presence of Se metal bonding, and also peaks with the matched binding energies of Se 3d3/2 and Se 3d5/2 of Se2− respectively.11,61,63

The presence of both the crystalline phases (cubic and rhombohedral) observed from XRD data was further scrutinized by high-resolution transmission electron microscopy (HRTEM). Fig. 3d shows the TEM image of crystalline NiSex. Moreover, the selected area electron diffraction (SAED) pattern (Fig. 3e) of the nanocrystals confirms the polycrystalline nature of NiSex along with different crystallographic planes, i.e. (210) and (211), for the respective orthorhombic NiSe2 and cubic NiSe phases which are in good agreement with XRD analysis.

Moreover, the morphological studies of as-synthesized rGO are shown in Fig. S2(a and b) at high and low magnifications where a 3D randomly oriented layered network was observed. The FESEM image shows a layer by layer arrangement of the rGO film with a large surface area in an orderly manner. For further confirmation of GO and rGO, Raman measurements were carried out as shown in Fig. S2(c). The Raman spectra show two characteristic peaks at 1347.7 cm−1 and 1589.3 cm−1 denoted as D and G bands, respectively.

Inspired by the interesting structure and composition of the synthesized NiSex, its charge storage performances were studied by using a three-electrode measurement set-up in a 6 M KOH aqueous electrolyte at room temperature. Fig. 4a shows the CV curves of NiSex as the working electrode at different scan rates varying from 5 mV s−1 to 100 mV s−1 within the potential window of −0.1 V to +0.65 V vs. Ag/AgCl. The CVD-grown NiSex nanostructured material exhibits excellent charge storage capacity comparable to other selenide samples synthesized by different processing routes like hydrothermal, electrodeposition etc.9,11,31,64,65 A pair of redox peaks is observed in the CV curves indicating the reversible faradaic reaction and the pseudocapacitive nature of NiSex different from the double layered capacitive electrode materials reported elsewhere.66 Upon increasing the sweep rate, the current response increases proportionally, thus retaining the CV shapes and showing better rate capability. Moreover, due to the polarization phenomenon, the anodic peak shifts towards a more positive and the cathodic peak to a more negative potential. The following possible redox reactions are responsible for the enhanced electrochemical pseudocapacitive charge storage in NiSex nanocrystalline thin films:67

NiSe2 + OH ↔ NiSe2 OH + e
and
NiSe2OH + OH ↔ NiSe2O + H2O + e


image file: c9dt03150b-f4.tif
Fig. 4 Electrochemical supercapacitor tests of the self-supported NiSex nanocrystal electrode in a three-electrode configuration: (a) the cyclic voltammogram test, (b) galvanostatic charge–discharge curves at different current densities, (c) a plot of specific capacitance from GCD curves at different current densities, and (d) the Ragone plot.

Then the specific capacitance of the material (C; F g−1) was obtained from the CV curve at different scan rates by using the following equation.38,68

image file: c9dt03150b-t1.tif
where ∫I(V)[thin space (1/6-em)]dV is the area under the CV curve (in AV), m is the mass of the active electrode material (in g), ϑ is the scan rate in mV s−1, and ΔV is the voltage window (in V) obtained from the CV curve. Here, the mass of the working electrode material was evaluated to be around 1.5 mg and the mass measurement procedure has been clearly discussed in the ESI. From the CV curve, it was found that the evaluated specific capacitance values at different scan rates indicated the better performance characteristics of NiSex (Fig. 4a). The high value of specific capacitance was found to be about 853 F g−1 at a scan rate of 20 mV s−1. In order to monitor the contribution of the nickel foam to the charge storage performance, we recorded the cyclic voltammogram of the bare nickel foam under similar electrochemical conditions at a sweep rate of 100 mV s−1 and the results are presented in Fig. S3. It shows a negligible contribution to the charge storage performance and the net specific capacitance observed in this work is mainly from the as-synthesized NiSex.

Furthermore, the galvanostatic charge–discharge profiles (Fig. 4b) have been recorded at various applied current densities and the specific capacitance values have been calculated as per the following equation:38,68

image file: c9dt03150b-t2.tif
where C is the specific capacitance (F g−1), I is the current density (A cm−2), Δt is the discharge time (s), and ΔV is the optimized potential window (V) obtained from the CV curve. Specific capacitances of 1333, 1111, 800 and 667 F g−1 were obtained at current densities of 30, 35, 40 and 45 A g−1 respectively (Fig. 4c). A high value of specific capacitance of 1333 F g−1 and a specific capacity of 78 C g−1 from the charge–discharge curve at a current density of 30 A g−1 for CVD-grown nanostructured NiSex are still challenging as compared to the other reported NiSex based materials for supercapacitors.9,31,32,65 It is obvious that upon enhancement of applied current densities during galvanostatic charge–discharge, the potential drop gets increased, leading to an incomplete faradaic reaction as observed in the previous reports.67,69 These excellent capacitive characteristics and high charge storage capacity make this material suitable for high performance energy storage applications.

The energy storage performance of this material for supercapacitor applications is also examined by determining its efficiency which is typically evaluated by energy and power density calculations through specific capacitance measurements at different current densities from the GCD characteristics (Fig. 4b). The energy density (ED) and power density (PD) were calculated from the specific capacitance values according to the following equations respectively.38,68

image file: c9dt03150b-t3.tif

image file: c9dt03150b-t4.tif

As depicted in Fig. 4d, the Ragone plot (log of energy density versus power density) shows a very good relationship between the energy and power densities of the as-synthesized self-supported NiSex electrode. Importantly the Ragone plot exhibits very high energy (105 W h kg−1) and power (86 kW kg−1) densities, thus demonstrating the as-synthesized NiSex as a promising material for future energy storage applications. These superior characteristics i.e. very high values of energy and power densities would be highly desired and compare favourably with those of other reported NiSex electrodes for energy storage applications.9,11,31,32,61,63

Furthermore, electrochemical impedance spectroscopy (EIS) was used to investigate the charge transfer process through electrochemical reaction in a 6 M KOH electrolyte. The electrochemical impedance spectrum shown as the Nyquist impedance plot is displayed in Fig. S4a. The impedance profile is composed of two regions: one is a semicircle in the high frequency regime and the other is almost a straight line in the low frequency regime. Furthermore the impedance spectrum is fitted by an equivalent circuit shown in Fig. S4(b), where Resr is the equivalent series resistance made up of the intrinsic resistance of the working electrode, contact resistances and the solution resistance; Rct is the charge-transfer resistance associated with the faradaic redox reaction; Cdl is the double layer capacitance of the constant phase element in the circuit; W represents finite length Warburg impedance caused by ion diffusion or intercalation through the electrode material; and CF is the faradaic pseudocapacitance. From the Nyquist plot, the Resr and Rct values were evaluated from the intercept to the ‘Z’ axis and the semicircle diameter in the high frequency regime respectively. The fitting data of the impedance spectrum show very low values of Resr (0.17 Ω) and Rct (0.04 Ω), which indicate a high-speed charge transfer process at a very low diffusion resistance. In addition, both values are very low in comparison with other reported NiSex electrodes that show higher electrochemical conductivity.35–37 Also, the as-obtained CVD grown crystalline NiSex structures contain very low defect density compared with other NiSex structures grown by hydrothermal methods and show metallic behavior with high electrical conductivity and negligible contact resistances. Moreover, the nearly vertical straight line in the low frequency regime represents a frequency independent ideal capacitor characteristic of the electrode material.64,65,68,70 In this context the NiSex nanostructured material would be in high demand and superior with a very fast response to charging and excellent capacitive performance for energy storage applications.

Hybrid supercapacitor prototype fabrication and measurement

Although in this work we observed higher capacitance values for the NiSex sample, the operational potential window remained very low (0.75 V), leading to lower energy and power densities. In order to increase the operational potential window, a hybrid supercapacitor in a coin cell configuration has been fabricated by using the as-grown NiSex as the anode and reduced graphene oxide as the cathode electrode material. Prior to device fabrication, the potential window optimization and mass ratio calculation for both the cathode and anode electrode materials have been carried out in 6 M KOH electrolyte. The free standing NiSex and rGO powder (on a glassy carbon electrode), aqueous Ag/AgCl, and bare platinum wire were used as the working, reference and auxiliary electrodes respectively and the CVs were recorded in a 6 M KOH electrolyte in a three-electrode electrochemical set-up. Here the CVs at a 10 mV s−1 sweep rate were recorded with a variable potential window and are presented in Fig. S5. In the case of rGO, the shape of the CV remained nearly rectangular up to −1.35 V showing EDLC behaviour with a slight inclination at higher potential. On the other hand, in the case of NiSex, a sudden rise in anodic current above 0.65 V was observed which may be due to the oxidation of the electrolyte. However, NiSex shows a pseudocapacitive behaviour within the potential range of −0.1 to 0.65 V. From these observations, the lower potential for the cathode material and the higher potential for the anode material are determined to be −1.35 and 0.65 V respectively. Thus, the optimized potential window for this hybrid supercapacitor was taken as 2 V. Then the mass ratio of the cathode and anode materials was determined as per the following equation:
image file: c9dt03150b-t5.tif

Here, m, Csp, and Δv are the mass of the electrode material, the specific capacitance and the potential window, respectively. The rGO to NiSex mass ratio was calculated to be 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5. Thereafter, with this mass ratio, the hybrid capacitor in a coin cell configuration was fabricated (the detailed fabrication procedure is presented in the Experimental section) and the electrochemical measurements (CV and GCD) were carried out in the optimized potential window of −1.35 to 0.65 V. The CV and GCD profiles were recorded at various sweep rates and applied current densities presented in Fig. 5a and b. Here we observe a specific capacitance of 40 F g−1 with an energy and power density of 22 W h kg−1 and 5.8 kW kg−1 respectively (Fig. 5c). Furthermore, the cycling stability of the hybrid supercapacitor was tested using GCD at an applied current density of 0.1 A g−1. Interestingly, the HSC shows 88% capacitance retention even after 10[thin space (1/6-em)]000 repeated cycles, demonstrating the robustness of the electrode material (Fig. 5d). Then the feasibility of the as-assembled HSC coin cell for practical applications was demonstrated by connecting it to a red LED as shown in the inset of Fig. 5d. In this present study we may presume that the excellent performance of the HSC may be ascribed to (a) the higher electrochemical conductivity of NiSex, (b) the three-dimensional array of NiSex on the Ni foam providing increased access of the electrolyte and facilitating charge and ion transport and (c) the presence of the active and stable cathode electrode material (rGO).


image file: c9dt03150b-f5.tif
Fig. 5 (a) CV and (b) GCD profiles of the hybrid supercapacitor (coin cell), (c) the corresponding Ragone plot and (d) the plot showing capacitance retention after 10[thin space (1/6-em)]000 repeated cycles. The inset in (d) is the picture showing the illumination of a red LED connected to the as-assembled coin cell.

Furthermore, the charge storage performances of as-prepared NiSex and the HSC have been compared with other reported nickel selenide and some selenide-based materials and are presented in Table S1. It has been observed that the present sample (CVD grown NiSex) reported in this work shows a better charge storage performance in terms of higher specific capacitance, ultrahigh specific energy/power density, excellent durability etc. compared with other reported nickel selenide-based materials.9,11,31,32,61,63 Therefore, we presume that this mixed phase of nickel selenide emerges as a promising material for the development of future energy storage devices.

Conclusions

The successfully designed self-supported polycrystalline NiSex nanocrystals show outstanding electrochemical performance owing to the heterogeneous orientation of cubic and rhombohedral crystals in a 3D matrix. Highly pure and highly crystalline NiSe2–NiSe nanostructures were directly synthesized by the one step CVD technique under high vacuum. In this context, the porous nature and discontinuous surface geometry of the Ni foam were responsible for creating a heterogeneous nanostructure under optimized conditions. The high-purity of the NiSex nanostructure was confirmed by XPS spectra in which no impurity or oxygen peaks were found. The surface morphology of different geometries confirmed the highly active large surface area that actively participated during electrochemical treatment. To the best of our knowledge, this novel self-supported NiSex with a close-packed cubic-rhombohedral lattice exhibited a higher specific capacitance of 1333 F g−1 and an ultra-high energy density of 105 W h kg−1 at a power density of 54 kW kg−1 compared with other reported NiSex nanostructures (Table S1). This outstanding stable performance to retain the charge for long cycles proves the reliability of the novel nanocrystal electrode. Furthermore, a prototype of the hybrid supercapacitor in a coin cell configuration was assembled and its energy storage performances were studied. The HSC showed a specific capacitance of 40 F g−1 with higher energy (22 W h kg−1) and power (5.8 kW kg−1) densities and excellent capacitance retention (88% of initial capacitance) even after 10[thin space (1/6-em)]000 repeated cycles. We assume that the novel synthetic protocol and excellent electrochemical behavior make NiSex a suitable material for future energy storage systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the DST-SERB Early Career Research project (Grant No. ECR/2017/001850) and a start up grant from Jain University (11 (39)/17/013/2017SG). Dr. S. M. Dinara acknowledges DST-SERB for the National Post-Doctoral Fellowship (Grant No. PDF/2017/000381).

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Footnote

Electronic supplementary information (ESI) available: Calculation of the mass of the working electrode material, the Raman spectrum of CVD grown polycrystalline NiSex, FESEM images of rGO at different magnifications, and the Raman spectra of GO and rGO. The Nyquist impedance plot with the corresponding equivalent circuit of a self-supported NiSex nanocrystal electrode. See DOI: 10.1039/C9DT03150B

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