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Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors
Accepted Manuscript Title: Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors Authors: N. Priyadharsini, R. Kalai Selvan PII: DOI: Reference:
S0013-4686(17)31338-5 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.100 EA 29738
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
4-4-2017 15-6-2017 17-6-2017
Please cite this article as: N.Priyadharsini, Kalai Selvan R., Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors N. Priyadharsini,a,b R. Kalai Selvana* a
Energy Storage and Conversion Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India. b Department of Physics, PSGR Krishnammal College for Women, Coimbatore, 641 004, Tamil Nadu, India.
*E mail: [email protected] (R.K.Selvan)
ABSTRACT A facile sol-gel thermolysis route was adopted to synthesize KNiPO4 nano-sheets for the design of hybrid supercapacitors. The phase purity, homogeneity, and functional groups present in the synthesized KNiPO4 were characterized through X-ray diffraction and FTIR measurements. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the nano-sheet-like particles were loosely stacked. The electrochemical properties of the KNiPO4 electrode were studied in various aqueous-based electrolytes such as 1 M LiOH, 1 M NaOH, and 1 M KOH to explore their superior performances. Among these electrolytes, the KNiPO4 electrode provided a maximum specific capacity of 278 C g-1 in 1 M KOH at 5 mV s-1. A hybrid supercapacitor was fabricated using the synthesized KNiPO4 as the positive electrode and activated carbon as the negative electrode in a 1 M KOH aqueous electrolyte. The supercapacitor exhibited a specific capacitance of 48 F g-1 in 1 M KOH at 0.6 mA cm-2 and energy density of 13 Wh kg-1 at a power density of 59 W kg-1. In addition, the hybrid system retained 93% of its initial specific capacitance even after 2000 cycles. A KNiPO4-based hybrid system thus exhibits superior characteristics and hence is a promising candidate for high-performance electrochemical energy storage devices.
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Key Words: Hybrid supercapacitor, sol-gel thermolysis, specific capacitance, energy density
1 Introduction It is well known that supercapacitors with high power density and moderate energy density have been extensively used in different applications in the past few decades because of the superior function of the porous electrode materials with large surface areas [1, 2]. Supercapacitors are broadly classified as electric double-layer capacitors (EDLC) and pseudocapacitors. The energy stored because of electrostatic charge diffusion and the amassing of charges at the dielectric interface of the electrode in EDLC results in quick delivery of charges. On the other hand, the energy stored using fast surface redox reactions is the principal reason for its high capacitance and an increase in energy density [3, 4]. Specific studies were carried out on supercapacitors to achieve high energy density without compromising the power density and rate capability by adopting various novel materials [5-8]. Over the years, many hybrid supercapacitors have emerged from the combination of carbonaceous electrodes as EDLC electrodes and metal oxides as battery electrodes. Both of these electrode materials are unique. They have a higher power density because of quick charge/discharge processes, and their higher specific capacitance leads to an increase in energy density. Because of these advantages of the two-electrode system, by combining battery-type electrode and capacitor-type electrode one can increase the working potential of the entire hybrid capacitor configuration. Recently, LiNi0.5Mn0.5O4, LiMn2O4, Li4Mn5O12, LiTi5O14, LiNi1/3Co1/3Mn1/3O2, and LiNi0.4Co0.6O2 [9-15] were used as possible electrodes in Li-ion hybrid systems. Many studies have focused on research in Na-based compounds, including NaNiPO4, Na3V2(PO4)3, Na3V2(PO4)3/C, Na2CoSiO4, etc., for hybrid capacitors [16-20] because of lower cost and abundance of sodium element.
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Considering the benefits of Li and Na-based compounds for hybrid capacitors, it would be advantageous to move toward another K-based compound as a possible electrode material in the fabrication of hybrid supercapacitors. Potassium holds higher electrochemical stability; its chemical diffusion coefficient is higher than that of lithium and sodium compounds because of a smaller stoke radius of solvated K+ ions; hence, potassium would be a better alternative. It is precisely in this context that we have carried out work on the synthesis of potassium nickel phosphate (KNiPO4) by sol-gel thermolysis for possible application in hybrid supercapacitors. KNiPO4 belongs to the same family of compounds as LiMPO4. It has an orthorhombic structure corresponding to the space group of Pna21 with a preferred screw axis of 21. It has been reported that the KNiPO4 unit cell contains four magnetic Ni2+ ions occupying the fourfold position 4a that provides the antiferromagnetic ordering at the Néel temperature (TN) of 25 K. KNiPO4 exhibits intriguing ferro-electric behavior due to its spontaneous electric polarization; this is mainly because KNiPO4 belongs to a special category of materials that have no center of symmetry [21]. In a past review that examined the symmetry of KNiPO4 single crystal, the magnetic ordering and presence of the domain wall were elucidated through single crystal X-ray measurements, and the results of neutron diffraction investigations were reported [22-24]. More aqueous-based devices like aqueous batteries, aqueous dye-sensitized solar cells, etc. are evoking increasing interest in the energy field because of the improved sustainability of the resulting devices [25-29]. These systems provide better solutions to the problems faced with the previously existing non-aqueous energy systems. They are inexpensive, environmental friendly, safer, and a potential redox mediator than the non-aqueous one. To the best of our knowledge, no study has been reported on the synthesis and characterization of KNiPO4 particles for aqueous hybrid capacitors. In this respect, the work reported in this paper is the first of its kind. We have made an attempt to prepare KNiPO4 by the facile synthetic technique of sol-gel thermolysis for the fabrication of hybrid supercapacitors as 3
the positive electrode in aqueous type. We believe that these types of materials would help switch over from the current technology to a new era of electrochemical energy devices.
2 Experimental methods and materials 2.1 Material synthesis KNiPO4 was synthesized through a facile self-propagating sol-gel thermolysis procedure using citric acid as both gelating agent and fuel for ignition. Stoichiometric amounts of potassium acetate (C2H3O2K) (1.01 g), nickel acetate (C4H6NiO4.4H2O) (2.58 g), and ammonium di-hydrogen phosphate (NH4H2PO4) (1.19 g) were dissolved in double-distilled water individually and mixed together to get a homogeneous solution. Subsequently, the required amount of citric acid (C6H8O7.H2O) (2.18 g) was dissolved with double-distilled water and added dropwise to the above mixed solution to form a metal citrate complex. Then, the homogenous mixture was continuously stirred for four hours with uniform heating at 100 ºC to form a viscous pale green gel. Finally, the as-prepared gel was fired at 400 ºC, at which the gel ignited with the rapid evolution of a large volume of dense fumes of gaseous products, resulting in a final foamy powder. The foamy powder was ground for one hour and the yield was calcinated at 700 ºC for five hours in air. 2.2 Characterization The structural and functional groups of the as-prepared compound were characterized by powder X-ray diffraction (XRD) using Cu Kα radiation (D8 Advance diffractometer, Bruker, Germany) and FTIR spectrum (FTIR spectrometer - ATRIR Affinity 1, Shimadzu), respectively. Field-emission scanning electron microscopy (FE-SEM, FEI-QUANTA-FEG 250) and transmission electron microscopy (TEM, JEM-2100, JEOL) with a 200-kV acceleration voltage were used for observing the morphological properties of the synthesized sample. 2.3 Electrode preparation and hybrid cell fabrication 4
For the preparation of working electrode, the active material, KNiPO4 (75 wt%), was mixed with acetylene black (20 wt%), and poly(vinylidene fluoride) (PVDF) binder (5 wt%). A volume of 0.2 ml of N-methyl-2-pyrrolidone solvent was added to the above mixture and homogenous slurry was prepared. The slurry was subsequently brush-coated onto stainless-steel (SS) electrodes. The bare stainless-steel electrode provides the capacitance of the electrode material. The SS electrode coated with the active material was dried at 60 °C in air overnight for the removal of the solvent.
A typical three-electrode glass cell equipped with a working
electrode, a platinum foil counter electrode, and an Hg/HgO reference electrode was used for electrochemical measurements. Cyclic voltammetry (CV) and galvanostatic charge−discharge measurements were performed using an electrochemical workstation (Bio-Logic SP 150) in 1 M KOH, 1 M NaOH, and 1 M LiOH aqueous electrolyte solutions. A hybrid supercapacitor was assembled with an integrated material of KNiPO4 as a positive electrode, an activated carbon (AC) as negative electrode, and 1 M KOH as the electrolyte. The hybrid system has positive and negative electrodes of KNiPO4║AC with a mass ratio of 1.6:1 mg of electro-active material; the geometric surface area is about 1 cm2. For the fabrication of hybrid capacitor, the two electrodes were separated by a microporous separator made of a polypropylene sheet and sealed in plastic cells to avoid evaporation of the aqueous electrolyte during long-term measurements. Before performing electrochemical analysis, the positive and negatives electrodes and propylene sheet were immersed in 1 M KOH electrolyte solution for 1 h. A series of electrochemical tests, including cyclic voltammetry (CV) and galvanostatic charge−discharge measurements were carried out. Electrochemical impedance spectroscopy (EIS) was conducted by sweeping frequencies from 10 MHz to 1 MHz for a hybrid supercapacitor, performed with a Bio-Logic SP 150. 3. Results and discussion 3.1 Structural and morphological analysis 5
The XRD pattern of KNiPO4 is shown in Figure 1a. It shows the narrow and highintensity diffraction peaks; these clearly show the high-crystalline nature of KNiPO4. The indexed (h k l) planes enumerate the phase purity of the synthesized KNiPO4. The unit cell parameters are calculated through unit cell refinement software, and the lattice constant values are a = 8.6130±0.0157 (Ȧ), b = 9.2667±0.0195 (Ȧ), and c = 4.8917±0.0158 (Ȧ) and the cell volume is 390.4236 (Ȧ)3. It is seen further that the structure is orthorhombic with space group Pna21(33) of KNiPO4, which is in agreement with the JCPDS (No. 86-0573) parameters (a = 8.6333(5) Ȧ, b = 9.2565 (5) Ȧ, c = 4.9064 (9) Ȧ and the cell volume is 392.09 (Ȧ)3). The FTIR spectrum (Figure 1b) of the calcinated KNiPO4 has been recorded at room temperature in the wavenumber range of 400–4000 cm-1. The strong bands at 922 cm-1 and 985 cm-1 are due to symmetric stretching of the P-O bond. Similarly, the two bands observed at 434 cm-1 and 457 cm-1 correspond to symmetric bending of the O-P-O bond. Anti-symmetric bending of the O-P-O bond is observed in the range of 627-565 cm-1; the strong bands between 1020 and 1150 cm-1are due to the anti-symmetric stretching of the P-O bond. [30]. The FTIR spectrum of KNiPO4 clearly explains the formation of the anhydrous KNiPO4 phase, as reported by Noisong et al. [31]. From these observations, it can be concluded that the PO43− ions in KNiPO4 are more distorted, leading to changes in the P-O bond length, which is in accordance with the results of Koleva et al. [32]. The surface morphological features of KNiPO4 were analyzed using FESEM images (Fig. 1c). The images clearly show that the side length is 1 μm. Also there are several nano-sheets that are loosely stacked. Normally, these types of nano-sheets or the nano-platelet-like structure provides for a higher surface area [33], which will increase the charge transfer between the electrode and electrolyte interface. The formation of nano-platelet morphology is further confirmed through TEM analysis (Fig. 1d), from which one can readily infer the particle morphology of the individual nano-sheets / nanoflakes having a thickness of approximately 1.5 6
μm. The well-defined lattice fringes from the HRTEM image (Fig. 1e) provide evidence for the highly crystalline nature of KNiPO4. The lattice spacing between the nearest crystal planes is 0.32 nm, which represents the (2 0 1) crystal plane of the orthorhombic KNiPO4 phase. The observed dot patterns in the SAED (Fig. 1f) image show the single crystalline nature of KNiPO4. The lattice planes inferred from these results exactly match the planes observed in the XRD pattern. 3.2 Electrochemical studies 3.2.1 Cyclic voltammetry (3-electrode system) The electrochemical performances of the KNiPO4 electrode were measured with cyclic voltammetry at various scan rates in three different aqueous electrolytes, including 1 M KOH, 1 M LiOH, and 1 M NaOH. Figure 2a shows the comparative CV curves of KNiPO4 in different electrolytes at 2 mV s-1. The current under the CV curve in 1 M KOH is apparently larger than the 1 M LiOH and 1 M NaOH electrolytes. The reason behind the expanded area for 1 M KOH could be mostly ascribed to the low hydrated radius of K+ ions (0.25 nm) compared to Na+ (0.36 nm) and Li+ (0.42 nm) ions. [34]. The smaller hydrated sphere radius provides more ion adsorption on the electrolyte/electrode interface to enhance the faraday reaction, which results in highest ionic mobility and conductivity [35]. These unique features of a less hydrated radius of K+ ions are responsible for the increase in ionic mobility of the electrode system and its interaction with the electrode resulting in superior electrochemical performance as reported by Ranjusha et al. [34]. The specific capacity of the electrode materials for CV data can be calculated using the following equation: E
1 2 C i( E )dE mv E1
(1)
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where, E1 and E2 are the cutoff potentials and i(E) is the current at each potential, m is the mass of the active materials, and v is the scan rate. The calculated specific capacity of the KNiPO4 electrode is 409, 323, and 270 C g-1 for the electrolytes of 1 M KOH, 1 M LiOH, and 1 M NaOH, respectively. The higher specific capacitance can be attributed to the nano-platelet structure of the electrode material in which it is apparent that most of the ions are taking part in the redox reaction. This is entirely because of the less hydrated radius of K+ ions, which enhances the ionic mobility of the electrode material. Figure 2b shows the CV curves of KNiPO4 at various scan rates in the optimized 1 M KOH electrolyte. The CV curve recorded for the 1 M KOH exhibits a couple of anodic and cathodic peaks at around 0.473 V vs Hg/HgO and 0.331 V vs Hg/HgO; it clearly indicates the reversible redox reaction of Ni (II) ↔ Ni (III) as given below:
KNiPO4 OH KNiPO4 OH e
(2)
The shape of the CV curve changes with an increase in the scan rate; this indicates that the electric potential of the anodic and cathodic peaks shifts toward positive and negative directions, respectively, which may be due to the increasing electric polarization and irreversible reactions of the electrode material as the scan rate rises [36]. In these types of metal oxides, the behavior of the supercapacitor is influenced by the formation of an electrical double layer and the faradaic redox reaction mechanism occurring at the surface of the electrode material. The hydroxyl ions adsorbed on the nonspecific sites leads to the formation of the electrical double layer, which creates a large increase in specific capacitance. This is primarily due to the successive oxidation by the hydroxyl ions on the surface of the electrode material due to the electron transfer across the double layer [37]. Figure 2c shows the plot of the calculated specific capacitance of the KNiPO4 electrode in 1 M KOH, 1 M NaOH, and 1 M LiOH electrolytes as a function of the scan rate. The specific capacitance diminishes significantly even at lower scan rates and remains almost the same when 8
the scan rate is higher than 20 mV s-1 in 1 M NaOH and 1 M LiOH electrolytes. On the other hand, the electrode performs well in 1 M KOH electrolytes. The obtained specific capacities of the KNiPO4 electrode in 1 M KOH are 409, 373, 323, 277, 199, 100, and 69 C g-1, corresponding to the scan rates 2, 3, 4, 5, 10, 20, and 30 mV s-1, respectively. The capacitance of the electrode decreases when the scan rate increases, which is the normal behavior of the materials. The reason behind this is that the restriction of the ion diffusion rate balances the neutralization of electrons during redox reaction, whereas with higher scan rates the inner active sites cannot support the redox transitions completely [38,39]. The surface of the electrode material is partially unreachable for the fast-moving ions at higher scan rates, which leads to a gradual decrease in the specific capacitance. Hence, the measured higher specific capacitance at lower scan rates renders the material advantageous for its usage as electrode material [40]. In order to calculate the available inner and outer charges of the electrode for kinetics, the trasatti plot of KNiPO4 was determined, as shown in Fig. 2 (d and e). By linear fitting the value of q* with ν-1/2, the value of charge at the outer surface was calculated and found to be 71.42 C/g (Fig. 2e). The total charge stored in the electrode can be directly calculated from the Trasatti plot by linearly fitting the values of 1/q* vs the square-root of the scan rates as shown Fig. 2d; the total charge was found to be 833 C g-1. The value of q* was derived by extrapolating the straight line to the square root of the scan rates [41]. The charge stored at the inner surface was calculated and found to be 762 C g-1. From these results, it is evident that the obtained specific capacitance at higher scan rates mainly arises from the outer surface charges. Similarly, the inner and outer charges were calculated for 1 M LiOH and 1 M NaOH electrolytes and are shown in Fig. 2(f); these explain that the electrode KNiPO4 stores more charges in the KOH electrolyte at both the inner and outer surfaces than in 1 M LiOH and 1 M NaOH, which in turn leads to higher specific capacitance. 3.2.2. Galvanostatic charge-discharge studies (3 electrode systems) 9
Figure 3a depicts the comparative galvanostatic charge-discharge curves of the KNiPO4 electrode at 1 A g-1. The highly nonlinear behavior of the discharge curve in all electrolytes clearly explains the supercapacitive behavior of the KNiPO4 electrode, which substantiates the CV results [42]. The specific capacitance can be calculated using the following equation:
C
i t m
(3)
where C (C g-1) is the specific capacity, I (A) is the discharge current, Δt (s) is the discharge time, and m (g) is the mass loading of the electrode material, KNiPO4. Among the studied electrolytes, the electrode exhibits a largest discharge time in 1 M KOH and the calculated specific capacitance was 263 C g-1. Similarly, the electrode provides 217 and 134 C g-1 in 1 M LiOH and 1 M NaOH electrolytes at 0.5 mA cm-2. Figure 3(b) shows the charge-discharge time profiles recorded at various current densities from 0.5 mA cm-2 to 5 mA cm-2 in the optimized electrolyte of 1 M KOH. These data are indicative of very good electrochemical performance. Figure 3c shows the calculated specific capacitance at various current densities in three different electrolytes. Among these, the electrode provides higher specific capacitance of 263, 204, and 170 C g-1 at 0.5, 1, and 2 mA cm-2, respectively, in the 1 M KOH electrolyte than in the other two electrolytes. The specific capacitance decreases with an increase in discharge current density; this is due to the inadequate time available for the faradaic reaction of the electrode material. Its superior performance can be attributed to the slackly bound 2D nano-platelets of electrode material, which can provide bulk accessibility of the faradaic reaction [43]. This ensures that most of the ions participate in the electrochemical reaction in the electrode system. 3.2.3. Electrochemical performances of fabricated (KNiPO4║AC) hybrid supercapacitor A hybrid capacitor was fabricated utilizing KNiPO4 as the positive electrode and activated carbon (AC) as the negative electrode in an optimized aqueous 1 M KOH electrolyte. Figure 4(a) shows the CV curves of the KNiPO4 and AC electrodes at a scan rate of 2 mV s-1, in 10
a three-electrode system. The CV curve of the AC negative electrode for a potential window of 1.0 to 0 V vs Hg/HgO shows an ideal rectangular shape without noticeable redox peaks. This is characteristic of charging/discharging of an electric double-layer capacitance. In the potential range of 0 to 0.6 V, there is a deviation from ideal rectangular behavior; it indicates the supercapacitive nature of the material. Prior to the fabrication of the hybrid capacitor, the masses of the positive and negative electrodes were balanced, based on the following equation:
m m
Cs V Cs V
(4)
where m is the mass, C s is the specific capacitance, and V is the potential window for the positive and negative electrodes, which have a mass balance value of 1.6:1 mg.
The
CV
curves of the KNiPO4║AC hybrid capacitor at different scan rates were measured and are presented in Fig. 4b; these data clearly show the ideal rectangular behavior of CV curves from 2 to 100 mV s-1, indicating typical capacitive behavior of the constructed hybrid capacitor. The hybrid supercapacitor exhibits higher specific capacitance within a potential window of 0-1.5 V. In the case of hybrid supercapacitor, the potential window can be enhanced by combining the electrode material of higher intercalation potential as the cathode and activated carbon as the anode. This type of hybrid system has the advantage of increased capacity of the positive electrode because of its wider potential; also only a reduced amount of electrolyte is required because of the ion concentration in the electrolyte [44]. The activated carbon (AC) that acts as a negative electrode will provide a synergistic effect in the hybrid configuration of the twoelectrode system. The relationship between the specific capacitance with different scan rates for hybrid supercapacitors is plotted in Fig. 4c. The specific capacitance values of the hybrid capacitor are 43, 34, and 31 F g-1, corresponding to 2, 5, and 10 mV s-1, respectively. In the hybrid supercapacitor of KNiPO4║AC, the specific capacitance is calculated using the total active masses of the electrode system. 11
Further, the performance of the hybrid supercapacitor was analyzed using galvanostatic charge-discharge curves to derive the specific capacitance. Fig. 5a shows the galvanostatic charge/discharge time profiles of the KNiPO4║AC hybrid supercapacitor at different current densities of 0.5 to 5 mA cm-2. The charge/discharge time profiles measured at all the current densities are almost symmetric to each other, indicating the excellent electrochemical reversibility [45]. Measurement of the current density is an important factor in the analysis of the power behavior of the supercapacitor. The hybrid supercapacitor of KNiPO4║AC delivers better specific capacitance starting from 48 to 30 F g-1 at various current densities of 0.5 to 5 mA cm-2 , as shown in Fig 5b. It is observed that the hybrid supercapacitor exhibits larger specific capacitance in GCD curves that readily demonstrates the superior electrochemical performance of the electrode materials due to the loosely stacked nano-platelets present in the electrode material. This platelet structure induces a facile diffusion of ions between the interlayer of the positive and negative electrodes. The decrease in specific capacitance at higher current densities is entirely due to an increase in ionic resistivity and a decrease in charge diffusion available in the inner active sites. Cycling stability and rate capability are some of the other significant factors determining the efficacy of hybrid supercapacitor electrodes for many concrete applications [46]. Excellent cycling stability is crucial for real-time supercapacitor operations. The cycle life test over 2000 cycles for the KNiPO4║AC hybrid supercapacitor was carried out by repeating galvano static charge-discharge analysis between 0 and 1.4 V at a current density of 1 mA cm-2. Figure 5c depicts the specific capacitance of the hybrid capacitor as a function of cycle number. The specific capacitance slightly decreases with an increase in cycling number and the hybrid system retains its specific capacitance values even after 2000 cycles; the capacity retention after 2000 cycles is 93%. This value is much superior to that of the previously reported hybrid capacitor systems of mesoporous carbon sphere @ nickel cobalt sulfide core–shell structures [47], nickel 12
cobalt hydroxide @ reduced graphene oxide hybrid nanolayers [48], and coral-like nanoporous β-Ni(OH)2 [49]. Initially, the specific capacitance of the hybrid supercapacitor was 44 F g-1 and it decreased to 41 F g-1 after 2000 cycles. Therefore, we can conclude that the KNiPO4 compound is an excellent positive electrode material for use in hybrid supercapacitors because of its better specific capacitance and high rate. In a commercial supercapacitor system, the energy and power densities are the two significant parameters for calculating the number of diffused ions and total number of ions involved in the charge transport. The inset of Fig. 5d shows the Ragone plot of the corresponding specific energy (E) and power density (P) of the as-fabricated hybrid supercapacitor at different current densities. The energy density and power density values of the hybrid supercapacitor can be obtained from the following equations, E
1 C sp V 2 2
(5)
P
E t
(6)
where C sp is the specific capacitance, V is the cell potential (i.e. 1.4 V), and t is the discharge time. The calculated energy density values for the hybrid supercapacitor of KNiPO4║AC at the current densities of 0.5, 1, and 5 mA cm-2 are 13, 12, and 8 Wh kg-1, respectively. The hybrid supercapacitor reaches a higher energy density of 13 Wh kg-1 at a power density of 59 W kg-1 and remains at 8 Wh kg-1 at a power density of 678 W kg-1, which is comparable and better than the previous results (Table 1). The power densities were found to be 59, 135, and 678 W kg-1 at current densities of 0.5, 1, and 5 mA cm-2, respectively. However, this energy and power densities of the KNiPO4-based aqueous hybrid supercapacitor can be further improved by particle size reduction [58], shape modification [59], preparing carbon composite
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[60] or r-GO composite [61], metal ion doping [10], using redox electrolytes [62], etc., as seen in previous studies. To investigate the microstructural and supercapacitive behavior of the fabricated hybrid supercapacitor, electrochemical impedance spectroscopy measurements were carried out in an open potential window in the frequency range of 0.1 MHz and 10 MHz with an ac perturbation of 5 mV. The Nyquist plot for the KNiPO4║AC hybrid supercapacitor measured before and after the 2000 cycle execution of charge discharge curves is shown in Fig. 5e. It exhibits a quasisemicircle in the high-frequency region and a sloped linear behavior in the low-frequency region. It is possible to measure a solution resistance (Rs) from this Nyquist impedance plot that comprises a different resistance, like the intrinsic resistance of the active material, the ionic resistance of the electrolyte, and contact resistance at the electrolyte/ electrode interface. The Rs values for the hybrid supercapacitor before and after cycling are 0.45 Ω and 0.9 Ω. The lower value of Rs explains the good contact between the electrode and electrolyte interface. The semicircle intersects with the real axis of the Nyquist plot and gives the charge transfer resistance (Rct) around 2 Ω corresponding to before cycling. It depicts that more number of ions are available for electrical conduction in the electroactive surface. After the execution of 2000 cycles in galvanostatic charge–discharge measurements, there is an increase in Rct value to 10.5 Ω. The increase in the resistance is due to the decrease in the number of ions involved in the redox reactions after the execution of discharge cycles. The slope of 45° angled straight line in the lower-frequency region is due to the supercapacitance of the electrodes and also the porous nature of the electrodes. The linear behavior explicitly shows less diffusion resistance of the electrode material and the frequency-dependence kinetics of the ion movements between the electrolyte and electrode surface. For real-time applications, the fabricated two hybrid capacitors of KNiPO4║AC are connected serially and charged for 30 s at a current density of 5 mA cm-2 and used to power the red LED source; the LED glows continuously for about 3 min (180 s), 14
which is shown in Fig. 5f. We conclude that the nano-sheet-structured KNiPO4 compound is a promising electrode material for energy storage applications.
4. Conclusions We investigated the compound of KNiPO4 as a spositive electrode material for the fabrication of a hybrid supercapacitor. KNiPO4 was synthesized by sol-gel thermolysis and was completely characterized by XRD, FTIR measurements. The SEM, TEM, and HRTEM images confirmed the formation of nano-sheet-like KNiPO4 particles. The prepared KNiPO4 electrode provided higher specific capacitance of 263 C g-1 at 0.5 mA cm-2 in 1 M KOH electrolyte. A hybrid cell was fabricated by combining KNiPO4 as a positive electrode and AC as a negative electrode and their performance was analyzed in aqueous electrolyte of 1 M KOH. A hybrid supercapacitor system of KNiPO4║AC exhibits superior electrochemical cycling stability of 93% compared to the previously reported nickel-based hybrid capacitors. This hybrid capacitor delivered a maximum energy density of 13 Wh kg-1 at a power density of 59 W kg-1 and retained 8 Wh kg-1 at a power density of 678 W kg-1. Hence we believe that potassium-based hybrid supercapacitors can be a new avenue for supercapacitor systems in various power delivery applications.
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Figure Captions: Figure 1. (a) XRD pattern, (b) FT-IR spectrum, (c) FESEM, (d) TEM and (e) HRTEM image and (f) SAED pattern of KNiPO4 nano-sheets. Figure 2. (a) CV curve of KNiPO4 in different electrolytes at 2 mV s-1 (b) CV curves at different scan rates in 1 M KOH (c) Specific capacitance vs scan rates, (d and e) Trassati plot using 1 M KOH at various scan rates, (f) the amount of capacitance contributed by inner and outer surface of the electrode at three different electrolytes. Figure 3. (a) Comparison of GCD curves, (b) GCD curves at various current densities and (c) Variation of specific capacitance with different current densities for three electrolytes. Figure 4. (a) Cyclic voltamogramms of AC and KNiPO4 electrodes in 1 M KOH aqueous solution at 2 mV s-1, CV curves of the fabricated hybrid (b) cells at various scan rates and (c) specific capacitance with scan rate curves of two different cells. Figure 5. Charge-discharge curves of (a) hybrid capacitor (b) Variation of specific capacitance with different current densities (c)
Cycling stability curve of hybrid
supercapacitor (d) Ragone plot of hybrid and symmetric supercapacitor (d) (f) Nyquist plot of KNiPO4║AC supercapacitor in the frequency range of 0.1 MHz-10 MHz for before cycling
and
after
cycling
at
open
circuit
voltage
(OCV)
and
(e) Photograph showing that two ASC devices in series can glow up a red LED.
24
Figure 1 25
Figure 2
26
Figure 3
27
Figure 4
28
Figure
5 29
Table – 1:
Hybrid capacitor
Ref
Ni-Co oxide║AC
Energy density /Wh kg-1 7.4
NixCo1-x LDH–ZTO ║AC
9.7
51
Ni-Co 8.1
52
MnO2║Fe3O4
50
MnO2║polyaniline
5.86
53
NaMnO2║AC
13.2
54
Ni(OH)2/GNs/NF ║AC
11.1
55
Ni(OH)2/UGF ║a-MEGO 13.4
56
MnFe2O4@C/LiMn2O4
5.5
57
KNiPO4║AC
12
Present Work
30
S0013-4686(17)31338-5 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.100 EA 29738
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
4-4-2017 15-6-2017 17-6-2017
Please cite this article as: N.Priyadharsini, Kalai Selvan R., Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-sheet-like KNiPO4 as a positive electrode material for aqueous hybrid supercapacitors N. Priyadharsini,a,b R. Kalai Selvana* a
Energy Storage and Conversion Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India. b Department of Physics, PSGR Krishnammal College for Women, Coimbatore, 641 004, Tamil Nadu, India.
*E mail: [email protected] (R.K.Selvan)
ABSTRACT A facile sol-gel thermolysis route was adopted to synthesize KNiPO4 nano-sheets for the design of hybrid supercapacitors. The phase purity, homogeneity, and functional groups present in the synthesized KNiPO4 were characterized through X-ray diffraction and FTIR measurements. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the nano-sheet-like particles were loosely stacked. The electrochemical properties of the KNiPO4 electrode were studied in various aqueous-based electrolytes such as 1 M LiOH, 1 M NaOH, and 1 M KOH to explore their superior performances. Among these electrolytes, the KNiPO4 electrode provided a maximum specific capacity of 278 C g-1 in 1 M KOH at 5 mV s-1. A hybrid supercapacitor was fabricated using the synthesized KNiPO4 as the positive electrode and activated carbon as the negative electrode in a 1 M KOH aqueous electrolyte. The supercapacitor exhibited a specific capacitance of 48 F g-1 in 1 M KOH at 0.6 mA cm-2 and energy density of 13 Wh kg-1 at a power density of 59 W kg-1. In addition, the hybrid system retained 93% of its initial specific capacitance even after 2000 cycles. A KNiPO4-based hybrid system thus exhibits superior characteristics and hence is a promising candidate for high-performance electrochemical energy storage devices.
1
Key Words: Hybrid supercapacitor, sol-gel thermolysis, specific capacitance, energy density
1 Introduction It is well known that supercapacitors with high power density and moderate energy density have been extensively used in different applications in the past few decades because of the superior function of the porous electrode materials with large surface areas [1, 2]. Supercapacitors are broadly classified as electric double-layer capacitors (EDLC) and pseudocapacitors. The energy stored because of electrostatic charge diffusion and the amassing of charges at the dielectric interface of the electrode in EDLC results in quick delivery of charges. On the other hand, the energy stored using fast surface redox reactions is the principal reason for its high capacitance and an increase in energy density [3, 4]. Specific studies were carried out on supercapacitors to achieve high energy density without compromising the power density and rate capability by adopting various novel materials [5-8]. Over the years, many hybrid supercapacitors have emerged from the combination of carbonaceous electrodes as EDLC electrodes and metal oxides as battery electrodes. Both of these electrode materials are unique. They have a higher power density because of quick charge/discharge processes, and their higher specific capacitance leads to an increase in energy density. Because of these advantages of the two-electrode system, by combining battery-type electrode and capacitor-type electrode one can increase the working potential of the entire hybrid capacitor configuration. Recently, LiNi0.5Mn0.5O4, LiMn2O4, Li4Mn5O12, LiTi5O14, LiNi1/3Co1/3Mn1/3O2, and LiNi0.4Co0.6O2 [9-15] were used as possible electrodes in Li-ion hybrid systems. Many studies have focused on research in Na-based compounds, including NaNiPO4, Na3V2(PO4)3, Na3V2(PO4)3/C, Na2CoSiO4, etc., for hybrid capacitors [16-20] because of lower cost and abundance of sodium element.
2
Considering the benefits of Li and Na-based compounds for hybrid capacitors, it would be advantageous to move toward another K-based compound as a possible electrode material in the fabrication of hybrid supercapacitors. Potassium holds higher electrochemical stability; its chemical diffusion coefficient is higher than that of lithium and sodium compounds because of a smaller stoke radius of solvated K+ ions; hence, potassium would be a better alternative. It is precisely in this context that we have carried out work on the synthesis of potassium nickel phosphate (KNiPO4) by sol-gel thermolysis for possible application in hybrid supercapacitors. KNiPO4 belongs to the same family of compounds as LiMPO4. It has an orthorhombic structure corresponding to the space group of Pna21 with a preferred screw axis of 21. It has been reported that the KNiPO4 unit cell contains four magnetic Ni2+ ions occupying the fourfold position 4a that provides the antiferromagnetic ordering at the Néel temperature (TN) of 25 K. KNiPO4 exhibits intriguing ferro-electric behavior due to its spontaneous electric polarization; this is mainly because KNiPO4 belongs to a special category of materials that have no center of symmetry [21]. In a past review that examined the symmetry of KNiPO4 single crystal, the magnetic ordering and presence of the domain wall were elucidated through single crystal X-ray measurements, and the results of neutron diffraction investigations were reported [22-24]. More aqueous-based devices like aqueous batteries, aqueous dye-sensitized solar cells, etc. are evoking increasing interest in the energy field because of the improved sustainability of the resulting devices [25-29]. These systems provide better solutions to the problems faced with the previously existing non-aqueous energy systems. They are inexpensive, environmental friendly, safer, and a potential redox mediator than the non-aqueous one. To the best of our knowledge, no study has been reported on the synthesis and characterization of KNiPO4 particles for aqueous hybrid capacitors. In this respect, the work reported in this paper is the first of its kind. We have made an attempt to prepare KNiPO4 by the facile synthetic technique of sol-gel thermolysis for the fabrication of hybrid supercapacitors as 3
the positive electrode in aqueous type. We believe that these types of materials would help switch over from the current technology to a new era of electrochemical energy devices.
2 Experimental methods and materials 2.1 Material synthesis KNiPO4 was synthesized through a facile self-propagating sol-gel thermolysis procedure using citric acid as both gelating agent and fuel for ignition. Stoichiometric amounts of potassium acetate (C2H3O2K) (1.01 g), nickel acetate (C4H6NiO4.4H2O) (2.58 g), and ammonium di-hydrogen phosphate (NH4H2PO4) (1.19 g) were dissolved in double-distilled water individually and mixed together to get a homogeneous solution. Subsequently, the required amount of citric acid (C6H8O7.H2O) (2.18 g) was dissolved with double-distilled water and added dropwise to the above mixed solution to form a metal citrate complex. Then, the homogenous mixture was continuously stirred for four hours with uniform heating at 100 ºC to form a viscous pale green gel. Finally, the as-prepared gel was fired at 400 ºC, at which the gel ignited with the rapid evolution of a large volume of dense fumes of gaseous products, resulting in a final foamy powder. The foamy powder was ground for one hour and the yield was calcinated at 700 ºC for five hours in air. 2.2 Characterization The structural and functional groups of the as-prepared compound were characterized by powder X-ray diffraction (XRD) using Cu Kα radiation (D8 Advance diffractometer, Bruker, Germany) and FTIR spectrum (FTIR spectrometer - ATRIR Affinity 1, Shimadzu), respectively. Field-emission scanning electron microscopy (FE-SEM, FEI-QUANTA-FEG 250) and transmission electron microscopy (TEM, JEM-2100, JEOL) with a 200-kV acceleration voltage were used for observing the morphological properties of the synthesized sample. 2.3 Electrode preparation and hybrid cell fabrication 4
For the preparation of working electrode, the active material, KNiPO4 (75 wt%), was mixed with acetylene black (20 wt%), and poly(vinylidene fluoride) (PVDF) binder (5 wt%). A volume of 0.2 ml of N-methyl-2-pyrrolidone solvent was added to the above mixture and homogenous slurry was prepared. The slurry was subsequently brush-coated onto stainless-steel (SS) electrodes. The bare stainless-steel electrode provides the capacitance of the electrode material. The SS electrode coated with the active material was dried at 60 °C in air overnight for the removal of the solvent.
A typical three-electrode glass cell equipped with a working
electrode, a platinum foil counter electrode, and an Hg/HgO reference electrode was used for electrochemical measurements. Cyclic voltammetry (CV) and galvanostatic charge−discharge measurements were performed using an electrochemical workstation (Bio-Logic SP 150) in 1 M KOH, 1 M NaOH, and 1 M LiOH aqueous electrolyte solutions. A hybrid supercapacitor was assembled with an integrated material of KNiPO4 as a positive electrode, an activated carbon (AC) as negative electrode, and 1 M KOH as the electrolyte. The hybrid system has positive and negative electrodes of KNiPO4║AC with a mass ratio of 1.6:1 mg of electro-active material; the geometric surface area is about 1 cm2. For the fabrication of hybrid capacitor, the two electrodes were separated by a microporous separator made of a polypropylene sheet and sealed in plastic cells to avoid evaporation of the aqueous electrolyte during long-term measurements. Before performing electrochemical analysis, the positive and negatives electrodes and propylene sheet were immersed in 1 M KOH electrolyte solution for 1 h. A series of electrochemical tests, including cyclic voltammetry (CV) and galvanostatic charge−discharge measurements were carried out. Electrochemical impedance spectroscopy (EIS) was conducted by sweeping frequencies from 10 MHz to 1 MHz for a hybrid supercapacitor, performed with a Bio-Logic SP 150. 3. Results and discussion 3.1 Structural and morphological analysis 5
The XRD pattern of KNiPO4 is shown in Figure 1a. It shows the narrow and highintensity diffraction peaks; these clearly show the high-crystalline nature of KNiPO4. The indexed (h k l) planes enumerate the phase purity of the synthesized KNiPO4. The unit cell parameters are calculated through unit cell refinement software, and the lattice constant values are a = 8.6130±0.0157 (Ȧ), b = 9.2667±0.0195 (Ȧ), and c = 4.8917±0.0158 (Ȧ) and the cell volume is 390.4236 (Ȧ)3. It is seen further that the structure is orthorhombic with space group Pna21(33) of KNiPO4, which is in agreement with the JCPDS (No. 86-0573) parameters (a = 8.6333(5) Ȧ, b = 9.2565 (5) Ȧ, c = 4.9064 (9) Ȧ and the cell volume is 392.09 (Ȧ)3). The FTIR spectrum (Figure 1b) of the calcinated KNiPO4 has been recorded at room temperature in the wavenumber range of 400–4000 cm-1. The strong bands at 922 cm-1 and 985 cm-1 are due to symmetric stretching of the P-O bond. Similarly, the two bands observed at 434 cm-1 and 457 cm-1 correspond to symmetric bending of the O-P-O bond. Anti-symmetric bending of the O-P-O bond is observed in the range of 627-565 cm-1; the strong bands between 1020 and 1150 cm-1are due to the anti-symmetric stretching of the P-O bond. [30]. The FTIR spectrum of KNiPO4 clearly explains the formation of the anhydrous KNiPO4 phase, as reported by Noisong et al. [31]. From these observations, it can be concluded that the PO43− ions in KNiPO4 are more distorted, leading to changes in the P-O bond length, which is in accordance with the results of Koleva et al. [32]. The surface morphological features of KNiPO4 were analyzed using FESEM images (Fig. 1c). The images clearly show that the side length is 1 μm. Also there are several nano-sheets that are loosely stacked. Normally, these types of nano-sheets or the nano-platelet-like structure provides for a higher surface area [33], which will increase the charge transfer between the electrode and electrolyte interface. The formation of nano-platelet morphology is further confirmed through TEM analysis (Fig. 1d), from which one can readily infer the particle morphology of the individual nano-sheets / nanoflakes having a thickness of approximately 1.5 6
μm. The well-defined lattice fringes from the HRTEM image (Fig. 1e) provide evidence for the highly crystalline nature of KNiPO4. The lattice spacing between the nearest crystal planes is 0.32 nm, which represents the (2 0 1) crystal plane of the orthorhombic KNiPO4 phase. The observed dot patterns in the SAED (Fig. 1f) image show the single crystalline nature of KNiPO4. The lattice planes inferred from these results exactly match the planes observed in the XRD pattern. 3.2 Electrochemical studies 3.2.1 Cyclic voltammetry (3-electrode system) The electrochemical performances of the KNiPO4 electrode were measured with cyclic voltammetry at various scan rates in three different aqueous electrolytes, including 1 M KOH, 1 M LiOH, and 1 M NaOH. Figure 2a shows the comparative CV curves of KNiPO4 in different electrolytes at 2 mV s-1. The current under the CV curve in 1 M KOH is apparently larger than the 1 M LiOH and 1 M NaOH electrolytes. The reason behind the expanded area for 1 M KOH could be mostly ascribed to the low hydrated radius of K+ ions (0.25 nm) compared to Na+ (0.36 nm) and Li+ (0.42 nm) ions. [34]. The smaller hydrated sphere radius provides more ion adsorption on the electrolyte/electrode interface to enhance the faraday reaction, which results in highest ionic mobility and conductivity [35]. These unique features of a less hydrated radius of K+ ions are responsible for the increase in ionic mobility of the electrode system and its interaction with the electrode resulting in superior electrochemical performance as reported by Ranjusha et al. [34]. The specific capacity of the electrode materials for CV data can be calculated using the following equation: E
1 2 C i( E )dE mv E1
(1)
7
where, E1 and E2 are the cutoff potentials and i(E) is the current at each potential, m is the mass of the active materials, and v is the scan rate. The calculated specific capacity of the KNiPO4 electrode is 409, 323, and 270 C g-1 for the electrolytes of 1 M KOH, 1 M LiOH, and 1 M NaOH, respectively. The higher specific capacitance can be attributed to the nano-platelet structure of the electrode material in which it is apparent that most of the ions are taking part in the redox reaction. This is entirely because of the less hydrated radius of K+ ions, which enhances the ionic mobility of the electrode material. Figure 2b shows the CV curves of KNiPO4 at various scan rates in the optimized 1 M KOH electrolyte. The CV curve recorded for the 1 M KOH exhibits a couple of anodic and cathodic peaks at around 0.473 V vs Hg/HgO and 0.331 V vs Hg/HgO; it clearly indicates the reversible redox reaction of Ni (II) ↔ Ni (III) as given below:
KNiPO4 OH KNiPO4 OH e
(2)
The shape of the CV curve changes with an increase in the scan rate; this indicates that the electric potential of the anodic and cathodic peaks shifts toward positive and negative directions, respectively, which may be due to the increasing electric polarization and irreversible reactions of the electrode material as the scan rate rises [36]. In these types of metal oxides, the behavior of the supercapacitor is influenced by the formation of an electrical double layer and the faradaic redox reaction mechanism occurring at the surface of the electrode material. The hydroxyl ions adsorbed on the nonspecific sites leads to the formation of the electrical double layer, which creates a large increase in specific capacitance. This is primarily due to the successive oxidation by the hydroxyl ions on the surface of the electrode material due to the electron transfer across the double layer [37]. Figure 2c shows the plot of the calculated specific capacitance of the KNiPO4 electrode in 1 M KOH, 1 M NaOH, and 1 M LiOH electrolytes as a function of the scan rate. The specific capacitance diminishes significantly even at lower scan rates and remains almost the same when 8
the scan rate is higher than 20 mV s-1 in 1 M NaOH and 1 M LiOH electrolytes. On the other hand, the electrode performs well in 1 M KOH electrolytes. The obtained specific capacities of the KNiPO4 electrode in 1 M KOH are 409, 373, 323, 277, 199, 100, and 69 C g-1, corresponding to the scan rates 2, 3, 4, 5, 10, 20, and 30 mV s-1, respectively. The capacitance of the electrode decreases when the scan rate increases, which is the normal behavior of the materials. The reason behind this is that the restriction of the ion diffusion rate balances the neutralization of electrons during redox reaction, whereas with higher scan rates the inner active sites cannot support the redox transitions completely [38,39]. The surface of the electrode material is partially unreachable for the fast-moving ions at higher scan rates, which leads to a gradual decrease in the specific capacitance. Hence, the measured higher specific capacitance at lower scan rates renders the material advantageous for its usage as electrode material [40]. In order to calculate the available inner and outer charges of the electrode for kinetics, the trasatti plot of KNiPO4 was determined, as shown in Fig. 2 (d and e). By linear fitting the value of q* with ν-1/2, the value of charge at the outer surface was calculated and found to be 71.42 C/g (Fig. 2e). The total charge stored in the electrode can be directly calculated from the Trasatti plot by linearly fitting the values of 1/q* vs the square-root of the scan rates as shown Fig. 2d; the total charge was found to be 833 C g-1. The value of q* was derived by extrapolating the straight line to the square root of the scan rates [41]. The charge stored at the inner surface was calculated and found to be 762 C g-1. From these results, it is evident that the obtained specific capacitance at higher scan rates mainly arises from the outer surface charges. Similarly, the inner and outer charges were calculated for 1 M LiOH and 1 M NaOH electrolytes and are shown in Fig. 2(f); these explain that the electrode KNiPO4 stores more charges in the KOH electrolyte at both the inner and outer surfaces than in 1 M LiOH and 1 M NaOH, which in turn leads to higher specific capacitance. 3.2.2. Galvanostatic charge-discharge studies (3 electrode systems) 9
Figure 3a depicts the comparative galvanostatic charge-discharge curves of the KNiPO4 electrode at 1 A g-1. The highly nonlinear behavior of the discharge curve in all electrolytes clearly explains the supercapacitive behavior of the KNiPO4 electrode, which substantiates the CV results [42]. The specific capacitance can be calculated using the following equation:
C
i t m
(3)
where C (C g-1) is the specific capacity, I (A) is the discharge current, Δt (s) is the discharge time, and m (g) is the mass loading of the electrode material, KNiPO4. Among the studied electrolytes, the electrode exhibits a largest discharge time in 1 M KOH and the calculated specific capacitance was 263 C g-1. Similarly, the electrode provides 217 and 134 C g-1 in 1 M LiOH and 1 M NaOH electrolytes at 0.5 mA cm-2. Figure 3(b) shows the charge-discharge time profiles recorded at various current densities from 0.5 mA cm-2 to 5 mA cm-2 in the optimized electrolyte of 1 M KOH. These data are indicative of very good electrochemical performance. Figure 3c shows the calculated specific capacitance at various current densities in three different electrolytes. Among these, the electrode provides higher specific capacitance of 263, 204, and 170 C g-1 at 0.5, 1, and 2 mA cm-2, respectively, in the 1 M KOH electrolyte than in the other two electrolytes. The specific capacitance decreases with an increase in discharge current density; this is due to the inadequate time available for the faradaic reaction of the electrode material. Its superior performance can be attributed to the slackly bound 2D nano-platelets of electrode material, which can provide bulk accessibility of the faradaic reaction [43]. This ensures that most of the ions participate in the electrochemical reaction in the electrode system. 3.2.3. Electrochemical performances of fabricated (KNiPO4║AC) hybrid supercapacitor A hybrid capacitor was fabricated utilizing KNiPO4 as the positive electrode and activated carbon (AC) as the negative electrode in an optimized aqueous 1 M KOH electrolyte. Figure 4(a) shows the CV curves of the KNiPO4 and AC electrodes at a scan rate of 2 mV s-1, in 10
a three-electrode system. The CV curve of the AC negative electrode for a potential window of 1.0 to 0 V vs Hg/HgO shows an ideal rectangular shape without noticeable redox peaks. This is characteristic of charging/discharging of an electric double-layer capacitance. In the potential range of 0 to 0.6 V, there is a deviation from ideal rectangular behavior; it indicates the supercapacitive nature of the material. Prior to the fabrication of the hybrid capacitor, the masses of the positive and negative electrodes were balanced, based on the following equation:
m m
Cs V Cs V
(4)
where m is the mass, C s is the specific capacitance, and V is the potential window for the positive and negative electrodes, which have a mass balance value of 1.6:1 mg.
The
CV
curves of the KNiPO4║AC hybrid capacitor at different scan rates were measured and are presented in Fig. 4b; these data clearly show the ideal rectangular behavior of CV curves from 2 to 100 mV s-1, indicating typical capacitive behavior of the constructed hybrid capacitor. The hybrid supercapacitor exhibits higher specific capacitance within a potential window of 0-1.5 V. In the case of hybrid supercapacitor, the potential window can be enhanced by combining the electrode material of higher intercalation potential as the cathode and activated carbon as the anode. This type of hybrid system has the advantage of increased capacity of the positive electrode because of its wider potential; also only a reduced amount of electrolyte is required because of the ion concentration in the electrolyte [44]. The activated carbon (AC) that acts as a negative electrode will provide a synergistic effect in the hybrid configuration of the twoelectrode system. The relationship between the specific capacitance with different scan rates for hybrid supercapacitors is plotted in Fig. 4c. The specific capacitance values of the hybrid capacitor are 43, 34, and 31 F g-1, corresponding to 2, 5, and 10 mV s-1, respectively. In the hybrid supercapacitor of KNiPO4║AC, the specific capacitance is calculated using the total active masses of the electrode system. 11
Further, the performance of the hybrid supercapacitor was analyzed using galvanostatic charge-discharge curves to derive the specific capacitance. Fig. 5a shows the galvanostatic charge/discharge time profiles of the KNiPO4║AC hybrid supercapacitor at different current densities of 0.5 to 5 mA cm-2. The charge/discharge time profiles measured at all the current densities are almost symmetric to each other, indicating the excellent electrochemical reversibility [45]. Measurement of the current density is an important factor in the analysis of the power behavior of the supercapacitor. The hybrid supercapacitor of KNiPO4║AC delivers better specific capacitance starting from 48 to 30 F g-1 at various current densities of 0.5 to 5 mA cm-2 , as shown in Fig 5b. It is observed that the hybrid supercapacitor exhibits larger specific capacitance in GCD curves that readily demonstrates the superior electrochemical performance of the electrode materials due to the loosely stacked nano-platelets present in the electrode material. This platelet structure induces a facile diffusion of ions between the interlayer of the positive and negative electrodes. The decrease in specific capacitance at higher current densities is entirely due to an increase in ionic resistivity and a decrease in charge diffusion available in the inner active sites. Cycling stability and rate capability are some of the other significant factors determining the efficacy of hybrid supercapacitor electrodes for many concrete applications [46]. Excellent cycling stability is crucial for real-time supercapacitor operations. The cycle life test over 2000 cycles for the KNiPO4║AC hybrid supercapacitor was carried out by repeating galvano static charge-discharge analysis between 0 and 1.4 V at a current density of 1 mA cm-2. Figure 5c depicts the specific capacitance of the hybrid capacitor as a function of cycle number. The specific capacitance slightly decreases with an increase in cycling number and the hybrid system retains its specific capacitance values even after 2000 cycles; the capacity retention after 2000 cycles is 93%. This value is much superior to that of the previously reported hybrid capacitor systems of mesoporous carbon sphere @ nickel cobalt sulfide core–shell structures [47], nickel 12
cobalt hydroxide @ reduced graphene oxide hybrid nanolayers [48], and coral-like nanoporous β-Ni(OH)2 [49]. Initially, the specific capacitance of the hybrid supercapacitor was 44 F g-1 and it decreased to 41 F g-1 after 2000 cycles. Therefore, we can conclude that the KNiPO4 compound is an excellent positive electrode material for use in hybrid supercapacitors because of its better specific capacitance and high rate. In a commercial supercapacitor system, the energy and power densities are the two significant parameters for calculating the number of diffused ions and total number of ions involved in the charge transport. The inset of Fig. 5d shows the Ragone plot of the corresponding specific energy (E) and power density (P) of the as-fabricated hybrid supercapacitor at different current densities. The energy density and power density values of the hybrid supercapacitor can be obtained from the following equations, E
1 C sp V 2 2
(5)
P
E t
(6)
where C sp is the specific capacitance, V is the cell potential (i.e. 1.4 V), and t is the discharge time. The calculated energy density values for the hybrid supercapacitor of KNiPO4║AC at the current densities of 0.5, 1, and 5 mA cm-2 are 13, 12, and 8 Wh kg-1, respectively. The hybrid supercapacitor reaches a higher energy density of 13 Wh kg-1 at a power density of 59 W kg-1 and remains at 8 Wh kg-1 at a power density of 678 W kg-1, which is comparable and better than the previous results (Table 1). The power densities were found to be 59, 135, and 678 W kg-1 at current densities of 0.5, 1, and 5 mA cm-2, respectively. However, this energy and power densities of the KNiPO4-based aqueous hybrid supercapacitor can be further improved by particle size reduction [58], shape modification [59], preparing carbon composite
13
[60] or r-GO composite [61], metal ion doping [10], using redox electrolytes [62], etc., as seen in previous studies. To investigate the microstructural and supercapacitive behavior of the fabricated hybrid supercapacitor, electrochemical impedance spectroscopy measurements were carried out in an open potential window in the frequency range of 0.1 MHz and 10 MHz with an ac perturbation of 5 mV. The Nyquist plot for the KNiPO4║AC hybrid supercapacitor measured before and after the 2000 cycle execution of charge discharge curves is shown in Fig. 5e. It exhibits a quasisemicircle in the high-frequency region and a sloped linear behavior in the low-frequency region. It is possible to measure a solution resistance (Rs) from this Nyquist impedance plot that comprises a different resistance, like the intrinsic resistance of the active material, the ionic resistance of the electrolyte, and contact resistance at the electrolyte/ electrode interface. The Rs values for the hybrid supercapacitor before and after cycling are 0.45 Ω and 0.9 Ω. The lower value of Rs explains the good contact between the electrode and electrolyte interface. The semicircle intersects with the real axis of the Nyquist plot and gives the charge transfer resistance (Rct) around 2 Ω corresponding to before cycling. It depicts that more number of ions are available for electrical conduction in the electroactive surface. After the execution of 2000 cycles in galvanostatic charge–discharge measurements, there is an increase in Rct value to 10.5 Ω. The increase in the resistance is due to the decrease in the number of ions involved in the redox reactions after the execution of discharge cycles. The slope of 45° angled straight line in the lower-frequency region is due to the supercapacitance of the electrodes and also the porous nature of the electrodes. The linear behavior explicitly shows less diffusion resistance of the electrode material and the frequency-dependence kinetics of the ion movements between the electrolyte and electrode surface. For real-time applications, the fabricated two hybrid capacitors of KNiPO4║AC are connected serially and charged for 30 s at a current density of 5 mA cm-2 and used to power the red LED source; the LED glows continuously for about 3 min (180 s), 14
which is shown in Fig. 5f. We conclude that the nano-sheet-structured KNiPO4 compound is a promising electrode material for energy storage applications.
4. Conclusions We investigated the compound of KNiPO4 as a spositive electrode material for the fabrication of a hybrid supercapacitor. KNiPO4 was synthesized by sol-gel thermolysis and was completely characterized by XRD, FTIR measurements. The SEM, TEM, and HRTEM images confirmed the formation of nano-sheet-like KNiPO4 particles. The prepared KNiPO4 electrode provided higher specific capacitance of 263 C g-1 at 0.5 mA cm-2 in 1 M KOH electrolyte. A hybrid cell was fabricated by combining KNiPO4 as a positive electrode and AC as a negative electrode and their performance was analyzed in aqueous electrolyte of 1 M KOH. A hybrid supercapacitor system of KNiPO4║AC exhibits superior electrochemical cycling stability of 93% compared to the previously reported nickel-based hybrid capacitors. This hybrid capacitor delivered a maximum energy density of 13 Wh kg-1 at a power density of 59 W kg-1 and retained 8 Wh kg-1 at a power density of 678 W kg-1. Hence we believe that potassium-based hybrid supercapacitors can be a new avenue for supercapacitor systems in various power delivery applications.
15
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23
Figure Captions: Figure 1. (a) XRD pattern, (b) FT-IR spectrum, (c) FESEM, (d) TEM and (e) HRTEM image and (f) SAED pattern of KNiPO4 nano-sheets. Figure 2. (a) CV curve of KNiPO4 in different electrolytes at 2 mV s-1 (b) CV curves at different scan rates in 1 M KOH (c) Specific capacitance vs scan rates, (d and e) Trassati plot using 1 M KOH at various scan rates, (f) the amount of capacitance contributed by inner and outer surface of the electrode at three different electrolytes. Figure 3. (a) Comparison of GCD curves, (b) GCD curves at various current densities and (c) Variation of specific capacitance with different current densities for three electrolytes. Figure 4. (a) Cyclic voltamogramms of AC and KNiPO4 electrodes in 1 M KOH aqueous solution at 2 mV s-1, CV curves of the fabricated hybrid (b) cells at various scan rates and (c) specific capacitance with scan rate curves of two different cells. Figure 5. Charge-discharge curves of (a) hybrid capacitor (b) Variation of specific capacitance with different current densities (c)
Cycling stability curve of hybrid
supercapacitor (d) Ragone plot of hybrid and symmetric supercapacitor (d) (f) Nyquist plot of KNiPO4║AC supercapacitor in the frequency range of 0.1 MHz-10 MHz for before cycling
and
after
cycling
at
open
circuit
voltage
(OCV)
and
(e) Photograph showing that two ASC devices in series can glow up a red LED.
24
Figure 1 25
Figure 2
26
Figure 3
27
Figure 4
28
Figure
5 29
Table – 1:
Hybrid capacitor
Ref
Ni-Co oxide║AC
Energy density /Wh kg-1 7.4
NixCo1-x LDH–ZTO ║AC
9.7
51
Ni-Co 8.1
52
MnO2║Fe3O4
50
MnO2║polyaniline
5.86
53
NaMnO2║AC
13.2
54
Ni(OH)2/GNs/NF ║AC
11.1
55
Ni(OH)2/UGF ║a-MEGO 13.4
56
MnFe2O4@C/LiMn2O4
5.5
57
KNiPO4║AC
12
Present Work
30