Title: Electrochemical properties of spray deposited nickel oxide (NiO) thin films for energy storage systems

Accepted Manuscript Title: Electrochemical properties of spray deposited nickel oxide (NiO) thin films for energy storage systems Authors: R.S. Kate, S.A. Khalate, R.J. Deokate PII: DOI: Reference: S0165-2370(17)30130-4 http://dx.doi.org/doi:10.1016/j.jaap.2017.03.014 JAAP 4001 To appear in: J. Anal. Appl. Pyrolysis Received date: Revised date: Accepted date: 1-2-2017 17-3-2017 18-3-2017 Please cite this article as: R.S.Kate, S.A.Khalate, R.J.Deokate, Electrochemical properties of spray deposited nickel oxide (NiO) thin films for energy storage systems, Journal of Analytical and Applied Pyrolysishttp://dx.doi.org/10.1016/j.jaap.2017.03.014 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. <AT>Electrochemical properties of spray deposited nickel oxide (NiO) thin films for energy storage systems <AFF>aR. S. Kate <AFF>aS. A. Khalate <AFF>aR. J. Deokate *rjdeokate@gmail.com Vidya Pratishthan’s, Arts, Science and Commerce College, Baramati-413 133 (MS), India. <PA>Tel.: +91- 02112- 243832, Fax: +91-02112- 243488. Highlights (for review)  The NiO thin films were deposited on the conducting glass substrates at different temperatures by using economical spray Pyrolysis technique for supercapacitor applications.  XRD results reveal that the as prepared films shows single cubic phase for all deposition temperatures.  Significantly specific capacitance 1000 F/g at 5 mV/s, supercapacitive performance 88.21 F/g, energy density 8.93 Wh/Kg at current density 0.4 mA/Cm2 was observed at 370 C.  The (EIS) study showed that the diffusion coefficient enhanced with substrate temperature and again decreases with increase in temperature.  The NiO thin films might be promising supercapacitor material. -HEAD>Abstract <ABS-P>Nanostructured nickel oxide (NiO) thin films were deposited on fluorine doped tin oxide (FTO) substrates by inexpensive spray pyrolysis. The influence of substrate temperature on structural, morphological and electrochemical properties has been investigated. The structural and morphological analysis showed all NiO thin films were found to be similar to the thin film synthesized by conventional methods From the electrochemical results specific capacitance of NiO thin films increased between 16 to 88 F/g with the substrate temperature. The specific 1 energy, specific power and coulomb efficiency are found to be 8.93 Wh/ kg, 0.52 KW/ kg and 85.08 %, respectively. Moreover, electrochemical impedance study showed higher conductivity and excellent Warburg impedance coefficient for NiO thin film deposited at 370 C electrode materials. <KWD>Keywords: NiO; Thin film; Electrical properties; AFM; Electrochemical properties. ________________________________________________________________________ <H1>1. Introduction Electrochemical energy conversions have attracted considerable attention in the field of energy storage over the past decade due to their high power density, long cycle life and energy density [1-3]. Electrochemical supercapacitors which are also known as supercapacitor, are considered to be an energy storage technology that can be acts as bridging function for the energy/power gap between batteries/fuel cells, which have high energy storage and conventional capacitors, which have high power output [4]. On the basis of the electrode materials used and the charge storage mechanisms, supercapacitors are classified into two type’s electric double layer capacitors (EDLC) and faradic redox reaction pseudocapacitors [5]. The pseudocapacitive (Faradaic) process relies on redox reactions that occur in the electrode materials and ions in the appropriate potential window [6]. Pseudocapacitors exhibits higher energy density than EDLC [7, 8]. Preceding investigations have allowed a better understanding of the origins of high specific capacitance of ruthenium oxide [9]. Ruthenium oxide could be replaced by cheaper transition metal elements like Ni, Co, Mn, Mo etc. [10-12]. Several alternatives electrode material has been investigated, including metal oxides, metal sulfides, conducting polymer and carbonaceous materials [13, 14]. The remarkable performance of pseudocapacitors materials compared to that EDLC materials in supercapacitors open up new research possibilities, particularly in transition metal oxides like nickel oxide [15, 16]. Being widely nickel oxide used in many applications such as, electrochromic device [17], solar thermal absorber [18], Tandem dye sensitized solar cell [19], battery cathode [20] and gas sensors [21]. Owing to its high theoretical specific capacitance (2584 F/g) NiO is considered to be a potential candidate for electrode materials in redox electrochemical capacitors [22, 23]. The electrochemical properties of nanosized NiO are considerably wide ranging according to its morphology such as from earlier study many techniques have been used in order to produce nickel oxide thin films: nano/microspheres, nanoflowers, nanosheets and nanomicro films, respectively [24, 25]. Structural morphologies show the variation in the rate of ion transfer during the charge-storage process [26]. Many techniques have been used to obtain nickel oxide thin films: electron beam evaporation [27], dc-magnetron sputtering [28], anodic electrodeposition [29], cathodic electrodeposition [10], chemical vapour deposition [30], sol- gel technology [31], spray pyrolysis method [32] and chemical bath deposition [33]. Here we report nanostructured NiO thin films by spray pyrolysis method applied as supercapacitor electrode material. The deposited NiO thin films were characterized to measure the structural, morphological and electrochemical properties. The electrochemical properties measured by performing cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy (EIS). <H1>2. Experimental Details <H2>2.1 Spray deposited nanostructured NiO thin films 2 Nickel oxide (NiO) thin films were deposited on to ultrasonically cleaned glass substrates and fluorine doped tin oxide (FTO) substrates at different temperatures within 350-390 C. The aqueous solution of 0.1 M nickel nitrate Ni (NO3)2 used as a source of nickel and automated spray pyrolysis homemade equipment was employed to deposit the thin films [34]. The fine aerosols of aqueous nickel nitrate solution sprayed through an atomizer undergo pyrolytic decomposition onto the preheated glass substrates forming thereby a thin solid film. During deposition, whole deposition parameters like concentration, nozzle to substrate distance (32 cm), spray angle (90), solution flow rate (5 ml/min) were kept constant for all experiments. The NiO thin films were prepared after evaporation of sprayed droplets, solute condensation and finally thermal decomposition. The formation of NiO thin films on the FTO substrates at various temperatures according to following reaction: 2NiNO3 2   2H 2O  2 NiO  4NO2   2H 2O  O2  (1) This reaction is simply the formation of NiO thin films on conducting glass substrates, formed thin films were used for further characterization. <H2>2.2 Characterization of NiO electrodes Structural studies of NiO thin films were carried out by Philips X-ray diffractometer PW-3710 with a CuKα (λ = 1.54 Å) target. The atomic force microscope (AFM) images were recorded in contact mode with a Nanoscope III (digital instruments using commercial n+ silicon cantilever with spring constant 0.2 N/m). The electrochemical measurements were performed in an electrolyte of 2 M KOH in a conventional three electrode arrangement comprising platinum as counter electrode and Ag/AgCl sensing as the reference electrode using scaling potentiostat (Model CHI 6002E) CH instrument USA. The electrochemical impedance spectroscopy (EIS) experiment is carried out in the three electrode cell consisting of platinum as counter electrode and Ag/AgCl as reference using potentiostat (CHI 6005E) instruments USA at room temperature. <H1>3. Results and discussion <H2>3.1 X–Ray Diffraction studies The X-ray diffraction patterns of NiO thin films deposited at different substrate temperatures (350-390 C) are shown in Fig.1. All diffraction peaks shown well polycrystalline nature with cubic phase and preferred orientation along (1 1 1) direction of pure NiO phase. Form all existed peaks estimated the d-spacing and lattice constant (a=4.17 Å) values which are good agreement with reported data (JCPDS: 004-0835) [35]. Some weak reflection peaks also observed like (2 0 0) and (2 2 0) might be ascribed to poor crystallization. At high substrate temperature crystallinity peak intensity of (1 1 1) plane attained maximum value for NiO thin film deposited at 370 C. The average crystallite size of NiO thin film is calculated using Debye- Scherer’s formula [36]: D 0.9  cos  (2) 3 where, D is average crystallite size, λ is the wavelength of the X-ray used (here, λ=1.5406 Å), β is the full width at half maximum (FWHM) of corresponding peak and is the Bragg’s angle. The average crystallite size of NiO thin film varies from 11 to 13.5 nm. The dislocations in crystal system can be represented as dislocation density (δ) which is defined as the length of dislocation per unit volume (lines/m2). The dislocation density of NiO thin films calculated using following equation [37]: δ 1 (3) D2 The variation of crystallite size (D) and dislocation density (δ) with different substrate temperatures of NiO films is shown in Fig. 2. The low value of dislocation density is obtained NiO thin film prepared at 370 C, corresponding to largest value of grain size. The particle size is an important parameter to enhance the faradic contribution of the electro active material [38, 39]. <H2>3.2 Morphological properties Fig. 3 (a-d) depicts 2D and 3D images shows the surface morphological features for all NiO thin films. It is seen that morphology of the NiO film is strongly depends on substrate temperature the images at low temperature 350 C reveals small grains are observed with nonuniform distribution. At higher temperature nucleation on the film is started whole thin film randomly distributed and covered with nanograins. NiO thin film prepared at 370 C shows the maximum grain size with homogeneity, slight decrease in non-uniformity and surface roughness which are useful as far as supercapacitor application. Further at high temperature surface of the thin film looks like densely packed and agglomeration of the grains. <H2>3.3 Electrochemical properties The temperature dependant electrochemical properties of spray deposited NiO thin films were studied using cyclic voltammetry in the presence of 2M KOH electrolyte in the applied potential range -1.016 to +0.7 V vs Ag/AgCl at various scan rates. The general reaction or charge storage for nickel oxide electrode in the NaOH or KOH electrolyte is [40], NiO  yOH-  yNiOOH  e  (4) Where y represents Na or K metal elements. In above reaction mechanisms involved a redox reaction between H2 and the Na, K metal ions present in the electrolyte. The redox reaction of nickel oxide is oxidized into metal nickel hydroxide. During oxidation-reduction process given in Eq.(4) in NiO, protons are exchanged with the electrolyte and electrode interface since proton transfer process is slow, higher scan rate laeds to either depletion or saturation of the protons in 4 the electrolyte inside the electrode [41]. The CV is measured in a potential window -1.05 to 0.75V at different scanning rates ranging from 5 to 100 mV/s to all NiO thin films which is shown in Fig. 4 (a-d). Anodic peaks in CV shifted towards negative potential while cathodic peaks shifted towards positive potential with increase in scan rate. All CV peaks shows the current density increases monotonically with increase in scan rate shows the direct relationship between CV current and scan rate suggesting an ideal capacitive characteristics [42]. The conversion of NiO to NiOOH, gives the oxidation peaks, while the reduction peak was due to reverse condition. Cyclic voltammetry (CV) curves were used to calculate the specific capacitance of NiO films deposited at various substrate temperatures using following equations [43]: C I C Ci , Ci  , Cs  dV / dt m A (5) Where, I (A) average charge deduced from CV, dV/dt ( mV/s) is the voltage scan rate, A (cm2) and m (g) are area of the electrode surface and mass of the active material deposited on the electrodes dipped in electrolyte, respectively. The variation of specific capacitance with scan rate for NiO thin film electrodes deposited at various substarte temperatures are shown in Fig. 5. With an increase in scan rate, ions do not have enough time to migrate to the double layer and hence the rate of double layer formation decreases which in turn decreases the specific capacitance by an increase in scan rates [44, 45]. The specific capacitance increases with substrate temperature reaching a maximum value 1000 F/g at 370 C, this may be a consequence of uniform distribution grain size with the homogenity, which offering more electroactive sites and nanochannels for easy intercalation and deintercalation of electrolyte ions. Galvonostatic charge discharge study is used to determine the charge/discharge stability and energy/power density. Fig.6 (a) shows the charge discharge curves of all thin films at constant current density of 0.4 mA/Cm2 in 2M KOH electrolyte in potential window 0 to + 0.45V. It is observed that with an increase in current density, discharge time decreases. In addition, IR drop is more at higher current density compared to at a lower discharge current density [46]. Fig. 6 (b) shows the nonlinear discharge curves and voltage plateues further verifies the pseudocapacitive behaviour of the NiO electrodes. The discharge profile usually contains two parts: firstly due to sudden voltage drop resistive component arising which repesenting the voltage charges due to the inernal resistance and secondly, a capacitive element correlated to the voltage change due to change in energy within the capacitor [47]. Fig. 7 shows the variation in specific capacitance of NiO thin films at different temperatures for 0.4 mA/cm2 current density. The current density is needed to charge up to the full potential window for two electrode materials. NiO thin films deposited at 370 C substrate temperature shows prolongated charging time than as compare to others, up to their full potential range at the constant current density. This is characteristics of the redistribution of charges inside highly nanostructured surface of NiO thin film deposited at 370 C substrate temperature, revealing good capacitive behaviour [48]. The constant current discharge curves of NiO thin film electrode at different current densities are shown in Fig. 6 (b). From these discharge curves the values of specific capacitance (Csp) are calculated according to the folowing relation [43]: Csp  I V  td m (6) 5 Where, I is the discharge current density, V is the working potrntial window, m is the actual mass deposited on the active area of the material, td is the discharging time measured in seconds. The maximum specific capacitance values obtained at specific current density of 0.4 mA/cm2 were 16.15, 19.56, 88.81 and 10.87 F/g at substrate temperature 350, 360, 370 and 390 C, respectively is shown Fig. 7. Specific energy (E), specific power (P) and coluomb efficiency (η%) are calculated using following formulas, E V  I d  Td W P V  Id W  (%)  (7) (8) Td 100 Tc (9) where Id, Tc, and Td are the discharge current, charge time and discharge time, respectively. The W is the mass of NiO film electrode. The values of E, P, and % are 8.93 Wh/ Kg, 0.η2 KW/ Kg and 85.08 % respectively. To determine charge transfer rate of electrolyte ions electrochemical impedance spectroscopy (EIS) were carried out. Fig. 8 shows the Nyquist plot of NiO thin film in aquious solution with the amplitude of 5 mV in 1 MHz to 10 Hz frequency range. The impedance spectra shown the partial semicircles (arc) in high frequency region and straight lines slopes in the lower frequency region, respectively. The distorted semicircle to higher frequencies resembles to charge-transfer resistance of the interface between the NiO electrode and electrolyte. The intercepts of this semicircle yields the electrolyte resistance (Re) and the diameter provides the faradaic charge transfer resistance (Rct). The electrical resistance of electrolyte (Re) which has value of 2 Ω in 2M KOH electrolyte at 370 °C. The lowest charge transfer resistance (Rct) value 28 Ω is observed at 370 °C substrate temperature. The Re and Rct shows the minimum values due to the uniform nanostructure of NiO thin film electrode assisting the effective access of electrolyte ions in the active electrode material and shortening the ion diffusion path [49]. The straight lines in Fig. 8 are related with diffusion of pottasium ions into the bulk of electrode material or Warburg diffusion. The plot of Zre vs. the reciprocal square root of the lower angular frequencies is illustrated in Fig. 9. The parameters shown in the Table 1 are calculated from following relations [50]: 0.5 Zre  Re  Rct     RT  D  0.5 2   AF   C  (10) 2 (11) 6  RT    nFRct  i0   (12) Where Rct, charge transfer resistance; Re, electrolyte resistance;  angular frequency in the low frequency region; D, diffusion coefficent; R, the gas constant; T, the absolute temperature; F, Faradays constant; A, the area of the electrode surface; and C, molar concentration of K+ ions [16, 50]. Higher values of exchange current densities represents the stronger charge transfer reactions however the role of temperature is not clear. <H1>4. Conclusion The influence of substrate temperatures on the structural, morphological, and electrochemical properties of the NiO thin film have successfully studied. The electrochemical and galvanostatic charge/discharge analysis exhibited that the NiO thin film electrodes have stable electrochemical capacitor properties. Due to uniform nanostructure morphology the NiO thin film deposited at 370 C shown maximum specific capacitance 88.81 F/g, specific energy, specific power and coulomb efficiency 8.93 Wh/ Kg, 0.52 KW/Kg and 85.08 %, respectively. These results suggest that such NiO electrode material is promising electrode for supercapacitor applications. <ACK> Acknowledgements Dr. R. J. 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Faulkner,;1; Electrochemical Methods, Second ed; <PN>John Wiley</PN> and sons, <PL>New York</PL>, 2001,p.231. </BIBL> <Figure>Fig. 1 X-ray diffraction patterns of NiO films deposited for different substrate temperatures. 11 <Figure>Fig. 2 Crystallite size and dislocation density of NiO thin film for different substrate temperatures. <Figure>Fig. 3 Atomic force microscope images of NiO thin films at different substrate temperature (a) 350 C (b) 360 C (c) 370C and (d) 390C. Temperature Re (Ω) (oC) <Figure>Fig. 4 Rct (Ω) σ (Ω s-0.5) D (cm2/s) Cdl io (mA/cm2) Cyclic Votamogram at different scan rates of NiO thin film prepared at different temperatures (a) 350C (b) 360C (c) 370C and (d) 390C. <Figure>Fig. 5 Plot of specific capacitance calculated from cyclic voltammograms versus scan rates of NiO thin film prepared at temperatures (a) 350C (b) 360C (c) 370C and (d) 390C. <Figure>Fig. 6 (a) Charging discharging curves of NiO thin film for 0.4 mA/Cm2 prepared at different substrate temperatures. <Figure>Fig. 6 (b) Plot of time discharging curves of all temperatures at 0.4 mA/Cm2. <Figure>Fig. 7 Plot of specific capacitance versus temperature at current density 0.4 mA/Cm2. <Figure>Fig. 8 Nyquist Plots of NiO thin films deposited at different substrate temperatures. <Figure>Fig. 9 Plot of real impedance versus low frequencies for NiO/FTO electrodes at different substrate temperatures. <Table>Table captions Table 1: The EIS parameters of NiO thin films prepared at temperatures 350 C, 360 C, 370 C and 390 C. 12 350 3 60 954.74 9.91× 10-21 3.623 × 10-7 8.61 × 10-4 360 2 46 414.52 5.214× 10-20 2.360 × 10-7 5.61 × 10-4 370 2 28 293.77 1.03 × 10-19 1.811 × 10-7 9.00 ×10-4 390 3 30 398.78 5.64 × 10-20 3.802 × 10-7 4.30 × 10-4 TDENDOFDOCTD 13