Electrochemical supercapacitive performance of spray deposited Co3O4 thin film nanostructures

Accepted Manuscript Title: Electrochemical supercapacitive performance of spray deposited Co3 O4 thin film nanostructures Authors: Abhijit A. Yadav, U.J. Chavan PII: DOI: Reference:

S0013-4686(17)30437-1 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.157 EA 29022

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-11-2016 17-2-2017 27-2-2017

Please cite this article as: Abhijit A.Yadav, U.J.Chavan, Electrochemical supercapacitive performance of spray deposited Co3O4 thin film nanostructures, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.157 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.

Electrochemical supercapacitive performance of spray deposited Co3O4 thin film nanostructures

Abhijit A. Yadav*, U.J. Chavan

Thin Film Physics Laboratory, Department of Physics, Electronics and Photonics, Rajarshi Shahu Mahavidyalaya, (Autonomous) Latur 413512, Maharashtra, India

*Corresponding author: [email protected], Phone: +919975213852 Fax: +912382253645

1

Graphical Abstract

92.56% retention

400 0.0

300 Potential /V

Specific capacitance /Fg

-1

0.2

200

-0.2 -0.4 -0.6

100

-0.8 752000

754000

756000

758000

760000

time /s

0

0

200

400

600

800

1000

Cycle /number

Research highlights  Electrochemical supercapacitive performance of Co3O4 nanostructures;  Porous morphology has been observed from SEM;  Highest specific capacitance of 425 Fg−1 for film deposited at 350ºC;  Retention of 92.56 % specific capacitance after 1000 cycles;

Abstract In this report, we have obtained cobalt oxide (Co3O4) nanostructures at various substrate temperatures by chemical spray pyrolysis using mixed aqueous/organic solvent. The influence of substrate temperature on the film properties has been studied. From X-ray diffraction studies the cubic phase of Co3O4 is confirmed. SEM shows porous morphology, which is advantageous for supercapacitor applications. Two direct allowed optical transitions are observed. Co3O4 nanostructures exhibit good electrochemical performance with excellent 2

cycle stability and high specific capacitance of 425 Fg-1 at a scan rate of 5 mVs-1. Specific capacitance of Co3O4 films decayed to 7.44% at 1 Ag−1 after 1000 charge/discharge cycling tests in 2M aqueous KOH electrolyte. The electrochemical impedance spectroscopy study indicates that Co3O4 nanostructures deposited at 350°C substrate temperature has better electrochemical properties. The present study is suitable in developing new class of portable energy storage devices.

Keywords: spray pyrolysis; cobalt oxide; supercapacitors; electrochemical impedance spectroscopy;

1. Introduction In day to day life, the problems of energy utilization and environmental concerns along with human development are facing challenges. Today, the world’s massive energy demands are mainly accomplished by the conventional and environmentally unfavorable fossil fuels. To replace this, pursuit of the renewable and clean energy sources, including hydrogen storage and supercapacitors have become important [1-3]. Supercapacitors have appealed significant attention due to their high power density; fast charge/discharge rate and good cycle stability against galvanostatic charge/discharge [4-9]. A balanced surface area and porous surface are highly desirable for electrode materials to be utilised in electrochemical supercapacitors, which is provided by metal oxides [10]. Out of the various metal oxides, ruthenium oxide is the most favorable electrode material with outstanding specific capacitance but its cost is huge. Cobalt oxide is one of the most studied oxides because of its use in various applications, such as solar selective absorbers, pigment for glasses and ceramics [11] and catalyst for oxygen evolution and reduction reaction [12]. It is also widely used as an electrochromic material, sensors, and electrochemical anodes [13], supercapacitors [14-16]. Cobalt oxide exists in three forms, CoO, Co2O3 and Co3O4, but Co3O4 is excessively reported having large applications due to its strong chemical stability and better electrochemical properties. Co3O4 in thin film form can be deposited by employing different deposition techniques such as sputtering [17], hydrothermal [18], coprecipitation [5, 19], chemical route [20], chemical vapor deposition [21], sol-gel [22] and spray pyrolysis [23, 24]. Amongst these, spray pyrolysis has numerous

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benefits including low cost, flexibility, ability for depositing porous and nanostructure thin films and appropriate for large area deposition. Kandalkar and coworkers [20] have prepared cobalt oxide films on a copper substrate at room temperature. These films showed a maximum specific capacitance of 165 Fg–1 in 1M aqueous KOH electrolyte at sweep rate of 10 mVs–1. Amin-Chalhoub et al. [21] have processed low reflective CoO coatings by CVD. A fractal cauliflower like morphology was observed. Ambare and colleagues [23] have studied spray deposited ruthenium incorporated cobalt oxide electrodes. The double-pseudo-capacitive behavior with specific capacitance of 628 Fg–1 at the scan rate 1 mVs–1 in 1M KOH has been observed. The spray deposition of cobalt oxide nanowires on stainless steel substrates is also reported in the literature [25]. From these and other [5, 12-16] studies, it is well understood that Co3O4 is the most beneficial material for the supercapacitor uses. Therefore in current investigation, it was decided to deposit good quality Co3O4 nanostructures for high-performance supercapacitors by chemical spray pyrolysis using mixed aqueous/organic solvent. In the current report, the effect of substrate temperature on various physical and electrochemical properties of Co3O4 thin films is studied. It is perceived that film thickness increases with increase in substrate temperature reaches maximum at 350°C and decreases thereafter. XRD study confirmed the cubic phase of Co3O4. Porous morphology is seen from SEM. The as deposited Co3O4 at 350ºC saw minimum electrical resistivity of 2.08×103 Ω cm. The films are highly capacitive with maximum specific capacitance of 425 Fg–1 at scan rate of 5 mVs–1. The noteworthy cycle stability of 92.56 % has been observed at a current density of 1 Ag–1 after 1000 charge/discharge cycles.

2. Experimental 2.1 Deposition of FTO substrates The 2M stannic chloride with 20 wt% doping of ammonium fluoride was used for deposition of fluorine doped tin oxide (FTO) glass substrates. For all depositions, 10 cc from stock solution is mixed with 10 cc of propane-2-ol to create the 20 cc final spraying solution. This 20 cc solution is sprayed on preheated substrates at 475°C. These FTO substrates have the sheet resistance of 8‒10 Ωcm‒2. 2.2 Deposition of Co3O4 thin films A mixture of 0.05M cobaltous chloride (CoCl2.6H2O) in a 1:1 (Propan-2-ol+Water) was used as precursor solution. Thoroughly cleaned amorphous glass microslides supplied by Blue Star Corporation and FTO coated glass were used as substrates. Substrates were retained 4

at 30 cm from tip of the nozzle. The computerized spray pyrolysis technique was in employment for the deposition [26]. The deposition temperature is varied at the interval of 25ºC from 300°C to 400°C. 2.3 Characterization of Co3O4 thin films The physical characterization of Co3O4 films is performed with X-ray diffractometer (wavelength λ=0.15406 nm for Cu-Kα radiation). The surface of Co3O4 is analyzed through JEOL-JSM-6360A analytical scanning electron microscope. Optical band gap of Co3O4 thin films is assessed by recording absorption spectra in the wavelength range 350‒1100 nm. The electrical study is performed with the help of a DC two-point probe method. The electrochemical characterization is carried out using electrochemical analyzer supplied by CH Instruments, USA in 2M aqueous KOH electrolyte. Prior to the electrochemical measurements, the electrodes were immersed in the electrolyte for up to 4 hours in order to eliminate the effect of the electrode preparation and obtain more stable values. 3. Results and discussion 3.1 Film formation The Co3O4 nanostructures were deposited at various substrate temperatures. The as deposited Co3O4 thin films appear blackish in color. The possible reaction mechanism can be written as: 3CoCl2 + 2H2O +O2 → Co3O4↓ + 3Cl2↑ + 2H2↑

(1)

The similar type of reaction was also reported earlier by Shinde and coworkers [24] for Co3O4. Film thickness of spray deposited cobalt oxide thin films is computed by gravimetric weight difference method using sensitive precision mass microbalance (LC = 0.01mg). Fig.1 shows the plot of film thickness versus substrate temperature. The lower substrate temperature (below 300°C) is not sufficient to carry out the chemical reaction and results into formation of non-adherent amorphous and powdery films; at lower temperatures the spray droplet hit on the hot substrate removing a lot of heat producing partial decomposition of the starting ingredients. Co3O4 thin films synthesized at and above 300°C are polycrystalline. It is realized that film thickness enhances with rise in substrate temperature from 300°C to 350°C, achieves terminal thickness at 350°C (273 nm), beyond which it drops. This can be described as follows: At 300°C the temperature may not be adequate to decompose the sprayed droplets of cobalt ions resulting in lower thickness. At 350°C decomposition takes place at the optimum rate ensuing in terminal thickness being reached. A clear drop in film thickness with rise in substrate temperature is witnessed after 350°C. This is ascribed to higher evaporation 5

rate of the starting ingredients. Similar type of behavior was earlier reported for spray pyrolyzed SnO2 thin films [27]. The deposited masses for these films are 0.12 mgcm‒2, 0.15 mgcm‒2, 0.17 mgcm‒2, 0.16 mgcm‒2 and 0.14 mgcm‒2 for films deposited at 300°C, 325°C, 350°C, 375°C and 400°C respectively. 3.2 Structural analysis Structural analysis of spray deposited cobalt oxide thin films is accomplished using X-ray diffraction (XRD) with CuKα radiation in the 2θ range between 10° and 90°. Fig. 2 shows the XRD patterns of Co3O4 nanostructures deposited at various substrate temperatures. From figure, it is observed that films are polycrystalline and the diffraction peaks are observed at 2θ angles 19.30º, 31.54º, 37.18º, 45.09º, 59.72º and 65.31º matching with (111), (220), (311), (400), (511) and (440) planes respectively. The comparison of observed and standard ‘d’ values approve cubic phase of Co3O4 with lattice parameters a = b = c = 0.8016 nm, which is marginally less than the standard JCPDS value 0.8084 nm [28]. The XRD shows that the peak (311) intensity grows with rise in substrate temperature up to 350°C, and then falls thereafter. The diminishing crystallinity after 350°C might be due to re-evaporation of the material from the film surface and decrease in the film thickness [29]. It has been reported that the preferred orientation of oxide films is influenced by source compound, solvent and growth parameters such as precursor concentration, spray rate, substrate temperature and carrier gas pressure [30-32]. Crystalline size (D) is estimated using Scherrer’s formula [26], 𝐷=

0.9 𝜆 𝛽·𝑐𝑜𝑠𝛳

(2)

where  is wavelength of X-ray used,  is the full width at half of the peak maximum in radians and  is Bragg’s angle. The crystalline size is calculated for (311) plane. It is seen that the crystalline size enhances with rise in substrate temperature (Table 1) reaches maximum 32 nm at 350ºC. This is due to the fact that the smaller grains tend to have surfaces with sharper convexity and progressively disappear by feeding into larger grains, with rise in substrate temperature. The net result is grain growth. Subsequently, film deposited at 350ºC has a greater crystalline size than other samples [33]. The crystalline size decreases to 21 nm with further increase in substrate temperature up to 400ºC. 3.3 Scanning electron microscopy In order to understand the surface of Co3O4 thin films the scanning electron microscopy analysis is carried out. Fig. 3(a-c) shows the SEM images of Co3O4 thin films deposited at 325ºC, 350°C and 375°C respectively. Inspection of this figure manifests that the 6

deposited Co3O4 shows a porous network as a consequence of the gases escaping during the thermal decomposition process. Co3O4 deposited at 350°C (Fig. 3(b)) shows large number of small pores allowing electrolyte ion enter into Co3O4 electrode effortlessly, which is essential for good performance in energy storage applications. Such porous morphology can potentially enable several applications such as supercapacitors and solar cells in near future. The similar morphology was also observed by Louardi et al. [34] for cobalt oxide thin films.

3.4 Optical properties Optical properties of Co3O4 thin films are tested in the wavelength range 350-1100 nm. The absorption data is analyzed to estimate the values of optical band gap energy of the Co3O4 using the Tauc relation [26]. 𝛼ℎ𝜈 = 𝐴(ℎ𝜈 − 𝐸𝑔 )𝑛

(3)

where A is the constant, Eg is the band gap energy, hν is the incident photon energy, n = ½ or 2 for allowed direct or indirect transitions, respectively. Fig. 4 shows the Tauc plots of (αhν)2 versus (hυ) for Co3O4 nanostructures. Two straight line portions are observed to each plot consistent with two direct allowed transitions. The optical band gaps (Eg) are in the range 1.40-1.45 eV for the lower energy region and 1.72-1.97 eV for the higher energy region, which are consistent with the values described in literature for Co3O4 [35, 36]. These values are given in Table 1.

3.5 Electrical resistivity Fig. 5 shows the variation of logρ with inverse of absolute temperature. The room temperature electrical resistivity is found to be in the range 2.08 × 103 Ω cm to 2.00 × 105 Ω cm for Co3O4 thin films. It is observed the room temperature electrical resistivity drops with a rise in the substrate temperature reaches a minimum at 350ºC and enhances for further rise in substrate temperature. The minimum electrical resistivity 2.08 × 103 Ω cm obtained in present study is better than 2.1 × 104 Ω cm reported by Patil et al. [35] for spray deposited cobalt oxide thin films. The witnessed minimum resistivity at 350ºC is beneficial for the high performance supercapacitor applications [37]. The plots have two regions which can be related to high temperature and low temperature, giving rise to two activation energies. Activation energy is calculated from the Arrhenius relation [26]; E

ρ = ρ0 exp ( a ) kT

(4)

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where, ρ represents the resistivity at temperature T, ρ0 is a constant, k is the Boltzmann constant, T is the absolute temperature and Ea represents the activation energy. The activation energies shrink with the rise in substrate temperature. The substrate temperature has a significant influence on orientation, structure, and stoichiometry of Co3O4 thin films. The crystallinity of the films is enhanced with rise in substrate temperature and the films grown at lower temperatures have a random orientation. The activation energies observed at 350ºC are in the range 0.08-0.15 eV. These values are close to 0.08 eV and 0.06 eV reported by Patil et al. [35] for Co3O4. The values of activation energy are listed in Table 1.

3.6 Electrochemical analysis 3.6.1 Cyclic voltammetry (CV) The CV analysis of Co3O4 nanostructures is accomplished using three electrode systems in 2M aqueous KOH solution at various scan rates from 5 to 100 mVs-1. The potential window is picked from -0.8 to +0.1 V vs Ag/AgCl reference electrode. Fig. 6 shows the CV performance at scan rate of 100 mVs-1 for spray deposited Co3O4 electrodes at various substrate temperatures. The shapes of the CV curves in all of the cases are not rectangular, and can be related to pseudocapacitance rather than the double-layer capacitance. The capacitive behavior of Co3O4 outcomes from the succeeding reactions [38]: Co3O4

+ OH− + H2O

↔

3CoOOH + e−

(5)

CoOOH

+ OH−

↔

CoO2 + H2O + e−

(6)

Specific capacitance ‘Csp’ was calculated from the relation [2, 27]. 𝐶𝑠 =

𝑉𝑐 1 ∫ 𝐼(𝑉)𝑑𝑉 𝑚𝑣(𝑉𝑐 −𝑉𝑎 ) 𝑉𝑎

(7)

where, Cs is the specific capacitance, v is the potential scan rate, (Vc-Va) is an operational potential window, I is the current response of the Co3O4 electrode dipped in 2M aqueous KOH electrolyte and m is mass of Co3O4 on 1cm2 surface. It is observed that the specific capacitance enhances with rise in substrate temperature up to 350°C and diminishes thereafter. This can be accounted with porous nature of Co3O4 thin films at 350ºC, showing more surface area for surface wetting of KOH electrolyte in Co3O4 electrode [39]. The improvement in the charge storage capacity of the Co3O4 electrode at higher substrate temperature may be due to activation of the Co3O4 sites and improved mobility of the electrolyte ions. The maximum value of specific capacitance is found to be 425 Fg–1 at scan rate of 5 mVs–1 for film deposited at 350ºC. This value of specific capacitance is better than the values reported in literature. Kandalkar et al. [20] have reported specific capacitance of 8

165 Fg–1 in 1M KOH for cobalt oxide on the copper substrate and 118 Fg–1 for chemically synthesized cobalt oxide thin films [36]. Shinde et al. [24] have reported a specific capacitance of 74 Fg–1. Wei Du et al. has obtained specific capacitance of 278 Fg–1 for hollow Co3O4 boxes [14]. Fig. 7 shows the variation of the specific capacitance with scan rate for Co3O4 electrodes. It is observed that the specific capacitance diminishes with increase in scan rate due to the presence of inner active sites, which cannot complete redox transitions fully at higher scan rates. At the lower scan rates the OH– has more time to transfer signifying that the more charge can be stored resulting higher specific capacitance [20]. 3.6.2 Galvanostatic charge/discharge To procure more information about supercapacitive performance, galvanostatic charge/discharge measurement is performed in the potential range of -0.8 to +0.1 V for Co3O4 electrode deposited at 350ºC and results are shown in Fig. 8 (a). The specific capacitance (Csp, Fg–1), specific energy (SE, Whkg–1), specific power (SP, kWkg–1) and coulomb efficiency (η%) are calculated by using the formulae [40, 41]. 𝐶𝑠𝑝 = 𝑆𝐸 = 𝑆𝑃 = 𝜂=

𝑡𝑑 𝑡𝑐

𝐼×∆𝑡 𝑚×∆𝑉 1/2𝐶𝑉 2 3.6 3600×𝑆𝐸 𝑡

× 100%

(8) (9) (10) (11)

where I is charge/discharge current at a discharge time t, ΔV is the potential window, ‘m’ is the mass of electrode, tc and td represent the time for charging and discharging, respectively. The specific capacitanÅÅce was 412 Fg–1 at a current density of 1 Ag–1, which is decreased to 383 Fg–1, at the high current density of 4 Ag–1. It is understood that the specific capacitance progressively decreases with the increase in the current density. This may be due to the diffusion limit of the OH− ion movement. This value of specific capacitance is higher than 304 Fg−1 at 0.5 Ag−1 recently reported for Co3O4 nanoparticles [42, 43]. Table 2 represents the specific capacitance, specific energy, specific power and coulomb efficiency, respectively. Coulomb efficiency is key requirement in the assessment of supercapacitors for practical applications. Table 1 gives the coulomb efficiency calculated at 1 Ag–1 for Co3O4 electrodes deposited at various substrate temperatures. It is observed that the coulomb efficiency increases with rise in substrate temperature becomes maximum at 350ºC and decreases thereafter. This can be attributed to enhancement in the crystalline size of Co3O4 9

with substrate temperatures. The higher coulomb efficiency signifies that less energy loss arise during charge/discharge processes for Co3O4 having greater (32 nm) crystalline size. From Table 1, it is discovered that the capacitance is meticulously related with the crystalline size, i.e., the greater the crystalline size, the highest is the specific capacitance. This suggests that the crystalline size should be large enough to allow a high-rate insertion/extraction of electrolyte ions for charge storage [44]. Thus the cell efficiency depends on the crystalline size of Co3O4 used in supercapacitors [45]. Fig. 8(b) shows the long-term charge/discharge cycle stability of the Co3O4 thin film electrode (deposited at 350ºC) at a current density of 1 Ag–1 for 1000 cycles. The specific capacitance maintains 92.56 % of its initial value, showing noteworthy cycle stability. The charge-discharge curves for last 10 cycles are given in the inset of Fig. 8 (b), which shows almost the same symmetric curves. The capacitance loss of 7.44% after 1000 cycles, is very good compared with 13.5% capacitance loss for as-prepared Co(OH)2 electrodes [46]. 3.6.3 Electrochemical impedance spectroscopy To understand the benefits of material, impedance spectra of Co3O4 electrodes were recorded. Impedance experiments were performed after keeping the cells at their open circuit potential for 4 h in 2M aqueous KOH electrolyte in order to eliminate the effect of the electrode preparation and obtain more stable values. In order to ensure the uniformity of testing conditions, the electrodes were cycled galvanostatically for 10 cycles to ensure complete formation of surface films over the electrode particles. Fig. 9 shows the complex plane impedance plots produced from EIS analyses of the Co3O4 electrodes. The equivalent series resistance (ESR) of all the materials is nearly 1Ω, which is the intercept of the plot with the real impedance (Z'), including both the solution resistance (Rs) and the DC resistance [47]. From Fig. 9, Nyquist plots are composed of a semicircle in the high mid frequency region. The high frequency region of the semicircle corresponds to the migration of the electrolyte (KOH) ions through surface film at the electrode/electrolyte interface and the mid frequency range of the semicircle is attributed to the charge transfer kinetics [48]. Since there is no distinct separation in the shape of semicircle between the high and mid frequency regions, the charge transfer kinetics and the migration through the interfacial film cannot be distinguished as two separate processes. Hence, the entire semicircle can be attributed to the resistance to charge transfer reaction, which also includes the migration resistance offered by the interfacial film formed over the electrode surface. The 45° sloped portion of the Nyquist plots in the low frequency is the Warburg resistance resulting from the frequency dependence

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of ion diffusion/transport in the electrolyte. The larger Warburg resistance indicates greater variations in ion diffusion path lengths and increases obstruction of ion movement. The numerical values of the charge transfer resistances (Rct) obtained by fitting the experimental results are given in Table 1. It can be seen that Rct varies with substrate temperature. The variation of Rct confirms a dependency Co3O4 electrode on the substrate temperature that is related to the different crystalline structures obtained at these substrate temperatures and therefore, the semiconductor/electrolyte interface for each substrate temperature is strongly influenced. The Co3O4 deposited at 350°C has resulted into the lowest Rct (Table 1). On the other hand, due to incomplete decomposition, substrate temperature 300°C has shown highest value of the Rct, whereas for 400°C the Rct is high due to decrease in film thickness. In addition, for substrate temperature 300°C, the curve was not completely semicircle; these results strongly suggest that the electrochemical reaction on the surface of such a sample would not be uniform [49]. These results indicate that Co3O4 film deposited at 350°C shows better electrochemical properties. The results are in agreement with the electrical resistivity results, which tell us that the Co3O4 electrode deposited at 350°C possess lowest electrical resistivity than other substrate temperatures. The reduced resistance of Co3O4 electrode deposited at 350°C facilitates the efficient access of electrolyte ions to the surface and shortens the ion diffusion path.

4. Conclusions Co3O4 nanostructures are successfully deposited with computerized spray pyrolysis. The XRD study endows cubic Co3O4 phase with polycrystalline nature and crystallinity enhances with rise in substrate temperature. SEM images show porous morphology. Optical band gap energies are in the range 1.40-1.45 eV and 1.72-1.97 eV for lower energy region and higher energy regions respectively. Cyclic voltammetry and EIS study shows the Co3O4 with improved electrochemical performance. The maximum specific capacitance is found to be 425 Fg–1 at a scan rate of 5 mVs–1 for film deposited at 350ºC. Galvanostatic charge/discharge study endows a maximum specific energy 46.25 Whkg–1, specific power 1.8 kWkg–1 and columbic efficiency 94.87%. The present study suggests that Co3O4 is good candidate for supercapacitor applications.

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Acknowledgements Dr. A. A. Yadav is grateful to the Science and Engineering Research Board, Department of Science and Technology, New Delhi, India for the financial assistance through the Project under the SERC Fast Track Scheme for Young Scientist (File No. SB/FTP/PS- 068/2013).

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16

275

t /nm

250

225

200 300

325

350

375

400

o

Ts/ C Fig.1 Variation of film thickness (t) with substrate temperature (Ts) for spray deposited Co3O4 thin films

17

(440)

(511)

Intensity /a.u.

(400)

(311)

(220)

(111)

JCPDS data card No. 76-1802

o

400 C o

375 C o

350 C o

325 C o

300 C 10

20

30

40

50

60

70

80

90

2/degree Fig. 2 X-ray diffraction patterns of Co3O4 thin films deposited at various substrate temperatures.

18

(a) 325 ºC

(b) 350 ºC

(c) 375 ºC Fig. 3 SEM images of Co3O4 thin films deposited at (a) 325ºC, (b) 350°C and (c) 375°C substrate temperatures respectively.

19

1.2 o

0.8 0.6

300 C o 325 C o 350 C o 375 C o 400 C

10

(h) x 10 / (eV cm )

-1 2

1.0

2

0.4 0.2 0.0 1.0

1.5

2.0

2.5

3.0

h/eV Fig.4 Tauc plots of (αhν)2 versus hν for Co3O4 thin films spray deposited at various substrate temperatures.

20

5.5

o

300 C

5.0

o

400 C o

325 C

Log cm)

4.5 4.0

o

375 C

3.5

o

350 C

3.0 2.5 2.0

2.2

2.4

2.6

2.8

(1000/T)/K

3.0

3.2

3.4

-1

Fig. 5 Variation of logρ versus inverse of absolute temperature for Co3O4 thin films spray deposited at various substrate temperatures.

21

12

j /mAcm

-2

6 0 o

300 C o 325 C o 350 C o 375 C o 400 C

-6 -12 -18

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 E vs (Ag/AgCl)/V

Fig.6 Cyclic voltammograms of spray deposited Co3O4 electrodes in 2M aqueous KOH electrolyte at scan rate of 100 mVs-1.

Specific capacitance /F g-1

400

300

200

300oC 325oC 350oC 375oC 400oC

100

0

0

20

40

60

Scan rate /mV s

80

100

-1

Fig. 7 Variation of specific capacitance with scan rate for spray deposited Co3O4 thin film electrodes at various substrate temperatures in 2M aqueous KOH electrolyte. 22

0.2 -1

1Ag -1 2Ag -1 3Ag -1 4Ag

Potential /V

0.0 -0.2 -0.4 -0.6 -0.8

0

150

300

450

600

750

900

time /s Fig. 8 (a) Galvanostatic charge/discharge curves for Co3O4 thin film electrode (deposited at 350ºC) supercapacitors at different current densities.

23

92.56% retention

400 0.0

300 Potential /V

Specific capacitance /Fg

-1

0.2

200

-0.2 -0.4 -0.6

100

-0.8 752000

754000

756000

758000

760000

time /s

0

0

200

400

600

800

1000

Cycle /number Fig. 8 (b) Long term cycling performance of the Co3O4 thin film electrode (deposited at 350ºC) at the current density of 1 Ag-1. The inset shows the charge/discharge curves of the last 10 cycles of the Co3O4 thin film electrode (deposited at 350ºC).

24

-300 o

300 C o 325 C o 350 C o 375 C o 400 C

-250

Z"

-200

-30

-150

o

300 C o 325 C o 350 C o 375 C o 400 C

-25 -20

Z" 

-100

-15 -10

-50

-5 0

0

0

10

20

30

40

50

Z' /

0

100

200

300

400

500

Z' / Fig. 9 Nyquist plot for Co3O4 electrode in 2M aqueous KOH electrolyte at various substrate temperatures. The inset is the enlarged Nyquist plots in high frequency region,

25

Table 1 Structural, optical, electrical and electrochemical data for Co3O4 thin films spray deposited at various substrate temperatures. Ts; substrate temperature, D; crystalline size, Eg; bandgap energy, Ea; activation energy, LT; low temperature, HT; high temperature, Csp; specific capacitance, η; coulomb efficiency at 1 Ag–1, Rs; solution resistance, Rct; charge transfer resistance Ts/°C

D/nm

Eg /eV I

Csp/F g–1

Ea /eV II

LT

η/%

Rct/Ω cm2

HT

300

20

1.97

1.45

0.15

0.12

260

90.45

30.05

325

29

1.92

1.43

0.13

0.09

325

92.85

18.19

350

32

1.72

1.40

0.12

0.08

425

94.87

10.45

375

28

1.75

1.41

0.13

0.10

343

93.56

14.15

400

21

1.87

1.42

0.14

0.11

283

91.81

22.10

26

Table 2 Various parameters obtained from galvanostatic charge/discharge measurements for Co3O4 thin films spray deposited at 350ºC. tdis/s

Csp/F g–1

SE/Wh kg–1

SP/W kg–1

η/%

CD/ A g–1 1

371

412

46.25

450

94.87

2 3 4

180 118 86

400 394 383

45 44.25 43

900 1350 1800

91.58 87.4 82.69

CD; Current density, tdis; discharge time, SE; specific energy, SP; specific power, η; Coulomb efficiency

27