Hausmannite Manganese oxide cathodes for supercapacitors: Surface Wettability and Electrochemical Properties

Accepted Manuscript Title: Hausmannite Manganese oxide cathodes for supercapacitors: Surface Wettability and Electrochemical Properties Authors: Shrikant Kulkarni, Dhanya Puthussery, Sanam Thakur, Arun Banpurkar, Shankar Patil PII: DOI: Reference:

S0013-4686(17)30204-9 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.165 EA 28832

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

21-11-2016 13-1-2017 25-1-2017

Please cite this article as: Shrikant Kulkarni, Dhanya Puthussery, Sanam Thakur, Arun Banpurkar, Shankar Patil, Hausmannite Manganese oxide cathodes for supercapacitors: Surface Wettability and Electrochemical Properties, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.165 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.

Hausmannite Manganese oxide cathodes for supercapacitors: Surface Wettability and Electrochemical Properties Shrikant Kulkarni,1 Dhanya Puthussery,2 Sanam Thakur,1 Arun Banpurkar,1 Shankar Patil,1* 1

Department of Physics, Savitribai Phule Pune University, Ganeshkhind road, Pune, 411007, India 2

National Chemical Laboratory, Homi J. Bhabha road, Pune 411008, India *Corresponding author: [email protected]

Graphical abstract

Highlights of paper: 1. 2. 3. 4. 5.

Mn3O4 thick films by spray pyrolysis Selection of electrolyte using Contact Angle Measurement Porous morphology of the films Study of two important spray pyrolysis method parameters Structural and morphological stability of the electrodes

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Abstract Mn3O4 thick film electrodes have been synthesized by spray pyrolysis method. Effect of its two important synthesis parameters viz. solution feed rate and substrate temperature has been studied. Solution feed rate was studied in the range of 150 to 250 cc.min-1 while substrate temperature was varied from 325 to 400°C. X-ray diffraction studies confirms Hausmannite crystal structure and SEM studies show spherical grains, uniformly distributed porous microstructure obtained for deposited Mn3O4 thick films. The contact angle measurements show these electrodes have very low contact angle in the range of 2-8° and high surface energy. Cyclic voltammetry study showed specific and interfacial capacitance of Mn3O4 electrodes increased with increase in solution feed rate, due to increased thickness of films. On the other hand, substrate temperature shows effect on film morphology, grain size and porosity, however, does not show any explicit effect on specific and interfacial capacitance. Highest values of specific and interfacial capacitance exhibited by Mn3O4 are 187.79 F.g-1 and 0.82 F.cm-2 respectively for sample prepared at 250 cc.min-1 solution feed rate and 325°C substrate temperature. The calculated energy density and power density for the same sample was 26.08 Wh.kg-1 and 1.01 kW.kg-1 respectively. Keywords: Manganese oxide, spray pyrolysis, surface wettability, cyclic voltammetry, supercapacitors Introduction: Electrochemical supercapacitors (ECS) are one of the most desirable candidates to store energy at high energy density as compared with the conventional capacitors. It has high power storage capacity in the range of thousands of Farads and long operational life compared to conventional batteries [1]. Its capability to deliver instantaneous electrical energy made these devices very useful in several applications, such as, portable electronic 2

devices, military, automobiles, space etc. [2]. A general assembly in electrochemical supercapacitors consists of an anode, which is a metal electrode, such as platinum, silver etc., electrolyte in liquid phase and cathode material in the form of porous oxides on the conducting substrate. This negative electrode governs the performance of the supercapacitor in terms of the charge storage capacity and charge discharge rate [3]. The cathode required to be dielectric having high electrical resistance as well as large specific surface area (SSA) [4]. Therefore, cathode required to be as porous as possible, having optimum cation vacancies and lattice sites allowing faradic reaction to improve charge storage and slow chargedischarge process of supercapacitors. This work is aimed to develop such an efficient electrodes using manganese oxide. There are several materials reported to be potential candidate as ECS cathode materials. Transition metal oxides, such as, RuO2, IrO2, are considered to be the best candidate for these applications. However, they are costly due to rare availability [5]. Therefore efforts are being taken to replace Ruthenium and Iridium by oxides of Titanium (Ti) [6], Nickel (Ni) [7], Cerium (Ce) [8], Tin (Sn) [9], Iron (Fe) [10], Manganese (Mn) [11] etc. Among these, Mn3O4 is one of the promising electrode materials for supercapacitor as it has several advantages over other materials. Manganese salts (chlorides, nitrates, acetates) are very cheap and easily available compared to rare earth salts. Manganese is non-toxic and have ample availability in earth’s crust [12]. Mn3O4 is most stable form of Manganese oxide having good redox performance and theoretical specific capacitance of 1100-1300 F.g-1 [13], [14]. It has high electrical resistivity in the range of 107-108 Ω.cm-1 [15], [16]. The pseudocapacitive charge storage [17], [18] of Mn3O4 in aqueous solution is; 𝑀𝑛𝑂𝑥 (𝑂𝐻)𝑦 + 𝛿𝐻 + + 𝛿𝑒 − ↔ 𝑀𝑛𝑂𝑥 − 𝛿(𝑂𝐻)𝑦+𝛿 3

This implies Mn3O4 possesses intrinsic properties to become promising candidate as cathode in supercapacitor applications. However, due to high resistivity, Mn3O4 shows low charge retention capabilities and faster damping capacitance [19]. To take the advantage of inherent properties of Mn3O4 material need to have an appropriate synthesis method to achieve high specific surface area and appropriate active faradic reaction sites. There are several methods reported in literature by which Mn3O4 has been synthesized viz. microwave assisted hydrothermal synthesis [20], vacuum evaporation [21], Metal Organic Chemical Vapor Deposition (MOCVD) [22], surfactant-assisted dispersion [23], and Pulse Laser Deposition (PLD) [24] etc. Mn3O4 thin films deposited on borosilicate glass substrate using Chemical Bath Deposition (CBD) method shows specific capacitance of 193 F.g-1 in 1 mole Na2SO4 at 10 mV.s-1 voltage scan rate [25]. When Mn3O4 synthesized by Successive Ionic Layer Adsorption and Reaction (SILAR) method on stainless steel substrate, Mn3O4 shows specific capacitance of 314 F.g-1 in 1 mole Na2SO4 at 5 mV.s-1 [26]. Mn3O4 synthesized by hydrothermal method on graphite paper showed maximum specific capacitance of 322 F.g-1 in 1 mole Na2SO4 electrolyte and scan rate of 5 mV.s-1 [27]. However, when Mn3O4 synthesized by hydrothermal method on glass carbon disk and electrolyte was 2 molar KCl, supercapacitor performance was lowered to 148 F.g-1 [28]. Therefore, it is seen that, synthesis method, selection of substrate and electrolyte also played vital role in optimizing performance of Mn3O4. Correct combination of these methods and parameters will definitely improve supercapacitor performance of Mn3O4. There are several preliminary tools to identify these combinations theoretically [29]. Selection of substrate is based on the lattice parameters of the targeted material. Similarly, electrode and electrolyte combination is also important to achieve optimum performance of the electrode materials. The electrode materials required to have lower surface tension and higher surface energy, so electrolyte diffuse through pores and electron storage facilitated on inner active surface of the pores.

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This will enhance the specific surface area of the deposited material and improve charge storage capacity of the cathode. A preliminary tool to identify electrode/electrolyte combination is to evaluate its surface wettability by contact angle measurements. Surface wettability is important property in ECS as the contact between solid electrode and liquid electrolyte decides the surface energy across the interface and hence charge transfer between two. Contact angle (surface wettability) measurement is reliable technique to interpret electrode and electrolyte combination for electrolytic cell [30]. Spray pyrolysis is very simple technique to deposit fine, porous, thin layers of oxide thin films. This method required metal ions precursors to be deposited on hot substrate by sprinkle it through a very fine nozzle with high velocity. These precursors may be acetates, chlorides or nitrates which are very cheap compared to other metal salts. This method is cost effective and very useful for large area and uniform deposition [31], [32]. From brief literature survey, and as far as we know, there is only one article recently published on synthesis of Mn3O4 thin films using spray pyrolysis method [33]. Yadav et al. reported deposition of Hausmannite phase of Mn3O4 using spray pyrolysis method. They have deposited Mn3O4 thin films using 0.8 Mole manganese acetate salt on FTO substrate and studied the supercapacitor properties in the context of substrate deposition temperature in the range of 300 to 375°C. They reports electrochemical properties of these films with reference to Ag/AgCl electrode and 1 Mole Na2SO4 aqueous electrolyte. This study reported for the working potential range from 0.1 to 0.5 V. It was reported that, film deposited at 350°C having maximum specific capacitance 394 F.g-1 at voltage scan rate of 10 mV.s-1. Present paper discusses about synthesis of Mn3O4 using spray pyrolysis method on stainless steel substrate by varying two of important synthesis parameters viz. solution feed rate and substrate deposition temperature.

Effects of these parameters on structural,

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morphological and surface wettability properties of the electrode are explored and its electrochemical properties are studied for supercapacitor applications. Experimental: Mn3O4 electrodes have been synthesized using spray pyrolysis method. A precursor of 1 mole Manganese chloride tetrahydrate [MnCl2.4H2O (Sigma Aldrich 99.99% trace metal basis)] was prepared in freshly prepared DI water (Millipore Elix 3). This solution was added to dispenser syringe to provide precursor solution supply to spray nozzle of locally designed and fabricated spray pyrolysis system. This solution was sprayed at different feed rates ranging from 150 to 250 cc.min-1 at temperature of 350˚C on preheated stainless steel (SS) substrates having dimensions of 25 × 75 mm2 and 1.5 mm thickness. The area of deposition of electrodes was 25 × 60 mm2. The distance between the spray nozzle and substrate was kept constant at 50 cm and nozzle air pressure was 1 kgf.cm-2. The substrate temperature was varied from 325 to 400˚C at the step of 25˚C at constant feed rate of 0.2 cc.min-1 to study its effect on structural and morphological properties of the electrodes. Deposited Mn3O4 electrodes are characterized by X-ray diffraction (Bruker axs D8 Advance) to confirm phase and crystal structure. The microstructure of these electrodes was studied by Scanning Electron Microscopy (SEM: JEOL JSM-6360A). The surface laser micrograph of electrodes were obtained using LASER tomographic thickness measurement unit (Tallysurf CLI 2000, Taylor Hobson, U.K.) to study surface roughness and thickness uniformity of electrodes. Surface wettability of electrodes was studied using contact angle goniometer OCA 20 (DataPhysics, Germany). The contact angle has been determined by sessile drop method and contact angle measured between base line and tangent to drop outline at both ends determined automatically using SCA 20 software. Both OCA and SCA 20 software was from

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DataPhysics Instruments, Germany. The fitting of drop outline was based on Young-Laplace method which is more reliable as it accounts physical properties of droplets. The electrochemical characterization of Mn3O4 electrodes were carried out using a potentiostat (Bio-Logic VMP3). Platinum was used as auxiliary electrode and Hg-HgSO4 was used as reference electrode. Electrolyte used was 1 mole Na2SO4. The working potential range was -0.1 to 0.9 V and scan rate for all measurements was kept at 5 mV.s-1. However, study of scan rate variation was done in the range of 5 to 100 mV.s-1 for sample prepared at 150 cc.min-1 solution feed rate and 325°C substrate temperature. Specific capacitance and interfacial capacitance, energy density, power density and coulombic efficiency were calculated using cyclic voltammetry and charge-discharge characteristics of these Mn3O4 electrodes. The electrochemical behavior of Mn3O4 films also have been studied by Electrochemical Impedance Spectroscopy (EIS) in the frequency range of 1 Hz to 100 kHz by applying 1 V potential and superimposed frequency perturbation of 0.2V. The EIS data was fitted and an equivalent circuit was found using ‘eisanalyser’ software. Results and discussion: Synthesized Mn3O4 thick film electrodes are brownish black in color and showed good adhesion properties when tested with adhesive tape pull taste method. Fig. 1 presents the XRD pattern measured for Mn3O4 electrode prepared at solution feed rate 250 cc.min-1 and substrate temperature 325°C. The diffraction peaks corresponding to Hausmannite crystal structure are present in the XRD pattern. XRD peaks of the pattern matches well with JCPDS data card number 00-024-0734, which is assigned to Hausmannite crystal structure of Mn3O4 [23], [25]. Electrodes crystallized in tetragonal spinel crystal structure with space group I41/amd where, lattice parameter ‘a’ and ‘b’ are 5.76 Å and ‘c’ is found to be 9.46 Å. Miller indices are assigned to each peak in Fig. 1 with reference to JCPDC data. Two prominent

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peaks at 2θ~ 36° and 74° are corresponding to stainless steel substrate (JCPDS card number: 00-033-0397). In the lattice, Mn3+ ions are located at octahedral sites surrounded by six oxygen ions [MnO6] and Mn2+ ions are located at tetrahedral sites surrounded by four oxygen ions [MnO4]. Mn3O4 lattice has at least one vacancy per unit cell due to which large cation vacancies are created at interstitial lattice sites [34]. Fig. 2 shows SEM images of the Mn3O4 electrodes synthesized at different synthesis conditions. Fig. 2 (a) and (b) presents surface microstructure of electrodes at two different solution feed rates 150 and 250 cc.min-1 and 325°C substrate temperature. At lower solution feed rates of 150 cc.min-1, electrode microstructure is observed to be non-uniform in thickness and large voids are observed on the film surface. Small spherical particles with average grain size measured for this sample was 100 nm. LASER surface micrograph in the inset of the Fig. 2 (a) confirmed lower and non-uniform thickness at lower solution feed rate. When solution feed rate increased to 250 cc.min-1, the thickness of electrode was seen to be increased and uniform electrode with smaller spherical grains and evenly distributed pores are observed on the film surface. Porous microstructure has larger surface area that gives more faradaic active sites and certainly higher pseudocapacitance [24]. Films deposited using solution feed rate greater than 250 cc.min-1 and substrate temperature 325°C has higher thicknesses but adhesion of the material with substrate was poor and pull out easily. Grain size was found to be larger in case of 150 cc.min-1 feed rate (160-180 nm) compared to 250 cc.min-1 feed rate sample (120-140 nm). The thickness of the electrodes was in the range of 45±5 micron to 86±5 micron for higher feed rate as shown in Fig. 3. Fig. 2 (c) and (d) shows SEM images for Mn3O4 electrode prepared at 325˚C and 400˚C respectively at solution feed rate of 150 cc.min-1. It is observed that, at lower substrate temperature, continuous film with smaller grain size / pores and uniform thickness was observed. However, when the temperature increased to 400°C, the uniformity of the film 8

disturbed and larger voids are observed over the electrode surface as seen in Fig. 2 (d). The thickness of these films remained almost same for both the samples. The grain size is seen to be slightly increased with substrate temperature. Average grain size measured using ImageJ tool for sample prepared at 325°C substrate temperature was 165 nm and it was 200 nm in case of sample prepared at 400°C substrate temperature. Surface wettability of electrodes with electrolyte is important to find out how much electrolyte penetrate through pores. The surface wettability measured by measuring contact angle of the electrode with electrolyte solution. To determine contact angle of electrode 1 molar Na2SO4 solution was dropped on the electrode surface and contact angle was measured in static mode. Fig. 4 shows, images of contact angle measurements for SS substrate and Mn3O4 electrode prepared at 150 cc.min-1 solution feed rate and substrate temperature 325°C. It was observed that contact angle for SS substrate-electrolyte was 40˚ and in case of Mn3O4-electrolyte contact angle was reduced to 8˚. For rest of samples, measured contact angle of Mn3O4 electrodes-electrolyte was in the range of 2-4˚ showing complete wetting of Na2SO4 electrolyte on Mn3O4 electrode surface [30]. The surface energy calculated using these contact angles is found to be greater than 72 mJ.m-2 for present electrode and electrolyte. This implies good wettability and low surface tension of the samples confirming diffusion of liquid electrolyte in the pores of electrode improving charge transfer across the electrode-electrolyte interface [35]. The spreading coefficient calculated using surface tension and contact angle values for different electrodes was in the range of (-)1.89 to (-)0.029 mN.m1

, which is close to zero. This implies Na2SO4 electrolyte solution completely spread over and

diffused through the pores of the Mn3O4 electrode surface. This shows ability of electrolyte to form electrical double layer or efficient ion transfer even in the pores of the electrodes, resulting in higher charge storage capacity [36].

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Supercapacitor properties of these electrodes were measured using cyclic voltammetry in 1 mole Na2SO4 solution in the potential range of -0.1 to 0.9 V. Fig. 5 shows CV curves measured for samples prepared at 325°C with different solution feed rate ranging from 150 to 250 cc.min-1 with scan rate of 5 mV.sec-1. It was seen that, with increase in solution feed rate, area under the curve increased. Small hump during oxidation and reduction cycle confirms the faradic charge storage due to electron transfer between electrode and electrolyte at interface [37], [38]. Maximum specific capacitance of 187 F.g-1 and interfacial capacitance of 0.82 F.cm-2 was observed in case of sample prepared with 250 cc.min-1 due to optimum porosity, roughness, greater surface area and promising surface wettability of the electrode surface [30], [39]. The energy density, power density and coulomb efficiency calculated was found to be 26.08 Wh.kg-1 and 1.01 kW.kg-1 and 62% respectively. Table 1 summarizes the thickness, contact angle, specific and interfacial capacitance, energy density, power density and coulomb efficiency obtained for samples prepared with different solution feed rates. Fig. 6 presents the effect of voltage scan rate on the cyclic voltammetry of the electrode prepared at 150 cc.min-1 solution feed rate and synthesized at 325°C substrate temperature. It is seen that as the scan rate increased, area under the C-V curves increased. However, due to higher scan rate values compared to area under curve, the capacitance measured using these plots are seen to be decreased with scan rate. Table 2 presents specific and interfacial capacitance of the electrode at different scan rate. It is seen that faradic charge transfer reduces dramatically due to increased equivalent series resistance of the cell with increase in scan rates [40]. Fig. 7 shows the charging and discharging curve for samples prepared using different solution feed rate and 325°C substrate temperature. It is seen that as the solution feed rate increases, due to the porous structure and better contact angle properties, longer charging and discharging cycles are observed. For example, sample prepared with 250 cc.min-1 solution 10

feed rate has longer charging and discharging time compared to other samples. Yang et al. reported such charging and discharging curves are symmetric with each other showing excellent capacitive reversibility of electrodes during electrochemical process [41]. Charging / discharging curves become symmetric, when the electrode material has excellent charge release behavior [42]. However, in our case the charging and discharging cycles are deviated from ideal triangular shape due to higher Ohmic losses and low charge throwing capabilities of Mn3O4 electrode [43]. In Fig. 7, the charging-discharging curves are seen asymmetric and charging time is greater than discharging time. It is seen that, due to higher porosity, the measured resistivity of these electrodes is very high. Due to which, charging/discharging curves are seen highly deviated from triangular shape. The stability of electrode prepared at solution feed rate of 250 cc.min-1 and substrate temperature of 325° C was studied up to 1000 cycles. The plot of specific capacitance and coulomb efficiency with respect to charge discharge cycle number is presented in Fig. 8. It is seen from the figure that, for first cycle specific capacitance was 135 F.g-1 and increased further subsequently up to 250 cycles. After that, specific capacitance starts reducing and touches to 104 F.g-1 at 400th cycle. However, after 400th cycle, electrode regain its charge storage capacity and shows specific capacitance value of 145 F.g-1, which is 7% more compared to its specific capacitance value for first cycle. Similar behavior was observed in case of coulomb efficiency, where, first cycle 50% coulomb efficiency was observed which lowered to 41% at 400th cycle and then regain it to 57% for rest of cycles. Such behavior in case Mn3O4 has been reported previously by Sankar et al. and it is due to structural or morphological modifications in the electrode material during long term charging - discharging processes [44]. Hence, it can be concluded that Mn3O4 is promising electrode material for long term applicability of supercapacitor. EIS studies of the sample prepared with different solution feed rate is presented in Fig. 9. It is observed that there is a linear relation between Z’ and –Z” for all samples in the given 11

frequency range of 1 Hz to 10 kHz. Z’ represents real part of impedance specially resistance, whereas Z” represents imaginary impedance i.e. reactance of the system. The linear relation between these two indicates faradic double layer capacitance at electrode - electrolyte interface, which is a characteristic behavior for supercapacitors [44]. An equivalent circuit is shown in the inset, which best fitted with the experimental data. The equivalent circuit shows series resistance corresponds to solution resistance (Rs) and a parallel combination of resistance (Rct) and constant phase element (Cdl) describing electrode-electrolyte interface. The resistance Rs corresponding to ionic resistance of electrolyte has very small value compared to resistance Rct at the electrode - electrolyte interface. Presence of Constant Phase Element (CPE) instead of capacitor confirms faradic double layer behavior of electrolyteelectrode surface [45]. The values of solution resistance (Rs), ionic charge transfer resistance (Rct) and faraday double layer capacitance (Cdl) were found by fitting of Nyquist plots for all samples. For example, in case of sample prepared at solution feed rate of 250 cc.min-1 and substrate temperature 325°C, these values of Rs, Rct and Cdl are 9.1 Ω, 37.9 kΩ and 96 µF respectively with exponent n=0.8. Cyclic voltammetry studies are also carried out for Mn3O4 samples prepared at different deposition temperatures at solution feed rate of 150 cc.min-1. Fig. 10 (a) presents cyclic voltammetry measured in the potential range of -0.1 to 0.9 V. However, it is seen that, no definite variation in the specific and interfacial capacitance was observed. Maximum capacitance observed at lower substrate temperature was attributed to appropriate porosity, larger specific surface area and wettability of the sample [46]. As the substrate temperature increases the grain size was seen to be increased and samples become denser lowering pore volume [47]. Similar effect was observed in charging discharging characteristics of these electrodes. Fig. 10 (b) presents charging discharging characteristics of these electrodes measured in the potential range of -0.1 to 0.9 V. Longer charging and discharging cycle was 12

observed in case of substrate temperature 325˚C. Charging time in this case was 95 seconds and discharging time was 56 seconds. It is seen that, for all samples charging time is higher than discharging time. Calculated specific and interfacial capacitances, energy density, power density and coulomb efficiency for these samples are tabulated in Table 3. Highest energy density of 6.07 Wh.kg-1 was observed for sample prepared at 325°C and solution feed rate of 150 cc.min-1. On other hand, highest power density observed in case of sample prepared at 400°C temperature due to high rate of charge-discharge process [48]. To summarize, the present study of electrodes, Mn3O4 has been synthesized successfully by spray pyrolysis method. It was observed that spray pyrolysis is one of the simple, low cost and feasible methods to synthesize porous electrochemical electrodes for supercapacitor applications. The morphology of the electrodes can be controlled easily using two of synthesis parameters i.e. solution feed rate and substrate temperature. By optimizing these parameters, interfacial capacitance as well as charging discharging properties of cathode can be improved. In the present study, it was seen that surface microstructure and contact angle measurement are efficiently used as preliminary tools to tailor and predict supercapacitor properties. These supercapacitor values obtained in this study are comparable to literature values for Mn3O4 electrode, however, there is lot of scope to improve surface microstructure properties of these electrodes. Conclusion Mn3O4 electrodes are synthesized by spray pyrolysis method. Effect of solution feed rate and substrate temperature on microstructure and contact angle of electrodes were studied. Electrical properties of these electrodes were tested using cyclic voltammetry to measure its charge storage capacity. Increase in solution feed rate increases thickness of electrodes and its contact angle lowered below 3˚ showing super hydrophilicity of the sample. The optimum

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specific capacitance values of 187.79 F.g-1 and interfacial capacitance of 0.82 F.cm-2 for sample prepared at higher solution feed rate of 250 cc.min-1 and substrate temperature of 325°C. The energy density and power density calculated for this sample was found to be 26.08 Wh.kg-1 and 1.01 kW.kg-1 respectively. Substrate temperature variation does not showed any specific variation on capacitance values, however, slower charging/discharging was observed in case of lower substrate temperature of 325˚C. Acknowledgement One of the authors, Shrikant Kulkarni is thankful to University Grant Commission (UGC), INDIA for providing research grant and financial support for research through D. S. Kothari post-doctoral fellowship [Award letter no.: F.4-2/2006 (BSR) / PH/ 14-15 / 0128]. He is also thankful to Prof. S. B. Ogale for providing necessary facilities to carry out this research work. References: [1] M. Beidaghi, Y. Gogotsi, Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors, Energy Environ. Sci., 7 (2014) 867-884. [2] J. Zhang, J. Jiang, H. Li, X. S. Zhao, A high-performance asymmetric supercapacitor fabricated with graphene-based electrodes, Energy Environ. Sci., 4 (2011) 4009-4015. [3] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li, L. Zhang, Progress of electrochemical capacitor electrode materials: A review, Int. J. Hydrogen Energ., 34 (2009) 4889-4899. [4] H. Wei, H. Gu, J. Guo, S. Wei, Z. Guo, Electropolymerized Polyaniline Nanocomposites from Multi-Walled Carbon Nanotubes with Tuned Surface Functionalities for Electrochemical Energy Storage, J. Electrochem. Soc., 160 (2013) G3038-G3045 14

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20

List of figures: Fig. 1

XRD pattern of Mn3O4 electrodes deposited on stainless steel substrate by spray

pyrolysis method

Fig. 2

SEM images showing surface microstructure of spray pyrolysis derived

electrodes taken at 25 kx (a) 150 cc.min-1 and (b) 250 cc.min-1 solution feed rate and substrate temperature (c) 325˚C and (d) 400˚C. Inset of Fig. (a) and (d) shows LASER surface micrograph of corresponding electrodes.

21

22

23

Fig. 3

Effect of solution feed rate on thickness of film.

24

Fig. 4

Images showing contact angle measurement of (a) substrate and (b) Mn3O4 thin

film with 1mole Na2SO4 electrolyte solution

25

26

Fig. 5

Cyclic voltammetry measured using cyclic voltammetry in the potential range

of -0.1 to 0.9V for electrodes deposited at various solution feed rates and 325°C substrate temperature

Fig. 6

Cyclic voltammetry measured at different scan rate for sample prepared at 150

cc.min-1 and 325°C substrate temperature

27

Fig. 7

Charging-discharging characteristics of the electrodes deposited at various

solution feed rate ranging from 150 to 250 cc.min-1 and substrate temperature 325°C

28

Fig. 8

Effect of 1000 charge discharge cycles on specific capacitance and Coulombic

efficiency of Mn3O4 electrodes prepared at 250 cc.min-1 solution feed rate and 325°C substrate temperature in 1 mole Na2SO4 electrolyte

29

Fig. 9

Nyquist plots measured in the frequency range of 1 Hz to 10 kHz along with

data fitting for Mn3O4 electrodes prepared at different solution feed rate ranging from 150 cc.min-1 to 250 cc.min-1

30

Fig. 10

Effect of substrate temperature on (a) cyclic voltammetry and (b) charging

discharging cycles of electrodes deposited at various substrate temperatures ranging from 325˚C to 400˚C and solution feed rate of 150 cc.min-1

31

32

33

Table 1: Thickness, contact angle, specific and interfacial capacitance measured for samples prepared at different solution feed rate and 325°C substrate temperature Solution Coulomb Contact Specific Interfacial feed Thickness Energy Power efficiency angle capacitan capacitance rate (μm) density density (%) (±1˚) ce (F.g-1) (F.cm-2) -1 -1 -1 (cc.m ) (Wh.kg ) (kW.kg ) 150

45±5

8

40.68

0.21

5.65

0.35

91.15

175

63±5

2-3

77.95

0.41

10.83

0.54

78.53

200

76±5

2-3

108.50

0.62

15.07

0.99

90.42

250

86±5

1-2

187.79

0.82

26.08

1.01

62.40

34

Table 2: Specific and interfacial capacitance measured at different scan rate for electrode sample prepared at 150 cc.min-1 solution feed rate and 325°C substrate temperature Interfacial capacitance Scan rate (mV.s-1) Specific capacitance (F.g-1) (F.cm-2) 5

40.68

0.21

10

26.49

0.12

25

16.35

0.07

50

10.92

0.05

100

6.77

0.03

35

Table 3: Specific and interfacial capacitance, charging/discharging time measured for samples prepared at different substrate temperature and solution feed rate of 150 cc.min-1 Average Substrate Specific Interfacial Average Energy Power Coulomb Dischargin temperatu capacitan capacitanc Charging density density efficiency g time re (˚C) ce (F.g-1) e (F.cm-2) time (sec) (Wh.kg-1) (kW.kg-1) (%) (sec) 325

43.67

0.26

91

57

6.07

0.16

62.76

350

31.75

0.27

59

42

4.41

0.19

70.65

375

31.19

0.18

53

40

4.33

0.19

75.28

400

37.83

0. 21

45

32

5.25

0.28

70.49

36