CHAPTER III Synthesis and Characterization Nanostructured RuO2 Thin Films Using Chemical Methods

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1 CHAPTER III Synthesis and Characterization of Nanostructured RuO 2 Thin Films Using Chemical Methods (A Bottom-Up Approach) and Their Supercapacitor Behavior 101

2 3. Introduction The literature survey on RuO 2 shows that, it is most important material used as a catalyst and electrode in supercapacitors [1]. Both hydrous and anhydrous ruthenium oxide have been reported to be promising electrode materials for supercapacitors due to their intrinsically high pseudocapacitance. Commonly, anhydrous ruthenium oxide with a crystalline structure (rutile) is fabricated by thermal decomposition from its metal chloride and its specific capacitance was found to be much lower. Consequently, hydrous ruthenium oxide (denoted as RuOx.nH 2 O) with amorphous structure is considered to be a potential material for use in supercapacitors [2]. Hydrous ruthenium oxide can be prepared by a sol-gel process from its chloride precursor in alkaline media or by electrochemical methods via polarizing the pure bulk metal to positive potentials [3-5]. On the other hand sol-gel process steps are very complicated and not easily controlled and in electrodeposition there is necessity of source for the generation of electricity to deposit the thin films. Thus, it is more desirable to develop new methods to replace the sol-gel and electrodeposition process for preparing RuO 2.xH 2 O for application in supercapacitors. So the high production cost of the supercapacitor requires a search in order to look for cheaper methods. Current research is focused in order to improve the electrochemical performance of RuO 2 electrodes in the devices, by designing the morphology. Chemical methods have been well developed to fabricate largearea metal oxide thin films. However, little work has been done to prepare nanoporous hydrous RuO 2 films by M-CBD and liquid phase growth method [6, 7]. 102

3 Section I Synthesis and characterization of nanocrystalline RuO 2 thin films using chemical methods (A bottom-up approach) 3.A Synthesis and characterization of nanostructured RuO 2 thin films using CBD method The development of the chemical bath deposition (CBD) method has gained lot of attention for the last few years as a thin film preparation method especially for metal chalcogenides/oxide compounds. Several investigations have been carried out in which the preparation and properties of transition metal oxide films are described. In some case, metal hydroxide thin films are deposited and subsequently converted into an oxide film. The deposition medium for the material consists of one or more metal salts, a source of oxygen with/without complexing agent. The deposit is a metal hydroxide or hydrated oxide which can be formed by reaction of the metal ions with slowly generated hydroxyl ions. In addition, the homogeneity and stoichiometry of the product are maintained by the solubility product of the material. CBD is a simple and cost effective method for preparation of thin films. The film thickness and deposition rate can be controlled by varying the solution ph, temperature and reagent concentration. The CBD also allied to coat the large areas in a reproducible and low cost process. Hence hydrous ruthenium oxide thin films were synthesized by low-cost and most feasible chemical bath (CBD) method. 3.A.1 Experimental details 3.A.1.1 Substrate cleaning Substrate cleaning plays an important role in the deposition of thin films. Extreme cleanness of the substrate is required for the deposition as the contaminated substrate surface provides nucleation sites facilitating growth resulting non-uniform films. Glass microslides and stainless steel used as substrates. 103

4 (a) The microslides by "Blue star" of the dimensions 75 mm X 25 mm X 1.35 mm have been used as substrates. The following procedure has been adopted for cleaning of glass substrate. 1 The micro slides are washed with double distilled water, 2 Then washed with concentrated chromic acid 0.5 M for 1 h and kept in it for 24 h, 3 The substrate were washed with double distilled water, 4 Then the substrates were ultrasonically cleaned for 10 min. and 5 Finally, the substrates were dried, degreased in AR grade acetone and were used for deposition. (b) To investigate supercapacitive performance of thin film electrode, electrically conducting substrate is the necessary requirement. Stainless steel was used as conducting substrates. Stainless steel substrates were cleaned according to following procedure. The substrates were mirror polished using zero grade polish paper. 1. The substrates were washed with detergent and double distilled water, 2. Then, the substrates were ultrasonically cleaned for 15 min, and 3. Finally the substrate were dried by air and used for the deposition. 3.A.1.2 Experimental setup for deposition of RuO 2 films by CBD method Fig. 3.1 shows the schematic diagram of CBD method for the deposition of RuO 2. Preparation of RuO 2 thin film by CBD method is based on the heating of an acidic bath of ruthenium (III) chloride containing the substrates immersed vertically in the solution. Acidic bath was prepared by using 0.01 M RuCl 3.xH 2 O as a source of ruthenium and 0.1 M ammonium chloride (NH 4 Cl). The ph of the resultant solution was 2.0. The glass microslide and stainless steel were used as substrates. These substrates were immersed in the bath and the bath was heated. When the bath attained the temperature of 333 K, the precipitation (blakish in color) started in the bath. During the precipitation, heterogeneous reaction occurred and deposition of 104

5 RuO 2 took place on the substrate. Preparative parameters are given in the table 3.1. Table 3.1 Preparative parameters for the deposition of RuO 2 using CBD method. Precursor 0.01 M RuCl 3, xh 2 O Complexing agent 0.1 M NH 4 Cl Deposition temperature 333 K Deposition time 2 h. Substrates Glass and stainless steel Fig. 3.1 Schematic experimental set up of CBD method for deposition of RuO 2 thin films 3.A.1.3 Characterization techniques The thickness of RuO 2 films was measured by weight difference method using a sensitive microbalance. The structure of RuO 2 thin films 105

6 identified by X-ray diffraction analysis were performed on a Philips (PW 3710) diffractometer with copper target (λ = Å). The FT-IR spectra of the samples were collected using a Perkin Elmer, FT-IR Spectrum one unit. Microstructural study was carried out using scanning electron microscopy (SEM-JEOL 6360). The optical properties were investigated within the wavelength range nm using a systronics spetrophotometer-119. To determine the electrical resistivity, d. c. two-point probe method was used. Contact angle measurements of as-deposited films were carried out by sessile drop method, in which water drop was observed through a microscope coupled goniometer (Phoenix 150, Surface Electro Optics, Korea). 3.A.2 Results and discussion 3.A.2.1 Growth mechanism In CBD method, film formation is observed when the solution is saturated. When the ionic product of anion and cation exceeds the solubility product of metal oxide, precipitation occurs and ions combine on the substrate and in the solution to form nuclei. The film growth can take place by ion-by-ion condensation (homogenous nucleation) of materials or by adsorption of colloidal particles (heterogeneous nucleation) from the solution on the substrate. The formation of solid phase from a solution involves two steps as nucleation and particle growth. The formation of nucleation is necessary for a precipitate formation. The concept of nucleation in the solution is that the clusters of molecules formed undergo rapid decomposition and particles combine to grown up to a certain thickness of the film. Generally, metal ions are complexes in such a way that reaction takes place between slowly released metal ions to form product in thin film. The RuO 2 thin films have been deposited on glass and steel substrates by slow hydrolysis of ruthenium chloride solution. This can be represented as follows: 106

7 2RuCl + 3H O 2Ru(OH) + 2HCl + H Ru(OH) + NH Cl (NH )Ru(OH) + HCl (NH )Ru(OH) + 6OH RuO + NH OH + 6H O + H A.2.2 Thickness measurement Variation of the weight of RuO 2 thin film deposited on the substrates with the deposition time is shown in Fig. 3.2 (a) The thickness in terms of weight of RuO 2 film deposited depends required deposition time. The terminal thickness, at which the highest amounts of RuO 2 were deposited on the glass substrate and steel substrate, was and mg.cm -2 after 2 hours. Different growth kinetics of RuO 2 was observed on to the different kind of substrates. Fig. 3.2 (b) shows photographs of the RuO 2 deposited on the glass and stainless steel substrate. Fig. 3.2 (a) Variation of RuO 2 film thickness with deposition time and (b) shows the photograph of the RuO 2 thin films on stainless steel and glass substrates. 3.A.2.3 Structural characterization Film crystallinity was analyzed using X-ray diffraction. The XRD patterns of RuO 2 films onto the glass substrate and stainless steel substrate are shown in fig 3.3 (a) and (b). XRD patterns of RuO 2 thin films consist of 107

8 broad hump (on glass substrate) and no well-defined diffraction peaks other than stainless steel (on steel substrate), indicating that ruthenium oxide film is amorphous. Practically no difference than amorphous structure is seen in RuO 2 thin films on glass as well as on stainless steel substrates. The amorphous phase is obtained which is feasible for supercapacitor application, since the protons can easily permeate through the bulk of the amorphous RuO 2 electrode and whole amount of electrode is utilized for energy storage [6]. Intensity (A.U.) 100 (a) θ (deg.) Intensity (A.U) Peak corrosponds to stainless steel (b) θ (deg.) Fig. 3.3 X-ray diffractograms of RuO 2 thin films on (a) glass and (b) stainless steel substrate. 3.A.2.4 FT-IR study The FT-IR absorption spectrum of as-deposited RuO 2 thin films in the range cm 1 is shown in fig From these spectra, it can be observed apparently that strong band (ν 1 ) around at 600 cm 1 is associated with the characteristic vibrational mode of rutile RuO 2 [8]. The absorption peak (ν 2 ) at around 1095 cm -1 is assigned to characteristic stretching vibration of peroxo groups. The absorption peak around at 1604 cm 1 may be due to the bending vibration of hydroxyl groups of molecular water (ν 3 ).The sharp absorption band at ν 4 : 3406 cm 1 is attributed to the stretching vibration stretching vibrations of OH -. Shoulder of the ν 4 bond 108

9 attributed to the hydrous nature of the RuO 2 material. This result indicated that, as deposited film contained hydroxide and other bonds, which indicates that formation of hydrous ruthenium oxide that may play important role in capacitive behavior [9]. Fig. 3.4 FT-IR spectrum of RuO 2 deposited by CBD method. 3.A.2.5 Surface morphology Fig. 3.5 shows the SEM images of ruthenium oxide thin film prepared by CBD method. At X 2,000 magnification, a spherical grained particle of RuO 2 is well seen, whereas, at higher magnification (X 10,000) a porous microstructure of ruthenium oxide is observed. The ruthenium oxide thin film had porous microstructures with fractal-like agglomerates of fine particles. 109

10 Fig. 3.5 SEM micrographs of RuO 2 thin films at (a) (X2,000) and (b) (X10,000) magnifications. Patake et al. and Lee et al. have been reported the compact structure of RuO 2 synthesized by liquid phase and M-CBD method [6, 7]. Such type of porous morphology will be helpful in the application of supercapcitor. 3.A.2.6 Optical absorption study The optical absorption spectrum of RuO 2 film deposited onto glass substrate was studied in nm wavelength range. The nature of the transition involved (direct or indirect) during the absorption process was determined by studying the dependence of absorption coefficient, (α) on photon energy (hν). The absorption spectrum of RuO 2 thin film is shown in inset of fig The spectrum revealed that RuO 2 film has high absorbance (10 5 cm -1 ). The optical data was further analyzed to determine the nature of transition that takes place in RuO 2 thin film. The plot of (αhν) 2 versus hν showed a straight line indicating a direct band gap, and is depicted in fig The straight-line portion was extrapolated to the energy axis to obtain the band gap of the RuO 2 thin film. The direct optical band gap of RuO 2 thin film was estimated to be 2.7 ev. The optical band gap of RuO 2, 2.4 and 2.2 ev was reported for spray pyrolysis and M-CBD methods [6, 10]. 110

11 Fig. 3.6 Plot of (αhν) 2 absorption (αt) with wavelength (λ) of RuO 2 film. vs. energy hν of RuO 2 film. Inset shows variation of 3.A.2.7 Electrical resistivity Fig. 3.7 shows the variation of log ρ with reciprocal of temperature for the RuO 2 thin films on glass substrate. The resistivity at room temperature (300K) was found to be 10 3 Ω-cm for RuO 2 thin films. The RuO 2 film resistivity decreased with increase in temperature indicating a semiconducting electrical behavior, while the metallic behavior was observed for metal organic chemically deposited (MOCVD) RuO 2 film [11]. Two trends of resistivity was observed in the graph at lower temperature (<420 K) region and at high temperature region (>420 K), corresponding activation energy for low temperature region is Ea=0.11 ev and that for high temperature region Ea = 0.98 ev. Change in the graphical trend attributed to the removal of hyroxyl species in the temperature regime I. Patake et al. reported similar semiconducting behavior of M-CBD deposited RuO 2 film. 111

12 log 'ρ' (Ω.cm) Regime I Regime II /T (K -1 ) Fig. 3.7 The variation of dark electrical resistivity (log ρ) with temperature of RuO 2 films. 3.A.2.8 Water contact angle measurement The wetting of solid with water, where air is the surrounding medium, is dependent on the relation between the interfacial tensions (water/air, water/solid and solid/air). The ratio between these tensions determines the contact angle (θ) between a water droplet on a given surface. A contact angle of 0 0 means complete wetting and a contact angle of corresponds to complete non-wetting. Both super-hydrophilic and super-hydrophobic surfaces are important for practical applications. In the present case, the water laid flat with contact angle of 10 0 on the surface of RuO 2 film in sheets instead of forming droplet as seen in fig This specific property is attributed to amorphous or nanocrystalline nature that is expected to possess very high surface energy, which increases with reduction in particle size. Direct relationship of surface energy with water surface tension is wellknown. High wettability gives rise to small water contact angle which has direct surface tension relation. It is reported that nanocrystalline thin films show high surface energy due to the presence of uniform and non-uniform strains [12]. Superhydrophilic nature of the RuO 2 might be due to the 112

13 hydrous nature of the films. Hydrous bonding on the substrate surface also causes the superhydrophilic nature [13]. Fig. 3.8 Measurement of water contact angle of RuO 2 thin films. 3.A.3 Summary and conclusions In conclusion, the amorphous hydrous ruthenium oxide (RuO 2 ) thin films have been successfully synthesized at low temperature on glass and stainless steel substrates using CBD method. Porous and spherical grained morphology was observed for CBD deposited RuO 2 thin films. The optical studies showed a direct band gap of 2.7 ev for hydrous RuO 2 thin films. The RuO 2 film shows semiconducting nature with 10 3 Ω.cm room temperature resistivity. 3.B Synthesis of nanostructured RuO 2 thin films using SILAR method Ristov et al. for the first time demonstrates the SILAR method for metal oxide thin film. The procedure of deposition of films involved the successive immersion of substrate in alkaline metal salt bath at room temperature and the distilled/deionized water maintained at high temperature ( K). The SILAR method was adopted for the deposition of binary/mixed metal oxides. The present section deals with the preparation of ruthenium oxide films employing SILAR method and studies of their structural, surface 113

14 morphological, electrical, surface wettability and optical properties. Different preparative parameters, such as concentration of bath, ph, adsorption and reaction periods, number of immersion cycles etc. were optimized to get feasible films for supercapacitor. 3.B.1 Experimental details 3.B.1.1 Substrate cleaning The glass microslide and stainless steel substrates cleaned by employing the procedure as described in section 3.A B.1.2 Experimental setup for deposition of RuO 2 films by SILAR method The RuO 2 thin films were grown on glass and stainless steel substrates at low temperature using SILAR method, which is based on the immersion of substrate in separate precursor solutions and rinsing it in distilled water. The aqueous RuCl 3 :xh 2 O (0.01 M) at ph 1 adjusted by using 0.1M NH 4 Cl, was used as cationic precursor kept at room temperature (300 K). The anionic precursor was double distilled water at 333 K temperature. For the deposition of RuO 2 thin film, the substrate was immersed in the cationic precursor for 20 s to adsorb ruthenium species on the substrate. Then, substrate was immersed in double distilled water kept at 333K for 30 s to react with the OH - ions. This completed 1 cycle for the formation of RuO 2. By repeating such deposition cycles, RuO 2 films were prepared up to 100 cycles. The growth kinetic was different on different kind substrates. Preparative parameters are given in the table 3.2. Fig. 3.9 shows the schematic representation of SILAR method for RuO 2 films by two beakers system. 114

15 Table 3.2 Preparative parameters for the deposition of RuO 2 using SILAR method. Cationic Precursor 0.01M RuCl 3. xh 2 O + 0.1M NH 4 Cl Anionic Precursor Double distilled water kept at 333K Adsorption Time 20 s Reaction Time 30 s No. of cycles 100 cycles Substrates Glass and stainless steel Fig. 3.9 Schematic experimental set up of SILAR method for deposition of RuO 2 thin films 3.B.1.3 Characterization techniques The SILAR deposited RuO 2 films were characterized for structural, surface morphological, electrical, optical and wettablity properties using the techniques described in section 3.A

16 3.B.2 Results and discussion 3.B.2.1 Growth mechanism In the present case, the precursor ruthenium (III) chloride dissolved in double distilled water forms Ru 3+ cations. These cations get complexed with NH 4. Complexed Ruthenium ions get adsorbed on the immersed substrate due to the attractive forces between ions in the solution and surface of the substrate. The forces of attraction may be cohesive forces or Van der Walls forces or chemically attractive forces. The substrate is then immersed in double distilled water (333 K) where the OH - anions combine to form ruthenium complexed ions (Ru(NH 3 ) n 3+ ). Further the excess anions reacting with ruthenium complex convert it into hydrous ruthenium oxide, which can be represented as, 2RuCl 2Ru + 3Cl NH 4Cl NH4 + Cl 3.5 Ru + nnh Ru(NH ) n Ru(NH ) + 6OH RuO.2H O + NH OH n In this process, the ammonium chloride is introduced as a complexing agent and exerts itself to control the release of Ru 3+ ions for deposition of hydrous RuO 2 thin films. 3.B.2.2 Thickness measurement Variation of the weight of RuO 2 thin film deposited on the substrate with the number of immersion cycles is shown in fig (a). The thickness in terms of weight of RuO 2 film deposited depends on the adsorption and reaction time periods. Moreover, the partial dissolution of the already deposited film in the ruthenium complex solution at the beginning of each deposition cycle, due to NH 4 Cl addition in the ruthenium (III) chloride solution, limits the deposition rate. Here, for deposition of RuO 2 films, the adsorption period of 20 s and the reaction period was 30 s. It was observed that the RuO 2 film thickness increased with an increase in the number of 116

17 immersion cycles. The terminated thickness, at which the highest amount of RuO 2 is deposited on the glass substrate and steel substrate, is and mg.cm -2 after 120 cycles. Different weights of RuO 2 on to the different kind of substrates, attributed to the different growth kinetics on different substrates. Fig (b) shows the photograph of the RuO 2 thin films on stainless steel and glass microslide substrates. Weight deposited (mg.cm -2 ) On glass On stainless steel Deposition cycles (a) Fig (a) Variation of RuO 2 film thickness with deposition cycles and (b) shows the photographs of the RuO 2 thin films on stainless steel and glass substrates. 3.B.2.3 Structural characterization Fig (a) and (b) shows the X-ray diffraction patterns of RuO 2 thin films grown on the glass and stainless steel substrate. It consists of broad hump (on glass substrate) and no well-defined diffraction peaks (on steel substrate). The XRD patterns reveal that formation of amorphous ruthenium oxide film on the both kind of substrate. This indicates that an amorphous phase formation can take place by SILAR method. The amorphous nature of RuO 2 material observed may be due to the formation of hydrous nature. Many researchers reported that, such kind of amorphous hydrous nature of RuO 2 is helpful in supercapacitor application [6]. 117

18 Intensity (A.U.) 80 (a) θ (deg.) Intensity (A.U.) 80 (b) 70 * 60 * * θ (deg.) Fig X-ray diffractograms of RuO 2 thin films on (a) glass and (b) stainless steel substrates. 3.B.2.4 FT-IR study The FT-IR absorption spectrum of as-deposited RuO 2 thin films in the range cm 1 is shown in fig The four sharp absorption bands at ν 1 : 710, ν 2 : 1009, ν 3 : 1601 and ν 4 : 3406 cm 1 are attributed to the stretching vibration of RuOx in rutile, peroxo group, vibration of hydroxyl groups of molecular water [14], and stretching vibrations of OH - [15], respectively. Shoulder of the ν 4 bond attributed to the hydrous nature of the RuO 2 material. This result indicated that, as deposited film contained hydroxide and other bonds, which indicates that formation of hydrous ruthenium oxide is the required key for pseudocapacitive behavior. 118

19 45 Transmittance (T%) ν 4 =3406 cm -1 ν 3 =1601 cm -1 ν 2 =1009 cm -1 ν 1 =710 cm Wavenumber (cm -1 ) Fig FT-IR spectrum of RuO 2 deposited by SILAR method. 3.B.2.5 Surface morphology The two-dimensional surface morphological study of the RuO 2 thin film has been carried out from SEM image. Fig shows the SEM image of RuO 2 thin film on the glass substrate at different magnifications X 5,000 and X 10,000. The morphology showed that the substrate is well covered with RuO 2 material. From the fig (a), one can see the cracks on the film surface. At higher magnification smooth and fine features of RuO 2 material is observed (fig (b)). A smooth surface with fine features frequently indicates as amorphous structure. 119

20 Fig The SEM micrographs of RuO 2 thin films at (a) X5,000 and (b) X10,000 magnifications. 3.B.2.6 Optical absorption study Inset of fig.3.14 shows the variation of optical absorbance (αt) RuO 2 thin films with wavelength (λ). The film has high absorbance in the visible range, indicating the applicability as an absorbing material. The variation of (αhν) 2 versus hν, as a straight line in the domain of higher energies is shown in fig. 3.14, indicate the presence of the direct optical transition having order of the 10 4 cm -1. From the graph, for ruthenium oxide film, the band gap value of 2.08 ev was obtained. 120

21 Fig Plot of (αhν) 2 vs. energy hν of RuO 2 film. Inset shows variation of absorption (αt) with wavelength (λ) of RuO 2 film on glass substrate. 3.B.2.7 Electrical resistivity The RuO 2 film on glass substrate used to determine the electrical behavior of the films. The D.C. two-probe method used to determine the electrical resistivity of the RuO 2 films. Fig shows the variation of electrical resistivity with temperature. The RuO 2 film resistivity decreased with increase in temperature indicating a semiconducting electrical behavior. In the temperature regime K and K two distinct trends are observed. Room temperature resistivity found to be 10-1 Ω.cm attributed to the quasy-metalic nature of the RuO 2 films. 121

22 Fig The variation of dark electrical resistivity (log ρ) with temperature of RuO 2 film. 3.B.2.8 Contact angle measurement Fig shows, the water laid flat with contact angle of 9 0 on the surface of RuO 2 film in sheets instead of forming droplet as seen in. This specific property is attributed to amorphous or nanocrystalline nature that is expected to possess very high surface energy, which increases with reduction in particle size. Superhydrophilic nature of the RuO 2 might be due to the hydrous nature of the films. Hydrous bonding on the substrate surface also causes the superhydrophilic nature. Fig Measurement of water contact angle of RuO 2 thin film. 122

23 3.B.3 Summary and conclusions In conclusion, the amorphous hydrous ruthenium oxide thin films have been successfully synthesized at low temperature on glass and stainless steel substrates using SILAR method. Smooth and fine features of RuO 2 material is observed for SILAR deposited thin films. The optical studies showed a direct band gap of 2.08 ev for hydrous RuO 2 thin films. The resistivity studies showed the semiconducting behavior having quassimetalic electrical conductivity. The SILAR method is useful for preparation of amorphous, hydrous RuO 2 electrodes. In the synthesis of amorphous, hydrous, quassi-metalic RuO 2 thin film various methods have been reported including electrodeposition, solgel, combustion synthesis, spray pyrolysis, etc. These synthesis methods restrict to large area deposition. Spray pyrolysis deposition technique unable to produce hydrous nature of RuO 2, while electrodeposition is limited for conducting substrate. On the other hand CBD and SILAR methods are consent to large area deposition of hydrous, amorphous, quassi-metalic RuO 2 thin films. 123

24 Section II Supercapacitive performance of the RuO 2 thin films 3.C Introduction Trasatti and Buzzanca was traced the concept and use of RuO 2 as a supercapacitor material in 1971 [16, 17]. Recent researchers focused on the studies of hydrous ruthenium oxide in the amorphous form (denoted as RuOx.nH 2 O). A maximum specific capacitance of 768 F/g has been obtained from an amorphous, hydrous ruthenium oxide prepared by sol-gel method [18]. The rapid release of protons and electrons in the RuO 2 leads to large pseudocapacitance, which in combination with a high specific surface area, results in the enhancement of charge storage by the material [19]. This pseudocapacitance is enhanced by the presence of structural water in the lattice, which provides nanostructured percolation pathways for proton conduction into the bulk of the material [20-23]. 3.C.1 Experimental The simple chemical methods chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) methods were employed to prepare the amorphous, hydrous, ruthenium oxide thin film electrodes. In order to study the performance of theses electrode the cyclic voltammetry (CV) experiments were carried out in 0.5M H 2 SO 4 electrolyte. The performance of electrodes was studied with respect to various parameters such as, films thickness, specific capacitance, scan rates, stability cycles, EIS study and efficiency. The preparation of electrode and electrolyte is an important factor in the supercapacitor study. In order to construct the electrode for the supercapacitor study, the 1 cm 2 area of the film was used and the remaining part of the film coated with the insulating tape. 124

25 3.C.2 Experimental set up for the supercapacitor study Fig (a) shows the schematic of experimental set up and (b) shows the actual experimental set up for the supercapacitor study. The electrochemical measurements for supercapacitor were carried out in a three electrode electrochemical cell, in which the hydrous ruthenium oxide thin film electrode was used as the working electrode, platinum as the counter, and saturated calomel electrode (SCE) as the reference electrode. The cyclic voltammetry (CV) experiments were performed using potentiostat/galvanostat (EG & G PAR 263-A) to determine the specific capacitance of the ruthenium oxide film electrode in 0.5 M H 2 SO 4 electrolyte. The capacitance C of film was calculated from the relation C = I /( dv / dt) 3.8 Where, I is the average current in amperes and (dv/dt) is the scan rate in mv/s. Similarly the interfacial capacitance (C i ) is obtained by dividing the capacitance by respective electrode area in the electrolyte. Ci = C / A 3.9 Where A is the area (1 cm 2 in this study) of electrode dipped in the electrolyte. The specific capacitance (C s ) of the electrode is obtained by dividing the capacitance by the weight of ruthenium oxide electrode dipped in the electrolyte. Cs = C / W 3.10 Where W is the weight of the ruthenium oxide electrode dipped in the electrolyte. The energy density of a supercapacitor decreases with increasing power density due to distributed characteristics of porous electrodes [24]. Usable energy stored in EDLCs diminishes as we extract it at higher discharge rates, which is also true for other electrochemical energy storage systems [25]. Therefore, it is important to understand the distributed characteristics and reveal their rationale. Electrochemical impedance spectroscopy is a useful tool to investigate the distributed characteristics of 125

26 an electrochemical system. The electrochemical impedance measurements were conducted with a versastat II frequency response analyzer (FRA) under Zplot program (Scribner Associates Inc.). The frequency range used was from 10 5 to 10-2 Hz under open circuit condition. 126

27 Fig (a) Schematic of experimental set up for the supercapacitor study Fig (b) Experimental set up for the supercapacitor study. Arrow shows the area (1cm 2 ) of RuO 2 electrode dipped in 0.5M H 2 SO 4 electrolyte. 127

28 3.D Results and discussion 3.D.1 Supercapacitive performance of CBD deposited RuO 2 electrode. 3.D.1.1 Effect of different electrolytes The electrolytes used in the pseudocapacitors must have a maximum possible decomposition voltage, a broad range of potentials of electrochemical stability. The resistance of the supercapacitor cell is strongly dependent on the resistivity of the electrolyte used and size of the ions from the electrolyte that diffuse in to and out of the pores of the microporous electrode particles. Organic electrolytes have a higher resistance, but the subsequent power reduction is usually offset by the gain in higher cell voltage. This is usually not a problem for an aqueous electrolyte, such as sodium hydroxide, potassium hydroxide or sulfuric acid, with the resistivity of 1-2 Ω.cm. Aqueous electrolytes are cheaper, easier to purify, and have a lower resistance, but they limit the cell voltage to typically 1 V, thereby limiting the maximum achievable power [26]. Furthermore, the acidic electrolytes have the problem of corrosion of electrode due to the acidic nature. For RuO 2 electrode, the aqueous electrolytes like NaOH, KOH, KCl, Na 2 SO 3, Na 2 SO 4 etc have been tested and 0.5 M H 2 SO 4 electrolytes was chosen to achieve maximum capacitance. Many researchers tested and reported that, the 0.5M H 2 SO 4 electrolyte is appropriate for RuO 2 electrode. With these references, we used 0.5 M H 2 SO 4 electrolyte for RuO 2 electrodes in further testing. 3.D.1.2 Optimization of potential window Typical voltammetric behavior of RuO 2.nH 2 O in 0.5M H 2 SO 4 at 20 mv/s with varying the upper potential limit of CV is shown in fig (a). The electrochemical evaluation of single electrode via cyclic voltammetry (CV) reveals the pseudocapacitive electrochemical properties of the RuO 2.nH 2 O. A voltage dependent CV plot reveals the transition peaks of Ru +4 with different shapes of curves. The shape of these CV curves is 128

29 rectangular-like, indicating the highly electrochemical reversibility of redox transitions oxide up to the upper potential limit 0 to +800 mv/sce. All the above results indicate that the redox transitions of oxyruthenium species within RuO 2 in 0.5M H 2 SO 4 is highly reversible, which meets the highpower requirement for the application of supercapacitors. A tail shaped I-E curve exhibited within the potential window 0 to mv/sce, indicates the non reversible redox reaction. Above discussion can be supported by the charge discharge plots shown in fig (b). The graph shows charge discharge (Q-t) plots within different potential windows. Charge discharge curve within potential 0 to +800 mv/sce window shows symmetric behavior. On the other hand, asymmetric charge discharge observed for the potential window 0 to +600 mv/sce and 0 to mv/sce. Within the potential window 0 to +600 mv/sce, electrode does not completely discharge whereas, in potential window 0 to mv/sce electrode discharged time is less than charging time. The highly ideal capacitive behavior within potential window 0 to +800 mv/sce, suggests that CBD synthesis favors the formation of hydrous oxides with a porous structure providing excellent pathways for electron hopping and proton diffusion/exchange during the redox processes. For the further analysis of electrode 0 to +800 mv/sce potential window is taken. 129

30 Current density (µa.cm -2 ) Potential window (mv/sce) Fig (a) Cyclic voltammetric behavior of RuO 2 with varying the upper potential limit in 0.5M H 2 SO 4 electrolyte at 20mV.s -1. Charge density (mc.cm -2 ) Time (s) mv/sce mv/sce mv/sce mv/sce Fig (b) Typical charge-discharge (Q-t) curves within different potential windows. 3.D.1.3 Effect of film thickness Accurate measurement of the film thickness was not possible due to the high porosity and hydrous nature of the films. Therefore, the weight of 130

31 RuO 2 deposited (g.cm -2 ) on the steel substrate was taken as an indication of the film thickness. The deposited weight of RuO 2 increases with deposition time and a maximum weight of mg.cm -2 were obtained for a deposition period of 2 h. The effect of thickness of the RuO 2 electrode on the capacitive behavior was studied in 0.5M H 2 SO 4 electrolyte at the scan rate of 20 mv.s -1. Fig (a) shows CV curves of RuO 2 electrode of different thicknesses which indicate that the current increased with thickness. The variation of interfacial and specific capacitance with thickness is shown in fig (b), it can be seen that the thickness increases from to mg.cm -2 the interfacial capacitance was increased from to F.cm -2 and specific capacitance was decreased 43 to 29 Fg -1. The interfacial capacitance is proportional to the specific surface area RuO 2 of the material. Such type of decrease in specific capacitance with increase in deposited weight behavior is reported by Park et al. [27]. Current density (µa.cm -2 ) mg.cm mg.cm mg.cm Potential window (mv/sce) Fig (a) Cyclic voltammograms in 0.5 M H 2 SO 4 electrolyte for different thicknesses of RuO 2 nh 2 O film electrodes mg.cm -2 at scanning rate 20 mv.s

32 Interfacial capacitance (F.cm -2 ) Specific capacitnce (F.g -1 ) Interfacial capacitance (F.cm -2 ) Specific capacitance (F.g -1 ) Deposited mass (mg.cm -2 ) Fig (b) Variation of interfacial and specific capacitance with thickness at scan rate 20 mv.s D.1.4 Effect of scan rate The voltammetric responses of RuO 2 electrode at different scan rates are shown in fig (a). The effect of the scan rate 2 to 100 mv.s -1 on supercapacitor formed by CBD deposited RuO 2 electrode was studied at a constant concentration 0.5 M H 2 SO 4 in voltage range of 0 to +800 mv/sce. It was found that the current under curve is slowly increased with scan rate. This showed that voltammetric currents are directly proportional to the scan rate of CV, indicating an ideally capacitive behavior [28]. Variation of specific capacitance and interfacial capacitance values with scan rate is shown in fig (b). The specific and interfacial capacitance values are decreased from 73 to 21 Fg -1 and to F.cm -2, respectively, as the scan rate was increased from 2 to 100 mv.s -1. The decrease in capacitance with the scan rate is attributed to the presence of inner active sites, which cannot precede the redox transitions completely at higher scan rate of CV, probably due to the diffusion effect of proton within the electrode. As the 132

33 scan rate is increased, there is a slight decrease in the specific capacitance that is comparable to previously reported data. Maximum capacitance obtained for hydrous ruthenium oxide at lower scan rate (2 mv.s -1 ) is ~73 Fg -1. Current density (µa.cm -2 ) mv.s -1 5 mv.s mv.s mv.s mv.s mv.s Potential window (mv/sce) Fig (a) The CV curves of RuO 2 electrode at different scanning rates in 0.5M H 2 SO 4. Interfacial capacitance (F.cm -2 ) Interfacial capacitance (F.cm -2 ) Specific capacitance (F.g -1 ) Scan rate (mv.s -1 ) Specific capacitance 30 (F.g-1) 20 Fig (b) Variation of interfacial and specific capacitance with scan rate. 133

34 3.D.1.5 Stability of electrode Stability of RuO 2 electrode in 0.5M H 2 SO 4 was tested by CV. Fig shows the CV curves of ruthenium oxide electrode at the scan rate of 20 mv.s -1 within the voltage range 0 to +800 mv/sce. Fig shows the CV curves for 1 st and 500 th number cycle. The current under curve is decreased by 16% up to 500 cycles. We found that our system can withstand about 500 cycles without a significant decrease in the capacity, illustrating the fairly stable nature of RuO 2 electrode in energy storage application. The specific and interfacial capacitance values are decreased in small amount with the number of cycles due to the loss of active material Current density (µa.cm -2 ) st Cycle 500 th Cycle Potential window (mv/sce) Fig The CV curves of RuO 2 electrode at a) 1 st and b) 500 th cycles. The scanning rate and concentration of H 2 SO 4 were 20 mv.s -1 and 0.5 M, respectively. 3.D.1.6 Electrochemical impedance analysis (EIS studies) The electrochemical impedance measurement (at open circuit voltage, in the frequency range 10 5 to 10-2 Hz) of RuO 2 was carried out in 0.5M H 2 SO 4. The complex-plane plot shown in fig (a) consists of a small semicircle at higher frequencies with a transition to a linear part at low 134

35 frequencies which corresponds to a capacitive behavior. An impedance arc of a semicircle is visible in the high frequency region ( 100Hz). The impedance arc in the high frequency regions is attributed to the complicated interfaces at the RuO 2 RuO 2 particles and RuO 2 substrate contacts since electron hopping at these interfaces occurs during the charge/discharge processes. Similar impedance spectra are also found for sol-gel prepared RuOx.nH 2 O and MnO 2 deposits [29, 30], which have been attributed to their mesoporous and 3D network nanostructures. In the low-frequency region, an approximately vertical increase in impedance on the imaginary part is visible with decreasing the frequency. This result demonstrates the fact that the lowfrequency impedance responses show the typical capacitive characteristics, which should be governed by the Faradaic reactions of RuOx nh 2 O between different oxidation states. The ESR of the hydrous ruthenium oxide film is found 2.6 Ω (shown in inset of fig (a)). Fig.3.22 (b) shows a Bode plot (i.e. plot phase angle vs frequancy); at a phase angle of 39 and 73 about 78% and 29% of the power corresponds to heat production at the internal resistance. A loss factor of electrode 0.30 and 1.26 is calculated at a frequency 0.25 Hz and of 1 KHz respectively in the lower and higher frequency region. The relaxation time constant (τ 0 ), can be calculated from plots of C (ω) verses frequency shown in fig (c). Relaxation time for the RuO 2 electrode (τ 0 ) is found to 0.63 s. 135

36 Z'' (ohm) Z" ESR= 2.6 Ω Z' (ohm) Fig (a) Nyquist plot of RuO 2 electrode in 0.5M H 2 SO Z' Phase angle (φ) Frequancy (log f) Fig (b) Bode plot of RuO 2 electrode in 0.5M H 2 SO

37 C' τ 0 =0.63 s Frequancy (log f) Fig (c) Plot of real capacitance (C ) vs frequency for RuO 2 electrode. 3.D.2 Summary and conclusions The electrochemical test shows that the CBD deposited RuO 2 thin films have electrochemical performance in the potential window from 0 to +800 mv/sce. A new synthesis technique based on CBD is introduced which yields to deposit hydrous ruthenium oxide with specific capacitance 73 Fg -1. CBD synthesized RuO 2 electrode also exhibits a good cycling performance and keep 84% of initial capacity over 500 cycles. Higher loss factor, heat production due to internal resistance and ESR limits the maximum specific capacitance. 3.E.1 Supercapacitor performance of SILAR deposited RuO 2 electrode. 3.E.1.1 Optimization of potential window The capacitive behavior of RuO 2 within potential window 0 to +600 mv/sce is shown in the fig (a). Fig (b) shows charge discharge (Q-t) plots of RuO 2 within different potential windows. The charge discharge curve within potential 0 to +600 mv/sce shows symmetric behavior. However, asymmetric charge discharge observed for the potential 137

38 window 0 to +400 mv/sce and 0 to mv/sce. In the potential window 0 to +600 mv/sce, electrode discharge time is very longer than charging time i.e. charges reside in the electrode which may cause to reduction in stability. Whereas, in potential window mv/sce electrode discharged time is less than charging time. The maximum utilization of the material will be obtained in the potential range mv/sce. Current density (ma.cm -2 ) Potential window (mv/sce) Fig (a) Typical voltammetric behavior of RuO 2 in 0.5M H 2 SO 4 at 20mV.s -1 Charge density (mc.cm -2 ) Time (s) mv/sce mv/sce mv/sce Fig (b) Typical charge-discharge (Q-t) curves within different potential windows. 3.E.1.2 Effect of film thickness The effect of thickness of the RuO 2 electrode on the capacitive behavior was studied in 0.5 M H 2 SO 4 electrolyte at the scan rate 20 mv.s

39 Fig (a) shows CV curves of RuO 2 electrode of different thicknesses which indicate that the current increased with thickness. The variation of interfacial capacitance and specific capacitance with thickness is shown in fig (b), it can be seen that the interfacial capacitance decreased from to F.cm -2 and specific capacitance was increased from 81 to 130 Fg -1 with thickness was decreased from to mg.cm -2. Current density (ma.cm -2 ) mg.cm mg.cm mg.cm Potential window (mv/sce) Fig (a) Cyclic voltammograms in 0.5 M H 2 SO 4 electrolyte for different thicknesses of RuO 2 film electrodes mg/cm 2 at scanning rate 20 mv.s

40 Interfacial capacitance (F.cm -2 ) Deposited mass (mg.cm -2 ) Specific capacitance (F.g -1 ) Fig (b) Variation of interfacial and specific capacitance with thickness at scan rate 20 mv.s E.1.3 Effect of scan rate The effect of scan rate on the RuO 2 electrode supercapacitor was studied in 0.5M H 2 SO 4. Fig (a) shows the cyclic voltammograms with different scan rates. The current under the curve increases slowly with the scan rate. This shows that the voltammetric current is directly proportional to the scan rate of CV, indicating an ideally capacitive behavior. Fig (b) depicted that, the specific and interfacial capacitance values are decreased from 192 to 90 F.g -1 and to F.cm -2, respectively, as the scan rate was increased from 2 to 100 mv.s

41 Current density (ma.cm -2 ) mv.s -1 5 mv.s mv.s mv.s mv.s mv.s Potential window (mv/sce) Fig (a) The CV curves of RuO 2 electrode at scanning rates in 0.5M H 2 SO 4. Interfacial capacitance (F/cm 2 ) Interfacial capacitance (F/g) specific capacitance (F/cm 2 ) Scan rate (mv/s) Specific capacitance (F/g) Fig (b) Variation of interfacial and specific capacitance with scan rate. 141

42 3.E.1.4 Stability of electrode The cycle life (stability) of RuO 2 electrode in 0.5 M H 2 SO 4 was tested by CV. Fig shows the CV curves for 1 st and 500 th number cycle. The current under curve is decreased by 4.4% up to 500 cycles. We found that our system can withstand about 500 cycles without a significant decrease in the capacity, illustrating the stable nature of RuO 2 electrode in energy storage application. The specific and interfacial capacitance values are decreased in small amount with the number of cycles due to the loss of active material. Current density (ma/cm 2 ) Scan rate 20 mv/sec 1 st cycle 500 th cycle Potential window (mv/sce) Fig The CV curves of RuO 2 electrode at a) 1 st and b) 500 th cycles. The scanning rate and concentration of H 2 SO 4 were 20 mv.s -1 and 0.5 M, respectively. 3.E.1.6 Electrochemical impedance analysis (EIS studies) Internal impedance of RuO 2 electrode is illustrated in fig The complex-plane plot shown in fig (a) consists of a dispersed semicircle at higher frequencies with a transition to a linear part is the 45 0 line, at low frequencies which corresponds to a capacitive behavior. Generally, the internal impedance involves two mechanisms including resistances of mass 142

43 transport and electrode materials [31]. In our case, the mass-transport resistance of the electrolyte is almost constant. The intermediate frequency region is the 45 0 line, which is the characteristic of diffusion into the electrode. The middle loop range (10 4 to 1Hz) shows pseudocharge transfer resistance, which is associated with the porous structure of the electrodes [32]. The overwhelming mesopores of the electrode materials may account for these smaller loops that reveal good charge-transfer performance. The quasi-linear profiles at low frequency ranges (less than 1Hz) uncover capacitive features of the electrode materials. Theoretically, the net doublelayer capacitance derived from reversible physical sorption should present a parallel line to the imaginary axis of Nyquist plot [33]. Here the deviation of experimental profiles from the theoretical behavior possibly attributes to two reasons. One is that the different penetration depth of the alternating current signal in virtue of pore size distribution at electrode occur abnormal capacitance [34,35], and on the other, the redox reaction at the RuO 2 electrode gives rise to pseudocapacitance. The ESR of the RuO 2. film is found 1.8 Ω. Fig.3.27 (b) shows a Bode plot; at a phase angle of 82 about 13% of the power corresponds to heat production at the internal resistance. A loss factor of electrode 0.14 is calculated at a frequency 0.5 Hz. The relaxation time constant (τ 0 ), can be calculated from plots of C (ω) verses frequency shown in fig (c). Relaxation time (τ 0 ) for the RuO 2.nH 2 O electrode is found to 0.39 s. 143

44 2000 Z'' (ohm) ESR= 1.8 Ω Z' (ohm) Fig (a) Nyquist plot of RuO 2 electrode in 0.5M H 2 SO Phase angle (φ) Log (f) Fig (b) Bode plot of RuO 2 electrode in 0.5M H 2 SO

45 τ 0 = 0.39 s C' Frequancy (Log f) Fig (c) Plot of C Vs frequency for RuO 2 electrode. 3.E.2 Summary and conclusions The present chapter concluded that the RuO 2 electrodes prepared by different chemical methods show supercapacitor behavior. Also it is noted that the amorphous and porous films are found to be useful in supercapacitor. The RuO 2 electrodes prepared by CBD method have low performance than that of the electrode prepared by SILAR method. Electrode prepared by SILAR method exhibits lower ESR and low power consumption due to heat production at internal resistance. The electrode has the maximum specific capacitance of 192 F.g -1 in 0.5 M H 2 SO 4 electrolyte for mg.cm -2 film thickness and stable up to 500 of cycles. Smaller relaxation time of RuO 2 electrode prepared by SILAR method can exhibit maximum power density. Hence the films prepared by SILAR method will be applicable for supercapacitor devices. There is scope to enhance the supercapacitance of RuO 2 electrode deposited by SILAR method by enhancing the surface area by loading on different nanostructures of TiO 2 material. 145

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