CHAPTER IV: PREPARATION AND CHARACTERIZATION OF MnO2 AND Fe: MnO2 FILMS By POTENTIOSTATIC MODE CHAPTER IV

Transcription

1 CHAPTER IV Sr. No. Title Page No. SECTION-A PREPARATION AND CHARACTERIZATION OF MnO2 AND Fe: MnO2 FILMS BY POTENTIOSTATIC 4.A.1 Introduction A.2 Experimental Setup For Deposition of MnO2 and Fe: 115 MnO2 Thin Films 4.A.3 Experimental Details A.4 Results and Discussion A.4.1 Potentiostatic Deposition of MnO2 and Fe: 117 MnO2 Thin Films 4.A.4.2 Thickness Measurement A.4.3 Structural Characterization: XRD Studies A.4.4 Surface Morphological Studies A Scanning Electron Microscopy and 121 compositional study (EDAX) 4.A Transmission Electron Microscopy 124 (TEM) and High Resolution Transmission Electron Microscopy (HRTEM) 4.A.4.5 FTIR Studies A.4.6 Surface Wettability Test

2 Section-B SUPERCAPACITIVE PERFORMANCE OF THE MnO2 AND Fe: MnO2 THIN FILMS BY POTENTIOSTATIC 4.B.1 Introduction B.2 Experimental Details of Evaluation of Supercapacitance B.3 Results and Discussion B.3.1 Effect of Electrolytes B.3.2 Effect of Fe Doping Concentration B.3.3 Effect of Different Working Potential Windows B.3.4 Effect of Electrolyte Concentration B.3.5 Effect of Scan Rate B.3.6 Stability Studies B.3.7 Galvanostatic Charge-Discharge Studies B.3.8 Electrochemical Impedance Analysis (EIS 137 studies) Conclusions 138 References

3 4.A.1 Introduction: As we know that, there are three different ways (i.e. varying electric field, constant voltage and constant current) by which we can apply the electric field in electrodeposition method. These three different types of applied electric fields significantly affect the surface morphology, lattice structure and hence the supercapacitive properties of deposited material. In earlier chapter we have successfully synthesized MnO2 and Fe: MnO2 thin films by potentiodynamic (i.e. under varying electric field) mode of electrodeposition and their supercapacitive properties have been investigated. The present chapter deals with the preparation and characterization of MnO2 and Fe: MnO2 thin film by potentiostatic (constant voltage) mode. The chapter is divided into two parts. Section A consists of potentiostatic deposition of MnO2 and Fe: MnO2 thin films. The effect of Fe doping on structural, morphological, compositional and wettability properties of MnO2 and Fe: MnO2 have been investigated. Section B focuses on the supercapacitive properties of MnO2 and Fe: MnO2 films have been studied. Huang et al. [1] prepared hydrous manganese oxide films on graphite substrates by anodic deposition and reported the maximum supercapacitance of 275 F.g -1. Hu et al. [2] also reported the supercapacitance of 265 F.g -1 for anodically deposited amorphous MnO2 thin films. Chang et al. [3] prepared cracked mud like MnO2 by anodic deposition on carbon substrate and achieved maximum supercapacitance of 240 F.g -1 with high reversibility and stability. Electrodeposition of the layered manganese oxide is carried out in a colloidal crystal template formed by self-assembly of polystyrene particles by Nakayama et al. [4] and reported maximum supercapacitance of 163 F.g -1. In order to further improve the pseudocapacitive properties of plain Mn oxide, the addition of other transition metal oxides has been attempted. Studies have found that the incorporation of Ni [2, 5-7], Co [8, 114

4 9], V [9], and Mo [10] oxides could enhance the specific capacitances of Mn-based oxides. Recently, crystalline MnFe2O4 with a spinel structure, which could exhibit pseudocapacitive behavior, was found to have wonderful cyclic stability. However, its specific capacitance was only around 100 Fg -1 [11]. In contrast, Lee et al [12] indicated that hydrous Mn Fe mixed oxide, in which the Mn oxide was nanocrystalline and the Fe oxide seemed to be amorphous in nature, can be prepared by anodic deposition. An optimum specific capacitance of the binary oxide was over 200 F.g -1. In present investigation, nano-nest like amorphous Fe: MnO2 thin films are successfully synthesized by potentiostatic deposition and used as potential candidate for supercapacitor. The effect of Fe doping on structural, morphological, optical, wettability and supercapacitive properties of MnO2 thin films have been investigated. The effect of electrolyte concentration and scan rate on specific capacitance of MnO2 and Fe: MnO2 electrodes have been studied. Also stability, chargedischarge and impedance of MnO2 and Fe: MnO2 electrodes have been studied. 4.A.2 Experimental Setup for Deposition of MnO2 and Fe: MnO2 Thin Films: Experimental setup for deposition of MnO2 and Fe: MnO2 thin films is described in chapter 3 (section 3.A.2) 4.A.3 Experimental Details The MnO2 and Fe: MnO2 thin films were electrodeposited at room temperature from aqueous alkaline bath. The bath consisted of 0.1 M manganese sulphate (MnSO4.4H2O) complexed with 0.1 M citric acid {C (OH)(COOH)(CH2COOH)2 H2O} maintained at a ph of 10.5 through the addition of 1 M sodium hydroxide (NaOH) solution. In order to dope Fe in 115

5 MnO2 thin films, four different concentrations (0.5, 1, 2 and 4 at %) were selected. The solution was prepared in freshly prepared double distilled water. Mirror polished and ultrasonically cleaned stainless steel was used as substrates. The optimized parameters for deposition of MnO2 and Fe: MnO2 films feasible for supercapacitor application are listed in Table 4.A.1 Table 4.A.1: Optimized preparative parameters of electrodeposition of MnO2 and Fe: MnO2 thin films. Sr. No. Film Manganese oxide Fe: manganese oxide 1 Medium Aqueous Aqueous 2 Bath composition MnSO4 [25 cc, 0.1 MnSO4 [25 cc, 0.1 M] + NaOH [1 M, 6 M] + FeSO4 [0.5, 1, cc] 2, 4at%]+ NaOH [1 M, 6 cc] 3 Complexing agent Citric acid [0.1 M, 25 cc] Citric acid [0.1 M, 25 cc] 4 Total quantity (ml) ph Potential (mv/sce) Temperature (K) Substrate Stainless steel Stainless steel 4.A.4 Results and Discussion 4.A.4 Electrodeposition of MnO2 and Fe: MnO2 Thin Films The deposition of MnO2 and Fe: MnO2 film was carried out from the bath containing MnSO4 complexed with citric acid and ph ~10.5 adjusted by addition of NaOH. The deposition potential and mechanism of film formation is studied from CV curves. The preparative conditions are optimized to get feasible MnO2 and Fe: MnO2 films for supercapacitor 116

6 application. The effect of Fe doping on structural, morphological, optical, wettability properties of MnO2 thin film is investigated. 4.A.4 Deposition Potential for MnO2 and Fe: MnO2Thin Films Deposition potential, redox reactions and the film formation were studied from CV curves discussed in chapter in 3 (section 3.A.4.1). 4.A.4.1 Potentiostatic Deposition of MnO2 and Fe: MnO2 Thin Films Fig. 4.A.1 shows potentiostatic curves during formation of MnO2 and Fe: MnO2 recorded on SS substrate with applied potential of mv/sce. At the initial stage of electrodeposition, drastic increase in current density was noted, indicating the forming Helmholtz layer of Mn(OH)2 + ions at the interface between SS substrate and electrolyte. Moreover, the initiation of electron transfer process corresponding to nucleation process of MnO2 on stainless steel shows the stage where current density was immensely reduced. Therefore, in presence of Fe, the current density of nucleation part was lower and the current density versus time variation was much steeper. In addition, the steady flow of current density after nucleation process, implying uniform growth of MnO2 crystallite was slightly lower in the Fe: MnO2. It is noted that there is now any oxygen or hydrogen bubbling was done. 2 Mn(OH) e - 2 MnOOH + H2 (1) 2 MnOOH + 2e - 2 MnO2 + H2 (+1100 mv/sce) (2) Therefore, this indicates that the nucleation formation and film growth processes can be altered in presence of Fe [12]. The higher potential and alkaline medium assist the formation of MnO2. Grayish black coloured, smooth, uniform and well adherent Fe: manganese oxide thin films were obtained by electrodeposition method as shown in Figure 4.A.2 Thus Figure 4.A.2 indicates the feasibility of electrodeposition for large area deposition. 117

7 1 Current density (ma/cm 2 ) (b) 2 at% Fe: M no 2 (a)m no Time (S) Fig. 4.A.1 Potentiostatic curves of MnO2 and 2 at% Fe: MnO2 thin films in aqueous alkaline bath at 1.1 V/SCE. 4.A.4.2 Thickness Measurement Thickness is the most important parameter, which alters the properties of the material due to surface phenomena. Thickness of Fe: MnO2 film was measured by the gravimetric weight difference method. Fig. 4.A.3 shows the variation of mass deposited of MnO2 with Fe doping concentration. It can be seen from the figure that as Fe doping concentration is increased the film deposited mass is decreased. The rate of decrease in deposited mass is non-linear. It is seen that as Fe doping concentration increases the deposited mass of the MnO2 thin film decreases up to 2 at % and after that slight increase in the deposited mass is noticed. From Fig. 4.A.1 it is confirmed that presence of Fe lowers the rate of deposition. 118

8 Fig. 4.A.2 Photographs of MnO2 and Fe: MnO2 thin films onto stainless steel substrates of 12 cm 2 area of each film on one side Mass deposited (mg/cm 2 ) Fe doping (at% ) Fig. 4.A.3 Variation of MnO2 film thickness with Fe doping concentration. 119

9 4.A.4.3 Structural Characterization: XRD Studies Fig.4.A.4 shows the XRD patterns of MnO2 and Fe: MnO2 thin films grown on stainless steel substrates with different Fe concentrations ranging from 0.5 to 4 at %. There is no any diffraction peak associated with MnO2, all the peaks observed in the XRD patterns are due to stainless steel substrate and are indexed by the triangles. Also there is no significant difference between XRD patterns of MnO2 and Fe: MnO2, which confirms that the incorporated Fe oxide could be amorphous and did not change the amorphous nature of deposited manganese oxide. Lee et al. [12] prepared Fe doped MnO2 thin films by electrodeposition method and reported similar kind of results. Amorphous phase of the oxide material is feasible for supercapacitor application due to easy penetration of ions through the bulk of the active material [13]. -Stainless steel 4.0at% Fe Intensity (A.U.) 2.0at% Fe 1.0at% Fe 0.5at% Fe Undoped θ (Degree) Fig. 4.A.4 The XRD patterns of MnO2 and Fe: MnO2 thin films prepared with different Fe concentrations onto SS substrate. 120

10 4.A.4.4 Surface Morphological Studies: In order to get information regarding the surface morphology of MnO2 and Fe: MnO2 films, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were studied. 4.A Scanning Electron Microscopy and Compositional studies (EDAX): Surface morphologies of MnO2 and Fe: MnO2 were examined by FESEM and the micrographs are demonstrated in Fig. 4.A.5 (a-e) at 50,000 magnification. Nucleation and growth processes are considered as the important factors in surface morphology alteration. The surface morphology of MnO2 can be altered by changing nucleation and growth stages of MnO2. Fig. 4.A.5 (a-e) illustrates the effect of presence of Fe on the surface morphology of MnO2 electrode. The surface of the MnO2 sample (Fig. 4.A.5 a) is made up of 3-D network of nano-nests like architecture. The approximate width of this nano-nest is about nm. For 0.5 at % Fe (Fig. 4.A.5b) doping there is no significant difference observed (may be due to less Fe doping concentration) whereas as Fe doping concentration increases to 1 at % (Fig. 4.A.5c), these nano-nests get converted into randomly oriented nanorods. In this case surface of MnO2 becomes slightly rough. As the Fe concentration in the plating solution continuously increased to 2.0 at %, nanorods converted into highly porous and rougher deposited oxide (Fig. 4.A.5d). Due to Fe addition, the surface of the MnO2 electrode becomes rough. However, as can be confirmed in Fig. 4.A.5e, the surface roughness began to decrease as the Fe addition in the plating solution is further increased. 121

11 Fig. 4.A.5 Scanning electron micrograph images of (a) MnO2 (b) 0.5 at% Fe (c) 1 at% Fe (d) 2 at% Fe (e) 4 at% Fe: MnO2 thin films at 50,000 magnification. The surface became flat and was even smoother than the plain MnO2. Thus the results indicated that up to 2 at %, the Fe addition modifies the surface of the MnO2 electrode. Note that shifting from MnO2 to Fe: 122

12 MnO2 materials lead to a drastic increase in the surface area. This type of highly amorphous and highly porous structures is expected to produce high supercapacitance values. Fig. 4.A.6 (a and b) shows typical EDAX patterns for MnO2 and Fe: MnO2 thin films on ITO substrate. The elemental analyses were carried out for Mn, Fe and O. Here some unexpected but physically present elements like Si and Sn were detected due to glass substrate and ITO conduction layer, respectively. The strong peaks for Mn and O were found in both the spectra (Fig. 4.A.6 (a and b)), and in Fig. 4.A.6 (b) there are few elemental peaks which are due to Fe. Thus the existence of Fe was confirmed from the EDAX spectrum. The average atomic percentage of Mn: Fe: O is listed in the Table 4.A.2. Fig. 4.A.6 The energy dispersive X-ray (EDAX) analyses of MnO2 and Fe: MnO2 (2 at % Fe) thin films on indium doped tin oxide (ITO) substrate. Table 4.A.2 Elemental composition analyses of MnO2 and Fe: MnO2 films Elements Mn-K Fe-K MnO2 Atomic % Fe: MnO2 (2 at % Fe) Atomic %

13 Thus from elemental analyses, it is confirmed that, for 2 at % Fe in plating bath only 1.02 at % Fe has resulted into MnO2 sample. 4.A Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM): In order to get the information regarding the size, shape and orientation of crystallites, transmission electron microscopic image was studied. Fig. 4.A.7 (a, b and c) shows transmission electron micrograph, corresponding SAED pattern and high resolution transmission electron micrographs of MnO2 film. Fig. 4.A.7 (a) shows that the growth has taken place cluster by cluster and randomly oriented nanocrystals are formed so that it looks like a rod along with amorphous matrix. The crystallites are grown together to form clusters where the crystallites are indistinguishable. Fig. 4.A.7 (a) TEM (b) SAED pattern and (c) HRTEM images of MnO2 thin film. 124

14 Fig.4.A.7 (b) shows corresponding selected area electron diffraction (SAED) pattern of MnO2 nano-nest. The blurred bright electron diffraction rings show that the MnO2 film is amorphous or poorly crystalline, supporting to X-ray diffraction results. The high resolution transmission electron micrograph of individual nano-nest indicates that the MnO2 nanonests produced is amorphous as no distinguished grains or clusters are seen. 4.A.4.5 FTIR Studies FTIR spectroscopy analyses can be an alternative to X-ray being sensitive also to amorphous components and to the structural environment of the hydrous components, which can be diagnostic for a specific mineral phase of MnO2 [14]. The region below 1400 cm -1 contains peaks due to fundamental vibrations of octahedral MnO6 and the region above 1400 cm -1 contains peaks primarily corresponding to OH vibrations [15]. The regions from cm -1 and cm -1 correspond to Mn- O stretching and bending vibrations. Fig.4.A.8 (a and b) shows the FTIR spectra of MnO2 and Fe: MnO2 (2 at %) samples, respectively. In both the spectra, there is broad and intense peak at 3340 cm -1 related to OH stretching vibrations. The absorption peaks around 1627 cm 1 and 1110 cm -1 may be attributed to -OH bending vibrations combined with Mn atoms [16]. The absorption of the Mn O lattice vibrations around at 765, 620 and 512 cm -1 in the spectrum is indicative of a tetragonally distorted cubic lattice. These absorption peaks may be associated with the coupling mode between Mn-O stretching modes of tetrahedral and octahedral sites [17, 16]. These bands for MnO2 associated with the coupling mode between Mn-O stretching modes of tetrahedral and octahedral sites shifted slightly towards lower wavenumbers after Fe doping. The IR result suggests the presence of somewhat bound water in the MnO2 and Fe: MnO2 structure, 125

15 which is generally believed to be favorable to the electrochemical activity of the material [18]. (b) Transmittance, % (a) 3340 cm cm cm cm cm cm W avenumber (cm -2 ) Fig. 4.A.8 FTIR spectra of (a) MnO2 and (b) 2 at % Fe: MnO2 films. 4.A.4.6 Surface Wettability Test Wettability involves the interaction between a liquid and a solid in contact. The wetting behavior is characterized by the value of the contact angle, a microscopic parameter. If the wettability is high, contact angle (θ), will be small and the surface is hydrophilic. On the contrary, if the wettability is low, θ will be large and the surface is hydrophobic. Fig.4.A.9 shows the systematic presentation of measurement of contact angles for MnO2 and Fe: MnO2 thin films. The nano-nest like MnO2 thin film showed a contact angle of about 64. Here the contact angle is high may be due to the air trapped inside pores of nano-nest network which prevents the water for adhering to the film. As the Fe doping concentration increased up to 2 at %, the water contact angle of MnO2 thin films decreased due to increased surface energy. Generally high surface energy exhibits low water contact angle [19]. The contact angles were found to be 52, 46 and

16 for 0.5, 1.0 and 2.0 at%, respectively as seen from Fig 4.A.9. This indicates that with increasing Fe doping concentration, the contact angle decreases and finally film surface becomes more hydrophilic. This is useful for making the intimate contact of electrolyte with MnO2 thin film (electrode). Hydrophilicity is attributed to amorphous nature [20]. Amorphous material with hydrophilic nature is one of the key requirements for supercapacitor electrode material. Further increase in Fe doping concentration decreases the porosity of the MnO2 electrode as seen from SEM images (Fig. 4.A.5e) and hence there is again slight increase in the contact angle. Fig. 4.A.9 Measurement of water contact angles for MnO2 and Fe: MnO2 thin films. 127

17 Section-B SUPERCAPACITIVE PERFORMANCE OF MnO2 AND Fe: MnO2 THIN 4.B.1 Introduction FILMS BY POTENTIOSTATIC In the present work, potentiostatically deposited MnO2 and Fe: MnO2 thin films have been used as supercapacitor electrodes. The effects of electrolyte concentration and scan rate on supercapacitance of MnO2 and Fe: MnO2 electrodes have been investigated. Additionally, stability, Charging discharging and impedance characteristics of MnO2 and Fe: MnO2 electrodes are studied. 4.B.2 Experimental Details of Evaluation of Supercapacitance: 4.B.2.A Experimental Set Up for Supercapacitor study: Experimental Setup for supercapacitive study of MnO2 and Fe: MnO2 Thin Films is described in chapter 3 (section 3.B.2.A) 4.B.3 Results and Discussion 4.B.3.1 Effect of Electrolytes The effect and requirements of the electrolyte are discussed in the earlier chapter 3 (Section 3.B.3.1) The Na2SO4 electrolyte gave the largest current than the other electrolytes. Hence further all the supercapacitive properties of MnO2 and Fe: MnO2 thin films are tested in Na2SO4 electrolyte. 4.B.3.2 Effect of Fe Doping Concentration Electrochemical behavior of the deposited oxides was evaluated using CV in 1 M Na2SO4 electrolyte with a potential scan rate of 100 mv.s -1. Fig. 4.B.1 shows the voltammograms of MnO2 and Fe: MnO2 electrodes prepared potentiostatically with different Fe concentrations ranging from 0.5 to 4 at %. The slightly rectangular shapes and mirror-image 128

18 characteristics of the CV curves reveal the ideal pseudocapacitive behavior of all the electrodes, indicating that the deposited binary Fe: MnO2 are promising electrode materials for use in supercapacitors [5, 18]. The values of supercapacitance calculated from the CVs are 120, 129, 147, 161 and 153 F.g -1 for undoped, 0.5, 1.0, 2.0 and 4 at % Fe, respectively. It is clear from Fig.4.B.2 that as Fe doping concentration increases up to 2at %, the supercapacitance of MnO2 electrode increases from 166 to 231 F.g -1. Further increase in Fe doping concentration (4at %) decreases the supercapacitance. This may be due to the decrease in the porosity of the MnO2 electrode which reduces the surface area of electrode. Current density (ma/cm 2 ) A-Undoped B-0.5at% Fe C-1.0at% Fe D-2.0at% Fe E-4.0at% Fe C D E Voltage (mv/sce) Fig. 4.B.1 The Cyclic voltammograms of MnO2 and Fe: MnO2 thin film electrode at different Fe concentrations (A-MnO2 B- 0.5 at%, C- 1.0 at%, D-2.0 at% and E- 4.0 at %) in the 1 M Na2SO4 electrolyte. A B In Fig.4.B.1, although the shapes of CV curves are similar, 2 at % Fe: MnO2 has the largest enclosed area, reflecting its superior charge-storage performance. The data indicates that the specific capacitance remarkably 129

19 increases from 120 F.g -1 for MnO2 to 161 F.g -1 for 2 at % Fe: MnO2. However, further increasing the Fe content in the binary oxide causes the reverse effect; the specific capacitance of 4 at % Fe: MnO2 is only 153 F.g -1. The experimental results, shown in Fig.4.B.2, clearly show that the amount of Fe oxide added significantly affects the overall capacitance of the deposited binary Mn Fe oxide. The value reported in this study is comparable with the values reported in the literature [21]. Supercapacitance (F.g -1 ) F.g F.g F.g F.g F.g Fe doping (at% ) Fig. 4.B.2 Variation of supercapacitance with Fe doping concentration. 4.B.3.3 Effect of Different Working Potential Windows The 2 at % Fe: MnO2 samples with different scanning potential ranges of , 0 0.9, , and V/SCE in a solution of 1 M Na2SO4 are shown in Fig. 4.B.3. Slightly rectangular and symmetric curves were observed within the potential range of V, indicating its high electrochemical reversibility at the scan rate of 20 mv.s 1. As the potential range was widened to 0.2 to 0.9 and -0.3 to 0.9 V, two pairs of relatively asymmetric peaks were clearly observed. The maximum and minimum values of polarization potential of the MnO2 electrodes are 130

20 limited by the decomposition potential and redox potentials of Mn 4+ to Mn 2+ and Mn 4+ to Mn 7+. Due to the solubility of both Mn 2+ and Mn 7+ in water, the reactions of Mn 4+ to Mn 2+ and Mn 4+ to Mn 7+ are irreversible. Therefore, at the test condition, the polarization potential should be controlled within the region of -0.1 to +0.9 V to ensure reversible pseudocapacitive behavior [18]. Current density (ma/cm 2 ) A- 0.9V to +0.1 V B-0.9 V to 0.0 V C-0.9 V to -0.1 V D-0.9 V to -0.2 V E-0.9 V to -0.3 V 2 at% Fe: MnO A Voltage (mv/sce) B C D E Fig. 4.B.3 The Cyclic voltammograms of 2 at % Fe: MnO2 thin film in different working potential windows (A- 0.9 to 0.1, B- 0.9 to 0.0, C- 0.9 to -0.1, D- 0.9 to -0.2 and E- 0.9 to -0.3) at the scan rate of 50 mvs B.3.4 Effect of Electrolyte Concentration The effect of electrolyte concentration on supercapacitance of 2 at % Fe: MnO2 was studied at a scan rate of 100 mv.s -1 within the potential range of -0.1 to +0.9 V. Fig.4.B.4 shows the CV curves of Fe: MnO2 electrodes in Na2SO4 electrolyte for different concentrations (i.e. from 0.1 to 1 M). The current under the curve increases as the Na2SO4 131

21 concentration is increased from 0.1 to 1 M. Thereafter the current under the curve was fairly constant [22]. Current density (ma/cm 2 ) 15 2 at % Fe: MnO 2 E 10 D B C A A- 0.1 M B M -10 C M D M -15 E- 1.0 M Voltage (mv/sce) Fig. 4.B.4 Cyclic voltammograms of 2 at% Fe: MnO2 electrode in different electrolyte concentrations (A- 0.1 M, B M, C M, D M, E- 1 M) of Na2SO4 electrolyte. Table 4.B.1 Effect of electrolyte concentration on interfacial and specific capacitances of MnO2 and 2 at % Fe: MnO2. Electrolyte MnO2 2 at% Fe: MnO2 Concentration Interfacial Specific Interfacial Specific (M) capacitance capacitance capacitance capacitance (F.cm -2 ) (F.g -1 ) (F.cm -2 ) (F.g -1 )

22 With increasing concentration of electrolyte, the specific capacitance is also increased which may be attributed to the increase in number of Na + ions in the electrolyte. Similar kind of behavior has been reported by Xu et al. [23] for MnO2 thin films prepared by micro-emulsion method. The specific and interfacial capacitances are increased with increasing concentration of Na2SO4 electrolyte. All the values of specific and interfacial capacitances of the MnO2 and Fe: MnO2 electrodes are listed in Table 4.B.1. 4.B.3.5 Effect of Scan Rate The high pulse-power property of 2 at % Fe: MnO2 electrode is examined by using cyclic voltammetry at different scan rates. Typical CV curves measured at 5, 10, 20, 50, 75 and 100 mv.s -1 in 1 M Na2SO4 electrolyte within voltage range of +0.9 to -0.1 V/SCE electrode are shown in Fig. 4.B.5 along with the current under the curve increases slowly with the scan rate. Current density (ma/cm 2 ) A-5 m V.s -1 B-10 m V.s -1 C-20 m V.s -1 D-50 m V.s -1 E-75 m V.s -1 F-100 m V.s -1 2 at% Fe: M no F 2 E D C B A Voltage (mv/sce) Fig. 4.B.5 Cyclic voltammograms of 2 at% Fe: MnO2 electrode at different scanning rates in 1 M Na2SO4 electrolyte. 133

23 The specific capacitance values decreased from 236 to 161 F.g -1 for Fe: MnO2 electrode. The maximum specific capacitance obtained for Fe: MnO2 was 236 F.g -1. The dependence of the voltammetric charge of Fe: MnO2 in 1 M Na2SO4 solution on the scan rate can be understood by the slow diffusion of Na + ions into the pores of Fe: MnO2 [24]. At high scan rates, diffusion limits the movement of Na + ions due to time constraint, and only the outer active surface is utilized for the charge storage. However, at lower scan rates, all the active surface area can be utilized for charge storage. Hence, the specific capacitance obtained at the slowest scan rate is believed to be closest to that of full utilization of the electrode material [13]. The values at different scanning rates for MnO2 and Fe: MnO2 electrodes are listed in Table 4.B.2. Table 4.B.2 Effect of scan rate on interfacial and specific capacitances of MnO2 and 2 at % Fe: MnO2. Scan rate MnO2 2 at% Fe: MnO2 (mv.s -1 ) Interfacial Specific Interfacial Specific capacitance capacitance capacitance capacitance (F.cm -2 ) (F.g -1 ) (F.cm -2 ) (F.g -1 ) B.3.6 Stability Studies The electrochemical stability of the Fe: MnO2 electrodes were evaluated by repeating the CV test for 1000 cycles at 100 mv.s -1 scan rate. The variation of specific capacitance with cycle number for the electrodes 134

24 is presented in Fig.4.B.6 which shows that specific capacitance decreases with increasing the cycle number. The specific capacitance of both the electrodes remains almost constant up to 1000 cycles but thereafter declines slightly. The capacitance retained ratios after 1000 cycles (capacitance at the 1000 th cycle/capacitance at the 1 st cycle) of the MnO2 and Fe: MnO2 electrodes were 89% and 92%, respectively. This implies an excellent long-term recycling capability. Lee et al [12] reported that the addition of Fe oxide effectively increased the capacitance retained ratio of MnO2 after cycling. Current density (ma/cm 2 ) 10 2 at % Fe: MnO th cycle 1 st cycle Voltage (mv/sce) Fig. 4.B.6 Cyclic voltammograms of 2 at% Fe: MnO2 electrode at different cycles at 100 mvs -1 scan rate. 4.B.3.7 Galvanostatic Charge-Discharge Studies The galvanostatic charge/discharge profile of Fe: MnO2 electrode in 1 M Na2SO4 electrolyte is presented in Fig. 4.B.7. The charge/discharge current rate is 0.2 ma.cm -2 and the operational potential range is between 0 and +0.8 V/SCE. It can be seen that the charge profile is slightly curved, 135

25 suggesting a pseudocapacitive characteristic. At the moment of electric current reversing from charging to discharging, a potential drop can be observed due to the electrode polarization at this high current rate. Except for the initial potential drop, the discharge profile is essentially linear. The electrical parameters, specific energy (SE), and specific power (SP) and coulomb efficiency (η%) are calculated using equation numbers (3.B.8), (3.B.9) and (3.B.10) in chapter 3 (Section 3.B.3.7): at% Fe: MnO 2 Potential (V) Time (S) Figure 4.B.7 Galvanostatic charge discharge curves of 2 at% Fe: MnO2 supercapacitor in 1.0 M Na2SO4 electrolyte. For a specific energy of 1.17 Wh/kg the specific power of the capacitor calculated to be W/kg for MnO2 thin film. After 2 at % Fe doping the specific energy increases to 2.19 Wh/kg but there is slight increase in specific power to W/kg. This increase in specific energy and power may be due to increased time for charging and discharging of capacitor. Coulomb efficiency (η%) is less (71.70 %) of Fe: MnO2 than that for MnO2. All the values of specific energy (SE), and specific 136

26 power (SP) and coulomb efficiency (η%) for MnO2 and Fe: MnO2 are listed in Table 4.B.3. Table 4.B.3 Values of power density, energy density and coulomb efficiency of MnO2 and 2 at % Fe: MnO2 thin films Name Specific Energy (Wh/kg) Specific Power (W/kg) Coulomb efficiency, η (%) MnO at % Fe: MnO B.3.8 Electrochemical Impedance Analysis (EIS studies) Fig.4.B.8 shows electrochemical impedance spectrum in the form of Nyquist plots for Fe: MnO2 electrode at potentials 0.5 V/SCE, where Z and Z are the real and imaginary parts of the impedance, respectively. As can be seen in Fig.4.B.8, the plot obtained is composed of a semi-circle at high frequencies, which is related to Faradaic reactions. The linear curve at the low-frequency region can be attributed to the diffusion controlled process in the electrolyte [25]. The initial non-zero intercept at Z at the beginning of the semicircle is almost identical in both the curves and is due to the electrical resistance of electrolyte, which has the average value of 1.1 Ω in 1.0 M Na2SO4 electrolyte. The resistance projected by semi-circle is due to the of the active electrode material (Re). Therefore, the resistance values of MnO2 and Fe: MnO2 thin films are 11 Ω and 7 Ω. This implies that iron addition in MnO2 lowers the charge transfer resistance and increases conductivity of MnO2. Thus Fe: MnO2 is most promising electrode material in supercapacitive technology. 137

27 -10 Z im '' (ohm cm 2 ) Z r ' (ohm cm 2 ) Figure 4.B.8 Nyquist plot obtained at 2 at % Fe: MnO2 electrodes at 0.8 V over the frequency range 1 mhz 100 MHz in 1 M Na2SO4. Conclusions Nano-nest like amorphous MnO2 and Fe: MnO2 thin films are successfully prepared by potentiostatic deposition. The Fe addition did not change the amorphous structure of the deposited MnO2. The addition of Fe significantly alters the surface morphology and lead to a drastic increase in porosity which increases the pseudocapacitive performance of MnO2. Fe doping is confirmed from EDAX patterns. Wettebility test shows that as Fe doping concentration increases the contact angle decrease which is useful in making intimate contact between electrode and electrolyte. The supercapacitance increases with increase in concentration of electrolyte and decreases with the scan rate. Maximum specific capacitance achieved for 2 at% Fe: MnO2 is 273 F.g 1. Also the specific power and specific energy increased from 2.9 to 3.1 Wh/kg and to kw/kg respectively due to the Fe addition. Moreover, capacitance-retained ratio of the Fe: MnO2 electrode after 1000 charge-discharge cycles was also improved 138

28 from 89% to 92% due to Fe addition. Impedance analysis shows that Fe addition improves conductivity of MnO2 by reducing the charge transfer resistance. Thus, all the above result suggests that due to Fe addition and also change in mode of electrodeposition there is significant change in surface morphologies of MnO2 thin films and hence supercapacitive performance of MnO2 thin films also improved. The values reported in this chapter are smaller as compared to that in earlier chapter which may be due to change in mode of deposition and also less Fe is doped in this case. 139

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