CHAPTER V: PREPARATION AND CHARACTERIZATION OF MnO2 AND Fe: MnO2 FILMS By GALVANOSTATIC MODE CHAPTER V

Transcription

1 CHAPTER V Sr. No. Title Page No. SECTION-A PREPARATION AND CHARACTERIZATION OF MnO2 AND Fe: MnO2 FILMS BY GALVANOSTATIC 5.A.1 Introduction A.2 Experimental Setup For Deposition of MnO2 and Fe: 145 MnO2 Thin Films 5.A.3 Experimental Details A.4 Results and Discussion A.4.1 Galvanostatic Deposition of MnO2 and Fe: 146 MnO2 Thin Films 5.A.4.2 Thickness Measurement A.4.3 Structural Characterization: XRD studies A.4.4 Surface Morphological Studies A Scanning Electron Microscopy and 150 compositional study (EDAX) 5.A Transmission Electron Microscopy 153 (TEM) and High Resolution Transmission Electron Microscopy (HRTEM) 5.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 GALVANOSTATIC 5.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 166 studies) Conclusions 167 References

3 5.A.1 Introduction: We know that, in electrodeposition the surface morphology, lattice structure and hence the supercapacitive properties of deposited material depend on applied electric field. In earlier chapter we have deposited MnO2 and Fe: MnO2 thin films by potentiostatic mode of electrodeposition and their supercapacitive properties have been investigated. In order to check the effect of constant current (galvanostatic mode of electrodeposition) on the structural and morphological properties of MnO2 and Fe: MnO2 films we have carried out electrodeposition using galvanostatic mode. MnO2 multilayer nanosheet clusters were prepared via galvanostatic mode by Feng et al. [1] and reported supercapacitance of 521 F.g -1 with high electrochemical stability. The electrochemical cyclability mechanism of nanocrystalline MnO2 electrodes prepared by galvanostatic mode with rock salt-type and hexagonal ε-type structures was investigated by Wei et al [2]. Wu et al. [3] fabricated nanostructured MnO2 electrodes by galvanostatic deposition and the effect of different current densities and annealing temperature on electrochemical performance of MnO2 electrode has been investigated. The present chapter deals with the synthesis and characterization of MnO2 and Fe: MnO2 thin films by galvanostatic mode of electrodeposition. The effect of Fe doping on structural, morphological, optical, wettability and supercapacitive properties of MnO2 thin films have been investigated. The effects of electrolyte concentration and scan rate on specific capacitance of MnO2 and Fe: MnO2 electrodes have been studied. Also stability, charge-discharge and impedance of MnO2 and Fe: MnO2 electrodes have been studied. 144

4 5.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) 5.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 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 SS were used as substrates. The optimized parameters for deposition of MnO2 and Fe: MnO2 films feasible for supercapacitor application are listed in Table 5.A.1 Table 5.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

5 6 Current density (ma/cm 2 ) Temperature (K) Substrate Stainless steel Stainless steel 5.A.4 Results and Discussion 5.A.4 Electrodeposition of MnO2 and Fe: MnO2 Thin Films The galvanostatic deposition of MnO2 and Fe: MnO2 films have been 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 application. The effect of Fe doping on structural, morphological, optical, wettability properties of MnO2 thin film is investigated. 5.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). 5.A.4.1 Galvanostatic Deposition of MnO2 and Fe: MnO2 Thin Films Fig. 5.A.1 shows galvanostatic curves during formation of MnO2 and Fe: MnO2 recorded on SS substrate with applied current density of 5 ma/cm 2. The nature of the plot determines the nucleation and growth of MnO2 and Fe: MnO2 thin films. Initially potential was raised suddenly to V/SCE due to the activation of space charge region and then started to decrease at one point at which the oxidation of Mn(OH) 2+ on substrate took place and after that a continuous growth of MnO2 and Fe: MnO2 is observed. In presence of Fe, the potential of nucleation part was lower and the current density versus time variation was much steeper. In addition, 146

6 the steady flow of potential after nucleation process, implying uniform growth of MnO2 crystallite was slightly lower in the Fe: MnO2. 2 Mn(OH) e - 2 MnOOH + H2 (5.A.1) 2 MnOOH + 2e - 2 MnO2 + H2 (5.A.2) Therefore, this indicates that the nucleation formation and film growth processes can be altered in presence of Fe [4]. The higher potential and alkaline medium assist the formation of MnO Potential (V/SCE) (b) 2 at % Fe: MnO 2 (a) MnO Time (S) Fig. 5.A.1 Galvanostatic curves of MnO2 and 2 at% Fe: MnO2 thin films in aqueous alkaline bath at 5 ma/cm 2. Grayish black coloured, smooth, uniform and well adherent Fe: manganese oxide thin films were obtained by electrodeposition method as shown in Figure 5.A.3 Thus Figure 5.A.3 indicates the feasibility of electrodeposition for large area deposition. 5.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. 147

7 Mass deposited (mg/cm 2 ) Fe doping (at %) Fig. 5.A.2 Variation of MnO2 film thickness with Fe doping concentration. Fig. 5.A.3 Photographs of MnO2 and Fe: MnO2 thin films onto SS substrates of 12 cm 2 area of each film on one side. Fig. 5.A.2 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 148

8 concentration increases the deposited mass of the MnO2 thin film decreases. From Fig. 5.A.1, it is confirmed that presence of Fe lowers the rate of deposition. 5.A.4.3 Structural Characterization: XRD Studies Fig.5.A.4 shows the XRD patterns of MnO2 and Fe: MnO2 thin films grown on SS substrates with different Fe concentrations ranging from 0.5 to 4 at %. No distinct diffraction peak other than SS substrate is observed in XRD pattern, which probably means that the film consisted of MnO2 and Fe: MnO2 colloidal particles in amorphous phase. All the peaks observed in the XRD patterns due to SS substrate are indexed by the triangles. Stainless steel 4.0at% Fe Intensity (A.U.) 2.0at% Fe 1.0at% Fe 0.5at% Fe Undoped θ (Degree) Fig. 5.A.4 The XRD patterns of MnO2 and Fe: MnO2 thin films prepared with respect to different Fe concentrations onto SS substrate. There is no any difference between XRD patterns of MnO2 and Fe: MnO2, which means that Fe oxide, could be amorphous and did not change the amorphous nature of deposited MnO2. Zhang et al [5] prepared Ni doped MnO2 thin films by solid state reaction route reported similar kind 149

9 of amorphous behavior. Thus, the earlier reports suggest that the amorphous oxides are very useful for high energy density applications [6, 7]. 5.A.4.4 Surface Morphological Studies: In order to get information regarding the surface morphology of MnO2 and Fe: MnO2 films, scanning electron microscope (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were studied. 5.A Scanning Electron Microscopy and Compositional studies (EDAX): The two-dimensional surface morphologies of MnO2 and Fe: MnO2 were examined by FESEM at 50,000 magnification and the micrographs are demonstrated in Fig. 5.A.5 (a-e). The scanning electron micrographs of MnO2 thin films doped with different Fe concentrations are displayed in Fig. 5.A.5 (a-e). As seen in micrograph (Fig. 5.A.5a), the warm like architecture is clearly observed for MnO2 thin film. The approximate size of which is about nm. For 0.5 at % Fe (Fig. 5.A.5b) doping this warm like architecture gets converted into interconnected web like network. Here the surface of MnO2 becomes slightly rough. Whereas as Fe doping concentration increases to 1 at % (Fig. 5.A.5c), the surface of MnO2 becomes slightly compact and spongy. Further as the Fe concentration in the plating solution continuously increased to 2.0 at %, the spongy surface of MnO2 converted into porous and rougher deposited oxide (Fig. 5.A.5d). Due to Fe addition, the surface of the MnO2 electrode becomes rough. However, as can be confirmed in Fig. 5.A.5e, the surface roughness began to decrease as the Fe addition in the plating solution is further increased. The surface became compact and is even smoother than the plain MnO2. Thus the results indicated that up to 150

10 2 at % the Fe addition modifies the surface of the MnO2 electrode. This type of amorphous and porous structures is expected to produce high supercapacitance values. Fig. 5.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. 151

11 Fig. 5.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 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. 5.A.6 (a and b)), and in Fig. 5.A.6 (b) there are few elemental peaks 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 5.A.2. Thus the elemental composition analyses showed that for 2 at % Fe doping in plating bath, only 0.69 at % Fe has resulted into MnO2 sample. Similar type of behavior has been reported by chang et al [8] for viologen doped MnO2 thin films using potentiodynamic and potentiostatic modes of electrodeposition. Fig. 5.A.6 The energy dispersive X-ray (EDAX) analysis of MnO2 and Fe: MnO2 (2 at % Fe) thin films on indium doped tin oxide (ITO) substrate. Table 5.A.2 Elemental composition analyses of MnO2 and Fe: MnO2 films Elements Mn-K Fe-K MnO2 Atomic % Fe: MnO2 (2 at Atomic % % Fe) 152

12 5.A Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM): Fig. 5.A.7 (a, b and c) shows transmission electron micrograph, corresponding SAED pattern and high resolution transmission electron micrograph of MnO2 film. Fig. 5.A.7 (a) shows that the growth has taken place cluster by cluster and randomly oriented nanocrystals are formed with amorphous matrix. The crystallites are grown together to form clusters where the crystallites are indistinguishable. Fig. 5.A.7 (b) shows corresponding selected area electron diffraction (SAED) pattern of warm like MnO2. 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 indicates that the warm like MnO2 has amorphous nature. Fig. 5.A.7 (a) TEM (b) SAED pattern and (c) HRTEM images of MnO2 thin film. 153

13 5.A.4.4 FTIR Studies Fig. 5.A.8 (a and b) shows the FTIR spectra of MnO2 and Fe: MnO2 (2 at %) samples, respectively. Samples showed several small peaks in the range of cm 1 and a broad peak at about 3416 cm 1 [9, 10]. These peaks are associated with the trace of surface-adsorbed moisture in the sample. It can be obviously observed that the stretching peak of OH groups is much more intensified, because the manganese oxide sample is synthesized in the aqueous solution which may form the tunnel structure during the reaction process, thus a small amount of H2O molecules is intercalated in the tunnel. Several small absorption peaks at around cm 1 are normally attributed to the bending vibrations of O H bonds connected with Mn atoms. The absorption peaks situated at 619 and 520 cm 1 as shown in Fig. 5.A.8 (a and b), are attributed to the vibrations of Mn O bonds [11]. The FTIR peaks in the range of cm 1 could reveal the existence of octahedral MnO6 [12]. Transmittance, T (%) (a) (b) 3416 cm cm cm cm cm Wave number (cm -1 ) Fig. 5.A.8 FTIR spectra of (a) MnO2 and (b) 2 at % Fe: MnO2 films. 154

14 The peaks associated with vibrations of Mn-O bonds are slightly shifted towards lower wavenumbers after Fe doping. Thus presence of Mn O bond and hydroxyl groups was confirmed from the FTIR spectrum of manganese oxide sample [13]. 5.A.4.5 Surface Wettability Test Wettability test is carried out to investigate the interaction between the electrolyte and MnO2and Fe: MnO2 electrode. If the wettability is high, contact angle (θ) is small and the surface is hydrophilic. On the contrary, if the wettability is low, θ is large and the surface is hydrophobic. Fig. 5.A.9 shows the water contact angles of MnO2 and Fe: MnO2 films for different Fe concentrations. For the MnO2 thin film, the water lies slightly flat on the warm-like structure with contact angle of 45. As the Fe doping concentration increases, the water contact angles of MnO2 thin films decreases. Fig. 5.A.9 Measurement of water contact angles for MnO2 and Fe: MnO2 thin films. 155

15 The contact angles were 38, 24, and 15 for 0.5, 1.0 and 2.0 at % Fe doping concentrations respectively were observed. Thus the result indicates that the contact angle decreases as the Fe doping concentration increases and finally film surface becomes more hydrophilic. Hydrophilicity is attributed to amorphous nature [14]. From the SEM images, it is confirmed that due to Fe doping the surface becomes porous. Thus, water placed on the surface of the film goes inside the pores due to which the contact angle decreases with increase in Fe doping concentration. This is useful for making the intimate contact of aqueous electrolyte with manganese oxide thin film (electrode). Thus, Na + /H + ions in aqueous electrolyte can easily intercalate or deintercalate, i.e., ionic conductivity increases. Further increase in Fe doping concentration decreases the porosity of the MnO2 electrode and hence there is again increase in the contact angle. 156

16 Section-B SUPERCAPACITIVE PERFORMANCE OF MnO2 AND Fe: MnO2 THIN FILMS BY GALVANOSTATIC 5.B.1 Introduction The present chapter deals with the synthesis and characterization of warm-like MnO2 and Fe: MnO2 thin films using galvanostatic mode. These films are further used for supercapacitor application. The effects of electrolyte concentration and scan rate on supercapacitance values of MnO2 and Fe: MnO2 electrodes have been investigated. Also the cycle life test (stability), charging discharging and impedance characteristics of MnO2 and Fe: MnO2 electrodes are studied. 5.B.2 Experimental Details of Evaluation of Supercapacitance: 5.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) 5.B.3 Results and Discussion 5.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. 5.B.3.2 Effect of Fe Doping Concentration Electrochemical behavior of the deposited oxides was evaluated using CV in 1.0 M Na2SO4 electrolyte with a potential scan rate of 100 mv.s - 1. Fig. 5.B.1 shows the voltammograms of MnO2 and Fe: MnO2 electrodes prepared galvanostatically with different Fe concentrations ranging from 157

17 0.5 to 4 at %. The rectangular shapes and mirror-image 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 [15-17]. The values of supercapacitance calculated from the CVs are 151, 159, 165, 173 and 169 F.g -1 for undoped, 0.5, 1.0, 2.0 and 4 at % Fe, respectively. It is clear from Fig.5.B.2 that as Fe doping concentration increases up to 2at %, the supercapacitance of MnO2 electrode increases from 151 to 173 F.g -1. Further increase in Fe doping concentration (4 at %) decreases the supercapacitance. This may be due to the decrease in the porosity of the MnO2 electrode which reduces the surface area of electrode. Similar type of behavior has been reported by Lee et al. [18] for anodically deposited Fe doped MnO2 thin films. In Fig.5.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. Current density (ma/cm 2 ) A A-Undoped B-0.5at% Fe C-1.0at% Fe D-2.0at% Fe E-4.0at% Fe B C Voltage (mv/sce) D E Fig. 5.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. 158

18 Supercapacitance (F.g -1 ) F.g F.g F.g F.g F.g Fe doping (at% ) Fig. 5.B.2 Variation of supercapacitance with Fe doping concentration. The data indicates that the specific capacitance increases from 151 F.g -1 to 173 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 169 F.g -1. The experimental results, shown in Fig.5.B.2, clearly show that the amount of Fe oxide added significantly affects the overall capacitance of the deposited binary Mn Fe oxide [18]. 5.B.3.3 Effect of Different Working Potential Windows The 2 at % Fe: MnO2 samples with different scanning potential ranges in a solution of 1.0 M Na2SO4 are shown in Fig. 5.B.3. The rectangular and symmetric curves are observed within the potential range of V, indicating its high electrochemical reversibility at the scan rate of 20 mv.s 1. The electrochemical evaluation of single electrode via cyclic voltammetry (CV) reveals the pseudocapacitive electrochemical properties of the Fe: MnO2. The shape of these CV curves is rectangularlike, indicating the highly electrochemical reversibility of redox transitions 159

19 oxide within -0.1 to +0.9 V/SCE potential limits. All the above results indicate that the redox transitions of oxymanganese species within MnO2 in 1.0 M Na2SO4 is highly reversible, which meets the high-power requirement for the application of supercapacitors. A tail shaped I-E curve exhibited within the potential window 0.9 to -0.3 V/SCE, indicates the non reversible redox reaction. 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 [17]. 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 B Voltage (mv/sce) C D E Fig. 5.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 concentration of Na2SO4 electrolyte was studied by keeping the scan rate and potential scanning range constant. The concentration of the Na2SO4 electrolyte was varied from 0.1 to 1.0 M. Fig. 5.B.4 shows the CV curves of Fe: MnO2 electrode at scan rate 100 mv.s

20 within the potential range of -0.1 to +0.9 V/SCE in Na2SO4 electrolyte of different concentrations. It is seen from the Fig. 5.B.4 that the current under curve increased as the concentration of Na2SO4 electrolyte increased from 0.1 to 1.0 M. The voltammograms were more distorted with the decrease in the electrolyte concentration. The continuous increase in the area under the CV was observed up to the 1.0 M Na2SO4 concentration; thereafter the current under the curve was fairly constant [19]. Current density (ma/cm 2 ) A B C D E 2 at % Fe: MnO 2 A- 0.1 M B M C M D M E- 1.0 M Voltage (mv/sce) Fig. 5.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. Xu et al. [20] prepared MnO2 thin films by micro-emulsion method and reported similar kind of behavior. 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. The specific and interfacial capacitances are increased with increasing the concentration of Na2SO4 electrolyte. All the values of specific and interfacial capacitances of the MnO2 and Fe: MnO2 electrodes are listed in Table 5.B

21 Table 5.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 ) B.3.5 Effect of Scan Rate The cyclic voltammetric (CV) curves for Fe: MnO2 electrode at different scan rates within voltage range of +0.9 to -0.1 V/SCE are shown in Fig. 5.B.5. It was found that, the area under curves was slowly increased with the scan rate. This shows that the CV currents are directly proportional to the scan rates of CV, which demonstrates, an ideal capacitive behavior. Since this proton transfer process is slow, higher scan rate leads to either depletion or saturation of the protons in the electrolyte inside the electrode during the redox process. This mainly results in the increase of ionic resistivity leading to drop in the capacitance of the electrode. The decreasing trend of the capacitance suggests that parts of the surface of the electrode material are inaccessible at high charging discharging rates. Hence, the specific capacitance obtained at the slow scan rates is believed to be closest to that of full utilization of the electrode material [21]. 162

22 Current density (ma/cm 2 ) D B C A E F 2 at% Fe: MnO 2 A-5 mv.s -1 B-10 mv.s -1 C-20 mv.s -1 D-50 mv.s -1 E-75 mv.s -1 F-100 mv.s Voltage (mv/sce) Fig. 5.B.5 Cyclic voltammograms of 2 at% Fe: MnO2 electrode at different scanning rates in 1 M Na2SO4 electrolyte. Table 5.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 )

23 The specific capacitance values decreased from 218 to 173 F.g -1 for Fe: MnO2 electrode. The maximum specific capacitance obtained for Fe: MnO2 was 218 F.g -1. The values of specific and interfacial capacitances at different scanning rates for MnO2 and Fe: MnO2 electrodes are listed in Table 5.B.2. 5.B.3.6 Stability Studies The electrochemical stability of the Fe: MnO2 electrode was 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 is presented in Fig.5.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. Current density (ma/cm 2 ) 6 2 at % Fe: MnO st cycle 1000 th cycle Voltage (mv/sce) Fig. 5.B.6 Cyclic voltammograms of 2 at% Fe: MnO2 electrode at different cycles at 100 mvs -1 scan rate. 164

24 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 84% and 91%, respectively. This implies an excellent long-term recycling capability. Lee et al. [22] prepared Fe doped MnO2 by potentiostatic deposition and reported that the capacitance ratio increased from 70 to 85 % due to Fe addition. 5.B.3.7 Galvanostatic Charge-Discharge Studies The galvanostatic charge-discharge profile of Fe: MnO2 electrode in 1.0 M Na2SO4 electrolyte is presented in Fig.5.B.7. The charge-discharge current rate is 0.2 ma and the operational potential range is between 0 and +0.8 V/SCE. It can be seen that the charge profile is slightly curved, 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 [23]. 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): For a specific energy of 1.2 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 1.6 Wh/kg but there is a 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 (76.14 %) of Fe: MnO2 than that for MnO2. All the values of specific energy (SE), and specific power (SP) and coulomb efficiency (η%) for MnO2 and Fe: MnO2 are listed in Table 5.B

25 0.8 2 at % Fe: MnO 2 Potential (V/SCE) Time (sec) Figure 5.B.7 Galvanostatic charge discharge curves of 2 at% Fe: MnO2 supercapacitor in 1.0 M Na2SO4 electrolyte. Table 5.B.3 Values of power density, energy density and coulomb efficiency of MnO2 and 2at % 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.5.B.8 shows electrochemical impedance spectrum in the form of Nyquist plot 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.5.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 166

26 low-frequency region can be attributed to the diffusion controlled process in the electrolyte [24]. 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 0.75 Ω in 1 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 are 24 Ω and 17 Ω, respectively. 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 at% Fe: MnO 2 -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 Warm-like MnO2 and Fe: MnO2 thin films are successfully prepared by galvanostatic 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 increase in 167

27 porosity which increases the pseudocapacitive performance of MnO2. Fe doping is confirmed from EDAX patterns. Wettability test shows that as Fe doping concentration increases the contact angle decreases. 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 218 F.g 1. Also the specific power and specific energy increased from 1.2 to 1.6 Wh/kg and to W/kg respectively due to the Fe addition. Moreover, capacitance-retained ratio of the Fe: MnO2 electrode after 1000 charge-discharge cycles was also improved from 88% to 95% 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. 168

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