Studies Of MnO2 As Active Material For Electrochemical Supercapacitors
Electrical double-layer formed at the interface between an electrode and an electrolyte has been a topic of innumerable studies. The electrical interface plays a crucial role in kinetics, mechanisms and applications in variety of electrochemical reactions. The electrical double-layer and electron-transfer reactions lead to many important applications of electrochemistry, which include energy storage devices, namely, batteries, fuel cells and supercapacitors. Electrochemical supercapacitors can withstand to higher power than batteries and deliver higher energy than the conventional electrostatic and electrolytic capacitors. A supercapacitor can be used as an auxiliary energy device along with a primary source such as a battery or a fuel cell for the purpose of power enhancement in short pulse applications. Among the various materials studied for electrochemical supercapacitors, carboneous materials, metal oxides and conducting polymers received attention. Among carboneous materials, various forms of carbon such as powders, woven cloths, felts, fibers, nanotubes etc., are frequently studied for electrochemical supercapacitors. Low cost, high porosity, higher surface area, high abundance and well established electrode fabrication technologies are the attractive features for using carboneous materials. However, specific capacitance (SC) of these materials is rather low. These electrodes store charge by electrostatic charge separation at the electrode/electrolyte interface. Electronically conducting polymers are interesting class of materials studied for supercapacitor application because of the following merits: high electronic conductivity, environmental friendliness, ease of preparation and fabrication, high stability, high capacitance and low cost. Polyaniline (PANI), polypyrrole and polythiophene are studied in this category. Transition metal oxides have attracted considerable attention as electrode materials for supercapacitors because of the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and handling. Hydrated RuO2 prepared by sol-gel process at low temperature has a specific capacitance as high as 720 F g-1 due to solid state pseudo faradaic reaction. However, high cost, low porosity and toxic nature limit commercialization of supercapacitors using this material. MnO2 is attractive as it is cheap, environmentally benign, its resources are abundant in nature and also it is widely used as a cathode material in batteries. An early study on capacitance behaviour of MnO2 was reported by Lee and Goodenough. Amorphous hydrous MnO2 synthesized by co-precipitation method exhibited rectangular cyclic voltammogram in various aqueous alkali salt solutions. A specific capacitance of 200 F g-1 was reported. Following this report, several reports appeared on capacitance characteristics of MnO2. According to the charge-storage mechanism reported, a specific capacitance of 1370 F g-1 is expected from MnO2. However, this value can be obtained in practice only when the mass of MnO2 is at the level of a few micrograms per cm2 area. At such a low thickness range, the utilization of the active material is high. As thin layers of MnO2 are uneconomical for practical capacitors, studies with a mass range of 0.4-0.5 mg cm-2 have been extensively reported. At this mass range, a maximum specific capacitance of about 240 F g-1 has been obtained. With an increase in mass per unit area, the specific capacitance of MnO2 decreases. The problem associated with low values of specific capacitance of thick layers of MnO2 is the following. The MnO2 deposits or coatings generally do not possess high porosity and the electrolyte cannot permeate into the coating. Only the outer layer of the electrode is exposed to the electrolyte. Consequently, the electrochemical utilization of the material decreases with an increase in thickness. Nevertheless, utilization of thick layers of the active materials is preferable for obtaining capacitance as high as possible in a given volume and area of the electrodes. Indeed, it would be ideal if specific capacitance of MnO2 is improved from its presently reported value of 240 F g-1 to a value equivalent to that of RuO2.xH2O, namely, 720 F g-1. In view of this, attempts are made to enhance specific capacitance of MnO2 by electrochemical deposition in presence of surfactants. Nanostructured MnO2 synthesized by inverse microemulsion route is also studied for electrochemical supercapacitors. The effect of crystallographic structure of MnO2 on the capacitance properties, studies on electrochemical deposition of MnO2 in acidic and neutral medium using electrochemical quartz crystal microbalance and capacitance characteristics of MnO2-polyaniline composites are also described in the thesis. Chapter 1 briefly discusses the importance of electrochemistry in energy storage and conversion, basics of electrochemical power sources, importance of MnO2, different synthetic procedures for MnO2 and its applications in energy storage and conversion in particular for electrochemical supercapacitors. Chapter 2 provides the experimental procedures and methodologies used for the studies reported in the thesis. In chapter 3, the effect of surface active agents, namely, sodium dodecyl sulphate (SDS) and Triton X-100 added to the electrolyte during electrodeposition of MnO2 on Ni substrate on capacitance properties is presented. Electrocrystallization studies show that MnO2 nucleates instantaneously under diffusion control and grows in three dimensions. The potentiodynamically prepared oxide provides higher specific capacitance than the potentiostatically and galvanostatically prepared oxides. Specific capacitance values of 310 and 355 F g-1 obtained for MnO2 electrodeposited in the presence of 100 mM SDS and 10 mM Triton X-100 are higher than the oxide electrodeposited in the absence of surfactants. Surfactant molecules adsorbed at the electrode/electrolyte interface alters structure of double-layer and kinetics of electrodeposition. Smaller particle size, greater porosity, higher specific surface area and higher efficiency of material utilization are the factors responsible for obtaining higher specific capacitance. Extended cycle-life studies indicate that the superior performance of MnO2 due to surfactants is present throughout the cycle-life tested. Chapter 4 pertains to electrochemical supercapacitor studies on nanostructured α-MnO2 synthesized by inverse microemulsion method and the effect of annealing. As synthesized nanoparticles of MnO2 was found to be in α-crystallographic structure with particles less than 50 nm size. Nanoparticles exhibited rectangular cyclic voltammograms between 0 and 1 V vs. SCE in aqueous 0.1 M Na2SO4 at sweep rates up to 100 mV s-1 due to the short diffusion path length. On annealing at different temperatures, a mixture of nanoparticles and nanorods with varying dimension is noticed. Specific capacitance of 297 F g-1 obtained during initial cycling decreases gradually on extended cycling. The capacitance loss is attributed to the increase in the resistance for intercalation/deintercalation of alkali cations into/from MnO2 lattice. MnO2 crystallizes into several crystallographic structures, namely, α-, β-, γ-, δ- and λ-structures. As these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or inter-layers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gap to accommodate these ions, are expected to be useful for capacitance studies. The effect of crystal structure of MnO2 on its electrochemical capacitance properties is also included in chapter 4. Specific capacitance of MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: α ≅ δ > γ > λ > β. A specific capacitance value of 240 F g-1 is obtained for α-MnO2, whereas it is 9 F g-1 for β-MnO2. A wide (~ 4.6 Å) tunnel size and large surface area of α-MnO2 are ascribed as favorable factors for its high specific capacitance. A large interlayer separation (~7 Å) also facilitates insertion of cations in δ-MnO2 resulting in SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into β-MnO2. As a result, it provides very small SC. In Chapter 5, capacitance characteristics of PANI synthesized using (NH4)2S2O8, nanostructured MnO2 (α- and γ-form) and also PANI-MnO2 composites are presented. Morphology of PANI synthesized resembles the morphology of the MnO2 used as the oxidant. Electrochemical capacitance properties of PANI and composites are studied in a mixed electrolyte of 0.1 M HClO4 and 0.3 M NaClO4 between 0 and 0.75 V vs. SCE. Specific capacitance of 394 F g-1 is obtained for PANI synthesized using γ-MnO2. Chapter 6 describes the electrocatalytic behaviour of Mn3[Fe(CN)6]2 synthesized by ion-exchange reaction between MnSO4 and K3[Fe(CN)6] and the effect of annealing on its electrochemical capacitance properties. As prepared Mn3[Fe(CN)6]2 and also the sample heated at 100 oC exhibit redox couple in 0.1 M Na2SO4 electrolyte, corresponding to Fe(CN)64-/Fe(CN)63- present in the matrix. Mn3[Fe(CN)6]2 samples annealed at 150 oC and above decompose to oxides of manganese and iron, and hence exhibit capacitance characteristics in 0.1 M Na2SO4 electrolyte. A maximum specific capacitance of 129 F g-1 is obtained for Mn3[Fe(CN)6]2 annealed at 300 oC. Electrochemical quartz crystal microbalance (EQCM) investigations of kinetics of electrodeposition of MnO2 in acidic and neutral media, and capacitance behaviour are presented in chapter 7. Oxidation of Mn2+ to MnO2 is characterized by an anodic cyclic voltammetric peak both in acidic and neutral media. During the reverse sweep, however, reduction of MnO2 into Mn2+ occurs in two steps in the acidic medium and in a single step in the neutral medium. From EQCM data of mass variation during cycling, it is observed that the rate of electrodeposition of MnO2 is higher in the neutral medium than in the acidic medium. Specific capacitance of MnO2 deposited from the neutral medium is higher than that deposited from acidic medium owing to different crystallographic structures. Reversible insertion/deinsertion of hydrogen in to the layers of δ-MnO2 is observed in hydrogen evolution region. Details of the above studies are described in the thesis.
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