Manganese oxide, an organic compound has a molecular formula MnO2. Originally exists in the form of brown or blackish solid form and mostly found in the form of minerals such as pyrolusite. It is mostly used in the dry cell batteries as well as alkaline and other batteries. Some other uses include pigments and precursor for many other manganese compounds. Organic synthesis involves the use of manganese oxide as regent i.e. synthesis of the allylic alcohol. MnO2 is very much effective in the form of α-polymorph and can incorporate the plenty of atoms and molecules via tunnels and channels with octahedral manganese oxide. This α-manganese oxide seeks an interest in the research field in order use it as a cathode to construct lithium ion batteries (Tompsett et al., 2013).
1.1.1. Crystal structures of Manganese oxide
Many polymorphs as well as dehydrated MnO2 have been reported. Beyond this many other dioxides have been reported. The MnO2 is also reported to crystalize in rutile crystal structure. This is termed as pyrolusite. It has the metal octahedral center and three coordinate oxides and has the nonstoichiometric characteristics. It is found to be deficient in the oxygen. Most importantly the chemistry of this material is found to be as the organically synthesized fresh MnO2. On the other hand alpha MnO2 consists of open structure in the channels which are able to adjust the other metallic atoms including silver. One of the most important accommodations is the barium alpha MnO2 which is called hollandite. Manganese oxides, including MnO, MnO2 and Mn3O4, are fascinating combinations and have been used in wastewater treatment, catalysis, super-capacitors, sensors, alkaline and rechargeable batteries
(Greenwood et al., 2012).
Figure 1.1: Different Crystal Structures of Manganese oxide
1.1.2. Properties and applications of Manganese oxides
Manganeseis a famous component of used in fertilizers and food additives. Thousands of tons is the range of manganese oxide in the industries such as fertilizers and food supplements due to effectiveness of this substance. It is also used as catalysis for the production of different things which include coloured glasses, allyl alcohol, ceramics and paints. Most important application is the dry cell batteries. The maximum amount of the manganese oxide is consumed in the production of dry cell batteries. However the alkaline batteries now a day are dominating the dry cell batteries but still manganese oxide is utilized in these batteries. Manganese oxide is also found naturally but contains many impurities and other oxides such as manganese III oxide. A small number of bonds contains γ-MnO2 which is nearly enough for the dry cells and batteries. Although batteries as well as ferrites are devised by manganese but it requires the high purity. Batteries require electrolyte manganese dioxide and ferrites require chemical manganese dioxide (Topmsett et al., 2013).
22.214.171.124. Chemical manganese dioxide
Many methods are used to synthesize chemical manganese dioxide. In one method natural manganese dioxide is used and a nitrate solution of water and di-nitrogen tetroxide is finalised for conversion into chemical oxide. Solution is evaporated and the crystalline nitrate salt is formed. The decomposition of salt occurs which results in formation of N2O2 and pure manganese dioxide at 400 degree temperature. In second method the manganese III oxide is reduced by using manganese dioxide carbo thermically. It is further dissolved in sulphuric acid and ammonium carbonate is used to treat with the filtered solution in form of precipitate MnCO3. Calcination of the carbonate is processed with air to get manganese II and IV oxides. For the completion of the proceedings the sulphuric acid with the suspension of material is treated with the sodium chlorate. Any Mn II or III oxide is converted to dioxide by the chloric acid and given out the residual in form of chlorine. Manganese heptoxide and monoxide are used in third process in 1:3 ratios for the reduction of manganese dioxide (Clapsaddle et al., 2004).
126.96.36.199. Electrolytic manganese dioxide
It is used in batteries made up of zinc carbon with ammonium and zinc chloride. Electrolytic manganese dioxide abbreviated as EMD is mostly used in rechargeable cells composed of zinc manganese dioxide. However most important is the purity of substance and synthesized in the pure form just like as electrolyte tough pitch copper (ETP). Sulphuric acid is used to dissolve manganese dioxide and solution is treated with heavy current by placing it between two electrodes. The dissolved MnO2 is found in the centered solution in form of sulphate and sticks to the positive anode (Clapsaddle et al., 2004).
Figure 1.2: Uses of MnO2 in Dry Battery
1.2. MnO2 Nanostructure
Manganese dioxide (MnO2) is well-thought-out to be one of the most capable materials for energy storage systems which includes batteries and super-capacitors. As compared to the batteries, super-capacitors have higher power density which is 10 to 100 times more than batteries, fast charging rate of a few seconds to minutes and excellent cycling stability of more than one million cycles. This makes super-capacitors ideal devices for many important applications like hybrid electric vehicles, consumer electronics and smart grid storage (Miller et al., 2008). However, their energy density is relatively low approximately 10 times than batteries which minimize the use of material for the production of energy storage devices. Carbon based super-capacitors have low charge storage bulk due to the restricted electrolyte ion adsorption and desorption on the surface of the carbon. Therefore, carbon materials in commercial super-capacitors replacement by high capability materials like MnO2 can be much more effective in order to increase the energy density of super-capacitors, thus flooring the way for their adoption in a variety of applications. However, oxide electrodes have poor cycling performance, which needs to be investigated (Toupin et al., 2002).
Various synthesis methods and a large variety of MnO2 with different morphologies and crystalline phases have been reported to prepare MnO2 nanostructures. However, a systematic study focused on studying the effect of MnO2 Nano scale morphology and crystal phase and its cycling performance on super-capacitor capacity has not been performed. MnO2 electrode with thick layer underwent no change of the Mn oxidation state because only a very thin layer of the material contributed to the energy storage process. Afterwards, a study carried out of the influence of MnO2 morphology on the electrochemical properties by controlling the material synthesis conditions. The electrochemical tests demonstrated that the storage capacity of MnO2 electrodes is highly related to its morphology, nanostructure, and surface area, but the study did not provide very detailed material analysis to elucidate the factors that control electrochemical performance of the devices. In order to achieve the best energy storage performance of MnO2 and better understand the energy storage mechanism, it is very important to study not only the general shape or nanostructure of MnO2 but also the evolution of the nanostructures with electrochemical cycling to the super-capacitor performance (Toupin et al., 2004).