Unveiling the Charge Storage Mechanism of Layered and Tunnel Structures of Manganese Oxides as Electrodes for Supercapacitors

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Abstract
The development of high performance active materials for supercapacitors that can outperform the state of art RuO2 is highly desirable to meet the next generation high power storage devices. In this regard, manganese oxide (MnO2) based materials have been intensively investigated in pseudocapacitors due to their high theoretical specific capacitance, natural abundance, good chemical and thermal stability, environmental benignity and low cost. Unfortunately, mechanism of electrochemical charge storage of MnO2 is not well understood. Here, we present a detailed charge storage mechanism of layered (δ-MnO2) and tunnel (α-MnO2) structures and the capacitance difference between layered and tunnel structures by preparing MnO2 nanostructures using permanganate and amino acids such as glycine, arginine and glutamic acid. Physico-chemical characterizations were carried out using powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. Field emission scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy were conducted to analyze the morphology and structural characteristics of the materials synthesized. The electrochemical performance of MnO2 nanostructures is assessed using cyclic voltammogram (CV) and galvanostatic charge-discharge analysis. Complete physico-chemical and electrochemical investigations strongly support that surface adsorption-desorption of cations is predominant in layered δ-MnO2, while tunnel α-MnO2 is more favorable for cation intercalation-deintercalation process.

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