Structural-chemistry of molybdenum oxide and its derivatives and their application in supercapacitors

Abstract

Electrochemical capacitors or supercapacitors are the alternative energy-storage (electrical as well as electrochemical) devices, which can deliver high power in a short period of time (ms-s). Supercapacitor devices are known to have high power density (1-10 kW/kg), but they have limited energy density (5-20 Wh/kg). The enhancement in the energy density without sacrificing the power density will be advantageous for various applications, such as portable electronic devices, industrial heavy vehicles, and energy back-up and so on. This dissertation mainly focuses on the structural-chemistry of molybdenum oxides and their derivatives to improve the pseudocapacitive performance of transition metal oxide based electrode materials. In this dissertation, several strategies are proposed in order to enhance the electrochemical performance of molybdenum oxide based supercapacitor devices. The corresponding strategies are as follows; firstly, three dimensional or open-structure of molybdenum trioxide (MoO3), i.e., hexagonal-MoO3 (h-MoO3) provides facile paths as well as additional intercalation sites for the electrolyte ions, which in response improves the specific capacitance. Secondly, incorporation of the metal or transition metal elements into the lattice of molybdenum oxide (metal or transition-molybdate) not only tunes the crystallographic structure but also improves the electrochemical properties, resulting in the high capacity and excellent energy density of supercapacitor device. Moreover, evaluation of the localized electrochemical activity of the electrode material is of great interest to provide the insight into the mechanism of charge transfer across the interface. Scanning electrochemical microscopy (SECM) is employed for the local analyses at the surface of electrode materials. Besides studying the charge transfer kinetics, SECM is also used to study the growth of the diffusion layer or propagation of the electronic or ionic charges. The work described herein contributes to the synthetic strategies to synthesize and to tune the crystal-chemistry of molybdenum oxide based nanomaterials for electrochemical energy-storage applications. Also, the work presented here has shed some light on the fundamental understanding and advancement to optimize the pseudocapacitive performance of the electrode materials, in order to realize an excellent supercapacitor device.