Chapter 17 - Metal oxide nanomaterials for supercapacitor applications

Abstract

The last decades have investigated many MeO supercapacitors and their role in energy devices; recently, many MeO supercapacitors such as NiO, MnO2, Co3O4, IrO2, TiO2, SnO2, V2O5, MnO, FeO, and RuO2. Among of them as Fe2O3 is cost effective and well known like non-toxic and eco-friendly, which is an attractive material for supercapacitor electrodes. Iron oxide has been studied to address the issue of low capacitance through structural development and complexation to maximize the use of surface pseudocapacitance. Nowadays, many researchers are interested in fixing these disadvantages like low conductivity and the low specific area to increase the specific capacitance of the Fe2O3. This chapter presents the synthesis of MeO supercapacitors and their various structures’ effect on the electrochemical performance of supercapacitor applications. Moreover, detailed information on synthesized α-Fe2O3 nanoflake and α-Fe2O3 nanotube with high specific surface area nanotube and limited performance of α-Fe2O3 nanotube was greatly improved by combination with polyaniline (PANI). However, the α-Fe2O3 nanoflake (NNF) electrode without any conducting material shows high specific capacitance. The α-Fe2O3 nanoflake samples were made with the following reaction times: NNF (30 min), NNF-1 (1 h), NNF-2 (2 h), and NNF-3 (6 h) and these samples were investigated for their effect on electrochemical performance. The electrochemical properties of α-Fe2O3 nanoflake plates yielded the highest specific capacitance of 171 F g−1 for the NNF-1 sample, in which the average flake size was 250 nm and the thickness was 30 nm. In the NNF-1 sample after 1000 cycles, the capacitance retention was 85% of the initial capacitance. And the α-Fe2O3 nanotubes (α-NT) were fabricated in a form in which the thickness and inner diameter of the tube were controlled by Fe(CO)5 vapor deposition using anodized aluminum oxide as a template. PANI was combined with the prepared α-NT in two forms: PANI@α-NT-a enclosed inside and outside with PANI, and PANI@α-NT-b containing PANI only on the inside. In contrast to α-NT, which showed a very low specific capacitance, these two composites showed significantly improved capacitances of 187 F g−1 for PANI@α-NT-a and 64 F g−1 for PANI@α-NT-b. In the electrochemical impedance spectroscopy analysis it was observed that the resistance of charge transfer was minimized in PANI@α-NT-a, and the pseudocapacitance on the entire surface of the α-Fe2O3 nanotubes was utilized with high efficiency through binding and conductivity improvement by PANI. PANI@α-NT-a exhibited a capacitance retention of 39% even when the current density was increased 10 times and showed excellent stability of 90.1% over 3000 charge–discharge cycles. This approach of incorporating conducting polymers through well-controlled nanostructures suggests a solution to overcome the limitations of α-Fe2O3 electrode materials and achieve performance improvement.