Nanoporous Anodic Bismuth Oxide for Electrochemical Energy Storage

Abstract

Electrochemical energy storage using nanostructured metal oxides has been a topic of intensive investigation among materials scientists and electrochemists. Several transition metal oxides such as TiO2, WO3, Fe2O3, and V2O5 in the form of ordered nanoporous or nanotubular structures have been considered as active electrode materials for battery and electrochemical capacitor applications. Bismuth oxide (Bi2O3), because of its reasonable conductivity, high dielectric constant, catalytic behavior, non-toxicity, and abundance finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, and superconductors. Electro-deposited bismuth oxide (on various substrates and in various morphologies) has been investigated as a super capacitor material. The specific capacitance of the electro-deposited Bi2O3 varies from 60 -160 F/g. In the present investigation, Bi2O3 nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. The electrochemical energy storage of the anodic bismuth oxide was investigated by carrying out cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), and Mott-Schottky measurements in different electrolytes in the pH range of 7-14.All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. The XRD analysis showed that the nanoporous anodic oxide film after annealing was tetragonal β-Bi2O3. The nanoporous Bi2O3 contained a defect concentration in the range of 7x1016 – 4x1018 cm-3 for various anodized conditions.Figure 2 shows the typical results of cyclic voltammetry of as-anodized bismuth oxide nanoporous in 1 M KOH at different scan rates in a wide potential range. Oxidation of Bi (III) was not observed until 1.0 VAg/AgCl. However, a reduction wave was observed at potentials more negative than -0.6 VAg/AgCl, which could be attributed to the hydrogen ion intercalation in the oxide. Figure 3 shows the potential transient profile during the galvanostatic charge-discharge that was carried out at 10 mA/cm2.The high current density resulted in a sharp potential drop. The capacitance was calculated to be 78 mF/cm2 and the charge retention efficiency was 92%. The capacitance reported for the electrodeposited α-Bi2O3 was in the range of 12 - 22 mF/cm2 [1]. The anodic nanoporous oxide layer showed at least four times higher capacitance than the electro-deposited bismuth oxide.A detailed discussion will be provided in the final presentation on the electrochemical behavior of the Bi2O3 nanoporous structure as a function of morphology, and defect and electronic structures.