Effect of Lattice Structure of Bismuth Sesquioxide on the Electrochemical Energy Storage Characteristics
Authors: Kersten Bethany R , Zillinger James , Utgikar Vivek , Day Brandon , Raja Krishnan S. 
Published: February 2020

Bismuth sesquioxide (Bi2O3) exists in different polymorphs such as monoclinic α phase, tetragonal β phase, face centered cubic (fcc) δ phase, and body centered cubic (bcc) γ phase. The stable phase at room temperature is α- phase, and at temperatures between 729-824 °C the δ phase is stable. Other phases are metastable which transform from the δ phase during the cooling cycle. The metastable phases can be stabilized at room temperature either by doping with aliovalent cations in the Bi2O3 lattice or by controlling the synthesis parameters.
Bi2O3 polymorphs are used in several engineering applications such as electrode materials for sensors, photocatalysts, and solid state electrolytes for fuel cells. Furthermore, Bi2O3 is a building block for advanced ferroelectric, and multiferroic materials. Our modeling calculations suggest that β-Bi2O3 could show auxetic behavior in certain crystallographic planes. Bi2O3 has been investigated as a potential supercapacitor electrode material because of its promising theoretical specific capacitance (1370 F/g). Depending on the morphology and the matrix in which a composite structure was formed, the reported capacity varied from 94 – 332 F/g [[1], [2]]. Most of the investigations focused on the morphology of the Bi2O3 or the composite structure of the electrode. The investigated material was either alpha [[3]] or delta phase [[4]], but no particular attention was given to the crystal structure. The effect of the lattice structure of the Bi2O3 on the energy storage properties was not investigated in detail to the best of our knowledge. A recent report focused on the engineered lattice defects to enhance the capacitance [4]. In this presentation, the results of electrochemical energy storage behavior of Bi2O3 electrodes prepared in the form of pure α-phase, a mixture of α+β, β-phase, and δ by electrodeposition will be reported.
The α-Bi2O3 thin film specimens were electrodeposited on to ITO-coated glass surfaces under galvanostatic condition at +5 mA/cm2 in a 100 ml solution containing 0.1 M bismuth nitrate, 0.2 M tartaric acid, and NaOH to adjust the pH to 12.0 at room temperature. In order to obtain β-Bi2O3 deposition, 0.05 M of Na2Cr2O7 was added to the solution used for electrodepositing the α-phase and the electrodeposition was carried out under galvanostatic condition at 45 °C. Thin film electrodeposits with mixed α+β phases were obtained by varying the dichromate concentration in the electrolyte. The δ-Bi2O3 was obtained by using a pH:14 solution containing bismuth salt on to a fcc lattice substrate (austenitic stainless steel) at 65 °C, following a similar procedure reported by Helfen et al. [[5]].
Cyclic voltammetry, and galvanostatic charge-discharge experiments were carried out on the thin film Bi2O3 specimens in 0.5 M LiCl + 0.1 M NaOH solution at room temperature. The β-Bi2O3 specimens showed significantly higher specific capacitance than the α-Bi2O3. The electrochemical behavior of different phases will be discussed based on the impedance spectroscopy, and Mott-Schottky results before and after charge-discharge cycles.
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