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.

[1] S. X. Wang, C. C. Jin, W. J. Qian, J. Alloys Compd. 2014, 615, 12

[2] M. Ciszewski, A. Mianowski, P. Szatkowski, G. Nawrat, J. Adamek, Ionics 2015, 21, 557.

[3] S.T. Senthilkumar, R. Kalai Selvan, M. Ulaganathan, J.S. Melo, Electrochimica Acta 115 (2014) 518– 524

[4] R. Liu, L. Ma, G. Niu, X.-L. Li, E. Li, Y.Bai, G. Yuan, Adv. Funct. Mater. 2017, 27, 1701635

[5] A. Helfen et al., Solid State Ionics 176 (2005) 629–633