To meet the global energy demand, a high-energy and high-power Energy Storage System (ESS) is the prime requirement for sustainable energy. Battery and Electrochemical Capacitor (EC) are two different kinds of electrochemical ESS and occupy two different regions in the Ragone plot. In terms of performance, both are complementary to each other. Therefore, the hybridization of both battery and EC can be a promising approach to gaining the combined merits of both ESSs. That means a Hybrid Electrochemical Capacitor (HEC), which is an assembly of a battery-type electrode with a capacitive electrode, one being as the negative electrode and the other as the positive electrode, could be a potential electrochemical ESS for the future. Such an HEC can extend the operating potential by utilizing the potential window from both electrodes. Many researches have been done to develop positive electrodes such as RuO₂, MnO₂, Co₂O₃, etc. In contrast, advances in negative electrodes are less. Besides carbon-based materials, iron oxides and bismuth oxides are explored as negative electrodes. Oxides of bismuth have gained attention due to their high theoretical specific capacity of 345 mAh g^-1 .
In this work, Polycrystalline β-Bi₂O₃ nanosheets with interacting network structures were synthesized by using tetrabutylammonium bromide as a structure-directing agent. The sample calcined at 300°C retained organic content and a polycrystalline structure, both crucial for the material's electrochemical performance as a negative electrode. The positive electrode was fabricated using a bio-derived activated carbon. Both electrode materials were thoroughly characterized electrochemically using three-electrode cells; where Ag/AgCl (KCl) was used as a reference electrode, Pt wire as a counter electrode, and aqueous Na₂SO₄ as electrolyte. The pouch-type HEC was assembled using an aqueous Na₂SO₄ electrolyte, operating within a wide potential window of 2.0 V. The device demonstrated good electrochemical performance, with areal capacitance of 131.5 mF cm^-2, energy density of 73.1 μW h cm^-2, and power density of 999.9 μW cm^-2 at a current density of 1 mA cm^-2. The interconnected network nanosheet morphology and polycrystalline structure of the β-Bi₂O₃ material resulted in cyclic stability of 97.8% capacitance retention after 10,000 galvanostatic charge-discharge cycles. Moreover, 72.9% diffusion contribution at 5 mV s^-1 to the total current of the device established its hybrid nature.

Debajyoti Mahanta

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