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Abstract:
A novel, template-assisted synthesis strategy for producing hollow polypyrrole (PPy) nanoparticles (H-PPy) with an average diameter of ≈70 nm is reported. Unlike conventional methods, the approach uniquely exploits an in situ reaction between FeCl3 and CaCO3 to simultaneously generate Fe(OH)3 nanoparticles that act as dynamic, self-decomposing templates for PPy deposition. This concurrent template formation and polymer growth restricts Fe(OH)3 particle size via rapid PPy surface encapsulation, facilitating the formation of uniformly dispersed PPy-coated Fe(OH)3 particles (T-PPy). Subsequent removal of unreacted CaCO3 and Fe(OH)3 yields hollow PPy nanoparticles (H-PPy) with a 30% size reduction due to contraction of the soft PPy, resulting in a high surface area morphology. The H-PPy exhibits excellent electrochemical performance as a cathode material in both pouch-type and all-solid-state asymmetric electrochemical capacitors. The specific capacitance of H-PPy calculated by using three electrode electrochemical cell is found to be 158.2 Fg−1 at 1 Ag−1 which is much higher than that of bulk PPy and T-PPy. Areal capacitances of 40.2 and 9.6 mF cm−2 at 0.2 mA cm−2 are obtained for the pouch and solid-state cells, respectively, where activated carbon electrodes are used as anode. The pouch cell demonstrates remarkable cycling stability, retaining 104.6% of its initial capacitance after 15 000 cycles

Abstract:
One of the foremost challenges in advancing aqueous electrochemical energy storage devices is improving their energy density and cyclic stability performance while preserving high power density. In this study, oxygen-deficient Bi2O2.33 nanosheets with interacting network structure are synthesised using tetrabutylammonium bromide (TBAB), which perfectly works as electrode material in the potential window from -1 to 0 V (vs. Ag/AgCl) in 0.5 M Na2SO4 electrolyte. Interestingly, skipping the addition of TBAB in the synthesis procedure leads to the formation of α-Bi2O3 with an irregular aggregated morphology, resulting in poor electrochemical performance in the three-electrode electrochemical cell as compared to Bi2O2.33. The specific capacity found for Bi2O2.33 electrode is 555.4 C g-1, while it is found to be 129 C g-1 for α-Bi2O3 at a current density of 1 A g-1. This Bi2O2.33 electrode, which is identified as a battery-type electrode, is further successfully combined with a bio-derived activated carbon electrode, a well-known capacitive electrode material, by balancing the charges to fabricate a pouch-type hybrid electrochemical capacitor (HEC). The pouch-type HEC, using aqueous Na2SO4 electrolyte with a 2.0 V potential window, delivers excellent performance: areal capacitance of 131.5 mF cm-2, volumetric capacitance of 526.1 mF cm-3, 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 fabricated device provides capacitance retention of 97.8 % after 10,000 continuous galvanostatic charge-discharge (GCD) cycles

Keywords:
Asymmetric supercapacitor, Pouch-type hybrid capacitor, Aqueous electrochemical capacitor, Polycrystalline metal oxide, Battery-type electrode