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Abstract:
Iron oxide (Fe2O3) is attractive for energy storage due to its low cost, abundance, and eco-friendliness, but suffers from poor cyclic stability and capacitance fading. Here, we systematically investigate how calcination temperature influences the structure, morphology, and electrochemical properties of surfactant-assisted self-assembled iron oxide nanoflowers. Variation of calcination temperature from 300 °C to 600 °C strongly affects the phase, crystal structure, morphology, surface area, porosity, and electrochemical properties of the oxides. The low-temperature calcination of iron oxide at 300 °C leads to a distinctive mesoporous flower-like morphology, high surface area (145.76 m2 g−1), and mixed-phase (maghemite and hematite) composition with low crystallinity, resulting in the highest specific capacitance (182.3 F g−1 at 1 A g−1), low internal resistance with enhanced capacitive behavior. In contrast, samples calcined at higher temperatures than 300 °C exhibit reduced surface area, enhanced phase purity (pure hematite), and diminished electrochemical activity. A low-cost pouch-type asymmetric capacitor is fabricated using Fe2O3 nanoflowers calcined at 300 °C and activated carbon, delivering 24.33 μWh cm−2 energy density, 448.91 μW cm−2 power density, and 78.8% capacitance retention with 97% coulombic efficiency after 10,000 cycles. These results underscore the pivotal role of calcination temperature in optimizing Fe2O3 nanostructures for efficient energy storage.

Keywords:
Iron oxide nanoflowers, Calcination temperaturę, Phase-morphology correlation, Pseudocapacitance, Asymmetric electrochemical capacitor

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