Green synthesis of high-performance gallium oxide supercapacitor: A path to outstanding energy density

Abstract

<p>Gallium oxide (Ga<sub>2</sub>O<sub>3</sub>) powder was prepared by mixing hydrogen peroxide (30% H<sub>2</sub>O<sub>2</sub>) with pure gallium metal (99.999%) inside a hydrothermal cell. The resulting white powder was subjected to different characterization techniques. X-ray diffraction (XRD) revealed the presence of multiple crystalline phases of gallium oxide, including monoclinic β-Ga<sub>2</sub>O<sub>3</sub>, and rhombohedral α-Ga<sub>2</sub>O<sub>3</sub>, as well as gallium suboxide Ga<sub>2</sub>O. X-ray photoelectron spectroscopy (XPS) divulged Ga<sup>3+</sup> as the dominant oxidation state of gallium in Ga<sub>2</sub>O<sub>3</sub>. However, a lower oxidation state, Ga<sup>1+</sup>, was also detected. Scanning electron microscopy (SEM) images showed a high degree of morphological diversity with a wide variety in shape and size of powder particles. Porosity measurements were performed to determine the total surface area, pore diameter, and pore volume of Ga<sub>2</sub>O<sub>3</sub> powder. The values were found to be 50 m<sup>2</sup> g<sup>−1</sup>, 2.6 nm, and 0.07 cm<sup>3</sup> g<sup>−1</sup>, respectively. Fascinatingly, gallium oxide powder was directly used to fabricate a symmetrical supercapacitor without any binder. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were performed to examine the electrochemical energy storage capabilities of Ga<sub>2</sub>O<sub>3</sub> powder using 1M KOH as electrolyte. The fabricated supercapacitor demonstrated a maximum specific capacitance of 1176 F g<sup>−1</sup>, at a current density of 2 A g<sup>−1</sup>, an energy density of 104.5 Wh kg<sup>−1</sup>, and a high-power density of 1.6 kW kg<sup>−1</sup>. Additionally, following 5000 charge-discharge cycles, the supercapacitor demonstrated outstanding capacitance retention stability of 91.18%. The obtained energy density of 104.5 Wh kg<sup>−1</sup> is among the highest reported for metal oxide-based supercapacitors. The presence of multiple crystalline phases in the gallium oxide powder likely contributed significantly to the remarkable results observed in this study.</p>