Agar-Activated Carbon\nCathode with Optimized Redox\nElectrolyte for High-Energy and Stable Aqueous Zinc Hybrid Battery–Capacitor

Recently, aqueous zinc hybrid battery–capacitors\n(AZHBCs)\nhave received significant attention owing to advantages such as low\ncost, high safety, high power, and a long cycle life. However, the\nlimited energy density of the current AZHBCs should be further improved\nby introducing cathode materials and optimized electrolytes to realize\ntheir large-scale applications. In this study, agar biopolymer-based\nactivated carbon optimized at 750 °C (AAC-750) with large specific\nsurface area, an optimized aqueous solution of ZnSO<sub>4</sub> +\nKI, and zinc metal were employed as the cathode material, redox electrolyte,\nand anode, respectively. An optimal AZHBC configuration, consisting\nof AAC-750//2 M ZnSO<sub>4</sub> + 0.3 M KI//Zn, exhibited an outstanding\nhigh capacity of 413 mAh g<sub>c</sub><sup>–1</sup>, a remarkable\nenergy density (∼508 Wh kg<sub>c</sub><sup>–1</sup>)\nthat breached the milestone of 500 Wh kg<sub>c</sub><sup>–1</sup> at 0.1 A g<sub>c</sub><sup>–1</sup> in the voltage range\nof 0.2–1.8 V, and an excellent cyclic stability with a capacity\nretention of 97% over 10 000 cycles at 10 A g<sup>–1</sup>. In terms of the mechanism, this remarkable performance can be ascribed\nto the polyiodide redox reaction (3I<sup>–</sup>/I<sub>3</sub><sup>–</sup>, 2I<sup>–</sup>/I<sub>2</sub>, and 2I<sub>3</sub><sup>–</sup>/3I<sub>2</sub>), the reversible ion adsorption\nof SO<sub>4</sub><sup>2–</sup>/I<sup>–</sup> at the\ncathode, and the reversible electrodeposition of Zn<sup>2+</sup> at\nthe anode, respectively. In addition, theoretical analyses were carried\nout to understand the molecular dynamics of various aqueous ZnSO<sub>4</sub> + KI electrolyte systems and the adsorption process of polyiodide\nions. The proposed strategy provides a way to design high-energy and\nstable AZHBCs with an appropriate electrolyte system and a biopolymer-derived\ncarbon cathode.