Agar-Activated Carbon Cathode with Optimized Redox Electrolyte for High-Energy and Stable Aqueous Zinc Hybrid Battery–Capacitor

Recently, aqueous zinc hybrid battery–capacitors (AZHBCs) have received significant attention owing to advantages such as low cost, high safety, high power, and a long cycle life. However, the limited energy density of the current AZHBCs should be further improved by introducing cathode materials and optimized electrolytes to realize their large-scale applications. In this study, agar biopolymer-based activated carbon optimized at 750 °C (AAC-750) with large specific surface area, an optimized aqueous solution of ZnSO 4 + KI, and zinc metal were employed as the cathode material, redox electrolyte, and anode, respectively. An optimal AZHBC configuration, consisting of AAC-750//2 M ZnSO 4 + 0.3 M KI//Zn, exhibited an outstanding high capacity of 413 mAh g c –1, a remarkable energy density (∼508 Wh kg c –1 ) that breached the milestone of 500 Wh kg c –1 at 0.1 A g c –1 in the voltage range of 0.2–1.8 V, and an excellent cyclic stability with a capacity retention of 97% over 10 000 cycles at 10 A g –1 . In terms of the mechanism, this remarkable performance can be ascribed to the polyiodide redox reaction (3I – /I 3 –, 2I – /I 2, and 2I 3 – /3I 2 ), the reversible ion adsorption of SO 4 2– /I – at the cathode, and the reversible electrodeposition of Zn 2+ at the anode, respectively. In addition, theoretical analyses were carried out to understand the molecular dynamics of various aqueous ZnSO 4 + KI electrolyte systems and the adsorption process of polyiodide ions. The proposed strategy provides a way to design high-energy and stable AZHBCs with an appropriate electrolyte system and a biopolymer-derived carbon cathode.