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

Abstract

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 ZnSO₄ + 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 ZnSO₄ + 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^–/I₃^–, 2I^–/I₂, and 2I₃^–/3I₂), the reversible ion adsorption of SO₄^2–/I^– at the cathode, and the reversible electrodeposition of Zn²⁺ at the anode, respectively. In addition, theoretical analyses were carried out to understand the molecular dynamics of various aqueous ZnSO₄ + 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.