A low cost Zn2+/I− redox active electrolyte for a high energy and long cycle-life zinc hybrid battery–capacitor

Abstract

The limited capacities and energy densities in zinc hybrid capacitors are due to the cathode materials and electrolytes that should be further improved to satisfy large-scale applications. To overcome this challenge, in this study, an aqueous zinc hybrid battery–capacitor (AZHBC) using low cost/concentration KI redox additive-added aqueous ZnSO₄ as the electrolyte, Zn metal as the anode, and heteroatom-containing porous carbon (PC) derived from gelatin bio-polymer as the cathode material is developed. The AZHBC configured as Zn//ZnSO₄ + KI//PC exhibited a high capacity of 399 mA h g_C⁻¹ and a high energy density of 479.1 W h kg⁻¹ in the voltage range of 0.2–1.8 V, which were higher than those of the high-cost ZnI₂ additive-containing Zn//ZnSO₄ + ZnI₂//PC AZHBC, respectively. This result can be attributed to the role of the KI additive, which enhances polyiodide redox reactions (3I⁻/I₃⁻, 2I⁻/I₂ and 2I₃⁻/3I₂) at the interfacial surface of the PC electrode, and the enhanced faradaic reaction. Additionally, 1 M ZnSO₄ + 0.08 M KI is identified as the optimal electrolyte concentration as it exhibited the best performance (high capacity retention (∼94.5%) after 6000 prolonged cycles). The strategy proposed in this correlation study will provide insight into the exploration of suitable and best redox additives for high-energy low-cost redox active-AZHBCs.