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

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(-)/I-3(-), 2I(-)/I-2 and 2I(3)(-)/3I(2)) 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 (similar to 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.