A high-capacity 2.5 V aqueous K+/Zn2+ mixed ions hybrid supercapacitor enabled by an electrochemically pre-intercalated α-MnO2/carbon black cathode

Abstract

Due to their inherent safety and environmental sustainability, aqueous zinc-ion hybrid supercapacitors (AZIHCs) continue to attract significant research attention. Among various cathode materials, manganese dioxide (MnO₂) stands out for its low cost and low toxicity. However, challenges such as limited operating voltage, poor cycling stability, and low energy density hinder its broader application. In this work, an α-MnO₂ functionalized low-cost carbon black composite, denoted as CB-MnO₂@Zn, is synthesized via a simple co-precipitation method followed by electrochemical Zn²⁺ modification, as a high-performance cathode material for efficient Zn²⁺ intercalation/deintercalation reaction. The composite was evaluated for both supercapacitor and aqueous Zn²⁺/K⁺ hybrid ions battery applications in a dilute 1 mM Zn²⁺ solution containing 0.1 M KCl (optimal). Comprehensive physicochemical (XRD, XPS, FTIR, and Raman) and electrochemical (operando EQCM and SECM) characterization methods confirmed the highly redox-active nature of the material, enabling highly reversible Zn²⁺ intercalation/deintercalation with minimal Mn²⁺ dissolution. The carbon black matrix effectively accommodates volume changes during cycling, thereby maintaining structural integrity and enhancing electrochemical stability. Formation of zinc hydroxy-sulphate (ZHS, M_w ∼ 416 g mol⁻¹) and Zn²⁺/Zn(OH)₂-like intermediate species was identified and visualized. The CB–MnO₂@Zn composite exhibited an outstanding specific capacitance of 811.6 F g⁻¹ at 1 A g⁻¹, corresponding to an energy density of 162.3 Wh kg⁻¹. Remarkably, it retained excellent capacitance and >95% efficiency over 5000 cycles. The assembled Zn²⁺/K⁺ hybrid battery (Zn‖CB–MnO₂@Zn) demonstrated a specific capacity of 315 mAh g⁻¹ at 0.25 A g⁻¹, a high energy density of 273.45 Wh kg⁻¹, and a power density of 1844.4 W kg⁻¹. Moreover, the battery exhibited superior rate capability and retained 94% capacity and 99% coulombic efficiency after 5000 cycles, highlighting its potential for durable and high-energy aqueous energy storage systems.