Capacitive-Dominant Ion Storage in Electronically Coupled Vanadium Hexacyanoferrate/rGO for Efficient Capacitive Deionization Devices

Abstract

The rational design of electrodes with hierarchically interconnected porosity and abundant redox-active channels is essential for high-performance capacitive deionization (CDI). Herein, vanadium hexacyanoferrate (VHCF) is integrated with reduced graphene oxide (rGO) to demonstrate a strongly coupled intercalation-type hybrid electrode that exhibits a pronounced morphological evolution from densely aggregated particles to a rosette-like nanosheet architecture, significantly improving electrolyte accessibility, active-site exposure, and ion-transport pathways. Electronic interaction at the rGO−VHCF interface facilitates charge redistribution, as indicated by binding-energy shifts and the stabilization of mixed-valence V and Fe states in XPS analysis. Notably, XPS reveals a ∼0.5 eV negative shift in the V3+ binding energy, confirming interfacial electron transfer from rGO to VHCF and increased electron density at vanadium centers. This electronic coupling activates additional redox centers, lowers charge-transfer resistance, and accelerates reaction kinetics, delivering enhanced charge storage (328.59 F/g at 1 A/g) with reduced polarization and reveals a dominant surface-controlled pseudocapacitance, indicating interfacial charge storage. Electrochemical quartz crystal microbalance (EQCM) measurements directly correlate charge transfer with reversible Na+-associated mass variations, confirming synchronized electron−ion transport and minimal ion trapping. In asymmetric CDI configuration, VHCF/rGO cathode and mesoporous carbon as anode achieve a high-salt adsorption capacity of 78.97 mg/g nearly double that of VHCF (37.40 mg/g). Additionally, the system demonstrates rapid electrosorption kinetic and low-energy consumption (0.65 kJ/mol) in Na2SO4. The synergistic integration of electronic coupling, defect-enriched active sites, and efficient ion-diffusion channels establishes VHCF/rGO as a robust, high-efficiency electrode for next-generation CDI systems.