Capacitive versus diffusion controlled mechanism in a mesoporous cobalt hexacyanoferrate/carbon composite for a high-performance capacitive deionization device: electrochemical quartz crystal microbalance (EQCM) studies

Abstract

A highly interconnected pore architectural design with readily accessible redox-active channels is crucial for enhancing the efficiency of a capacitive deionization (CDI) device. Herein, hierarchically mesoporous cubic cobalt hexacyanoferrate (meso-CoHCF) demonstrated enhanced ion transport properties, possessing a high surface area of 160.88 m² g⁻¹ compared to CoHCF (121.78 m² g⁻¹). The electrochemical quartz crystal microbalance (EQCM) technique was employed to monitor real-time ion-associated mass variations in meso-CoHCF, establishing a direct correlation between ion intercalation behaviour and CDI performance. The higher Co³⁺/Co²⁺ ratio, coupled with enhanced lattice oxygen content, improved structural stability of the Co–NC–Fe framework, thereby promoting faster and more efficient redox processes. Furthermore, meso-CoHCF delivered the highest specific capacitances of 835 F g⁻¹ (NaCl) and 518 F g⁻¹ (Na₂SO₄) at 0.5 A g⁻¹. Kinetic studies revealed that mesostructuring significantly enhances diffusion-assisted charge storage, with diffusion contributions increasing from 12.89% to 24.92% in NaCl (∼1.9-fold) and from 8.19% to 25.32% in Na₂SO₄ (∼3-fold), confirming accelerated ion transport and deeper bulk redox utilization. When assembled in an asymmetric configuration with mesoporous carbon, the CDI device achieved high salt adsorption capacity (SAC) values of 64.31 mg g⁻¹ and 53.71 mg g⁻¹, with a low energy consumption of 0.55 kWh m⁻³, and thus acts as a promising electrode material for advanced CDI water purification systems.