Structurally engineered ZnCo ₂ O ₄ spinel nanoparticles on ZIF-derived hierarchically porous graphitic carbon for high-performance flow capacitive deionization

Flow capacitive deionization (FCDI) operated under continuous flow is a promising strategy for energy-efficient desalination. However, its large-scale implementation is restricted by the inherent limitations of conventional electrode materials, including poor electronic conductivity, limited pseudocapacitive redox activity, and inefficient ion transport pathways. In this study, a hierarchical hybrid electrode, ZnCo₂O₄@GPC, was synthesized through controlled pyrolysis followed by oxidative transformation of a bimetallic Zn/Co-zeolitic imidazolate framework (Zn/Co-ZIF) precursor. This approach yields uniformly dispersed ZnCo₂O₄ spinel nanocrystallites embedded within a nitrogen-doped graphitic carbon framework, possessing hierarchically interconnected micro- and mesopores. Structural and spectroscopic analyses confirm the retention of conductive graphitic domains, the coexistence of Co²⁺/Co³⁺ redox couples within the spinel lattice, and strong interfacial coupling between the oxide nanoparticles and carbon matrix. When deployed in an FCDI cell, ZnCo₂O₄@GPC exhibits a salt adsorption capacity (SAC) of approximately 31.5 mg g^-1 and achieves 87% salt removal (SR) at 1.0 V and 20 mL min^-1, clearly outperforming pristine ZIF-derived carbons and commercial activated carbons. Electrochemical investigations reveal a predominantly surface-controlled charge storage mechanism with enhanced electric double-layer capacitance, accelerated ion transport through hierarchical pores, and excellent cycling durability. These integrated physicochemical and electrochemical characteristics establish ZnCo₂O₄@GPC as a scalable and high-efficiency electrode platform for advanced FCDI technologies.