Flowers on leaves: Rationally designed zeolitic imidazolate framework (ZIF)-on-ZIF hierarchical...

Capacitive deionization (CDI) is an energy-efficient desalination technology for low-salinity water, yet its practical application is hindered by the inherent trade-off between high adsorption capacity and fast ion transport in electrode materials. Herein, a synergistic strategy of morphology engineering and precise carbonization regulation is proposed to fabricate a binder-free integrated electrode derived from a "flowers-on-leaves" zeolitic imidazolate framework (ZIF)-on-ZIF hierarchical heterostructure. Vertically aligned ZIF-L leaf arrays grown in situ on carbon cloth (CC) act as the conductive backbone, with ZIF-RD polyhedra further deposited on the leaf surfaces via secondary heterogeneous growth. Systematic investigation of carbonization temperatures identifies 700 °C as the optimal "structural reconstruction window", preserving moderately curled leaf skeletons and embedded polyhedra to form a 352.3 m²/g microporous-mesoporous structure with balanced nitrogen doping (pyridinic/graphitic N) and cobalt chemical states (Co-N_X/Co⁰). The optimized ZIF-L@RD/CC-7 electrode delivers a salt adsorption capacity of 48.28 mg/g in 1000 mg/L NaCl solution, with 83.64% capacity retention after 50 cycles, outperforming counterparts carbonized at 500 °C and 900 °C. Density functional theory (DFT) calculations unravel the electronic origin of the enhanced performance: carbonization transforms the semiconducting precursor (band gap ≈ 2.5 eV) into a metallic system with continuous density of states at the Fermi level (D_total (E_F) = 10.844 states/eV), where Co 3d orbitals constitute efficient conductive channel. Charge density difference analysis shows that Cl^- forms strong electronic coupling with Co clusters/Co-N_X units (2.35-2.44 Å) with 1.325 e charge redistribution (vs. 0.735 e for the precursor), significantly reducing adsorption energy. Experimental and theoretical results demonstrate that synergistic morphology-temperature regulation is key to breaking the classic "capacity-rate" deadlock in CDI.