Optimizing the integration of nickel hexacyanoferrate with hollow mesoporous carbon spheres (HMCSs) for highly efficient capacitive deionization

Capacitive deionization (CDI) is a promising water desalination technology known for its energy efficiency and low environmental impact. Charge-transfer materials (e.g., Prussian blue analogues, PBAs) have gained much attention as electrodes for CDI due to their much higher salt adsorption capacities (SACs) beyond conventional carbon-based electrodes, though they face challenges such as conductivity, stability, and ion transfer kinetics. Integrating these charge-transfer materials with carbon offers a promising strategy to enhance CDI performance and address these limitations. Herein, using a stepwise "ship-in-the-boat" approach, we incorporated a typical PBA, i.e., NiHCF, with hollow mesoporous carbon spheres (HMCS) to yield hierarchical composite materials (i.e., PBA@HMCS). This composite combines the protective and conductive roles of carbon materials with the high ion storage capacity of PBAs. The resulting PBA@HMCS electrodes demonstrated exceptional CDI performance, with a maximum salt adsorption capacity of 80.5 mg g- 1 in 500 mg L- 1 NaCl solution at 1.2 V. Notably, the PBA@HMCS-1 electrode exhibited enhanced cycling stability, while the unwrapped PBA micropellets showed reduced performance. Furthermore, our investigation revealed the high affinity of PBA@HMCS electrodes for Na+ over other ions in synthetic brine, and particularly, the yolk-shell PBA@HMCS-3 electrode demonstrated high repulsion to K+, highlighting its potential for selectively extracting specific ions from dicationic brines with K+ ions. This study highlights the potential of hierarchical yolk-shell PBA@HMCS as a promising CDI electrode and underscores the need for continued exploration into the hierarchically structural design of composite materials for high-performance CDI platforms.