Integrating of Nickel Hexacyanoferrate with Hollow Mesoporous Carbon Spheres (HMCSs) for Highly Efficient Capacitive Deionization

Capacitive deionization (CDI) is a promising technology for seawater desalination, offering a green, low-cost, and low-energy alternative to traditional methods. However, its widespread adoption relies on the development of high-performance electrode materials. Prussian blue analogs (PBAs), such as NiHCF, are attractive candidates due to their high theoretical ion storage capacity but often suffer from poor conductivity and structural integrity. To address these limitations, we designed a group of hierarchical composite materials (i.e., PBA@HMCS) by integrating NiHCF with hollow mesoporous carbon spheres (HMCS) using a stepwise "ship-in-the-boat" approach. 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 core-shell 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 vast potential of hierarchical yolk-shell PBA@HMCS as a promising CDI electrode design and underscores the need for continued exploration into hierarchical structured PBA-based materials with tunable features to further enhance their CDI performance.