Synergy from the Stepped Hollow FeHCFe Nanocubes and the Dimorphic Polypyrrole for High‐Performance Electrochemical Water Desalination

Saline water desalination serves as an important route to increase freshwater supply, while capacitive deionization (CDI) has emerged as a promising technique to tackle this issue. To boost the successful application of CDI, development of advanced electrode materials is vital. Herein, we innovatively designed and synthesized a sophisticated Prussian blue/dimorphic polypyrrole composite for the hybrid CDI (HCDI). Specifically, the nanoparticle-like polypyrrole (PPy) in situ distributed on the surface of stepped hollow FeHCFe nanocubes, while the coexisting nanotube-like PPy interconnected the discrete stepped hollow FeHCFe nanocubes. The introduction of PPy nanoparticles/nanotubes promoted both electron and ion dynamics, and improved the electrochemical activity of FeHCFe. Meanwhile, the FeHCFe nanocubes with concave stepwise architecture on each side offered large accessible contact area for electrolyte, reduced Na⁺ migration path, improved tolerance for lattice expansion, and optimized redox sites for Na⁺ storage. Through such unique structural design and synergistic combination, the FeHCFe/PPy with appropriate component ratio achieved a remarkable desalination capacity and a fast desalination rate along with excellent cycling performance, outperforming other related materials. Moreover, the treated solution can meet the drinking water standard within 6 min via the use of six tandem HCDI cells. Density functional theory (DFT) revealed the mechanism of Na⁺ capture process and provided a fundamental understanding of the rapid Na⁺ migration and large Na⁺ storage capability. This study provides insights into ration design of superior electrode materials for high-performance electrochemical water desalination.