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

Saline water desalination is an important route to increase freshwater supply, while capacitive deionization (CDI) has emerged as a promising technique to tackle this issue. To boost the success of CDI, development of advanced electrode materials is vital. Herein, we innovatively designed a sophisticated Prussian blue/dimorphic polypyrrole composite for hybrid CDI (HCDI). Specifically, the nanoparticle‐like polypyrrole (PPy) in‐situ distributed on the stepped hollow FeHCFe nanocubes' surface, while coexisting nanotube‐like PPy interconnected the discrete stepped hollow FeHCFe nanocubes. The introduction of dimorphic PPy promoted both electron and ion dynamics, and improved the electrochemical activity of FeHCFe. Meanwhile, the unique FeHCFe nanocubes 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 achieved a remarkable desalination capacity and a fast desalination rate outperforming related materials. Moreover, the treated solution can meet drinking water standard within 6 min via six tandem HCDI cells. Density functional theory (DFT) revealed Na+ capture mechanism and provided fundamental understanding of the rapid Na+ migration and large Na+ storage. This study provides insights into ration design of superior electrode materials for high‐performance electrochemical water desalination.