Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization.

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (TiCT) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl-capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl-capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl-capturing in electrochemical desalination.