Thermal conversion of polypyrrole nanotubes to nitrogen-doped carbon nanotubes for efficient water desalination using membrane capacitive deionization

[Display omitted] •Nitrogen-doped carbon nanotubes are synthesized by thermal conversion of polypyrrole nanotubes.•Nitrogen-doped carbon nanotubes are applied membrane capacitive deionization electrode materials for the first time.•Nitrogen-doped carbon nanotubes exhibit a maximum desalination capacity of 17.18 mg g−1. The exploration of new family of nitrogen-doped carbon materials with superior performance is of significant interests in capacitive deionization (CDI) or membrane CDI (MCDI) field. In this work, nitrogen-doped carbon nanotubes (nit-CNTs) were synthesized by thermal conversion of polypyrrole nanotubes in nitrogen atmosphere. Thanks to their interconnected nanotube structure providing more accessible space for ion accommodation, shortened ion diffusion pathway for fast ion adsorption/desorption, and optimized nitrogen doping species for improved electrical conductivity and increased sodium ion adsorption, the nit-CNTs exhibit a maximum desalination capacity of 17.18 mg g−1 and a good cycling stability with little capacity fading after 20 cycles, highlighting their practicality for water desalination. This work not only showcases a new case of nitrogen-doped carbon materials for MCDI application, but also highlights the significance of one-dimensional hollow nanotubes and nitrogen doping chemistry.