Nitrogen doped graphene quantum dot-decorated earth-abundant nanotubes for enhanced capacitive deionization

Abstract

The combination of capacitive carbon nanomaterials with metal oxides as electrode materials for capacitive deionization (CDI) has emerged as a promising strategy for water desalination. Herein, environmentally friendly nanocomposites of nitrogen doped graphene quantum dots (N-GQDs) decorated onto halloysite nanotubes (HNTs) have been developed for enhanced CDI. The decoration of as-prepared 8–10 nm N-GQDs onto the surface of HNTs (N-GQDs@HNT) significantly reduces the electric resistance. The N-GQDs@HNT nanocomposites exhibit an ideal electric double layer feature with high specific capacitances of 335 and 201 F g⁻¹ at 0.5 and 10 A g⁻¹, respectively. In addition, more than 95% of the original capacitance can be retained after 3000 charge/discharge cycles at 1 A g⁻¹, which indicates the superior rate capability and stability of the nanocomposites. The symmetric N-GQDs@HNT electrode shows an excellent specific electrosorption (SEC) of 20.1 mg g⁻¹ (874 μmol g⁻¹) and high stability toward Na⁺ removal after 3 consecutive charge/discharge processes. The CDI Ragone plot indicates that the SEC of N-GQDs@HNT is highly dependent on the flow rate, applied voltage and initial NaCl concentration. The superior electrochemical performance of N-GQDs@HNT is mainly attributed to the increased charge storage sites and good electrical conductivity, resulting in the improvement of the charge transfer and ion transport. These superior performance features make the N-GQDs@HNT a promising electrode material which can pave the way for the development of next generation and environmentally friendly technology using naturally occurring clay minerals as high performance electrode materials for water desalination.