Maximized ion accessibility in the binder-free layer-by-layer MXene/CNT film prepared by the electrophoretic deposition for rapid hybrid capacitive deionization

Abstract

Two-dimensional (2D) pseudo-capacitive material MXene has attracted great consideration in the capacitive deionization (CDI) process for its prominent ion storage capacity and reversible ion intercalation/deintercalation mechanism. Unfortunately, the accessibility of active sites for ion intercalation is greatly limited by the self-stacking of MXene flakes for the strong van der Waals forces. What’s worse, the addition of binder in the MXene electrode prepared by the conventional coating method for CDI blocks ion accessibility channels and increases ion transfer resistance, hindering the efficient capture of ions by electrodes. Herein, the binder-free layer-by-layer MXene/CNT film electrode is flexibly constructed by the electrophoretic deposition (EPD) method to effectively alleviate the above situation. In this design, CNT not only acts as the conductive interlayer spacer to restrain MXene nanosheets self-stacking but also builds interconnected conductive channels to minimize the tortuosity of charge transport path. Furthermore, the binder-free EPD method for electrode preparation maximizes ion accessibility to ion intercalation sites, simplifies ion transfer path, accelerates ion transfer rate, and improves ion diffusion kinetics. Consequently, the resulting deposited MXene/CNT film electrode displays a dominant specific capacitance (178 F g−1), reduced charging resistance, rich Na+ diffusion channels, and maximized Na+ diffusion coefficient (5.8 × 10−8 cm2 s−1). Thus, the asymmetric hybrid CDI cell CNT//MXene/CNT adopting the deposited CNT electrode as anode and deposited MXene/CNT film electrode as cathode delivers the prominent desalination capacity (34.5 mg g−1), rapid average desalination rate (3 mg g−1 min−1), low energy consumption (0.26 kWh kg−1-NaCl), and excellent cyclic intercalation-deintercalation stability (89% retention rate). This rational design provides an alternative strategy to solve the tricky problems of MXene and develops new approach towards maximized ion accessibility in CDI field.