Capacitive deionization (CDI), a water desalination technology based on the electro-deposition of salt ions in porous electrodes, is considered a simple, non-energy intensive method to produce clean water. This work introduces a new current collector-free CDI architecture based on electrodes consisting of a porous metal oxide (MOx) network interpenetrated into porous fibres of carbon nanotubes (CNTf). The full CDI device, comprising a stack of γ-Al₂O₃/CNTf and SiO₂/CNTf anodes and cathodes, respectively, has a large salt adsorption capacity of 6.5 mg g⁻¹ from brackish water (2.0 g_NaCl L⁻¹) and very high efficiency of 86%, which translates into a low energy consumption per gram of salt removed (∼0.26 W h g⁻¹). This is an 80% improvement compared with reference devices based on activated carbon electrodes and titanium foil current collectors. The remarkable efficiency obtained is due to the morphology of the electrodes, in which the CNT fibres act simultaneously as a current collector, active material and support for the metal oxide. Such architecture leads to high capacitance while minimizing internal resistance, as confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. The fact that full electrodes can be made continuously, as demonstrated on 4 km of CNTf, makes the fabrication process more attractive.

Currently, according to conventional charge–discharge profiles, energy consumed in charging Capacitive Deionization (CDI) systems is always a function of different parameters (current used for charging, capacitance and current employed for discharging) making it difficult to separate the effect of these parameters on CDI performance and energy efficiency. Thus, energy efficiencies are strongly influenced by the current in the preceding charge or discharge stage of the process. We find consistently that this phenomenon, which to our knowledge has not been addressed in previous CDI communications, is much more intense when different currents are applied for each of the charging and discharging cycles. The investigation reported here provides a mechanistic analysis of the operational aspects of CDI and develops a new procedure that allows for a precise evaluation of performance and energy efficiency. Furthermore, the model developed here allows one to separate charge and discharge cycles, and therefore contributes to the possibility of defining an operational mode for real-world devices in which effective separation of deionization and regeneration steps needs to be implemented. This method of analysis could be useful not only for CDI but also for other electrochemical systems such as in secondary batteries and supercapacitors where charge and discharge are typically employed.