Capacitive deionization, CDI, is considered as a versatile and promising electrochemical technology useful for both water desalination and remediation.¹ CDI works under operational conditions equivalent to the charging/discharging steps in a generic electric double-layer capacitor (EDLC). Thus, it allows reducing ion concentration of an aqueous stream by the electrosorption of ions in porous electrodes and formation of the EDL, resulting in simultaneous salt withdrawal and energy accumulation. Once the deionization (charging) step is completed, the ions can be desorbed in regeneration (discharging) step. This leads to energy release and leaves the electrode surface available for the next deionization cycle. Accordingly, CDI is considered a simple, non-energy intensive method to produce clean water.²

In this work, a novel current collector-free electrode for CDI devices is designed based on electrodes consisting of a porous metal oxide (MOx) network interpenetrated into porous fibres of carbon nanotubes (CNTf). The fabrication of electrodes is based on the continuous impregnation of CNTf with MOx precursors in-line as they are spun from the chemical vapour deposition reaction. SiO₂ and γ-Al₂O₃ were chosen, which have the following main benefits: i) developing an electrostatic EDL surface potential in the pH range of seawater and drinking water; ii) increasing specific surface area; iii) enabling wetting of electrodes by aqueous electrolytes.³ The content of MOx can be varied by adjusting the concentration of sol in the dispersion. γ-Al₂O₃/CNTf and SiO₂/CNTf electrodes from 6 wt.% to 60 wt.% of MOx have been manufactured and characterized (TG-analysis, electrical conductivity, N₂ adsorption isotherms, SEM and TEM images and RAMAN spectroscopy). The hybrid structure consisting of two interconnected porous networks with a uniform distribution of MOx on the supporting CNTf (see Fig 1a and Fig 1b) leads to high capacitance while reducing internal resistance (see Fig 1c), as confirmed by the electrochemical characterization (cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy).

The use of a relatively simple fabrication process, particularly the infiltration of fabrics with sol particles in-line during fibre spinning, enables fabrication of large electrode samples and testing of a full CDI flow cell for brackish water desalination, 2.0 g_NaCl L^-1 (see Fig 1d). The full current collector-free CDI cell (see Fig 1e and Fig 1f), 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 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 and titanium foil current collector electrodes. The exceptional desalination properties (see Fig 1g) are directly linked to the interconnected nanoparticle MOx/CNTf hybrid network. Accordingly, we believe that these CNTf-based electrodes fabricated in a large scale could contribute to a significant improvement in the engineering manufacturing process of desalination reactors through more energy-efficient systems. Considering the high flexibility in bending of these hybrid electrodes, the possibility to build current collector-free desalination devices with complex non-planar shapes has also been opened up.

References

1. M. A. Anderson et al. Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete?, Electrochim. Acta, 55, 3845–3856 (2010). http://dx.doi.org/10.1016/j.electacta.2010.02.012.
2. M. E. Suss et al., Water desalination via capacitive deionization: what is it and what can we expect from it?, Energy Environ. Sci., 8, 2296–2319 (2015).http://dx.doi.org/10.1149/2.0271805jes.
3. J. J. Wouters et al., Influence of Metal Oxide Coatings, Carbon Materials and Potentials on Ion Removal in Capacitive Deionization J. Electrochem.Soc., 165, 148–161 (2018). http://dx.doi.org/10.1149/2.0271805jes.