Increasing demand for the limited resource of fresh water for the large urban populations and development of agriculture and industry draws public concern. Removal of heavy metals such as lead, cadmium, chromium and mercury is crucial in environmental improvement of water and industrial wastewater treatment. Great efforts have been made through chemical precipitation, adsorption, ion exchange, filtration and electrochemical treatment. However, a large volume of sludge residue, expensive and complex matrix materials and low efficiency are still problems that need to be improved.

Capacitive deionization (CDI) is a promising energy-efficient technology for water desalination, which is easy to handle and environmentally friendly producing no secondary contaminants through the water purifying process [1]. In order to effectively remove ions, the porous electrodes with large surface area, good chemical stability, high electronic conductivity, and hydrophility are key factors in the selection of CDI materials. Highly porous carbon materials represent the typical electrodes to store the ions through surface ion adsorption/desorption, which is generally categorized as electrochemical double layer. By contrast, pseudocapacitors that consist of conducting polymers and transition metals, store more charge through redox reactions.

Among the alternative candidates, the natural abundant and environmental benign MnO₂ is of particular interest for research, due to its high theoretical specific capacitance and the ability to be use in mild aqueous electrolytes which expand its practical application [2-3]. MnO₂ can be fabricated easily and its morphology can be controlled during simple hydrothermal growth processes. Direct growth on carbon cloth, which is an excellent flexible and conductive substrate, could enhance the regeneration and reuse property of MnO₂ as an ideal CDI electrode.

Porous MnO₂@cabon cloth composites were prepared via a facile hydrothermal method (Figure a). The BET result showed that the average pore width is 18.2 nm. To investigate the CDI property of removing the heavy metal ions, one piece of MnO₂@CC and one piece of activated carbon@graphite paper were assembled as working and counter electrodes respectively. This work confirmed the potential of using MnO₂@CC as a good CDI electrode material for removal of heavy metal ions from water (Figure b).

References

1. S. Porada, R. Zhao, A. Wal, V. Presser, and P. M. Biesheuvel, Prog. Mater. Sci., 58, 1388 (2013).
2. W. Wei, X. Cui, W. Chen, and D. G. Ivey, Chem. Soc. Rev., 40, 1697 (2011).
3. J. Wang, F. Kang, and B. Wei, Prog. Mater. Sci., 74, 51 (2015).