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Elucidating the capacitive desalination behavior of NaxCoO2: the significance of electrochemical pre-activation

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Abstract

Hybrid capacitive deionization (HCDI) has emerged as a promising desalination technique due to its ultra-high salt removal capacity in high brine water. However, the mechanism behind HCDI is seldom discussed anywhere. Herein, we perform a comprehensive investigation to have some insight into the HCDI behavior of NaxCoO2 by varying x as 0.2, 0.5, 0.7, 1.0 and 1.6. Regardless of x, NaxCoO2 are classified as a representative P63/mmc space group with a P2 layered structure. With the increase of the sodium content, the (002) crystal plane of NaxCoO2 shifts significantly toward a high angle as the distance between CoO2 layers decreases. This results from the variation of the Na–O bonding length as well as the bonding energy according to the first-principles simulation. Moreover, it is observed that the Na–O bond broke once the input energy is higher than the Na–O bonding energy, leading to the electrochemical pre-activation of NaxCoO2. As a result, Na0.7CoO2 exhibits the best HCDI performance, i.e. a salt removal capacity of 63.0 mg g−1 and a charge efficiency of 97% in NaCl solutions with an initial conductivity of 2000 μS cm−1. Besides, the intercalation of sodium ions into NaxCoO2 has been confirmed by differentiating the respective contributions of pseudo-capacitance together with crystal phase transformation. Our results show that the desalination behavior of NaxCoO2 can be mediated by controlling the sodium content and electrochemical pre-activation.

Graphical abstract: Elucidating the capacitive desalination behavior of NaxCoO2: the significance of electrochemical pre-activation

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Supplementary files

Article information


Submitted
09 Jan 2020
Accepted
05 Feb 2020
First published
06 Feb 2020

Nanoscale, 2020, Advance Article
Article type
Paper

Elucidating the capacitive desalination behavior of NaxCoO2: the significance of electrochemical pre-activation

Z. Liu, W. Ma and H. Li, Nanoscale, 2020, Advance Article , DOI: 10.1039/D0NR00248H

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