The capacitive deionization behaviour of a carbon nanotube and reduced graphene oxide composite

Abstract

In this work, a carbon nanotubes (CNTs) and reduced graphene oxide (RGO) composite (CNTs–RGO) has been synthesized by a facile chemical approach for capacitive removal of salty ions from synthetic salty water. The as-prepared RGO–CNTs composite exhibits a network structure and strong hydrophilicity which are favourable for electrosorption. Cyclic voltammetry results illustrate that CNTs with 10 wt% RGO (CNTs–RGO-10) have the highest electrochemical capacity among all CNTs–RGO samples, implying the high capacitive deionization (CDI) performance. Meanwhile, it is found that the interfacial electron transfer resistance derived from the electrochemical impedance spectra for CNTs–RGO-10 in 1 M NaCl solution is 0.58 Ω which is much lower than that of either pure CNTs or RGO, implying that the introduced CNTs inhibit the aggregation and increase the conductivity by forming a network structure. Furthermore, the CDI performance of CNTs–RGO-10 is demonstrated through a batch mode desalination experiment in a given NaCl solution. The results reveal that the electrosorptive capacity of CNTs–RGO-10 is higher than that of pure CNTs regardless of the applied voltage when the experiment is conducted in NaCl solution with an initial conductivity of 100 μS cm⁻¹. This is due to the optimized synergistic effect. In addition, the electrosorption rate constant as well as charge efficiency of the CNTs–RGO-10 electrode is found to be proportional to the electrical voltage and higher than that of the pure CNTs electrode at each electrical voltage, confirming the enhanced conductivity in the CNTs–RGO-10 electrode. Besides, the result of charge efficiency also demonstrates that CDI is envisioned to be a very energy efficient water desalination technology when the ionic content is relatively low. Finally, the repeatability of the CNTs–RGO-10 electrode is investigated, demonstrating that the CNTs–RGO composite electrode could be regenerated very well without any decay after several cycles.