Abstract
Capacitive deionization (CDI) based on ion electrosorption has recently emerged as a promising desalination technology due to its low energy consumption and environmental friendliness compared to conventional purification technologies. Carbon-based materials, including activated carbon (AC), carbon aerogel, carbon cloth, and carbon fiber, have been mostly used in CDI electrodes due their high surface area, electrochemical stability, and abundance. However, the low electrical conductivity and non-regular pore shape and size distribution of carbon-based electrodes limits the maximization of the salt removal performance of a CDI desalination system using such electrodes. Metal-organic frameworks (MOFs) are novel porous materials with periodic three-dimensional structures consisting of metal center and organic ligands. MOFs have received substantial attention due to their high surface area, adjustable pore size, periodical unsaturated pores of metal center, and high thermal and chemical stabilities. In this study, we have synthesized ZIF-67 using CNTs as a substrate to fully utilize the unique advantages of both MOF and nanocarbon materials. Such synthesis of ZIF-67 carbon nanostructures was confirmed by TEM, SEM, and XRD. The results showed that the 3D-connected ZIF-67 nanostructures bridging by CNTs were successfully prepared. We applied this nanostructured ZIF-67@CNT to CDI electrodes for desalination. We found that the salt removal performance was significantly enhanced by 88% for 30% ZIF-67@CNTs-included electrodes as compared with pristine AC electrodes. This increase in salt removal behavior was analyzed by electrochemical analysis such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, and the results indicate reduced electrical impedance and enhanced electrode capacitance in the presence of ZIF-67@CNTs.
Highlights
The demand and consumption of natural resources have substantially increased over the last few decades due to the rapid growth of the global population and industrialization
We found that the salt removal performance was significantly enhanced by 88% for 30% zeolitic imidazolate frameworks (ZIFs)-67@carbon nanotubes (CNTs)-included electrodes compared with pristine activated carbon (AC) electrodes
The ion diffusion coefficients of Capacitive deionization (CDI) electrodes followed the order of 20% ZIF-67@CNT > 10% ZIF-67@CNT > 10% CNT > pristine AC since the diffusion coefficient is inversely proportional to the WO-T and the square of WO-R, and this is consistent with previously reported desalination experiments [43]
Summary
The demand and consumption of natural resources have substantially increased over the last few decades due to the rapid growth of the global population and industrialization. The low electrical conductivity and non-regular pore shape and size distribution of carbon-based electrodes limits the maximization of the salt removal performance of a CDI desalination system using such electrodes. Nanostructured carbon materials, including carbon nanotubes (CNTs) and graphene, have been widely investigated as highly conductive agents, which can significantly enhance the electrical interconnectivity between individual electrode particles, including AC [11,33–36]. We found that the salt removal performance was significantly enhanced by 88% for 30% ZIF-67@CNTs-included electrodes compared with pristine AC electrodes. This increase in salt removal behavior was analyzed by electrochemical analysis such as CV and EIS measurements, and the results indicate reduced electrical impedance and enhanced electrode capacitance in the presence of ZIF-67@CNTs
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