Enhanced Electrochemical Stability of a Zwitterionic-Polymer-Functionalized Electrode for Capacitive Deionization

Abstract

In capacitive deionization, the salt-adsorption capacity of the electrode is critical for the efficient softening of brackish water. To improve the water-deionization capacity, the carbon electrode surface is modified with ion-exchange resins. Herein, we introduce the encapsulation of zwitterionic polymers over activated carbon to provide a resistant barrier that stabilizes the structure of electrode during electrochemical performance and enhances the capacitive deionization efficiency. Compared to conventional activated carbon, the surface-modified activated carbon exhibits significantly enhanced capacitive deionization, with a salt adsorption capacity of ∼2.0 × 10 mg/mL and a minimum conductivity of ∼43 μS/cm in the alkali-metal ions solution. Encapsulating the activated-carbon surface increased the number of ions adsorption sites and the surface area of the electrode, which improved the charge separation and deionization efficiency. In addition, the coating layer suppresses side reactions between the electrode and electrolyte, thus providing a stable cyclability. Our experimental findings suggest that the well-distributed coating layer leads to a synergistic effect on the enhanced electrochemical performance. In addition, density functional theory calculation reveals that a favorable binding affinity exists between the alkali-metal ion and zwitterionic polymer, which supports the preferable salt ions adsorption on the coating layer. The results provide useful information for designing more efficient capacitive-deionization electrodes that require high electrochemical stability.

Low-Resistant Ion-Exchange Membranes for Energy Efficient Membrane Capacitive Deionization

“…In situ EIS was leveraged to understand the current distribution within MCDI. , This technique allowed quantitative analysis of the individual resistance contributions in the cell and helped us correlate ex situ properties of the IEMs to MCDI performance. EIS was performed under different current loads to examine how the resistance contributions evolved as a function of applied current to the cell.…”

“…Additionally, the reduction in membrane resistance allowed for higher current density operation of the MCDI cell, which is important for enhancing the rate of salt removal from water. Reducing the overall cell resistance with improved IEMs maintained a low cell voltage at a constant higher current density, and this minimized unwanted, parasitic side reactions like electrochemical splitting of water or carbon corrosion − that hinder the cell’s Coulombic efficiency. In situ electrochemical impedance spectroscopy (EIS) experiments, analyzed with a transmission line electric circuit equivalent model, investigated the current distribution within MCDI, and they revealed the following: (i) the poly­(arylene ether) (PAE) and perfluorinated IEMs gave lower high-frequency resistance values when compared to the Tokuyama IEMs, and this aided a more energy efficient MCDI operation; and (ii) the new IEMs increased the charge-transfer resistance barriers for unwanted, parasitic Faradaic reactions, and this caused greater Coulombic efficiency of the MCDI cell.…”

“…Reducing the overall cell resistance with improved IEMs maintained a low cell voltage at a constant higher current density, and this minimized unwanted, parasitic side reactions like electrochemical splitting of water or carbon corrosion − that hinder the cell’s Coulombic efficiency. In situ electrochemical impedance spectroscopy (EIS) experiments, analyzed with a transmission line electric circuit equivalent model, investigated the current distribution within MCDI, and they revealed the following: (i) the poly­(arylene ether) (PAE) and perfluorinated IEMs gave lower high-frequency resistance values when compared to the Tokuyama IEMs, and this aided a more energy efficient MCDI operation; and (ii) the new IEMs increased the charge-transfer resistance barriers for unwanted, parasitic Faradaic reactions, and this caused greater Coulombic efficiency of the MCDI cell. Finally, it is important to point out that commercially available IEMs and polymers were selected because they could be easily adopted by industryi.e., buy the IEM off the shelf or convert the low-cost base polymer into an IEM using straightforward and low-cost reaction schemes.…”

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Additionally, they also used a Na-birnessite electrode [128] to remove Mg 2+ with a removal capacity of 527 µmol g −1 (50.2 mg g −1 ) in the same solution. Other than this, Jung et al [318] prepared a zwitterionic polymer@AC composite for Ca 2+ and Mg 2+ removal. Due to the binding affinities between the zwitterionic polymer and alkalimetal ions as calculated by DFT, the CDI cell with this electrode obtained a higher ion removal capacity of 30-35 mg g −1 compared to pure AC.…”

“…Other than this, Jung et al. [ 318 ] prepared a zwitterionic polymer@AC composite for Ca 2+ and Mg 2+ removal. Due to the binding affinities between the zwitterionic polymer and alkali‐metal ions as calculated by DFT, the CDI cell with this electrode obtained a higher ion removal capacity of 30–35 mg g −1 compared to pure AC.…”