Improved Desalination Performance of Flow- and Fixed-Capacitive Deionization using Redox-Active Quinone

Abstract

Capacitive deionization (CDI) has newly emerged as a desalination technology because of its energy and costeffectiveness. In particular, CDI using a flow electrode (FCDI) significantly increased salt removal by continuous desalination even at salt concentrations of both brackish and seawater. Since CDI mainly uses the electrosorption of salt ions onto an electric double layer on electrodes, carbon materials and their derivatives have been widely used. Even with various approaches, including modification and synthesis of new carbon-based electrode materials, the salt-removal capacity of CDI and FCDI is still limited by the accessible surface area and charge-transfer properties of carbon electrodes. Hydroquinone (HQ) is a redox-active organic molecule with highly reversible properties in electrochemical reactions and is a low-cost and non-toxic material. In this study, we introduced quinone in the activated carbon (AC) electrodes for CDI and FCDI desalination systems. It exhibited a significant increase in salt-removal performance (for CDI, more than a 65% increase for 300 mM HQ, and for FCDI, more than a 19% increase for 500 mM HQ). Even though the specific surface area of an HQ-added AC electrode is reduced because of the penetration of small HQ molecules into pores in the AC surface, the pseudocapacitive contribution from HQ and benzoquinone (BQ) increased the saltremoval performance. We analyzed such electrochemical properties by using cyclic voltammetry and electrochemical impedance spectroscopy. We expect that such an approach will open a new door for realizing excellent deionization of saline water, even of seawater, and will have a strong potential for large-scale desalination and energy-storage systems.

Comparing energy demands and longevities of membrane-based capacitive deionization architectures

“…1). 43–45,47–54 Using a saline electrolyte can increase the electrodes' salt adsorption capacities and/or salt adsorption rates relative to those achieved in MCDI cells, while potentially decreasing deionization energy demands. 43,47 Recirculating the electrolyte and flowing it over both electrodes can also facilitate continuous cell operation.…”

“…43,47 Recirculating the electrolyte and flowing it over both electrodes can also facilitate continuous cell operation. 44,49–54 In some cases, the addition of a soluble redox-active compound to the electrolyte decreased energy demands. 49,52 Note that systems using suspended carbon particles or a soluble redox-active compound in a recirculated electrolyte begin to, or do, resemble ED cells, as salt is stored in the electrolyte, not at or in the electrode.…”

Flow Electrode Capacitive Deionization (FCDI): Recent Developments, Environmental Applications, and Future Perspectives

“…Electron-mediators are another popular conductive additive which rely on fast and reversible redox reactions occurring at the electrode/electrolyte and current collector/electrolyte interfaces in order to accelerate electron transport between the active materials and the current collectors (Figure d). Most of these compounds possess proton-coupled multielectron exchange ability and prefer basic (PPD and MPD) or acidic (H 2 Q) conditions to facilitate electron/proton transfer. − Carbon slurry containing PPD in an alkaline environment exhibited a 130% increase in capacitance and 75% reduction in ohmic resistance compared to those of the pristine carbon slurries at neutral pH . In other work by Ma et al, facile redox transformation between hydroquinone (H 2 Q)/benzoquinone (Q) (eq ) within the slurry electrodes significantly accelerated the electron and ionic transfer, boosting the average salt removal rate by 131% (Figure d).…”

Recent Advances in Faradic Electrochemical Deionization: System Architectures versus Electrode Materials

“…Most of the faradaic electrode materials used in HCDI systems were inorganic compounds, which have several drawbacks, such as decomposition and dissolution, which limit the cycling performance of the system. Therefore, researchers have coupled these faradaic inorganic compounds with organic components or even replaced them with redox-active polymers to reinforce the structural stability of the electrode and enhance the long-term stability of HCDI. − …”