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

Abstract

With the increasing severity of global water scarcity, a myriad of scientific activities is directed toward advancing brackish water desalination and wastewater remediation technologies. Flow-electrode capacitive deionization (FCDI), a newly developed electrochemically driven ion removal approach combining ion-exchange membranes and flowable particle electrodes, has been actively explored over the past seven years, driven by the possibility of energy-efficient, sustainable, and fully continuous production of high-quality fresh water, as well as flexible management of the particle electrodes and concentrate stream. Here, we provide a comprehensive overview of current advances of this interesting technology with particular attention given to FCDI principles, designs (including cell architecture and electrode and separator options), operational modes (including approaches to management of the flowable electrodes), characterizations and modeling, and environmental applications (including water desalination, resource recovery, and contaminant abatement). Furthermore, we introduce the definitions and performance metrics that should be used so that fair assessments and comparisons can be made between different systems and separation conditions. We then highlight the most pressing challenges (i.e., operation and capital cost, scale-up, and commercialization) in the full-scale application of this technology. We conclude this state-of-the-art review by considering the overall outlook of the technology and discussing areas requiring particular attention in the future.

Membrane-Current Collector-Based Flow-Electrode Capacitive Deionization System: A Novel Stack Configuration for Scale-Up Desalination

“…29 Once entering the next chamber, the negatively (i.e., Na + /carbon) or positively (i.e., Cl − /carbon) charged flow electrodes will be neutralized by oppositely charged ions and electrodes (Figure 1c). 5,26,32 Thus, simultaneous ion adsorption and electrode regeneration can occur in the shared electrode chamber. For the transport of brackish water, the feed stream is sequentially driven through the spacer chamber by pumping; therefore, the dilute water quality is improved after multistage deionization.…”

“…When WR = 76%, the increase in stacked units leads to an increase in TEE from 1.1% to 4.6% (Figure 3d), which is similar to typical CDI variants. 6,11,26 Like a membrane-stacked ED system, in which the total applied potential is divided equally among the stack during constant voltage operation, the thermodynamic energy consumption of the gradient FCDI system for ion separation will significantly increase with an increase in the number of stacked units (i.e., at high SR). 5,20,43 Without considering the effect of pumping pressure, different numbers of working units can be assembled in the gradient FCDI system according to actual treatment requirements.…”

“…However, graphite plates not only occupy a large amount of weight and space in FCDI cells but also require high material cost and complex processing, which significantly limits the amplification of FCDI in practical applications. 26 Based on an ED configuration, a membrane-stacked FCDI cell was recently introduced by Ma et al 27 Unfortunately, inserting more than two membrane pairs requires higher voltages, which promotes parasitic reactions and diminishes the energy efficiency for salt removal. 20 Cho et al showed a three-dimensional (3D) FCDI system utilizing honeycomb-shaped porous lattice structures.…”

Making Waves: Zero Liquid Discharge for Sustainable Industrial Effluent Management

“…While traditional wastewater treatment technologies such as coagulation and biological processes may contribute to decontamination, advanced treatment units are required to refine the treated water to meet the criteria for ZLD. Recent progress in reverse-osmosis (RO) has highlighted the viability of ZLD in the management of industrial effluent as compared to the early systems based on stand-alone thermal processes [4,6], and continuous innovation of technologies such as electrochemical and novel membrane-based processes provides opportunities to expand the applicability of ZLD at a reduced energy cost [7,8]. In 2020, the global ZLD market size has registered 6.37 billion USD, which is projected to reach 11.77 billion USD in 2028 [9].…”

Synthesis and Characterization of Activated Carbon Co-Mixed Electrospun Titanium Oxide Nanofibers as Flow Electrode in Capacitive Deionization

“…Flow electrode CDI (FCDI) is a promising second-generation water desalination method based on the principle of ions adsorption. When a specific voltage is applied, it entails using a polarized flow electrode (liquid electrode) travelling via a flow channel to adsorb ions from brackish or seawater via electrical double layer (EDL) formation (interface between electrolyte and electrode) [4].…”