Flow-electrode capacitive deionization (FCDI) scale-up using a membrane stack configuration

A novel two-stage continuous capacitive deionization system with connected flow electrode and freestanding electrode

In flow-electrode capacitive deionization (FCDI) systems, the spacer serves as the core structural element governing ion migration pathways. Concentration polarization emerges as a critical factor determining system performance, with spacer architecture directly influencing ion transport dynamics. This study introduces a novel ventilated spacer (V-FCDI) featuring a cage-like design surrounded by a thin nylon fabric layer, positioned exclusively near the membrane interfaces, in contrast to thick woven mesh spacers (W-FCDI). This ventilated architecture strategically mitigates concentration polarization through localized flow disruption at the membrane surfaces, while simultaneously lowering hydraulic and ohmic resistance in the bulk region and extending feed residence time. A computational fluid dynamics (CFD) simulation of a periodic unit cell validated the hydrodynamic behavior within the spacer, revealing enhanced near-surface flow in the ventilated configuration. The experimental investigation systematically assessed the efficacy of the V-FCDI against W-FCDI by varying flow rates and applied voltage. The V-FCDI design demonstrates a marked improvement, with a 9.0% increase in average salt removal rate, a 9.5% higher voltage-driven desalination capability, an 11.2% reduction in specific electrical energy consumption, and an 18.8% reduction in specific pumping energy consumption compared to the W-FCDI. The findings establish new design principles for high-performance FCDI systems, offering a pathway toward energy-efficient brackish water desalination.

The stack configuration in flow-electrode capacitive deionization (FCDI) has been verified to be an attractive and feasible strategy for scaling up the desalination process. However, challenges still exist when attempting to simultaneously improve the desalination scale and the cell configuration. Here, we describe a novel stack FCDI configuration (termed a gradient FCDI system) based on a membrane-current collector assembly, in which the charge neutralization enables the in situ regeneration of the flow electrodes in the single cycle operation, thereby realizing a considerable increase in the desalinating performance. By evaluating standardized metrics such as the salt rejection, productivity (P), average salt removal rate (ASRR), energy-normalized removed salt (ENRS), and TEE, the results indicated that the gradient FCDI system could be a performance-stable and energy-efficient alternative for scale-up desalination. Under optimal operating conditions (carbon content = 10 wt %, feed salinity = 3000 mg L-1, cell voltage = 1.2 V, and productivity = 56.7 L m-2 h-1), the robust desalination performance (ASRR = 1.07 μmol cm-2 min-1) and energy consumption (ENRS = 7.8 μmol J-1) of the FCDI system with a desalination unit number of four were verified at long-term operation. In summary, the stacked gradient FCDI system and its operation mode described here may be an innovative and promising strategy capable of enlarging the scale of desalination while realizing performance improvement and device simplification.

Enhanced salt removal performance using nickel hexacyanoferrate/carbon nanotubes as flow cathode in asymmetric flow electrode capacitive deionization

Enhanced capacitive deionization for Cr(VI) removal from electroplating wastewater: Efficacy, mechanisms, and high-voltage flow electrodes

Carbon nanotubes/activated carbon hybrid as a high-performance suspension electrode for the electrochemical desalination of wastewater

Redox flow deionization using Prussian blue and functionalized ion exchange membrane for enhanced selective ion recovery

Equivalent film-electrode model for flow-electrode capacitive deionization: Experimental validation and performance analysis

New insights into the performance analysis of flow-electrode capacitive deionization using ferri/ferrocyanide redox couples for continuous water desalination

Construction of MS-Ti3C2TX MXene continuous conductive structure for highly efficient fluoride removal in flow-electrode capacitive deionization

Scale-up desalination: Membrane-current collector assembly in flow-electrode capacitive deionization system

Towards long-term operation of flow-electrode capacitive deionization (FCDI): Optimization of operating parameters and regeneration of flow-electrode

Enhancing charge transfer utilizing ternary composite slurry for high-efficient flow-electrode capacitive deionization

Superiority of a novel flow-electrode capacitive deionization (FCDI) based on a battery material at high applied voltage

Flow electrode capacitive deionization (FCDI) is a newly developed water treatment technology that offers better performance than the original, solid electrode capacitive deionization (CDI). By eliminating the need for discharging process, FCDI provides much higher desalination capacity with a continuous desalination operation. Along with applications for salt removal, selective removal and recovery of specific ions are also prominent features of FCDI technology. Characteristics of electrodes and ion exchange membranes play a crucial role in controlling the selectivity of ions in an FCDI system. In this study, we demonstrated selective and continuous ion recovery by combining redox-active Prussian blue as flow electrodes and functionalized ion exchange membranes in FCDI configuration. Prussian blue selectively absorbs monovalent Na+ ions from Na+/Ca2+ mixtures through intercalation reaction, in which a smaller Stokes radius of Na+ ion is more favored to fit in the crystal lattice. Furthermore, such selectivity is much more enhanced through functionalization of a cation-exchange membrane (CEM) with polymer multilayers. A layer-by-layer method was used to deposit poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) on the CEM. Results showed that polymer multilayers changed the CEM from divalent to monovalent ion-affinity due to different ion sizes and charge density. The combination of Prussian blue and functionalized CEM raised the selectivity for sodium around 17 times compared to a control system (0.2 to 3.35). Furthermore, our results showed a highest Li selectivity of 12.88 with an extremely low energy consumption of 0.57 Wh/molLi. We believe our approach can lead to new technologies that address the shortcomings of existing lithium recycling method and expand the application not only to lithium recycling, but also to future lithium production technologies and the removal of specific ions, such as toxin removal.