Flow-electrode capacitive deionization: A review and new perspectives

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

Influence of particle size distribution on carbon-based flowable electrode viscosity and desalination efficiency in flow electrode capacitive deionization

Slurry of activated carbon particles mixed with an aqueous electrolyte solution has been used as “flowable electrode” in a few recent electrochemical systems, e.g., electrochemical flow capacitors (EFCs) for energy storage, and flow-electrode capacitive deionization (FCDI) for water treatment. In these applications, the porous carbon particles with very large specific surface area adsorb ions from the electrolyte and meanwhile store electrical charges when a voltage source is added in the charging process. Under the flow condition, the motion of the particles and their mutual contact form a dynamically varying electrical network for the charge transport through the bulk material. We introduce a novel particle-based computational model using the Stokesian dynamics to simulate the hydrodynamic interaction of the carbon particles and the charge transport. An analogous electrical circuit model is developed by approximating the particles with many interconnected resistor-capacitor units, and the circuit’s topology is temporally changing depending on the instantaneous particle configuration. The Stokesian model and the circuit model are solved simultaneously to study how the hydrodynamic interaction and cluster formation affect the charge transport process of the slurry. The presence of the stationary current collector can be included to incorporate the near-wall effects on particle mobility. In the simulation, we vary the particle concentration as well as the ratio of the particle charging time to the hydrodynamic interaction time. The results shows that the charge transport in the carbon slurry is enhanced by increasing the concentration of the particles and faster particle charging. In addition, cluster formation of the particles plays an important role for the electronic transport process. After scaling, the transient electrical current from the present study generally agrees with that from previous experimental and modeling studies.

In redox flow electrode capacitive deionization (FCDI), the solubility of the redox electrolyte and flowability of the carbon slurry have a great influence on the salt removal rate and energy consumption. In this work, a mixed solvent electrolyte is proposed for FCDI, which consists of iodide/triiodide redox couples and a carbon slurry in a mixed solvent of water and ethanol (1:1). At a current density of 5 mA cm−2, the salt removal rate in the mixed solvent can reach up to 2.72 μg cm−2 s−1, which is much higher than the value of 1.74 μg cm−2 s−1 and 2.37 μg cm−2 s−1 obtained in aqueous and ethanol solutions, respectively. This is attributed to the fast transport of ions during the redox reaction in organic solvents and the excellent flowability of the carbon slurry under aqueous conditions, which can provide more reaction sites for iodide/triiodide redox reactions and faster electron transportation. This unique FCDI with organic and aqueous mixed solvent electrolytes provides a new perspective for the development of redox flow electrochemical desalination.

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

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

Process model for flow-electrode capacitive deionization for energy consumption estimation and system optimization

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.

This study demonstrates the efficacy of a symmetrically designed flow-electrode capacitive deionization (FCDI) system for the electrochemical defluorination of photovoltaic (PV) wastewater, with a systematic investigation conducted to optimize operational parameters and analyze key factors influencing system performance, offering valuable insights into enhancing FCDI efficiency. The results revealed that an optimal applied voltage of 1.2 V yielded a fluoride removal efficiency of 92.9% with an energy consumption of 6.49 kWh/mol. Increasing the electrode content to 0.75 wt% enhanced the removal efficiency to 98.3%; however, further increases in electrode content led to higher energy consumption due to elevated viscosity. Optimizing the flow rate to 45 mL/min resulted in a removal efficiency of 98.6%, accompanied by improved adsorption rates and reduced energy consumption. Adding 1 g/L of electrolyte substantially enhanced system performance, achieving a fluoride removal efficiency of 92.9%. In mixed-ion wastewater, competitive adsorption between NO 3 − and F⁻ was observed. Doubling the NO 3 − concentration relative to F⁻ decreased the F⁻ removal efficiency from 96.2% to 87.3%. Nonetheless, the FCDI system demonstrated robust fluoride removal performance under high ion concentrations and complex matrix conditions, offering an efficient and sustainable approach for industrial wastewater treatment.

As pressure mounts on global water systems due to climate change, growing populations, and resource constraints, the need for more sustainable and adaptive water treatment solutions is more apparent than ever. Electrified water treatment offers a compelling path forward. By using electrical energy to drive separation and redox reactions, these systems reduce dependence on chemical additives while allowing for fine control over treatment performance. Their ability to pair with renewable energy sources like solar and wind also makes them especially suitable for decentralized and off-grid settings.Among the various electrochemical methods, technologies such as electrocoagulation, electrooxidation, and electro-disinfection are gaining traction for their compact footprint and on-demand operation. In particular, Capacitive Deionization (CDI) and its advanced form, Flow-Electrode CDI (FCDI), are showing strong promise, not just for desalinating brackish water, but for tackling a much broader set of challenges. Recent developments in FCDI have pushed the boundaries of what electro-deionization can achieve. Beyond conventional desalination, FCDI has been successfully used to recover valuable resources like lithium and rare earth elements from battery leachates and acid mine drainage. It has also been applied to remove naturally occurring metals such as iron and manganese from groundwater. In some cases, FCDI has even been combined with advanced oxidation processes to create hybrid systems that handle both ionic and organic contaminants in a single step. These diverse applications highlight the flexibility of the technology and its potential to adapt to a wide range of water treatment needs.This talk will explore how FCDI is helping to shape the next generation of low-carbon, electrified water treatment systems. We will look at how it can be integrated with renewable energy, how it fits into modular and scalable treatment architectures, and how it works in synergy with other electrochemical processes. As the water sector moves toward net-zero goals, electro-deionization technologies like FCDI are emerging as key tools, offering energy efficiency, chemical reduction, and resilience in the face of today’s evolving environmental and resource challenges

Enhancing the electronic and ionic transport of flow-electrode capacitive deionization by hollow mesoporous carbon nanospheres

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

Flow electrode capacitive deionization (FCDI) employs carbon-based materials for electrochemical separation. Redox additives like vanadium, ferricyanide, and organic molecules have boosted desalination rates in FCDI. However, these additives must be low-cost, have chemical stability in electrolytes, and have low toxicity. We investigate two iron-based redox couples, iron chloride, and iron citrate, for their cost-effectiveness and reduced health risks. We incorporate them with activated carbon (AC) in flow electrodes, resulting in a significant enhancement of the salt removal rate, with iron chloride increasing it by 100 % and iron citrate by 23 % compared to standard AC slurries. In our system, the AC exhibits a salt removal rate of 7.8μmolcm−2min−1, while the iron chloride system reaches 15.6μmolcm−2min−1. Iron citrate flow electrodes achieve a desalination rate of 9.6μmolcm−2min−1, maintaining a pH suitable for potable water. We observe that iron chloride concentration in the flow electrode influences iron crossover and pH in the water streams, leading to acidity. Conversely, the iron citrate-based slurry maintains minimal iron crossover and a neutral pH. This study highlights iron-based redox additives as promising for FCDI slurries and the need for tailored flow electrode composition based on specific ion removal requirements.

Three-dimensional titanium mesh-based flow electrode capacitive deionization for salt separation and enrichment in high salinity water

Can flow-electrode capacitive deionization become a new in-situ soil remediation technology for heavy metal removal?