Flow-electrode Capacitive Deionization Research Articles

Separation of Mono-/Divalent Ions via Controlled Dynamic Adsorption/Desorption at Polythiophene Coated Carbon Surface with Flow-Electrode Capacitive Deionization.

Capacitive deionization for selective separation of ions is rarely reported since it relies on the electrostatic attraction of oppositely charged ions with no capability to distinguish ions of different valent states. Using molecular dynamic simulation, a screening process identified a hybrid material known as AC/PTh, which consists of activated carbon with a thin layer of polythiophene (PTh) coating. By utilizing AC/PTh as electrode material implementing the short-circuit cycle (SCC) mode in flow-electrode capacitive deionization (FCDI), selective separation of mono-/divalent ions can be realized via precise control of dynamic adsorption and desorption of mono-/divalent ions at a particular surface. Specifically, AC/PTh shows strong interaction with divalent ions but weak interaction with monovalent ions, the distribution of divalent ions can be enriched in the electric double layer after a couple of adsorption-desorption cycles. At Cu2+/Na+ molar ratio of 1:40, selectivity toward divalent ions can reach up to 110.3 in FCDI SCC mode at 1.0V. This work presents a promising strategy for separating ions of different valence states in a continuously operated FCDI device.

Comparison of current collector architectures for flow-electrode capacitive deionization

Flow-electrode Capacitive Deionization (FCDI) is a relatively young technology that allows energy-efficient desalination and salt concentration. FCDI employs flow electrodes, suspensions of conductive particles capable of storing ions in the electric double layer at their surface upon charging. To facilitate charging, the flow electrode particles must be electrically contacted using current collectors. Current collectors must guide the flow electrode along a membrane surface, provide a surface area for electrical contact with the particles, and be inert at high salt concentrations. In this study, we compare different current collector architectures for FCDI. Graphite plates and membrane-electrode assemblies made of carbon fiber fabric are two architectures described in the recent literature. We introduce a new current collector architecture based on a bipolar plate made of expanded graphite. These novel current collectors show up to 55 % higher salt transfer than graphite plates, rendering them a valuable new architecture for FCDI modules. Comparative analysis of different current collector architectures shows that the side walls of a flow channel are more effective at charge transport than the bottom wall. Furthermore, surface roughness impacts charge transport, with smoother surfaces leading to higher salt removal rates.

Flow electrode capacitive desalination of industrial RO reject.

Flow electrode capacitive deionization (FCDI) is a promising technology for efficiently treating industrial brine with high salt content. However, its desalination performance is currently limited by internal resistance. Achieving an effective FCDI system relies on active electrode materials with high conductivity. This study compares the desalination performances of the widely used flow electrode activated carbon (AC) with more conductive materials, reduced graphene oxide (rGO), and ZnO/rGO composite. Additionally, the lack of particle-to-particle contact in the flow electrode contributes to internal resistance and to address this, a cationic surface-active agent is introduced. This agent forms a stable dispersion, creating a space for enhanced mass loading of the active material. This modification enhances the conductive network and particle contact, reducing the diffusion path and promoting rapid ion transport. With a 5 wt% loading, ZnO/rGO achieved a 73% salt removal efficiency, surpassing AC at 63%. Furthermore, the surfactant-modified ZnO/rGO flow electrode with a 7 wt% loading demonstrated an 81% salt removal efficiency.

Energetic Comparison of Flow-Electrode Capacitive Deionization and Membrane Technology: Assessment on Applicability in Desalination Fields.

Flow-electrode capacitive deionization (FCDI) is a promising technology for sustainable water treatment. However, studies on the process have thus far been limited to lab-scale conditions and select fields of application. Such limitation is induced by several shortcomings, one of which is the absence of a comprehensive process model that accurately predicts the operational performance and the energy consumption of FCDI. In this study, a simulation model is newly proposed with initial validation based on experimental data and is then utilized to elucidate the performance and the specific energy consumption (SEC) of FCDI under multiple source water conditions ranging from near-groundwater to high salinity brine. Further, simulated pilot-scale FCDI system was compared with actual brackish water reverse osmosis (BWRO) and seawater reverse osmosis (SWRO) plant data with regard to SEC to determine the feasibility of FCDI as an alternative to the conventional membrane processes. Analysis showed that FCDI is competent for operation against brackish water solutions under all possible operational conditions with respect to the BWRO. Moreover, its distinction can be extended to the SWRO for seawater conditions through optimization of its total effective membrane area via scale-up. Accordingly, future directions for the advancement of FCDI was suggested to ultimately prompt the commercialization of the FCDI process.

Pine-derived porous carbon for efficient capacitive deionization and the role of its hierarchical pore structure

Capacitive deionization (CDI) is a low-energy, eco-friendly, and economically viable technology for seawater desalination. In the desalination process of CDI, the pore structure characteristics of electrode materials play a crucial role. In this study, the carbon materials with a hierarchical pore structure were prepared through high-temperature carbonization and KOH activation using pine powder as raw material. When the pine-derived porous carbon (PPC) was used as an electrode material, it was found that its specific capacitance is remarkably consistent with the specific surface area of the mesopores. This discovery provides guidance for further optimization and regulation of biomass-derived carbon electrode materials. The desalination performance of the PPC electrode was studied in detail by CDI and flow-electrode capacitive deionization (FCDI) devices. In NaCl solution with an initial concentration of 700 ppm, the equilibrium adsorption capacities in CDI and FCDI systems are 24.96 mg g−1 and 23.12 mg g−1, respectively. Particularly in FCDI tests, the average adsorption rate reaches 23.3 μg cm−2 min−1, and the desalination rate reaches 98.96 %. The PPC shows an excellent desalination performance, making it a promising efficient electrode material for electrochemical desalination.

Efficient phosphorus removal from ultra-low concentration wastewater by flow-electrode capacitive deionization

During the 14th Five-Year period, the stringent phosphorus emission standard puts forward higher requirements for the phosphorus removal. In this study, lotus leaf-based biochars were prepared by hydrothermal carbonization method and used as low-cost FCDI electrode materials to remove ultra-low concentration phosphorus in wastewater at the first time. The governing factors and optimal adsorption conditions were systematically investigated as 1.2 V applied voltage, 24 s HRT, 10 wt% content of biochars. The maximum removal efficiency reached 98.55 % under 20 mg/L initial phosphorus concentration, which ensured that the phosphorus concentration of the effluent could meet discharge standard (0.3 mg/L) of WWTPs. Furthermore, the microscopic and spectroscopic characterization revealed that phorphorus removal mechanism included physical adsorption, electrosorption, ion exchange, precipitation and ligand exchange, which may take place simultaneously or supplementing each other. Using in situ potential measurement method, the potential gradient and energy consumption contribution of each FCDI component were described. The results showed that total specific energy consumption was very low and the flow electrodes modules dominated (more than 50 %) in all FCDI components. The findings opened up a promising way for managing biowastes and confimed the application of engineering biochar in environmental pollution treatment field.

Enhanced ammonium removal and recovery from wastewater using an innovative redox-active flow electrode: The significant role of redox charge compensation and hydrogen bond network reconstruction

Flow-electrode capacitive deionization (FCDI) is an ideal electrochemical technology for NH4+ recovery but suffers from inefficient charge transfer, leading to high energy consumption and unsatisfactory recovery performance. In this study, we developed an innovative redox-active flow electrode (RAFE) comprising redox-active electrolyte ((NH4)3Fe(CN)6) and activated carbon (AC) to enhance the removal and recovery of NH4+. The ion capture and charge transfer mechanism of NH4+ within this electrode were further investigated. FCDI with RAFE cathode exhibited a rapid removal (31 mgN/m2/min) and recovery rate (26 mgN/m2/min), with about 32 % increment as compared to the FCDI with AC cathode. The lower removal (2.3 kWh/kgN) and recovery energy consumption (5.76 kWh/kgN) were also achieved. The CV curves and electrode characterization indicated that NH4+ was capacitive captured and released through electric double layer (EDL) and redox charge compensation. Dynamic charge efficiency analysis showed that EDL capacitive capture mechanism plays a dominant role during the charging stage while the redox compensation capture mechanism dominated the charge transfer during the discharging stage. Moreover, electrode suspension characterization demonstrated that the migration of NH4+ was facilitated by hydrogen bond network reconstruction. This work provided an energy-saving system for efficient NH4+ removal and recovery by coupling FCDI with RAFE cathode.

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

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). We believe that our approach can provide insight into various areas including resource production, recovery, removal of toxic ions, and material recycling using an FCDI system.

Flow-electrode capacitive deionization (FCDI) with microfiltration membranes for water reclamation from highly saline and dye-polluted wastewater

Textile wastewater, in addition to being highly polluted by organics like dyes, contains large amounts of salts, e.g., ≈10gL−1, that need to be removed before discharged to the environment or reused. This work introduces a new FCDI process that uses microfiltration membranes to desalinate and decolor synthetic textile wastewater. Synthetic wastewater simulates the characteristics of real Indian textile wastewater, containing a mixture of different salts (NaCl, Na2SO4, NaHCO3), with a total concentration of 6.9gL−1, and tartrazine (MW =534.3gmol−1) as the large organic pollutant model, with a concentration of 2.1gL−1. FCDI modules with different membrane configurations, including microfiltration membranes (MF), are investigated, giving insights into the desalination and dye removal efficiencies under steady-state operation. At wastewater flow rates below 0.08mLmin−1, the conductivity of treated water reaches 0.2mScm−1, with a tartrazine concentration below 50mgL−1, fulfilling the requirement for water reuse in textile industries. This work reveals the potential of the FCDI process for both desalination and organics removal that can be applied to a wide range of wastewater streams.

Performance and mechanism of chromium removal using flow electrode capacitive deionization (FCDI): Validation and optimization

The contamination of wastewater with hexavalent chromium (Cr (VI) poses significant health and environmental risks. To alleviate this challenge, our research explores the potential of flow electrodes capacitive deionization (FCDI) technique, an energy-efficient and environmentally friendly approach, for Cr (VI) removal and detoxification. We incorporated varying weight loadings of carbon nanotubes (CNTs) into activated carbon (AC) flowable electrodes, and assessed the system’s performance under different operational conditions. Our findings revealed that by adding just 1.5 wt% CNT to AC and applying a voltage of 0.9 V, the current response increased 3.4 times compared to that of pure AC flowable electrode, resulting in a remarkable Cr (VI) removal efficiency of 99.5 %. This study also revealed that the presence of high concentrations of co-existed chloride ions (Cl−) did not significantly influence the efficiency of Cr (VI) removal. Incorporation of CNT in AC flow electrodes reduced the inter-tube aggregation and enhanced the contact efficiency between the AC particles, forming a connectingpercolationnetwork and enhancing the conductivity of the slurry electrodes for Cr (VI) removal. In addition to Cr (VI) removal, FCDI technique transformed extremely dangerous Cr (VI) to less toxic Cr (III) from 5.4 to 49.8 % during multiple cycles. In summary, we presented an innovative, efficient, eco-friendly approach for Cr (VI) removal and detoxicity by using CNTs incorporated electrodes, underscoring the potential FCDI technology in heavy metal disposal.

Flow electrode capacitive deionization with iron-based redox electrolyte

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.

Carbon felt (CF) acted as an “ionic capacitor” to enhance flow electrode capacitive deionization (FCDI) desalination performance

Flow electrode capacitive deionization (FCDI) is a new ion removal technology with sustainable deionization performance. However, the interface resistance has a significant impact on ions migration in FCDI. We therefore introduced a novel FCDI device that incorporated a layer of carbon felt as an “ionic capacitor” between the flow electrode and the ion exchange membrane, termed CF-FCDI. Mechanism exploration results revealed that the addition of carbon felt reduced device resistance, shorten electron transfer pathways, improved charge efficiency, and increased ion migration rate. Experimental results demonstrated that the improved CF-FCDI device achieved 44 % higher desalination rate compared to traditional device. In addition, long term operation experiments confirmed the stability of CF-FCDI device, highlighting its potential as a viable desalination solution. The findings of this study provided valuable insights for further development of FCDI and offered a promising solution for its applications such as seawater desalination.

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

Flow electrode capacitive deionization (FCDI) is a highly promising desalination technique known for its exceptional electrosorption capacity, making it suitable for efficient salt separation in high salinity water. However, the unsatisfactory charge transfer process between the flow electrode and current collector severely curtails the salt separation and enrichment performance of the FCDI device. To address this issue, three-dimensional titanium mesh (3D-TM) was proposed as a novel current collector for FCDI device, which significantly amplifies the charge transfer area and exhibits excellent salt separation performance. The 3D-TM current collector promotes the electron transfer, charge percolation, and ion migration processes through the electroconvection generated by the turbulence effect on the flow electrode. In the specific case of the 20-mesh 3D-TM, which is composed of 12 stacking layers of titanium mesh, the remarkable average salt removal rate and charge efficiency were achieved 5.06 μmol cm−2 min−1 and 92.9 % under an appropriate applied voltage of 2.0 V, respectively. Dramatically, the desalination performance maintained above 76.4 % over 100 desalination cycles at 2.0 V, demonstrating the exceptional cyclic stability of the 3D-TM FCDI cell. In the seawater desalination, the 3D-TM FCDI cell exhibited an impressive salt removal efficiency of 97.5 % (from 34.2 g L−1 to 0.84 g L−1) for 1 L East China seawater at 2.0 V for 24 h. For lithium-ion enrichment, the FCDI continuous desalting system achieved an astonishing concentration of 17.3 g L−1 for Li+ ions enrichment from an initial concentration of 1.30 g L−1, obtaining the average salt treating rate of 23.6 g m−2h−1 and charge efficiency of 80.0 %. Moreover, the lithium-sodium ions and lithium-magnesium ions enrichments were both conducted, yielding an enriched concentration of 10.4 g L−1 and 7.30 g L−1 for Li+ ions, respectively. These findings highlight the enormous potential of FCDI technology in industrial engineering applications, further establishing it as a highly viable solution.

Flexible construction of three-dimensional continuous conductive structure by hollow carbon sphere and CNT for promoted ions transport in flow-electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) emerges as a promising desalination technique, characterized by its unlimited desalination capacity, seamless operation, and easy scalability. However, challenges such as discontinuous electronic network, inadequate dispersion of suspended carbon particles, and hindered electron/ion transport pathways have impeded the full potential of FCDI. This study addresses the above issues by synergistically leveraging the advantage of hollow carbon sphere (HCS) and CNT to construct a flexible, continuous three-dimensional conductive network (HCS@CNT) as the flow-electrode for FCDI. The incorporation of HCS substantially improves the fluidity and effective collision of the flow-electrode slurry. Simultaneously, the bridging effect of CNT facilitates smooth ion transport routes and rapid electron/ion transfer. Moreover, the spherical structure of HCS as a node can effectively alleviate the agglomeration of CNT due to its inherent good fluidity. As a result, the meticulously designed HCS@CNT flow-electrode demonstrates outstanding rheological behavior with excellent hydrophilicity, a large specific capacitance, and low ion diffusion resistance. During desalination tests, the HCS@CNT flow-electrode exhibited a high average salt removal rate (0.56 µmol min−1 cm−2), appropriate energy-normalized removal salt (15 µmol J−1), and notable charge efficiency (89 %) in a 1 g L-1 NaCl solution. Impressively, it maintained good stability and continuity over a continuous 10-hour desalination process, showcasing substantial potential for treating highly concentrated saline water. In addition, density functional theory (DFT) calculations provided additional insights, confirming that the construction of HCS@CNT facilitated interfacial charge transfer and electron transport from HCS to CNT. This innovative approach sheds light on enhancing electron/ion transfer by harnessing the unique structural characteristics of the flow-electrode.

Ionic liquid redox flow desalination of seawater

The solubility feature of redox electrolyte is a key factor to influence the performance of salt removal in redox flow electrode capacitive deionization. Herein, a novel redox flow desalination using ionic liquid (IL) as the eco-friendly solvent is proposed, i.e., 1-ethyl-3-methylimidazolium acetate. The high salt removal rate of 4.82 μg·min−1·cm−2 was obtained at the current density of 11 mA·cm−2. The low energy consumption of 254.12 kJ·mol−1 was achievable in IL, compared with 466.3 kJ·mol−1 in aqueous solution and 317.86 kJ·mol−1 ethanol. In addition, a long-term test with 11 cycles was conducted, and the real seawater was desalinated to a freshwater standard in batch mode. This work is significant for development of low-energy consumption in redox flow desalination via the usage of novel ionic liquids as solvent.

Polyaniline/activated carbon composite based flowing electrodes for highly efficient water desalination with single-cycle operational mode

Flow-electrode capacitive deionization (FCDI) is a promising technique for water desalination and has received considerable attention. The desalination efficiency of FCDI is significantly influenced by the flowing electrode material. Therefore, it is necessary to develop highly efficient flowing electrode materials to obtain excellent desalination capability. Polyaniline (PANI) is regarded as a promising pseudocapacitive material with high conductivity and desirable hydrophilicity. Herein, in order to improve the stability of PANI, a rod-like polyaniline loaded on activated carbon (AC) was prepared by a simple in-situ polymerization to obtain the PAC composite for FCDI electrode. The composite successfully combined the characteristics of AC and polyaniline. The desalination performance of PAC composite for FCDI single-cycle mode was explored, and the mechanism was ascribed to the synergy between electric-double layer capacitance of AC and pseudocapacitance of PANI. The PAC-9 composite with the best electrochemical performance achieved a salt removal efficiency of 94.3 % and an average salt removal rate of 0.963 μmol cm−2 min−1 at 1.2 V. In addition, the PAC-9 composite did not decay after 12 h of long-term operation, showing that the composite is a promising active material for water desalination.

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

This study systematically optimized the key operating parameters and interpreted their effecting mechanisms in a flow-electrode capacitive deionization (FCDI) system. The optimal voltage, activated carbon electrode content, electrolyte concentration, feedwater flowrate, and electrode flowrate for desalinating low salinity feedwater (1.0 g L−1 NaCl) were determined to be 1.8 V, 2.0 wt%, 10.0 g L−1, 80 mL min−1, and 60 mL min−1, respectively. The variations of the above parameters can affect the system conductivity, the thickness and stability of the electric double layers, and/or the degree of concentration polarization, thereby influencing the desalination performance. Moreover, a sensitivity analysis identified the operating voltage as the dominant parameter with the most significant influence on the FCDI system. Subsequently, a long-term operation was carried out under single-pass mode. The results showed that the lab-scale FCDI system was able to constantly maintain the desalination efficiency of 1.0 g L−1 feedwater (NaCl) at 40–60 % for multiple operating cycles. Over 99.8 % of electrode material regeneration and desalination efficiency recovery was able to be obtained during a 60-h operation, demonstrating that the FCDI system showed strong stability and long-term operation potential.

Simultaneous removal of tetracycline and copper ions from wastewater by flow-electrode capacitive deionization

ABSTRACT To effectively solve the problem of tetracycline (TC) and Cu2+ contamination in wastewater, this study innovatively proposed a low-energy flow-electrode capacitive deionization (FCDI) technology to simultaneously remove TC and Cu2+ from wastewater. The removal efficiencies of TC and Cu2+ using FCDI was investigated under various voltages, electrode flow rates, influent flow rates, and electrode liquid concentrations. The results showed that the removal efficiency of TC and Cu2+ was 60.78% and 84.43%, respectively. The energy consumption for TC and Cu2+ removal was only 1.76 and 1.10 kWh kg−1, which was lower than other electrochemical systems. The ion removal performance of the FCDI system remained stable after six cycles of continuous operation. These findings demonstrated the promising potential of FCDI as an innovative technology for the simultaneous removal of TC and Cu2+, presenting a significant prospects for application in the water treatment field.

Designing a Stable and Electrically Conductive Colloidal Suspension for Improved Flow Electrode Capacitive Deionization

With increasing effects from major droughts and water shortages, water desalination has garnered significant technological interest. Like many technologies, desalination suffers from high energy consumption, capital cost, process inefficiencies, and material degradation. Flow electrode capacitive deionization (FCDI) is a relatively new water desalination technology that combines aspects of electrodialysis and capacitive deionization to deliver a device which can achieve >90% water recovery at high productivity and thermodynamic efficiency. A fundamental challenge in FCDI, though, is engineering an electrically conductive slurry with excellent colloidal stability. Here, we explore the tradeoff between electrical conductivity and sedimentation rate using commercially available surfactants. We find that the optimal surfactant is non-intuitive and strongly depends on the type of carbon used in the slurry (e.g., activated carbon, carbon black, etc.).This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Funding was provided by Lawrence Livermore National Laboratory Directed Research and Development (LDRD) Grant #22-DR-011. IM release number: LLNL-ABS-846651.

Exploring the Use of Static Mixers for Enhanced Flow-Electrode Capacitive Deionization

Water desalination plays a major role in alleviating water scarcity issues, but it faces major barriers due to high energy use, low water utilization, and significant environmental impacts. Flow electrode capacitive deionization (FCDI) is a new ultra-efficient electrochemical water treatment technology capable of desalinating saline streams (e.g., brackish water, seawater) with less than 10% of the feed water rejected as waste. Here we use a structured flow plate design to maximize charge transfer while incurring a negligible pressure drop penalty. Different 3D printed flow plate structures were characterized individually in a flow capacitor arrangement to quantify their relative merits in terms of the tradeoff between conductivity and flow characteristics with carbon slurries containing 5-9 wt% activated carbon and 0-1 wt% carbon black.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Funding was provided by Lawrence Livermore National Laboratory Directed Research and Development (LDRD) Grant 22-DR-011. IM #LLNL-ABS-846456