Capacitive Deionization System Research Articles

Novel flow-electrode capacitive deionization system employing modified MOF-derived carbon electrodes for metal ion removal

Activated carbon (AC) as an electrode for capacitive deionization (CDI) is constrained by its low specific surface area and specific capacitance, leading to electrical double layer overlap (EDL). To address this issue, a flow electrode capacitive deionization system (MDC-FCDI) was developed, which used modified MOFs-derived carbon (MDC) as the electrode. The MDC electrode exhibited enhanced specific capacitance (145.78 F/g) and specific surface area (2458.44 m2/g). Compared to conventional FCDI, the MDC-FCDI system demonstrated a 35–39 % improvement in heavy metal removal efficiency from wastewater containing Pb2+, Cu2+, Cd2+, and Zn2+. The selectivity of MDC-FCDI for heavy metals exhibited a unique “two-step phenomenon” that differed from what was observed in conventional CDI systems. In the MDC-FCDI system, high-valent ions experienced stronger electric field forces and preferentially occupied active sites on the electrode surface prior to the adsorption of ions with lower valence. The selectivity of the MDC-FCDI system for heavy metal ions is determined by three factors in the following order of priority: ionic charge, hydration radius, and ion feed concentration. This study offers a practical approach for expanding the range of MDC-FCDI applications.

Effective electro-removal of glyphosate and glufosinate from aqueous solutions via capacitive deionization

Both glyphosate (Gly) and glufosinate (Glu) are commonly used as herbicides, which has caused rising concerns about their impact on the environment and human health. Capacitive deionization (CDI) is a newly developed energy-efficient water treatment technique that effectively removes inorganic and small organic molecules under the electric field. In this work, a carbon electrode-based CDI device was used in the electro-removal of Gly and Glu in an aqueous solution with an excellent removal rate of up to 95 % under various conditions. Furthermore, maximum electro-removal rates of 1.99 μmol∙cm−2∙min−1 for Gly and 1.33 μmol∙cm−2∙min−1 for Glu were also achieved, except with super-high removal rates of 100 % (Gly) and 97.6 % (Glu) in actual Gly and Glu-containing lake water, demonstrating the feasibility of the CDI device in real aqueous systems. In-situ electro-oxidation that occurred during the electrosorption process was confirmed by analyzing the three-electrode system-based electrochemical behaviors and tracing the time-dependent changes of the suspicious degradation products. Transition state (TS) search and molecular dynamics (MD) simulations were employed to study the microscopic processes theoretically, revealing the degradation pathways and thermodynamic characteristics of Gly and Glu. This research presents a novel avenue for the decomposition and removal of herbicides from water, leveraging the inherent side reactions of CDI systems as an advantage to decontaminate organic pollutants.

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

The desalination of seawater/wastewater utilizing flow-electrode capacitive deionization (FCDI) has received significant interest. However, challenges like the low electrical conductivity of flow electrode and excessive energy consumption have prevented the scaling up of desalination operations. To overcome these issues, this study introduces a new FCDI system termed the two-stage FCDI (TS-FCDI) system, in which desalination modules are equipped with freestanding and flow electrode. The TS-FCDI system offers a graded desalination module design that produces an average salt removal rate (ASRR) of 113 µg cm2·min−1. However, the TS-FCDI system requires substantially reduced infrastructure investment, device size, and energy expenses. Notably, the design allows for the simultaneous operation of freestanding and flow electrode and enables the transport of ions and charges in the electrode region. Experimental evidence from multiple device configurations (CDI, FCDI, and TS-FCDI) supports this claim. Notably, the TS-FCDI system shows about twice the desalination performance over FCDI, with approximately 50 h of single-cycle (SC) operation. This enhancement links the freestanding electrode with the flow electrode, enabling the device to be well-suited for practical applications. The continuous flow of electrode particles in contact with the freestanding electrode initiates secondary ion transport, merging two different types of electrodes to enhance the active space of the electrode, effectively improving ion adsorption performance. In summary, the results of this study indicate the potential of the TS-FCDI system as a suitable desalination technology for scaling up applications.

Designing NiFe-metal organic frameworks@NiFe-layered double hydroxide/carbon fiber self-supporting electrode for enhanced capacitive deionization

Self-supporting electrode benefits to enhance the desalination capacity and maximize the capacitance retention by avoiding the usage of organic binders and conductive additives in capacitive deionization (CDI). In this work, we design a self-supporting electrode of NiFe-metal organic framework@NiFe-layered double hydroxides/carbon fiber (MOF@LDH/CC) for highly efficient CDI. The cross-linked CC provides a unique network structure to promote the diffusion of salty ions, leading to high desalination kinetics. On the other hand, the NiFe-MOF acts as seed to induce the growth of NiFe-LDH which not only block the agglomeration of NiFe-LDH layers, but also strengthen the linkage between the CC and NiFe-LDH. As expected, the optimized MOF@LDH/CC//activated carbon CDI system achieves high initial salt removal capacity of 0.033 mg/cm2 in 1000 μS/cm NaCl solution, which decreases to 0.026 mg/cm2 after 30 cycles. The capacity decline is ascribed to the salt deposition during the cycling, which can be observed from the structural evolution. Further, the X-ray photoelectron spectra verifies the reversible chloride ions electro-adsorption/desorption by MOF@LDH/CC. Meanwhile, the porous fibrous structure of MOF@LDH/CC electrode is remaining after few cycles, highlighting the robust structure.

Capacitive deionization electrode fabrication using activated carbon derived from agriculture by-products toward industry application: A review

Capacitive deionization (CDI), membrane capacitive deionization (MCDI), and flow capacitive deionization (FCDI) are emerging technologies applied to remove ions from brackish water or saline wastewater based on electrode double layer (EDL) mechanisms. The electrode itself can be seen as the most important factor that determines the deionization efficiency of the CDI system. Electrode fabrication using activated carbon derived from agriculture byproducts is a useful solution to decrease cost, restrict environmental pollution, and enable agriculture waste recovery. In this review, the primary parameter's impact on activated carbon (AC) and the electrode fabrication process from the agriculture byproducts is detailed in a discussion to find the most optimal conditions. With excellent characteristics for the activated carbon such as a surface area of up to 2500 m2/g, it can be used to fabricate electrodes that have a high adsorption capacity of approximately 25 mg/g, able to be advanced to 40 mg/g with a modified AC. It can achieve a cycle performance of 95–97 %. The deionization efficiency of the CDI, MCDI, and FCDI systems can be up to 90 % with an input salt concentration from 2000 to 3000 mg/L where the water recovery ratio achieved is 50–90 %, alongside a salt rejection rate of about 7–75 %. AC can be modified to increase its special properties such as surface area, conductivity, and ion selectivity using different methods. The drawbacks and challenges of the CDI technology are also mentioned along with useful solutions such as electrode coating techniques using an anilox roller and a spray, for use in industry applications.

Scaling behavior of Si in capacitive deionization (CDI) systems for brackish groundwater desalination

Capacitive deionization (CDI) is a potentially cost-effective method of desalinating brackish groundwaters though the technology has not yet been widely utilised for this particular application. Silicon (Si) is a common element in groundwaters which may cause problems due to its tendency to induce severe fouling of equipment used for water treatment. In this study we focus on the impacts of the presence of Si on the performance of CDI. A systematic evaluation procedure is proposed, which is used to distinguish the “dynamic” and residual Si scaling and the impacts of these types of fouling on desalination performance. Results show that Si has minimal influence on CDI performance within low concentration ranges (<0.5 mM). Sorption of deprotonated silicic acid species is largely avoided in CDI, which contributes to sustaining the removal of major ions and stable operation of the system. However, at high concentrations of Si (>2.0 mM), scaling was observed to form on the electrode surface, leading to noticeable deterioration in CDI electrode performance. The presence of ion exchange membrane in CDI (i.e., membrane CDI or MCDI) can significantly alleviate Si scaling of the desalination system. Results of this study clearly demonstrate the scaling behavior of CDI and its derivatives when dealing with Si-laden water matrices, providing insights into the design and maintenance of CDI systems in practical applications.

Comprehensive investigation of capacitive deionization cells by response surface methodology

Freshwater resources are limited; thus, different technologies for desalination of brackish and seawater have been developed. Capacitive deionization (CDI) is one of the most promising desalination technologies owing to its low cost, simplicity, nonmoving parts, and low energy consumption. However, its energy consumption significantly increases with desalination of high-salinity water. In this regard, a good understanding of the effects of different process parameters on CDI energy consumption is crucial for further performance improvement. Therefore, the surface response methodology (RSM) was adopted to systematically study the effects of applying voltage, Stern layer capacity, volume and concentration of feed water, and volume of micropores on energy consumption and desalination time. The results showed that the Stern layer capacity and volume of micropores reduced desalination time by approximately 25% without significant changes in energy consumption. With the design of an improved electrode (improving the micropore structure for ion adsorption), the desalination time (Cycle) was reduced and a slight reduction in energy consumption was achieved. It can be claimed that any improvement in the physical (porosity) and electrical properties of the electrodes, results in an important increase in the performance of the CDI system.

Critical review of capacitive deionization: design, operation consideration and real-world environmental applications

Capacitive Deionization (CDI) has emerged as a promising and environmentally sound solution for addressing pressing water treatment challenges. This critical review meticulously examines the design, operational considerations, and performance metrics of CDI systems, particularly highlighting their significance in environmental applications under real-world conditions for handling different water sources such as groundwater, surface water, sea water. Various CDI configurations, electrode materials, and operational parameters are scrutinized through an extensive analysis of literature, with their direct influence on system efficiency, ion removal capacities, and long-term stability being elucidated. The reduced energy consumption, minimal chemical usage, and potential for sustainable water treatment of CDI, as compared to traditional methods, are emphasized from an environmental perspective. Moreover, this review underscores the pivotal role of CDI in practical environmental applications, including desalination, wastewater treatment, and remediation of inorganic pollutants (heavy metals and hardness) and resource recovery (nutrient, uranium, lithium…). By presenting compelling case studies, CDI's efficacy in mitigating environmental impacts is illustrated, offering a cleaner and more cost-effective alternative to conventional treatment methods. By outlining challenges and charting future directions, this review serves as a roadmap for researchers, engineers, and policymakers, facilitating the development of sustainable and efficient water treatment strategies. In an era marked by escalating environmental concerns, CDI emerges as a potential technology for responsible and effective water management, poised to shape a more sustainable future.

Selection of graphene as a conductive additive for biomass-based activated carbon electrode in capacitive deionization: acid-treated as a practical approach to reduce graphene content

Abstract The use of graphene (GE) as a conductive additive for activated carbon (AC) electrode in capacitive deionization (CDI) has attracted much attention due to its high electrical conductivity. However, self-agglomeration by π–π interactions between individual GE sheets required the use of high content to ensure an efficient conductive network in the electrode, which limited its practical application. This work proposed an approach to reduce the GE content in the fabrication of biomass-based activated carbon electrode for CDI application. The stacking effect of GE sheets is inhibited when GE sheets functionalize with oxygen groups via acid treatment under ultrasonic condition. This helps to reduce GE content to less than 2 wt% in the fabrication of CDI electrodes. The electrode fabricated using 2 wt% treated GE (T-GE) achieved the superior capacitance of 54 F/g, twice that of the pristine AC. The desalination process using as-prepared electrodes was evaluated by batch-mode CDI system with the initial NaCl solution of 2000 ppm and 3000 ppm at various applied voltages. The AC/T-GE 2 % electrode showed excellent performance with a desalination capacity greater than 10 mg/g, and reached an adsorption rate of 1.93 mg/g.min in both solutions under an applied voltage 1.2 V.

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.

Deionization performance, desalination enhancement mechanism, desorption behavior and energy consumption analysis of biochar-based membrane capacitive deionization

Four different configurations of capacitive deionization (CDI) were assembled to comparatively investigate the contribution of different ion exchange membranes (IEMs), including anion exchange membrane (AEM) and cation exchange membrane (CEM), to the enhancement of desalination performance. The results showed that the low charge efficiency of membrane-free biochar-based CDI system was mainly attributed to co-ion repulsion rather than Faraday reactions, and a single IEM was not effective in suppressing the co-ion repulsion effect thereby exhibiting a similar low charge efficiency. Compared with AEM, CEM was more beneficial for high coulombic efficiency by restraining the oxygen reduction reactions. In addition, the electrosorption and desorption behaviors of the biochar-based membrane capacitive deionization (MCDI) under different operating conditions were explored. The results showed that zero voltage desorption (ZVD) was unfavorable to the desorption capacity of the electrode, especially in high concentration solutions, the reverse voltage desorption (RVD) led to higher desorption efficiency but required higher energy consumption. Therefore, energy consumption and regeneration efficiency need to be a trade-off in practical applications to realize energy saving and efficient desalination.

Prussian blue analogue based integrated membrane electrodes for desalination and selective removal of ammonium ions in a rocking-chair capacitive deionization

Capacitive deionization (CDI) technology has shown significant promise in selective removal of nutrients from wastewater. In this study, a novel inorganic integrated membrane electrodes (i-IMEs) were developed with Prussian blue analogues (PBAs), cation insertion materials, as a coating on activated carbon (AC) electrode to replace cation exchange membrane. When the i-IMEs were operated in a rocking-chair CDI (RCDI) system for continuous desalination, a high specific adsorption capacity (SAC) of 42.7 mg/g and an impressive charge efficiency of 71.6 % were obtained. Moreover, taking advantage of PBAs' ion-selective properties for monovalent cations especially NH4+, this IME-RCDI achieved highly selective removal of cations compared to a RCDI with bare AC electrodes. The selectivity coefficients (Si/j) of NH4+ to divalent cations (such as Ca2+) and monovalent cations (such as Na+) were determined to be 2.15 and 1.95, respectively. These values are much higher than those obtained for AC electrodes, which were 0.3 and 1.07, respectively. Overall, this work demonstrates that the incorporation of PBAs in the i-IME-RCDI configuration offers superior desalination performance and selective removal capabilities for specific ions such as NH4+.

3D interconnected porous graphene architecture as a high-performance capacitive deionization electrode

Capacitive deionization (CDI) is a promising technology for brackish water desalination because of its high efficiency, no secondary pollution and low cost. However, the ion removal capacity of CDI system for brackish water desalination is still insufficient. Herein, a new in-situ porous Cu template method is used to prepare a unique open three-dimensional mesoporous graphene foam (3DMG) with high CDI performance. The developed 3DMG material has rich mesoporous structure, interconnected pore structure, high specific surface area, good hydrophilicity and excellent electrical conductivity, which can achieve effective solution/pore contact, rapid ion migration and high electrosorption capacity. Therefore, the specific capacitance of 3DMG as high as 141.37F/g at 1 M NaCl and 10 mV/s, and the maximum salt adsorption capacity is as high as 32.08 mg/g (1000 mg/L, 1.2 V), which is significantly higher than that of graphene-based electrode materials reported in most literatures. It also shows excellent regeneration performance and stability after repeated electrosorption-desorption cycles. The developed 3DMG undoubtedly provides a promising electrode material platform for CDI devices, effectively solving the urgent problem of freshwater resource shortage.

Preparation of heterogeneous cation exchange membrane and its contributions in enhancing the removal of Ni2+ by capacitive deionization system

Capacitive deionization (CDI), an emerging method to eliminate ions from water at a low cost, has garnered significant interest in recent years. This study evaluates the implication of cation exchange resin loading on the membrane via the nonsolvent-induced phase inversion method. After determining the quantity of resins efficiently loaded on the membrane, it was subsequently utilized as a cation exchange membrane in the membrane capacitive deionization (MCDI) unit to examine the performance removal of Ni2+. The results show that the amount of resins influenced the membrane structure and significantly improved the efficiency of Ni2+ removal. The sulfonic acid group show a strong intensity directly proportional to the quantity of resins based on the FTIR measurement. In conjunction with the enhanced resin amount, ion exchange capacity and water content were increased. Simultaneously, there was an observed elevation in the water contact angle and the roughness of the membrane surface with increased resin amount. In the MCDI unit, membrane M20 (20% by weight resin) was employed to elucidate its roles in the CDI unit, encompassing an examination of various concentrations and flow rates, with Ni2+ utilized as a test contaminant. The results demonstrated that using membrane M20 in the MCDI (MCDI-M20) unit consistently exhibited higher adsorption levels than the CDI unit, reaching 19.80 mg g−1 ACC in the MCDI-M20 unit, while CDI unit achieved 10.27 mg g−1 ACC at 200 mg L−1 Ni2+ concentration and a flow rate of 10 mL min−1 at 1.2 V. Additionally, Ni2+ concentrations and flow rates in CDI system had an evident impact on the duration of adsorption due to the mechanisms of ions transport on the membrane. This study suggests that employing the prepared membrane in the MCDI unit enhanced the removal of Ni2+ from the solution, contributing to sustainable development goals.

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.

Exploration of selective copper ion separation from wastewater via capacitive deionization with highly effective 3D carbon framework-anchored Co(PO3)2 electrode

The increasing amount of heavy metal copper ions (Cu2+) in industrial emissions, poses a serious threat to human health, biological environment, and resource scarcity. Capacitive deionization (CDI) is considered as a green and efficient method for desalination. It is crucial to develop high-performance electrodes for efficient operation of CDI that go beyond conventional carbon and yield considerable environmental benefits. Here, metal organic frameworks (MOFs) derived carbon-loaded cobalt metaphosphate (NC-Co(PO3)2) was prepared by low-temperature gas–solid phosphating for Cu2+ removal as CDI electrode for the first time. NC-Co(PO3)2 demonstrated superior electrode structure and function due to the synergistic effects of electric double layer coupling P-O bonds, the binding tendency of metaphosphate groups with Cu2+, and interfacial redox reactions induced by the labile valence state of cobalt. The optimal electrosorption capacity of NC-Co(PO3)2 was 95.41 mg g−1 at 1 V in 50 mL Cu2+ solution with splendid cyclic regeneration capability. Moreover, NC-Co(PO3)2 exhibited excellent selectivity and outstanding electrosorption performance in the presence of multiple coexisting ions and this CDI system realized the purification of actual copper-containing wastewater. A series of characterizations further revealed the specific mechanism of Cu2+ in adsorption–desorption process. Our finding strongly supported NC-Co(PO3)2 electrode can extend the CDI platform's capability for effectively removing and retrieving Cu2+ from wastewater.

Nanomaterial grafted polymorphous activated carbon cloth surface for antibacterial, capacitive deionization and oil spill cleaning applications

This work reports the development of multifunctional or polymorphous surfaces using zinc oxide (ZnO) nanorods, silica (SiO2), and fluoropolymer functionalization in a sequential process. Firstly, zinc oxide nanorods were grown on activated carbon cloth (ACC) using a simple low-temperature synthesis process. ZnO nanorods-coated ACC substrate was applied to investigate the antimicrobial properties, and the results showed inhibition of 50% for Escherichia coli (E.coli) and 55% for Bacillus subtilis (B.subtilis) over 48 h of incubation time. Subsequent in-situ modification of silica nanoparticles like layer on ZnO nanorods-coated ACC surface was developed and used as an electrode for brackish water desalination in a capacitive deionization system. ZnO–SiO2 modified ACC surface enhanced the desalination efficiency by 1.6 times, the salt removal rate (SRR) by threefold, and the durability (fouling prevention) for long-term usage compared to pristine ACC. Further modification of the ZnO–SiO2-ACC surface using fluoropolymer rendered the surface superhydrophobic and oleophilic. Vegetable (1.4 g/g) and crude oil (1.6 g/g) adsorption capacities were achieved for modified surface which was 70% enhancement compared with pristine ACC. The dynamic oil spill adsorption test exhibited the complete removal of oil spills on water surfaces within a few seconds, suggesting a potential application in oil spill cleaning.

Bifunctional Deionization and Lithium Recovery Via Four-Step Constant Voltage Operation of Hybrid Capacitive Deionization with Composite Electrodes

The efficient recovery of valuable resources from wastewater has become a crucial challenge in the era of a carbon-neutral society. Capacitive deionization (CDI) has been employed in electrochemical lithium recovery (ELR) processes, based on selective Li+ intercalation to LiMn2O4 (LMO). In this study, we propose a four-step constant voltage operation (forward/zero/reverse/zero) of bifunctional hybrid CDI to produce deionized water as well as Li+-rich concentrate within a cycle. This unique performance was enabled by composite electrodes with capacitive activated carbon cloth (ACC) impregnated with redox-active LMO and Ag, respectively, which were characterized and evaluated through material characterization and electroanalysis. The proposed CDI system demonstrated deionization (step 1; forward voltage) and selective Li+ enrichment (step 3; reverse voltage) without influent replacement. The performance of the CDI system was evaluated through single pass experiments with binary NaCl and LiCl solutions (10 mM each) under 1.0/0/-1.0/0 V operation, revealing supreme performance with mass loadings of 32.6 mg/cm2 for LMO and 3.3 mg/cm2 for Ag. Furthermore, the long-term stability of the system was confirmed, indicating its potential for practical implementation. The proposed bifunctional CDI system, combining ELR with water treatment, paves the way for advancing resource recycling from wastewater.

Chicken Feathers-Derived Carbon Electrodes for Capacitive Deionization

Capacitive deionization (CDI) is an emerging technology which is being developed as a promising desalination alternative for charged species from salt water, being electrode materials as key drivers for highly efficient process. In this study, we report sustainable, scalable and stable carbon electrodes for CDI. Electrodes were synthesized from poultry chicken feathers waste using pyrolysis followed by chemical activation with potassium hydroxide at a 1:4 ratio to produce highly porous carbon. Poultry chicken feathers-activated carbon (CF-AC) were prepared and fabricated into electrodes using poly(vinyl alcohol) (PVA) binder and later crosslinked with glutaraldehyde (GA) to produce a PVA-GA polymer due to its hydrophilicity by acetylation reaction to improve the resistance of carbon material. Prepared materials were characterized using scanning electron microscopy, Fourier transform infrared, X-ray diffraction, Brunauer-Emmett-Teller and cyclic voltammetry. Fabricated CDI electrodes were used to investigate their performance for desalination, and exhibited different electrosorption capacity at different applied potentials. Modified electrode possessed good stability in shear conditions and CDI process was stable and reproducible around 16 electrosorption cycles. Salt removal capacity of PVA-GA-bonded CF-AC (PVA-GA-CF-AC) electrodes was found to be 3.89 mg g-1. The obtained outcomes offer important considerations of ions electrosorption and help advancing CDI system for water treatment and desalination.

Organic Faradaic Desalination Cell Based on Polyimide Symmetric Electrodes

Capacitive deionization (CDI) is an energy-efficient and environmentally friendly deionization technology. However, the maximum desalination capacity of conventional carbon-based CDI systems is in the range of 5–25 mg g-1, which compromises the use of this technology in practical applications [1]. Faradaic deionization (FDI) is a promising electrochemical technique, based on storing the ions within the particles/layers of the electrode materials either by intercalation and/or faradic reactions. By this mechanism, FDI has expanded the deionization values above 50 mg g-1. In the last ten years, research on FDI has primarily focused on battery electrode materials, especially in inorganic intercalation compounds, typically used for energy storage [2]. However, inorganic materials have several drawbacks, such as decomposition and dissolution issues, which affect long-term performance along with safety and sustainability aspects and consequently might limit their practical deployment.As an alternative, we introduce in this work for the first time an all-polymer deionization cell in combination with a rocking-chair desalination (RCD) mechanism with the aim of boosting the deionization performance [3]. A proof-of-concept next generation technology is accomplished by using a redox-active naphthalene-polyimide (poly[N,N′-(ethane-1,2-diyl)-1,4,5,8-naphthalenetetracarboxiimide], named as PNDIE), which is a well conductive and free-standing electrode material, as anode and cathode in a symmetric FDI system. Our approach relies on the synergistic combination of smart redox polymer selection i.e. PNDIE, and the advance configuration of this material into a buckypaper electrode architecture. A detailed electrochemical characterization under various operational conditions in three electrodes standard half-cell and in two electrode coin-cell configurations was performed. The results of those experiments provide critical information to build an all-polymer rocking chair system with practical PNDIE electrodes (92.2 mgPNDIE; 9.6 cm2). This novel FDI lab-cell demonstrated a remarkable desalination capacity (155.4 mg g-1 at 0.01 A g-1) together with a high salt-removal rate and productivity (3.42 mg g-1 min-1 at 0.01 A g-1 and 62 L h-1 m-2, respectively). In addition, long-term stability experiments provided salt adsorption capacity retention values over 95 % after 100 cycles (23 days). Compared to the state-of-the-art deionization technologies (CDI, Hybrid CDI and FDI) the reported technique exhibits superior electrochemical and deionization performance. These results support the idea of using organic materials as a viable alternative for robust and sustainable “water-energy” electrochemical applications.