Asymmetric Capacitive Deionization Research Articles

Efficient removal of short-chain perfluoroalkyl substances (PFAS) using asymmetric membrane capacitive deionization

Concerns have been raised over the persistence and potential health risks of perfluoroalkyl substances (PFAS). PFAS pose significant challenges in water treatment owing to their chemical stability and the difficulty of treating them with conventional purification methods. Although traditional water treatment methods for contaminant removal have been explored, their effectiveness against PFAS remains limited. This gap in knowledge highlights the need for innovative approaches. This study introduces asymmetric membrane capacitive deionization (ACDI) as a novel method for short-chain PFAS removal to address the shortcomings of existing techniques. ACDI refers to a configuration in which an anion exchange membrane is removed from a conventional membrane capacitive deionization with both a cation exchange membrane and an anion exchange membrane. Among the PFAS, perfluorobutanesulfonic acid (PFBS), which is known to be difficult to remove because of its short chain, has been the focus. The ACDI system demonstrated superior deionization capacity (28.75 mg/g) and energy efficiency (energy consumption of 0.46 Wh/g) compared to conventional CDI and membrane CDI systems. In addition, the effects of key operating parameters, such as voltage (0.5 ∼ 1.2 V), initial concentration (50 ∼ 500 mg/L), and flow rate (1 ∼ 5 mL/min), on the PFBS removal were systematically investigated and superior at 1.2 V, 500 mg/L, and 2 mL/min. This study suggests the need for further exploration of real-world applications considering the presence of competing ions in wastewater. This study provides valuable insights into advancing CDI technology for the sustainable treatment of industrial wastewater containing PFAS.

Asymmetric capacitive deionization based on pore structures of biochar

Biochar is an affordable and sustainable material for the application of capacitive deionization. Although the desalination capability of biochar is significantly impacted by its pore size distribution, the correlation between them is challenging to explore due to the uncontrollable pore size. This study exclusively utilizes different activation mechanisms to regulate the pore size distribution of carbon derived from the same biomass precursor, and explores the role of pore structures in capacitive deionization. The specific capacitance of microporous biochar (MiB) can reach up to 319.1 F/g (0.2 A/g), with an Epzc of 0.11 V and high oxygen content, while mesoporous biochar (MeB) has a specific capacitance of only 170.4 F/g, an Epzc of 0 V, and a small charge transfer impedance. The MeB//MiB desalination cell is designed based on these characteristics and demonstrates a higher salt adsorption capacity of 17.04 mg/g at 1.2 V, compared to MiB//MiB (15.72 mg/g) and MeB//MeB (6.07 mg/g), alongside a stability of 81 % after 50 cycles. These capabilities can be attributed to the expanded voltage range resulting from the different Epzc, and the high ion storage capacity of MiB, as well as the fast ion transport capability of MeB. The oxygen/carbon ratio of the anode in MeB//MiB increased by only 42 %, whereas that in MiB//MiB increased by 370 %, confirming the oxidation inhabitation ability of the asymmetric structure and the oxidation resistance of MeB. Therefore, constructing ideal pore structures and more importantly reasonable deployment is an effective strategy for improving desalination capacity and stability.

A single-electrode evaluation method used for analyzing the working mechanism and capability of integrated membrane capacitive deionization

The evaluation of capacitive deionization (CDI) often relies on indicators like salt adsorption capacity and rate. However, these indicators encompass the entire system, including the anode and cathode. In practice scenarios, differences in specific capacitance, weight, and potential of zero charge result in varying theoretical ion adsorption capacity (IAC) and electrode potential. Hence, it is crucial to assess the deionization performance of individual electrodes. In this study, by introducing a reference electrode into the desalination device and enhancing the effective area and mass loading of the counter electrode, a single-electrode evaluation device was established to specifically analyze the deionization performance of the working electrode. Through this evaluation method, the single-electrode deionization performances of the anodic and cathodic integrated membrane electrodes (IMEs) were investigated, respectively, shedding light on the impact of IME on the Faradaic side reactions and co-ion effect. The results indicate that IME maintains an effective working voltage window and improves its stability. The cathodic IME effectively curbs the transport of dissolved oxygen, and the anodic IME prevents the carbon oxidation, thereby bolstering their IACs. This research not only elucidates the operational mechanism of IME but also highlights the feasibility of the single-electrode evaluation strategy in appraising asymmetric CDI.

Simultaneous desalination and molecular resource recovery from wastewater using an electrical separation system integrated with a supporting liquid membrane

Separating molecular substances from wastewater has always been a challenge in wastewater treatment. In this study, we propose a new strategy for simultaneous desalination and selective recovery of molecular resources, by introducing a supported liquid membrane (SLM) with molecular selectivity into an asymmetric flow-electrode capacitive deionization. Salts and molecular substances in wastewater are removed after passing through the ion separation chamber and the molecular separation chamber, respectively. Faradaic reactions, i.e., the electrolysis of water with OH–, occurred in the electrochemical cathode electrode provides a sufficient and continuous chemical potential gradient for the cross-SLM transport of phenol (a model molecule substance). By optimizing the formulation of the liquid membrane and the pore size of the support membrane, we obtained the SLM with the best performance for separating phenol. In continuous experiment tests, the electrochemical membrane system showed stable separation performance and long-term stability for simultaneous salts removal and phenol (sodium phenol) recovery from wastewater. Finally, we demonstrate the potential application of this technology for the recovery of different carbon resources. Overall, the electrochemical system based on SLM is suitable for various wastewater treatment needs and provides a new approach for the recovery of molecular resources in wastewater.

Extreme Monovalent Ion Selectivity Via Capacitive Ion Exchange

Capacitive deionization (CDI) is an emerging technology applied to brackish water desalination and ion selective separations. A typical CDI cell consists of two microporous carbon electrodes, where ions are stored in charged micropore via electrosorption into electric double layers. For typical feed waters containing mixtures of several cations and anions, some of which are polluting, models are needed to guide cell design for a target separation, given the complex electrosorption dynamics of each species. An emerging application for CDI is brackish water treatment for direct agricultural use, for which it is often important to selectively electrosorb monovalent Na+ cations over divalent Ca2+ and Mg2+ cations. Recently, it was demonstrated that utilizing constant-voltage CDI cell charging with sulfonated cathodes and short charging times enabled monovalent-selective separations. Here, we utilize a one-dimensional transient CDI model for a flow-through electrode CDI cell to elucidate the mechanisms enabling such separations. We report the discovery that an asymmetric CDI cell with a chemically functionalized cathode induces electric charges in the pristine anode at 0 V cell voltage, which has important implications for monovalent cation selectivity. Leveraging our mechanistic understanding, with our model we uncover a novel operational regime we term “capacitive ion exchange”, where the concentration of one ion species increases while competing species concentration decreases. This regime enables resin-less exchange of monovalent cations for divalent cations, with chemical-free electrical regeneration.

Unleashing the power of capacitive deionization: Advancing ion removal with biomass-derived porous carbonaceous electrodes

Capacitive deionization (CDI) is a promising electrochemical technique for the removal and recycling of ions from micro-polluted wastewater but is still hindered by the co-ion expulsion effect and anode oxidation. In this study, these issues are addressed through optimization of both materials and electrochemical systems. A diverse set of porous carbons are prepared using biomass as a precursor and KOH as the activation agent. It is found that direct carbonization and/or KOH activation induce a negative surface charge, whereas intense nitrogen-doping results in an inverse surface charge for all biomass-derived carbons, characterized by the potential of zero charge (Epzc). Density functional theory calculations suggest that the carboxyl group and quaternary N contribute most among other functional groups to the negative and positive charges, respectively. A Epzc-matching asymmetric CDI system is constructed employing negatively charged and positively charged carbons as the cathode and anode, respectively. This configuration, coupled with precise optimization of the cathode-to-anode mass ratio (m-/m+), unlocks a high adsorption capacity of 17.2 mg g−1 for NaCl, surpassing the symmetric system by 84.7 %. Further fine tuning of the m-/m+ ratio results in a removal capacity of 167.4 mg g−1 for Cu2+ ions, which is the highest reported for carbonaceous materials to date.

Hierarchical nickel cobaltite nanoneedle arrays armored flexible electrospinning carbon nanofibers membrane for electrochemical deionization

Crafting an ideal capacitive deionization (CDI) cathode presents a significant challenge, owing to the myriad requisites it must fulfill. These entail a high specific capacitance, commendable conductivity, stability, and the counteraction of the shortcomings inherent in additive/binder electrodes. Realizing all these attributes from a single-component material, however, is a daunting task. In this investigation, we present our findings on the in-situ cultivation of ultrathin NiCo2O4 nanoneedle arrays on freestanding electrospinning nitrogen-enriched carbon nanofibers devised to tackle these hurdles. The resultant material boasts remarkable features that not only alleviate tension triggered by phase transition but also accentuate electric conductivity. Electrochemical evaluations authenticate the effectiveness of structural modulation, reflected through a markedly augmented specific capacitance, superior stability, and diminished electrochemical resistance. Furthermore, the assembled pliable, asymmetric CDI cell incorporating electrospinning carbon nanofiber showcases superior desalination capabilities (38.86 mg g−1 at 1.2 V), a swift desalination rate (0.65 mg g−1 min−1), and improved cycle stability. This work finding contribute valuable interpretations towards proficiently designing and developing advanced CDI electrode materials capable of meeting the stringent requisites for high specific capacitance, good conductivity, and stability. It further propels progress towards industrialization, thereby expediting the realization of practical applications.

Amino acids functionalized vascular-like carbon fibers for efficient capacitive deionization

Porous carbons have attracted great attention in capacitive deionization (CDI), benefiting from their high surface areas and abundant adsorption sites. However, the sluggish adsorption rate and poor cycling stability of carbons are still concerns, which are caused by the insufficient ion-accessible networks and the side reactions (the co-ion repulsion and oxidative corrosion). Herein, inspired by the blood vessels in organisms, mesoporous hollow carbon fibers (HCF) were successfully synthesized via a template assisted coaxial electrospinning strategy. Subsequently, the surface charge of HCF was modified by various amino acids (arginine (HCF-Arg) and aspartic acid (HCF-Asp)). Combining structure design and surface modulation, these freestanding HCFs present enhanced desalination rate and stability, in which the hierarchal vasculature facilitates electron/ion transport, and the functionalized surface suppresses the side reactions. Impressively, when HCF-Asp and HCF-Arg serve as cathode and anode respectively, the asymmetric CDI device provides an excellent salt adsorption capacity of 45.6 mg g−1, a fast salt adsorption rate of 14.0 mg g−1 min−1 and a superior cycling stability up to 80 cycles. In short, this work evidenced an integrated strategy to exploiting carbon materials with outstanding capacity and stability for high-performance capacitive deionization.

Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization.

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (Ti3C2Tx) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl--capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl- intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl- de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl--capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g-1 at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl--capturing in electrochemical desalination.

Coal-based activated carbon functionalized with anionic and cationic surfactants for asymmetric capacitive deionization of nitrate

Capacitive deionization (CDI) is a promising desalination technology, and activated carbon is a most commonly used electrode material. In this work, coal-based activated carbon (AC) was prepared from residual carbon of coal gasification fine slag (CGFS) and functionalized with cationic surfactant (cetyltrimethyl ammonium bromide, CTAB) and anionic surfactant (sodium dodecyl sulfate, SDS), respectively. The prepared activated carbon named CTAB-AC and SDS-AC was assembled into the asymmetric CDI electrodes to remove low concentration nitrate in water. The results show that the average pore size, surface Zeta potential and specific capacitance of AC after modification by CTAB and SDS respectively. The surfactant modification slightly decreased the surface hydrophilicity of AC, but increased the electrical conductivity and improved the charge-discharge cycle stability. Compared with the symmetrical electrode AC, the asymmetric electrodes of CTAB-AC and SDS-AC largely improved the CDI performance, the total capacitive deionization capacity and the desalination efficiency for NaNO3 increased by 38.01% and 25.08%, and the continuous batch CDI process were shortened by 10 cycles. The final nitrate concentration could meet the drinking water requirement. This work proposed a new raw material for carbon electrode and a method for high value utilization of residue carbon in the coal gasification slag.

Fully 3D Modeling of Electrochemical Deionization.

Electrochemical deionization devices are crucial for meeting global freshwater demands. One such is capacitive deionization (CDI), which is an emerging technology especially suited for brackish water desalination. In this work, we extend an electrolytic capacitor (ELC) model that exploits the similarities between CDI systems and supercapacitor/battery systems. Compared to the previous work, we introduce new implementational strategies for enhanced stability, a more detailed method of describing charge efficiency, layered integration of leakage reactions, and theory extensions to new material and operational conditions. Thanks to the stability and flexibility the approach brings, the current work can present the first fully coupled and spatiotemporal three-dimensional (3D) CDI model. We hope that this can pave the way toward generalized and full-scale modeling of CDI units under varying conditions. A 3D model can be beneficial for investigating asymmetric CDI device structures, and the work investigates a flow-through device structure with inlet and outlet pipes at the center and corners, respectively. The results show that dead (low-flow) areas can reduce desalination rates while also raising the total leakage. However, the ionic flux in this device is still enough under normal operating conditions to ensure reasonable performance. In conclusion, researchers will now have some flexibility in designing device structures that are not perfectly symmetric (real-life case), and hence we share the model files to facilitate future research with 3D modeling of these electrochemical deionization devices.

Solar reduced graphene oxide decorated with manganese dioxide nanostructures for brackish water desalination using asymmetric capacitive deionization.

Capacitive deionization (CDI) has emerged as a low-cost, reagent-free technique for the desalination of water. This technique is based on the immobilization of dissolved ions on the electrically charged electrodes, by the electrosorption phenomenon. The electrosorption of dissolved ions by using CDI is limited for feed water having a low concentration of salts. To address this problem, we employ an asymmetric capacitive deionization (Asy-CDI) architecture having solar reduced graphene oxide decorated with manganese dioxide nanostructures (SRGO-MnO2 composite). The Asy-CDI possesses an SRGO-MnO2 composite as the cathode and SRGO as the anode with an anion exchange membrane. The cathode formed from the SRGO-MnO2 composite serves the purpose of immobilization of cations, whereas the anode formed from SRGO is responsible for anion removal. The crystal structure, chemical composition and morphology of the as-synthesized SRGO-MnO2 composite electrode materials are characterized by several techniques, confirming that the surface of SRGO is successfully loaded with α-MnO2 nanostructures. The electrochemical characterization reveals a high specific capacitance of the as-synthesized SRGO-MnO2 composite (419.9 F g-1) at 100 mV s-1. The Asy-CDI provides a higher salt adsorption capacity (40.2 mg g-1) compared to Sy-CDI (28.3 mg g-1) with feed water containing a salt concentration of 2000 mg L-1. These results indicate that the Asy-CDI may be employed as an efficient technique for the desalination of high concentration salt water.

Selective capacitive removal of Pb2+ ion contaminants from drinking water via nickel foam/Mn2CoO4@tannic acid-Fe3+ electrodes.

The health hazards caused by low concentrations of Pb2+ ions in drinking water systems are of significant concern. In order to remove Pb2+ ions and retain Na+, K+, Ca2+ and Mg2+ as harmless competitive ions without simultaneous removal, nickel foam (NF)/Mn2CoO4@tannic acid (TA)-Fe3+ electrodes were prepared by a hydrothermal method and a coating method and an asymmetric capacitive deionization (CDI) system is established using the prepared electrodes and a graphite paper positive electrode. The designed asymmetric CDI system exhibited a high Pb2+ adsorption capacity of 375 mg g-1 with high removal efficiency and significant regeneration behavior at 1.4 V at neutral pH. When the asymmetric CDI system is used to enrich a hydrous solution of mixed Na+, K+, Ca2+, Mg2+ and Pb2+ ions each with a concentration of 10 ppm and 100 ppm by electrosorption at a 1.4 V operating voltage, the removal rate of Pb2+ is as high as 100% and 70.8% respectively, and the relative selectivity coefficients are 4.51-43.22. According to the different adsorption mechanisms of lead ions and coexisting ions, the separation and recovery of ions can be realized by a two-step desorption process, thereby providing a new method for removing Pb2+ ions from drinking water with excellent application potential.

Zn/Fe/Al Modified Carboxymethyl Cellulose Biomass Carbon Aerogel for Capacitive Deionization

Biomass carbon aerogels have attracted increasing interest worldwide for capacitive deionization (CDI) desalination due to their hierarchical pore structure distribution, high specific surface area and modifiability. Herein, carboxymethyl cellulose (CMC) is used as a raw material, and Zn2+, Fe3+, and Al3+ are used as crosslinking agents to prepare carbon aerogels through “sol-gel, freeze-drying, high-temperature pyrolysis.” A CMC//AC asymmetric CDI electrode device is constructed for desalination. The results showed that coordination involving metal ions and carboxyl groups formed a carbon aerogel with a three-dimensional network structure; moreover, the addition of metal ions significantly increased the surface charge and graphitization of the material. Among the systems studied, CMC-Fe showed abundant pseudocapacitance due to redox reactions of the Fe. Gasification of Zn further increased the pore volume (2.11 cm3 g−1), specific surface area (1844 m2 g−1) and total specific capacitance (365 F g−1) of CMC-Zn. Al exhibited no obviously favorable behavior. Additionally, the prepared CMC-Zn showed good cycling stability, and the capacitance remained at 98% after 100 charging and discharging cycles. The CMC-Zn carbon aerogel electrode achieved a significantly high adsorption capacity of 25.8 mg g−1, showing that it has great potential among carbon materials for desalination.

Polystyrene sulfonate coated activated graphene aerogel for boosting desalination performance using capacitive deionization

Capacitive deionization (CDI) has proven to be a cost-effective and environmentally friendly desalination process. However, the high salt adsorption capacity is hampered by the poor surface wettability of the electrode materials and a lack of charge in the surface electrode. Despite its unique chemical and physical properties, pure reduced graphene oxide is still far from the expected potential desalination performance. In this work, graphene aerogel (GA) and activated graphene aerogel (AGA) with an ultrahigh specific surface area were successfully synthesized. The AGA was functionalized using a charged polystyrene sulfonate (PSS) and utilized as a negative electrode for the asymmetric CDI configuration cell. The synthesized charged PSS-activated graphene aerogel (PSS-AGA) achieved outstanding hydrophilic behavior, a high specific surface area (1401.53 m2/g), and a typical specific capacitance of 178F/g, furthermore excellent capacitance retention of 97.2 % over 5000 cycles. The salt adsorption capacity of the fabricated electrodes is evaluated by conducting a single-pass mode (SP-Mode) experiment under various conditions. The fabricated PSS-AGA//AGA asymmetric cell configuration delivers an exceptional salt adsorption capacity of 26.33 mg/g, excellent charge efficiency of 88 % with low energy consumption at 1000 mg/L, and full regeneration with long-term stability.

Porous 3D-graphene functionalized with MnO2 nanospheres and NiO nanoparticles as highly efficient electrodes for asymmetric capacitive deionization: Evaluation by impedance-derived capacitance spectroscopy

Capacitive deionization (CDI) is a novel desalination technology for brackish water purification. Here, we report the rational design of an efficient asymmetric CDI system considering several steps. First, three-dimensional graphene nanostructures (3DG) were synthesized using the hydrothermal method, followed by the freeze-drying process, and then drop casting onto the carbon fiber paper (CFP) substrate. Second, a facile and controllable electrodeposition technique was used for the in-situ synthesis of MnO2 nanospheres and NiO nanoparticles onto the CFP/3DG surface to prepare CFP/3DG/MnO2 cathode and CFP/3DG/NiO anode. Third, the prepared nanocomposite electrodes were used to fabricate an asymmetric CDI cell. Here, MnO2 (pI = 4.5) and NiO (pI = 10) nanostructures provide opposite surface charges due to their dissimilar isoelectric points, enhancing cations and anions electrosorption. The surface morphology, chemical and electrochemical characteristics, as well as the capacitive performance of the fabricated CFP/3DG/MnO2 and CFP/3DG/NiO electrodes, were investigated using SEM, EDX, XRD, FTIR, TGA, BET/BJH, and electrochemical techniques. Besides, impedance-derived capacitive analysis was used to evaluate the ions retention time on the fabricated electrodes. Finally, the fabricated CFP/3DG/NiO||CFP/3DG/MnO2 asymmetric CDI cell was used in water desalination with high reversibility, good regeneration rate, and high electrosorption capacity (21.01 mg g−1) for the initial salt concentration of 1000 mg L−1 in 1.2 V. These results suggest that the proposed asymmetric CDI system is a promising candidate for the efficient capacitive desalination process.

Efficient and Durable Sodium, Chloride-doped Iron Oxide-Hydroxide Nanohybrid-Promoted Capacitive Deionization of Saline Water via Synergetic Pseudocapacitive Process.

Recently, the rational design and development of efficient faradaic deionization electrodes with high theoretical capacitance, natural abundance, and attractive conductivity have shown great promise for outstanding capacitive deionization (CDI)‐based desalination applications. Herein, the construction of novel FeOOH hybrid heterostructures with Na and Cl dopants (e.g., Na‐FeOOH and Cl‐FeOOH) via a robust hydrothermal strategy is reported, and an asymmetric CDI cell (Na‐FeOOH//Cl‐FeOOH) comprising Na‐FeOOH and Cl‐FeOOH working as the cathode and anode, respectively, is assembled. The multiple coupling effects of the specific structural features (e.g., enriched porosity, hierarchical pore alignment, and highly open crystalline framework), enhanced electrochemical conductivity, and optimized ion‐transfer property endow the FeOOH hybrid electrode with improved electrochemical performance. Impressively, the Na‐FeOOH//Cl‐FeOOH cell demonstrates a superior salt adsorption capacity (SACNaCl) of 35.12 mg g−1 in a 500 mg L−1 NaCl solution, a faster removal rate, and remarkable cycling stability. Moreover, the pseudocapacitive removal mechanism from the synergetic contribution of the Na‐FeOOH cathode and Cl‐FeOOH anode account for the significant desalination promotion of the Na‐FeOOH//Cl‐FeOOH cell.

Nickel hexacyanoferrate incorporated with reduced graphene oxide for highly efficient intercalation desalination

In this study, nickel hexacyanoferrate (NiHCF) was incorporated with reduced graphene oxide (rGO) as a pseudocapacitive intercalation cathode in asymmetric capacitive deionization (ACDI) for desalination. The electrochemical characterization results demonstrated that the NiHCF@rGO nanocomposite exhibited an excellent specific capacitance of 158.2 F g−1 at a scan rate of 5 mV s−1 and high electrical conductivity. The desalination experiments were carried out in single-pass mode for the performance analysis under different operational parameters. For desalting 4 g L−1 NaCl solution at 1.2 V, NiHCF@rGO-assembled ACDI exhibited a large salt adsorption capacity of 80.2 mg g−1 and a high charge efficiency of 86.6%, associated with an energy consumption of 0.053 kWh m−3. It is noted that the NiHCF@rGO-assembled ACDI significantly outperformed the activated carbon-based membrane capacitive deionization for desalination. More importantly, the incorporation of rGO can improve the 27.7% salt adsorption capacity of NiHCF. The significant improvement can be ascribed to rGO uniformly distributing NiHCF particles on its surface via conductive bridges, which enable a low charge transfer resistance and facilitate the transport of ions. The X-ray photoelectron spectroscopy analysis further verified the Na+ intercalation/deintercalation of the NiHCF@rGO electrode, accompanied by the Fe3+ to Fe2+ redox reaction in a cyclic charging–discharging process. Overall, this work demonstrates the great potential of NiHCF@rGO as a cathode for efficiently improving desalination performance.

Anion-kinetics-selective graphene anode and cation-energy-selective MXene cathode for high-performance capacitive deionization

Capacitive deionization (CDI) is one of the most promising energy-efficient technologies for water desalination, however its industrial translation is slow and impeded by limited electrosorption capacity, slow electrosorption rate and poor cycling stability. Herein we address the above challenges by developing and validating a new concept of unimpeded and selective full-cell anion-cation separation using custom-designed anion-kinetics-selective anode and cation-energy-selective cathode. Nanoporous graphene anode enables the selective anion kinetics, while cation selectivity on the functionalized MXene cathode is due to the difference in ion adsorption energy. These mechanisms are validated by electrochemical quartz crystal microbalance measurements. The finely-tuned balance between ion transport and adsorption causes unimpeded ion diffusion, leading to the higher electrosorption rates. These effects are guided by the atomistic and quantum chemistry simulations, and also confirmed experimentally by tuning the pore size of graphene anode and the functionalization of MXene cathode. The fabricated asymmetric CDI cell exhibits superior electrosorption capacity of 49 mg g−1 and high electrosorption rate of 2.92 mg g−1 min−1 in 5000 mg L−1 NaCl solution as well as excellent cycling stability (100 cycles), which are among the best of the current state-of-the-art CDI studies. These results demonstrate the design of advanced electrode materials for the effective control of ion kinetics and energies to achieve selective ion transport, thus providing new insights for a broader range of energy-related applications.

Facile fabrication of intercalation-type pseudocapacitive S-Ti3C2Tx/PANI/F-Ti3C2Tx cathode for asymmetric capacitive deionization

Although tremendous efforts have been paid on Ti3C2Tx as a cathode for capacitive deionization (CDI), its structural instability and easy agglomeration have always limited such a promising application. In this study, a hybrid of hollow Ti3C2Tx spheres and flakes was synchronously fabricated by a novel sacrificial-templated method for the first time, and then immobilized/bonded together by in-situ polymerization of polyaniline, which was labelled as S-Ti3C2Tx/PANI/F-Ti3C2Tx. The highest specific capacitance of S-Ti3C2Tx/PANI/F-Ti3C2Tx in 1 M NaCl solution was measured to be 102 F·g−1, and diffusion capacitance of the as-obtained S-Ti3C2Tx/PANI/F-Ti3C2Tx was calculated to be 63%. Thus, a high salt adsorption capacity of 76 mg·g−1 with a maximum salt adsorption rate of 11.45 mg·g−1·min−1 was obtained, which is much higher than that of Ti3C2Tx-related materials in literature for asymmetric CDI system. Moreover, such a superior desalination performance was verified stable at almost 100% after 30 cycles. Besides the ion intercalation and fluoro/hydroxyl groups of Ti3C2Tx, the high performance and stability of the as-obtained S-Ti3C2Tx/PANI/F-Ti3C2Tx should also be ascribed to the beneficial effects of immobilization/bonding role of conductive PANI between S-Ti3C2Tx and F-Ti3C2Tx. This study gives a new perspective to promote the real applications of CDI by designing highly capacitive and stable electrode materials.