Covalent triazine-based frameworks as electrodes for high-performance membrane capacitive deionization

Heterointerface regulation of covalent organic framework-anchored graphene via a solvent-free strategy for high-performance supercapacitor and hybrid capacitive deionization electrodes

Covalent organic frameworks (COFs) with customizable geometry and redox centers are an ideal candidate for supercapacitors and hybrid capacitive deionization (HCDI). However, their poor intrinsic conductivity and micropore-dominated pore structures severely impair their electrochemical performance, and the synthesis process using organic solvents brings serious environmental and cost issues. Herein, a 2D redox-active pyrazine-based COF (BAHC-COF) was anchored on the surface of graphene in a solvent-free strategy for heterointerface regulation. The as-prepared BAHC-COF/graphene (BAHCGO) nanohybrid materials possess high-speed charge transport offered by the graphene carrier and accelerated electrolyte ion migration within the BAHC-COF, allowing ions to effectively occupy ion storage sites inside BAHC. As a result, the BAHCGO//activated carbon asymmetric supercapacitor achieves a high energy output of 61.2 W h kg-1 and a satisfactory long-term cycling life. More importantly, BAHCGO-based HCDI possesses a high salt adsorption capacity (SAC) of 67.5 mg g-1 and excellent long-term desalination/regeneration stability. This work accelerates the application of COF-based materials in the fields of energy storage and water treatment.

Solvent-Free Synthesis of Covalent Organic Framework/Graphene Nanohybrids: High-Performance Faradaic Cathodes for Supercapacitors and Hybrid Capacitive Deionization

Covalent organic frameworks (COFs) with flexible periodic skeletons and ordered nanoporous structures have attracted much attention as potential candidate electrode materials for green energy storage and efficient seawater desalination. Further improving the intrinsic electronic conductivity and releasing porosity of COF-based materials is a necessary strategy to improve their electrochemical performance. Herein, the employed graphene as the conductive substrate to in situ grow 2D redox-active COF (TFPDQ-COF) with redox activity under solvent-free conditions to prepare TFPDQ-COF/graphene (TFPDQGO) nanohybrids and explores their application in both supercapacitor and hybrid capacitive deionization (HCDI). By optimizing the hybridization ratio, TFPDQGO exhibits a large specific capacitance of 429.0 F g-1 due to the synergistic effect of the charge transport highway provided by the graphene layers and the abundant redox-active centers contained in the COF skeleton, and the assembled TFPDQGO//activated carbon (AC) asymmetric supercapacitor possesses a high energy output of 59.4 Wh kg-1 at a power density of 950 W kg-1 and good cycling life. Furthermore, the maximum salt adsorption capacity (SAC) of 58.4 mg g-1 and stable regeneration performance is attained for TFPDQGO-based HCDI. This study highlights the new opportunities of COF-based hybrid materials acting as high-performance supercapacitor and HCDI electrode materials.

The closed-loop recycling strategy of Li and Co metal ions based on aqueous Zn-air desalination battery

Nowadays, it is a global problem to recycle LiCoO₂ from waste lithium-ion batteries (LIBs) due to the deficiency of high business cost and environmental pollution. Here, a novel three-channel ion recovery device based on a Zn-air desalination battery (ZADB) is proposed which can supply energy while separating Li⁺ and Co²⁺ from the recovered solution. The three-channel ZADB device consists of a Zn foil anode chamber with ZnSO₄ anolyte stream, an intermediate chamber with Li⁺ and Co²⁺ recovered stream and an air cathode chamber with LiOH and Co(OH)₂ catholyte stream, chambers are separated by anion exchange membrane (AEM) and cation exchange membrane (CEM) respectively. It can be described by the finite element simulation (FES) of physics field that, the Li⁺ and Co²⁺ in the recovered solution move to the cathode chamber, where the OH^- are produced by absorbing O₂ from the air combined with electronic in the discharge process. At the same time, the SO₄^2- moves to the other end of the Zn foil anode chamber according to the law of charge conservation, which combined with the Zn²⁺ removed from the Zn foil. The results show that the recovery efficiency of the ZADB device is closely related to the discharge current density and the concentration of the recovered stream. The best recovery effect has achieved when 0.2 mol L^-1 recovered solution is run for 24 h at the discharge current density of 0.2 mA cm^-2. The average recovery rate is 0.275 mg min^-1 with the highest recovery rate is 40.73 mg h^-1, and the output energy density is 102.5 Wh Kg^-1 during the experiment process. In addition, the ZADB device has the excellent long-term cycling performance and recycling stability. By comparing this device with other ion recovery methods, which provides that it is a splendid way to recycle Li⁺ and Co²⁺ from waste LIBs.

Desalination Using the Capacitive Deionization Technology with Graphite/AC Electrodes: Effect of the Flow Rate and Electrode Thickness

Capacitive deionization (CDI) is an emerging water desalination technology whose principle lies in ion electrosorption at the surface of a pair of electrically charged electrodes. The aim of this study was to obtain the best performance of a CDI cell made of activated carbon as the active material for water desalination. In this work, electrodes of different active layer thicknesses were fabricated from a slurry of activated carbon deposited on graphite sheets. The as-prepared electrodes were characterized by cyclic voltammetry, and their physical properties were also studied using SEM and DRX. A CDI cell was fabricated with nine pairs of electrodes with the highest specific capacitance. The effect of the flow rate on the electrochemical performance of the CDI cell operating in charge–discharge electrochemical cycling was analyzed. We obtained a specific absorption capacity (SAC) of 10.2 mg/g and a specific energetic consumption (SEC) of 217.8 Wh/m³ at a flow rate of 55 mL/min. These results were contrasted with those available in the literature; in addition, other parameters such as Neff and SAR, which are necessary for the characterization and optimal operating conditions of the CDI cell, were analyzed. The findings from this study lay the groundwork for future research and increase the existing knowledge on CDI based on activated carbon electrodes.

Nitrogen-enriched micro-mesoporous carbon derived from polymers organic frameworks for high-performance capacitive deionization

Nitrogenization is an effective method for improving the capacitive deionization (CDI) performance of porous carbon materials. In particular, polymer organic frameworks with heteroatom doping, containing an ordered pore structure and excellent electrochemical stability, are ideal precursors for carbon materials for high-performance CDI. In this study, a nitrogen-enriched micro-mesoporous carbon (NMC) electrode was fabricated by carbonizing a Schiff base network-1 at 500, 600, and 700 °C. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, N₂ adsorption-desorption, the contact angle of water, cyclic voltammetry, and electrochemical impedance spectroscopy were used to characterize the morphological structure, wettability, Brunauer-Emmett-Teller surface areas, and electrochemical performance of the NMCs. The results showed that the NMC carbonized at 600°C achieved the best specific capacitance (152.33 F/g), as well as a high electrosorption capacity (25.53 mg/g) because of its chemical composition (15.57% N) and surface area (312 m²/g). These findings prove that NMC is viable as an electrode material for desalination by high-performance CDI applications.

Recent Advances in Electrocatalysts for Alkaline Hydrogen Oxidation Reaction

With the rapid development of anion-exchange membrane technology and adequate supply of high-performance non-noble metal oxygen reduction reaction (ORR) catalysts in alkaline media, the commercialization of anion exchange membrane fuel cells (AEMFCs) become possible. However, the kinetics of the anodic hydrogen oxidation reaction (HOR) in AEMFCs is significantly decreased compared to the HOR in proton exchange membrane fuel cells (PEMFCs). Therefore, it is urgent to develop HOR catalysts with low price, high activity, and robust stability. However, comprehensive timely reviews on this specific subject do not exist enough yet and it is necessary to update reported major achievements and to point out future investigation directions. In this review, the current reaction mechanisms on HOR are summarized and deeply understood. The debates between the mechanisms are greatly harmonized. Recent advances in developing highly active and stable electrocatalysts for the HOR are reviewed. Moreover, the side reaction control is for the first time systematically introduced. Finally, the challenges and future opportunities in the field of HOR catalysis are outlined.

Faradic capacitive deionization (FCDI) for desalination and ion removal from wastewater

Capacitive deionization (CDI) is one of the emerging desalination technologies that attracted much attention in the last years as a low-cost, energy-efficient, and environmentally-friendly alternative to other desalination technologies, such as multi-stage flash desalination (MSF) and multiple effect distillation (MED). The implementation of faradaic electrode materials is a promising method for enhancing CDI systems' performance by achieving higher salt removal characteristics, lower energy consumption, and better ion selectivity. Therefore, a novel CDI technology named Faradaic CDI (FCDI) that implements faradaic electrode materials arose as a high-performance CDI cell design. In this work, the application of FCDI cells in desalination and wastewater treatment systems is reviewed. First, the progress done on using various FCDI systems for saline water desalination is summarized and discussed. Next, the application of FCDI in wastewater treatment applications and selective ion removal is presented. A thorough comparison between FCDI and conventional carbon-based CDI is carried out in terms of working principle, electrode material's cost, salt removal performance, energy consumption, advantages, and disadvantages. Finally, future research consideration regarding FCDI technology is included to drive this technology closer towards practical application.

Facile Designed Manganese Oxide/Biochar for Efficient Salinity Gradient Energy Recovery in Concentration Flow Cells and Influences of Mono/Multivalent Ions

Development of effective, environmentally friendly, facile large-scale processing, and low-cost materials is critical for renewable energy production. Here, MnO_x/biochar composites were synthesized by a simple pyrolysis method and showed high performance for salinity gradient (SG) energy harvest in concentration flow cells (CFCs). The peak power density of CFCs with MnO_x/biochar electrodes was up to 5.67 W m^-2 (ave. = 0.91 W m^-2) and stabilized for 500 cycles when using 1 and 30 g L^-1 NaCl, which was attributed to their high specific capacitances and low electrode resistances. This power output was higher than all other reported MnO₂ electrodes for SG energy harvest due to the synergistic effects between MnO_x and biochar. When using a mixture with a molar fraction of 90% NaCl and 10% KCl (or Na₂SO₄, MgCl₂, MgSO₄, and CaCl₂) in both feed solutions, the peak power density decreased by 2.3-40.1% compared to 100% NaCl solution with Ca²⁺ and Mg²⁺ showing the most pronounced negative effects. Our results demonstrated that the facile designed MnO_x/biochar composite can be used for efficient SG energy recovery in CFCs with good stability, low cost, and less environmental impacts. When using natural waters as the feed solutions, pretreatment would be needed.

N-Doping Carbon-Nanotube Membrane Electrodes Derived from Covalent Organic Frameworks for Efficient Capacitive Deionization

Capacitive deionization (CDI) is an energy-efficient and environmentally friendly electrochemical desalination technology which has attracted increasing attention in recent years. Electrodes are crucial to the performance of CDI processes, and utilizing a carbon-nanotubes (CNTs) membrane to fabricate electrodes is an attractive solution for advanced CDI processes. However, the strong hydrophobicity and low electrosorption capacity limit applications of CNTs membranes in CDI. To solve this problem, we introduce crystalline porous covalent organic frameworks (COFs) into CNTs membranes to fabricate N-doping carbon-nanotubes membrane electrodes (NCMEs). After solvothermal growth and carbonization, CNTs membranes are successfully coated with imine-based COFs and turned into integrated NCMEs. Comparing with the CNTs membranes, the NCMEs exhibit an ∼2.3 times higher electrosorption capacity and superior reusability. This study not only confirms that COFs can be used as high-quality carbon sources but also provides a new strategy to fabricate high-performance CDI electrodes.

Pseudocapacitive Behaviors of Polypyrrole Grafted Activated Carbon and MnO2 Electrodes to Enable Fast and Efficient Membrane-Free Capacitive Deionization

Capacitive deionization (CDI) has emerged as a promising technique for brackish water desalination. Here, composites of polypyrrole grafted activated carbon (Ppy/AC) were prepared via in situ chemical oxidative polymerization of pyrrole on AC particles. The Ppy/AC cathode was then coupled with a MnO₂ anode for desalination in a membrane-free CDI cell. Both the Ppy/AC and MnO₂ electrodes exhibited pseudocapacitive behaviors, which can selectively and reversibly intercalate Cl^- (Ppy/AC) and Na⁺ (MnO₂) ions. Compared to AC electrodes, the specific capacitances of Ppy/AC electrodes increased concurrently with the pyrrole ratios from 0 to 10%, while the charge transfer and ionic diffusion resistances decreased. As a result, the 10%Ppy/AC-MnO₂ cell showed a maximum salt removal capacity of 52.93 mg g^-1 (total mass of active materials) and 34.15 mg g^-1 (total mass of electrodes), which was higher than those of conventional, membrane, and hybrid CDI cells. More notably, the salt removal rate of the 10%Ppy/AC-MnO₂ cell (max 0.46 mg g^-1 s^-1 to the total mass of active materials and 0.30 mg g^-1 s^-1 to the total mass of electrodes) was nearly 1 order of magnitude higher than those in most previous CDI studies, and this fast and efficient desalination performance was stabilized over 50 cycles.

Microwave-assisted production of CO2-activated biochar from sugarcane bagasse for electrochemical desalination

A high-performance carbon electrode is desirable for promoting electrochemical desalination efficiency in the membrane capacitive deionization (MCDI). Sugarcane bagasse (food waste) was employed in this study to prepare hierarchically porous biochars by microwave-assisted carbonization and activation with potassium hydroxide in N₂ or CO₂ atmosphere under varying flow rates (100-600 cm³ min^-1). The sugarcane bagasse-derived biochars activated under CO₂ flow of 300 cm³ min^-1 (denoted as SBB-CO₂-300) possessed the ratio of mesopores to total pore volume (V_meso/V_total) of 56.7% with a specific surface area of 1019 m² g^-1. The electrochemical behavior of SBB-CO₂-300 was demonstrated by a surpassing specific capacitance of 208 F g^-1 at 5 mV s^-1 by means of cyclic voltammetry. The desalination tests using a batch-mode MCDI at 1.2 V in a 5 mM NaCl solution indicated that the SBB-CO₂-300 electrode exhibited an excellent electrosorption capacity of 28.9 mg g^-1. The improvement in the electrochemical deionization performance of SBB-CO₂-300 was attributed to the superior V_meso/V_total ratio, high surface area, excellent capacitance behavior, and hierarchical pore structure. The biowaste-derived biochars prepared via facile microwave-assisted carbonization and CO₂ activation route can provide a sustainable and high-efficiency carbon electrode for electrochemical deionization of brackish water.

Free-standing flexible film as a binder-free electrode for an efficient hybrid deionization system

In recent years, capacitive deionization (CDI) has emerged as an energy efficient and cost-effective technology for the desalination of brackish water. However, high energy consumption and poor desalting efficiency at high salinity levels have hampered the application of CDI for seawater (∼35 000 mg L-1). A novel method of CDI termed hybrid capacitive deionization (HCDI) employs the use of a faradaic electrode paired with a capacitive electrode. Doing so increases the salt removal capacity to approximately three times that of conventional activated carbon (AC) materials (∼10 mg g-1). Herein, we report experimental results of our HCDI cell using free-standing Na2Ti3O7-CNT@reduced graphene oxide (NCNT@rGO) film as a binder-free negative and activated carbon@reduced graphene oxide (AC@rGO) film as the positive electrode. The HCDI cell is operated under a constant current mode. During desalination, sodium ions are intercalated into the negative electrode (NCNT@rGO) whereas chloride ions are adsorbed onto the surface of the positive electrode (AC@rGO). We observed a high removal capacity of 129 mg g-1 at the low energy consumption of 0.4 W h g-1 for a salt concentration of ∼3000 mg L-1 at 50 ml min-1 flow rate. The higher performance of our electrodes over conventional ones (∼109 mg g-1, 0.68 W h g-1) is attributed to the absence of binders or conductive additives and the unique nano-architectured sandwich structure of NCNT@rGO. The advantageous features of our electrodes shed new insight into the development of CDI materials and show promise for low-cost, scalable systems.