Enhanced Electrochemical Stability of a Zwitterionic-Polymer-Functionalized Electrode for Capacitive Deionization

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.

Faradaic Electrodes Open a New Era for Capacitive Deionization

Capacitive deionization (CDI) is an emerging desalination technology for effective removal of ionic species from aqueous solutions. Compared to conventional CDI, which is based on carbon electrodes and struggles with high salinity streams due to a limited salt removal capacity by ion electrosorption and excessive co-ion expulsion, the emerging Faradaic electrodes provide unique opportunities to upgrade the CDI performance, i.e., achieving much higher salt removal capacities and energy-efficient desalination for high salinity streams, due to the Faradaic reaction for ion capture. This article presents a comprehensive overview on the current developments of Faradaic electrode materials for CDI. Here, the fundamentals of Faradaic electrode-based CDI are first introduced in detail, including novel CDI cell architectures, key CDI performance metrics, ion capture mechanisms, and the design principles of Faradaic electrode materials. Three main categories of Faradaic electrode materials are summarized and discussed regarding their crystal structure, physicochemical characteristics, and desalination performance. In particular, the ion capture mechanisms in Faradaic electrode materials are highlighted to obtain a better understanding of the CDI process. Moreover, novel tailored applications, including selective ion removal and contaminant removal, are specifically introduced. Finally, the remaining challenges and research directions are also outlined to provide guidelines for future research.

Chemical Tuning of Specific Capacitance in Functionalized Fluorographene

Owing to its high surface area and excellent conductivity, graphene is considered an efficient electrode material for supercapacitors. However, its restacking in electrolytes hampers its broader utilization in this field. Covalent graphene functionalization is a promising strategy for providing more efficient electrode materials. The chemistry of fluorographene is particularly attractive as it allows scalable chemical production of useful graphene derivatives. Nevertheless, the influence of chemical composition on the capacitance of graphene derivatives is a largely unexplored field in nanomaterials science, limiting further development of efficient graphene-based electrode materials. In the present study, we obtained well-defined graphene derivatives differing in chemical composition but with similar morphologies by controlling the reaction time of 5-aminoisophthalic acid with fluorographene. The gravimetric specific capacitance ranged from 271 to 391 F g-1 (in 1 M Na2SO4), with the maximum value achieved by a delicate balance between the amount of covalently grafted functional groups and density of the sp2 carbon network governing the conductivity of the material. Molecular dynamics simulations showed that covalent grafting of functional groups with charged and ionophilic/hydrophilic character significantly enhanced the ionic concentration and hydration due to favorable electrostatic interactions among the charged centers and ions/water molecules. Therefore, conductive and hydrophilic graphitic surfaces are important features of graphene-based supercapacitor electrode materials. These findings provide important insights into the role of chemical composition on capacitance and pave the way toward designing more efficient graphene-based supercapacitor electrode materials.

Membrane-Free Hybrid Capacitive Deionization System Based on Redox Reaction for High-Efficiency NaCl Removal

Capacitive deionization (CDI) is a promising technology for desalination due to its advantages of low driven energy and environmental friendliness. However, the ion removal capacity (IRC) of CDI is insufficient for practical application because such a capacity is limited by the available surface area of the carbon electrode for ion absorption. Thus, the development of a novel desalination technology with high IRC and low cost is vital. Here, a membrane-free hybrid capacitive deionization system (HCDI) with hollow carbon@MnO₂ (HC@MnO₂) to capture sodium via redox reaction and hollow carbon sphere with net positive surface charges (PHC) for chloride adsorption is introduced. The as-obtained HC@MnO₂ with unique structure and high conductivity can improve the utilization of MnO₂ pseudocapacitive electrodes. Meanwhile, the PHC can selectively adsorb Cl^- and prevent the adsorption of Na⁺ due to electrostatic repulsion. As expected, the membrane-free HCDI system demonstrates excellent desalination performance. The system's IRC and maximum removal rate are 30.7 mg g^-1 and 7.8 mg g^-1 min^-1, respectively. Moreover, the proposed system has a low cost because of the absence of expensive ion exchange membranes (IEM), which is suitable for practical application. The excellent performance of this HCDI makes it a promising desalination technology for future use.

Flow-electrode capacitive deionization with highly enhanced salt removal performance utilizing high-aspect ratio functionalized carbon nanotubes

Flow-electrode-based capacitive deionization (FCDI) has attracted much attention owing to its continuous and scalable desalination process without the need for a discharging step, which is required in conventional fixed-electrode capacitive deionization. However, flow electrode slurry is poorly conductive, which restricts desalination performance, but higher carbon mass loading in the slurry could improve salt removal capacity due to enhanced connectivity. However, increased viscosity restricts higher loading of active materials. Herein, we report a significant increase in salt removal performance by introducing functionalized carbon nanotubes (FCNTs) into activated carbon (AC)-based flow electrodes, which led to the generation of conducting bridges between AC particles. The salt removal rate in the presence of 0.25 wt% FCNT with 5 wt% AC improved four-fold from that obtained with only 5 wt% AC, which is the highest value reported in the literature so far (from 1.45 to 5.72 mmol/m²s, at a saline water concentration of 35.0 g/L and applied potential of 1.2 V). Further, FCNTs with a high aspect ratio (∼50,000) can more effectively enhance salt removal than low-aspect ratio FCNTs (∼1300). Electrochemical analysis further confirms that the addition of FCNTs can efficiently form a connecting percolation network, thus enhancing the conductivity of the flow electrode slurry for the practical application of highly efficient desalination systems.