A review on free-standing electrodes for energy-effective desalination: Recent advances and perspectives in capacitive deionization

Recent advanced freestanding pseudocapacitive electrodes for efficient capacitive deionization

The effective desalination and purification devices for seawater/ brackish water treatment are crucial in sustainable progress. Techniques that render high salt removal efficiency and water purification ability at low applied potentials play a central role in sustainable water supplies. One of them is capacitive deionization (CDI) which has drawn significant consideration as a promising deionization technology since the last decade. Desalination efficiency profoundly depends on the utilized electrode material. The most widely used CDI electrodes are carbons due to their cost effectiveness and good stability. However, to acquire high electrosorption capacity, extensive researches are reported with modified carbon materials. CDI cell architectures are equally important for practical high salt removal performance. This review focuses on carbon materials in CDI along with other emerging trends in diverse carbon types, e.g., carbon nanotubes and their composites. Various architectures reported in the literature to improve desalination efficiency are also included here.

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

As a promising desalination technology, capacitive deionization (CDI) have shown practicality and cost-effectiveness in brackish water treatment. Developing more efficient electrode materials is the key to improving salt removal performance. This work reviewed current progress on electrode fabrication in application of CDI. Fundamental principal (e.g. EDL theory and adsorption isotherms) and process factors (e.g. pore distribution, potential, salt type and concentration) of CDI performance were presented first. It was then followed by in-depth discussion and comparison on properties and fabrication technique of different electrodes, including carbon aerogel, activated carbon, carbon nanotubes, graphene and ordered mesoporous carbon. Finally, polyaniline as conductive polymer and its potential application as CDI electrode-enhancing materials were also discussed.

Freshwater reserves are being polluted every day due to the industrial revolution. Man-made activities have adverse effects upon the ecosystem. It is thus the hour of need to explore newer technologies to save and purify water for the growing human population. Capacitive deionization (CDI) is being considered as an emerging technique for removal of excess ions to produce potable water including desalination. Herein, cost-effective activated carbon incorporated with carbon nanotubes (CNT) was used as a freestanding electrode. Further, the desalination efficiency of the designed electrodes was tuned by varying binder concentration, i.e., polyvinylidene difluoride (PVDF) in the activated carbon powder and CNT mixture. PVDF concentration of 5, 7.5, 10, and 12.5 wt% was selected to optimize the freestanding electrode formation and further applied for desalination of water. PVDF content affected the surface morphology, specific surface area, and functional groups of the freestanding electrodes. Moreover, the electrical conductivity and specific surface area changed with PVDF concentration, which ultimately affected the desalination capacity using the freestanding electrodes. This study paves the way to produce cost effective carbon-based freestanding electrodes for capacitive deionization and other applications including battery electrodes.

Perchlorate removal from brackish water by capacitive deionization: Experimental and theoretical investigations

Exploring MXene’s role in capacitive deionization: Advances, challenges, and future directions

Comprehensive Summary Access to safe and clean water is fundamental to human health and economic development. While the practical impact of emerging technologies depends on their successful demonstration at large scales, capacitive deionization (CDI) has garnered significant attention as a promising approach for efficient desalination of seawater and brackish water. Among the various 2D materials explored for CDI (e.g., graphene, MXenes, covalent organic frameworks), their derived 2D/2D heterostructures, with unique lamellar morphology and interfacial engineering, offer an ideal platform for effectively modulating charge transfer behavior and ion diffusion. Despite a variety of 2D/2D heterostructures with diverse construction modes have been developed as CDI electrodes in recent years, a dedicated review focusing on the design strategies, synergistic effects, water desalination performance, and prevailing challenges remains lacking. In this review, we highlight the cutting‐edge research progress of 2D/2D heterostructures for CDI applications. After an overview of 2D materials and synthetic strategies of 2D/2D heterostructures, the relationships between the morphology/structure/composition and the water desalination performance are discussed in detail. Thereafter, we discuss current limitations and propose future directions for the rational design of 2D/2D heterostructures. This review will promote exploitation of 2D/2D heterostructures with an ideal performance of CDI towards water remediation. Key Scientists Significant progress has been made in the development of 2D/2D heterostructures for capacitive deionization (CDI) applications towards versatile ion capture. This collection of pioneering work underscores a clear trajectory in the field: the strategic construction of 2D/2D heterostructures is a powerful and versatile paradigm for advancing CDI. By intelligently combining different 2D materials, researchers have successfully engineered heterointerfaces with enhanced ion adsorption capacity, superior selectivity, and improved stability, paving the way for next‐generation, high‐performance desalination and water remediation technologies.

Each year approximately 1.3 billion tons of food is either wasted or lost. One of the most wasted foods in the world is bread. The ability to reuse wasted food in another area of need, such as water scarcity, would provide a tremendous sustainable outcome. To address water scarcity, many areas of the world are now implementing desalination. One desalination technology that could benefit from food waste reuse is capacitive deionization (CDI). CDI has emerged as a powerful desalination technology that essentially only requires a pair of electrodes and a low-voltage power supply. Developing freestanding carbon electrodes from food waste could lower the overall cost of CDI systems and the environmental and economic impact from food waste. We created freestanding CDI electrodes from bread. The electrodes possessed a hierarchical pore structure that enabled both high salt adsorption capacity and one of the highest reported values for hydraulic permeability to date in a flow-through CDI system. We also developed a sustainable technique for electrode fabrication that does not require the use of common laboratory equipment and could be deployed in decentralized locations and developing countries with low-financial resources.

Study of sugar cane bagasse fly ash as electrode material for capacitive deionization

Impact of molecular size and electrical polarity on fouling of capacitive electrodes during water desalination

There is a large push to develop appropriate technologies to treat not only seawater but brackish waters in order to increase the availability of water. Brackish water treatment requires significantly less energy than seawater desalination. To put the energy demand into perspective, the thermodynamic minimum energy require for separation, estimated by the Gibbs free energy of mixing, is between 1-3 kWh per m3 for seawater (35 g/l) and only 0.2-0.9 kWh per m3 for brackish streams (5 g/l). This energy savings quickly compounds at industrial and power generation sites which require over 1 billion m3 of water daily1. Furthermore, brackish waters are not geographically limited, available at both coastal and non-coastal regions.The treatment of brackish rather than seawater can yield energy savings; however, achieving high energy efficiency during low saline separations processes remains a significant challenge. This has been demonstrated using state of the art desalination technology (reverse osmosis and distillation) that in general thermodynamic efficiencies are low (~5%)2. Furthermore, most common desalination technologies require additional components (heat or pressure exchangers) to recover energy. One emerging technology which aims to improve thermodynamic efficiencies associated with brackish water and simultaneously has the capability to recovery energy is termed capacitive deionization (CDI)3. CDI removes minority ions from a mixture through electroadsorbing the charged constituents when a potential is applied across an electrochemical cell. The ions stored in the electric double layer (EDL) can then be discharged into a brine solution to regenerate active surface area. During this discharge process energy can be recovered. To date, the majority of CDI energy efficiency studies have focused on evaluating systems levels losses associated with the materials, or have evaluated individual processes (charge or discharge) in order to improve performance. While optimizing individual processes provides significant insight into mechanistic losses, evaluating a complete cycle is critical in order to evaluate 2nd law limitations. Here we will evaluate the challenges associated with discharging into a brine solution, and the potential energetic and exegetic savings which can occur through non-adiabatic system operation. Maupin, Molly A., et al. Estimated use of water in the United States in 2010. No. 1405. US Geological Survey, 2014. Demirel, Yaşar. "Thermodynamic analysis of separation systems." Separation science and technology 39, no. 16 (2004): 3897-3942. Hatzell, Kelsey B., Marta C. Hatzell, Kevin M. Cook, Muhammad Boota, Gabrielle M. Housel, Alexander McBride, E. Caglan Kumbur, and Yury Gogotsi. "Effect of oxidation of carbon material on suspension electrodes for flow electrode capacitive deionization." Environmental science & technology 49, no. 5 (2015): 3040-3047.

Among various desalination technologies, capacitive deionization (CDI) has rapidly developed because of its low energy consumption and environmental compatibility, among other factors. Traditional CDI stores ions within the electric double layers (EDLs) in the nanopores of the carbon electrode, but carbon anode oxidation, the co-ion expulsion effect, and a low salt adsorption capacity (SAC) block its further application. Herein, the Faradaic-based electrode is proposed to overcome the above limitations, offering an ultrahigh adsorption capacity and a rapid removal rate. In this paper, the open framework structure Na3V2(PO4)3@C is applied for the first time as a novel Faradaic electrode in the hybrid capacitive deionization (HCDI) system. During the adsorption and desorption process, sodium ions are intercalated/deintercalated through the crystal structure of Na3V2(PO4)3@C while chloride ions are physically trapped or released by the AC electrode. Different concentrations of feedwater are investigated, and a high SAC of 137.20 mg NaCl g-1 NVP@C and low energy consumption of 2.157 kg-NaCl kWh-1 are observed at a constant voltage of 1.0 V, a concentration of 100 mM, and a flow rate of 15 mL min-1. The outstanding performance of the Na3V2(PO4)3@C Faradaic electrode demonstrates that it is a promising material for desalination and that HCDI offers great future potential.

Activated carbon derived from the date palm leaflets as multifunctional electrodes in capacitive deionization system

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. During galvanostatic operation, a current is applied between porous carbon electrodes until a limiting voltage is reach. The cell is then discharged by applying a reverse current, generating brine and recovering stored charge. However, CDI desalination and energy efficiency can be limited by parasitic side reactions, and selective adsorption of counter-ions (anions at the positive electrode, and cations at the negative electrode). Several material additions and electrode configurations have been proposed to overcome these limitations, with the most prominent being the addition of ion exchange membranes (IEMs) promote counter ion flux out of the desalination chamber and incorporation of carbon slurry electrodes to increase system adsorption capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of functionalized carbon, IEMs in membrane capacitive deionization (MCDI), and the use of slurry electrodes in flow capacitive deionization (FCDI) have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space. In this study, an equivalent circuit (M)CDI model, with porous electrode sub-models, was used to measure the sensitivity of CDI performance to material selection, design, and operating choices. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Constrained system parameters were randomly selected via latin hypercube sampling (LHS) across multiple electrode geometries and influent salt concentrations. The resultant model outputs were then correlated with input values to quantify parameter sensitivity. Our sensitivity analysis shows that operating current density, electrode specific capacitance, and contact resistance were the parameters which most significantly dictated (M)CDI performance. These parameters where then used to construct an operational space for CDI, MCDI, and FCDI. The results of our operational space were then used to develop a simplified, operational model for both capacitive and faradic materials in CDI. Using this model we conducted a techno-economic analysis (TEA) of proposed improvement to CDI. From the TEA we are able more directly set operating parameters under which CDI might favorably compete with the primary technology for desalination, reverse osmosis (RO). We are able to evaluate materials lifetimes and costs necessary to economically operate CDI. Lastly, goals for operating parameters highlighted in the sensitivity analysis (specific capacitance, voltage limits, charge efficiency, and cell resistance) were set. These benchmarks will provide target for the continued development of CDI towards and economic and viable alternative to RO for brackish water desalination.