Enhanced Rocking Chair Capacitive Deionization for Accelerated Water Desalination

Coupling ion-exchangers with inexpensive activated carbon fiber electrodes to enhance the performance of capacitive deionization cells for domestic wastewater desalination

Microbial desalination cell (MDC) was considered inefficient to desalinate salt water with low salt concentration, therefore, the feasibility of using capacitive deionization (CDI) and membrane capacitive deionization (MCDI) as a post-processing technologies for MDC was investigated in this study, as well as the possibility of using MDC as the power supply for CDI and MCDI. The internal resistances of MDC with different salt concentration, the desalination rate and fresh water yield during a typical desalination cycle under initial salt concentration of 35 g/L were investigated in order to find out the deadline salt concentration for the MDC to desalinate effectively. The internal resistance increased from 21.7 to 602 Ω as the concentration of salt water decreased from 35 g/L to 0.1g/L. The salt water volume increased from 42 to 48 ml when the salt concentration decreased from 35 to 15 g/L, then decreased to 38 ml at the end of one desalination cycle when the salt concentration achieved 0.05 g/L due to the salt gradient (osmotic pressure). The maximum desalination rate during one typical desalination cycle in our experiment reached 5.65 mg/h when salt concentration decreased from 27.26 to 26.32 g/L, while the minimum desalination rate was 0.534 mg/h when salt concentration decreased from 0.38 to 0.05 g/L. It was concluded that MDC was not suitable to desalinate salt water with salt concentration less than 1 g/L. When CDI and MCDI were used as the post-processing technologies for MDC, a better performance in term of electrosorption capacity was obtained from MCDI with an influent salt concentration of 1 g/L. The experimental result also showed that the electrosorption capacity of MCDI with MDC as power supply was more than that with potentiostat as power supply at 0.8V, this suggests that MDC could be an alternative power supply for MCDI.

Temporal and spatial distribution of pH in flow-mode capacitive deionization and membrane capacitive deionization

Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: A review

Brackish groundwater is an invaluable yet overlooked water resource which can be leveraged to mitigate the unprecedented water scarcity experienced around the globe. Capacitive Deionization (CDI) represents a highly efficient and economical alternative to the common, energy intensive desalination methods for low salinity water streams. In CDI a small voltage is applied to separate ionic species from aqueous solutions and adsorb them into a highly porous material. Extraction of these ions lowers the solution concentration at the exit. The electrodes can then be discharged and regenerated by desorbing the adsorbed ions. CDI is a complex system comprised of several highly coupled transport mechanisms taking place at different length and time scales. In this work, a comprehensive theoretical and experimental study is conducted to further understand the underlying physics of CDI and to improve its the performance. First, I conduct a parametric study to investigate the interrelationship between the multiscale transport phenomena in CDI. The findings demonstrate that despite the superiority of the systems with high mass Péclet number in the desalination performance, units at low mass Péclet number show higher energetic efficiency. Based on these findings, next, I introduce an operational method aimed at enhancing the overall output of CDI. The numerical and experimental data prove the improved performance of the proposed configuration. I then present a new multi-cycle arrangement and investigate the effects of the regeneration feed stream in this cyclic configuration. The results indicate trade-offs between the desalination performance and energy efficiency of the proposed arrangements. Additionally, a dynamic two-dimensional model is developed incorporating the adsorption/desorption resistance in the porous electrodes. The obtained results accentuate the significant effect of this resistance limiting the transfer rate of the ionic species, especially in systems subject to ion starvation. Moreover, to achieve systems with better desalination and energetic performance, activated carbon electrodes with engineered microtopology are manufactured by employing magnetic field manipulation and are tested for desalination using a CDI cell. This work provides deeper insights into the transport mechanisms of CDI and allows for better guidelines on design and implementation of efficient systems with improved overall desalination performance.

Capacitive deionization (CDI) is a developing technology for water desalination applications with benefits over current methods such as reverse osmosis (RO) and distillation due to its operation at lower temperatures and pressure. CDI uses small applied potentials (<2.0 V) to selectively separate ions from feed water streams by reversibly adsorbing these ions in the electric double layer of conductive, high surface area, and porous carbon electrodes. This reversible absorption process can be used in a variety of configurations including flow-by, flow-through, and flowable electrode arrangements.1 Classic CDI systems have charge efficiency (equivalent charge of salt adsorbed per electronic charge input) values well below 100% due to parasitic discharge processes and Faradaic reactions.2,3 Therefore, ion-exchange membranes have been added into these systems to increase the effective adsorption capacity of the electrodes and limit the extent of parasitic processes. Overall, CDI systems are considered to be more efficient than RO membranes for many lower concentration salt streams (<5000 ppm) due to differences in its separation mechanism.4 Shown in Figure 1 is an energy cost comparison of CDI and RO technologies as a function of salt concentration. CDI is shown with energy recovery values of 0, 25, and 50%. As the concentration of NaCl decreases, there is a distinct region where the energy cost of this separation can best RO. The more recent addition of ion-exchange membranes in a membrane capacitive deionization (MCDI) process can further increase the charge efficiency to over 100% in certain instances and aid in further decreasing energy costs.5 The efficiency of both the CDI and MCDI process make them attractive options for many larger scale applications where energy cost becomes a dominant factor in choosing the water treatment system. In this talk, a comparison between the energy costs of RO and CDI/MCDI will be given. The use of recent surface chemistries to increase the charge efficiency will be shown to decrease the energy cost of CDI/MCDI processes further. Finally, applications will be reviewed where CDI and MCDI can be used as a standalone option as well as interface with existing membrane processes. Acknowledgements The authors are grateful to the U.S. - China Clean Energy Research Center, U.S. Department of Energy for project funding (No. DE-PI0000017).

Comparison of faradaic reactions in flow-through and flow-by capacitive deionization (CDI) systems

Various cell architectures of capacitive deionization: Recent advances and future trends

Theory of pH changes in water desalination by capacitive deionization

The rapid and efficient exchange of ions between porous electrodes and aqueous solutions is important in many applications, such as electrical energy storage by supercapacitors, water desalination and purification by capacitive deionization, and capacitive extraction of renewable energy from a salinity difference. Here, we present a unified mean-field theory for capacitive charging and desalination by ideally polarizable porous electrodes (without Faradaic reactions or specific adsorption of ions) valid in the limit of thin double layers (compared to typical pore dimensions). We illustrate the theory for the case of a dilute, symmetric, binary electrolyte using the Gouy-Chapman-Stern (GCS) model of the double layer, for which simple formulae are available for salt adsorption and capacitive charging of the diffuse part of the double layer. We solve the full GCS mean-field theory numerically for realistic parameters in capacitive deionization, and we derive reduced models for two limiting regimes with different time scales: (i) in the "supercapacitor regime" of small voltages and/or early times, the porous electrode acts like a transmission line, governed by a linear diffusion equation for the electrostatic potential, scaled to the RC time of a single pore, and (ii) in the "desalination regime" of large voltages and long times, the porous electrode slowly absorbs counterions, governed by coupled, nonlinear diffusion equations for the pore-averaged potential and salt concentration.

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.

Capacitive deionization (CDI) has shown great promise as a water treatment and conditioning technique. However, a large majority of studies using this technology have focused solely on NaCl removal, while other ions commonly found in natural waters might pose unaddressed challenges for CDI, such as specific adsorption, diffusional limitations and scaling precipitation. For instance, precipitation of inorganic salts such as calcium carbonate is an issue associated with brackish waters and RO plants. Similarly, high concentrations of sulfate and silicate salts in groundwater may lead to membrane scaling due to their low solubility [1]. Therefore, there is still a need to identify the effect of both individual ions and mixtures on CDI performance. In this work, calcium and sulfate sorption was studied using a CDI reactor outfitted with low-cost, high surface area carbons coated with two different metal oxides (SiO2 on the cathode and Al2O3 on the anode). The metal oxide coatings were chosen because of previous efforts in the literature showing improved electrode performance in the CDI process with the addition of these coatings [2-4]. Two calcium and two sodium salts (CaSO4, CaCl2, NaCl and Na2SO4) were tested to examine and compare the sorption characteristics of different anions and cations. Previous testing of this CDI system in a “Single Pass” (passing the water through the device only once) operational mode using a CaSO4 solution showed reductions in both Ca2+ and SO4 2- concentrations [5]. However, issues related with ion desorption and faradaic reactions (due to high cell potentials) were also observed. In this work, cycles of consecutive adsorption/desorption tests were performed to evaluate the long-term performance of this CDI system. Ion sorption and electrode stability were analyzed in terms of variables such as influent concentration, flow rate, and regeneration potential. These analyses also included performance metrics related with charge efficiency and energy consumption. For comparison purposes, CDI cells with uncoated carbon electrodes were evaluated following similar protocols. The results showed that increasing the concentration of CaSO4 in the feed solution (from 1.5 to 4.5 mM) increased the ion removal to values up to 3.29 ± 0.25 mg·g-1 and reduced energy consumption from 0.16 down to 0.09 kWh·mol-1. It was also observed that electric charge efficiencies increased by 15-30 % when asymmetrically coated configurations were employed. Nevertheless, the presence of SO4 2- significantly hindered salt desorption; conversely, recovery values close to 95 % were achieved using NaCl electrolyte. Finally, the evaluation of the long-term CDI performance revealed a slight but progressive loss in the ion electrosorption capability, probably due to the oxidation of the positive electrodes.

Novel ecofriendly cation exchange membranes for low-cost electrodialysis of brackish water: Desalination and antiscaling performance

Capacitive deionization (CDI) is an emerging water desalination technology that offers unique advantages for energy efficient water desalination due to its design flexibility, long cycle life, low energy demand, reversible operation at low pressure and room temperature, simultaneous ion adsorption, and high coulombic efficiency. Despite their potential, a series of technical challenges hinder the widespread utilization of CDI systems. Among these, limited desalination performance has been identified as the key issue. This issue is primarily governed by the underutilization of electrodes, slow adsorption kinetics, and diminished energy efficiency of the system. Presently, the desalination performances of the CDI systems are limited by the lack of fundamental understanding of effects of operating conditions and electrode properties on the electrosorption kinetics. This PhD study seeks to address this critical issue by exploring fundamental mechanisms responsible for ion adsorption in electrodes. The overall objective of this dissertation is to establish a fundamental understanding of the effects of operating conditions and electrode properties on the desalination performances of the CDI systems by investigating the dependence of effective ion delivery on the composition and nanoscale morphology of the electrodes and flow directionality of the saline water. To achieve these goals, series of experiments were designed. Several materials and electrochemical characterization techniques were used to characterize electrode materials and study the effects of various operating conditions on the desalination performances of CDI systems. Using these experimental data, a fundamental understanding of the effects of electrode properties and operating conditions on the desalination kinetics and performance of the CDI systems has been obtained. Finally, this understanding was utilized to identify strategies for more effective delivery of ions, for improved utilization of the electrodes. Keywords: Activated Carbon Cloth, Capacitive Deionization, Electrosorption Kinetics, MXene, Salt Adsorption, Water Desalination

Eco-friendly and energy efficient flowable surface modified carbon electrode materials in flow electrode capacitive deionization for heavy metal adsorption and energy recovery.