Activated Carbon Cloth for Desalination of Brackish Water Using Capacitive Deionization

Brackish water desalination using reverse osmosis and capacitive deionization at the water-energy nexus

Can capacitive deionization outperform reverse osmosis for brackish water desalination?

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.

Desalination and disinfection of inland brackish ground water in a capacitive deionization cell using nanoporous activated carbon cloth electrodes

Desalination technologies are expected to play an important role in producing clean water in the future, resulting a surge in the research and development of energy efficient and cost effective technologies for desalination of seawater and brackish water. Membrane-based desalination technologies such as reverse osmosis (RO) are the most commercially used desalination methods for seawater and treatment of agricultural water, but are highly energy intensive and the future development of desalination is dependent on finding more energy efficient technologies. Capacitive deionization (CDI) is an emerging technology for water desalination, and is based on the phenomenon of ion electrosorption. Simple CDI is an energy efficient technology and its applications for desalination of low molar concentration streams, like brackish water, is demonstrated. However, to expand the applications, research is focused on improving the efficiency and salt removal capacity of CDI systems. To this end, the important requirements are finding electrode materials with higher capacities and CDI systems with higher efficiencies. Also, the large-scale application of CDI for personal and industrial applications is dependent on designing CDI systems with potential to be implemented at different capacities and length scales. In this work, we have studied the CDI for removal of salt and nutrients from agricultural water. Various high surface area electrode materials were considered and the efficiency of CDI for removal is measured via downstream analyses on batch experiments and reported, allowing development of preliminary parameters for optimizing CDI systems for high-strength agricultural wastewaters.

Capacitive deionization (CDI) process is a novel approach for desalination of an aqueous salt solution. In the present study, an activated carbon cloth (ACC) is proposed as effective electrode material. Initially the carbon cloth was activated in 1 M and 8 M HNO3 for 9 hours at room temperature. The untreated and chemically activated carbon cloth (ACC) electrode materials were subjected to BET surface area measurements in order to get information about their specific surface area, average pore size, total pore volume and micropore area. The above materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) also. The electrochemical studies for the electrodes were done using cyclic voltammetry (CV) in 0.1 M Na2SO4 medium. From the studies, it was found that resistivity of the activated carbon cloth electrodes (treated in 1 M and 8 M HNO3) was decreased significantly by the chemical oxidation in nitric acid at room temperature and its capacitance was found to be 90 F/g (1 M HNO3) and 154 F/g (8 M HNO3) respectively in 0.1 M Na2SO4 solution. The capacitive deionization behavior of a single cell CDI with activated carbon cloth electrodes was also studied and reported in this work. Keywords: Activated carbon cloth, capacitive deionization (CDI), desalination, chemical activation, cyclic voltammetry, chronocoulometry

Brackish water desalination by capacitive deionization using zinc oxide micro/nanostructures grafted on activated carbon cloth electrodes

Capacitive Deionization (CDI) has applications ranging from water purification to biofuels processing. For CDI to be a viable scalable technology, electrode stability is critical. In extended experiments the activated carbon cloth (ACC), used for the electrode, has been previously observed to degrade as a result of surface oxidation reactions. Carbon cloth oxidation can reduce the surface area and alter the pore structure, in turn reducing the adsorption capacity. While the behavior of oxidation is known, a reliable method of quantifying the extent of degradation is lacking. To quantify it, cyclic voltammetry was explored to measure the capacitance of pristine and oxidized carbon cloth samples. A three-electrode electrochemical cell was used to quantify capacitance of activated carbon cloth in 1M NaCl solution. The specific capacitance of carbon cloth that underwent anodic oxidation was lower than cathodic and pristine carbon cloth. Further tests analyzed the change in the severity of the anodic degradation on whether the initial CDI separations were performed on organic or inorganic ions. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) was utilized to verify and quantify surface oxidation. Cyclic voltammetry is demonstrated as a simple and reliable method for analyzing and quantifying anodic oxidation on activated carbon cloth electrodes in CDI stacks.

Capacitive deionization (CDI) is a low energy desalination technology with long-cycle life, which utilizes high-porosity capacitive electrodes for capturing ions from flowing saline water [1, 2]. Though still suffering from relatively low desalination capacity, one major advantage of CDI technology is its low energy requirement and high rate operation for desalination. In recent years, much effort has been put on improving electrode materials for CDI applications. These studies have mostly focused either on the use of novel electrode materials or on surface modifications (e.g. metal oxide growth, chemical treatment, and thermal treatment) of the existing CDI electrode materials [1, 3, 4]. Although thermal treatment of carbon electrodes for improved charge storage is a well-established approach, the effects of various thermal treatment procedures on the performance of CDI electrodes still remain unexplored. Inherent similarities between the operating principles of supercapacitors and CDI technology might make one to think a similar correlation could be established between thermal treatment and the CDI performance. However, due to major differences in required charge storage mechanisms, a detailed study on various treatment conditions should be conducted to understand which conditions specifically promote better ion adsorption in CDI electrodes. Motivated by this, the effects of different thermal treatment conditions (i.e., temperature and gases) on salt adsorption performances of the activated carbon cloth (ACC) electrodes were investigated. Major discrepancy between stored charge versus salt adsorption capacity (SAC) was observed for different treatment conditions. To better assess these effects, additional BET and Raman tests on the ACC electrodes were also conducted. Results indicated interesting observations regarding charge storage capacity and SAC for different treatment conditions, which highlights the importance of selecting a suitable thermal treatment condition for enhancing the CDI performance of ACC electrodes.

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

Capacitive deionization (CDI), a class of electrochemical separation technologies, has been proposed as an energy-efficient brackish water desalination method. Previous studies have focused on improving capacity and energy consumption through material (e.g., ion-selective membranes [IEMs], charged carbon) and operational modifications, but there has been no analysis that directly links lab-scale experimental performance to capital and operating costs of full-scale water production. In this study, we developed a parameterized process model and technoeconomic analysis framework to project capital and operating costs at the million gallon per day scale based on reported material and operational characteristics for constant current CDI with and without low ($20 m-2)- and high-cost ($100 m-2) IEMs. Using this framework, we conducted global sensitivity and uncertainty analyses for water price across the reported CDI design space. Our results show that the operating constraints of brackish water desalination lead to capital costs 2-14 times greater than operating costs (particularly for MCDI). While MCDI outperforms CDI, IEM prices dictate the threshold at which MCDI is more cost-effective. The high relative capital costs highlight the importance of achieving system lifetimes at 2 years or beyond. Last, we set performance and areal cost benchmarks for material-based CDI performance and lifetime improvements.

Due to the increasing demand for clean and potable water stemming from population growth and exacerbated by the scarcity of fresh water resources, more attention has been drawn to different and innovative methods for water desalination. Capacitive deionization (CDI) is a relatively new, low maintenance, and energy efficient technique for desalinating brackish water. In this technique, an electrical field is employed to adsorb ions into a high-porous media. After the saturation of the porous electrodes, their adsorption capacity can be restored through a regeneration process. Various parameters affect the overall performance of CDI. The flow rate at which water is purified in CDI plays an essential role in its ultimate performance. Many studies have shown that desalination percentage decreases as flow rate increases in CDI, since the advection of ions in the flow becomes more dominant than their diffusion toward the electrodes. However, herein, based on a physical model previously developed, we conjecture that for a given amount of time and volume of water, multiple desalination cycles in a high flow rate regime will outperform desalinating in a single cycle at a low flow rate. Moreover, splitting a CDI unit into two sub-units, with the same total length, will lead to higher desalination. Based on these premises, we introduce a new approach aimed at enhancing the overall performance of CDI. An array of CDI cells are sequentially connected to each other with intermediate solutions placed in between them. These intermediate solutions act as buffers to homogenize the outlet concentration of the preceding cell and maintain a constant inlet concentration for the following cell. Desalination tests were conducted to compare the performance of the proposed system, consisting of two CDI units and one intermediate solution buffer, with a two-cascaded-CDI unit system with no intermediate solution. Desalination tests were performed in a high flow rate regime with a low salinity initial solution of NaCl in water. In the buffered arrangement, the concentration of the solution buffer was set at the minimum average outlet concentration of the first CDI test. Experimental data demonstrated the improved performance of the buffered system over the non-buffered system, in terms of desalination percentage and energy consumption. Increasing the number of CDI units and solution buffers in a buffered system, the new proposed method will lead to lower amount of energy consumed per unit volume of the desalinated water.

Research progress of sodium super ionic conductor electrode materials for capacitive deionization

Wastewater treatment and freshwater production are integral to sustaining an increasing global population. Presently, techniques like reverse osmosis (RO), and multi- stage distillation (MSD), which are extensively used for water cleanup are cost and energy intensive due to an operational focus on removing a majority constituent water from salts. Capacitive de-ionization (CDI) is an emerging technique for water treatment whereby dissolved ionic content in the bulk solution is stored in highly porous electrodes upon the application of an electric field, i.e., removing salts from water. When the potential is released, a concentrated waste stream is produced (1, 2). A lot of research has been devoted to optimizing the electrodes, usually carbon to possess high surface area, large porosity, and high conductivity for use in CDI. Other key research areas include membrane assisted capacitive deionization and chemically functionalized electrodes. While CDI has been touted as an efficient process for brackish water desalination, there remains intensive energy consumption to keep the traditional CDI process from becoming a transformational technology, and reverse osmosis continues to be more economical when desalinating higher concentration streams such as sea water. Many authors have discussed the possibility of parallel CDI operation with energy recovery to reduce energy requirements. However, beyond discharging a charged CDI cell into a resistive load, there has been little demonstration of CDI to CDI energy recovery. The interfacing of CDI cells offers not only the ability to lower the energy requirements, but also the possibility of semi-continuous CDI operation. In this work, parallel CDI cells are integrated with a multi-input bi-directional DC/DC converter to show active energy recovery from a discharging cell and subsequent deionization of a secondary cell using energy stored in the initial charged cell. Deionization experiments are conducted by closed-loop recirculation of a saline stream through the CDI cell to achieve a batch-mode configuration. The test system is equipped with inline conductivity probes for continuous monitoring of concentration. Along with the converter, the deionization system is connected to a power supply to compensate for losses and maintain operation. Initial testing was done by charging a primary cell at 1.2 V (Figure 1), and then directly connecting to a secondary cell. There were 12 and 7 µS/cm drops in concentration for the primary and secondary cells respectively. Future studies will track inter-cell energy transfer and show recovery of more than 50 % of the energy stored in the initial cell using this integrated CDI-converter setup.AcknowledgementsThe authors are thankful for the support of the State of Wyoming Advanced Conversion Technologies Task Force for supporting this research.

Capacitive deionization (CDI) can desalinate brackish or lower salt content water at energy costs competitive with reverse osmosis [1]. The brief lifetime of CDI anodes has recently been overcome by operating in an inverted CDI mode (i-CDI), whereby chemical surface charges adsorb ions and an applied potential is subsequently used to repel them to form a concentrated stream [2]. The work here presents new insights on these next generation CDI cells with a focus on in situ anode pre-treatments, energy use, and system longevity for practical considerations. Electrochemically conditioning pristine anodes for 12 hours at +2.5 V was performed in a water stream, i.e. in situ, and immediately deliver peak salt capacities of approximately 6 mg g-1 for SpectraCarb activated carbon cloth electrodes in i-CDI mode (see conditions in Fig. 1). This is in contrast to common ex situ anode pretreatment methods using chemical oxidation in HNO3, which can require lengthy immersion durations, extensive neutralization, and up to 50 cycles (100 h) to reach a similar salt capacity (Fig. 1). This new electrochemical conditioning method can thus save days of processing time to attain steady desalination performance compared to the chemical pretreatment of electrodes and limit the volume of waste stream produced from the treatment process. Moreover, the charge efficiency of electrochemically prepared i-CDI cells is superior to chemical treatment until convergence after 80 cycles. Energy consumption of the cells reaches a minimum of 0.21 kWh m-3, which is approximately 80% lower than reverse osmosis at the feed concentration used here, and approaches the expected energy consumption by CDI desalination [1]. The long-term experiments shown in Figure 1 are run from feed tanks that are not purged of dissolved oxygen in order to more accurately simulate real-world operation. This gives rise to an observed accelerated performance loss with time, independent of the electrode pre-treatment method used. Surface area and sheet resistance measurements suggest that anode degradation (or further oxidation) may be responsible for the decline in performance: anode resistance (HNO3-treated) increased from 60 Ω □-1 before cycling to 600 Ω □-1 after cycling while specific surface area declined from 1390 to 1050 m2 g-1. Frequently, during capacitive deionization experiments, anode degradation is obscured or delayed by the use of membranes, by desalinating for only several cycles, and/or by purging the influent water stream of dissolved O2 with an inert gas such as nitrogen. While these controlled conditions have unveiled critical insights and allowed CDI to flourish in the recent literature, steady performance decline due to anode chemistry may continue to stall commercial adoption, and strategies to mitigate anode oxidation will be reviewed. Figure 1. Long-term salt adsorption capacity and charge efficiency for i-CDI cells operated at +0.8/0 V after electrochemical and nitric acid anode pre-treatments; 4.5 mM NaCl solution was circulated at 20 ml min-1 in single-pass mode through 3.5 g of SpectraCarb electrodes. [1] Oren Y. Capacitive delonization (CDI) for desalination and water treatment - past, present and future (a review). Desalination 2008;228:10-29. [2] Gao X, Omosebi A, Landon J, Liu KL. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior. Energ Environ Sci 2015;8:897-909. Acknowledgements These authors are grateful to the U.S.-China Clean Energy Research Center, U.S. Department of Energy (DE-PI0000017) for project funding. Figure 1