Efficiency Of Capacitive Deionization Research Articles

Porous carbonized N-doped MOF-199 modified with MWCNTs for the deionization of uranium(VI)

When applying Capacitive Deionization (CDI) for water desalination, discovering effective techniques to attract more salt ions poses significant challenges. Herein, a 3D porous carbonized MOF-199@PVP/MWCNTs (CMPC) composite for enhanced electrosorption of U(VI) in water had been developed. Further investigation revealed that the incorporation of polyvinylpyrrolidone (PVP) enabled the CMPC composite to maintain its distinctive octahedral porous structure and rich N-doping to reduce particle size, providing more active sites. Additionally, the interpenetrating of multi-walled carbon nanotubes (MWCNTs) had formed a localized conductive network, further enhancing its properties. As a result, the CMPC electrode had been demonstrated a remarkable removal ratio of 95.2 % with the uptake electroadsorption capacity of 410.3 mg g−1 for UO22+ aqueous solution (1.2 V). The exceptional CDI efficiency achieved by the CMPC electrode is attributed to the collaborative influence of electrochemical double layers (EDLs) and pseudocapacitance derived from the unique assembly of MOF-199, PVP, and MWCNTs. Besides, the results of the experiments on the effect of ionic strength showed that the electrosorption ratio still remained at 80.1 % when 60 mg/L NaNO3 was added to the UO22+ solution. And the removal performance of U(VI) remained over 85.7 % even after five CDI cycles. Overall, CMPC is a highly prospective electrode material in CDI for efficient extraction of uranium from water.

Modification of Metal-Organic Framework-Derived Nanocarbons for Enhanced Capacitive Deionization Performance: A Mini-Review.

Capacitive deionization (CDI) is a promising electrochemical water treatment technology. Development of new electrode materials with higher performance is key to improve the desalination efficiency of CDI. Carbon nanomaterials derived from metal–organic frameworks (MOFs) have attracted wide attention for their porous nanostructures and large specific surface areas. The desalination capacity and cycling stability of MOF-derived carbons (MOFCs) have been greatly improved by means of morphology control, heteroatom doping, Faradaic material modification, etc. Despite progress has been made to improve their CDI performance, quite a lot of MOFCs are too costly to be applied in a large scale. It remains crucial to develop MOFCs with both high desalination efficiency and low cost. In this review, we summarized three modification methods of MOFCs, namely morphology control, heteroatom doping, and Faradaic material doping, and put forward some constructive advice on how to enhance the desalination performance of MOFCs effectively at a low cost. We hope that more efforts could be devoted to the industrialization of MOFCs for CDI.

MXene as a Cation-Selective Cathode Material for Asymmetric Capacitive Deionization.

Capacitive deionization (CDI) has become a promising method to solve the shortage of freshwater resources recently. However, the co-ion expulsion effect obviously hinders electrosorption capacity and charge efficiency of CDI. In this work, an asymmetric CDI cell is assembled in which Na+-intercalated Ti3C2Tx (NaOH-Ti3C2Tx) serves as a cation-selective cathode, while the activated carbon (AC) serves as the anode. The NaOH-Ti3C2Tx with negatively charged surface groups (-OH, -O, and -F) is adopted to weaken the co-ion expulsion effect. Benefited from the synergistic effect of the reduced co-ion expulsion effect and expanded interlayer space, the asymmetric CDI cell achieves a higher electrosorption capacity of 12.19 mg g-1 and a higher charge efficiency of 0.826 compared with the symmetric one composed of AC (4.55 mg g-1 and 0.306) in 100 mg L-1 NaCl solution. High cyclic stability of the as-prepared asymmetric CDI cell is also observed. The improved desalination performance indicates that NaOH-Ti3C2Tx is a promising alternative as cation-selective cathode material for asymmetric CDI cells. The desalination mechanism is discussed in detail to lay the foundation for further improvement of the CDI performance of other 2D materials like MXene.

Understanding capacitive deionization performance by comparing its electrical response with an electrochemical supercapacitor: Strategies to boost round-trip efficiency

While Electrical Double-Layer Capacitors (EDLC’s) have been proven to achieve round-trip efficiencies of over 95%, electrical efficiencies of Capacitive Deionization (CDI) systems are still far from those values. Since this issue seems to be crucial for CDI to commercially compete with other water treatment processes including Reverse Osmosis (RO) it is essential that these efficiencies be improved. In this work, the electrical response of a commercial EDLC’s and a CDI system are compared. Results of asymmetric constant current experiments reveal that the main electrical inefficiency occurs during the discharge stage (i.e. in CDI this refers as salt desorption or electrode’s regeneration). Here, we propose an operational mode of CDI focused on improving performance of regeneration by simply conducting this stage in a more concentrated solution (3.5M NaC) brine water. The experimental findings suggest that this operational strategy led to 1) an increase from ≈40% to 75% in the round-trip efficiency of CDI, approaching (although admittedly not reaching) those of EDLC’s; 2) a drop in ohmic resistance in the discharge cycle of 92% (from 19.77 Ω to 1.45 Ω) consequently increasing the amount of energy that could be potentially recovered from 0.24 J to 0.50 J, and thus, reducing significantly the energy consumption of the process; 3) a wider range of deionization/regeneration current density ratios (0.1–1.25) in which high efficiency values (65–75%) are still maintained. In general, we find that this operational mode might have practical relevance as it implies that CDI systems may be operated with a higher degree of flexibility allowing them to be coupled more readily with renewable energy technologies. Furthermore, the use of a highly concentrated electrolyte (i.e. not deionized water) during regeneration offers an innovative solution for brine concentration.

The effect of crystal phase of manganese oxide on the capacitive deionization of simple electrolytes

MnO2 is a common material for the fabrication and design of capacitive deionization (CDI) devices but there is little information on the role of MnO2 crystal phase on CDI performance. A series of MnO2 (α, β, γ, and δ phase) were synthesized and fabricated as cathodes for studying the CDI performance as affected by pH in simple batch mode experiments. Our results revealed that the deionization efficiency decreased with increased negative surface charge as a result of the deprotonated surface. Importantly, this correlation was pH independent and the surface heterogeneity due to different MnO2 phase was likely responsible for the different degree of surface ionization and consequently the CDI efficiency. Results of electrochemical impedance spectroscopy analyses further implicated that a highly ionized surface would result in a diffusion layer with a great resistance that conversely inhibited the access of co-ions in the CDI process. This indicated the applied potential was mainly responsible for driving ions transporting through the double layer resistance instead of accommodating them (electrosorption). Based on our results, the surface heterogeneity as a result of different spatially distributed MnO6 octahedral would be accounted for the varying degree of surface ionization and consequently the discrepancy in CDI efficiency among different MnO2 phases.

Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis

Capacitive deionization (CDI), which is based on the electrosorption of ions by porous electrodes, is an emerging technology for brackish water desalination. Understanding the key drivers of energy consumption in CDI and benchmarking CDI with reverse osmosis (RO), the current state-of-the-art for brackish and seawater desalination, is crucial to guide the future development of desalination technologies. In this study, we develop system-scale models to analyze the energy consumption and energy efficiency of CDI and RO over a wide range of material properties and operating conditions. Using our models, we explore how the energetic performance of CDI and RO compare as a function of feed salinity, water recovery, salt rejection, and average water flux, which is normalized by electrode and membrane area in CDI and RO, respectively. Our analysis shows that RO is significantly more energy efficient than CDI, particularly when targeting higher salinity feed streams and higher salt rejection values. For brackish water with a salt concentration of 2000 mg L−1, achieving 50% water recovery and 75% salt rejection, with an average water flux of 10 L m−2 h−1 using CDI requires a specific energy consumption of 0.85 kWh m−3, more than eight times that of RO (0.09 kWh m−3). Importantly, our results also indicate that current efforts to improve electrode materials can only marginally reduce the energy consumption of CDI. We conclude with a discussion highlighting other important factors, such as capital cost, electrode stability, and membrane fouling, which affect the efficacy of CDI and RO for low-salinity desalination.

Effect of Surface Ionization of Doped MnO2 on Capacitive Deionization Efficiency.

Associating MnO2 with carbonaceous supports profoundly enhances capacitive deionization (CDI) efficiency. A fundamental question of how the surface chemistry of MnO2 itself influences CDI efficiency is not yet fully understood. In this study, the effect of surface ionization on the CDI efficiencies of Fe-, Co-, and Ni-doped α-MnO2 (<0.1 mol %) as a model cathode material was studied. A pattern that CDI efficiency decreased with increasing negative surface charge density resulting from surface deprotonation was noted. This is likely attributed to the appreciable co-ion expulsion occurring at a highly ionized surface in the mesopores of MnO2. It is thus concluded that the combination of surface charge modification and a microporous environment would be important for CDI efficiency enhancement by minimizing co-ion exclusion effect. In the former case, structural stress adjustment by doping elements would be a practical route to regulate the p Ka1 and p Ka2 values and consequently the degree of surface ionization of MnO2.

Thermodynamics of Ion Separation by Electrosorption.

We present a simple, top-down approach for the calculation of minimum energy consumption of electrosorptive ion separation using variational form of the (Gibbs) free energy. We focus and expand on the case of electrostatic capacitive deionization (CDI). The theoretical framework is independent of details of the double-layer charge distribution and is applicable to any thermodynamically consistent model, such as the Gouy-Chapman-Stern and modified Donnan models. We demonstrate that, under certain assumptions, the minimum required electric work energy is indeed equivalent to the free energy of separation. Using the theory, we define the thermodynamic efficiency of CDI. We show that the thermodynamic efficiency of current experimental CDI systems is currently very low, around 1% for most existing systems. We applied this knowledge and constructed and operated a CDI cell to show that judicious selection of the materials, geometry, and process parameters can lead to a 9% thermodynamic efficiency and 4.6 kT per removed ion energy cost. This relatively high thermodynamic efficiency is, to our knowledge, by far the highest thermodynamic efficiency ever demonstrated for traditional CDI. We hypothesize that efficiency can be further improved by further reduction of CDI cell series resistances and optimization of operational parameters.

Design of novel electrode for capacitive deionization using electrospun composite titania/zirconia nanofibers doped-activated carbon

Well-dispersed TiO2/ZrO2 nanofibers-doped activated carbon (AC/TiZr) were successfully synthesized as an electrode for the capacitive deionization (CDI) technique via electrospinning that was followed by a hydrothermal treatment. SEM, TEM, XRD, XPS and EDX measurements were employed to characterize the morphology, crystal structure and chemistry composition of the as-synthesized material. The electrochemical performance of the AC/TiZr nanocomposite modified electrode was measured using systematic cyclic voltammetry (CV) experiments. The highest specific capacitance for the AC/TiZr-based electrode is 251.3 Fg/g, which is higher than those of the AC (207.4 Fg/g) and TiO2/ZrO2 NFs (0.4 Fg/g)-based electrodes, and the interior resistance was also reduced. A desalination performance enhancement was obtained in the case of the AC/TiZr and the electrosorptive is 2.96 mg g−1. The implementation of the AC/TiZr nanocomposite provided a high potential for improving the efficiency of CDI.

Development and Optimization of Capacitive Deionization for Treatment of High-Strength Agricultural Waters

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.

Effect of surface modification of carbon felts on capacitive deionization for desalination

Surface modified carbon felts were utilized as an electrode for the removal of inorganic ions from seawater. The surfaces of the carbon felts were chemically modified by alkaline and acidic solutions, respectively. The potassium hydroxide (KOH) modified carbon felt exhibited high Brunauer-Emmett-Teller (BET) surface areas and large pore volume, and oxygen-containing functional groups were increased during KOH chemical modification. However, the BET surface area significantly decreased by nitric acid (<TEX>$HNO_3$</TEX>) chemical modification due to severe chemical dissolution of the pore structure. The capability of electrosorption by an electrical double-layer and the efficiency of capacitive deionization (CDI) thus showed the greatest enhancement by chemical KOH modification due to the appropriate increase of carboxyl and hydroxyl functional groups and the enlargement of the specific surface area.

정전기적 흡·탈착 공정에서의 탄소 전극

With the world population's continuous growth and urban industrialization, capacitive deionization (CDI) has been proposed as a next-generation water treatment technology to augment the supply of water. As a future water treatment method, CDI attracts significant attention because it offers small energy consumption and low environmental impact in comparison to con- ventional methods. Carbon electrodes, which have large surface area and high conductivity, are mainly used as electrode mate- rials of choice for the removal of ions in water. A variety of carbon materials have been investigated, including their adsorp- tion-desorption behavior in accordance to the specific surface area and pore size distribution. In this review, we analyzed and highlighted these carbon materials and looked at the impact of pore size distribution to the overall CDI efficiency. Finally, we propose an optimal condition in the interplay between micropores and mesopores in order to provide the best electro- sorption property for these carbon electrodes.

Effects of Pore Structure on the High-Performance Capacitive Deionization Using Chemically Activated Carbon Nanofibers

Capacitive deionization (CDI) electrodes were constructed from activated carbon fibers prepared using electrospinning and chemical activation. The CDI efficiencies of these electrodes were studied as a function of their specific surface areas, pore volumes and pore sizes via salt ion adsorption. The specific surface areas increased approximately 90 fold and the pore volume also increased approximately 26 fold with the use of greater amounts of the chemical activation agent. There was a relative increase in the mesopore fraction with higher porosity. A NaCI solution was passed through a prepared CDI system, and the salt removal efficiency of the CDI system was determined by the separation of the Na+ and Cl- ions toward the anode and cathode. The CDI efficiency increased with greater specific surface areas and pore volumes. In addition, the efficiency per unit pore volume increased with a reduction in the micropore fraction, resulting in the suppressed overlapping effect. In conclusion, the obtained improvements in CDI efficiency were mainly attributed to mesopores, but the micropores also played an important role in the high-performance CDI under conditions of high applied potential and high ion concentrations.

Capacitive deionization by ordered mesoporous carbon: electrosorption isotherm, kinetics, and the effect of modification

AbstractIn this study, an ordered mesoporous carbon (OMC) was synthesized and investigated as electrodes for capacitive deionization (CDI). The influence of working parameters on salt removal was studied and the electrosorption isotherm and kinetics were investigated. It was found that the electrosorption process fitted both Langmuir isotherm and Freundlich isotherm, and followed pseudo-first-order kinetics. The maximum electrosorptive capacity was 10.1 mg/g at the optimized applied voltage of 1.2 V and flow rate of 40 ml/min. In order to investigate the effect of surface groups on CDI efficiency, OMC was modified by H2O2 solution or thermal treatment under N2 atmosphere, without significantly changing its textural properties. X-ray photoelectron spectroscopy measurements showed that the amount of surface oxygen-containing groups was decreased and increased by the thermal and chemical treatments, respectively. CDI experiments revealed that OMC modified by H2O2 solution performed better than original OMC, ...

Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process

Desalination experiments were performed by constructing a capacitive deionization (CDI) unit cell with a carbon electrode prepared from activated carbon powder (ACP). Through CDI experiments, the mechanism of adsorption, desorption and electrode reactions were investigated by measuring conductivity, effluent pH, and the current passed through the cell under different electrode potentials. The salt-removal efficiency increased with increasing potential at the range of 0.8–1.5 V. Additionally, the pH of the solution varied significantly with a change in potential. At potentials less than 1.0 V, the pH increased due to the reduction of dissolved oxygen and the pH decreased at potentials over 1.2 V due to oxidation reactions at the anode. The change in current revealed that adsorbed ions were not completely desorbed and a fraction of ions were retained at the carbon electrode. These accumulated ions were re-adsorbed at the electrode surface when a potential was re-applied, which led to a decrease in the salt-removal efficiency of CDI.

탄소에어로젤 복합전극의 전기용량적 탈이온 공정 특성

전기화학적으로 이온을 흡착시켜 이온을 제거하는 capacitive deionization(CDI)공정용 전극으로 탄소에어로젤에 실리카젤이 첨가된 다공성 탄소에어로젤 복합전극을 사용하여 1,000ppm NaCl수용액에서 탈염 특성에 대한 충전과 방전시 시간에 따른 전류 변화, CDI효율을 조사하였다. Paste rolling법으로 제조된 <TEX>$10\times10cm^2$</TEX>다공성 탄소에어로젤 복합전극은 촉매 분야에서 활용되고 있는 다공성 지지체인 실리카젤을 첨가함으로써 CDI 반응진행에 대한 전극활물질 탈락이 없이 전극의 성형성이 크게 향상되었고, 친수성과 전극의 기계적 강도 증가 및 CDI 효율을 증가시킬 수 있었다. 이러한 45개의 전극을 하나의 묶음으로 네 개의 단을 직렬연결 하여, CDI 시스템을 구성하였고 충전 시에는 1.2V, 방전 시에는 0.001V를 각각 10분간 인가하여 실험한 결과 <TEX>$99\%$</TEX> 이상의 CDI 효율을 달성하였다. Porous-composite electrodes have been developed using silica gel, which reduce carbon aerogel usage with high cost. Silica gel powder was added to the carbon aerogel to simplify the manufacturing procedure and to increase the wet-ability, the mechanical strength and the CDI efficiency. Porous composite electrodes composed of carbon aerogel and silica gel powder were prepared by paste rolling method. Carbon aerosol composite electrodes with <TEX>$10\times10cm^2$</TEX> are placed face to face between spacers, and assembled the four-stage series cells for CDI process. Each stage is composed of 45 cells. Four-stage series cells (flow through cells) for CDI process are put in continuous-system reactor containing 1,000ml-NaCl solution bath of 1,000 ppm. The four-stage series cells with carbon aerogel electrodes are charged at 1.2V and are discharged at 0.001V, and then read the current. Conclusively, removal efficiencies of ions using the four-stage series cells composed of carbon aerogel composite electrodes show good removal efficiency of <TEX>$99\%$</TEX> respectively.