Ion-selective asymmetric carbon electrodes for enhanced capacitive deionization

Selectivity adsorption of sulfate by amino-modified activated carbon during capacitive deionization

ABSTRACTCapacitive deionization (CDI) is an environmentally friendly desalination technique with low energy consumption. However, unmodified carbon electrode materials have poor sulfate selectivity and adsorption capacity. In this work, to improve sulfate selectivity, we prepared activated carbon materials loaded with different amino contents by grafting amino groups via acid treatment for different times. In the competitive ion adsorption experiments, the sulfate selectivity of AC was only 0.64 and the amino-modified AC increased by 1.98-2.52 times due to the formation of stronger hydrogen bonds between the amino group and sulfate. AC-NH₂-4 had the best selectivity and the sulfate selective coefficient was 2.25. The desorption of sulfate was 92.46% within one hour. In addition, the surface of the amino-modified activated carbon showed significantly improved electrochemical properties and better capacitance. The specific capacitance of amino-modified AC in different electrolyte solutions was consistent with the competitive adsorption results. The specific capacitance of amino-modified AC in Na₂SO₄ electrolyte solution was the highest. The modified electrode material also had the advantages of a higher adsorption capacity and excellent regeneration performance after continuous electric adsorption-desorption cycles. Therefore, it may have development potential to selectively adsorb sulfate in practical applications.

Ultrahigh Content Boron and Nitrogen Codoped Hierarchically Porous Carbon Obtained from Biomass Byproduct Okara for Capacitive Deionization

Capacitive deionization (CDI) is an environmentally friendly, energy efficient, and low cost water purification technique in comparison with other conventional techniques, and it has attracted considerable attention in recent years. Here, we use biomass byproduct okara as the starting material to fabricate a boron and nitrogen codoped hierarchically porous carbon (BNC) with ultrahigh heteroatom contents and abundant in-plane nanoholes for CDI application. With the interconnected hierarchical porous structure, the BNC not only exhibits a large surface area (647.0 m³ g^-1) for the adsorption of ions but also offers abundant ion transport channels to access the entire internal surface. Meanwhile, the ultrahigh dopants' content of B (11.9 at%) and N (14.8 at%) further gives rise to the increased surface polarity and enhanced capacitance for BNC. Owing to these favorable properties, BNC exhibits top-level salt adsorption capacity (21.5 mg g^-1) and charge efficiency (59.5%) at the initial NaCl concentration of ∼500 mg L^-1. Moreover, we performed first-principle simulations to explore the different effects between N-doping and N,B-codoping on the capacitive property, which indicate that the boron and nitrogen codoping of carbon can largely increase the quantum capacitance over the double layer capacitance. The results of this work suggest a promising prospect for the BNC material in practical CDI application.

Perfect divalent cation selectivity with capacitive deionization

Capacitive deionization (CDI) is an emerging membraneless water desalination technology based on storing ions in charged electrodes by electrosorption. Due to unique selectivity mechanisms, CDI has been investigated towards ion-selective separations such as water softening, nutrient recovery, and production of irrigation water. Especially promising is the use of activated microporous carbon electrodes due to their low cost and wide availability at commercial scales. We show here, both theoretically and experimentally, that sulfonated activated carbon electrodes enable the first demonstration of perfect divalent cation selectivity in CDI, where we define "perfect" as significant removal of the divalent cation with zero removal of the competing monovalent cation. For example, for a feedwater of 15 mM NaCl and 3 mM CaCl₂, and charging from 0.4 V to 1.2 V, we show our cell can remove 127 μmol per gram carbon of divalent Ca²⁺, while slightly expelling competing monovalent Na⁺ (-13.2 μmol/g). This separation can be achieved with excellent efficiency, as we show both theoretically and experimentally a calcium charge efficiency above unity, and an experimental energy consumption of less than 0.1 kWh/m³. We further demonstrate a low-infrastructure technique to measure cation selectivity, using ion-selective electrodes and the extended Onsager-Fuoss model.

Intrinsic Pseudocapacitive Affinity in Manganese Spinel Ferrite Nanospheres for High-Performance Selective Capacitive Removal of Ca2+ and Mg2

Pseudocapacitor-type hybrid capacitive deionization (PHCDI) has been developed extensively for deionization, which enables to address the worldwide freshwater shortage. However, the exploitation of selective hardness ion removal in resourceful hard water via the intrinsic pseudocapacitive effect, rather than the ion-sieving or ion-swapping effect based on the electric double layer (EDL) of porous carbon, is basically blank and urgent. Herein, manganese spinel ferrite (MFO) nanospheres were successfully fabricated by one-step solvothermal synthesis and used as the cathode for PHCDI assembled with commercial activated carbon. The MFO electrode exhibited prominent capacities of 534.6 μmol g^-1 (CaCl₂) and 980.4 μmol g^-1 (MgCl₂), outperforming those of other materials ever reported in the literature. Fascinatingly, systematic investigation of binary and ternary ion solutions showed the high electro-affinity of hardness ions (Ca²⁺ and Mg²⁺) toward Na⁺, especially the leading affinity of Mg²⁺, in which the superhigh hardness selectivity of 34.76 was achieved in the ternary solution with a molar ratio of Na-Ca-Mg as 20:1:1. Unexpectedly, the ion-swapping trace in a multi-ion environment was also first detected in our pseudocapacitive-based electrode. The electrochemical response in unary and multiple electrolytes disclosed that the unique pseudocapacitive affinity based on the cation (de)intercalation-redox mechanism was from the synergistic effect of the relative redox potential, ionic radius, and valence, in which the redox potential was the dominant factor.

Electrocatalytic desalination with CO2 reduction and O2 evolution

Multifunctional electrocatalytic desalination is a promising method to increase the production of additional valuable chemicals during the desalination process. In this work, a multifunctional desalination device was demonstrated to effectively desalinate brackish water (15 000 ppm) to 9 ppm while generating formate from captured CO₂ at the Bi nanoparticle cathode and releasing oxygen at the Ir/C anode. The salt feed channel is sandwiched between two electrode chambers and separated by ion-exchange membranes. The electrocatalytic process accelerates the transportation of sodium ions and chloride ions in the brine to the cathode and anode chamber, respectively. The fastest salt removal rate to date was obtained, reaching up to 228.41 μg cm^-2 min^-1 with a removal efficiency of 99.94%. The influences of applied potential and the concentrations of salt feed and electrolyte were investigated in detail. The current research provides a new route towards an electrochemical desalination system.

Carbon-Based Capacitive Deionization Electrodes: Development Techniques and its Influence on Electrode Properties

Capacitive deionization (CDI) is a potential technology to provide cost efficient desalinated and/or softened water. Several efforts have been invested in the fabrication of CDI electrodes that not only has outstanding performance but also high chance of large scalability. In this personal account, the different techniques in developing carbon-based materials are presented together with its actual effect on the surface and electrochemical properties of carbon. The categories presented are based on the studies done by the Electrochemical Reaction and Technology Laboratory, the Ertl Center, different research groups in South Korea, and selected papers from the past three years. Our perspective about research gaps and prospects are also included with the aim to increase interest for CDI research.

Asymmetrical removal of sodium and chloride in flow-through capacitive deionization

Capacitive deionization (CDI) is an electrochemical method of removing salt ions from brackish water. A common assumption in CDI is that monovalent ions (e.g., Na⁺, Cl^-) are removed in a 1:1 symmetry on the electrodes. Validation of this assumption with techniques such as ion chromatography is not commonly performed, but is important to better understand how parasitic process, such as faradaic reactions, affect ion removals. In this study, we quantified the removals of Na⁺ and Cl^- as a function of electrode orientation in flow-through CDI. When the cathode was positioned upstream, Na⁺ and Cl^- removals approached a 1:1 symmetry, but when the anode was located upstream, we observed a significant drop in Na⁺, but not Cl^-, removals. We attributed this drop to oxygen reduction reactions at the cathode that competed with Na⁺ adsorption. Oxidation of carbon in the upstream anode yielded H⁺ that enhanced the reduction of oxygen to H₂O₂ at the downstream cathode, which in turn diverted electrons from Na⁺ adsorption. In the absence of oxygen, Na⁺ removals increased in the upstream anode orientation and were comparable to Cl^- removals, confirming that competition with oxygen reduction reactions was the primary reason for decreased Na⁺ removal. In the upstream cathode orientation, we show that H₂O₂ generated at the cathode can be oxidized at the downstream anode, possibly enhancing Na⁺ removals via internal electron recycling. Salt adsorption capacities calculated using actual ion removals did not always agree with those estimated using changes in solution conductivity, with the largest disagreement observed when conductivity data were corrected for pH changes. Our results highlight that faradaic reactions, particularly oxygen reduction reactions, can contribute to asymmetrical removals of monovalent ions in flow-through CDI.

Capacitive Deionization of Divalent Cations for Water Softening Using Functionalized Carbon Electrodes

Water softening is a relatively untapped area of research in capacitive deionization (CDI). In this work, we demonstrate how an asymmetric combination of oxidized and aminated carbon can be used for selective removal of divalent cations for water softening. We first show how higher electrosorption performances can be achieved in single-salt experiments involving NaCl, KCl, MgCl2, and CaCl2 before proceeding to multi-salt experiments using different combinations of the four salts. The salt combinations are chosen to investigate one of the three factors: (1) ionic mass, (2) ionic charge, or (3) concentration. We show how divalent selectivity can be achieved due to high local electrostatic attraction between negatively charged oxygen moieties and divalent cations. Additionally, an ion-exchange process between the oxidized carbon surface and cations can result in lower pH values, which prevent the precipitation of scale-forming ions.