A mini review on metal-organic framework-based electrode materials for capacitive deionization.

Capacitive deionization (CDI) is an electrochemical method of extracting ions from solution at potentials below electrolysis. It has various applications ranging from water remediation and desalination to heavy metal removal and selective resource recovery. A CDI device applies an electrical charge across two porous electrodes to attract and remove ions without producing waste products. It is generally considered environmentally friendly and promising for sustainability, yet ion removal efficiency still falls short of more established filtration methods. Commercially available activated carbon is typically used for CDI, and its ion adsorption capacity is low at approximately 20-30 mg g-1. Recently, much interest has been in the highly porous and well-structured family of materials known as metal-organic frameworks (MOFs). Most MOFs are poor conductors of electricity and cannot be directly used to make electrodes. A common workaround is to pyrolyze the MOF to convert its organic components to carbon while maintaining its underlying microstructure. However, most MOF-derived materials only retain partial microstructure after pyrolysis and cannot inherit the robust porosity of the parent MOFs. This review provides a systematic breakdown of structure-performance relationships between a MOF-derived material and its CDI performance based on recent works. This review also serves as a starting point for researchers interested in developing MOF-derived materials for CDI applications.

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.

A Study of MnO2 with Different Crystalline Forms for Pseudocapacitive Desalination.

Recent research on materials for capacitive deionization (CDI) has shown that intercalation materials have salt removal capacities (>40 mg g-1) much higher than those of carbon materials (∼15 mg g-1). However, little work has been done to elucidate the relationship between the microstructure of an intercalation material and its desalination performance. Herein, we report the desalination performances of various crystalline forms of MnO2 in a hybrid CDI setup without the use of ion-exchange membranes. MnO2 materials used in our experiments were either poorly crystalline or crystalline forms of 1D hollandite α-MnO2, 2D birnessite δ-MnO2, and 3D spinel λ-MnO2. X-ray photoelectron spectroscopy performed on electrochemically cycled MnO2 showed redox changes associated with intercalation processes in crystalline MnO2, whereas poorly crystalline MnO2 showed no such changes. It was further shown that surface adsorption is dominant in poorly crystalline MnO2 and that poorly crystalline forms of α-MnO2 and δ-MnO2 exhibited the highest salt removal capacities of 0.17 and 0.16 mmol g-1 (9.93 and 9.35 mg g-1), respectively. These performances are comparable to state-of-the-art carbon materials and are remarkable considering the low surface areas (<400 m2 g-1) of MnO2 materials.

A Prussian blue anode for high performance electrochemical deionization promoted by the faradaic mechanism.

Desalination is a sustainable process that removes sodium and chloride ions from seawater. Herein, we demonstrate a faradaic mechanism to promote the capacity of capacitive deionization in highly concentrated salt water via an electrochemical deionization device. In this system, ion removal is achieved by the faradaic mechanism via a constant current operation mode, which is improved based on the constant voltage operation mode used in the conventional CDI operation. Benefiting from the high capacity and excellent rate performance of Prussian blue as an active electrochemical reaction material, the designed unit has revealed a superior removal capacity with an ultrafast ion removal rate. A high removal capacity of 101.7 mg g-1 has been obtained with proper flow rate and current density. To further improve the performance of the EDI, a reduced graphene oxide with nanopores and Prussian blue composite has been synthesized. The PB@NPG has demonstrated a high salt removal capacity of 120.0 mg g-1 at 1 C with an energy consumption of 6.76 kT per ion removed, which is much lower than most CDI methods. A particularly high rate performance of 0.5430 mg g-1 s-1 has been achieved at 40 C. The faradaic mechanism promoted EDI has provided a new insight into the design and selection of host materials for highly concentrated salt water desalination.

Nitrogen-doped graphene oxide for effectively removing boron ions from seawater.

Elemental boron exists in the form of boric acid or borate salts in aqueous solution. The human body is very sensitive to the amount of boron, and boron contamination in drinking water affects our health adversely. However, boron is not easily removed due to its small ionic size and is a problem to water treatment systems. Herein, we report a new method to remove boron using nitrogen-doped graphene oxide (N-GO). The maximum adsorption capacity we have obtained is 58.7 mg g-1 and this makes N-GO one of the best materials to adsorb boron. Real seawater with 5 mg L-1 as boron is used as a feed for testing and the absorption capacity is shown to be up to 2.42 mg g-1. This high adsorption capacity is mainly attributed to the large amount of hydroxyl groups distributed across the high surface area of graphene oxide and the enhanced adsorption that results from nitrogen-doped sites. Once N-GO is saturated with boron ions, it can be easily regenerated via acid treatment. Our proposed technique has high commercial value and we believe that it is very valuable to the water treatment industry.

Ultrahigh Performance of Novel Capacitive Deionization Electrodes based on A Three-Dimensional Graphene Architecture with Nanopores.

In order to achieve optimal desalination during capacitive deionization (CDI), CDI electrodes should possess high electrical conductivity, large surface area, good wettability to water, narrow pore size distribution and efficient pathways for ion and electron transportation. In this work, we fabricated a novel CDI electrode based on a three-dimensional graphene (3DG) architecture by constructing interconnected graphene sheets with in-plane nanopores (NP-3DG). As compared to 3DG, NP-3DG features a larger specific surface area of 445 m(2) g(-1) and therefore the higher specific capacitance. The ultrahigh electrosorptive capacity of NP-3DG predicted from Langmuir isotherm is 17.1 mg g(-1) at a cell potential of 1.6 V. This can be attributed to the interconnected macropores within the graphene networks and nanopores on graphene sheets. Both of macropores and nanopores are favorable for enhancing CDI performance by buffering ions to reduce the diffusion distances from the external electrolyte to the interior surfaces and enlarging the surface area.