Controlled synthesis of NaTi2(PO4)3/Carbon composite derived from Metal-organic-frameworks as highly-efficient electrodes for hybrid capacitive deionization

Ternary-metal Prussian blue analogues as high-quality sodium ion capturing electrodes for rocking-chair capacitive deionization

Prussian blue analogs (PBAs) have gained much attention in the capacitive deionization (CDI) field because of their rigid open structure and good energy storage capacity. However, their desalination performance is still to be improved for practical application. Herein, we reported the NiCoFe ternary-metal PBAs materials and explored their application as Na⁺ capturing electrode in rocking-chair capacitive deionization (RCDI) system. On the one hand, the introduction of Ni²⁺ into CoFe PBA can effectively reduce the lattice changes in the (dis)charging process.On the other hand, the RCDI system with symmetrical structure could avoid the performance deficiency caused by the unbalanced capacity of common HCDI system. Due to the rationalized RCDI cell configuration and ternary-metal PBAs with improved stability, the NiCoFe-PBAs-based RCDI exhibits amazing desalination performance with maximum capacity of 131.4 mg·g^-1 and rate of 0.46 mg·g^-1·s^-1 as well as optimum stability with 90.7 % capacity retention over 300 cycles, surpassing those of PBAs based CDI system reported previously. The special strategy in this work offers inspiration via optimizing the cell structure and electrode materials for the promising development of CDI systems.

Solvothermal Engineering of NaTi2(PO4)3 Nanomorphology for Applications in Aqueous Na-Ion Batteries

Aqueous Na-ion batteries are among the most discussed alternatives to the currently dominating Li-ion battery technology, in the area of stationary storage systems because of their sustainability, safety, stability, and environmental friendliness. The electrochemical properties such as ion insertion kinetics, practical capacity, cycling stability, or Coulombic efficiency are strongly dependent on the structure, morphology, and purity of an electrode material. The selection and optimization of materials synthesis route in many cases allows researchers to engineer materials with desired properties. In this work, we present a comprehensive study on size- and shape-controlled hydro(solvo)thermal synthesis of NaTi₂(PO₄)₃ nanoparticles. The effects of different alcohol/water synthesis media on nanoparticle phase purity, morphology, and size distribution are analyzed. Water activity in the synthesis media of different alcohol solutions is identified as the key parameter governing the nanoparticle phase purity, size, and shape. The careful engineering of NaTi₂(PO₄)₃ nanoparticle morphology allows control of the electrochemical performance and degradation of these materials as aqueous Na-ion battery electrodes.

Turning waste into valuables: In situ deposition of polypyrrole on the obsolete mask for Cr(VI) removal and desalination

The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g^-1) and economic value (811.42 mg $^-1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO₂ (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g^-1) and ultrafast salt adsorption rate (6.96 mg g^-1 min^-1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination.

Polyaniline coated MOF-derived Mn2O3 nanorods for efficient hybrid capacitive deionization

Mn-based oxides are efficient pseudocapacitive electrode materials and have been investigated for capacitive deionization (CDI). However, their poor conductivity seriously affects their desalination performance. In this work, polyaniline coated Mn₂O₃ nanorods (PANI/Mn₂O₃) are synthesized by oxidizing a Mn-based metal organic framework (MOF) and subsequent in-situ chemical polymerization. The polyaniline not only acts as a conductive network for faradaic reactions of Mn₂O₃, but also enhances the desalination rate. PANI/Mn₂O₃ has a specific capacitance of 87 F g^-1 (at 1 A g^-1), superior to that of Mn₂O₃ nanorod (52 F g^-1 at 1 A g^-1). The hybrid CDI cell constructed with a PANI/Mn₂O₃ cathode and an active carbon anode shows a high desalination capacity of 21.6 mg g^-1, superior recyclability with only 11.3% desalination capacity decay after 30 desalination cycles and fast desalination rate of 2.2 mg g^-1 min^-1. PANI/Mn₂O₃ is a potential candidate for high performance CDI applications.

In situ anchor of Na2Ti3O7 in nitrogen-rich carbon hollow red blood cell-like structure as a 0D-3D hierarchical electrode material for efficient electrochemical desalination

Reasonable design of the structure and complementary compounding of electrode materials is helpful to enhance capacitive deionization (CDI) performance. Herein, a novel 0D-3D hierarchical electrode material containing Na2Ti3O7 nanoparticles anchored at hollow red blood cell (HRBC)-like nitrogen-rich carbon (HRBC-NTO/N-C-60) was prepared via selective protection, pyrolysis, and alkalization. Specifically, a HRBC-like NH2-MIL-125-based material (HRBC-MOF-60) was first constructed by a selective protection approach of tannic acid (TN), which addresses the shortcomings of using sacrificial templates or corrosive agents. Afterwards, HRBC-NTO/N-C-60 was obtained in situ by annealing and alkalization of HRBC-MOF-60. The nitrogen-rich carbon with a HRBC-like structure has the ability to rapidly transport electrons, and its porous structure enables remarkable charge transfer. Benefiting from the grafted 3D N-doped porous carbon with a HRBC-like structure, well-dispersed 0D Na2Ti3O7 nanoparticles, and satisfactory bonding effects, HRBC-NTO/N-C-60 exhibited high specific capacitance and fast ionic and electronic diffusion kinetics. Moreover, HRBC-NTO/N-C-60 was well-suited for desalination by functioning as a cathode material for capacitive deionization (CDI), and delivering a high desalination capacity of 66.8 mg g-1 in 200 mg L-1 NaCl solution at 1.4 V. This work introduces an excellent high-performance candidate for electrochemical deionization as well as affording afflatus for accurately inventing OD-3D hierarchical materials with hollow structures.

Recent progress in materials and architectures for capacitive deionization: A comprehensive review

Capacitive deionization is an emerging and rapidly developing electrochemical technique for water desalination across the globe with exponential growth in publications. There are various architectures and materials being explored to obtain utmost electrosorption performance. The symmetric architectures consist of the same material on both electrodes, while asymmetric architectures have electrodes loaded with different materials. Asymmetric architectures possess higher electrosorption performance as compared with that of symmetric architectures owing to the inclusion of either faradaic materials, redox-active electrolytes, or ion specific pre-intercalation material. With the materials perspective, faradaic materials have higher electrosorption performance than carbon-based materials owing to the occurrence of faradaic reactions for electrosorption. Moreover, the architecture and material may be tailored in order to obtain desired selectivity of the target component and heavy metal present in feed water. In this review, we describe recent developments in architectures and materials for capacitive deionization and summarize the characteristics and salt removal performances. Further, we discuss recently reported architectures and materials for the removal of heavy metals and radioactive materials. The factors that affect the electrosorption performance including the synthesis procedure for electrode materials, incorporation of additives, operational modes, and organic foulants are further illustrated. This review concludes with several perspectives to provide directions for further development in the subject of capacitive deionization. PRACTITIONER POINTS: Capacitive deionization (CDI) is a rapidly developing electrochemical water desalination technique with exponential growth in publications. Faradaic materials have higher salt removal capacity (SAC) because of reversible redox reactions or ion-intercalation processes. Combination of CDI with other techniques exhibits improved selectivity and removal of heavy metals. Operational parameters and materials properties affect SAC. In future, comprehensive experimentation is needed to have better understanding of the performance of CDI architectures and materials.

Freestanding Ti3C2Tx MXene/Prussian Blue Analogues Films with Superior Ion Uptake for Efficient Capacitive Deionization by a Dual Pseudocapacitance Effect

Exploring and designing high-performance Faradaic electrode materials is of great significance to enhance the desalination performance of hybrid capacitive deionization (HCDI). Herein, open and freestanding films (MXene/Prussian blue analogues (PBAs), specifically, MXene/NiHCF and MXene/CuHCF) were prepared by vacuum filtration of a mixed solution of PBAs nanoparticles and a Ti₃C₂T_x MXene dispersion and directly used as HCDI electrodes. The conductive MXene nanosheets bridge the PBAs nanoparticles to form a three-dimensional (3D) conductive network structure, which can accelerate the salt ion and electron diffusion/transport kinetics for HCDI. Additionally, the PBAs nanoparticles can prevent the restacking of MXene nanosheets, expand their interlayer spacing, and facilitate the rapid diffusion and storage of ions. Benefiting from the dual pseudocapacitance and synergistic effect of PBAs and MXene, the obtained MXene/PBAs films show superior properties, with a high desalination capacity (85.1 mg g^-1 for the MXene/NiHCF film and 80.4 mg g^-1 for the MXene/CuHCF film) and an ultrafast salt-removal rate, much higher than those of other Faradaic electrodes. The synergistic effect, the adsorption of Na⁺ ions, and the enhanced conductivity of MXene/PBAs films were demonstrated through first-principles calculations. This paper offers a simple and convenient method for the design of freestanding HCDI electrodes and promotes the rapid development of HCDI technology.

A binder-free δ-MnO2@reduced graphene oxide composite film as a bi-functional electrode for aqueous rechargeable sodium-ion batteries and hybrid capacitive deionization

The freestanding δ-MnO2@rGO/CC electrode exhibits stable operation performance for both ASIBs and HCDI.

Layered double hydroxide coated electrospun carbon nanofibers as the chloride capturing electrode for ultrafast electrochemical deionization

Slow desalination kinetics and poor durability of the electrodes are two key limitations of electrochemical deionization (EDI) that are considered to be the next generation of capacitive desalination (CDI). Herein, we report the design of a high-efficiency chloride removal electrode material for accelerating the desalination kinetics and concurrently improving the durability of EDI, which is based on coating NiMn-Cl layered double hydroxides (LDHs) on the surface of electrospun carbon nanofibers (CNFs@LDHs). The salient features of the as-developed CNFs@LDHs are that applying layer-structured LDHs with abundant redox-active sites to accelerate the pseudo-capacitive ion storage via fast ion intercalation/deintercalation, and leveraging the rigid CNF backbone to strengthen its durability by preventing the potential aggregation of LDHs. As expected, the CNFs@LDH based EDI system displays an ultrafast desalination rate of 0.51 mg g^-1 s^-1 and outstanding long-term stability (only 10.66 % desalination capacity reduction after 35 cycles), which is achieved without sacrificing its excellent desalination capacity (72.04 mg g^-1). This work could be inspirational for the future design of ultrafast yet durable EDI approaching industrial desalination applications.