Well-dispersed Prussian blue analogues connected with carbon nanotubes for efficient capacitive deionization process

Additive-Free, In Situ Rapid Repair of Vacancies in Fe[Fe(CN)6] Electrodes for Efficient Capacitive Deionization

Fe[Fe(CN)6] (FeHCF) is considered a promising material for capacitive deionization-desalination of saline wastewater due to its excellent structure. However, additives are usually introduced during the synthesis of FeHCF in order to avoid [Fe(CN)6]3- vacancy defects filled by ligand water, which can result in the appearance of harmful byproducts and additional water treatment costs. In this study, an additive-free in situ vacancy repair strategy is proposed for the rapid synthesis of high-quality FeHCF in a saturated K3Fe(CN)6 solution. During the process of synthesizing FeHCF in solution, a high concentration of [Fe(CN)6]3- is found to facilitate the binding of Fe3+ to [Fe(CN)6]3- and hinder the hydrolysis and coordination reaction of Fe3+. After undergoing repair, FeHCF4 demonstrates an increased capacity and highly reversible electrochemical performance due to the robust structure. When utilized as Faraday cathodes in hybrid capacitive deionization (HCDI) systems, FeHCF4 exhibits a higher salt removal capacity (65.67 mg g-1) and lower energy consumption (0.68 kWh kg-1-NaCl) compared to unrepaired FeHCF1, while still maintaining excellent cycling performance. This environmentally friendly approach of repairing vacancies serves as a source of inspiration for the advancement of high-performance Prussian Blue analogues as capacitive sodium-removing materials.

Entropy Engineering Constrain Phase Transitions Enable Ultralong-life Prussian Blue Analogs Cathodes

Prussian blue analogs (PBAs) are considered as one of the most potential electrode materials in capacitive deionization (CDI) due to their unique 3D framework structure. However, their practical applications suffer from low desalination capacity and poor cyclic stability. Here, an entropy engineering strategy is proposed that incorporates high-entropy (HE) concept into PBAs to address the unfavorable multistage phase transitions during CDI desalination. By introducing five or more metals, which share N coordination site, high-entropy hexacyanoferrate (HE-HCF) is constructed, thereby increasing the configurational entropy of the system to above 1.5R and placing it into the high-entropy category. As a result, the developed HE-HCF demonstrates remarkable cycling performance, with a capacity retention rate of over 97% after undergoing 350 ultralong-life cycles of adsorption/desorption. Additionally, it exhibits a high desalination capacity of 77.24 mg g-1 at 1.2 V. Structural characterization and theoretical calculation reveal that high configurational entropy not only helps to restrain phase transition and strengthen structural stability, but also optimizes Na+ ions diffusion path and energy barrier, accelerates reaction kinetics and thus improves performance. This research introduces a new approach for designing electrodes with high performance, low cost, and long-lasting durability for capacitive deionization applications.

Switchable NaCl cages via a MWCNTs/Ni[Fe(CN)6]2 nanocomposite for high performance desalination

With the application of nanomaterials in seawater desalination technology increasing, the adjustable characteristics of carbon-based nanomaterials make it possible to use multiwalled carbon nanotube (MWCNT) materials in seawater desalination technology. In this study, Ni[Fe(CN)6]2 is loaded onto the inner wall of MWCNTs by the co-precipitation method to prepare MWCNTs with variable pore size, making it a switchable cage for NaCl. During the procedure, most of the Ni[Fe(CN)6]2 is transferred to the outer surface of the MWCNTs after adsorption, and NaCl is stored inside the MWCNTs (which have been proved by characterization); at the same time, Ni can improve the cell stability of Ni[Fe(CN)6]2. The effect of adsorbent reaction time and addition amount on the desalination performance of MWCNTs/Ni[Fe(CN)6]2 has been tested. According to the results, the best desalination performance of MWCNTs/Ni[Fe(CN)6]2 is 1354.6 mg g-1 when the reaction time is 0.5 h and the addition amount is 20 mg. After 3 cycles of adsorption and desorption, its desalting performance decreased to 242.3 mg g-1.

Efficient lithium extraction using redox-active prussian blue nanoparticles-anchored activated carbon intercalation electrodes via membrane capacitive deionization

Global demand for lithium (Li) resources has dramatically increased due to the demand for clean energy, especially the large-scale usage of lithium-ion batteries in electric vehicles. Membrane capacitive deionization (MCDI) is an energy and cost-efficient electrochemical technology at the forefront of Li extraction from natural resources such as brine and seawater. In this study, we designed high-performance MCDI electrodes by compositing Li⁺ intercalation redox-active Prussian blue (PB) nanoparticles with highly conductive porous activated carbon (AC) matrix for the selective extraction of Li⁺. Herein, we prepared a series of PB-anchored AC composites (AC/PB) containing different percentages (20%, 40%, 60%, and 80%) of PB by weight (AC/PB-20%, AC/PB-40%, AC/PB-60%, and AC/PB-80%, respectively). The AC/PB-20% electrode with uniformly anchored PB nanoparticles over AC matrix enhanced the number of active sites for electrochemical reaction, promoted electron/ion transport paths, and facilitated abundant channels for the reversible insertion/de-insertion of Li⁺ by PB, which resulted in stronger current response, higher specific capacitance (159 F g^-1), and reduced interfacial resistance for the transport of Li⁺ and electrons. An asymmetric MCDI cell assembled with AC/PB-20% as cathode and AC as anode (AC//AC-PB20%) displayed outstanding Li⁺ electrosorption capacity of 24.42 mg g^-1 and a mean salt removal rate of 2.71 mg g min^-1 in 5 mM LiCl aqueous solution at 1.4 V with high cyclic stability. After 50 electrosorption-desorption cycles, 95.11% of the initial electrosorption capacity was retained, reflecting its good electrochemical stability. The described strategy demonstrates the potential benefits of compositing intercalation pseudo capacitive redox material with Faradaic materials for the design of advanced MCDI electrodes for real-life Li⁺ extraction applications.

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.

Energy Consumption in Capacitive Deionization for Desalination: A Review

Capacitive deionization (CDI) is an emerging eco-friendly desalination technology with mild operation conditions. However, the energy consumption of CDI has not yet been comprehensively summarized, which is closely related to the economic cost. Hence, this study aims to review the energy consumption performances and mechanisms in the literature of CDI, and to reveal a future direction for optimizing the consumed energy. The energy consumption of CDI could be influenced by a variety of internal and external factors. Ion-exchange membrane incorporation, flow-by configuration, constant current charging mode, lower electric field intensity and flowrate, electrode material with a semi-selective surface or high wettability, and redox electrolyte are the preferred elements for low energy consumption. In addition, the consumed energy in CDI could be reduced to be even lower by energy regeneration. By combining the favorable factors, the optimization of energy consumption (down to 0.0089 Wh·g_NaCl^-1) could be achieved. As redox flow desalination has the benefits of a high energy efficiency and long lifespan (~20,000 cycles), together with the incorporation of energy recovery (over 80%), a robust future tendency of energy-efficient CDI desalination is expected.