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.

Probing the Electronic Structure of Prussian Blue and Analog Films by Photoemission and Electron Energy Loss Spectroscopies

X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS) were employed to characterize the electronic properties of Prussian blue (PB) and its analogs when electrodeposited over metal-decorated carbon nanotubes (CNTs). Through an investigation of the influence of carbon nanotubes (CNTs) and preparation conditions on the electronic structure, valuable insights were obtained regarding their effects on electrochemical properties. XPS analysis enabled the probing of the chemical composition and oxidation states of the film materials, unveiling synthesis-driven variations in their electronic properties. REELS provided information on energy loss and electronic transitions, enabling further characterization of the changes in the electronic structure induced by different preparation methods. Such findings emphasize the importance of surface characterization to understand how the unique electronic properties of such materials can be harnessed to enhance their performance and functionality.

Density-functional-theory-predicted symmetry lowering from cubic to tetragonal in nickel hexacyanoferrate

Density-functional-theory (DFT) computations on a Prussian blue analogue (PBA), nickel hexacyanoferrate, Ni2+ 3[Fe3+(CN)6]2·nH2O, predict the existence of a tetragonal (P 4 m2) crystal structure that is energetically degenerate with the previously reported cubic (F 43m) structure for this PBA. The proposed tetragonal structure satisfies observations, such as X-ray diffraction and magnetic measurements, that have been reported previously. A van der Waals corrected exchange-correlation functional is used in the DFT+U computations for an improved description of hydrogen bonding. The results provide strong support for a revised and simplified crystallographic description of Ni2+ 3[Fe3+(CN)6]2·nH2O, and show how H2O molecules stabilize the crystal structure and affect its magnetic and electronic properties. The symmetry lowering in nickel hexacyanoferrate is attributed to the hydration shell of the interstitial nickel cation. Calculations strongly suggest a maximum of n = 7 interstitial H2O molecules per formula unit for nickel hexacyanoferrate at room temperature, and a higher water content at temperatures below T ≃ 200 K. Since the symmetry lowering relies on the presence of interstitial H2O molecules, this revised crystallographic description may be applicable more generally to the large class of F 43m-structured PBAs.

First Principles Study on the Effect of Pressure on the Structure, Elasticity, and Magnetic Properties of Cubic GaFe(CN)6 Prussian Blue Analogue

The structure, elasticity, and magnetic properties of Prussian blue analogue GaFe(CN)6 under external pressure ranges from 0 to 40 GPa were studied by first principles calculations. In the range of pressure from 0 to 35 GPa, GaFe(CN)6 not only has the half-metallic characteristics of 100% spin polarization, but also has stable mechanical properties. The external pressure has no obvious effect on the crystal structure and anisotropy of GaFe(CN)6, but when the pressure exceeds 35 GPa, the half-metallicity of GaFe(CN)6 disappears, the mechanical properties are no longer stable, and total magnetic moments per formula unit are no longer integer values.

Porous 3D Prussian blue/cellulose aerogel as a decorporation agent for removal of ingested cesium from the gastrointestinal tract

In the present study, we successfully synthesized a porous three-dimensional Prussian blue-cellulose aerogel (PB-CA) composite and used it as a decorporation agent for the selective removal of ingested cesium ions (Cs⁺) from the gastrointestinal (GI) tract. The safety of the PB-CA composite was evaluated through an in vitro cytotoxicity study using macrophage-like THP-1 cells and Caco-2 intestinal epithelial cells. The results revealed that the PB-CA composite was not cytotoxic. An adsorption study to examine the efficiency of the decorporation agent was conducted using a simulated intestinal fluid (SIF). The adsorption isotherm was fitted to the Langmuir model with a maximum Cs⁺ adsorption capacity of 13.70 mg/g in SIF that followed pseudo-second-order kinetics. The PB-CA composite showed excellent stability in SIF with a maximum Cs⁺ removal efficiency of 99.43%. The promising safety toxicology profile, remarkable Cs⁺ adsorption efficacy, and excellent stability of the composite demonstrated its great potential for use as an orally administered drug for the decorporation of Cs⁺ from the GI tract.

On the Mechanism of the Improved Operation Voltage of Rhombohedral Nickel Hexacyanoferrate as Cathodes for Sodium-Ion Batteries

We reported a rhombohedral Na-rich nickel hexacyanoferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na⁺ ions in the rhombohedral framework in parallel with the low charge density at the Fe²⁺ ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g^-1 at the current density of 80 mA g^-1, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral-cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/discharge and the DFT calculations, respectively.

A Database of the Structural and Electronic Properties of Prussian Blue, Prussian White, and Berlin Green Compounds through Density Functional Theory

Prussian blue and its related compounds are formed by cheap and abundant metals and have shown their importance in the generation of new fuels by renewable sources. To optimize these compounds it is important to understand their electronic structure and thus establish robust structure-activity relationships. To this end, we employed theoretical simulations based on density functional theory, employing functionals of different degree of complexity, including pure generalized gradient approximation (GGA) and GGA+U functionals, which introduce self-interaction correction terms through the Hubbard parameter, and compared those to the hybrid functionals HSE03 and HSE06. With this robust setup, we can identify an appropriate computational scheme that provides the best compromise between computational demand and accuracy. A complete database considering Berlin green and Prussian blue and white for all alkaline cations is presented.

Water-mediated cation intercalation of open-framework indium hexacyanoferrate with high voltage and fast kinetics

Rechargeable aqueous metal-ion batteries made from non-flammable and low-cost materials offer promising opportunities in large-scale utility grid applications, yet low voltage and energy output, as well as limited cycle life remain critical drawbacks in their electrochemical operation. Here we develop a series of high-voltage aqueous metal-ion batteries based on 'M(+)/N(+)-dual shuttles' to overcome these drawbacks. They utilize open-framework indium hexacyanoferrates as cathode materials, and TiP2O7 and NaTi2(PO4)3 as anode materials, respectively. All of them possess strong rate capability as ultra-capacitors. Through multiple characterization techniques combined with ab initio calculations, water-mediated cation intercalation of indium hexacyanoferrate is unveiled. Water is supposed to be co-inserted with Li(+) or Na(+), which evidently raises the intercalation voltage and reduces diffusion kinetics. As for K(+), water is not involved in the intercalation because of the channel space limitation.

A low-temperature sintered heterostructure solid film of coordination polymer nanoparticles: an electron-rectifier function based on partially oxidised/reduced conductor phases of Prussian blue

Double-layer films of Prussian blue (PB) and its analogue nanoparticles act as an electron-rectifier controlled by the PB bottom layer with a 20-nm thickness.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.