Is the Hexacyanoferrate(II) Anion Stable in Aqueous Solution? A Combined Theoretical and Experimental Study

Ni-Containing Electrolytes for Superior Zinc-Ion Aqueous Batteries with Zinc Hexacyanoferrate Cathodes

A one-step coprecipitation process is designed to synthesize zinc hexacyanoferrate (ZnHCF) cathodes in aqueous zinc-ion batteries (ZIBs). The morphology of the cathode is influenced by the concentration of the precursor solution and valence of iron ions. The rhombohedral ZnHCF sample exhibits high crystallinity on the microscale in the cut-angle cubic structure, whereas Na-rich NaZnHCF contains many interstitial water molecules in the rhombic nanoplates. Both samples show effective insertion of Zn ions in the aqueous ZnSO4 solution. ZnHCF shows a specific capacity of 66.7 mA h g-1, a redox voltage of 1.73 V, and fast decline in a few cycles. On the other hand, NaZnHCF has a lower specific capacity of 48.2 mA h g-1, showing two voltage platforms and robust cycling stability. However, owing to serious side reactions, both samples have low Columbic efficiency. To improve the properties such as Coulombic efficiency, specific capacity, and cycling stability, Ni ions are introduced by adding 10 wt % NiSO4 to the ZnSO4 electrolyte. The ZnHCF cathode in the Ni-containing electrolyte has the best properties such as a high specific capacity of 71.2 mA h g-1 at a current density of 100 mA g-1, 93% retention of the Coulombic efficiency, and a good rate performance manifested by a reversible capacity of 58.2 mA h g-1 at 1 A g-1. The results reveal a strategy to improve the electrochemical properties of aqueous ZIBs by modifying the electrolytes.

A High Rate and Stable Hybrid Li/Na-Ion Battery Based on a Hydrated Molten Inorganic Salt Electrolyte

Taking into the consideration safety, environmental impact, and economic issue, the construction of aqueous batteries based on aqueous electrolyte has become an indispensable technical option for large-scale electrical energy storage. The narrow electrochemical window is the main problem of conventional aqueous electrolyte. Here, an economical room-temperature inorganic hydrated molten salt (RTMS) electrolyte with a large electrochemical stability window of 3.1 V is proposed. Compared with organic fluorinated molten salts, RTMS is composed of lithium nitrate hydrate and sodium nitrate with much lower cost. Based on the RTMS electrolyte, a hybrid Li/Na-ion full battery is fabricated from cobalt hexacyanoferrate cathode (NaCoHCF) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) anode. The full cell with the RTMS electrolyte exhibits a fantastic performance with high capacity of 139 mAh g^-1 at 1 C, 90 mAh g^-1 at 20 C, and capacity retention of 94.7% over 500 cycles at 3 C. The excellent performances are contributed to the unique properties of RTMS with a large electrochemical window, solvated H₂ O free and high mobility of Li⁺ , which exhibits excellent Li-ions insertion and extraction capacity of NaCoHCF. This RTMS cell provides a new economic choice for large-scale energy storage.

A novel catalytic kinetic method for the determination of mercury(ii) in water samples

Mercury(ii) ions act as catalyst in the substitution of cyanide ion in hexacyanoruthenate(ii) by pyrazine (Pz) in an acidic medium. This property of Hg(ii) has been utilized for its determination in aqueous solutions. The progress of reaction was followed spectrophotometrically by measuring the increase in absorbance of the yellow colour product, [Ru(CN)₅Pz]³⁻ at 370 nm (λ_max, ε = 4.2 × 10³ M⁻¹ s⁻¹) under the optimized reaction conditions; 5.0 × 10⁻⁵ M [Ru(CN)₆⁴⁻], 7.5 × 10⁻⁴ M [Pz], pH 4.00 ± 0.02, ionic strength (I) = 0.05 M (KCl) and temp. 45.0 ± 0.1 °C. The proposed method is based on the fixed time procedure under optimum reaction conditions. The linear regression (calibration) equations between the absorbance at fixed times (t = 15, 20 and 25 min) and [Hg(ii)] were established in the range of 1.0 to 30.0 × 10⁻⁶ M. The detection limit was found to be 1.5 × 10⁻⁷ M of Hg(ii). The effect of various foreign ions on the proposed method was also studied and discussed. The method was applied for the determination of Hg(ii) in different wastewater samples. The present method is simple, rapid and sensitive for the determination of Hg(ii) in trace amount in the environmental samples.

Mercury(ii) ions act as catalyst in the substitution of cyanide ion in hexacyanoruthenate(ii) by pyrazine (Pz) in an acidic medium.

Elucidation of the photoaquation reaction mechanism in ferrous hexacyanide using synchrotron x-rays with sub-pulse-duration sensitivity

Ligand substitution reactions are common in solvated transition metal complexes, and harnessing them through initiation with light promises interesting practical applications, driving interest in new means of probing their mechanisms. Using a combination of time-resolved x-ray absorption spectroscopy and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations and x-ray absorption near-edge spectroscopy calculations, we elucidate the mechanism of photoaquation in the model system iron(ii) hexacyanide, where UV excitation results in the exchange of a CN^- ligand with a water molecule from the solvent. We take advantage of the high flux and stability of synchrotron x-rays to capture high precision x-ray absorption spectra that allow us to overcome the usual limitation of the relatively long x-ray pulses and extract the spectrum of the short-lived intermediate pentacoordinated species. Additionally, we determine its lifetime to be 19 (±5) ps. The QM/MM simulations support our experimental findings and explain the ∼20 ps time scale for aquation as involving interconversion between the square pyramidal (SP) and trigonal bipyramidal pentacoordinated geometries, with aquation being only active in the SP configuration.

Breakthrough Potential in Near-Infrared Spectroscopy: Spectra Simulation. A Review of Recent Developments

Near-infrared (12,500–4,000 cm⁻¹; 800–2,500 nm) spectroscopy is the hallmark for one of the most rapidly advancing analytical techniques over the last few decades. Although it is mainly recognized as an analytical tool, near-infrared spectroscopy has also contributed significantly to physical chemistry, e.g., by delivering invaluable data on the anharmonic nature of molecular vibrations or peculiarities of intermolecular interactions. In all these contexts, a major barrier in the form of an intrinsic complexity of near-infrared spectra has been encountered. A large number of overlapping vibrational contributions influenced by anharmonic effects create complex patterns of spectral dependencies, in many cases hindering our comprehension of near-infrared spectra. Quantum mechanical calculations commonly serve as a major support to infrared and Raman studies; conversely, near-infrared spectroscopy has long been hindered in this regard due to practical limitations. Advances in anharmonic theories in hyphenation with ever-growing computer technology have enabled feasible theoretical near-infrared spectroscopy in recent times. Accordingly, a growing number of quantum mechanical investigations aimed at near-infrared region has been witnessed. The present review article summarizes these most recent accomplishments in the emerging field. Applications of generalized approaches, such as vibrational self-consistent field and vibrational second order perturbation theories as well as their derivatives, and dense grid-based studies of vibrational potential, are overviewed. Basic and applied studies are discussed, with special attention paid to the ones which aim at improving analytical spectroscopy. A remarkable potential arises from the growing applicability of anharmonic computations to solving the problems which arise in both basic and analytical near-infrared spectroscopy. This review highlights an increased value of quantum mechanical calculations to near-infrared spectroscopy in relation to other kinds of vibrational spectroscopy.

Comprehensive Experimental and Computational Spectroscopic Study of Hexacyanoferrate Complexes in Water: From Infrared to X-ray Wavelengths

We present a joint experimental and computational study of the hexacyanoferrate aqueous complexes at equilibrium in the 250 meV to 7.15 keV regime. The experiments and the computations include the vibrational spectroscopy of the cyanide ligands, the valence electronic absorption spectra, and Fe 1s core hole spectra using element-specific-resonant X-ray absorption and emission techniques. Density functional theory-based quantum mechanics/molecular mechanics molecular dynamics simulations are performed to generate explicit solute-solvent configurations, which serve as inputs for the spectroscopy calculations of the experiments spanning the IR to X-ray wavelengths. The spectroscopy simulations are performed at the same level of theory across this large energy window, which allows for a systematic comparison of the effects of explicit solute-solvent interactions in the vibrational, valence electronic, and core-level spectra of hexacyanoferrate complexes in water. Although the spectroscopy of hexacyanoferrate complexes in solution has been the subject of several studies, most of the previous works have focused on a narrow energy window and have not accounted for explicit solute-solvent interactions in their spectroscopy simulations. In this work, we focus our analysis on identifying how the local solvation environment around the hexacyanoferrate complexes influences the intensity and line shape of specific spectroscopic features in the UV/vis, X-ray absorption, and valence-to-core X-ray emission spectra. The identification of these features and their relationship to solute-solvent interactions is important because hexacyanoferrate complexes serve as model systems for understanding the photochemistry and photophysics of a large class of Fe(II) and Fe(III) complexes in solution.

Supramolecular inter-ionic charge-transfer complexes between derivatives of pyridinium-4-oxime cations and hexacyanoferrate(ii) anions

Mono- and bis-pyridinium-4-oxime compounds are introduced as new electron acceptors for the formation of colored, supramolecular, inter-ionic charge-transfer complexes with hexacyanoferrate(ii) as a donor.

Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops

Formation of the small, highly charged tetraanion ferrocyanide, Fe(CN)₆^4–, stabilized in aqueous nanodrops and its influence to the surrounding hydrogen-bonding network of water is reported.

Formation of the small, highly charged tetraanion ferrocyanide, Fe(CN)₆^4–, stabilized in aqueous nanodrops is reported. Ion–water interactions inside these nanodrops are probed using blackbody infrared radiative dissociation, infrared photodissociation (IRPD) spectroscopy, and molecular modeling in order to determine how water molecules stabilize this highly charged anion and the extent to which the tetraanion patterns the hydrogen-bonding network of water at long distance. Fe(CN)₆^4–(H₂O)₃₈ is the smallest cluster formed directly by nanoelectrospray ionization. Ejection of an electron from this ion to form Fe(CN)₆^3–(H₂O)₃₈ occurs with low-energy activation, but loss of a water molecule is favored at higher energy indicating that water molecule loss is entropically favored over loss of an electron. The second solvation shell is almost complete at this cluster size indicating that nearly two solvent shells are required to stabilize this highly charged anion. The extent of solvation necessary to stabilize these clusters with respect to electron loss is substantially lower through ion pairing with either H⁺ or K⁺ (n = 17 and 18, respectively). IRPD spectra of Fe(CN)₆^4–(H₂O)_n show the emergence of a free O–H water molecule stretch between n = 142 and 162 indicating that this ion patterns the structure of water molecules within these nanodrops to a distance of at least ∼1.05 nm from the ion. These results provide new insights into how water stabilizes highly charged ions and demonstrate that highly charged anions can have a significant effect on the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells.

The structural influence of Ca(2+) counter-ions on uranyl(VI) tricarbonate in aqueous solution

The presented study elucidates the influence of calcium(II) counter-ions on the structure of the environmentally relevant uranyl tricarbonates using hybrid quantum mechanical/molecular mechanical (QM/MM) MD simulations. Since experimental investigations may be subject to limitations in detecting the presence of counter-ions in solution, the present study is of importance to obtain a profound understanding of the effects counter-ions may have on coordination complexes. It can be concluded from the obtained simulation data that two calcium(II) ions are essential for stabilizing the experimentally observed uranyl tricarbonate complex in aqueous solution. Including only one calcium(II) ion in the coordination sphere was found to be insufficient to form a six-fold equatorial coordination of carbonates, but a five-fold coordination is adopted similar to the counter-ion free case in aqueous solution reported in a previous study.