Aqueous Electrolyte Solutions Research Articles

Elucidation of Molecular‐level Mechanisms of Oxygen Atom Reactions with Chlorine Ion in NaCl Solutions using Molecular Dynamics Simulations Combined with Density Functional Theory

AbstractMolecular dynamics simulations combined with density functional theory were performed to investigate the reaction mechanisms for oxygen atom and chlorine ion in NaCl solutions. Considering the ground and first excited states of the oxygen atom, four bulk systems, O(3P)‐NaCl‐63H2O, O(1D)‐NaCl‐63H2O, O(3P)‐4NaCl‐63H2O, and O(1D)‐4NaCl‐63H2O were studied. The main reaction pathway depended on the concentration of the aqueous electrolyte solution. For a dilute solution with a 1 : 64 ratio of NaCl to H2O, the singlet oxygen atom first attached a water molecule to produce a neutral OH radical, and then the OH radical and chlorine ion approach each other to form an intermediate structure [Cl…OH]−. Synchronously, the intermediate transferred charge to the solvated OH radical by accessing the hydrogen bond network. Finally, neutral HOCl and OH− ions were formed. For a dense solution with a 4 : 64 ratio of NaCl to H2O, a singlet oxygen atom was directly combined with Cl− to produce ClO− via an oxygen transfer reaction. The subsequent ab‐initio energy computations indicated the most probable mechanism, where the O atom directly combines with Cl− ion to OCl−. This study provides insights into the fundamental mechanisms of the behavior of dissolved oxygen radicals in aqueous electrolyte solutions.

Structural and electrochemical properties of Cu doped NiCo2O4 prepared by microwave hydrothermal method

A series of NiCuxCo2-xO4 with x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10 samples were synthesized using microwave hydrothermal method (MH). The structural, microstructural, and spectroscopic characterizations were carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). All the samples are in Cubic phase with space group Fd3m. The samples x ≥ 0.08 shows some impurity phases. The average grain size for all the samples is in the range of 30–55 nm. The specific capacitance (Csp) 250.66 Fg−1, energy density (E) 22.22 Whkg−1 and power density (P) 757.80 Wkg−1 was observed for x = 0.06 with a mass loading of 7 mgcm−2 and in 2 M aqueous KOH electrolyte solution at a current density of 1 Ag−1. Therefore, the Cu doping with x = 0.06 shows good redox activities and hence can be used in electric batteries and supercapacitors.

Use of Pitzer’s model to calculate thermodynamic properties of aqueous electrolyte solutions of sulfuric acid

Over this study we calculated the coefficients of activity of different species and the concentration in the presence of H+ ion when H2SO4 is alone in solution. The coefficient of average activity was calculated using the model of Pitzer. The constants of dissociation of these reactions were calculated from the values of the free standard energies. The reaction reaches a balance while producing H+, HSO4- and SO42-. The research reported here concentrated on the effect of some important operational parameters on dissolution process. The parameters were investigated and their ranges are as follows: initial molality ranging from 0.001 to 5 mol.kg-1 and temperatures between 25 and 200°C. Fortran 90 (Mathematical formula translating system) was used to perform all mathematical calculations. The numerical code can be used to calculate the molarity of ion H+ of an aqueous solution of sulfuric acid. The objective of the present work is to optimize the parameters of leaching such as activity of sulfuric acid on the dissociation phenomena.

Molecular simulations of alkali metal halide hydrates

It has been known that classical molecular simulations of concentrated aqueous electrolyte solutions are limited by the ability of microscopic models (force fields) to predict the correct values of salt solubility, which also requires their ability to faithfully model crystalline salts. Previous simulation studies have often focused on the solubility of anhydrous crystalline salts, but virtually never on crystalline hydrates, except for hydrohalite, NaCl⋅2H2O, despite there are at least 23 experimentally known different hydrates that can precipitate from alkali-halide solutions. This work attempts to fill this gap in hydrate simulation studies by systematically investigating the ability of the best force fields selected to qualitatively capture the stability of the individual phases of various alkali-halide hydrates and to quantitatively predict their lattice parameters. First, we show that the nonpolarizable force fields studied often fail to model hydrates containing the Li+ cations, whereas the polarizable force fields recently refined by us are able to model all the hydrates except for LiCl⋅H2O. Second, we further refine our FFs for Li+ to yield stable LiCl⋅H2O. Third, our simulations clarify the positions of the Li+ cations in the β phases of LiBr⋅H2O and LiI⋅H2O, whose distributions were previously described only as stochastic. As a byproduct, a simple and reliable simulation methodology suitable also for complex polarizable models and nonorthorhombic crystal lattices is proposed and tested, based on simulations of finite crystals floating in vacuum.

Mass transport limitations in concentrated aqueous electrolyte solutions: Theoretical and experimental study of the hydrogen–bromine flow battery electrolyte

Modelling and simulation is a powerful tool to support the development of novel flow cells such as electrolysers and flow batteries. Electrolytes employed in such cells often consist of aqueous solutions of highly concentrated solutes at elevated temperatures. Such conditions pose numerous challenges in conventional model parametrisation because of non-ideal behaviour of the electrolytes. The aim of this work is to study mass transport of electroactive species in highly-concentrated media.We selected the hydrogen–bromine flow battery posolyte, HBr (aq) and Br2, as an exemplary flow battery electrolyte and we leveraged chronoamperometric techniques involving ultramicroelectrodes to study diffusion and migration of bromide and bromine at high concentration and temperature. We successfully simulated the current densities of HBr/Br2 redox reactions in solutions up to 8 mol L–1 using advanced mass transport theory which agreed well with the results obtained with ultramicroelectrodes.While uncharged species transport (Br2) can be credibly modelled using conventional theories such as Fick’s law, charged species (Br–) require special treatment as the diffusion coefficient vary with concentration up to 50 % with respect to the limiting value at infinite dilution. The transport of charged species without added supporting electrolyte occurs via both migration and diffusion and the contribution of migration current may be up to 50 % of the total current. At HBr concentration > 0.6 mol L–1 migration appears to be suppressed due to the “self-screening” effect of the electrolyte.Proper experimental electrolyte characterisation under operating conditions similar to the actual flow cell applications is indispensable to establish predictive models and digital twins of electrochemical devices. Straightforward transfer of concepts known in electro-analytical chemistry to flow cells modelling may lead to erroneous simulations or model overfitting.

Self-Activated SiO2/C Nanocomposite from Silica-Rich Marine Mucilage for Supercapacitor Applications

Bioresources are continually being explored as potential precursors for sustainable supercapacitor electrode materials. In this work, diatom-rich marine mucilage (MM) collected from the Marmara Sea, Turkey was used as a biomass source and converted to SiO2/Carbon nanocomposite (SiO 2 /C) through pyrolysis and acid etching. Diatom frustules acted as a highly porous silica source while algal organic matter delivered the carbonaceous part of the composite. The natural salts found in marine mucilage served as a self-activating agent and avoided the use of corrosive chemicals for the development of pores. The SiO 2 /C exhibited a competitive capacitance of 210 F g–1 at a current rate of 0.5 A g–1 in 1 M sodium sulfate (Na2SO4) aqueous electrolyte solution. The porous and durable silica skeleton improved capacitance by expanding the electrode/electrolyte interface, and the interconnected hierarchical pores ensured high electrochemical stability during long-term cycling. The mucilage-derived nanocomposite retained 80% and 70% of its capacitance after 4000 and 10,000 charge–discharge cycles, respectively. This work presents a potential solution for the management of marine mucilage by converting it into a high-value electrode material.

The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond.

Quantum sensing with spin defects in diamond, such as the nitrogen vacancy (NV) center, enables the detection of various chemical species on the nanoscale. Molecules or ions with unpaired electronic spins are typically probed by their influence on the NV center's spin relaxation. Whereas it is well-known that paramagnetic ions reduce the NV center's relaxation time (T1), here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the T1 time of near-surface NV center ensembles compared to pure water. To elucidate the underlying mechanism of this surprising effect, single and double quantum NV experiments are performed, which indicate a reduction of magnetic and electric noise in the presence of diamagnetic electrolytes. In combination with ab initio simulations, we propose that a change in the interfacial band bending due to the formation of an electric double layer leads to a stabilization of fluctuating charges at the interface of an oxidized diamond. This work not only helps to understand noise sources in quantum systems but could also broaden the application space of quantum sensors toward electrolyte sensing in cell biology, neuroscience, and electrochemistry.

Thermal Conductivity of Ternary Aqueous H2O + Li2SO4 + Zn(NO3)2 Solutions at High Temperatures and Pressures

The thermal conductivity of ternary aqueous H2O + Li2SO4 + Zn(NO3)2 solutions at high temperatures (from 298 to 573 K) and at two isobars of 10 and 30 MPa is reported in the present work. The measurements were made for five solutions with concentration ratios of Li2SO4/Zn(NO3)2 of 1:1, 2:1, 1:2, 1.75:2.00, and 0.7:0.3 (mol·kg–1) using a concentric cylinder (steady) method. The combined expanded relative uncertainties of thermal conductivity, pressure, concentration, and the absolute uncertainty of temperature measurements (at 0.95 confidence level, a coverage factor of k = 2) were estimated to be less than 0.02, 0.0005, 0.0002, and 20 mK, respectively. The measured data were used to confirm the reliability, accuracy, and predictive capability of the existing correlation methods for multicomponent aqueous electrolyte solutions. It was shown that the temperature and pressure behavior of the aqueous solutions is controlled by the water. The effect of Li2SO4 and Zn(NO3)2 concentrations on the thermal conductivity of H2O + Li2SO4 + Zn(NO3)2 solutions at various temperatures and pressures has been studied.

Controlling the Molar Ratios of Cation to Anion of Precursors for High Performance Capacitive Properties of MnO2 Hybridized Carbon-Based Materials Electrode

Controlling the cation to anion (Mn2+/MnO4−) molar ratios of the precursors was used to obtain a highly performance capacitive properties of nanostructural MnO2 hybridized carbon-based materials on nickel foam (NF) through successive ionic layer adsorption and reaction technology. SEM, XRD, BET, and XPS analyses are utilized to investigate the influence of cation/anion molar ratios of precursors on the as-obtained MnO2 electrode materials. At a lower molar ratio of cation/anion of 1, the prepared manganese oxide deposited on the NF with obvious δ-MnO2 phase. The average pore size distribution of BET analysis of the as-obtained δ-MnO2 is about 4.6 nm, the specific surface area is 155.7 m2 g−1, exhibiting a mesoporous structure. However, when the molar ratio of cation/anion is higher than 5, the deposited film produced by the reaction exhibits a γ-MnO2 crystal phase. The capacitance of δ-MnO2/NF electrode is 280 F g−1 at 1 A g−1 in a 1 M Na2SO4 aqueous electrolyte solution. In addition, reduced graphene oxide (rGO) mixed with multi-wall carbon nanotube (MWCNT) was added to synthesize γ-MnO2/rGO-MWCNT/NF electrode, which has a high capacitance of 377.4 F g−1 under the charge/discharge current density at 1 A g−1.

High-temperature and high-pressure thermodynamic properties of aqueous zinc sulfate solutions

A new method has been proposed to estimate the osmotic and activity coefficients of aqueous electrolyte solutions under non-ambient conditions on the basis of the Pitzer ion-interaction approach using the volumetric data. Following this method, a compact set of Pitzer ion-interaction parameters, valid within the temperature range 293.15 - 373.15 K, and pressure range 0.1 - 10 MPa have been obtained for aqueous ZnSO4 solutions. These parameters helped evaluate the osmotic and activity coefficients of this system to 3.0 mol∙kg−1. The pressure dependences of osmotic and activity coefficients have also been estimated using the equations derived by Rogers and Pitzer (1982). The results obtained by the method proposed in this research agree very well with those calculated using Rogers and Pitzer approach. The advantage of the method proposed here is that it directly provides a compact set of Pitzer ion-interaction parameters required for describing the osmotic and activity coefficients, in contrast to the other method. This study also provided important insight into the ion-association behavior of aqueous ZnSO4 solution with respect to temperature and pressure. Further, the apparent molar enthalpies have been estimated using appropriate temperature derivatives of the Pitzer ion-interaction parameters obtained in this study.

Modification of Single-Walled Carbon Nanotube Networks Anodes for Application in Aqueous Lithium-Ion Batteries

The changes in global energy trends and the high demand for secondary power sources, have led to a renewed interest in aqueous lithium-ion batteries. The selection of a suitable anode for aqueous media is a difficult task because many anode materials have poor cycling performance due to side reactions with water or dissolved oxygen. An effective method for improving the characteristics of anodes in aqueous electrolyte solutions is adding carbon nanotubes (CNTs) to the electrode materials. For a better comprehension of the mechanism of energy accumulation and the reasons for the loss of capacity during the cycling of chemical current sources, it is necessary to understand the behaviour of the constituent components of the anodes. Although CNTs are well studied theoretically and experimentally, there is no information about their behaviour in aqueous solutions during the intercalation/deintercalation of lithium ions. This work reveals the mechanism of operation of untreated and annealed single-walled carbon nanotubes (SWCNT) anodes during the intercalation/deintercalation of Li+ from an aqueous 5 M LiNO3 electrolyte. The presence of -COOH groups on the surface of untreated SWCNTs is the reason for the low discharge capacity of the SWCNT anode in 5 M LiNO3 (3 mAh g−1 after 100 cycles). Their performance was improved by annealing in a hydrogen atmosphere, which selectively removed the -COOH groups and increased the discharge capacity of SWCNT by a factor of 10 (33 mAh g−1 after 100 cycles).

ОБЧИСЛЕННЯ ПОПРАВОК ДИФЕРЕНЦІЙНИХ ДВОХЕЛЕКТРОДНИХ КОНДУКТОМЕТРИЧНИХ КОМІРОК ІЗ РОЗРАХУНКОВОЮ КОНСТАНТОЮ

The object of study is a differential two-electrode conductivity cell with a calculated constant, designed for the practical realization of the unit of length – a meter in national measurement standards of electrolytic conductivity of liquids. The differential cell consists of two tubes of the same diameter but different lengths. The article presents an idealized model for calculating cell constants and determining electrolytic conductivity. The presence of holes for filling the cell, caus-ing a distortion of the uniformity of the current density distribution in the middle of the cell, and a significant dielectric permeability of aqueous solutions lead to significant errors when measuring the resistance of liquid columns. The arti-cle presents mathematical expressions for calculating two types of corrections. First, the correction caused by the dis-tortion of the field uniformity due to the presence of the inlet and outlet holes of the cell. Secondly, the correction caused by the presence of bias currents in aqueous solutions of electrolytes. The appearance of a differential cell based on two tubes with a diameter of 9 mm and a length of 50 and 100 mm, respectively, which is being tested as a part of the national measurement standard of Ukraine, is given. References 7, figures 5, tables 2.

Fabrication of high energy density symmetric polyaniline/functionalized multiwalled carbon nanotubes supercapacitor device with swift charge transport in different electrolytic mediums

This work is conducted with aim of enhancing the energy density and cycling performance of PANI/fMWCNTs within a very low molar concentration of the mixed acidic/neutral electrolyte where not only the medium, but the structure too facilitates easy charge transfer because of increased ionic mobility. A promising optimized flower-like morphology of symmetric supercapacitor electrode of functionalized Multi-walled Carbon Nanotubes (f-MWCNTs) with Polyaniline (PANI) composites are prepared by in-situ chemical oxidative polymerization method with different doping concentrations of f-MWCNTs (5 wt% and 10 wt%). The synthesized PANI/f-MWCNTs (5 wt%) composite provides a large surface area (133.55 m2/g), high porosity (11.24 nm), low internal resistance (6.16 Ω) and charge transfer property (2.74 Ω) between the electrode within a mixed (0.1 M H2SO4 + 0.1 M Na2SO4) electrolyte interface. To overcome the cracking/swelling of PANI during electrochemical performance, the electrodes of PANI are incorporated with different doping concentration of f-MWCNT (5 wt% or 10 wt%). With a specific capacitance of 867 F g−1 at 10 mV/s, energy density ~77.0 Whkg−1 and power density ~801 Wkg−1, PFC1 nanocomposite indicates ~20 % more stability than pure PANI. Also, the nanocomposite electrode cell shows a columbic efficiency of 97 % and 98.5 % of its initial value of specific capacitance after 10,000 GCD cycles in a mixed acidic/neutral (0.1 M H2SO4 + 0.1 M Na2SO4) aqueous electrolytic solution.

High efficiency Al-based multicomponent composites for low-temperature hydrogen production and its hydrolysis mechanism

In this study, a series of multicomponent Al-based composites were prepared by mechanical ball milling of Al powder, low-melting-point metals (Ga, In, Sn), NaCl, g-C3N4 and carbon nanospheres. The composites were thoroughly analyzed using state-of-art characterization techniques with respect to morphologies, structures, chemical compositions, thermal, and electrochemical properties, etc. The hydrogen production performance of Al-based composites has been investigated under three low-temperature conditions, including deionized water (275.15 K), snow-melting agent aqueous solution (253.15–273.15 K), and aqueous electrolyte solutions (KOH, FeCl3, CaCl2, ethylene glycol) at 233.15 K. The best hydrogen production performance with a hydrogen yield of 1133 mL·gAl−1 at 275.15 K and 1095 mL·gAl−1 at 253.15 K was obtained. Especially, no induction time was observed in either condition. Even at 233.15 K in KOH solution, its reaction induction time was around 6 s and final hydrogen yield reached 1057 mL·gAl−1. The observed hydrogen production capabilities of composites have been analyzed and understood in depth, and a tentative multicomponent activation mechanism for low-temperature hydrogen production was proposed as well.

Development of rechargeable high-energy hybrid zinc-iodine aqueous batteries exploiting reversible chlorine-based redox reaction

The chlorine-based redox reaction (ClRR) could be exploited to produce secondary high-energy aqueous batteries. However, efficient and reversible ClRR is challenging, and it is affected by parasitic reactions such as Cl2 gas evolution and electrolyte decomposition. Here, to circumvent these issues, we use iodine as positive electrode active material in a battery system comprising a Zn metal negative electrode and a concentrated (e.g., 30 molal) ZnCl2 aqueous electrolyte solution. During cell discharge, the iodine at the positive electrode interacts with the chloride ions from the electrolyte to enable interhalogen coordinating chemistry and forming ICl3-. In this way, the redox-active halogen atoms allow a reversible three-electrons transfer reaction which, at the lab-scale cell level, translates into an initial specific discharge capacity of 612.5 mAh gI2−1 at 0.5 A gI2−1 and 25 °C (corresponding to a calculated specific energy of 905 Wh kgI2−1). We also report the assembly and testing of a Zn | |Cl-I pouch cell prototype demonstrating a discharge capacity retention of about 74% after 300 cycles at 200 mA and 25 °C (final discharge capacity of about 92 mAh).

Investigation of Plasma Electric Oxide Coating Formed on the Prototype Samples of Aluminum Plates Made of 1050 Grade

Today, the technology of hardening the surface layers of parts and the creation of protective coatings on the surface with high physical, mechanical and chemical properties are especially efficient in many industries. The article presents the studies of the most promising innovative technology for surface hardening of 1050 grade aluminum plates by plasma electrolytic oxidation method. This method allows to obtain the materials with an ultra-high melting point, high hardness, and wear resistance. Possible conditions and mechanisms for the formation of protective layers on the surface of aluminum plates to improve reliability and safety in the production of chemical industry products are considered. The influence was studied concerning the main technological parameters (alloying elements, electrical parameters, electrolyte composition) on the properties and structure of oxide ceramic coatings. The qualitative characteristics of the finished products from aluminum alloys and the surface layer of the samples showed the efficiency of the plasma electrolytic oxidation technology, which allows to obtain ceramic coatings with increased hardness, wear and corrosion resistance, and strength. When processing by plasma electrolytic oxidation in an aqueous electrolyte solution, all the industrial safety requirements are met. An alternative approach to processing by plasma electrolytic oxidation is considered, in which 1050 grade aluminum plates were used in a molten nitrate salt at a temperature of 280 °C. The microstructure, phase, chemical composition, and microhardness were studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and microhardness tests. The formed coating was found to be free from electrolyte contamination, cracks and pinholes commonly found in coatings formed during plasma electrolytic oxidation treatment in an aqueous electrolyte solution.

Mechanical and corrosion-resistant coatings prepared on AZ63 Mg alloy by plasma electrolytic oxidation

Using the plasma electrolytic oxidation (PEO) process conducted in galvanostatic mode at a constant current density of 0.2 A/cm2 in aqueous aluminate, silicate, and phosphate electrolytic solutions, we successfully produced magnesium oxide coatings (up to 182 μm) on AZ63 alloy. Structural, chemical composition, and morphological evaluations of the coatings were investigated by XRD, XPS, and SEM. The results indicated that mixed-crystalline magnesium oxide grown on pure AZ63 alloy forms porous 8–30 μm PEO layers with increased hardness and improved corrosion resistance. At the same time, values of 26.9 GPa and 2.0 μA/cm2 yield noticeable performance characteristics.

Sulfolane-containing aqueous electrolyte solutions for producing efficient ampere-hour-level zinc metal battery pouch cells

Aqueous zinc metal batteries are appealing candidates for grid energy storage. However, the inadequate electrochemical reversibility of the zinc metal negative electrode inhibits the battery performance at the large-scale cell level. Here, we develop practical ampere-hour-scale aqueous Zn metal battery pouch cells by engineering the electrolyte solution. After identifying the proton reduction as the primary source of H2 evolution during Zn metal electrodeposition, we design an electrolyte solution containing reverse micelle structures where sulfolane molecules constrain water in nanodomains to hinder proton reduction. Furthermore, we develop and validate an electrochemical testing protocol to comprehensively evaluate the cell’s coulombic efficiency and zinc metal electrode cycle life. Finally, using the reverse micelle electrolyte, we assemble and test a practical ampere-hour Zn||Zn0.25V2O5•nH2O multi-layer pouch cell capable of delivering an initial energy density of 70 Wh L−1 (based on the volume of the cell components), capacity retention of about 80% after 390 cycles at 56 mA g−1cathode and ~25 °C and prolonged cycling for 5 months at 56 mA g−1cathode and ~25 °C.

Fast Ion Transfer Associated with Dehydration and Modulation of Hydration Structure in Electric Double-Layer Capacitors Using Molecular Dynamics Simulations and Experiments

Carbon materials, such as graphite and activated carbon, have been widely used as electrodes in batteries and electric double-layer capacitors (EDLCs). Graphene, which has an extremely thin sheet-like structure, is considered as a fundamental carbon material. However, it was less investigated as an electrode material than graphite and activated carbons. This is because graphene is a relatively new material and is difficult to handle. However, using graphene electrodes can enhance the performance of nanodevices. Here, the performance of EDLCs based on single-layer and bilayer graphene electrodes in LiCl, NaCl, and KCl aqueous electrolyte solutions was evaluated using cyclic voltammetry, and the charging mechanism was evaluated using molecular dynamics simulations. KCl aqueous solution provided the highest capacitance compared to LiCl and NaCl aqueous solutions in the case of single-layer graphene electrodes. In contrast, the dependence of the capacitance on the ion species was hardly observed in the case of bilayer graphene. This indicates that Li and Na ions also contributed to the capacitances. The high EDLC performance can be attributed to the fast ion transfer promoted by the dehydration and modification of the second hydration shell on the bilayer graphene because of the relatively strong interaction of ions with the bilayer graphene.

On the Transition-State theory approach to the Jones-Dole’s viscosity B-coefficient: A novel molecular-based interpretation, assessment of its implications, and experimental evidence

We report a novel statistical mechanics interpretation of the frequently invoked Feakins’ transition-state theory approach to the Jones − Dole’s Bi- coefficient, and its temperature–pressure derivatives, in the context of their suitability as structure-making/breaking markers for the behavior of solutes in dilute electrolyte solutions. We provide additional rigorous molecular-based thermodynamic evidence to highlight the non-uniqueness of the Bi- and ∂Bi/∂TP- responses to the solute–solvent intermolecular asymmetries in these systems, and to illustrate the resulting outcomes according to a fundamental structure-making/breaking function involving the available experimental data for aqueous electrolyte solutions. Finally, we discuss the implications of our findings while providing a cautionary outlook on the pervasive use of the viscosity-based structural markers.