Nonaqueous Electrolyte Solutions Research Articles

Preparation and characterisation of manganese–TiO2 nanocomposites for solar water splitting

In this paper, Mn–TiO2 nanocomposite films were successfully fabricated by the anodising of pure titanium plates in a non-aqueous electrolyte solution containing manganese ions. For the first time, potassium permanganate was used as the Mn source in anodising process. The morphology, structure and composition of these nanocomposite films were characterised by scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, elemental mapping and UV–Vis absorption spectrum respectively. Characterisation of the as prepared films indicated that potassium permanganate concentration in anodising solution significantly influenced the morphology of the surface, structure and photocatalytic activity of these films. So, to access a specific morphology, a proper concentration of KMnO4 is required. Photocatalytic activity of samples was evaluated by testing the water splitting. The Mn–TiO2–1 nanocomposite exhibited better photocatalytic activity than did the bare TiO2 film and other Mn–TiO2 nanocomposites.

Electrochemical Properties of Lithium Air Secondary Batteries with Nonaqueous Electrolyte Solution Containing Manganese-Based Organic Complexes As Solution-Phase Catalysts

Introduction Lithium air secondary batteries (LABs) have the highest theoretical energy density among secondary batteries reported so far. However, major problems are poor cycle characteristics and large discharge/charge overpotential. To improve these properties, various kinds of solid-phase catalysts loaded into air electrodes have been reported [1]. One of the significant problems is that the solid-phase catalysts become inactivated gradually because discharge product Li2O2 accumulats on the surface of the air electrode as a result of imperfect decomposition during the charging process after a large number of cycles [2]. Since a solution-phase catalyst dissolved in the electrolyte solution could work stably during the cycles, it would overcome these issues associated with the solid-phase catalyst. Recently, as solution-phase catalysts, transition metal complexes such as manganese phthalocyanine (MnPc) [3], cobalt phthalocyanine (CoPc) [4] have been reported [3-5]. It is necessary to investigate the electrocatalytic activities for various kinds of solution-phase catalysts because the catalytic mechanism is unclear. We focused on manganese-containing salen-type complexes (MnSl) as a new solution-phase catalyst (Fig. 1). The electrochemical properties would be improved by involving oxygen adsorbed on the central Mn-ion site with the reaction over the air electrode [6]. Here, we report discharge/charge properties in LABs using MnSl as solution-phase catalyst. Experimental MnSl [(R,R)-(−)-N,N-Bis(3,5–di–tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)chloride] and MnPc [manganese(II) phthalocyanine] as a reference catalyst were purchased from Sigma-Aldrich Co. LLC. A 2.0 wt% of both catalysts vacuum-dried at 90 °C for 12 h was added to an electrolyte solution of 1.0 mol/l LiTFSA / TEGDME (tetraglyme). Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) was used as the air electrode. A Li metal sheet was used as the negative electrode. The experimental LAB cell was assembled using a commercial cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were conducted under galvanostatic condition at 200 mA/g in dry air atmosphere. Current density and discharge/charge capacity were normalized by the mass of the reaction layer (KB + PVdF) in the air electrode. Results and discussion Figure 2 shows the first discharge/charge curves of LAB cells with MnSl, with MnPc catalyst, and whithout the solution-phase catalysts. The cells with MnSl catalyst and MnPc show higher average discharge voltages of 2.64 and 2.55 V and larger discharge capacities of 4903 and 3309 mAh/g, respectively, compared to the cell without the catalysts. In addition, the cell with MnSl catalyst shows lower average charge voltage of 3.93 V and larger charge capacity of 6390 mAh/g than the other cells. These results suggest that the solution-phase catalyst could have electrocatalytic activities. Moreover, the MnSl catalyst exhibits rather higher activity than the conventional catalyst, MnPc. However, with the MnSl catalyst, the first charge capacity was larger than the discharge capacity by about 2000 mAh/g. This behavior suggests that some side reactions might co-occur with the charge (oxygen evolution) reaction in the high-voltage region above 4.0 V. Figure 3 shows SEM images of the air electrodes as prepared and after the first discharge without and with MnSl catalyst. Compared to the electrode surface as prepared shown in Fig. 3(a), the fine sub-micrometer discharge products with similar structures were deposited in a part of the electrode surface both without and with MnSl catalyst as shown in Fig. 3(b) and (c). In particular, there were spherical micrometer structures of the discharge products only with MnSl as shown in Fig. 3(c). MnSl catalyst three-dimensionally catalyzed deposition and decomposition of Li2O2 over the air electrode, which led to significant increase in the capacity. According to the above results, the MnSl catalyst exhibited higher activities than MnPc. One of the reasons might be that the oxygen adsorption on the central metal Mn containing N-Mn-N bonds is stronger in MnSl than in MnPc. At the IMLB meeting, we will also discuss the cycle characteristics of LABs with the MnSl catalyst.

Correlation Between Free Volume and Ionic Conductivity of Non-Aqueous Lithium Battery Electrolyte Solutions over a Wide Concentration Range

There is little known about the transport behavior of ions in electrolyte solutions at very high concentration and there is currently no satisfactory theory/equation to describe it [1,2]. In this work electrolyte solutions of lithium salts of various anions: [(CF3SO2)2N]°¥, (TFSI), [PF6]°¥ and [BF4]°¥ dissolved in polar solvents and solvent mixtures up to saturation have been prepared. The solvents are propylene carbonate (PC), acetonitrile (ACN), adiponitrile (ADN) and ethylene carbonate:dimethyl carbonate (1:1 v/v) (EC:DMC). The Ionic conductivity and density of the electrolyte solutions have been measured over a wide concentration range (up to 5.4 M). The specific ionic conductivity (κ) vs. C plots show a non-ideal behavior with very strong dependence on concentration (a typical Gaussian-like function) while the molar conductivity (Λ) vs. C plots show an exponential decrease similar to the behavior of weak electrolyte solutions. The conductivity vs. concentration data were fitted to various equations of known models (Debye-Hückel-Onsager’s square-rate law: Λ vs. C^1/2, Cubic-root law: Λ vs. C^3, Casteel-Amis equation: κ/κmax vs. C/Cmax , or the corresponding state-law equation based on the pseudo-lattice theory: κ/κmax vs. C/Cmax .The models show good fit only in the low concentration, with the last two models fail to show universal behavior beyond the concentration of maximum conductivity, Cmax. However, we have derived a new, semi-empirical equation that shows a very good fit over the whole concentration range and can correlate molar ionic conductivity to changes in free volume within the liquid solution: Λ = A' exp [-B C], B =-γ Vo/Vf, and Vf = V-Vo Where Vƒ is the free volume which can be calculated from the difference between the measured volume, V, of the liquid (from density measurements) and the Van der Waal “molecular “volume, Vo, which can be obtained from XRD data at low temperatures or calculated using chemical models. C is concentration, A’ is a pre-exponential factor that can be related to limiting molar conductivity and square root of temerature. γ is a correction factor for overlapping holes with a value between 0.5 and 1. These preliminary results are encouraging in bringing us closer to a free volume-based approach to explain the behavior of ionic conductivity of electrolyte solutions at very high concentrations. However, more work is underway to study more solutions with different chemical and physical properties, including aqueous solutions, to test the validity of this approach. [1] S. I. Smedley, The Interpretation of Ionic Conductivity in Liquids, Springer, 1980 [2] C.A. Angell, J. Phys. Chem., 70, 1966, 3988

Measurement and modeling of solubility of MgCl2 in 2-methylpropan-2-ol + water + glycerol + MgCl2 system: A solid–liquid equilibrium study based on symmetric eNRTL model and evaluation of thermodynamic functions of solutions

A comprehensive solid–liquid phase equilibrium to measure the solubility of MgCl2 in a 2-methylpropan-2-ol (2M2P)+water+glycerol system is investigated in the present study. The saturation concentrations of MgCl2 at different temperatures are reported for two different binary systems; 2M2P+MgCl2 and glycerol+MgCl2, a ternary system; 2M2P+water+MgCl2 and a quaternary system; 2M2P+water+glycerol+MgCl2 for a temperature range of 303.15–343.15K at the local atmospheric pressure of 94.9kPa. The thermodynamic functions of the solutions viz. Gibbs free energy change, dissolution enthalpy, dissolution entropy and isobaric heat capacity are evaluated by combining the modified Apelblat equation with the Clark–Glew equation to analyze the dissolution process at the molecular level. The molality based solubility product of MgCl2 in the different systems explored presently, is calculated using the mean ionic activity coefficients at different saturation temperatures. The experimental solubility data is fitted to the symmetric eNRTL model embedded in Aspen Plus in order to validate the thermodynamic consistency of the data. The high goodness of fit between experimental and predicted solubility data establishes the accuracy of the symmetric eNRTL model to represent the solid–liquid equilibria of MgCl2 in the 2M2P+water+glycerol+MgCl2 system. Hence, this model can suitably be adopted for the process modeling and simulation of aqueous as well as non-aqueous electrolyte solutions involving MgCl2.

Easy synthesis of titania–tungsten trioxide nanocomposite films by anodising method for solar water splitting

Titania–tungsten trioxide (W-TiO2) films were in situ fabricated by the electrochemical anodising of titanium in a non-aqueous electrolyte solution containing sodium tungstate as the tungsten source. Glycerol was used as a green solvent. The morphology, structure and composition of synthesised films were characterised by SEM, X-ray diffraction, energy dispersive X-ray spectroscopy and UV–visible respectively. The results indicated that sodium tungstate concentration in anodising solution significantly influenced the morphology of the surface, structure and photocatalytic activity of these films. So to access an optimum morphology, a proper concentration of sodium tungstate is required. Photocatalytic activity of samples was evaluated by testing the solar water splitting. The W1-TiO2 sample exhibited better photocatalytic activity than the bare TiO2 film and other W-TiO2 samples.

Tuning the Stability of Organic Active Materials for Nonaqueous Redox Flow Batteries via Reversible, Electrochemically Mediated Li+ Coordination

We describe an electrochemically mediated interaction between Li+ and a promising active material for nonaqueous redox flow batteries (RFBs), 1,2,3,4-tetrahydro-6,7-dimethoxy-1,1,4,4-tetramethylnaphthalene (TDT), and the impact of this structural interaction on material stability during voltammetric cycling. TDT could be an advantageous organic positive electrolyte material for nonaqueous RFBs due to its high oxidation potential, 4.21 V vs Li/Li+, and solubility of at least 1.0 M in select electrolytes. Although results from voltammetry suggest TDT displays Nernstian reversibility in many nonaqueous electrolyte solutions, bulk electrolysis reveals significant degradation in all electrolytes studied, the extent of which depends on the electrolyte solution composition. Results of subtractively normalized in situ Fourier transform infrared spectroscopy (SNIFTIRS) confirm that TDT undergoes reversible structural changes during cyclic voltammetry in propylene carbonate and 1,2-dimethoxyethane solutions contain...

Transport Phenomena of Nonaqueous Electrolyte Solutions at High Concentrations: A Comparison between the Li- and Na-Systems

In order to understand the solution chemistry of electrolyte solutions at concentrations far from infinite dilution, the solution density, viscosity and ionic conductivity of Li- and Na-TFSI dissolved in GBL and PC were measured at 0.1 ⩽ C/mol · dm− 3 ⩽ 2.0 and 278 ⩽ T/K ⩽ 328, where TFSI = bis(trifluoromethanesulfonyl)imide, GBL = γ-butyrolactone and PC = propylene carbonate. The results are compared to those of previously determined perchlorate salts. The partial molar volume of the solute, derived from the density, confirmed that the Na-systems occupy more volume in the solution than the Li-systems. On the other hand, the viscosity and ionic conductivity suggested that the Na-systems are more fluid and conductive than the Li-systems. The relative viscosity vs. the molarity follows a modified empirical Jones-Dole equation. The molar conductivity linearly decreased with respect to the cube-root of the molarity, which was analyzed by the pseudolattice model. The Raman spectroscopy revealed that, while the solvation number is comparable at 1-2 for either the Li- or Na-systems, Li+ is more tightly bound to the solvent molecules than Na+. The higher fluidity and conductivity of the Na-systems than those of the Li-systems result from the less occurrence of the solvent-shared ion pairs in the former than in the latter.

Thermodynamic parameters of the first order in low-concentration binary alloys

Thermodynamic parameters of the first order (Wagner interaction parameter e2(2), enthalpy, and entropy parameter σ2(2)) in low-concentration liquid binary alloys are considered. The values of these parameters for 32 binary systems are estimated from experimental data. A system of classification is proposed for the obtained data. These data are compared to similar data for aqueous solutions of nonelectrolytes. A qualitative explanation of the obtained differences is given.

Electrochemical performance of activated carbons prepared from rice husk in different types of non-aqueous electrolytes

Activated carbons (ACs) prepared from rice husk (RH), an agricultural byproduct, have mesoporosity that is obtainable from leaching of the mineral component of silica. To verify the suitability of RH-derived ACs for the use of electrode materials of electrical double-layer capacitors, we evaluated the electrochemical performance of three RH-derived ACs (two micro- and mesoporous ACs and one mesoporous AC). Evaluation was done by using the non-aqueous ionic electrolyte solutions 1 mol dm−3 triethylmethyl ammonium tetrafluoroborate/propylene carbonate (PC) solution, 1.5 mol dm−3 spiro-(1,1′)-bipyrrolidinium tetrafluoroborate/PC (SBP·BF4/PC) solution, and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm·BF4). Under low voltage scan rate (1 mV s−1) and low current density (<1 mA cm−2), mesoporous AC, which had the highest specific surface area, showed the highest specific capacitance (120 F g−1) in EMIm·BF4. However, its specific capacitance considerably decreased because of the increase in scan rate and current density. Under high scan rate (10 and 100 mV s−1) and high current density (>10 mA cm−2), micro- and mesoporous AC in 1.5 mol dm−3 SBP·BF4/PC showed the highest specific capacitance and highest retention of specific capacitance, even though its specific surface area was not the highest. Mesoporous AC showed voltage-dependent specific capacitance, indicating that ionic transport in the mesoporous structure was sensitive to electric field. It was finally shown that micro- and mesoporosity developed by utilizing natural structure and composition of RH was useful for the electrode materials of advanced electrical double-layer capacitors requiring more viscous non-aqueous electrolytes.

Calculation and fabrication of two-dimensional complete photonic bandgap structures composed of rutile TiO2 single crystals in air/liquid

Photoelectrochemical applications of photonic crystals are gathering great interests both from physicists and chemists. Here, we theoretically and experimentally present two-dimensional photonic bandgap (2D-PBG) structures based on rutile titanium dioxide (TiO2) single crystal that is a famous material because of the photoelectrochemical ability. The structures were the arrays of hollow hexagonal rutile TiO2 pillars in contact with air or a typical nonaqueous electrolyte solution, acetonitrile. Since the TiO2 refractive indices exhibit a strong dispersive behavior, the bandgap width was discussed from the viewpoint of the refractive index map that would be helpful for the real application of this structure. The 2D-PBG structures for both infrared light and visible light were fabricated by our established lithography technique for rutile TiO2 with and without Nb doping, i.e., photocatalytic TiO2 and high electron conductive TiO2, respectively. These structures show characteristic absorbance peaks or reflectance dips at wavelengths predicted by our theoretical calculations.

THERMODYNAMIC PROPERTIES OF NONAQUEOUS SINGLE SALT SOLUTIONS USING THE Q-ELECTROLATTICE EQUATION OF STATE

The correlation of thermodynamic properties of nonaqueous electrolyte solutions is relevant to design and operation of many chemical processes, as in fertilizer production and the pharmaceutical industry. In this work, the Q-electrolattice equation of state (EOS) is used to model vapor pressure, mean ionic activity coefficient, osmotic coefficient, and liquid density of sixteen methanol and ten ethanol solutions containing single strong 1:1 and 2:1 salts. The Q-electrolattice comprises the lattice-based Mattedi-Tavares-Castier (MTC) EOS, the Born term and the explicit MSA term. The model requires two adjustable parameters per ion, namely the ionic diameter and the solvent-ion interaction energy. Predictions of osmotic coefficient at 298.15 K and liquid density at different temperatures are also presented.

Parameter Interpretation and Reduction for a Unified Statistical Mechanical Surface Tension Model.

Surface properties of aqueous solutions are important for environments as diverse as atmospheric aerosols and biocellular membranes. Previously, we developed a surface tension model for both electrolyte and nonelectrolyte aqueous solutions across the entire solute concentration range (Wexler and Dutcher, J. Phys. Chem. Lett. 2013, 4, 1723-1726). The model differentiated between adsorption of solute molecules in the bulk and surface of solution using the statistical mechanics of multilayer sorption solution model of Dutcher et al. (J. Phys. Chem. A 2013, 117, 3198-3213). The parameters in the model had physicochemical interpretations, but remained largely empirical. In the current work, these parameters are related to solute molecular properties in aqueous solutions. For nonelectrolytes, sorption tendencies suggest a strong relation with molecular size and functional group spacing. For electrolytes, surface adsorption of ions follows ion surface-bulk partitioning calculations by Pegram and Record (J. Phys. Chem. B 2007, 111, 5411-5417).

A Thermodynamic Perspective for Formation of Solid Electrolyte Interphase in Lithium-Ion Batteries

This work studied the formation and growth of solid electrolyte interphase (SEI) under the assumption that the formation of an SEI layer is associated with the nucleation and growth of disk-like islands at the interface between an active material and nonaqueous electrolyte solution (NES). The supersaturation of NES favors the nucleation of the islands via lowering the Gibbs free energy with the contribution of interfacial energy and mismatch strain enegy to the nucleation process. Using a modified Lifshitz-Slyozov-Wagner model, the growth of the disk-like islands is analyzed. Explicit kinetic estimation for the change of the density and mean dimension of the islands as well as the degree of supersaturation of NES with time is obtained. The degree of supersaturation at time t is found to be proportional to the square root of time.

Electrochemical Characterization of the Bi(111) | 1-Butyl-3-Methylimidazolium Iodide Interface

The need for alternative high energy and power density energy sources has led scientists to study extensively the electrochemical energy conversion/storage devices such as electrochemical double layer capacitors (EDLC), batteries, fuel cells, etc. The values of power and energy densities of EDLC depend strongly on the properties of the electrode material and electrolyte used. EDLCs based on non-aqueous electrolytes or room temperature ionic liquids have a wider region of ideal polarizability (ΔE) compared to those based on traditional aqueous electrolytes 1-4. The wider ΔE in turn improves the energy and power density values of EDLCs. The specific adsorption of halide ions at single crystal electrodes from aqueous and non-aqueous electrolyte solutions has been demonstrated before 5,6. However, the specific adsorption of ions is a complicated process depending on the electrode potential, chemical composition and concentration of halides etc., and the kinetics of formation/rearrangement of electrical double layer between electrode and ionic liquid (IL) needs an extended examination. The main aim of this work was to characterize the adsorption of iodide ions from ionic liquid media without added electrolytes. Electrochemical impedance spectroscopy and cyclic voltammetry methods have been applied to study the electrochemical characteristics of the interface between electrochemically polished Bi(111) electrode and 1-butyl-3-methylimidazolium iodide (BMImI). All measurements were carried out inside a glovebox using a 3-electrode electrochemical cell: a Bi(111) as a working, Pt net as a counter, and Ag wire coated with AgCl as a reference electrodes. Current density (i) vs. electrode potential (E) dependencies show that the system is stable within a wide range of potentials. ΔE is 1.7V i.e from ─1.4 V to +0.3 V (vs. Ag | AgCl), which is extremely high for the bismuth electrode, known as a catalytically, chemically and electrochemically inactive electrode material. The shape of the complex impedance plane, i.e. Nyquist plots, total impedance vs. log frequency and phase angle vs. log frequency dependences depend on the electrode potential applied. The values of phase angle are high as -85o within a wide range of potentials at medium and low ac frequencies. Also the Nyquist plots show only a slight deviation from the ideal capacitive behavior of the system at f < 1 Hz within wide ΔE applied. The values of series differential capacitance (C s) were calculated from Nyquist dependencies at different fixed ac frequencies (C s = (Z''i2πf)−1; where i = √−1and Z’’ is the imaginary part of impedance). In the studied system C s depends strongly on ac frequency and electrode potential, increasing at lower ac frequencies and at anodic range of potentials. C s values for Bi(111) | BMImI are much higher compared to Bi(111) | 1-ethyl-3-methylimidazolium tetrafluoroborate system. References L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, J. Electroanal. Chem., 719, 133 (2014).L. Siinor, C. Siimenson, K. Lust, E. Lust, Electrochem. Commun. 35, 5 (2013).R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochim. Acta, 85, 139 (2012)., T. Tooming, T. Thomberg, L. Siinor, K. Tõnurist, A. Jänes, E. Lust, J. Electrochem. Soc. 161, A222 (2014).L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem., 601, 39 (2007).M. Väärtnõu, E. Lust, J. Electroanal. Chem., 565, 211 (2004). Acknowledgements Estonian Energy Technology Program project SLOKT10209T, Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117, Functional Micro/Mesoporous Nanomaterials for Novel Energy Conversion and Storage Systems IUT 20-13, Functionalization of carbon nanomaterials: synthesis, characterization and application SLOKT12180T.

In Appreciation of Professor Robert H. Wood

This special issue of the Journal of Solution Chemistry is dedicated to Professor Robert H. Wood on the occasion of his 80th birthday. It contains work presented at the 68th Calorimetry Conference in Atlantic City, New Jersey, USA, August 18–22, 2013, that is in the symposium on Aqueous Solutions, Fluid Mixtures and Polymers—A Tribute to Robert H. Wood on the Occasion of His 80th Birthday. The different scientific topics of the lectures made for an interesting and memorable symposium for all in attendance; it was highly appreciated by Professor Wood, though unexpected health problems unfortunately precluded his participation. Because of Bob Wood’s prominent role in solution chemistry, a Festschrift in the Journal of Solution Chemistry is most appropriate, and I was greatly pleased by the enthusiastic acceptance of my suggestions concerning Symposium and Festschrift by Professor Jean-Pierre E. Grolier (Universite Blaise Pascal, Clermont-Ferrand, France), Symposium Co-Organizer, by Professor David P. Remeta (Rutgers University, New Brunswick, NJ, USA), Program Chair for CALCON 2013, and by the Editors-in-Chief of the Journal of Solution Chemistry, Professor Joseph A. Rard and Professor W. Earle Waghorne. Bob Wood’s interest in the physical chemistry of solutions now spans about five decades and is documented by his more than 150 published technical papers, the majority of which is devoted to the physical chemistry of aqueous solutions of electrolytes and nonelectrolytes. When the focus is on water as solvent, one has to realize that water is not only the most abundant substance on the surface of the earth, it is also the only substance on earth occurring naturally in all three states of matter. Water is important for the structure, the stability and the function of biomolecules, that is to say, it has a controlling influence on biochemical processes and is thus a prerequisite for the emergence of life as we know it. Professor Wood’s scientific work may be subdivided roughly into six topics listed below. For each topic I have included one or more representative publications:

Impedance Spectra of Energy-Storage Electrodes Obtained with Commercial Three-Electrode Cells: Some Sources of Measurement Artefacts

ABSTRACT The information derived from electrochemical ac impedance spectroscopy (EIS) measurements on Li-ion batteries is important for the non-destructive diagnostics of the battery state of health, as well as for identifying the best cell materials for automotive electrification and characterizing their electrochemical performance. The present work aims to identify and determine some of the sources of systematic errors in EIS measurements on small (“coin” type) cells. The data reveal perfect capacitive behavior of both electrodes at low frequencies and kinetic limitation at higher frequencies. Artefacts due to a small misalignment between the working electrode (WE) and the counter electrode (CE) in symmetric cells, or to an unfavorable ratio of characteristic frequencies for the WE and CE in mixed cells, are negligible only in aqueous electrolyte solutions of high conductivity. In contrast, measurement artefacts seriously complicate impedance measurements in non-aqueous, low conductivity solutions, as is the case for non-aqueous electrolyte solutions used in Li-ion batteries and supercapacitors. Our study shows that the necessary condition for a good CE in 3-electrode impedance measurements, namely, a small impedance (relative to the WE) at low frequencies is not sufficient to ensure the absence of high-frequency artefacts, since it must also have a small impedance at high frequencies. When this is not the case (a common practical situation), a severe contamination of the measured WE impedance by the large, high-frequency CE impedance will occur during 3-electrode impedance measurements. The report ends with recommendations regarding best practices for electrochemical ac impedance measurements.

Hydration constants of simple non-electrolytes in aqueous solutions determined by the acoustic method

New quasi-chemical models are developed to estimate semi-quantitatively the equilibrium constants of hydrate formation, which also explain the parabolic-shaped dependences of sound speed or compressibility on concentration of non-electrolytes in their aqueous solutions. It was shown that only two fitted parameters, the hydration number nh and the formation constant of the hydrate β, are able to reproduce satisfactorily the experimental data. The results obtained are spectacular and consistent with former observation concerning hydration of non-electrolytes.

Electrochemical Characterization of the PG | EMImBF4 + x% EMImI Interface

The need for alternative high energy and power density energy sources has led scientists to study extensively the electrochemical energy conversion/storage devices such as electrochemical double layer capacitors (EDLC), batteries, fuel cells, etc. The values of power and energy densities of EDLC depend strongly on the properties of the electrode material and electrolyte used. EDLCs based on non-aqueous electrolytes or room temperature ionic liquids have a wider region of ideal polarizability (ΔE) compared to those based on traditional aqueous electrolytes 1-4. The wider ΔE in turn improves the energy and power density values of EDLCs. The specific adsorption of halide ions at single crystal electrodes from aqueous and non-aqueous electrolyte solutions has been demonstrated before 5,6. However, the specific adsorption of ions is a complicated process (depending on the electrode potential, chemical composition and concentration of halides etc.) and the kinetics of formation/rearrangement of electrical double layer between electrode and ionic liquid (IL) needs an extended examination. The main aim of this work was to characterize the adsorption of iodide ions from the mixture of two ionic liquids based on the same electrochemically stable 1-ethyl-3-methylimidazolium cation. Electrochemical impedance spectroscopy and cyclic voltammetry methods have been applied to study the electrochemical characteristics of the interface between pyrolytic graphite (PG) and mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide (1-10 wt%). All measurements were carried out in a 3-electrode electrochemical cell using a PG as a working, Pt net as a counter, and Ag wire coated with AgCl as a reference electrodes. Current density (i) vs. electrode potential (E) dependencies (Fig. 1) show that two peaks in CV are caused by the addition of x wt% of EMImI into EMImBF4 (for PG | EMImBF4 interface there is no peaks within the potential region from ─1.5 V to +1.5 V (vs. Ag | AgCl)). Reduction and oxidation processes are more intensive in more concentrated solutions. A strong dependence of current density on the potential scanning rate (υ) for PG | EMImBF4+EMImI interface was observed in the region of peaks. Also the capacitance values calculated from CV data (Ccv= i / υ) show the strong dependence on υindicating a very slow adsorption process occurs. The shape of the complex impedance plane, i.e. Nyquist plots, total impedance and phase angle vs. frequency dependences depend strongly on the electrode potential and on the concentration of the I-ions in the solution. The values of series differential capacitance (C s) were calculated from Nyquist dependencies at different fixed ac frequencies (C s = (Z''i2πf)−1; where i = √−1and Z’’ is the imaginary part of impedance). C s depends strongly on ac frequency and electrode potential, increasing at lower ac frequencies and at anodic range of potentials. C s values depend also on the added amount of I-(1-10%) to the solution, increasing from 1 to 5wt% and diminish in more concentrated solutions. References L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, J. Electroanal. Chem., 719, 133 (2014).L. Siinor, C. Siimenson, K. Lust, E. Lust, Electrochem. Commun. 35, 5 (2013).R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochim. Acta, 85, 139 (2012).R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochem. Commun. 22, 203 (2012).L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem., 601, 39 (2007).M. Väärtnõu, E. Lust, J. Electroanal. Chem., 565, 211 (2004). Acknowledgements Estonian Energy Technology Program project SLOKT10209T, Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117, Functional Micro/Mesoporous Nanomaterials for Novel Energy Conversion and Storage Systems IUT 20-13, Functionalization of carbon nanomaterials: synthesis, characterization and application SLOKT12180T.

V2O5-Based Composite As Mg-Ion Battery Electrode Material Studied By Electrochemical SPM at the Nanoscale

The science and technology of rechargeable Li-ion batteries have dominated the field of advanced power sources and replaced many other batteries in the market, particularly in the areas of communication, computers, electronics, and in very high power devices such as power tools and electric vehicles.1 The critical challenges for rechargeable Li-ion batteries are cost, safety, and energy density.2 Since an innovative Mg-ion battery was demonstrated successfully by the Aurbach et al.,3 it has received much attention as a potential rechargeable battery to replace the Li-ion battery because of safety, abundance and inexpensiveness of Mg. Moreover, Mg anodes, being divalent and elemental, theoretically provide a higher volumetric capacity of up to ~3800 mAh cm-3 compared to ~2050 mAh cm-3 of Li.2 Despite many promising aspects, there is few research work associated with Mg-ion batteries so far. Indeed, the development of rechargeable Mg-ion batteries has been impeded from various intrinsic limitations relating to the use of Mg metal anodes and Mg-ion intercalation cathodes.4 Passivation of Mg metal in the conventional non-aqueous electrolyte solution is a serious challenge, and Mg-ion intercalation is very slow or even impossible for most of the transition metal oxides serving as Li-ion insertion electrodes. In spite of these difficulties, it has been shown that Mg-ion may insert into transition metal oxides even in conventional non-aqueous Mg salt electrolyte solutions with a certain degree of reversibility.4 Specifically, nanostructured V2O5-based composites are known as high power and high energy density materials that might insert Mg ions reversibly, and they have already been studied intensively as anode and cathode materials for Li-ion batteries demonstrating a high reversible capacity, excellent cycling performance, and good rate capability.5 However, it is important to note that the research efforts dedicated to Mg-ion batteries are still in infancy. Recently, Gershinsky et al.6 explored that highly reversible Mg-ion insertion/extraction was possible with highly pure V2O5 thin-film electrodes using various electrochemical and spectroscopic analytical tools. A growing practical experience related to nanostructured materials suitable for Mg-ion battery applications requires a fundamental insight at nanoscale. In virtue of the Electrochemical Scanning Probe Microscopy (ECSPM) technique, especially Electrochemical Scanning Tunneling Microscopy (ECSTM), our approach to study magnesium intercalation is to use single-crystalline V2O5 to obtain an insight and accurate prediction of reaction mechanisms for the electrode of rechargeable Mg-ion batteries. In order to analyze the intercalation mechanisms of Mg2+, topographical changes of V2O5 will be analyzed under stepwise polarization to different potentials by ECSTM. The results will be described in terms of the combination among physicochemical phenomena, electrochemical metal-ion intercalation into single-crystalline host structures with different electrolyte systems, and electrochemical performance in rechargeable Mg-ion batteries. These studies will be complemented by three electrode electrochemical studies at nanostructured VOxcomposites, and compared to results obtained for Li-ion insertion.

The challenge of developing rechargeable magnesium batteries

Abstract