Study on electrochemical performance of activated carbon in aqueous Li2SO4, Na2SO4 and K2SO4 electrolytes

The electrochemical performances of activated carbon (AC) in 0.5 mol/l Li₂SO₄, Na₂SO₄ and K₂SO₄ aqueous electrolytes were investigated. The cyclic voltammetric results at different scan rates show that the rate behaviors of AC in the three electrolytes improve in the order of Li₂SO₄ < Na₂SO₄ < K₂SO₄. This improvement can be mainly ascribed to the following two reasons: (1) the decreasing equivalent series resistance in the order of Li₂SO₄ > Na₂SO₄ > K₂SO₄, which is the main factor influencing the maximum output power, and (2) the increasing migration speed of hydrated ions in the bulk electrolyte and in the inner pores of AC electrode in the order of Li⁺ < Na⁺ < K⁺. Their cycling behaviors do not show any differences in capacitive fading. The above results provide valuable information to explore new hybrid supercapacitors.     

The PVT properties of concentrated aqueous electrolytes IX. The volume properties of KCl and K2SO4 and their mixtures with NaCl and Na2SO4 as a function of temperature

The densities of KCl and K₂SO₄ were measured from dilute solutions to saturation from 5 to 95°C. The data were combined with literature data to produce density and apparent molal volume, V_φ, equations from 0 to 100°C and to saturation. The standard deviations of the density equations were 30×10⁻⁶ g-cm⁻³ and 32×10⁻⁶ g-cm⁻³, respectively, for KCl and K₂SO₄. Pitzer equations were used to fit the V_φ data. The resulting infinite dilute partial molal volumes, V^o, were in reasonable agreement with literature data. The densities of the mixtures of the six combinations of the salts KCL, K₂SO₄ NaCl and Na₂SO₄ were measured at I=2.0 and t=5, 25, 55 and 95°C. The resulting volumes of mixing were fitted to equations of the form

Viscosity of aqueous Na2SO4 solutions at temperatures from 298 to 573 K and at pressures up to 40 MPa

Viscosities of nine (1.5, 3, 5, 7, 10, 15, 20, 23, and 26) mass% of aqueous Na₂SO₄ solutions have been measured in the liquid phase with a capillary flow technique. Measurements were made at five isobars 0.1, 10, 20, 30, and 40 MPa. The range of temperatures was from 298.15 to 573.5 K. The total uncertainty of viscosity, pressure, temperature, and concentration measurements was estimated to be less than 1.5%, 0.05%, 15 mK, and 0.015%, respectively. The reliability and accuracy of the experimental method was confirmed with measurements on pure water for four selected isobars 5, 10, 20, and 40 MPa and at temperatures between 296.7 and 573.7 K. The experimental and calculated values from IAPWS (International Association for the Properties of Water and Steam) formulation for the viscosity of pure water show excellent agreement within their experimental uncertainty (AAD = 0.41%). The temperature, pressure, and concentration dependences of the relative viscosity (η/η₀) where η₀ is the viscosity of pure water are studied. The values of the viscosity A-, B-, and D-coefficients of the extended Jones–Dole equation for the relative viscosity (η/η₀) of aqueous Na₂SO₄ solutions as a function of temperature are studied. The maximum of the B-coefficient near the 323 K isotherm has been found. The behavior of the concentration dependence of the relative viscosity of aqueous Na₂SO₄ solutions is discussed in terms of the modern theory of transport phenomena in electrolyte solutions. The derived values of the viscosity A- and B-coefficients were compared with the results predicted by Falkenhagen–Dole theory of electrolyte solutions and calculated with the ionic B-coefficient data. Different theoretical models for the viscosity of electrolyte solutions were stringently tested with new accurate measurements on aqueous Na₂SO₄. The quality and predictive capability of the various models was studied. The measured values of viscosity were directly compared with the data reported in the literature by other authors.     

Water activity,osmotic and activity coefficients of aqueous solutions of Li2SO4, Na2SO4, K2SO4, (NH4)2SO4, MgSO4, MnSO4, NiSO4, CuSO4, and ZnSO4 at T=298.15 K

The water activities for aqueous solutions of Li₂SO₄(aq), Na₂SO₄(aq), K₂SO₄(aq), (NH₄)₂SO₄(aq), and sulphates MgSO₄(aq), MnSO₄(aq), NiSO₄(aq), CuSO₄(aq), and ZnSO₄(aq) were determined experimentally at a temperature of 298.15 K with a hygrometric method, at molalities in the range from 0.1 mol·kg⁻¹ to saturation. The osmotic coefficients are calculated from these results. The coefficients of Pitzer’s model was used to fit the osmotic coefficients for each salt solution. These parameters were used to predict solute activity coefficients for the salts studied.     

Gas phase dissociation of H2SO4: A computational study

The decomposition of gaseous sulfuric acid has been investigated computationally. In particular the role of the hydrated gaseous coordination adducts of SO₃(g) and H₂SO₄(g) in the (dissociation + decomposition) process has been evaluated. A first principles study of the gaseous coordination complexes SO₃(H₂O)_n (n = 1 to 3) and H₂SO₄(H₂O)_m (m = 1 to 2) has been carried out deriving equilibrium ground state structures, vibrational frequencies and energetic stabilities by the Moller–Plesset perturbation approximation. These results have been used to derive the enthalpy of formation at 0 K and the Gibbs energy functions of these molecules. A new thermodynamic modeling of the decomposition of H₂SO₄(g) has been therefore performed considering the effect of temperature, pressure and initial composition of the gas (hydration conditions).     

Thermodynamics of electrolytes. Binary mixtures formed from aqueous NaCl,Na2SO4, CuCl2, and CuSO4, at 25°C

Osmotic coefficients to high ionic strengths are reported for five of the binary mixtures formed from NaCl, Na₂SO₄, CuCl₂, and CuSO₄; the sixth system studied, NaCl–Na₂SO₄, is one studied by Wu, Rush, and Scatchard. The equations recently developed by Pitzer are used successfully in the interpretation of the experimental results. Revised values are given for the activity and osmotic coefficients for pure CuSO₄ and CuCl₂ solutions.     

Theoretical study of the first acid dissociation of H2SO4 at a model aqueous surface

Electronic structure calculations on the H2SO4.(H2O)(4,6) model system embedded at the surface of an aqueous layer have been performed to examine the feasibility of the first acid dissociation of H2SO4 to an H2SO4-.H3O+ contact ion pair over a wide temperature range, with a special focus on the 190-250 K range relevant for atmospheric sulfate aerosols. The results indicate that the acid dissociation can be either thermodynamically favored or disfavored depending on the degree of solvation of the acid and the produced ions, as well as on the temperature.     

Phase compositions,viscosities and densities of systems PPG425 + Na2SO4+H2O and PPG425 + (NH4)2SO4 + H2O at 298.15 K

Phase diagrams of PPG425+Na₂SO₄+H₂O and PPG425+(NH₄)₂SO₄+H₂O systems at 298.15 K were measured. The densities and viscosities of two-phase systems were also measured. The improved regular solution theory was used to correlate the equilibrium data. In this theory a mixing entropy term and the Fowler-Guggenheim long-range electrostatic term has been used.     

Comparative study of vibrational spectra of Rb4LiH3(SO4)4, K4LiH3(SO4)4, K4LiH3(SeO4)4, Na5H3(SeO4)4.2H2O and Na2SeO4.H2SeO3.H2O crystals. Polarized infrared and Raman studies

Vibrational spectra of M4LiH3(XO4)4 family, where M=K, Rb, X=S, Se together with Na5H3(SeO4)4.2H2O and Na2SeO4.H2SeO3.H2O crystals were compared. Similarities and differences are described. The spectroscopic manifestation of the presence of hydrogen bonds is discussed. Position of the bands corresponding to bending type of vibrations (in-plane and out-of plane) of hydrogen bonds is analyzed in the function of temperature. Small dynamic splitting of the bands due to weak interactions between ions is noticed.     

Volumes of aqueous alcohols, ethers, and ketones to T = 523 K and p = 28 MPa.

Densities of dilute aqueous solutions of isopropanol, 1,5-pentanediol, cyclohexanol, benzyl alcohol, diethyl ether, 1,2-dimethoxyethane, acetone, and 2,5-hexanedione were measured by means of a vibrating-tube flow densimeter at temperatures near T = (302, 373, 423, 473, and 521) K at a pressure of p = 28 MPa. At the lowest and highest temperatures, measurements were also made close to the saturation vapour pressure of water to investigate the effect of pressure on the volumes of solutes. Apparent molar volumes were calculated for each solute and extrapolated to give partial molar volumes at infinite dilution. The variation of the volume with temperature, pressure, and structure of solute is discussed qualitatively, and group contributions are determined at the temperatures of measurements and p = 28 MPa. Several equations proposed in the literature for correlating the partial molar volumes at infinite dilution as a function of state parameters are tested. Parameters of one selected equation are tabulated allowing calculation of the partial molar volumes at infinite dilution at temperatures and pressures up to T = 573 K and p = 40 MPa. respectively.     

Activated Carbon by KOH and NaOH Activation: Preparation and Electrochemical Performance in K2SO4 and Na2SO4 Electrolytes

Russian Journal of Electrochemistry - Activated carbons were successfully prepared from rice husk (RH) by chemical activation using KOH (RH-K4) or NaOH (RH-N3) as activating agents and...     

Electrical conductances of tetrabutylammonium bromide and tetrapentylammonium bromide in 2-ethoxyethanol + water mixtures at 308.15, 313.15, 318.15 and 323.15 K

The electrical conductances of the solutions of tetrabutylammonium bromide (Bu₄NBr), and tetrapentylammonium bromide (Pen₄NBr) in 2-ethoxyethanol (1) + water (2) mixed solvent media containing 0.25, 50 and 0.75 mass fractions of 2-ethoxyethanol (w ₁) have been reported at 308.15, 313.15, 318.15 and 323.15 K. The conductance data have been analyzed by the 1978 Fuoss conductance–concentration equation in terms of the limiting molar conductance (Λ⁰), the association constant (K _A) and the association diameter (R). These two electrolytes are found to exist essentially as free ions in the solvent mixtures with w ₁ = 0.25 and 0.50 over the entire temperature range; however, slight ionic association was observed in the mixed solvent medium richest in 2-ethoxyethanol. The electrostatic ion–solvent interaction is found to be very weak for the tetraalkylammonium ions in the aqueous 2-ethoxyethanol mixtures investigated.     

Thermodynamics of saturated electrolyte mixtures of NaCl with Na2SO4 and with MgCl2

The ion=interaction equation is used to calculate the mean activity coefficients for the saturated aqueous mixtures, NaCl+Na₂SO₄ and NaCl+MgCl₂. A comparison between these values with those obtained from solubility shows a good agreement over a wide temperature range as well as at high ionic strengths for both mixed systems.     

Thermodynamic properties of aqueous mixtures of LiCl and Li2SO4 at different temperatures

Emf measurements were made on the cell without liquid junction: Li?ISE LiCl(m₁), Li₂SO₄(m₂) Ag/AgCl. The performances of the electrode pairs constructed in our laboratory were tested and exhibited near-Nernstian behavior. The mean activity coefficients of LiCl for the system Li⁺?Cl^??SO ₄ ^2? ?H₂O have been investigated by the emf values at temperatures of 0, 15, 35°C and constant total ionic strengths of 0.05, 0.1, 0.5, 1.0, 2.0, 3.0 and 5.0 mol·kg^?1. The activity coefficients decrease with increasing temperature and the ionic strength fraction of Li₂SO₄ in the mixtures. The thermodynamic properties are interpreted by use of Harned's empirical equations and Pitzer's ion interaction approach including the contribution of higher order electrostatic terms. The experimental results obey Harned's rule and are described by using Pitzer equations satisfactorily. The activity coefficients of Li₂SO₄, the osmotic coefficients and the excess free energies of mixing for the system in the experimental temperature range were reported.     

Structural organization of cetyltrimethylammonium sulfate in aqueous solution: The effect of Na2SO4

We used dynamic light scattering (DLS), steady-state fluorescence, time resolved fluorescence quenching (TRFQ), tensiometry, conductimetry, and isothermal titration calorimetry (ITC) to investigate the self-assembly of the cationic surfactant cetyltrimethylammonium sulfate (CTAS) in aqueous solution, which has SO(2-)4 as divalent counterion. We obtained the critical micelle concentration (cmc), aggregation number (N(agg)), area per monomer (a0), hydrodynamic radius (R(H)), and degree of counterion dissociation (alpha) of CTAS micelles in the absence and presence of up to 1 M Na2SO4 and at temperatures of 25 and 40 degrees C. Between 0.01 and 0.3 M salt the hydrodynamic radius of CTAS micelle R(H) approximately 16 A is roughly independent on Na2SO4 concentration; below and above this concentration range R(H) increases steeply with the salt concentration, indicating micelle structure transition, from spherical to rod-like structures. R(H) increases only slightly as temperature increases from 25 to 40 degrees C, and the cmc decreases initially very steeply with Na2SO4 concentration up to about 10 mM, and thereafter it is constant. The area per surfactant at the water/air interface, a0, initially increases steeply with Na2SO4 concentration, and then decreases above ca. 10 mM. Conductimetry gives alpha = 0.18 for the degree of counterion dissociation, and N(agg) obtained by fluorescence methods increases with surfactant concentration but it is roughly independent of up to 80 mM salt. The ITC data yield cmc of 0.22 mM in water, and the calculated enthalpy change of micelle formation, Delta H(mic) = 3.8 kJ mol(-1), Gibbs free energy of micellization of surfactant molecules, Delta G(mic) = -38.0 kJ mol(-1) and entropy TDelta S(mic) = 41.7 kJ mol(-1) indicate that the formation of CTAS micelles is entropy-driven.     

Sequential hydration energies of the sulfate ion, from determinations of the equilibrium constants for the gas-phase reactions: SO4(H2O)(n)2- = SO4(H2O)(n-1)2- + H2O

Sequential hydration energies of SO4(H2O)(n)2- were obtained from determinations of the equilibrium constants of the following reactions: SO4(H2O)(n)2- = SO4(H2O)(n-1)2- + H2O. The SO4(2-) ions were produced by electrospray and the equilibrium constants Kn,n-1 were determined with a reaction chamber attached to a mass spectrometer. Determinations of Kn,n-1 at different temperatures were used to obtain DeltaG0n,n-1, DeltaH0 n,n-1, and DeltaS0n,n-1 for n = 7 to 19. Interference of the charge separation reaction SO4(H2O)(n)2- = HSO4(H2O)(n-k)- + OH(H2O)(k-1)- at higher temperatures prevented determinations for n < 7. The DeltaS0n,n-1 values obtained are unusually low and this indicates very loose, disordered structures for the n > or = 7 hydrates. The DeltaH0n,n-1 values are compared with theoretical values DeltaEn,n-1, obtained by Wang, Nicholas, and Wang. Rate constant determinations of the dissociation reactions n,n - 1, obtained with the BIRD method by Wong and Williams, showed relatively lower rates for n = 6 and 12, which indicate that these hydrates are more stable. No discontinuities of the DeltaG0n,n-1 values indicating an unusually stable n = 12 hydrate were observed in the present work. Rate constants evaluated from the DeltaG0n,n-1 results also fail to indicate a lower rate for n = 12. An analysis of the conditions used in the two types of experiments indicates that the different results reflect the different energy distributions expected at the dissociation threshold. Higher internal energies prevail in the equilibrium measurements and allow the participation of more disordered transition states in the reaction.     

Dynamics of vibrational overtone excitations of H2SO4, H2SO4-H2O: hydrogen-hopping and photodissociation processes

Photochemical processes of sulfuric acid (H2SO4) and sulfuric acid monohydrate (H2SO4-H2O) following overtone excitation of the OH stretching mode are studied by classical trajectory simulations using the semiempirical PM3 potential suface in "on the fly" calculations. The main results are the following: (1) In the excitation of H2SO4 to the fifth OH-stretch overtone, hopping of the H atom between oxygen atoms is found to take place in 22% of the trajectories, only once during simulations of 400 ps. (2) All the trajectories for H2SO4 show a rapid cis-trans isomerization. (3) The photolysis of H2SO4 into SO3 + H2O takes place in 5% of the trajectories on a time scale of approximately 9 ps. (4) Only low overtone levels of H2SO4-H2O have sufficiently long lifetimes to be spectroscopically relevant. Excitation to these OH stretching overtones is found to result in the dissociation of the cluster. H hopping or dissociation of H2SO4 does not take place.