State of hydration of the proton and the transition between molten-salt and aqueous-solution double-layer and kinetic behaviour

The influence of the state of hydration of the proton, from H₃O⁺ in the room-temperature melt, H₃O⁺. CF₃SO⁻₃, to 1 M aqueous in CF₃SO₃H solution in water, on the interfacial capacitance behaviour of the Au electrode is reported. The results allow the transition from “molten-salt” to “aqueous-solution” double-layer capacitance behaviour to be evaluated.

At positive potentials, near the potential for onset of surface oxidation of Au by OH, the interfacial capacitance is dominated by a pseudo-capacitance associated with chemisorption of the anion CF₃SO⁻₃, with charge transfer. At negative potentials in the range in which cathodic H₂ evolution occurs, the capacitance varies from ca 23 to ca 50 μF cm⁻², with the maximum arising around a degree of proton hydration corresponding to H₇O⁺₃.

Cathodic H₂ evolution kinetics have been studied from the same proton sources and show characteristic dependences of log i_o and the heat of activation on the hydration-state of H⁺. The Tafel slopes decrease with increasing temperature, giving a further example of non-conventional behaviour of the transfer coefficient.     

Anwendung der coulostatischen methode zur bestimmung der doppelschichtkapazität einer bleielektrode

The coulostatic method used in determining the double-layer capacitance of the mercury electrode was applied for measurements on a lead electrode in sodium sulfate solutions. The curves: differential capacitance of the double-layer electrode potential are shown. As a result, the sulfate ions do not adsorb on the lead. The double-layer capacitance values in 10⁻³ N Na₂SO₄ at different potentials of the lead electrode have been calculated.

The theoretical results have been compared with the experimental data. The application of Grahame's model for the double-layer structure in the case of lead electrodes has been confirmed.     

Rapid discharge performance of composite electrode of hydrated sodium manganese oxide and acetylene black

Hydrated sodium manganese oxide was synthesized by reducing permanganate ion using ethanol by a sol–gel method. By including acetylene black in the synthetic reaction, we obtained composite materials in which sodium manganese oxide hydrate particles were small and mixed well with the acetylene black. We evaluated those composites as a lithium battery cathode and found that they showed 170 mA h g⁻¹ under 5 mA g⁻¹ and 117 mA h g⁻¹ under 5 A g⁻¹ on the basis of composite weight. This rapid discharge performance was probably caused by the favorable contact condition of the composite constituents.     

Differential capacitance of the platinum/aqueous-solution interphase

The differential capacitance of polycrystalline platinum in aqueous H₂SO₄ and NaOH solutions has been investigated as a function of bias potential. A capacitance minimum is observed at 0·56 V in H₂SO₄ and − 0·34 V in NaOH solution. There is strong evidence for the irreversible adsorption of oxygen/OH⁻ ions and oxide-film formation. Addition of n-butylamine hinders the adsorption of oxygen/OH⁻ ions and it is itself strongly adsorbed.     

Chloride penetration and the ratio of Cℓ/OH in the pores of cement paste

Hardened Portland cement paste samples were immersed in sodium chloride solution. Their pore solution was expressed and analysed for Cℓ⁻ and OH⁻ ions concentrations. The development of free Cℓ⁻ and of Cℓ⁻/OH⁻ ratio is discussed in relation to the limits previously established for the onset of depassivation of steel reinforcement. It was found that within a relatively short period penetration of the chloride ions could cause the Cℓ⁻/OH⁻ ratio in the pores to exceed the depassivation limits.     

Preparation of highly dispersed PEM fuel cell catalysts using electroless deposition methods

A methodology for the electroless deposition (ED) of PtCl₆²⁻ using dimethylamine borane (DMAB) on a Rh-seeded carbon support has been developed for electrochemical and fuel cell applications. This procedure required seeding the carbon with a Rh-precursor catalyst via wet impregnation prior to the exposure of an aqueous ED bath containing PtCl₆²⁻, DMAB, and sodium citrate (complexing/stabilizing agent). Kinetic parameters that affect the extent and rate of PtCl₆²⁻ deposition include concentrations of PtCl₆²⁻, DMAB, and sodium citrate as well as pH and concentrations of Rh seed sites. A linear relationship between rate and extent of PtCl₆²⁻ deposition and DMAB and Rh concentrations was found while the citrate concentration had little effect on rate and a modest effect on extent. Lastly, extent of PtCl₆²⁻ deposition showed a maximum with respect to pH. Characterization of the Rh-seeded, carbon support by transmission electron microscopy (TEM) shows that the Rh particle diameters remain constant at 33–43 Å as the Rh weight loading increases from 0.4% to 2.2% to 4.4%. Further, after deposition of similar loadings of Pt, TEM analysis shows Pt particle diameters decrease with increasing Rh loading, since equal amounts of Pt were deposited on greater numbers of Rh seed particles. This pattern suggests a shell-core geometry, where Pt is deposited more or less uniformly around a Rh core.     

Preparation of polyaniline nanowire arrayed electrodes for electrochemical supercapacitors

Polyaniline (PANI) nanowire arrayed electrodes were successfully synthesized by means of anodic deposition technique using the membrane-template synthesis route. The desired three-dimensional architecture of PANI nanowire arrayed electrodes was characterized with field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) techniques. It was found that the co-deposition nanowires were regular size and continuous with an average diameter of about 30 nm, and the nanowires have an amorphous nature. The electrochemical characterization was performed in 2 M H₂SO₄ aqueous solution. The capacitance of the PANI nanowire arrayed electrode at a charge–discharge current density of 5 Ag⁻¹ is 1142 Fg⁻¹. The nanowire arrayed electrodes showed an excellent capacitive ability for the facility of electrolyte penetration, the ease of proton exchange/diffusion and the metallic conductivity of PANI nanowire arrayed electrodes.     

Voltammetric study of halide ion adsorption on platinum in perchloric acid solutions

Periodic current/potential curves were taken potentiostatically at 30 mV/sec on smooth platinum in 1 N HClO₄ with different additions of HCl, HBr, and HI, in the potential range between hydrogen evolution and deposition of oxygen or of the respective halide ion. The effect of increasing halide ion concentration, starting at 10⁻⁶ M, on hydrogen adsorption and oxygen adsorption was studied systematically. Impedance measurements were made at 1000 c/s under the same conditions as the measurements of the current/potential curves. It is concluded from the dependence of the double layer capacitance upon the bulk concentration of halide ions in the double layer region that nearly a monolayer of adsorbed ions is present there for ₀c_Cl⁻ ≈ 10⁻² M, ₀c_Br ≈ 10⁻⁴ M, and ₀c_HI ≈ 10⁻⁵ M. The measurements at larger concentrations, especially for I⁻, show additional effects which are due to the deposition of halides and to the reverse reaction.     

Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method

Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zirconyl chloride and sodium salts (chloride, carbonate, nitrate or sulfate) are found to exhibit effective catalytic performances in the oxidative coupling of methane. The highest C₂₊ yield (16.6%) and ethylene-to-ethane ratio are obtained over the Na₂CO₃-added catalyst having an appropriate Na/Zr ratio; however, the active promoting species in this catalyst are actually identified to be Na⁺ and Cl⁻. The mixed solution method is considered to be an effective preparation method when proper precursor compounds of zirconia and promoters are chosen, giving a desirable interaction between them. The reaction and characterization results indicate that the chlorine ion plays a more prominent role than the sodium ion, and the X-ray photoelectron spectroscopy results show the presence of multiple chlorine species on the surface. It is suggested that the Cl⁻ bound to Na⁺ is capable of activating both methane and ethane whereas the Cl⁻ bound to Zr⁴⁺ is only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion.     

Preparation and electrochemical properties of pitch-based activated carbon aerogels

The porous structure of pitch-based carbon aerogels (P-CAs) can be modified by KOH activation. It is found that decreasing the carbonization temperature of precursor CAs and increasing the mass ratio of KOH to CAs help to the formation of 0.7 nm-sized micropores and 2.7 nm-sized mesopores, respectively. The origin and the pore size of micropores play an important role in controlling electrochemical properties. The carbonization-forming micropores have stronger energy storage efficiency than activation-forming micropores, and only those with diameter below 1 nm (Microp < 1 nm) are the crucial place to storage energy. Due to the substantive increase of the number of the Microp < 1 nm, the highest specific capacitance of the as-prepared activated samples can reach 187.2 F g⁻¹ at 5 mA cm⁻², 1.8 times as large as that of their precursor CAs. Furthermore, this capacitance is still up to 173.3 F g⁻¹ when increasing the current density to 50 mA cm⁻², indicating that the activated samples have a high-rate charge–discharge performance.     

Untersuchung über die anionenadsorption mit hilfe markierter ionen

From previous work, the adsorption of anions is regarded as an essential factor for the different corrosion behaviour of metals in solutions containing different anions. Adsorption is measured by means of ³⁶Cl⁻, ⁸²Br⁻, ¹⁸ F⁻, ³⁶ClO₄⁻, ³⁵O₄²⁻, H³⁵S⁻ and ¹⁴CN⁻ on Pt, Ni and Fe in the form of sheets and evaporated films. Besides the determination of the adsorption after dipping into the solution, a method has been developed for the measurement of adsorption in contact with the solution and for the determination of its kinetics. The method can also be applied to O₂-free metal surfaces produced under vacuum. In this case, however, very rapid adsorption is observed, whereas normally saturation is reached only after many hours. It is concluded that, in general, exchange between oxygen on the metal and the anion takes place rather than simple adsorption.

The distribution of the anions adsorbed on the metal surface has been studied by autoradiography; adsorption takes place preferentially at the grain boundaries and increases when the crystal size decreases.

These results confirm the interpretation of passivation as a competition between various processes: metal dissolution, coverage by a passivating oxide film, and displacement of oxygen by anions.     

A new tantalum sulfur compound as electrode material for non-aqueous alkali metal batteries

Polycrystalline (PbS)_1.14(TaS₂)₂, a misfit layer sulfide, was used as cathodic material for lithium secondary battery. One molar LiClO₄ in propylene carbonate (PC) was used as electrolyte. The cell could be galvanostatic discharged down to x = 4.6 [Li_x(PbS)_1.14(TaS₂)₂] when the current density was 65 μA cm⁻² and the cell was cycled more than 100 times between 3.5 and 1.5 V at a current density of 260 μA cm⁻². Lattice expansion increased linearly with lithium content and was less than that reported for the Li/TaS₂ system. Chemical diffusion coefficients were determined by a modified version of the galvanostatic intermittent titration technique and they were fairly constant in the composition range 0.2 < x < 1, and an average value of 8.1 × 10⁻¹¹ cm² s⁻¹ was calculated. Sodium intercalation was also accomplished, but the uptake of this ion resulting in a significant lattice expansion compared with that observed for lithium ions. Moreover, a similar dependence of the sodium chemical diffusion coefficient on the composition was observed with an average value of 1.4 × 10⁻¹⁰ cm² s⁻¹, somewhat higher than that of lithium ion. We believe that differences in lattice expansion may be responsible for the differences found in the chemical diffusivity values.     

On the simultaneous role of Cr in the passivity of stainless steel and in the sorption of HSO4 and Cl ions in the passive film

The sorption behaviour of HSO⁻₄ and Cl⁻ ions on stainless steel is discussed in the light of recent theories of passivity attributing a significant role to the Cr content and the state of Cr species in the passive layer. It is shown through experimental evidences that a link between the formation of Cr³⁺ species in the passive film and HSO⁻₄ sorption can be formulated.     

Electrodissolution of cobalt in carbonate/bicarbonate media

The electrodissolution of cobalt in carbonate/bicarbonate solutions was studied at room temperature by steady state polarisation, interfacial pH measurements and Raman spectroscopy. The active dissolution of cobalt leads to an initial CoO film formation. The metal passivation occurs by a slow transformation of the CoO into a Co₃O₄ oxide. The influence of HCO₃⁻ and CO₃²⁻ anions was investigated. Two different parallel electrochemical processes were proposed to account for the anion role on the electrochemical steady state behaviour of cobalt in the studied solutions.     

Study of the impedance of a platinum electrode in the system Br2/Br (HClO4, aq.)—II. Mechanism of the electrode reaction

The Randles circuit well represents impedance measurements carried out with activated Pt electrodes. This enables us to study the variation of j_o for redox reactions with concentration of the reactants, at constant potential, and also the variation of j_o with potential, keeping constant the concentration of one of the reactants. The results thus obtained indicate that the step Br₂ + e Br₂⁻ is rate-determining; it is followed or preceded by the rapid equilibria Br₂⁻ Br⁻ + Br 2Br Br₂. The mechanisms proposed hitherto for the electrochemical behaviour of the halogen/halide systems at inert electrodes are discussed, and it is reasoned that the ‘reversibility’ of these systems increases in the order Cl₂/Cl⁻ < Br₂/Br⁻ < I₂/I.     

Etude electrochimique de l''iode et de ses derives en solution dans la pyridine variation des proprietes d''oxydo-reduction en fonction du pH

The electrochemical behaviour of iodine and redox derivatives in solution in pyridine was studied by the use of voltammetric method. By controlling the pH value, it is possible to establish the normal apparent thermodynamic diagram of stability of iodine in this solvent. In acid media, the four chemical species I⁻, I⁻₃, I₂ and I⁺ are stable; their respective stability constants are evaluated. In basic media, one observes the disproportionation reaction of iodine, at first into triodide and into a base complexing with iodine in (I⁺), then into iodide and iodate. A salt of positive univalent iodine was isolated by neutralizing I⁺ with an excess of tetraethylammonium hydroxide.     

Thermodynamics of ionic-association in aqueous solutions of Ca and Mg organic salts using ion-selective electrode technique—I. Formates, acetates, propionates and butyrates

The divalent selective electrode together with high precision solid state, digital pH -mv -meter makes broader application of potentiometry in physical and inorganic chemistry a certainty. The above set-up is used to determine the stoichiometric constants, K, for Ca and Mg ions association with formates, acetates, propionates and butyrates at 25°, 35° and 45°C in aqueous media. The K-values were converted to infinite dilution K_A values were found to be 8.4 LM⁻¹, 10.4 LM⁻¹, 19.1 LM⁻¹ and 19.3 LM⁻¹ for calcium salts of formate, acetate, propionate and butyrate respectively. Also K_A values for Mg salts of formate, acetate, propionate and butyrate were found to be 7.8 LM⁻¹, 9.5 LM⁻¹, 13.1 LM⁻¹ and 13.1 LM⁻¹ respectively. Other thermodynamic parameters such as ΔG°, ΔH° and ΔS° are also obtained from the variation of K_A with temperature for each salt. The data are interpreted relative to each other on basis of pK_a of the corresponding organic acid. Their temperature behaviour is similar to those salts derived from strong acids such as sulphates, rather than weak acids.     

A study on chloride-induced depassivation of mild steel in simulated concrete pore solution

The effect of chloride ions on passivity breakdown of steel in simulated concrete pore (SCP) solution was studied using electrochemical techniques. In this regards, the sensitivity of cyclic potentiodynamic parameters such as ΔE (difference between E_pit and E_rep), i_peak and AC-impedance parameters like R_ct, R_f, C_i, R_ad and C_ad to chloride ion concentration was investigated. Adsorption of OH⁻ ions on the metal surface in free chloride SCP solution and also displacement of those ions by Cl⁻ ions were demonstrated in high frequency part of Nyquist plots. In addition, a severe decrease in resistance of interfacial reaction (R_f+R_ct) was observed through breakdown of passive film in the [Cl⁻]/[OH⁻] ratio of 0.6. The interfacial capacitance, C_i, was abruptly raised when localized corrosion changed to general one.     

The investigation of passivating oxide layers on zirconium metal using small single shot anodic current pulses

Passivated metals such as Al, Nb, Ta and particularly Zr were anodically polarized in aqueous sulphate solution by single shot intensiostatic charge pulses, and the overtensions recorded on CRO photograms.

The initial slope of the overtension/time curves is given by the capacitance of the passivating oxide film.

The slope decreases when, with rising electrical field strength across the oxide film, the ionic current flow increases. In agreement with the known theories (Wagner; Mott; Verwey et al.) a straight line results, from plots of the overtension measured at charge densities of 10⁻⁴ C/cm² vs. the logarithm of the current density. From the slope of this straight line the thickness of the passivating layer can be obtained without interference of the surfaces roughness.

For the measurement of the thickness of the passivating oxide film of an individual specimen surface, two successive pulses were applied, the first of 3·3 × 10⁻⁵ A/cm² and the second of 3·3 × 10⁻³ A/cm² current density; the pulse duration was so chosen that each pulse represented the same charge density, 10⁻⁴ C/cm². Thus the modifications of the original surface state of the electrode metal caused by each pulse were negligible. From the difference of the maximum overtensions and the difference of the logarithms of the current densities of the two pulses the thickness of the passivating oxide film of the investigated sample can be calculated.

The method was checked measuring zirconium samples after gravimetrically controlled oxidation. The results obtained applying the method to differently prepared metal surfaces are discussed.