G. H. Kelsall

Growth kinetics of bubbles electrogenerated at microelectrodes

The growth kinetics of electrogenerated hydrogen, oxygen and chlorine gas bubbles formed at microelectrodes, were determined photographically and fitted by regression analysis to the equation;r(t)=βtx, wherer(t) is the bubble radius at timet after nucleation,β the ‘growth coefficient”, andx the ‘time coefficient’. The coefficientx was found to decrease from a short time (< 10 ms) value near unity, typical of inertia controlled growth, through 0.5, characteristic of diffusional control, to 0.3, expected for Faradaic growth, at long times (\s> 100 ms). The current efficiency for bubble growth increased with bubble lifetime, reflecting the decrease in local dissolved gas supersaturation. The pH dependency of the bubble departure diameter indicated that, in surfactant-free electrolytes, double layer interaction forces between the negatively charged hydrogen evolving cathode or positively charged oxygen/chlorine evolving anode and positively (pH \s< 2) or negatively (pH \s> 3) charged bubbles, were the determining factor. The effect of addition of an increasing concentration of cationic (DoTAB) or anionic (SDoS) surfactant was to progressively reduce the pH effect on departure diameter, due to surfactant adsorption on the bubble and, to a lesser extent, on the electrode.

Surface Oxidation of Chalcopyrite ( CuFeS2 ) in Alkaline Solutions

The surface oxidation of chalcopyrite in solutions of pH 9.2 and 12.7 has been investigated using electrochemical techniques, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The results show that the oxidation process consists essentially of three potential- and pH-dependent stages. In 0.1 M Na 2 B 4 O 7 (pH 9.2), on increasing the electrode potential from -0.6 to +0.02 V vs. SCE, the iron in the top layer of the chalcopyrite surface is oxidized, forming a monolayer of Fe(OH) 3 and Fe 2 O 3 . The copper and sulfur remain unoxidized as a phase we designate CuS 2 * , which together with Fe(OH) 3 and Fe 2 O 3 forms a film retarding the oxidation. As the potential is increased further, deeper layers are involved in the oxidation, but the passivating film is not destroyed. At this stage, the oxidation process is controlled by solid-state mass transport. When the applied potentials are higher than 0.4 V vs. SCE, CuS 2 * is no longer stable and is oxidized to CuO, S, and SO 2 4 ions. The passivating film then decomposes, greatly accelerating the oxidation rate of the underlying CuFeS 2 . In 0.05 M NaOH (pH 12.7), the oxidation mechanism is similar to that in 0.1 M borax solution. However, because the equilibrium potentials are lower, the corresponding current peaks appear at less positive potentials. In addition, higher concentrations of OH - ions enhance the dissolution rates of iron and copper oxides and hydroxides, so increasing reaction rates.

Interfacial electrical properties of electrogenerated bubbles

Electrophoresis measurements on bubbles of electrogenerated hydrogen, oxygen and chlorine rising in a lateral electric field, are reported. In surfactant-free solutions, all bubbles displayed a point of zero charge of pH 2–3, i.e. they were negatively charged at pH > 3 and positively charged at pH < 2. The bubble diameter and electric field strength dependence of the electrophoretic mobilities, coupled with bubble rise rate measurements, indicated that the gas—aqueous solution interface was mobile, such that classical electrophoresis theory for solid particles could not be applied. Adsorption of anionic or cationic surfactants, in addition to modifying the apparent bubble charge, also tended to rigidify the bubble surface, so that at monolayer coverage the bubbles behaved as solid particles.

Redox chemistry of H2S oxidation in the British Gas Stretford Process Part I: Thermodynamics of sulphur-water systems at 298 K

The thermodynamics of aqueous sulphur-water systems are summarized in the form of potential-pH diagrams, calculated from recently reported critically assessed standard Gibbs energies of formation of the species considered. However, there is convincing evidence from the literature that a value of pKa(HS−) = 17–19 is appropriate, whereas a value of 13 is widely accepted; hence, the higher value of 19, corresponding to ΔGf0(S2−) = 120.5 kJ mol−1 , was used in these calculations, rather than ΔGf0(S2-) = 86.31 kJ mol−1 quoted in the main data source.Under ambient conditions, only − 2 (sulphide), 0 (elemental sulphur) and + 6 (sulphate) oxidation states are thermodynamically stable in water, which is predicted to be oxidized by peroxodisulphate (H2 S2 O8/SO82− and peroxomonosulphate (HSO5−/SO52−). However, when sulphate is excluded from the calculations to allow for the large energy of activation/slow kinetics of its formation from sulphide, then other sulphoxy species appear on the diagram for what is then a metastable system. Similarly, if all sulphoxy species (i.e. any species with oxidation states > 0) are excluded, then polysulphide ions (Sn2−, 2 ⩾ n ⩾ 5) have areas of predominance at high pH, each with a narrow potential window of predominance. Hence, this information is complemented with Sn2−/HS− activity-potential diagrams at pH 9 and 14.Some species have no area of stability even on the metastable diagrams. Hence, potential-pH diagrams are also presented for the sulphite-dithionite system (excluding elemental sulphur), and that involving peroxomonosulphate (HSO5−/SO52−) in place of peroxodisulphate (H2S2O8/SO8¨−).

Thermodynamics and electrochemical behaviour of HgSClH2O systems

Abstract The thermodynamics of mercury–sulfur–chlorine–water systems are summarised in the form of potential–pH and activity–pH diagrams. The potential–pH diagram for HgClH 2 O shows that chloride ions greatly extend the areas of stability of Hg 2 2+ and Hg 2+ ions, forming Hg 2 Cl 2 (c) and HgCl 4 2− ions. While the potential–pH diagram for the HgSH 2 O system predicts oxidation of HgS to S(VI) and elemental Hg, this was not observed experimentally with bulk and particulate HgS, due to the strong interaction of sulfur with mercury, and the large activation energy involved. Hence, metastable potential–pH diagrams were calculated, which provide a good prediction of the observed behaviour of HgS and Hg in sulfide solutions. Voltammetry and chronoamperometry were used to investigate the oxidation and reduction behaviour of an HgS electrode. A novel process for the treatment of mercury-bearing effluents is described, based on the indirect reduction of HgS precipitates using electrogenerated Cr(II) as a reductant.

Electrochemical and Surface Analytical Studies of Enargite in Acid Solution

The electrochemical oxidation and reduction of the surface of natural enargite (Cu3AsS4) was investigated in 0.1 M HCl solution using cyclic voltammetry and chronoamperometry. Surface analysis by ex situ X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy, together with aqueous phase analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES), were used to aid in the interpretation of the electrochemical behavior of this complex system. XPS analyses on enargite oxidized at potentials >0.2 V (SCE) detected the presence of Cull surface species, with associated sulfate and chloride. At potentials 0.2 V (SCE), whereas dissolved arsenic concentrations were negligible over the entire potential range investigated. Raman spectroscopy provided additional evidence of elemental sulfur formation at oxidizing potentials at which sulfate forming reactions occurred in parallel. The sulfur was responsible for an active-passive transition observed at ca. 0.3 V (SCE) in voltammograms

Redox chemistry of H2S oxidation by the British Gas Stretford process part IV: V-S-H2O thermodynamics and aqueous vanadium (v) reduction in alkaline solutions

The thermodynamics of V-H2O and V-S-H2O systems at 298 K are summarized in the form of potential-pH and activity-pH diagrams calculated from recently published critically assessed standard Gibbs energies of formation. At pH 9, as used in Stretford Processes, V-H2O potential-pH diagrams predicted that the V(v)/V(iv) couple involves HV2O73−/V4O92− ions. However, in neutral and alkaline solutions there is difficulty in discriminating between kinetic intermediates and thermodynamically stable solution species, some with V(v)/V(iv) mixed oxidation states. Hence, V18O4212− ions may be the stable V(iv) species, depending on the concentration, though no thermodynamic data are available to enable them to be included in potential-pH calculations. Potential-pH diagrams for V-S-H2O systems predicted an area of stability for VS4 in the pH range ≈2–8 and over a restricted potential range; neither VS nor VS2 were predicted to be stable under any conditions considered. In cyclic voltammetric experiments at Hg, Au and vitreous carbon electrodes, reduction of vanadium (v) species (probably HV2O73− ions) was found to be irreversible on a variety of electrode surfaces and, at lower potentials, led predominantly to the formation of solid oxide films (V3O5, V2O3 and VO) rather than to V(iv) solution species, of which V18O4212− ions probably predominate at equilibrium. In the presence of the large excess of HS− ions required to form VS43− ions, the electrochemical behaviour of a gold electrode was dominated by the former species.

Redox chemistry of H2S oxidation by the British Gas Stretford Process Part. II: Electrochemical behaviour of aqueous hydrosulphide (HS−) solutions

Electrochemical techniques were used to study the oxidation of HS− ions at pH 9.3. Voltammetry of gold electrodes in HS− -containing solutions showed that multilayers of sulphur and soluble oxidation products were formed. As a known HS− oxidation product, thiosulphate solutions were also studied voltammetrically, but found to be electro-inactive at mildly oxidising potentials. The voltammetric behaviour of polysulphide ions, Sn2−(n = 2 to 5), was similar to that of HS− solutions on oxidation, though they could be reduced to HS− ions at low potentials. Ring-disc electrode experiments, extending the HS− concentration range that had been studied previously, confirmed that polysulphide ions were produced on reduction of anodically deposited elemental sulphur. This was demonstrated in both cyclic Volammetry and potential step experiments. By comparison of the charges passed producing polysulphides from sulphur and reducing them to HS− ions, an average polysulphide chain length of 1.8 was calculated, indicating a mixture of species was produced. Ion chromatography confirmed that polysulphide solutions do contain a number of species, consistent with thermodynamic predictions.

CO2 splitting into CO and O2 in micro-tubular solid oxide electrolysers

Micro-tubular solid oxide electrolysers for electrochemical CO2 reduction of the form Ni–YSZ|YSZ|YSZ–LSM|LSM have been fabricated using a two-step method: dual layer co-extrusion phase inversion to produce electrode-supported| electrolyte precursors and subsequent coating with the outer electrode, whereby each step was followed by (co-)sintering. The microstructures and physical properties of the fibres were characterized and the electrochemical performance of the fabricated electrolysers determined. Electrolyte thicknesses of 19 (±2), 26 (±2) and 49 (±3) μm were achieved. Electrolysis performance increased with increasing temperature (700–800 °C) and with decreasing electrolyte thickness. The maximum performance achieved was 1.0 A cm−2 at 1.8 V cell potential difference at 800 °C for a 15 μm electrolyte. Electrical impedance spectroscopy revealed that only 5–28% of the ohmic polarization resistance was due to the electrolyte resistance; most of the resistance was due to electrical connections and contact potential losses. The feasibility to operate the solid oxide cells in electrolysis and fuel cell modes was demonstrated, revealing that no unique gas composition existed that would optimise the performance in both modes simultaneously.

Mathematical Models for Time-Dependent Impedance of Passive Electrodes

Three mathematical models are reported for the analysis of time-dependent electrode impedance data of passive electrodes. These models describe the complex relationships between the impedance, the components of the electrical equivalent circuit, the characteristics of the passive layer, the ac frequency, and the measurement time, Hence, they can be used to analyze time-dependent electrode impedance data obtained from the anodic oxidation of metals, alloys, conducting or semiconducting metal compounds in aqueous solutions. Both single and double passive layer models predict that the real impedance and negative imaginary impedance are approximately proportional to the square root of the resistivity of the passive layer and the measurement time. A solid-state transport model predicts that the impedance increases with measurement time, and with a decrease in either the concentration and/or transport coefficient of the electroactive ions.