M. W. Khalil

Anodic oxidation of bismuth in H2SO4 solutions

Abstract Anodic charging curves have been measured on polycrystalline Bi in H 2 SO 4 solutions (0·01–6·0 N) at 30°C. The effect of various experimental procedures, eg electrode treatment and stirring, has been established. The anodic behaviour of Bi depends markedly on the acid concentration. In the high range, the results indicate dissolution which follows a Tafel relation. However, in dilute solutions, growth of oxide film occurs, and the potential rises rapidly with time, reaching over 100 V at high cds. Sparking and oxide breakdown start at about 150 V. Oxide growth follows the high field ionic conduction. The reciprocal capacitance is linearly related to the logarithm of cd. The constants of the exponential law, the half-barrier width, and the field strength have been calculated The last lies between 1 and 3 × 10 6 V/cm at 1 mA/cm 2 . The effect of F − and Cl − ions on oxide growth in H 2 SO 4 has been also investigated.

Platinum–tin alloy electrodes for direct methanol fuel cells

Pt electrodes, modified by partial electrodeposited tin, were used as anodes for the catalytic electrooxidation of methanol in acid medium. Sn was electrodeposited galvanostatically and potentiostatically. Cyclic voltammetry was used to study the methanol electrooxidation. Pt modified with Sn proved superior to pure platinum as shown from the methanol peak current densities. Sn also improved the performance regarding the stability of the anode over repeated cycles. It was found that electrodeposited Sn facilitates the oxidation of intermediate poison products through a mixed homogeneous–heterogeneous catalytic mechanism.

Anodic oxidation of bismuth in sulphate, carbonate and hydroxide solutions

Abstract Anodic charging curves have been measured by the galvanostatic technique on Bi in 0·05 M solutions of Na 2 SO 4 , Na 2 CO 3 and NaOH at 30°C. Oxide films can be grown to 100 V and more. The nature of the film depends on the pH and the type of anion present. Formation rates are highest in sulphate solutions, and decrease in both acids and alkalis. The theory of galvanostatic anodization accounts for the experimentally observed dependence of formation rate and reciprocal capacitance on cd. Oxide growth follows the highfield approximation in the cd range 10–300 μA/cm 2 . Both the field strength and the activation distance depend on the nature of the electrolyte. Increase of temperature retards film growth due to the increased solubility of oxide.

Anion incorporation and its effect on the dielectric constant and growth rate of zirconium oxides

Mechanically polished zirconium electrodes were potentiodynamically polarized in phosphate buffer solutions of various pH values and in 0.5 lvl NaOH. The results show that the shape of the I-E curves is independent of the solution pH. At relatively low scan rates, oxygen gas evolution was observed. The oxide film thickness was calculated from the values of the charge consumed in the anodic process assuming 100% current efficiency for oxide formation below oxygen evolution (lower values for the current efficiency are assumed for potentials above oxygen evolution). Capacitance measurements, together with the calculated oxide thickness, were used to estimate values for the dielectric constant of the oxide. Two different values of the dielectric constant were obtained for the oxides formed in the range of potential below and above oxygen evolution. Also, higher dielectric constant values were obtained with increasing solution pH. Anion incorporation was assumed to increase the conductivity of the oxide films and, hence, decrease the dielectric constant. A two-layer structure is proposed for the anodically formed oxide on zirconium in aqueous solutions; an anion-free layer near the metal and an outer layer containing the incorporated anions.

Adsorption of bromide ion on polarized iron

Abstract Electrochemical polarization measurements have been carried out in order to investigate the effect of bromide ion on the dissolution of iron in air-free aqueous sulphuric acid. The surface coverage has been calculated from the observed cathodic shift. Bromide ion affects both the cathodic hydrogen evolution and the anodic dissolution of iron. Deviations from the Langmuir isotherm have been observed. Comparison is made with the effect previously reported for other halides.

A review on the electrochemistry of tin

Abstract The recent literature on the electrochemistry of tin has been reviewed since 1970. In order to avoid unnecessary length, only papers which contain significant contributions are included. The review has been written in a bibliographical manner, and papers are arranged chronologically in each section. In this manner, the reader can follow the developments in each section as new techniques and new ideas are introduced, and can easily draw conclusions regarding the agreement and contradictions between the results of various authors. Furthermore, he has the liberty of formulating plans for future research, without being biased by the reviewers personal opinions. The contents of this review have been classified into twelve sections: 1. (1) Anodic dissolution of Sn and tinplate. 2. (2) Corrosion of Sn and tinplate. 3. (3) Passivity and anodic passivation of Sn, tin alloys, and tinplate. 4. (4) Cathodic passivation in dichromate. 5. (5) Electrodeposition (electroplating) of Sn and Sn alloys. 6. (6) Electrochemical properties of electroplated Sn alloys. 7. (7) Semi-conducting properties of SnO 2 . 8. (8) Some industrial aspects of tinplate. 9. (9) Cathodic processes. 10. (10) Electron transfer studies between soluble Sn species. 11. (11) Photo-effects. 12. (12) Miscellaneous studies. Each section written as concisely as possible, and 190 references are included.

Kinetics of the ferro—ferricyanide electron transfer reaction on anodically formed tin oxide

Abstract The kinetics of electron transfer reaction across a layer of tin oxide formed on tin electrodes were measured in borate buffer pH = 8. The effect of concentration of the redox system, Fe(CN)3−6/Fe(CN)4−6, on the rate of the process and also the effect of oxide thickness were measured by cathodic and anodic polarization curves. A reaction order of 0.7 with respect to the redox system was found. An exponential dependence of the exchange c.d, i0, on film thickness was interpreted on the basis of direct tunnelling through the oxide. Thick films formed at potentials within the O2-evolution range showed increase of i0 as function of film thickness.

Anodic passivation of iron in alkaline solutions containing halides

Passivation of Fe in pure NaOH solutions, and in alkaline solutions containing Clt-, Brt-, and It- ions has been studied by measuring the galvanostatic relations during anodic polarization to the O2-evolution potential, followed by cathodic polarization to the H2evolution potential. Halides decrease the charge needed for each of the two arrests observed in the anodic curve, and for the arrest in the cathodic curve. At the same halide concentration, the effect increases with increase of atomic number. However, it increases with increase of concentration for the same halide. The phenomena are explained on the basis of adsorption. The possible role of XOt- ions has been also considered. In pure NaOH solutions, the effect of pH on the potential E of the two anodic arrests is given by dE/dpH = -70 to -75 mV, in agreement with the suggested mechanisms for iron dissolution in alkaline solutions.

Evidence for electrosorption of halides on polarized iron

Abstract The effect of temperature on the cathodic polarization of Fe is reported in H 2 SO 4 solutions containing Br − and I − . Both ions decrease the rate of the hydrogen-evolution reaction (her). The results are discussed in connexion with (i) changes in the structure of the double layer which influence both the apparent heat of activation Δ H * for the her, and the pre-exponential term of the exchange cd, and (ii) blocking of the surface. While their effect on Δ H * is relatively small, halides produce a considerable decrease in the pre-exponential term. Because double-layer contributions to the latter cannot be calculated separately, the whole change is related to surface blocking. This allows the calculation of apparent surface coverage as a function of temperature, and based on the Langmuir isotherm estimates are obtained for the enthalpy and entropy of adsorption. Positive values are found in both cases. The results provide evidence for electrosorption of halides on Fe.