M. López Teijelo

The electrochemical response of binary mixtures of hydrous transition metal hydroxides co-precipitated on conducting substrates with reference to the oxygen evolution reaction

Abstract The potentiodynamic behaviour of colloidal electrodes obtained by co-precipitation of binary mixtures of hydrous transition metal hydroxides on conducting substrates in alkaline solutions is described as well as some aspects about single precipitated hydroxides. The mixed interphases investigated can be classified into three groups according to the interaction between the components. The catalytic activity of the mixtures towards the oxygen evolution reaction depends also on that interaction. The interphases involving the co-precipitation of hydrous Ni(OH) 2 with either Mn(OH) 2 or Fe(OH) 2 give the best performance as electrocatalysts for O 2 -evolution; in both mixed electrodes the rate of this reaction is higher than that of the corresponding individual components.

Complex potentiodynamic response of silver in alkaline Electrolytes in the potential range of the Ag/Ag2O Couple

Abstract The potentiodynamic behaviour of Ag in 0.1 M NaOH in the potential range of Ag 2 O electroformation is investigated through the application of different potential/time perturbation programmes. The splitting of the electroreduction profile which is produced under well-defined perturbation conditions and the shift of the Ag 2 O electroreduction E/i profile under a constant charge condition, suggest that ageing-type processes are involved in the overall electrochemical reaction. These processes are related to a probable dehydration of the Ag 2 O multilayer.

The electroformation and electroreduction of anodic films formed on silver in 0.1 M sodium hydroxide in the potential range of the Ag/Ag2O couple

Abstract The complex electrochemical reactions related to the electroformation and electroreduction of silver(I) oxide films in base electrolyte were investigated by means of combined potentiodynamic techniques and potential steps. The anodic process involves the initial fast OH− ion discharge yielding OH-adsorbed species and later continues to form the silver(I) oxide layer. The latter behaves as a duplex structure anodic layer with a degree of hydration changing with depth. The electroreduction characteristics of the anodic film depend both on potential sweep and anodic charge, and the relationships of the corresponding kinetic parameters fit qualitatively with those predicted by nucleation and growth mechanisms. Activity increases of silver caused by the potential sweeps are interpreted by the simultaneous contribution of the increase in the concentration of silver active surface sites and the change in size distribution of silver electrodeposited from silver(I) oxide.

The hanging-meniscus rotating disk (HMRD) Part 1. Dependence of hydrodynamic behavior on experimental variables

Abstract The hydrodynamic behavior of the hanging-meniscus rotating disk (HMRD) has been studied under different experimental conditions. The HMRD is a configuration that allows an electrode material to be studied without being mounted in an insulating shroud. The influence of experimental variables such as electrode material, electrode diameter and meniscus height is discussed. A correction term which accounts for the differences in hydrodynamic boundary conditions introduced in the standard Levich equation allows the slope and the small negative intercept of Levich plots for the HMRD as well as the dependence on experimental variables to be explained. Conditions for the proper use of this configuration are also established.

The effect of iron hydroxide on nickelous hydroxide electrodes with emphasis on the oxygen evolution reaction

The electrochemical response of mixed Ni(OH)2 and Fe(OH)2 electrodes prepared either by chemical coprecipitation or layer by layer precipitation in alkaline solutions at 25°C was studied. Ni(OH)2 coprecipitated with Fe(OH)2 forms a mixed hydrated oxide with specially high catalytic activity towards the oxygen evolution reaction (OER). In layer by layer precipitated electrodes, the catalytic activity for the OER depends on the sequence of precipitation of the hydroxides and there is a loss of efficiency when the inner layer is made of Fe(OH)2 because of a poisoning of the electrode. Kinetic and mechanistic aspects are discussed. Some long term experiments at controlled potential were also carried out.

Electrochemical behaviour of tungsten in alkaline media: Part I. NaOH solutions

Abstract The electrochemical behaviour of tungsten in alkaline NaOH solutions in the pH range 7–13 was investigated using single and repetitive triangular potential sweeeps and the rotating disc electrode. In the −0.7 to 0.3 V (SHE) potential range the anodic process involves the formation of soluble tungstate species and at higher potentials the electroformation of different tungsten oxides occurs up to the completion of the electrooxidation process to give the WO3 oxide. For potentials more positive than 1 V, the anodic WO3 film grows according to a high-field mechanism as in a typical valve metal. The onset of oxygen evolution is observed at ca. 8.7 V. The electroreduction processes correspond to the formation of a tungsten and hydrogen “bronze” on the electrooxidised surface prior to the hydrogen evolution reaction.

The hanging meniscus rotating disk (HMRD) Part 2. Application to simple charge transfer reaction kinetics

Abstract The hanging meniscus rotating disk (HMRD) is a configuration in which the electrode material is studied without first being mounted in an insulating mantle. It was recently shown that the hydrodynamic behavior of this system is similar to that of the conventional rotating-disk electrode. In this paper the applicability of this configuration to the study of simple charge transfer reaction kinetics is analyzed and some experimental data are presented. The kinetic parameters for simple charge transfer reactions can be obtained using the HMRD as well as on conventional rotating-disk electrodes.

Comparative voltammetric behaviour of the silver/silver oxide electrode prepared on vitreous carbon and silver substrates

The voltammetric behaviour of vitreous carbon/chemically precipitated silver hydroxide layer and silver/electrochemically formed silver oxide layer electrodes are compared. As the former type of electrode is free from silver substrate contributions during the oxidation-reduction cycles (ORC) the voltammetric data indicate the occurrence of soluble Ag(I) species in the gel-like silver hydroxide matrix. Soluble species can be formed during the ORC as a consequence of AgOH formation at the early stages of the oxidation and reduction processes. Nucleation and growth of new phases produce a non-homogeneous layer structure and a decrease in the amount of active material participating in the ORC under preset conditions.

Electrochemical and ellipsometric data related to the formation of soluble silver (I) during the potentiodynamic polarization of polycrystalline silver in basic solutions

Abstract Soluble Ag(I) species is formed in the early stages of voltammetric silver electrooxidation in 0.1 M NaOH and 0.1 M NaOH + 1 M NaClO 4 solutions at 25°C, as demonstrated by the rotating ring-disc technique. This result allows a reasonable interpretation of ellipsometric data obtained for the anodization of silver under different voltammetric conditions. Correspondingly, two anodic layers of different optical constants are electroformed. The hydrous Ag(I) oxide layer behaves as a porous structure with an average thickness of about 2.5 times greater than that of the anhydrous oxide.

Ellipsometry of silver electrodes in base solutions under different potential controlled perturbation conditions

Abstract The ellipsometric and reflectance response at 5461 A of silver in 0.1 M NaOH at 25°C, is studied in the Ag(0)/Ag(I) potential range when the electrode has been subjected to different complex potentiodynamic perturbations. The optical response depends remarkably on the potential sweep rate, on the accumulated anodic charge and on the number of potential cycles. The data of the complex anodic film can be interpreted in terms of a single film model in two limiting cases defined in terms of the anodic charge involved as thin and thick films, respectively: (i) for thin anodic films ( Q a −2 ) − n f - 1.50 –0.12 i and (ii) for thick anodic films ( Q a > 50 mC cm −2 )− n f = 1.50 − 0.22 i . Time dependence of the optical parameters of both reformed silver and anodic films are shown. Optical results correlates with electrochemical data recently reported and are discussed in terms of different degree of hydrations for each type of anodic film.