Arnaldo Visintin

Changes in real surface area, crystallographic orientation and morphology of platinum electrodes caused by periodic potential treatments: phenomenological approach

Abstract The changes in surface structure of polyfaceted single crystal platinum spheres subjected to repetitive square-wave potential treatments in acid solution have been studied. The dependence of the prevailing type of change on symmetry, potential limits and frequency of the periodic potential has been determined. Results are discussed on the basis of different reactions occurring within definite potential windows and distinct rate controlling processes depending on the operating conditions. Accordingly, the surface restructuring involves a predominant change in either the real surface area, the crystallographic orientation or the entire surface morphology.

The electroformation of thick hydrous nickel hydroxide films through the application of periodic potential signals

Abstract Nickel hydroxide films in alkaline solutions resulting from the application of fast periodic square-wave potential signals to polycrystalline nickel electrodes have been studied. This procedure promotes the growth of thick hydrous nickel hydroxide films and, correspondingly, a marked increase in the charge storage capacity of the nickel hydroxide electrode. The optimal characteristics of the square-wave potential treatment for growing hydrous nickel hydroxide films are described and a mechanism of the overall process is discussed.

Changes in the surface morphology of platinum electrodes produced by the application of periodic potential treatments in alkaline solution

Abstract The development of preferentially oriented crystalline surfaces of Pt with rough topographies in alkaline solution, produced by the application of periodic square wave potential treatments, is described. The influence of the different electrical variables is studied systematically and the characteristics of the different growth modes are followed through voltammetry in the H-atom electrosorption potential range and SEM micrographs. These effects are explained in terms of a strong OH− ion-metal surface interaction which favours the formation of hydrous Pt oxide layers. The electroreduction of the latter yields Pt overlayers with distinguishable growth modes and preferred crystalline orientations.

Smooth and rough platinum deposits resulting from the electroreduction of hydrous oxide platinum overlayers—a mechanistic approach

The electroreduction of thick platinum oxides accumulated by the application of a fast square wave perturbing potential in 0.5 M H2SO4 was studied by using potentiodynamic and potentiostatic techniques complemented with scanning electron miscroscopy. The electroreduction process can be unambigously explained through an instantaneous nucleation and 3-D growth of right circular cones under charge transfer control where diffusion of reacting species to the growing centres is essential for further expansion. The growth mode of the Pt crystallites either smooth or rough depends on the electroreduction conditions. The rought/smooth modes of growth result from the influence of the electric field on the transport of particles in the reaction region and on the supersaturation concentration operating during the electrocrystallization process.

Hydrogen absorption characteristics and electrochemical properties of Ti substituted Zr-based AB2 alloys

Abstract The hydrogen absorption characteristics of Zr 1− x Ti x CrNi alloys, with x ranging between 0.1 and 0.4, were studied by volumetric and electrochemical techniques in order to determine their possible application as the negative electrode in nickel/metal–hydride rechargeable batteries. Electrodes of alloy powders mixed with different compacting additives were tested with respect to charge and discharge cycles in KOH solution. The effects of the electrode-compacting additive, discharge rate and temperature on the electrode discharge capacity were also investigated. The results obtained show that: (i) at 80°C the equilibrium pressures of Zr 1− x Ti x CrNi alloys with x =0.1 are 2.6 times higher than those for the unsubstituted ZrCrNi alloy, while maintaining a good hydrogen storage capacity; (ii) the Zr 0.9 Ti 0.1 CrNi alloy electrodes exhibit the highest discharge capacities and the best performances during fast charge–discharge cycling; (iii) the Ti substituted alloy electrodes using copper powder as compacting material show faster activation, while carbon–Teflon compacted electrodes exhibit the best cycle life behavior; (iv) for all Ti substituted alloy electrodes the discharge capacities increase with temperature.

The development of facetting and roughening at platinum polyfacetted single-crystal electrodes in a chloroplatinic acid solution

Abstract The description of a procedure for producing preferred crystallographic orientation and roughness development at Pt single-crystal microspheres is the main topic of this paper. It is based on the application of a repetitive square-wave-potential routine to Pt microsphere electrodes in chloroplatinic acid solution. The operation conditions can be adjusted to assure that the Pt electrodeposition and the Pt electrodissolution are the main reactions taking place at the cathodic and anodic half-cycle of the periodic potential routine, respectively. Surface oxide formation on Pt can be largely avoided in the complex-ion-forming solution. Depending on the characteristics of the potential routine different Pt surface textures with specific reactivities for the H-adatom electrosorption processes can be obtained. The procedure discussed in this work can be extended to the preparation of dispersed Pt electrodes with a defined morphology.

The electrochemical performance of Pd-coated metal hydride electrodes with different binding additives in alkaline solution

The performance characteristics of Pd-coated mischmetal-based alloy electrodes containing either copper powder or Teflonized carbon additives were evaluated in alkaline solution at 25 C. The Pd coating was very effective for increasing both the discharge capacity and rate capability and for decreasing the galvanostatic cycling time for activating the alloy electrodes. The best performances were attained with electrodes with 6 wt % Pd content. The electrodes with the Teflonized carbon additive also exhibited greater stability and longer cycle life than those with the copper additive. The improve performance of Pd-coated alloy electrodes over that of an uncoated alloy electrode can be attributed to the catalytic effect of Pd on the charge-transfer step at the electrode surface.

The development of a cobalt oxide spinel structure overlayer on cobalt electrodes: A modified electrode surface of electrocatalytic interest

Abstract A new electrochemical procedure to obtain spinel-type cobalt oxide overlayers on cobalt electrodes is presented. The procedure involves the application of periodic potential treatments to cobalt electrodes immersed in alkaline solutions. The optimal conditions for growing thick cobalt oxide overlayers were determined. The cobalt oxide overlayers were characterized by IR spectroscopy, EDAX, X-ray analysis and differential thermal analysis, and their catalytic activity for the oxygen evolution reaction was evaluated.

Electrochemical characterization of a hydride-forming metal alloy surface-modified with palladium

The effect of surface palladium films on the electrochemical properties of hydride-forming metal alloys used as cathode materials in Ni-metal hydride cells has been studied. The results indicate that the presence of surface Pd improved the discharge capacity and rate capability of the hydride electrodes. The enhancement in performance may be explained on the basis of the electrocatalytic effect of Pd on the kinetics of the hydrogen absorption/desorption (hydriding/dehydriding) reactions.

Development of a novel metal hydride–air secondary battery

A laboratory metal hydride/air cell was evaluated. Charging was via a bifunctional air gas-diffusion electrode. Mixed nickel and cobalt oxides, supported on carbon black and activated carbon, were used as catalysts in this electrode. At 30mAcm−2 in 6m KOH, the air electrode potentials were −0.2V (oxygen reduction) and +0.65V (oxygen evolution) vs Hg/HgO. The laboratory cell was cycled for 50 cycles at the C/2 rate (10mAcm−2). The average discharge/charge voltages of the cell were 0.65 and 1.6V, respectively. The initial capacity of the metal hydride electrode decreased by about 15% after 50 cycles.