Diffusion alloy electrode

Various pc electrode models have been tested.827 Using the independent diffuse layer electrode model74,262 the value of E n = -0.88 V (SCE) can be simulated for Cd + Pb alloys with 63% Pb if bulk and surface compositions coincide. However, large deviations of calculated and experimental C,E curves are observed at a 0. Better correspondence between experimental and calculated C,E curves was obtained with the common diffuse-layer electrode model,262 if the Pb percentage in the solid phase is taken as 20%. However, the calculated C, at a Ois noticeably lower than the experimental one. It has been concluded that Pb is the surface-active component in Cd + Pb alloys, but there are noticeable deviations from electrical double-layer models for composite electrodes.827... 

To counteract the (vexing) convection effects on kinetic experiments, Aogaki and co-workers, having developed a special electrode assembly to separate mass transport and kinetic effects, report a marked decrease in the exchange current density (about 25%) in magnetic fields imposed on a copper deposition cell. Virtually no effect on the transfer coefficient (a 0.44) was observed. Experimental results obtained in nickel-phosphorus alloy deposition, cupric ion reduction in ethylenediamine solutions, and the electrolytic reduction of acetophenone " are further demonstrations of the interaction of the magnetic fields with polarization characteristics, and point to the difficulty of fully eliminating the effect of convection and/or diffusion on electrode kinetics. 

From this equation, it becomes obvious that the ratio of the equilibrium ion activities in the solution is linked with the alloy composition as expressed by the bulk atom fractions of the components, Xa and Xb = 1 — Xa- In general, therefore, the establishment of complete equilibrium for an alloy electrode requires a change of composition both of the alloy phase and of the electrolyte solution [1]. For solid alloys at ambient temperature, compositional changes (due to the preferential dissolution of one alloy component) will be restricted to the uppermost atomic layers. Further equilibration between the surface and the bulk of the alloy is prevented by solid-state diffusion limitations. Complete thermodynamic equilibrium for both components is therefore expected to evolve only with liquid alloys in which the diffu-sivity at ambient temperature is extremely high (for dilute Zn-amalgams, e.g., inter-diffusion coefficients t>zn of the order of 10 cm s have been reported under these conditions [2]). 

For most engineering alloys, the ambient temperature only corresponds to a small fraction of the melting temperature, Tm-As outlined above, this implies a very low solid-state diffusivity under these conditions that impedes the establishment of complete equilibrium of the alloy electrode according to Eq. (3). At anodic... 

Figure 5.16 Diffusion of a metal in an alloy electrode, metal wire of radius r, concentration profiles after potential or current pulses n and n+1.

Two limiting equations for the concentration-time dependence on the interface between melt and alloy electrode are obtained. For shorter times than for the characteristic diffusion time (t r /D) the Sand equation is obtained which in this case is... 

Under certain conditions, it will be impossible for the metal and the melt to come to equilibrium and continuous corrosion will occur (case 2) this is often the case when metals are in contact with molten salts in practice. There are two main possibilities first, the redox potential of the melt may be prevented from falling, either because it is in contact with an external oxidising environment (such as an air atmosphere) or because the conditions cause the products of its reduction to be continually removed (e.g. distillation of metallic sodium and condensation on to a colder part of the system) second, the electrode potential of the metal may be prevented from rising (for instance, if the corrosion product of the metal is volatile). In addition, equilibrium may not be possible when there is a temperature gradient in the system or when alloys are involved, but these cases will be considered in detail later. Rates of corrosion under conditions where equilibrium cannot be reached are controlled by diffusion and interphase mass transfer of oxidising species and/or corrosion products geometry of the system will be a determining factor. 

Metal-air cells are developed with air gas-diffusion cathodes and Mg-anodes. Non-aggressive NaCl-solution is used as electrolyte. Carbon based catalysts for the oxygen reduction are selected and tested in the air gas-diffusion electrodes. Various Mg-alloys are tested as anodes. The V-A, power and discharge characteristics of the Mg-air cells are investigated. 

Magnesium-air air cells with NaCl-electrolyte were developed and investigated. The current-voltage and the discharge characteristics of the cells with were studied. Air gas-diffusion electrodes suitable for operation in NaCl-electrolytes were designed. Various carbon-based catalysts for the electrochemical reduction were tested in these air electrodes. Magnesium alloys suitable for use as anodes in Mg-air cells were found. 

Diffusion barriers are coatings that serve in that role specifically, protection against undesirable diffusion. One of the best examples is that of a 100- tm-thick electrode-posited copper layer that serves as an effective barrier against the diffusion of carbon. Another example is that of nickel and nickel alloys (notably, electrolessly deposited Ni-P) that block diffusion of copper into and through gold overplate. This is achieved by the deposition of a relatively thin Ni-P layer (less than 1 /mm) between the copper and its overlayer. Naturally, the effectiveness of the diffusion barrier increases with its thickness. Other factors in the effectiveness of a diffusion barrier... 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

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