Nickel oxide electrodes thickness

Electrochemically generated nickei(lll) oxide, deposited onto a nickel plate, is generally useful for the oxidation of alcohols in aqueous alkali [49]. The immersion of nickel in aqueous alkali results in the formation of a surface layer of nickel(ll) oxide which undergoes reversible electrochemical oxidation to form nickel(lll) oxide with a current maximum in cyclic voltammetry at 1.13 V vj. see, observed before the evolution of oxygen occurs [50]. This electrochemical step is fast and oxidation at a prepared oxide film, of an alcohol in solution, is governed by the rate of the chemical reaction between nickel oxide and the substrate [51]. When the film thickness is increased to about 0.1 pm, the oxidation rate of organic species increases to a rate that is fairly indifferent to further increases in the film thickness. This is probably due to an initial increase in the surface area of the electrode [52], In laboratory scale experiments, the nickel oxide electrode layer is prepared by prior electrolysis of nickel sulphate at a nickel anode [53]. It is used in an undivided cell with a stainless steel cathode and an alkaline electrolyte. 

Figure 8.4 Potential excursion (upper part) and deflection angle variation during the activation of pristine nickel oxide electrode performed by galvanostatic cycles (1= 2 xA cm ). Sample thickness 2000A. Counter-electrode Li foil. Electrolyte iM LiClO -PC. From [26] by permission of Elsevier Science Publishers, Amsterdam.

Figure 8.6 Cyclic voltammetry and optical transmittance of a lithiated nickel oxide electrode in a LiC104-PC solution. Light source He-Ne laser (6328A). Scan rate 20 mV s Sample thickness 6(X)A. From [22] by permission of the Electrochemical Society, Inc.

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

Using the unique four-electrode STM described above, Bard and coworkers (Lev, 0. Fan, F-R.F. Bard, A.J. J. Electroanal. Chem.. submitted) have obtained the first images of electrode surfaces under potentiostatic control. The current-bias relationships obtained for reduced and anodically passivated nickel surfaces revealed that the exponential current-distance relationship expected for a tunneling-dominated current was not observed at the oxide-covered surfaces. On this basis, the authors concluded that the nickel oxide layer was electrically insulating, and was greater than ca. 10 A in thickness. Because accurate potential control of the substrate surface is difficult in a conventional, two-electrode STM configuration, the ability to decouple the tip-substrate bias from... 

The uncertainties regarding che actual topography of the layer obtained after emersion and subsequent transfer into the main UHV chamber precludes a precise determination of its thickness and composition based solely on the AES data. If one assumes, however, that the layer consists of a smooch nickel oxide and/or oxyhydroxide film interposed between the underlying metal and an equally smooch KOH overlayer che K/Ni signal is consistent with the presence of the equivalent of up to three molecular layers of KOH. This indicates chat the film of electrolyte that remains on the electrode surface after the electrolyte absorption step is on the order of a few nm. It must... 

Passivity has been attributed, since the time of Faraday, to an oxide film. In a few cases the presence of this has been directly demonstrated, and the response of a passive electrode to pH changes is very much that of a metal-metal oxide electrode. Uncertainties still exist, however, as to the nature and thickness of the surface film. On platinum, nickel and iron the onset of passivation may only require a monolayer of oxide, or of adsorbed oxygen atoms, although a layer several Angstroms thick may be produced in time. Evidence for these very thin layers, which are quite undetectable visibly, comes from the very small cathodic pulse of electricity that is sufficient to remove them. In other cases, e.g. lead, quite thick films are needed to preserve the passive state. Partial protection is obtained for many metals in a variety of electrolytes so long as conditions favour the precipitation of a film of insoluble hydroxide or salt (see Anodising),... 

They measured the zero potential point of the system, which depends on the halide concentration and type as described by the equations above. They were able to measure the effect of diffusion of species to the electrode and were able to determine a mixed potential of the electrode where the forward transfer of electrons from the silver was equal to the rate of reduction of the oxidant. This is shown diagram-matically in Figure 31. Nickel et al. found that the adsorption of halide to the silver surface did not influence the potential if the halide layer was less than 5 pm thick, but they did note that at a clean silver surface the silver halide formation began at a higher potential than expected, which they attributed to the high solubility of microscopic silver on the surface. 

Disadvantages of thermally grown silicon dioxide are the high deposition temperature of about 960 °C and the exigency of a silicon substrate. Moreover, an adhesion layer of 4-8 nm of nickel is necessary to improve the adhesion of the 30 nm thick gold electrodes deposited on top of the oxide. 

Hic purity partially sintered nickel sheets were obtained from ERC (Electric Research Ciorp.). The porosity of these sheets varied firom 60% to 70%. The proprietary nature of these materials prohibits any release of the pore size distribution. Disks with a thickness of 30 mils and a diameter of 11/4" were cut from this material. This disks were soaked in 1 molar liOH, dried at 100X1, and oxidized in room air at 650 C for at least 8 hours to form a lithiated NiO stmcture. Gravimetric analysis of these electrodes showed that they were greater than 98% converted to NiO. 

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