Metal-aqueous systems, electrode-electrolyte

The facilitated transfers of Na+ and K+ into the NB phase were observed by the current-scan polarography at an electrolyte-dropping electrode [12]. In the case of ion transfers into the DCE phase, cyclic voltammetry was measured at an aqueous gel electrode [9]. Both measurements were carried out under two distinctive experimental conditions. One is a N15C5 diffusion-control system where the concentration of N15C5 in the organic phase is much smaller than that of a metal ion in the aqueous phase. The other is a metal ion diffusion-control system where, conversely, the concentration of metal ion is much smaller than that of N15C5. Typical polarograms measured in the both experimental systems are shown in Fig. 2. 

It must be remembered that in aqueous systems the redox process occurs over the entire electrode area, whereas in solid electrolyte systems the redox process occurs only in the three-phase or charge-transfer region. The technique has been used with solid electrolyte systems for sometime to study the oxidation and reduction of metals and metal oxides in inert atmospheres,94,95 the behaviour of solid oxide fuel cell (SOFC) electrodes and has also been applied to the in-situ study of catalysts.31,32,95... 

Using supporting electrolytes such as tetraalkylammonium salts, one may apply potentials as negative as -2.6 V vs. SCE in aqueous solutions, while in some nonaqueous systems even -3.0 V vs. SCE (aqueous) is accessible. Unfortunately, mercury electrodes have serious limitations in applications at positive potentials (with the exception of passivated mercury electrodes, which are described in Section VI), and this has led to extensive research in the development of solid metal and carbon electrodes. Oxidation of mercury occurs at approximately +0.4 V vs. SCE in solutions of perchlorates or nitrates, since these anions do not form insoluble salts or stable complexes with mercury cations. In all solutions containing anions that form such compounds, oxidation of the mercury proceeds at potentials less than +0.4 V vs. SCE. For example, in 0.1 M KC1 this occurs at +0.1 V, in 1.0 M KI at -0.3 V, and so on. 

It is well known that the product distribution also depends on the electrode material used [13, 14]. We have examined the effect of various electrode materials on the product distribution in the CO -methanol system. The current efficiencies of the reduction products are shown in Figure 6. The production of formate was fairly favorable at all electrodes, in comparison with that in aqueous systems. For example, the production of formate was more than 20% on Pt and Ni electrodes, which is much higher than that observed in aqueous systems [14]. At the metals Sn and Sb, formate production was favoured, as in the aqueous systems, but CO formation was also somewhat favored. This is due to the effect of supporting electrolyte. The electrodes Ag, Zn and Pd showed similar activities for CO production, as in aqueous systems. The efficiency of hydrocarbon formation at the Cu electrode was found to be lower, whereas that at the Ni electrode was found to be higher than that in aqueous systems. The balance of hydrogen and carbon atom concentrations on the electrode surface may explain this difference. 

Nickel/cadmium batteries (line 8 in Table 1.1) have been in technical use nearly as long as lead-acid batteries. They belong to a whole family of secondary batteries that are based on aqueous, but alkaline electrolyte, usually diluted potassium hydroxide. Nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries are the most important members of this group. A further common feature of these battery systems is that they employ the nickel-hydroxide electrode as the positive one. Some of their basic features will be described in the following. 

The electrochemical reactions associated with the discharge of a zinc/silver oxide battery as a primary system are generally considered to proceed as follows. The cathode or positive electrode is silver oxide and may be either Ag20 (monovalent), AgO (divalent), or a mixture of the two. The anode or negative electrode is metallic zinc, and the electrolyte is an aqueous solution of potassium hydroxide. The chemical reactions and the associated voltages at standard conditions are... 

Zinc is also attractive for electrically rechargeable metal/air systems because of its relative stability in alkaline electrolytes and also because it is the most active metal that can be electrodeposited from an aqueous electrolyte. The development of a practical rechargeable zinc/air battery with an extended cycle life would provide a promising high-capacity power source for many portable applications (computers, communications equipment) as well as, in larger sizes, for electric vehicles. Problems of dendrite formation, nonuniform zinc dissolution and deposition, limited solubility of the reaction product, and unsatisfactory air electrode performance have slowed progress toward the development of a commercial rechargeable battery. However, there is a continued search for a practical system because of the potential of the zinc/air battery. 

As discussed in Chapter 1, three main types of electrolytes are used in lithium-ion batteries. Liquid electrolytes are most frequently used. Since the potential of the negative electrode of lithium-ion batteries is close to that of lithium metal, the negative electrode is relatively active and unstable in aqueous solutions. Therefore, a nonaqueous, aprotic organic solvent should be used as the lithium-ion carrier. The mixture of organic solvents and lithium salts constitutes the nonaqueous liquid electrolyte, also referred to as the organic liquid electrolyte, which is an indispensable ingredient of lithium-ion batteries and an important component of gel polymer electrolytes. The same electrode material may perform differently in different electrolyte systems. 

The investigation of various aqueous and nonaqueous systems, including adsorption of molecules and ions at the electrode/electrolyte interface and oxide films on a metal substrate. 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

Several technical arrangements have been used successfully to immobilize this catalyst on an electrode surface as thin films.80-85 In such arrangements the metal sites in films show a dramatic increase in reactivity and stability toward C02 reduction into CO. Moreover, this kind of modified electrode (for instance [Re(bpy)(CO)3Br] incorporated in Nafion membrane) appeared as a good electrocatalyst in pure aqueous electrolyte.86 However, in such systems both CO and HCOO are also produced, and the total current yield of C02 reduction is lowered by the concurrent H+ reduction into H2. 

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