Electrodes, passive

Subsequent deposition and dissolution of lithium at the anodically passivated electrode by CV in the range -1000 mV to 6000 mV versus Li was successful, showing that the passivating film at the electrode is only impermeable for the anions, but not for lithium ions however, the amount of lithium deposited decreases, with cycle number. 

Upon an increase of the anodic reverse potential finally up to 8 V versus Li the cyclic voltammogran corresponding to Fig. 9 remains unchanged, showing that the passivating layer at the electrode also protects the solvents (PC and DME) from being oxidized. Subsequent deposition and dissolution of lithium at the passivated electrodes remains possible when the electrode is passivated but the cycling efficiency decreases. 

Figure 16 shows the effect of the potential of passivated electrode and the interfacial tension of film-free metal/electrolyte interface on the activation barrier for film breakdown. From Eq. (22), the minimum potential for film breakdown AE corresponding to A b = is given by... 

Some electrodes are made of substances that participate in the redox reactions that transfer electrons. These are active electrodes. Other electrodes serve only to supply or accept electrons but are not part of the redox chemistry these are passive electrodes. In Figure 19-7. both metal strips are active electrodes. During the redox reaction, zinc metal dissolves from the anode while copper metal precipitates at the cathode. The reactions that take place at these active electrodes are conversions between the metals contained in the electrodes and their aqueous cations. 

In contrast with these active electrodes, a passive electrode conducts electrons to and from the external circuit but does not participate chemically in the half-reactions. Figure 19-8 shows a redox setup that contains passive electrodes. One compartment contains an aqueous solution of iron(III) chloride in contact with a platinum electrode. Electron transfer at this electrode reduces Fe " (a q) to Fe " ((2 q) ... 

Passive electrodes serve only to conduct electrons to and from the interfaces. They do not take part in the redox reactions. 

Platinum metal is often used as a passive electrode because platinum is one of the least reactive elements. Platinum has a large ionization energy, so it can act as an electron shuttle without participating in redox chemistry. 

When an electrolytic cell is designed, care must be taken in the selection of the cell components. For example, consider what happens when an aqueous solution of sodium chloride is electrolyzed using platinum electrodes. Platinum is used for passive electrodes, because this metal is resistant to oxidation and does not participate in the redox chemistry of the cell. There are three major species in the solution H2 O, Na, and Cl. Chloride ions... 

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

The nature of the passive film has been the object of innumerable further studies with arguments over its thickness, composition, structure and electronic properties raging. What has become very clear is that that removal from solution and drying of the passivated electrode alters the film profoundly and any studies of the film ex situ must be treated with considerable caution. For that reason we will concentrate here primarily on in situ studies or at the least those studies carried out on passive films that retain their hydration. 

There are a great many names given to these types of electrode. Some texts call them non-passive electrodes and others melal indicator electrodes. 

The electrode potential of such a solution-phase system is best followed with an inert electrode such as platinum or gold. An inert electrode is so called because it is not involved in the redox reaction except as a probe of the electrode potential E. An inert electrode is also called a passive electrode, flag electrode or indicator electrode. 

The membrane surface may become passivated by some solution components that are strongly adsorbed. This effect is often encountered in measurements on biological fluids containing proteins. These adsorption effects can sometimes be prevented by selecting a suitable compoation of the sample and standard solutions for example by adding trypsin and triethanolamine to dissolve proteins [108]. Passive electrodes can sometimes be reactivated by soaking in suitable solutions (for example pepsin in 0.1M HCl [68]) and in more serious cases the membrane must be replaced or a solid membrane be repolished. 

An effort has been made to apply mercury electrodes at positive potentials. Such electrodes are constructed from mercury covered by a thin layer of insoluble mercury compound. This electrode was used for the first time by Kuwana and Adams [65,66]. In their experiments mercury was covered by a thin layer of calomel produced by the oxidation of mercury in chloride solutions. 7V,7V -Dimethyl-/ -phenylenediamine and ferrocyanide were oxidized at these passivated electrodes. 

The older literature on the electrochemistry of dioxygen in acidic media attributes the difference between the thermodynamic potential for the four-electron reduction (02/H20, +1.23 V vs. NHE Table 9.3) and the observed value at a freshly activated platinum electrode [+0.67 V vs. NHE, Eq. (9.13)] to overvoltage (or kinetic inhibition). Likewise, the difference between the thermodynamic potential for the two-electron reduction (02/H00H, +0.70 V vs. NHE, Table 9.3) and the observed value at passivated electrodes [+0.05 V vs. NHE, Eq. (6.12)] was believed to be due to the kinetic inhibition of the two-electron process. 

Figure 4 Evans diagram illustrating the influence of solution velocity on corrosion rate for a passivating electrode exhibiting an active-passive transition. The cathodic reaction shown is under mixed charge transfer-mass transport control.

Ateya and Pickering have been concerned with the cathodic polarization of a crevice or recess (21,22). Note that while the term crack has been used, the crack half-angle is zero, which is not realistic for actual cracks. They focus on the situation where the external surface is either anodically or cathodically polarized for various metals. Active/passive electrodes and actively corroding metals have both been considered in these analyses. 

Active Electrode Porous Support Passive Electrode Filter Paper... 

The system comprising the resistor Re and capacitor C in series provides an example of a class of systems for which, at the zero-frequency or dc limit, current cannot pass. Such systems are considered to have a blocking or ideally polarizable electrode. Depending on the specific conditions, batteries, liquid mercury electrodes, semiconductor devices, passive electrodes, and electroactive polymers provide examples of systems that exhibit such blocking behavior. 

Assuming the fluid has no component acting toward the passive electrode and all particles achieve their saturation charge, the equation of motion of a charged spherical particle in an electric field is... 

Curves 1 and 2 of Fig. 11 have been taken on passive electrodes in hexamethylphosphotriamide solutions of different composition (sodium and lithium salts) and curve 6 — on a nonpassivated electrode (tetramethyl-, tetraethyl- and tetrabutylammonium salts). The rate of the process is equal for sodium and lithium, and is independent of the alkyl radical chain lenght. The generation rates in tetraalkylammonium salt solution were close to those in hexamethylphosphotriamide solutions of lithium salts on a copper electrode specially activated by anodic treatment (curve 3 in Fig. 11). The practical equality of the currents in these systems opposes the idea that intermediate adatoms are participating. 

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