Metallic electrode behavior

Models have been proposed to reproduce the curves in Fig. 8. Behavior at metal electrodes was discussed by Frumkin and Damaskin in this... 

The behavior of metal electrodes with an oxidized surface depends on the properties of the oxide layers. Even a relatively small amount of chemisorbed oxygen will drastically alter the EDL structure and influence the adsorption of other snb-stances. During current flow, porous layers will screen a significant fraction of the surface and interfere with reactant transport to and product transport away from the surface. Moreover, the ohmic voltage drop increases, owing to the higher current density in pores. All these factors interfere with the electrochemical reactions, particularly with further increase in layer thickness. 

Capon A, Parsons R. 1973b. The oxidation of formic acid on noble metal electrodes. Part II. A comparison of the behavior of pure electrodes. J Electroanal Chem 44 239-254. 

The previous analysis indicates that although the voltammetiic behavior suggests that the aqueous phase behaves as a metal electrode dipped into the organic phase, the interfacial concentration of the aqueous redox couple does exhibit a dependence on the Galvani potential difference. In this sense, it is not necessary to invoke potential perturbations due to interfacial ion pairing in order to account for deviations from the Butler Volmer behavior [63]. This phenomenon has also been discarded in recent studies of the same system based on SECM [46]. In this work, the authors observed a potential independent ket for the reaction sequence. 

Figure 11.6. Crossed nanowire p-n diode, (a) A typical SEM image of a crossed NW p-n diode, (b) Current-voltage (I-V) relation of the crossed p-n diode. Linear or nearly linear I-V behavior of the p-type and n-type NWs indicates good contact between NWs and metal electrodes. I-V curves across the junction show clear current rectification, (c) An SEM image of an NW p-n diode array, (d) I-V behavior for a 4(p) x l(n) multiple junction array. [Adapted from Ref. 57.]... 

Charge transfer reactions represent an important category of electrochemical behavior. As already pointed out above, an appropriate investigation of kinetic parameters of electrochemical reactions in aqueous electrolytes suffers from the small temperature range experimentally accessible. In the following, some preliminary results using the FREECE technique are presented for the Fe2+/Fe3+ redox reaction and for hydrogen evolution at various metal electrodes. 

Based on the discussion above, it seems evident that a detailed understanding of kinetic processes occurring at semiconductor electrodes requires the determination of the interfacial energetics. Electrostatic models are available that allow calculation of the spatial distributions of potential and charged species from interfacial capacitance vs. applied potential data (23.24). Like metal electrodes, these models can only be applied at ideal polarizable semiconductor-solution interfaces (25)- In accordance with the behavior of the mercury-solution interface, a set of criteria for ideal interfaces is f. The electrode surface is clean or can be readily renewed within the timescale of... 

Helmholtz [71] first described the interfacial behavior of a metal and electrolyte as a capacitor, or so-called electrical double layer, with the excess surface charge on the metallic electrode remaining separated from the ionic counter charge in the electrolyte by the thickness of the solvation shell. Gouy and Chapmen subsequently... 

D. Aurbach, The electrochemical behavior of achve metal electrodes in nonaqueous soluhons in Nonaqueous Electrochemistry (Ed. D. Aurbach), Marcel Dekker, New York, 1999. 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

One of the interesting measurements among the bundle experiments was done by Rakitin et al. [71]. They compared the conductivity of a A-DNA bundle to that of an M-DNA [71-74] bundle (DNA that contains an additional metal ion in each base pair, developed by the group of Jeremy Lee from Saskatchewan). The actual measurement was performed over a physical gap between two metal electrodes in vacuum (see Fig. 10). Metallic-like behavior was observed for the M-DNA bundle over 15 /um, while for the A-DNA bundle a gap of -0.5 V in the I-V curve was observed followed by a rise of the current. 

At present there is a sufficiently complete picture of photoelectrochemical behavior of the most important semiconductor materials. This is not, however, the only merit of photoelectrochemistry of semiconductors. First, photoelectrochemistry of semiconductors has stimulated the study of photoprocesses on materials, which are not conventional for electrochemistry, namely on insulators (Mehl and Hale, 1967 Gerischer and Willig, 1976). The basic concepts and mathematical formalism of electrochemistry and photoelectrochemistry of semiconductors have successfully been used in this study. Second, photoelectrochemistry of semiconductors has provided possibilities, unique in certain cases, of studying thermodynamic and kinetic characteristics of photoexcited particles in the solution and electrode, and also processes of electron transfer with these particles involved. (Note that the processes of quenching of photoexcited reactants often prevent from the performing of such investigations on metal electrodes.) The study of photo-electrochemical processes under the excitation of the electron-hole ensemble of a semiconductor permits the direct experimental verification of the applicability of the Fermi quasilevel concept to the description of electron transitions at an interface. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

It is debatable whether electrochemical reactions should be discussed here. There are two reasons for including them First, electrochemical and catalytic reductions have many common features and parallels [37]. Second, electrochemical methods have been used to determine the amount of adsorbed tartrate on Ni [12] and it might be possible to study the adsorption behavior of certain modifiers on metallic electrodes using methods recently described by Soriaga et al. [38]. 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

It is interesting to compare this behavior with that expected at a metal electrode. If the Fermi level lies above the highest normally filled dye level, but below the lowest excited level, no dark redox process will occur. If the dye is... 

Whether or not Chi is regarded intrinsically as an organic semiconductor, the solid Chi layer in contact with a metal does display a p-type photovoltaic effect, and its efficiency depends significantly on the morphology of the Chi layer as well as the nature of the metal. The effect corresponding to a p-type photoconductor can also be expected at the junction of a metal / Chi / liquid in a photoelectrochemical system. Such a presumption is in fact compatible with the photoelectrochemical behavior observed for most of Chl-coated metal electrodes, as will be shown later. 

Strongly potential dependent spectral features observed in the optical linear elec-troreflectance spectroscopy of various single crystal noble metal electrodes have been attributed to Stark shifts in surface states [105, 106, 140]. For the Ag(l 1 l)/electrolyte interface, an energy state is presumed to shift from about 5 eV at -0.20 V bias potential to about 4eV at -0.80 V. An analogous explanation is suggested to account for the behavior of Ag(l 10), which is reported to have two surface states [105,140] in... 

Ferroelectric thin films considerably gain in interest within the last couple of years due to their potential application in nonvolatile random-accessmemory devices (FeRAM). Among potential candidates, PbZr. n i, (>> (pzt) is one of the most promising materials because of its large remanent polarization and low coercive field. However, pzt is also well known for its poor fatigue behavior on metal electrodes [1,2] and occurrence of size effects [3-5] which are well due to the ferroelectric/electrode interface properties [1-5]. 

The silver-silver Ion electrode. Of the reversible metal electrodes, silver has been most often employed. There is only one stable oxidation state of silver above 300°C there is no danger of oxide formation because Ag20 is unstable.57 The metal has no observable tendency to dissolve in molten silver salts and is highly reversible in mixed chloride and nitrate eutectics. The Ag(I) ion can be introduced into the melt by either adding silver nitrate to a nitrate melt (AgCl to a chloride melt) or by anodizing a silver electrode. The potentials of silver nitrate concentration cells show ideal thermodynamic behavior up to 0.5 mol % in (Na,K)N03 eutectic and in NaN03.58... 

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