Oxygen Electrode Process

3 General Description of Cathode Reaction and Polarization 7.3.1 Oxygen Electrode Process 

Oxygen incorporation reaction at a cathode can be broken down into several processes that are connected in series and in parallel, i.e., gas-phase diffusion, adsorption, dissociation, surface or bulk diffusion, and incorporation into the electrolyte (Fig. 7.1). At every step, a driving force is required to promote the reaction or the mass transport, and this causes an energy loss, which is called overpotential or polarization. Although a large number of studies have been published so far, complete understanding has not been attained on the cathode reaction mechanism [3]. Most work is based on dc/ac electrochemical measurements, which provide only macroscopic and averaged information on the whole electrode process. In actuality, however, a reaction site is not uniform and distributes three dimensionally around the triple-phase boundaries of electrode, electrolyte, and gas phases. 

In modeling and analyzing the electrode process, it is often assumed that the local equilibrium is held at every point inside the electrode and electrolyte, where local chemical potential of oxygen is determined using electrochemical potentials of oxide ion and electron as follows  

Additionally, in most cases, quasi-equilibrium is assumed between the electrons in the electrode and in the electrolyte at the interface [17-19]. Possible high tunneling current at the interface of ionic conductors might rationalize this assumption [20]. Based on these assumptions, overpotential is attributed to the variation of oxygen potential at the interface from the equilibrium with the surrounding atmospheres  

Oxygen potential profile inside a eathode, although it is important, is not easy to be determined. Since transient response of current or potential reflects the process of forming the potential gradient, the ac impedance signal contains useful information. For understanding the ac response of the cathode-electrolyte system, equivalent eireuit analysis based on the mass transport equation is useful [23]. 

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Ho KC, Hung WT (2001) An amperometric N02 gas sensor based on Pt/Nafion electrode. Sens Actuators B 79 11-18 Ho KC, Liao JY, Yang CC (2005) A kinetic study for electrooxidation of NO gas at a Pt/membrane electrode-apphcation to amperometric NO sensor. Sens Actuators B 108 820-827 Imaya H, Ishiji T, Takahashi K (2005) Detection properties of electrochemical acidic gas sensors using halide-halate electrolytic solutions. Sens Actuators B 108 803-807 Ives DJG, Janz GJ (eds) (1961) Reference electrodes theory and practice. Academic, New York, NY Jordan LR, Hauser PC, Dawson GA (1997) Humidity and temperature effects on the response to ethylene of an amperometric sensor utilizing a gold-Nafion electrode. Electroanalysis 9 1159-1162 Katayama-Aramata A, Nakajima H, Fujikawa K, Kita H (1983) Metal electrodes bonded on sohd polymer electrolyte membranes (SPE)—the behaviour of platinum bonded on SPE for hydrogen and oxygen electrode processes. Electrochim Acta 28 777-780... 

It is not appropriate here to consider the kinetics of the various electrode reactions, which in the case of the oxygenated NaCl solution will depend upon the potentials of the electrodes, the pH of the solution, activity of chloride ions, etc. The significant points to note are that (a) an anode or cathode can support more than one electrode process and b) the sum of the rates of the partial cathodic reactions must equal the sum of the rates of the partial anodic reactions. Since there are four exchange processes (equations 1.39-1.42) there will be eight partial reactions, but if the reverse reactions are regarded as occurring at an insignificant rate then... 

Although Table 2.16 shows which metal of a couple will be the anode and will thus corrode more rapidly, little information regarding the corrosion current, and hence the corrosion rate, can be obtained from the e.m.f. of the cell. The kinetics of the corrosion reaction will be determined by the rates of the electrode processes and the corrosion rates of the anode of the couple will depend on the rate of reduction of hydrogen ions or dissolved oxygen at the cathode metal (Section 1.4). 

The electrode processes of oxygen represent a further important group of electrocatalytic processes. The reduction of oxygen to water... 

The above experimental data (Figs. 1 and 2) allow us to estimate the possibility for taking place the oxygen reduction process on PANI catalyst. If the special procedure is not taken to withdraw oxygen out of solution, PANI electrode in 1 M HC1 usually demonstrates a potential of about 0.6 V (SHE). In compliance with the Fig. 1 the position of the reduction peak might be at 0. 28 V. From the data of the standard potentials for the following reaction ... 

Oxygen reduction can be accelerated by an application of electrodes with high surface area, e.g. the porous electrodes [9, 13]. The porous electrodes usually consist of catalysts, hydrophobic agent (polytetrafluoroethylene-PTFE) and conductive additive. Electrode kinetics on the porous electrodes is complicated by the mass and charge transfer in the pores and is called the macrokinetics of electrode processes . 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

The chemical reduction of the same 4-cyano-3-phenylcinnoline in the presence of oxygen in air leads to quite different products when compared to the electrode process. The reactions were carried out in a dilute solution in which the chemicals were evenly distributed. On action of zinc dust in acetic acid at 25°C, the parent compound gives 3-cyano-2-phenylindole in quantitative yield. Reaction of the same starting material with sodium borohydride in ethanol under reflux provides 3-phenylcinnoline at almost quantitative yields (Matsubara et al. 2000, Ref. 13). The products mentioned are depicted in Scheme 2.32. 

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

The measurement of open circuit potential of the catalyst during the liquid phase oxidation of alcohols provides a unique insight into the redox processes taking place on the catalyst surface. A Pt catalyst stored in air contains surface oxides and in an aqueous Na2C03 solution it behaves as an oxygen electrode. Its potential is 250-280 mV when referred to a Ag/AgCl/KCl(3 ) electrode (Figure 3). When the catalyst is... 

Pb Lead, and particularly underpotentially deposited Pb, exhibits electrocatalytic properties in numerous electrode processes. The model reaction can be oxygen reduction with slow step of peroxide reduction ]374-376] or reduction of nitrobenzene and other nitrocompounds [377, 378]. In the case of... 

The oxygen electrode has been the subject of intensive study for many years. The electrode reaction is complex and is greatly affected by the electronic conductor and electrolyte used. In basic solution, it may be considered as a two stage process only the first of these is reversible. The two steps may be written as... 

In addition, other processes may occur such as the reaction of the hydroperoxide ion with the conductor to form metal-oxygen bonds which in turn may be reduced. The hydroperoxide ion may itself decompose to reform oxygen, etc. The potential of an oxygen electrode is invariably a mixed potential with a value of about 1.0 V on the standard hydrogen scale, at zero current drain. 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

Clarke-Othmer process Clark oxygen electrode Clary sage oil [8016-63-5] Class A direct dyes Classification of dyes Classified removal Classifiers Clathrate... 

In the case of the ternary eutectic Li2C03-Na2C03-K2C03 at 605°C saturated with pure C02 (p02 = 6), the anodic limit is about 0.27 V vs. the oxygen electrode however, when saturated with Li20 (pO2- = 0) this melt is reported to exhibit an anodic limit of only -0.23 V [5]. The cathodic limit of ternary eutectic carbonate melts with p02 = 2 to 6 is about -1.9 to -2.1 V [5]. The reduction process produces elemental carbon according to the reaction... 

In this review, however, only recent studies of the reactions of macromolecule-metal complexes will be reviewed. There have been some exellent reviews recently on macromolecule-metal complexes regarding syntheses, formation, characterization and catalytic activities l solar energy conversion 2), artificial oxygen carriers 3), and electrode processes 4). Furthermore, the preprints of the 1st International Conference on Macromolecule-Metal Complexes Tokyo Seminar on Macromolecule-Metal Complexes were published in 19875). These reviews and the preprints give useful information about the recent development of the basic and applied chemistry of macromolecule-metal complexes. 

This equation can be solved numerically. At concentrations of oxygen below 0.2%, the limiting current plateau becomes narrow and the onset of another electrode process contributes to positive error. It has been attributed to the reduction of CO2 or to the increase of unspecified electronic current (Pham and Glass, 1997). Excellent agreement between the calculated and experimental values has been observed for a wide range of temperatures (Fig. 7.20). 

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