Electrode potentials thermodynamic

The ORR mechanism is very complex and involves a number of different adsorbate intermediates. In addition to its complex reduction mechanism, the relatively high thermodynamic electrode potential of the ORR causes electrocatalytic surfaces... 

Aluminium-air cells are attractive in principle because of the high thermodynamic electrode potential and theoretical capacity of aluminium. Because of corrosion, however, the only commercial power sources based on this couple are either reserve systems or are mechanically rechargeable, i.e. the anodes are replaced after discharge. A typical cell may be written as... 

Ihe actual from the thermodynamic electrode potential. The polarization is Ihe result of the irreversibility of the electrode process, that is, the activation polarization and the voltage loss, which develops from concentration gradients nf the reactants. This leads to the current-voltage characteristics as shown in Fig. 2,... 

Standard thermodynamic electrode potential at 25 °C and unit activity (V)... 

For a better comprehension of the ED processes it is necessary to refresh a few basic concepts and definitions regarding the electrolytic cell and thermodynamic electrode potential, Faraday s laws, current efficiency, ion conduction, diffusivity, and transport numbers in solution. 

The Nemst equation is a thermodynamic expression of the equilibrium state of an electrochemical reaction. It can give the value of the thermodynamic electrode potential for electrochemical reactions as well as point out the reaction direction. However, it cannot show the reaction rate. To connect the reaction rate and the electrode potential, one needs to use the Butler-Volmer equation. 

The thermodynamic electrode potentials of these two reactions at pH 7.0 are -e0.06 and -e0.60 V vs. SCE, respectively. However, reduction of dissolved oxygen in water at conventional electrodes involves kinetic complications so that, at graphite electrodes, very weak reduction signals appear. 

The working and reference electrodes are thus poised at a set potential, which will be between 3 V, since almost the whole of organic electrochemistry occurs within this 6-V range. (These are true thermodynamic electrode potentials and 4-3 V corresponds roughly to the oxidation of benzene, while -3 V corresponds roughly to the reduction of benzene.) Pyrrole electro-oxidation typically takes place above +0.8 V (versus SCE reference). 

It is also possible for only one electrode to consist of dopable polymer and for the other be of metal, usuaUy Kthium (Figure 5.25), which is preferred because of its low weight and high thermodynamic electrode potential. 

Thermodynamically, electrode reactions can be either reversible or irreversible. In case of a reversible reaction, the electrode potential is called reversible (thermodynamic electrode potential). The corresponding OCV is traditionally called electromotive force (EMF) and is denoted as e. 

The oxygen reduction reaction is a multi-electron process involving numerous steps and intermediate species. As stated above, ORR may proceed via four or two electron transfer in aqueous acidic medium. The most relevant reactions pathways and their thermodynamic electrode potentials in acidic medium are shown below ... 

Table 4.1 presents the selected ORR processes and their corresponding thermodynamic electrode potentials at standard conditions. All the potentials are against the standard hydrogen... 

Table 4.1. Thermodynamic Electrode Potentials of Oxygen Reduction Reaction in Two Aqueous Electrolyte Solutions at 25 ""C and 1.0 aW ...

Electrochemical Oxygen Reduction Reactions Thermodynamic Electrode Potential (V vs SHE) Electrolyte Condition... 

The thermodynamic electrode potentials listed in Table 4.1 are those at standard conditions (25 °C, 1.0 atm). However, if these reactions are not at standard conditions, the electrode potentials can be expressed as the Nernst forms. For example, for the reaction... 

This cathode reaction is actually the same as Reaction (4-1). Both Reactions (4-II) and (4-III) have their own thermodynamic electrode potentials, h2/h+ standard conditions. As... 

Table 2.1 lists several typical ORR processes with flieir corresponding thermodynamic electrode potentials at standard conditions. The mechanism of the electrochemical O2 reduction reaction is quite complicated and involves many... 

Table 2.1. Thermodynamic electrode potentials of electrochemical O2 reductions [2, 3]...

It is desirable to have the O2 reduction reaction occurring at potentials as close as possible to the reversible electrode potential (thermodynamic electrode potential) with a satisfactory reaction rate. The current-overpotential is given in Equation 2.1 [3] ... 

During the reverse potential sweep in CV experiments on the other hand, the product of the initial oxidation or reduction of the electroactive species is complementary reduced or oxidized (Figure 2.5). In this context, and correspond to the anodic and cathodic potential peaks, respectively [1, 6, 7]. It is important to note that for reactions characterized by fast electron transfer events relative to the time-scale of the potential sweep rate (reversible reactions, where the difference pa - E is independent of v), the formal thermodynamic electrode potential E° can be readily estimated using Equation 2.2. 

In a sulfuric acid solution, the reaction between Pb and protons can also occur due to the difference between the thermodynamic electrode potentials of HVHj and Pb/PbS04 couples ... 

Reaction 5.5 is the self-discharge reaction at the negative electrode, causing a loss of capacity in a lead-acid battery. This reaction is normally slow, but it will occur quickly if impurities are present or the battery temperature is high. Another selfdischarge reaction also can happen on the positive electrode in a sulfuric acid solution due to the difference between the thermodynamic electrode potentials of Pb02/ PbS04 and O2/H2O couples ... 

Thermodynamic Electrode Potential and Cell Voltage of a Ha/Air Fuel Cell... 

Note that the two-directional arrows in these reaction expressions indicate that aU these reactions are chemically or electrochemically reversible, although they are not thermodynamically reversible due to their limited reaction rate in both reaction directions. Assuming that these reactions are in equilibrium states, the thermodynamic electrode potentials for the half-electrochemical Reactions (1.1) and (l.II) and the overall Reaction (1. III) can be expressed using the following Nemst equations ... 

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