Electrode potentials different

If components of the solution phase are prone to electrochemical reactions (e.g. reduction of dissolved oxygen, reduction of oxidising anions) their presence may also cause Faradaic reactions and the subsequent establishment of an electrode potential different from iipzc. 

According to Eq. (3.21) and taking into account that the electrode potential differs by a constant term from the metal-solution Galvani potential, we thus have an expression for the equilibrium potential of this electrode and, at the same time, for the equilibrium potential, redox of this redox system ... 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

Since the absolute and the conventional electrode potentials differ only by an additive constant, the absolute potential depends on the concentration of the reactants through the familiar Nernst s equation. This dependence is implicitly contained in Eq. (2.6) the real Gibbs energies of solvation contain an entropic term, which depends on the concentration of the species in the solution. 

Initially, both electrodes are at equilibrium. Since the anode has accumulated electrons and the cathode has depleted electrons, electrons begin to flow from electrode from the anode to the cathode. The thermodynamic driving force for the electron flow is the electrode potential difference, which for the fuel cell reaction is 1.23 Y at standard conditions. In addition to electron flow, H + ions produced at the anode diffuse through the bulk solution and react at the cathode. The reaction is able to continue as long as H2 is fed at the anode and 02 at the cathode. Hence, the cell is not at equilibrium. The shift in electrode potential from equilibrium is called the overpotential (>/). 

Moreover, due to the rise in the electrode potential difference galvanic interaction between galena, sphalerite and pyrite, it will further accelerate the oxidation of the sulphides. 

The economic target of an electrochemical process is to keep the electrical consumption to a minimum. On the basis of Equation (7.15), this implies that a process should be performed at the maximum I with the minimum AV In industrial processes, it is the current to be fixed rather than the electrode potential difference. 

Consider again the electrode potential difference, V, of the cell diagram of Eq. (6.46). It can be written as the energy difference of the electrons between the two electrodes through the external circuit ... 

By combination of eqns (104) and (105a) by replacing the concentration of O at the plane of reaction or pre-electrode plane, the apparent rate constant can be expressed in terms of the applied electrode potential difference (0m — 0S)... 

If both electrode processes operate under standard conditions, this voltage is E°, the equilibrium standard electrode potential difference. Values of E and E° may be conveniently measured with electrometers of so large an internal resistance that the current flow is nearly zero. Figure 3.1.6 illustrates the measurement and the equilibrium state. The value of E° is a most significant quantity characterizing the thermodynamics of an electrochemical cell. Various important features of E and E° will be addressed in the following chapters. 

Considering Equation (4), it is clear that one has to operate a galvanic cell at the maximum possible cell voltage in order to maximize the electric energy yield. As shown previously (Figure 3.1.6), the maximum value of U is the equilibrium electrode potential difference E or E°. Thus, one may formulate the fundamental relationship between chemical and electric energy ... 

Here Eq is the equilibrium electrode potential difference between the two electrodes of the galvanic cell. It is now possible to choose the standard reference electrode as the hydrogen electrode, for which the chemical potentials jXj of the constituents and H2 are taken to be zero at standard temperature and pressure. The equilibrium condition 7.14 then becomes... 

Other in chemical properties. Both are amphoteric, but gallium is less electropositive as indicated by its lower electrode potential. Differences in the chemistry of the two elements can be related to the presence of a filled set of 3d orbitals in gallium. 

Much effort has been devoted to the study of manganese oxides as positive electrode materials. Manganese oxides are very promising materials due to their abundance, environmental compatibility, low cost, favorable charge density, relatively high electronic conductivity, and suitable electrode potentials. Different structures of manganese oxides have been investigated and the best known, and the first to be... 

IR drop, and the overpotential. As a consequence, the only practical way of achieving separation of species whose electrode potentials differ by a few tenths of a volt is to measure the cathode potential continuously against a reference electrode whose potential is known. The applied cell potential can then be adjusted to maintain the cathode potential at the desired level. An analysis performed in this way is called a controlled-potential electrolysis. Controlled-potential methods are discussed in Sections 22C-2 and 22D-4. 

Wyatt s apparatus with approaching-needle rubber-base electrodes. With a steel-base electrode, however, a value of less then 3 X 10" J was obtained for RD1333 and a value between 5 X 1(10 and 1.1 X 10" J for the mixture. (The 3 X 10" J value was due to contact initiation since the electrode potential difference was only 250 V, which is less than the threshold voltage for gap breakdown in air.)... 

The final experimental approach to IT kinetics reported recently involves the use of a thin film of organic solvent covering a graphitic electrode the whole assembly is then immersed in aqueous solution, as depicted in Fig. 17 [124]. Potentiostatic control is maintained over the aqueous/electrode potential difference, which is the sum of the aqueous/organic and organic/electrode potential differences. The aqueous/organic term is assumed to be controlled by... 

During current passage, the electrode potential differs from its equilibrium value, E, since the concentration of metal... 

In the case of the slightest defects in the coating corrosion elements appear on the metal substrate. As a result, areas of the substrate insulated by the coating from the corrosive medium function as the cathode and the naked areas of the substrate near the coating defects serve as the anode and acquire a more negative potential than the cathodic areas. The electrode potential difference between the anodic and cathodic areas of the substrate is a function of the ohmic potential drop on the cathode and can reach 200-300 mV. This is commensurate with the contact potential difference assumed dangerous for metal structures (0.25 V) [1]. 

Pairs of Electrodes Ml and M2 Current, pA Electrode Potential Difference, V... 

Fig. 4.13. Dependence of maximum current imax (1) in PVB samples and maximum temperature Tmax (2) on electrode potential difference AUox of oxidized metal pairs Ml and M2...

The maximum current max shown in Fig. 4.12 is conditioned by the nature of the electrode metals. This value imax is related to the electrode potential difference of the oxide metal pairs (Fig. 4.13, Table 4.5). It is evident (Fig. 4.13) that extrapolation of line / to the ordinate gives zero current at AUok = 0- This is a qualitative confirmation of the insignificant current in the M-P-M system with electrodes of similar metals. There is, however, a certain deviation of experimental points from the linear dependence /, which is explained by the formation of numerous oxides by such metals as, e.g. Ag, Cu, etc., while the A17ox values presented in [50] are a result of averaging of the potentials of these oxides. 

Some basic aspects of alloy dissolution are best illustrated by the behavior of a liquid binary alloy A-B. This is due (1) to the absence of crystallization overvoltage and dissolution induced structural surface modifications [6] as well as (2) to the high diffusivity in the alloy phase that provides for the reactant supply at the alloy/electrolyte interface if one alloy component dissolves preferentially (at a higher rate than the other) (7). Provided that the standard electrode potential difference of the components, AE = E — El, is large AE > RT/F) and their charge transfer reactions are fast, one expects a schematic polarization curve as shown by Fig. 1(a). For Ea < E < Eb, only the less noble component. A, dissolves ( selective dissolution or deaUoying ), the partial anodic... 

Identical metals in contact with different concentrations In this case, the metal immersed in a dilute solution is dissolved from the electrode and deposited on the electrode immersed in a more concentrated solution. The other type of electrochemical concentration ceU is known as a differential aeration cell. In this case, the electrode potential difference occurs when the electrode is immersed in the same electrolyte with different oxygen partial pressures. Differential aeration initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

Because most applications of (photo)elec-trochemical systems involve the transfer of electrons across an interface (Sect. 2.1.1), current density-potential techniques are commonly used in (photo)electrochemis-try. In this case, the difference in electrochemical potential of electrons across the interface of interest (accessible via the working electrode - reference electrode potential difference) and the current density through this interface are used as the perturbation and the response (or vice versa). Two approaches can be distinguished. When (quasi) steady state signals are used, one speaks of current density versus potential measurements whereas harmonically modulated signals, superimposed on a bias, are involved in electrochemical impedance spectroscopy (EIS). We introduce these two approaches on the basis of the kinetics of the simple system shown in Fig. 1. 

The difference of this view, in explaining the potential difference across biological interfaces, compared with that of the earlier workers, is profound. Thus, ion transport through the membrane is a consequence of the electrode potential differences referred to above. Electron transfer at the solid/liquid interface sites is rate determining (one of the two kinds of sites will dominate). The concentration of alkali metal ions on the two sides of the membranes may be the result, rather than the cause, of the potential... 

The absolute electrode potential of the fuel cell is difficult to measure. However, only the electrode potential difference between the cathode and anode is important in fuel cells. The reversible cell potential can be obtained from the difference between the reversible electrode potentials at the cathode and anode. 

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