Equilibrium cathode potential

Diffusion oodSdcnt of species i Measured or applied electrode potential Equttibrium electrode potential Standard electrode potential Equilibrium cathode potential Equilibrium anode potential Cathode potential Anode potential... 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

No gas phase equilibrium exists between O2 and CO2 in the oxidant gas that could alter the composition or cathode potential. 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

Since in the course of measurement of the anode and cathode potentials the influence of the electrolyte resistance is eliminated the current curves during the ideal reversible course of the electrolysis should rise vertically upwards after transgressing the cathode or anode equilibrium potentials. Actually, however, this is never exactly the case as electrolysis is always accompanied by polarization phenomena, at least by concentration polarization. Owing to this polarization the rising parts of the current curves are always somewhat inclined to the voltage axis. From the magnitude of their slope it can be found... 

When plotting the relation between concentration polarization and current density the resultant curve will be of the form, illustrated in Fig. 22. The curve starts to move upwards at point A which gives the value of the equilibrium potential (see equation (VII-15) for Ih = 0). Proportionally with increased current density the cathode potential increases almost linearly at the beginning, later it increases more rapidly, until finally the curve at point B breaks into... 

By a reducing phase is meant hydrogen or any metal phase which is supposed to stand in a dynamic equilibrium with it, so that the action of the different cathode materials can be taken as equal, a condition which can be experimentally obtained by the choice of a cathode potential which remains always the same. 

For each 0.059/n =23RTInF) volt of increasingly negative potential, the surface concentration is diminished 10-fold. Thus, if the cathode potential is maintained at a value 3 x 0.059/n V more negative than the equilibrium potential, the surface concentration is only 0.1% of the bulk concentration, the concentration at the surface being maintained in accordance with the potential of the electrode. Further increase in cathode (negative) potential can cause no appreciable further increase in the quantity Cm + — C +, the difference between the bulk concentration and the surface concentration. 

Equation (78) assumes that the deposition reactions are activation controlled, rj = rjct (see Sect. 3.1.3). Figure 28 shows current-potential curves for the case when the deposition of metal Mi is limited by transport (iy = i 11 diffusion control). It can be seen that in this case, at the cathode potential E(i), metal M2 with a more negative equilibrium potential ( eq,2 < h eq/i) is deposited to a greater extent than metal Mi, i2 > i. ... 

Polarization has various meanings and interpretations depending on the system under study. For an electrochemical reaction, this is the difference between actual electrode potential and reaction equilibrium potential. Anodic polarization is the shift of anode potential to the positive direction, and cathodic polarization is the shift of cathode potential to the negative direction. In an electrochemical production system driven with an external current source, polarization is a harmful phenomenon. It will increase the cell voltage and therefore production costs. A system that polarizes easily will not pass high currents even at high overpotentials. The reaction rates are therefore small. 

The integrated peak areas between 1650-1700 cm, at several bias potentials, are shown in Fig. 11. The adsorption of the anion, CO, increases in the cathodic direction. If the adsorbed CO radial were in equilibrium with C0 in solution a decrease in adsorption coverage would be expected when the potential is moved in the cathodic direction. However, increase in anion concentration at cathodic potentials is consistent with CO2 as an intermediate radical in the electrochemical reduction of CO. Thus, from (l)-(3). 

We now estimate the anode/cathode potential difference during current flow for driven and self-driven cells. First, we define the cell potential (V) as the difference in the anode and cathode potentials. For a self-driven cell, the cell voltage at a given current density will be less than the difference in equilibrium electrode potentials (AEJ due to the presence of various overpotentials. 

The description of corrosion kinetics in electrochemical terms is based on the use of potential-current diagrams and a consideration of polarization effects. The equilibrium or reversible potentials Involved in the construction of equilibrium diagrams assume that there is no net transfer of charge (the anodic and cathodic currents are approximately zero). When the current flow is not zero, the anodic and cathodic potentials of the corrosion cell differ from their equilibrium values the anodic potential becomes, more positive, and the cathodic potential becomes more negative. The voltage difference, or polarization, can be due to cell resistance (resistance polarization) to the depletion of a reactant or the build-up of a product at an electrode surface (concentration polarization) or to a slow step in an electrode reaction (activation polarization). 

If we measure the potential of the dissolving iron,we will not obtain an equilibrium potential (as in the Pourbaix diagram) but a value somewhere between the anodic and cathodic potentials. This number will depend on such factors as how the potentials... 

Te can be reduced at more cathodic potentials to form H2Te, and the corresponding equilibrium potential for this process is... 

The molar Gibbs energy of formation of CdTe is -92kJmoT , so that CdTe can be deposited at less cathodic potentials than Cd. The relevant equilibrium potential is... 

The cathodic polarization curve is constructed using the oxygen electrode equilibrium potential and the cathodic slope, 6<- = —0.05V/decade. The equilibrium cathode potential, geq,c is calculated by applying the Nemst equation to the oxygen reduction reaction, Eq. (4.13), for an oxygen concentration of 1.0x10 mol/1 at pH= 11. 

Equilibrium potential for cathodic reaction = Equilibrium potential for anodic reaction = Eoa... 

Controlled potential electrogravimetry, using a po-tentiostat, is commonly used. The applied cell potential, Eappi, is distributed between the equilibrium anode and cathode potentials (Ea and Ec), the anodic and cathodic overpotentials (fja and rjc) and the solution ohmic drop (iR) ... 

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