Overvoltage minimum

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

Sodium ion acts as a spectator ion and is not involved in the electrode reactions. Thus, the sodium chloride solution is converted to a sodium hydroxide solution as the electrolysis proceeds. The minimum potential required to force this nonspontaneous reaction to occur under standard-state conditions is 2.19 V plus the overvoltage. 

As measurement of minimum overvoltage is marked by an inherent error due to indefinable and thus problematic determination of the beginning of a visible evolution of gases, it is more exact to quote the values whioh are valid for a certain current density (see Tab. 13a, b). 

Overvoltage associated with the evolution of oxygen at the anode is even less reproducible than the hydrogen overvoltage because of its greater variability with time and because of the formation of metal oxides on the surface of the anode. Approximate values of minimum overvoltage as measured by Caspari on oxygen evolved from a normal NaOH solution at 25 °C at different metals are contained in Table 14. 

Cathode potential (including overvoltage) Maximum 1.260 Minimum 1.160... 

The cathode process is also influenced by current density because hydrogen overvoltage decreases with decreased current density which results in greater liberation of hydrogen for this reason the load on the electrolyzer should not be allowed to drop below a certain minimum value. 

Line- neutral voltage V System voltage V Impulse voltage kV Overvoltage category III IV Minimum clearances mm Pollution degree 3 4 Minimum Voltage clearance V mm ... 

Most metals can be deposited at an appreciable rate with relatively small overvoltages, but this is not so for iron, cobalt and nickel at ordinary temperatures the minimum cathode potentials necessary to produce... 

For the hep e-phases of various intermetallic compounds, the Hj overpoten-tiai passes through a well-defined minimum (optimum electrocatalysis). Correspondingly, the bonding energy between metals passes through a maximum where H2 overvoltage is a minimum for an interatomic distance of approximately 0.27 nm for both bcc and fee structures. 

In refs. 30-32 this dependence was explained by participation of the proton quantum shift—along with reorganization of classical medium—in the reaction. The dependence of on A was thoroughly analyzed elsewhere [14, 27, 32] in connection with the theory of electrode reactions for which the dependence of the rate constant on the energy difference of initial and final states (equal to the overvoltage) can be measured directly. As follows from relations (12) and (14), the probability of the nonradiative transition is maximal when the final-state potential curve crosses the initial-state term close to its minimum. Such reactions were called activationless . The transition probability is described by... 

Theoretical estimates show that the E° value for the overall process is about 4 V, which means that this is a nonspontaneous process. Therefore, a minimum of 4 V must be supplied by the battery to carry out the reaction. In practice, a higher voltage is necessary becanse of inefficiencies in the electrolytic process and because of overvoltage, to be discussed shortly. 

Up to now, reversible thermodynamics has been assumed. The electrical energy demand for water electrolysis under real conditions is significantly higher than the theoretical minimum energy derived above. The total voltage of an electrolysis cell imder operation depends on the current in the cell, the voltage drop caused by Ohmic resistance and the anodic and cathodic overvoltages (see Eq. (5.16)). 

The theoretical tools discussed in this contribution address various optimization tasks in PEMFC research (i) highest system efficiencies and fuel cell power densities and, thus, minimum overvoltage losses in CCLs (ii) optimum catalyst utilization and, thus, minimal Pt loading (and minimal cost), and (iii) waterhandling capabilities of CCLs and their impact on the water balance of the complete fuel cell. Structural parameters, as well as operating and boundary conditions that control the complex interplay of processes enter at three major levels of the theory. 

Table V-7—DC Live-Line Minimum Approach Distance (in Meters) With Overvoltage Factor ...

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