Decrease overvoltage

Fig. 6.8. Delay time as a function of applied voltage for two temperatures showing the statistical spread of tp increase with decreasing overvoltage. After Lee, Henisch, and Burgess (1972).

Temperature the hydrogen overvoltage decreases with increasing temperature. It can be zero at higher temperatures. For example, the hydrogen overvoltage on nickel is zero at 90 °C. 

The pH the hydrogen overvoltage, with other conditions remaining unchanged, varies with the pH of the electrolyte. First it increases with increasing pH value and then decreases again in alkali solutions. 

Gas phase composition effects As shown in figure 6 the increase in the rates Ar. depends not only on the overvoltage AV but also on gas phase composition. This has been reported also in a previous communication where Ar./r. increase with decreasing P, (17. 10 constant P -------J ... 

Consequently, for the transfer reaction of redox electrons via the conduction band mechanism, the anodic current is constant and independent of the electrode potential whereas, the cathodic current increases with increasing cathodic overvoltage (decreasing electrode potential). 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

This complex reduction reaction leads to a relatively high overvoltage - at least 0.3 V- thus decreasing the cell voltage of the fuel cell by the same quantity. Pt-X binary catalysts (with X = Cr, Ni, Fe,. ..) give some improvements in the electrocatalytic properties compared vdth pure Pt dispersed on Vulcan XC72 [17]. 

Any factor which tends to decrease the velocity of Reaction 2, i.e., which increases the overvoltage, will decrease the corrosion represented by Reaction 1. Watts and Whipple have found that, contrary to statements in the chemical literature, a decrease of the external pressure will produce a decrease of corrosion of metals in adds. These workers, however, attribute the decreased corrosion to the absence of air in the solutions which were under reduced pressure. As it seemed probable to us that the effect is, largely at least, due to an increase in the overvoltage with decreased pressure, we repeated their experiments, taking care to exdude oxygen from the add. Table 1, which contains TABLE 1... 

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