Cathodic overvoltage, 6.32

Fig. 8-5. Electron state density in a metal electrode and in hydrated redox particles, and anodic and cathodic currents of redox electron transfer under cathodic polarization n s cathodic overvoltage (negative) i = anodic current i = cathodic current. [From (lerischer, I960.]...

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). 

Fig. 10-26. Energy diagram for a cell of photoelectrolytic decomposition of water consisting of a platinum cathode and an n-type semiconductor anode of strontium titanate of which the Fermi level at the flat band potential is higher than the Fermi level of hydrogen redox reaction (snao > epM+zHj) ) he = electron energy level referred to the normal hydrogen electrode ri = anodic overvoltage (positive) of hole transfer across an n-type anode interface t = cathodic overvoltage (negative) of electron transfer across a metallic cathode interface.

From the energy diagram shown in Fig. 10-33, the operating cell voltage, V,, is obtained, as expressed in Eqn. 10-60, by subtracting from the electromotive force AEph the potential barrier of the space charge layer, the cathodic overvoltage t h, and the iR drop in the electrolyte ... 

Thus for an overvoltage of — 118 mV, iJiA = 100/1. For overvoltages of 200 mV and above, only the cathodic or anodic contribution to the total current need be considered. For a large cathodic overvoltage,... 

For an electrolyser, instead of an isentropic coefficient, we take into account the overvoltage necessary for driving the electrolysis (ohmic resistance, anodic-cathodic overvoltage) here the exergy loss is simply connected to this overvoltage by Faraday s law ... 

The cathode process has received much less attention than the anode process, probably because the reactions occurring at the cathode have been considered to be simple. However, the cathode reactions are important not only because of cathodic overvoltage but also because they may be connected with the formation of dissolved metal and thereby with the current efficiency of the process. 

A 0.1 molar solution of copper sulfate in 1.0 n sulfuric acid is electrolyzed what will be the potential of the cathode when the cupric ion concentration has been reduced to 10 g.-ion per liter How much further could the cathode potential be increased and hydrogen evolution still be avoided The cathodic overvoltage at copper may be taken from Table LXXVIII. 

Therefore, during the charge process of an electrochemical cell, a potential Fj, which is the sum of the cell equilibrium potential Voc, the cathodic overvoltage ( /e,c) Ihe anodic overvoltage ( /j. g) and the ohmic drop (Eq. (8)),... 

At high overvoltage, t]J > 100 mV, only the cathodic or anodic current contributes significantly to the net current e.g., for cathodic overvoltages, Eq. (11) becomes... 

The sequence i)-iv) corresponds to increasing inhibition of the electrocrystallization process accompanied by increasing cathodic overvoltage [6.27, 6.28, 6.37]. Examples are shown in Fig. 6.1 [6.8]. A special texture type is produced by the so-called rhythmic-lamellar crystal growth, representing an oscillation reaction (Fig. 6.2) [6.38]. 

Here we assumed that velocity does not change dramatically with z, so that the estimate vc(z) c(° is valid. The latter assumption is justified if u) , the velocity at the inlet, is large enough. In addition we will assume, hereafter, that the cathode overvoltage i)c only weakly depends on z and that the only /-dependent quantity in Eq. (143) is co2. As simulations show, such assumption works amazingly well but only if the membrane is well humidified. 

Horita, T., Yamaji, K., Sakai, N., Yokokawa, H., Kawada, T., Kato, T. Oxygen reduction sites and diffusion paths at Lao <>Sro iMnO j x/yttria-stabilizcd zirconia interface for different cathodic overvoltages by secondary-ion mass spectrometry. Solid State Ionics 2000,127, 55-65. 

Nafion-315 is currently used in the SPE cell for brine electrolysis. The SPE electrolyzer exhibits a 15-20% energy savings when compared to conventional brine electrolyzers, primarily due to the decrease in ohmic and cathode overvoltages. Figure 5 shows the schematic of the SPE electrolyzer along with a typical membrane electrolyzer. The current distribution across the membrane of an SPE electrolyzer is more uniform than that of a typical brine electrolyzer. 

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