Overpotential Ohmic

The latter equals IRwc where RWc is the ohmic resistance between the working and counter electrode. Experimentally it is rather easy to measure the riohmic.wc term using the current interruption technique as shown in Figure 4.9. Upon current interruption the ohmic overpotential r 0i,mjCtwc vanishes within less than 1 ps and the remaining part of the overpotential which vanishes much slower is t w+T c (Eq. 4.9). 

Figure 4.9. Use of the current interruption technique to measure the ohmic overpotential, r ohmic,wc> between the working (W) and counter (C) electrode.

The ohmic overpotential q0hmic,w is also negligible, provided the catalyst-electrode is sufficiently conductive. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

For hydrogen production from water, pure water (pH=7.0) is seldom used as an electrolyte. Water is a poor ionic conductor and hence it presents a high Ohmic overpotential. For the water splitting reaction to proceed at a realistically acceptable cell voltage the conductivity of the water is necessarily increased by the addition of acids or alkalis. Aqueous acidic and alkaline media offer high ionic (hydrogen and hydroxyl) concentrations and mobilities and therefore possess low electrical resistance. Basic electrolytes are generally preferred since corrosion problems are severe with acidic electrolytes. Based on the type of electrolytes used electrolyzers are... 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

The equations for multistep electron transfer according to the quasiequilibrium treatment can be derived on the basis of Parsons general and rigorous treatment [47]. For simplicity, mass transport limitations, double layer effects, ohmic overpotential, and specific adsorption or chemisorption are neglected in the present formalism. 

At higher current densities than those referred to in Eq. (13.20), the activation-overpotential terms in this equation change much less with current than the ohmic overpotential owing to the internal resistance of the cell. Under these conditions, when (I/A)/iL continues to remain negligible and the variation of Vand I (but not its absolute value) is dominated by the IRt term, one has (Fig. 13.7)... 

Diffusion overpotential — The diffusion overpotential means the extra voltage which could compensate the difference between bulk concentration and surface concentration, and is called -> concentration overpotential [i]. As a performance of industrial electrolysis or batteries, it has been used along with -> activation overpotential and - ohmic overpotential. It not only varies compli-catedly with cell configuration, current, applied voltage, and electrolysis time but also cannot be separated from activation and ohmic overpotentials. 

Ohmic overpotential is also named - IR drop, resistance overpotential, or ohmic drop. 

Water electrolysis is an electrochemical reaction where water is split into hydrogen and oxygen in the presence of a catalyst and applied electric field. As current density increases the cell losses due to membrane, electrode, and interfacial resistances dominate and are referred to as ohmic overpotential. 

Ohmic losses occur because of resistance to the flow of ions in the solid electrolyte and resistance to flow of electrons through the electrode materials. Because the ionic flow in the electrolyte obeys Ohm s law, the ohmic losses can be expressed by Ohm s law. The ohmic overpotential, r o of Eq. 19 is a function of the stack current density (i), membrane thickness (cp), and the conductivity of the stack (ct),... 

Once the ohmic overpotential, 0hmic w> has been eliminated or computed via current interruption, one is left with the activation and concentration (or diffusion) overpotentials only. The activation overpotential, rjac,w, is due to slow charge-transfer... 

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