Diffusion overpotential elimination

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... 

Measurement of the Tafel lines for different concentrations, the partial electrochemical reaction orders, can be detennined from Eqs. (6.34) and (6.35). This implies that the concentration at the electrode surface remains approximately equal to the bulk concentration (elimination of the diffusion overpotential). It should be pointed out that for this evaluation the logarithm of current must be plotted versus the electrode potential and not versus the overpotential. 

Elimination of diffusion overpotential with a rotating disc electrode... 

Impedance spectroscopy in the high-frequency region is another way of eliminating diffusion overpotential. The equations were given in Chapter 5. The combination of the double-layer equivalent circuit with the diffusion impedance was described in Section 5.23 and examples for the determination of the charge transfer resistance at high frequencies were given. 

Two impedance arcs, which correspond to two relaxation times (i.e., charge transfer plus mass transfer) often occur when the cell is operated at high current densities or overpotentials. The medium-frequency feature (kinetic arc) reflects the combination of an effective charge-transfer resistance associated with the ORR and a double-layer capacitance within the catalyst layer, and the low-fiequency arc (mass transfer arc), which mainly reflects the mass-transport limitations in the gas phase within the backing and the catalyst layer. Due to its appearance at low frequencies, it is often attributed to a hindrance by finite diffusion. However, other effects, such as constant dispersion due to inhomogeneities in the electrode surface and the adsorption, can also contribute to this second arc, complicating the analysis. Normally, the lower-frequency loop can be eliminated if the fuel cell cathode is operated on pure oxygen, as stated above [18],... 

Thus, instead of a limiting diffusion current density plateau, a curve inflection point or a short inclined plateau can be expected on the polarization curve in Ohmic-controlled electrodeposition of metals, as observed in the case of silver electrodeposition from nitrate solutions. The exchange current density of the silver reaction in nitrate electrolytes is sufficiently large to permit Ohmic-controlled deposition as well as dendritic growth at low overpotentials.27 After a linear increase of the deposition current density with increasing overpotential, an exponential increase after the inflection point appears, meaning the elimination of mass-transfer limitations due to the initiation of dendritic growth. 

At overpotentials larger than 175 my the current density is considerably larger than the one expected from the linear dependence of current on overpotential. The formation of dendritic deposits (Fig. 16d-f) confirms that the deposition was dominantly under activation control. Thus, the elimination of mass transport limitations in the Ohmic-controlled electrodeposition of metals is due to the initiation of dendritic growth at overpotentials close to that at which complete diffusion control of the process on the flat part of the electrode surface occurs. 

Figure 6.16 Elimination of diffusion contributions to the overpotential on the rotating disc eiectrode by a Koutecky-Levich plot, extrapolation to vJf = 0 (rotation frequency / = 10 mol dm ...

Elimination of diffusion contribution to the overpotential in chronoamperometry and chronopotentiometry... 

Elimination of the diffusion contribution to the overpotential is also possible by time-dependent measurements. 

Elimination of diffusion contributions to the overpotential by impedance spectroscopy... 

This equation can be used to predict the current when the mass-transfer limitation is eliminated. In a well-stirred solution diffusion to the electrode is no longer the limiting factor in the experiment. In a cell at 25 °C, if a = 0.5 and i = 10 pA, for an overpotential of 200 mV the resultant current calculated with equation (1.4.25) is 0.49 mA. 

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