Overpotential experimental measurement

Oxidant The oxidant composition and utilization are parameters that affect the cathode performance, as evident in Figure 2-3. Air, which contains -21% Oi, is the oxidant of choice for PAFCs. The use of air with -21% Oi instead of pure Oi results in a decrease in the current density of about a factor of three at constant electrode potential. The polarization at the cathode increases with an increase in Oi utilization. Experimental measurements (38) of the change in overpotential (Aric) at a PTFE-bonded porous electrode in 100% H3PO4 (191°C, atmospheric pressure) as a function of O2 utilization is plotted in Figure 5-4 in accordance with Equation (5-7) ... 

The first term on the right-hand side of Eq. (16) is the ohmic potential difference between any point on the electrode and the reference electrode, the second term is the local surface overpotential for the reaction, and the third term is the local concentration overpotential. These three terms may vary from point to point on the electrode, but their sum must always be AT. Distinguishing among the three components of the experimentally measured voltage of the electrode is a complicated problem, but necessary if one wishes, for example, to obtain fundamentally meaningful values for the surface overpotential as a function of current density for a gas-evolving electrode. Consider the following cases. 

Substituting the experimentally measured value ofi).= ic—jca) and the calculated value of 3 into Eq. (8.35), the hydrogen surface coverage can be estimated as a function of the overpotential. 

Hydrogen surface coverage On is calculated as a function of hydrogen overpotential by substituting the calculated value of estimated in exercise E8.2 and experimentally measured ir = ic—joo into Eq. (8.35) ... 

In equation (3), E (V) is the experimentally measured potential at current i Eact (V) is the activation overpotential Er (V) is Ohmic potential loss at current i E ass (V) is overpotential caused by mass transfer, Ecross is an additional potential loss at the cathode because of methanol crossover. In equation (4), Er (V) is the reversible potential for the... 

We assume here that the resistance between WE and CE is negligible. Otherwise, the solution resistance should be experimentally measured and taken into account in estimating the single electrode overpotential. One of the best methods to measure this resistance is electrochemical impedance spectroscopy. 

Here, the single cell element is modeled by a number of voltage sources representing the half-cell potentials. It is to be emphasised that the electrical equivalent shown in Figure 9.9 is only approximate, and errors in accurate determination of overpotentials cannot be entirely eliminated. The only way an accurate estimate of overpotential can be obtained is by solving the appropriate transport equations for the appropriate boundary conditions - coupled with experimental measurements. 

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

The autocorrelation distance is determined by the total overpotential (0Q) of the double layer, which is measured from the critical pitting potential and the coverage 0 of the passive film. From the experimental results which will be discussed later, the actual function form is determined as... 

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). 

It is difficult to measure kinetic currents at high overpotentials, since then the reaction is fast and usually transport controlled (see Chapter 13). At small overpotentials only Butler-Volmer behavior is observed, and the deviations predicted by theory were doubted for some time. But they have now been observed beyond doubt, and we will review some relevant experimental results in Chapter 8. 

However, as mentioned previously, gas-diffusion electrodes usually deviate substantially from traditional electrochemical—kinetic behavior, often being limited by multiple rate-determining factors and/or changes in those factors with overpotential or other conditions. In attempting to analyze this type of electrode, one of the most influential experimental techniques to take hold in the solid-state electrochemical literature in the last 35 years is electrochemical impedance spectroscopy (EIS)—also know as a.c. impedance. As illustrated in Figure 6, by measuring the sinusoidal i— response as a function... 

The passage of a net current through an electrode implies that the electrode is no longer at equilibrium and that a certain amount of overpotential is present at the electrode-electrolyte interface. Since the overpotential represents a loss of energy and a source of heat production, a quantitative model of the relationship between current density and overpotential is required in design calculations. A fundamental model of the current-overpotential relationship would proceed from a detailed knowledge of the electrode reaction mechanism however, mechanistic studies are complicated even for the simplest reactions. In addition, kinetic measurements are strongly influenced by electrode surface preparation, microstructure, contamination, and other factors. As a consequence, a current-overpotential relation is usually determined experimentally, and the data are often fitted to standard models. 

Electrochemical kinetic measurements show that many reactions have a rather constant Tafel slope over a wide overpotential range. This is true for both redox79 and combined electron- and atom-transfer reactions, particularly proton transfer.3,80 None of the approaches discussed above account for the experimental facts. References 41-50, 55-59, and 67 all... 

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