Charge-transfer overpotential, combined

By combining the Nernst equation with the expressions for charge-transfer overpotential (r CT) and diffusion overpotential (r D), equations can be written for the total experimental polarization behavior, E(iex ox) and E(iex red), of a single half-cell reaction ... 

It is now necessary to take a more unified view by considering situations in which the rate of the electrodic process at the interface is subject both to activation and to transport limitations. One refers to a combined activation-transport control of the electrodic reaction. Under such conditions, there will be, in addition to the overpotential T)c produced by the concentration change (from c° to c ) at the interface, an activation overpotential because the charge-transfer reaction is not at equilibrium. The total overpotential rj is the difference between the interfacial-potential difference... 

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],... 

There are two types of problems in the analysis of electrocatalytic reactions with mixed control kinetics reactant adsorption and combined considerations of mass and charge transfer processes in the current vs. potential profiles. The dependence of the current density, j, with the overpotential, x, can be expressed under r values larger than 0.12 V (in absolute values) through the Tafel expression corrected by the mass transfer effects ... 

In chronoamperometry, after switching on an overpotential, the time dependence of the current is monitored. For purely diffusion-controlled processes, the current depends on time according to the Cottrell equation in Chapter 5, Eq. (5.20). The current decreases proportional to 1/Vt. The combination with charge transfer control leads to the following... 

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. 

The equivalent circuit for a complete PEM fuel cell can be applied initially to a "symmetrical" gas supply while varying current densities. At low current densities (< 400 mA/cm ) the mass-transport limitations are very minor, and a simple Nyquist diagram with two relaxation times emerges. The overall impedance is dominated primarily by the cathodic process, with resulting higher cathodic overpotentials. The equivalent circuit is represented by two parallel combinations of charge-transfer resistances R and doublelayer capacitors C 20 pF/cm for the anode and cathode processes. The... 

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