Reduction overpotential

Together with carbon, mercury is one of the most attractive electrode materials. It has high proton reduction overpotential and a very well-defined and smooth surface. For these reasons it has been widely used as electrode material in organic electrochemistry. The preparation of mercury microelectrodes has thus been an interesting and challenging problem that has been addressed by several groups (29-31). The technique of making mercury... 

With a reduction overpotential, the supply of electrons to the electrode makes it more negative relative to the solution, an effect that the electrode attempts to counteract by increasing the rate of transfer of electrons to the metal ions in solution (i.e., ired M is increased and iox M is decreased). These concepts are considered in more detail in the following discussion. 

With the values of reversible and actual electrode potentials in L8-L11, the respective cathode and anode overpotentials are given in fines L12 and LI 3. Note that overpotential has units of Volts, since the effect of a reference electrode cancels in Eqs. 1 and 7. As mentioned above, overpotential is defined with respect to an increase in thermodynamic driving force. Thus, cathode (reduction) overpotentials are negative and anode (oxidation) overpotentials are positive. The cell overpotential will therefore be negative, according to Eq. 7. 

The ORR is well recognized as sluggish. The slow kinetics constitutes one of the major voltage losses due to a high reduction overpotential in the performance of a... 

Compared with oxygen reduction overpotential, both kinetic and mass-transport losses of the hydrogen electrode can be neglected. Therefore, the iR-free cell voltage, Ec(iR.jree), of a fuel cell operating on H2/O2 at low current densities (0.1 A/cnr) is controlled by voltage loss due to the O2 reduction kinetics, i.e., by the qact term in Equation 23.6. The cathode overpotential term, qact, for Pt catalysts at low current density (< 0.1 A/cm ) follows the Tafel equation ... 

The main classifications of terms in Equation 3.56 are related to the effects of oxygen concentration and electrode potential. With regard to [G2], Equation 3.56 contains a mix of terms that are of 0th and first-order in [O2]. Thus, the effective order should lie between 0 and 1. At sufficiently small [O2], first-order terms will prevail. In this case, the rate of adsorptive OOHarf formation is the kinetic process with the highest activation barrier, that is, -C 4. Experimental findings consistently show that the [G2] dependence of the ORR is of first order in the normal potential range for the ORR (Ihonen et al., 2002). If this adsorption step represents the highest barrier at equilibrium (or small overpotential), it must remain the step with the highest activation barrier at any reduction overpotential. 

It should also be noted that at high reductive overpotential, exp[(l — a)9] —> 0. This reduces Eq. 2.7 to that given below, and thus provides an expression for the limiting current... 

Fig. 9 (a) Cell impedance measurements (1-kHz high-frequency resistance) of the continuously operated cell and the start/stop-cycled cell, (b) Changes in oxygen reduction overpotentials versus lifetime for a MEA in constant operation (circles) and start/stop cycling mode (squares). (c) Changes in mass-transport overpotentials in Hj/air operation versus lifetime for a MEA in constant operation (circles) and start/stop cycling mode (squares) at 0.2 and 0.8 A cm , respectively... 

By proper calculation of the oxygen reduction overpotential can be deter-... 

By proper calculation of E°(px, T), the oxygen reduction overpotential can be determined using Eq. 4 from the measured oxygen polarization curve, which follows an oxygen partial pressure dependent Butler-Volmer expression [121,122]. The theoretical Tafel line for air polarization should be just... 

Fig. 24 Oxygen reduction overpotentials in V as a function of current density and MEA runtime...

On the cathode electrode, oxygen reduction prefers alkaline conditions to obtain a lower reduction overpotential. Both Puig et al. [85] and He et al. [86] reported the highest current and power generation at pH 9.0-9.5 in two-chamber air-cathode MFCs. Differences in the above results may arise from the different operating conditions (such as substrates, electrolyte conductivity, temperature) and MFC configurations in these studies. 

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