Influence of Mass Transport on Electrode Kinetics

At the value of the limiting (maximum) current density, the species Mz+ are reduced as soon as they reach the electrode. 

At these conditions, the concentration of the reactant Mz+ at the electrode is nil and the rate of deposition reaction is controlled by the rate of transport of the reactant Mz+ to the electrode. If an external current greater that the limiting current il is forced through the electrode, the double layer is further charged, and the potential of the electrode will change until some process other than the reduction of Mz+ can occur. 

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In this chapter we derive the Butler-Vohner equation for the current-potential relationship, describe techniques for the study of electrode processes, discuss the influence of mass transport on electrode kinetics, and present atomistic aspects of electrodeposition of metals. 

Figure 12(b) shows the local current distribution of first and second order reactions and applied over potentials ° for the coupled anode model without the mass transfer parameter y. The figure also shows the effect of a change in the electrode kinetics, in terms of an increase in the reaction order (with respect to reactant concentration) to 2.0, on the current distribution. Essentially a similar variation in current density distribution is produced, to that of a first order reaction, although the influence of mass transport limitations is more severe in terms of reducing the local current densities. 

Fig. 4 Schematic representation of the influence of mass transport limitations on electrode kinetics.

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

It will be assumed in this section that the mass transport is much more rapid than the redox kinetics, such that the activities or concentrations of species O and R at the electrode-solution interface can be considered as identical to their bulk values (i.e., a = a °l and c = c 1 with i = O, R). The influence of the mass transport on the current-potential response is treated in Sect. 1.8. 

An aim of the model is to determine the influence of the various mass transport parameters and show how they influence the polarization behavior of three-dimensional electrodes. In the model we have adopted relatively simple electrode kinetics, i.e., Tafel type, The approach can also be applied to more complicated electrode kinetics which exhibit non-linear dependency of reaction rate (current density) on reactant concentration. 

There are very few studies of the ORR under hot and dry fuel cell operating conditions. Recently, methods have been devised to separate the mass transport effects from the kinetic effects [74, 75], but none of these have been applied to hot and dry fuel cell operation. These studies showed that, under fully humidified conditions up to 70°C, oxygen reduction had a tenfold higher specific performance for platinum black at 0.90 V compared to Pt on carbon as has been previously reported in the literature [76]. However, this significant benefit of platinum black is shown to rapidly decrease when the potential is shifted to lower, more fuel cell relevant potentials. This is manifested in the Tafel slope, which decreased from 360 to 47 mV/decade in the region where the overpotential was <0.35 V. The effect of hot and dry conditions has been studied in a 5 cm MEA where mass transport and kinetics are difficult to separate [73]. At 120°C, the Tafel slope is found to increase inversely with RH. It is speculated that this is due to the decrease in ionic conductivity in the electrode. RH can also influence water oxidation to form Pt-OH and Pt-O and thereby change the surface condition of the platinum crystals. 

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