Density gradients, high currents

Upon comparison of eq 32 to 28, it is seen that the proton—water interaction is now taken into account. This interaction is usually not too significant, but it should be taken into account when there is a large gradient in the water (e.g., low humidity or high-current-density conditions). Upon comparison of eq 33 to 31, it is seen that the equations are basically identical where the concentration and diffusion coefficient of water have been substituted for the chemical potential and transport coefficient of water, respectively. Almost all of the models using the above equations make similar substitutions for these variables 15,24,61,62,128... 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

The deviation of the flux from a purely radial configuration can lead to several consequences on the operational conditions of the fuel cell, thus affecting the resultant performance. First of all, if the gas is not well distributed within the cell surface, the reaction rate varies from area to area. This implies the existence of preferred zones for the electrochemical reaction, and, consequently, of local high current density, thus reducing the overall cell voltage. Secondly, different reaction rates throughout the cell causes temperature gradients and, consequently, thermal stresses, which can cause mechanical failure of the cell [2-4], Finally, the existence of (some) preferred zones for the electrochemical reaction implies that part of... 

Convection may occur due to density gradients (natural convection). A density gradient may arise at high currents due to the production or depletion of matter, especially in technical electrolysis and in coulometric experiments. Heating or cooling may also cause density gradients. 

In direct electrochemical reactions the substrate undergoes a heterogeneous redox reaction at the electrode surface within the Helmholtz layer. The thus formed reactive intermediate, i.e. a radical ion, undergoes the chemical follow-up reaction to the product in die reaction layer. There are steep concentration gradients near the electrode. Second-order reactions of the intermediates can thus be obtained at high current densities. 

Depending on the current density a rather non uniform utilization of the catalyst layer can be expected. While at low current density negligible effects are caused by the ionic resistance in the catalyst layer and almost all platinum particles can be used, the reaction concentrates close to the membrane interface at high current densities causing underutilization of the platinum present in the electrode [54, 55]. Optimization of electrode performance can be expected from microstructural optimization for example by designing catalyst layers having gradients in noble metal concentration and porosity. 

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