Electrocatalyst electric potential

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

This is the simplest model of an electrocatalyst system where the single energy dissipation is caused by the ohmic drop of the electrolyte, with no influence of the charge transfer in the electrochemical reaction. Thus, fast electrochemical reactions occur at current densities that are far from the limiting current density. The partial differential equation governing the potential distribution in the solution can be derived from the Laplace Equation 13.5. This equation also governs the conduction of heat in solids, steady-state diffusion, and electrostatic fields. The electric potential immediately adjacent to the electrocatalyst is modeled as a constant potential surface, and the current density is proportional to its gradient ... 

This expression demonstrates that the electric potential changes logarithmically with the radial distance of the cylindrical electrocatalyst. When the current flows from the outer to the inner cylinder, the electric potential difference is negative in this spatial system the current density on the outer cylinder is lower than that of the inner because of the larger area. On the other hand, the electric potential difference is positive from the left side to the right side of the cylindrical system. 

In the case of an electrocatalyst element, as shown in Figure 13.1b, we can say that there are only electric potential variations along the x axis thus, the electric potential across the electrolyte, , will be... 

It is clear that the maximum change in the electric potential in the solution occurs for 0 = 0° and 180°, since a direct interaction between the ions and the electrocatalyst takes place. 

When the primary distribution does not illustrate the current or electric potential distribution well, an additional resistance, that is, the charge transfer electrode resistance, has to be considered. In such cases, we need to account for the electrode kinetics, and the secondary current and potential distributions emerge from the models. For industrial purposes the porous or tortuous electrocatalyst has to be considered as a dynamic system. This means that its porosity shape and density besides the surface roughness and the real geometric area changes all the time. This point makes us think that it... 

In these cases, the electric potential near the surface of the electrocatalyst is governed by the Butler-Volmer equation of a pure charge transfer process ... 

Electrodeposition. In this process, a conductive substrate is placed in an electrolyte solution (typically aqueous) that contains a salt of the material of interest. When an electrical potential is apphed between the substrate and a counter electrode, redox chemistry takes place at the surface of the substrate which deposits material. Complex pulse trains and/or high-pulse frequencies are sometimes used to direct current flow and favor desired reactions. A postsynthesis calcination is often performed to reach a desired material phase. Electrodeposition is restricted to deposition of electrically conductive materials and produces polycrystaUine and amorphous films. This process is also appropriate for thin film surface treatment of PEC electrodes, such as electrocatalyst deposition. 

Of course, clectrocalalytic reactions are potential dependent in rate, as are all other electrode reactions,87 and one of the subjects to which attention will be given in the following discussion is the reference potential at which a comparison of electrocatalysts should be made. Table 7.17 contains a comparison of chemical (thermal) and electrochemical (electrical) catalysis. 

Let us suppose that the total current from all the fuel cells used in the electricitygenerating plant is I A. The number of seconds in a year is 3.1 x 107 and (as It is coulombs since t is the time in seconds), / = 3.1 x 107 = 1.8 x 1014 or 1 = 6.6 x 106 A. Now, all this current would be converted to electrical energy in the fuel cells at (say) about 0.7 V, which is a reasonable potential in the oxidation of methanol in a fuel cell with a good electrocatalyst. Hence, we should produce 4 x 106 W or about 4000 kW. 

One of the distinctions of electrocatalysts, which differs from that of conventional heterogeneous catalysis, is that the electron transfer processes between the oxidant and the reductant are separated into two half-reactions which are carried out in separate reaction zones. This enables the transfer of electrons through an external electrical circuit which can potentially power a load by doing useful work. An obvious advantage of such electrochemical cells is that it becomes possible to use different types of catalyst materials for the respective half-cell reactions and can be subjected to different types of environments. 

Low temperature fuel cells are electrochemical devices that convert chemical energy directly to electricity. They have great potential for both stationary and transportation applications and are expected to help address the energy and environmental problems that have become prevalent in our society. Despite their great promise, commercialization has been hindered by lower than predicted efficiencies and the high cost of electrocatalysts in the electrodes. 

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