Reaction overvoltage

Concentration overvoltage (reaction overvoltage and diffusion overvoltage) If a... 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

FIGURE 9.2 Current density vs. electrode potential curves for the H2-02 and the CH30H-02 fuel cells showing the reaction overvoltages T a and T c at different catalytic electrodes (Pt, Pt-Ru,...). 

However under working conditions with a current density j (different from zero), the cell voltage E(J) becomes smaller than the equilibrium voltage Eeq as the result of reaction overvoltages T a and r c at both electrodes (r is defined as the difference between the working electrode potential E(i) and the equilibrium potential fi ) and ohmic losses (Figure 9.2). 

The coin has its reverse, however. The broadening of the potential window that is often bordered by the solvent electrochemical decomposition potentials (e.g. cathodic hydrogen evolution and anodic oxygen evolution from water) is due to an increase in reactions overvoltage. This may be caused by a diamond s lower electrochemical activity, as compared with the glassy carbon and like electrode materials. On the whole, this conclusion is corroborated by the kinetics studies on diamond electrodes... 

Reaction overvoltage is due to secondary reactions of, e.g., the electrolysis products (for example, the association of gaseous atoms into molecules, the escape of gas bubbles, the formation of a crystalline lattice in the case of metal deposition, also called crystallization overvoltage). 

Chemical reaction rates, controlled by the reaction overvoltage... 

Chemical reaction within the electrolyte or at the electrode surface reaction overvoltage, JJr... 

Chemical reaction (homogeneous, heterogeneous) Reaction overvoltage, tir... 

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