Trends in Electrocatalytic Activity The Limiting Potential Map

Consider a reduction reaction like the ORR, where electrons are transferred from the snrface to the reactant (this corresponds to a + sign in Equation (11.8) and a - sign in Equation (11.9)). The minimum of the (7. s for the elementary steps making up the reaction defines the potential where all steps are exergonic, and this potential is termed the limiting potential for the reaction 

Likewise, for an oxidation reaction like the OER, the maximum value of for which aU reaction steps are exergonic will define the limiting potential for the full reaction  

Note that this is not the same as the measured overpotential (Eq. 11.7), which depends on the current density. However, is a measure of the activity and has been found 

Because of the scaling relations, each of the reaction free energies in the ORR will scale with the O or the OH adsorption energy. That means that we can define the limiting potential for each step as a function of the OH adsorption energy  

FIGURE 11.16 Scaling relation between the OOH and OH adsorption energy on different metals (left) and metal oxides (right). Adapted from Viswanathan et al. (2012) and Man et al. (2011). 

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