In a complete description of the dependence of the rate of an electrocatalytic process on electrode potential, the effect of the potential has to include two components: acceleration of the electrode reaction by lowering the activation energy of any step involving electronic charge transfer and, modification of the composition of the catalyst/electrolyte interface by the change in electrode potential. The latter effect has only rarely been included rigorously in quantitative descriptions of the rate dependence on electrode potential, thereby missing in the description an important factor determining the dependence of the rate on overpotential and , consequently, determining the power output and conversion efficiency observed in a fuel cell or electrolyzer given some combination of electrocatalyst and electrolyte.
This talk will cover examples of ORR , OER and HOR processes taking place at electrocatalysts which undergo significant changes in surface composition with change of the potential within the window of relevance to fuel cells or water electrolyzers. The examples cover several types of effects on the rate of the faradaic process as result of the dependence of the interfacial composition on overpotential, including:
* rate acceleration by potential-induced generation of active surface sites
* rate decay by potential-induced deactivation of surface sites
* rate effects by potential-induced electrolyte layer formation
Complete quantitative description of these effects in mathematical form, is, in many cases, not straightforward. Nevertheless, recognition of such effects in the general expression for the rate of the electrocatalytic process can and should be reflected by a pre-exponential factor which is potential dependent [1,2]. Assuming a potential independent pre-exponential factor, as typically done, results in misinterpretation of fundamental aspects of the kinetics of electrocatalytic process, for example the reason for commonly observed increase in "Tafel slope" with the increase in overpotential.
Finally, the argument made frequently that electrocatalytic processes can be fully understood using a single “descriptor”, e.g., a M-O or M-H bond strength (where M stands for an active metal surface site) , does not recognize the change of the number of active M sites with overpotential as result of a potential-induced surface modification process. This dependence of the number of active sites on potential has to be covered by , at least, a second “descriptor” , for a full description of the electrocatalytic process.
References:
1. S.Gottesfeld, ECS Trans. 2014 61(31): 1-13
2. S.Gottesfeld, ECS Trans. 2015 66(29): 13-30