Toward improved descriptors by refining the complex reaction network in electrocatalysis

Electrocatalysis is one of the key technologies for developing sustainable and fossil resource free routes to produce fuels and chemicals. The limiting potential (U-L), defined by the reaction free energy of the most difficult electrochemical step in a given pathway, is an effective descriptor for establishing the activity trend of a set of electrocatalysts, allowing high throughput screening of new catalysts. However, the reaction network of electrocatalytic processes is rather complex, especially for the reactions with necessary thermochemical steps, e.g., the synthesis of valuable C-N bond-containing chemicals. Thermochemical steps cannot be significantly enhanced by electrode potentials, where kinetics is a non-negligible issue at even high overpotentials. This makes it challenge by using limiting potential to accurately describe activity trends for the reactions with necessary thermochemical steps. To this end, we propose an effective scheme to determine an improved descriptor. We suggest refining the rather complex reaction network at first. In particular, it is suggested to decouple electro- and thermochemical steps and exclude the unfavorable pathways with an excessively high thermochemical barrier. Then, a global comparison among the other pathways can be made, to determine the optimal pathway and the improved descriptor (the reaction free energy of the most difficult step of the optimal pathway, defined as Delta G(rRPD)-limiting). In addition, the studies on reaction kinetics are also suggested to understand the exception of the best catalysts and provide the direction of experimental optimization. This scheme is of a great compromise between practical efficiency and the accuracy toward the rational design of electrocatalysts.