Exploring mesoscopic mass transport effects on electrocatalytic selectivity

Electrocatalytic selectivity is often discussed at the atomic level on the basis of the active site, while ignoring more subtle effects of mesoscopic mass transport. Here we show how transport controls selectivity through the exchange of surface-bound reaction intermediates between the electrode and bulk electrolyte. We argue that the arising kinetic competition changes with the catalyst’s surface area and can become relevant for technologically important reactions including, for example, different products during the electrochemical CO2 reduction on Cu-based catalysts. Combining microkinetic and transport modelling in a multi-scale approach, we specifically explore and quantify this effect for various showcase examples in the experimental literature. Despite its simplicity, our model correctly reproduces selectivity trends with respect to catalyst roughness on all meso-, micro- and atomic scales. The resulting insight provides an alternative or, at least, complementary explanation to changes in electrocatalytic selectivity that have otherwise been attributed to nano-structuring of active sites or electronic effects due to doping or alloying. Mesoscopic mass transport is often ignored but it can influence electrocatalytic processes. This Analysis introduces a simple multi-scale model that couples diffusion to electrochemical surface kinetics and shows how mesoscopic mass transport determines product selectivity through catalyst morphology.