Exploring mesoscopic mass transport effects on electrocatalytic selectivity

Electrocatalytic selectivity has shown a puzzling dependence on experimental parameters related to catalyst morphology or the reactor design. In this study, we explore the proposition that these effects are due to mesoscopic mass transport. Basis for the underlying mechanism is the kinetic competition that arises from exchanging surface-bound, yet volatile, reaction intermediates between the electrode and the bulk electrolyte. The electrocatalyst's morphology can be decisive in driving this competition since its surface area directly affects the probability that a diffusing species will return to the surface for continued reaction, rather than escape as an early partially-converted product. We argue that this competition is relevant for a number of technologically important reactions, including e.g. different products during the electrochemical CO2 reduction on Cu-based catalysts. Combining microkinetic and transport modeling 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 electrode potential and catalyst roughness. Comparing against experimental data further establishes catalyst roughness as a descriptor that unifies the effects of meso-, micro- and atomic-scale morphology on selectivity through transport. 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.