This analysis explores how mesoscopic mass transport influences electrocatalytic selectivity, highlighting the role of surface-bound reaction intermediates in determining reaction pathways and product formation. The study argues that kinetic competition between surface kinetics and diffusion plays a critical role in electrocatalytic selectivity, particularly in reactions such as CO₂ reduction on Cu-based catalysts, where different products can be formed depending on catalyst morphology. By combining microkinetic and transport modeling in a multi-scale approach, the researchers demonstrate that catalyst surface roughness significantly affects selectivity across mesoscopic, microscopic, and atomic scales. The model correctly reproduces selectivity trends observed in experimental studies, offering an alternative or complementary explanation to changes in selectivity previously attributed to nanostructuring or electronic effects. The desorption–re-adsorption–reaction mechanism is central to this analysis, describing how volatile intermediates can desorb from the catalyst surface and either re-adsorb or diffuse away, influencing the final product selectivity. The model incorporates parameters such as catalyst roughness (ρ), which reflects the ratio of electrochemically active surface area to geometric surface area, and other factors like activation free energies and diffusion properties. The model shows that catalyst roughness has a significant impact on selectivity, with higher roughness leading to increased re-adsorption and thus higher selectivity for later reaction products. The study examines the effects of catalyst loading, particle shape, surface corrugation, and alloying on electrocatalytic selectivity. For example, increasing catalyst loading leads to higher surface roughness, which reduces selectivity for early products. Similarly, changes in catalyst particle shape and surface morphology affect selectivity in CO₂ reduction on Cu electrodes, with different morphologies leading to distinct selectivity trends. Alloying Cu with Pd or Ag lowers catalyst roughness, which in turn affects selectivity for products like acetate. The analysis also highlights the importance of surface roughness in catalyst degradation and performance over time, as morphological changes can alter selectivity. The study concludes that mesoscopic mass transport is a critical factor in determining electrocatalytic selectivity, and that surface roughness serves as a key descriptor that captures the influence of catalyst morphology across multiple length scales. The findings provide a new perspective on electrocatalyst design, emphasizing the role of mesoscopic effects in controlling reaction outcomes.This analysis explores how mesoscopic mass transport influences electrocatalytic selectivity, highlighting the role of surface-bound reaction intermediates in determining reaction pathways and product formation. The study argues that kinetic competition between surface kinetics and diffusion plays a critical role in electrocatalytic selectivity, particularly in reactions such as CO₂ reduction on Cu-based catalysts, where different products can be formed depending on catalyst morphology. By combining microkinetic and transport modeling in a multi-scale approach, the researchers demonstrate that catalyst surface roughness significantly affects selectivity across mesoscopic, microscopic, and atomic scales. The model correctly reproduces selectivity trends observed in experimental studies, offering an alternative or complementary explanation to changes in selectivity previously attributed to nanostructuring or electronic effects. The desorption–re-adsorption–reaction mechanism is central to this analysis, describing how volatile intermediates can desorb from the catalyst surface and either re-adsorb or diffuse away, influencing the final product selectivity. The model incorporates parameters such as catalyst roughness (ρ), which reflects the ratio of electrochemically active surface area to geometric surface area, and other factors like activation free energies and diffusion properties. The model shows that catalyst roughness has a significant impact on selectivity, with higher roughness leading to increased re-adsorption and thus higher selectivity for later reaction products. The study examines the effects of catalyst loading, particle shape, surface corrugation, and alloying on electrocatalytic selectivity. For example, increasing catalyst loading leads to higher surface roughness, which reduces selectivity for early products. Similarly, changes in catalyst particle shape and surface morphology affect selectivity in CO₂ reduction on Cu electrodes, with different morphologies leading to distinct selectivity trends. Alloying Cu with Pd or Ag lowers catalyst roughness, which in turn affects selectivity for products like acetate. The analysis also highlights the importance of surface roughness in catalyst degradation and performance over time, as morphological changes can alter selectivity. The study concludes that mesoscopic mass transport is a critical factor in determining electrocatalytic selectivity, and that surface roughness serves as a key descriptor that captures the influence of catalyst morphology across multiple length scales. The findings provide a new perspective on electrocatalyst design, emphasizing the role of mesoscopic effects in controlling reaction outcomes.