Apart from the electrocatalyst roughness, we include six additional parameters in our model. Three of these parameters enter the microkinetic model as the activation free energies for each of the competing surface reaction steps: the redox barrier ∆G ‡ rdx (U ) for further electrocatalytic conversion of X * at the surface, the barrier ∆G ‡ des for X * desorption, and the reverse barrier ∆G ‡ ads for X ( * ) adsorption. Among these barriers, we assume that only ∆G ‡ rdx depends on the applied electrode potential U , while desorption/adsorption are treated as purely 'chemical' steps and thus entirely independent of potential. This is an important distinction to make as it suggests that U can change selectivity by shifting the competition toward or against the surface redox reaction. Again, this is a dependence that we will rely upon in the following when comparing against experimental selectivity data. Finally, the remaining three model parameters are part of the transport modeling: the diffusion coefficient D diff and diffusion length L diff of species X ( * ) (which simplify the otherwise complex hydrodynamic dimensionless constants), as well as Henry's solubility constant H s which is used to convert between activity and concentration. Further details on our kinetic model can be found in the SI, Sec. S1.