We next explore selectivity with varying catalyst particle shape and surface corrugation, i.e. the influence of morphological features on the mesoscopic/microscopic range. These effects are demonstrated here for the eCO 2 RR on Cu electrodes; a model system of immense technological importance due to its unique ability to synthesize both a variety of one-carbon (C 1 ), but also high-value multicarbon (C 2+ ) products. A study by the group of Jaramillo et al. identified and enumerated a total of 16 different possible products, of which we classify about half as the closed-shell intermediates that may be relevant to the "desorptionre-adsorption-reaction" mechanism. It is this intricate reaction network, along with the plethora of available experimental data, that make Cu-based eCO 2 RR an ideal playground to explore the effects of mass transport on electrocatalytic selectivity. In this section, we specifically focus on two such intermediates in the reaction mechanism: CO as the prominent first product following CO 2 electroreduction, and acetaldehyde (MeCHO) which appears further down the reaction path and competes with various other C 2+ products. Figure compiles selectivity data from different experimental studies, with very different catalyst morphologies. A Cu(111) single crystal, polycrystalline Cu foil (pc-Cu), as well as two sets of data for oxygen-derived Cu (OD-Cu) are all included in Fig. The generality of the "desorption-re-adsorption-reaction" mechanism is further strengthened by our second case study in Fig. ). This figure panel plots the selectivity towards MeCHO (vs. later C 2+ products) as a function of U SHE , while similarly compiling experimental data on different Cu morphologies: a pc-Cu sample of ρ=1, an OD-Cu sample of ρ=87, and a highly porous Cu nanoflower (Cu-flower) of ρ=390. The exponential drop of MeCHO selectivity with reducing potentials is not as clear here as in the previous example due to the limited U SHE range of the available experimental data. This relation is nevertheless predicted by our kinetic model (solid lines in Fig. )), along with the inverse relation with respect to catalyst roughness. While quantitative experiment-theory agreement is certainly far from perfect, the underlying trends are clearly compatible. Taken together, Fig. confirms the prevalent notion that copper's meso-or microscopic is critical to the resulting eCO 2 RR selectivity. These effects have so far mostly been associated, however,