Fig. Schematic of the different mass transport zones in front of a cathode surface during CO 2 ER in an aqueous electrolyte. Zone 1: Stern layer, Zone 2: potential screening layer, Zone 3: Nernst diffusion layer and Zone 4: bulk. The red dotted line in the electrolyte is a typical potential profile with φ M and φ OHP being the potential of the metal cathode and at the outer Helmholtz plane (OHP), respectively with the potential at the point of zero charge (PZC) of the catalyst surface as the reference potential (description in section 2.2). a cat and a an are the effective diameters of a solvated cation (in dark blue) and anion (in orange), respectively. L Stern , L Screening and L Nernst depict the dimensions of the Stern layer, potential screening layer and the Nernst diffusion layer, respectively and t relax is the relaxation time (order of magnitude) relevant for the mass transport processes in the respective zone. J i depicts the incoming and outgoing flux of species (i) OH -and CO 2 , respectively.