Electrocatalytic CO₂ reduction has become a highly promising route to sustainable and economical carbon-neutral fuels. So far, however, a basic understanding of the underlying chemical processes is still lacking, mainly due to the highly complex, large reaction network involving a variety of C1 and C2 products. Experimental investigations are additionally aggravated by apparent transport limitations of CO₂ to the surface which are also determined by buffer equilibria in solution. Additionally, pH gradients at the solid-liquid interface can be important to consider, since local pH effects have shown to critically influence product selectivities. Such effects overlay (electro-)chemical kinetics which can be modeled e.g. by Density Functional Theory (DFT) in combination with micro-kinetic modeling approaches. However, in order to utilize these methods for the transport limited regime, these need to be extended by equations governing diffusion and migration effects as well as chemical reactions in the electrolyte. Using a coupled kinetic-transport model based on Poisson-Nernst-Planck theory and ab initio calculations of reaction and activation energies, we show transport limitations to be crucial to determining C2 to C1 product selectivity. We present strategies for catalyst structural design to optimize the selectivity towards higher value C2 products.