The immediate environment of the CO 2 electro-reduction (CO 2 ER) catalyst is an extremely important handle to rationally optimize the overall performance of the system . The concentrations of reactive as well as non-reactive species in the vicinity of the catalyst surface, and their mutual interactions, have a direct influence on catalytic behaviour, and thus on the achievable activity and selectivity . Mass transport becomes a crucial factor for CO 2 ER due to the limited solubility of CO 2 in aqueous elec-trolytes, the participation of CO 2 in the buffer reactions, as well as the changes in the local environment during reaction due to the continuous consumption of CO 2 and production of hydroxide (OH -) ions. Designing more favorable reaction interfaces has helped to achieve remarkable performance gains and the approach highlights the significance of mass transport as an influential optimization parameter for the CO 2 ER system. What constitutes the local reaction environment in such an electrocatalytic system however, is still an open question. Further understanding needs to be developed to define the physical phenomena that result in the local environment of the catalyst and that sufficiently capture the parameters most important for the performance.