the reduction reaction itself greatly disturbs the local environment. At the root of this reaction-driven sensitivity is the requirement for both CO2 and protons (H + ) to participate in the CO2 reduction process. The ever-present, and in many cases more thermodynamically favourable, hydrogen evolution reaction (HER) then simultaneously competes with CO2 reduction for protons and electrons. At extremely low current densities (<1 mA/cm 2 ), these protons can be supplied to either reaction directly by hydronium contained within the local electrolyte (Fig. ). As hydronium is depleted, water reduction fills the role as a hydrogen source while the unused hydroxide molecule generated as a by-product, rapidly increases the local electrolyte pH (Fig. ). At slightly more moderate CO2 reduction current densities in aqueous-fed systems (~35 mA/cm 2 for C1 products, ~100 mA/cm 2 for C2 products ), CO2 ultimately becomes depleted by a combination of diffusion limitations from the bulk electrolyte, and the now unfavourable bicarbonate-equilibrium conditions as a result of the increased local pH. Thus, as the reaction proceeds from 0 mA/cm 2 to CO2-depletion (Fig. ), the surface coverage and binding energies of key intermediates on a catalyst's surface are ultimately impacted through these changes in the local environment; not dissimilar from the surface effects incurred by varying material composition, structure or morphology. The activity of a catalyst is then identified by the environment around it, as much as its physical make-up.