Main text: Electrochemical reduction of CO2 to produce chemicals and fuels is widely studied as a potential solution for renewable energy storage and CO2 recycling. Because the hydrogen evolution reaction (HER) dominates in acidic aqueous solutions, CO2 electroreduction is conducted in an alkaline or near neutral medium. These reaction media, however, create one of the most important obstacles for high-efficient steady-state CO2 electrolysis: the facile reaction of CO2 with hydroxide (OH -) to form carbonate (CO3 2-) . In alkaline medium where the overpotentials of many catalysts are minimized thanks to a rate-influencing protondecoupled one-electron CO2 reduction step, the carbonate problem obliges continuous refreshing of the OH -electrolytes in a flow-cell configuration in order to obtain a stable performance. The regeneration of CO2 and 2 OH -from aqueous carbonate is energy demanding and leads to a low or even negative energy efficiency for CO2 electroreduction. In near neutral media such as bicarbonate solutions, steady-state CO2 electrolysis is possible. However, CO2 is still consumed by OH -anions electrochemically generated in CO2 electroreduction. The (bi)carbonate is protonated near the anode to regenerate CO2, which leads to a low carbon efficiency. In addition, in near neutral media the high resistance of the solution as well as higher overpotential for oxygen evolution reaction (OER) leads to a high cell voltage and low energy efficiency. In strongly acidic media the resistance and overpotential for OER are lower, and the carbonate problem can be solved since carbonate will not cross the medium to the anode. An acidic medium is also essential to obtain formic acid from CO2 reduction. In near neutral and alkaline media, the same reduction leads to formate, which requires energy-intensive downstream processes for separation and conversion.