Ag/AgCl (sat. KCl) the catholyte chamber was continuously purged with pure N2 for 30 min, after which the CO2 concentration was gradually increased to 10, 20, 50 and 100 vol% every 45 min (Figure ). Under pure N2, the current density was the lowest at approximately -0.5 mA cm -2 and around ~0.08 µmolH 2 min -1 evolved as the main gaseous product, with a minor CO background (<0.01 µmolCO min -1 ) likely caused by the chemical equilibrium between carbonic acid (H2CO3) and CO2. Upon increasing the CO2 concentration (in vol%), the current density gradually increased, from approximately -0.8 mA cm -2 (10 vol%), to -1.2 mA cm -2