A thin nonporous disk of YSZ (solid electrolyte) is sandwiched between two porous electrodes. CO2 diffuses through the porous electrode (cathode) and reaches the vicinity of the electrode-electrolyte boundary. Through a combination of thermal dissociation and electrocatalysis, an oxygen atom is liberated from the CO 2 molecule and picks up two electrons from the cathode to become an oxide ion (O2–).