CO2 gas (grey bubbles) is fed to the cathode, where electrons
supplied by the applied cell voltage reduce it at the catalyst
surface — visible as the bubble colour turning into whichever
product molecule the catalyst favours. At the anode, water is
oxidised to O2 (small blue bubbles) to balance the charge, with the
two compartments separated by an ion-exchange membrane. Which
carbon product wins is set almost entirely by the catalyst's surface
chemistry, not just the voltage.
Cathode: CO2 + 2H⁺ + 2e⁻ → CO + H2O (or → C2H4, HCOOH, …)
Anode: 2H2O → O2 + 4H⁺ + 4e⁻
Faradaic efficiency = charge → product / total charge passed
- Cell voltage — more negative cathode potential drives higher current density, but past a point the competing hydrogen evolution reaction (H2 side product) starts winning.
- CO2 feed rate — too little CO2 near the surface starves the reaction and H2 evolution takes over; too much has diminishing returns.
- Electrolyte pH — alkaline conditions suppress hydrogen evolution and favour CO2 reduction; acidic conditions favour H2.
- Catalyst — Ag and Au bind the *CO intermediate weakly, so it desorbs as CO; Cu binds it just right to let further reduction proceed toward C2+ products like ethylene.