2.1 Mechanism of PEC CO2RR
PEC systems offer promising avenues for CO2 reduction
using solar energy. In a fundamental approach, photocathodes utilizing
p-type semiconductor materials harness solar energy to generate
photovoltage, serving as the primary driving force to counter the
necessary potential for CO2 reduction. A standard
three-electrode PEC CO2RR system consists of a p-type
semiconductor photocathode for CO2 reduction, an n-type
semiconductor photoanode for the oxygen evolution reaction (OER), and a
reference electrode. Immersing a p-type semiconductor photocathode in a
CO2-saturated aqueous electrolyte creates a
semiconductor-electrolyte interface. The difference in Fermi levels
across this interface induces a built-in electric field, causing the
semiconductor’s energy bands to bend downward. Upon light absorption,
electron-hole pairs
(e–/h+pairs) are generated by the promotion of an electron from the
semiconductor’s valence band (VB) to the conduction band (CB), which
then separate into free carriers. Photogenerated electrons from the
photoanode migrate to the photocathode, while photogenerated holes
transfer from the photocathode to the photoanode. This organized
movement of electrons and holes involves the participation of electrons
in the CO2RR and holes in the OER at the interface
between the electrode and the electrolyte.[17,26]A schematic mechanism of PEC CO2RR is depicted in Figure
1B.