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.