Photoelectrochemical reduction of carbon dioxide

Already in 1912 he stated that "[b]y using suitable catalyzers, it should be possible to transform the mixture of water and carbon dioxide into oxygen and methane, or to cause other endo-energetic processes."

Single electron reduction of CO2 to CO2●− radical occurs at E° = −1.90 V versus NHE at pH = 7 in an aqueous solution at 25 °C under 1 atm gas pressure.

The reason behind the high negative thermodynamically unfavorable single electron reduction potential of CO2 is the large reorganization energy between the linear molecule and bent radical anion.

The reduction of redox species happens at less negative potential on illuminated p-type semiconductor compared to metal electrode due to the band bending at semiconductor/liquid interface.

Various p-type semiconductors have been successfully employed for CO2 photo reduction including p-GaP, p-CdTe, p-Si, p-GaAs, p-InP, and p-SiC.

Mechanism proposed by Hori[6] based on CO2 reduction on metal electrodes predicts formation of both formic acid (in case of no adsorption of singly reduced CO2●− radical anion to the surface) and carbon monoxide (in case of adsorption of singly reduced CO2●− radical anion to the surface) in aqueous media.

This same mechanism can be evoked to explain the formation of mainly formic acid on p-GaP, p-GaAs and p+/p-Si photocathode owing to no adsorption of singly reduced CO2●− radical anion to the surface.

In case of p-InP and p-CdTe photocathode, partial adsorption of CO2●− radical anion leads to formation of both carbon monoxide and formic acid.

Maximum catalytic current density for CO2 reduction that can be achieved in aqueous media is only 10 mA cm−2 based solubility of CO2 and diffusion limitations.