Photoelectrocatalytic Reduction of Carbon Dioxide

The light driven electrochemical reduction of CO₂ can be accomplished in a monolithic device composed of a semiconductor cathode coupled to anode forming a photoelectrochemical cell, or in a multiunit system in which a carbon dioxide electrolyzer is coupled to a photovoltaic array. The photoelectrochemical cell employs a semiconductor to absorb light energy, generate charge, and carryout the interfacial charge transfer leading to CO₂ reduction. As such, the semiconductor electrode has both optical and heterogeneous catalytic roles. In a solar driven electrolyzer system, metal electrodes are employed and thus, the electrode processes are limited electrocatalysis. In both systems, the impingent light energy must overcome both the positive δG_rxn associated with the multielectron reduction of CO₂, and the free energy of activation, δG^‡. This latter quantity, when converted to an electrochemical potential is referred to as the reaction "overpotential". The properties of semiconductor and metal cathodes are considered in light of the energetic and kinetic parameters noted here. Attention is directed toward semiconductor materials that exhibit cathodic properties under illumination (i.e. p-type semiconductors), along with the physics of semiconductor-electrolyte interfaces, and molecular species that can electrocatalyze CO₂ reduction to CO, formate, and methanol. Areas of limitation and chemical advances that will enhance the efficiency of CO₂ reduction are identified.