How efficient could photocatalytic CO2 reduction with H2O into solar fuels be?

Photocatalytic carbon dioxide (CO2) reduction in aqueous media provides a potential and convenient way for fulfilling increasing fossil energy demand and relieving global warming problems. However, the efficiency limit for the photocatalytic CO2 reduction system still remains unclear. Here a comprehensive model of the photo catalytic CO2 reduction system is established to theoretically evaluate the efficiency limit, in which the light absorption, charge carrier recombination behaviors, and surface reaction processes are considered. Effects of Auger coefficient and the gas coverage ratio on the energy conversion efficiency are discussed to provide feasible enhancement approaches. Experimental efficiencies are compared with theoretical results to analyze the origin of low efficiencies. The photocatalytic reduction of CO2 into methanol is taken as an example, the energy conversion efficiency limit amounts to be 46.7%. To further improve the utilization of solar spectrum, up conversion materials are incorporated into the photocatalytic system. The maximum efficiency of photocatalytic reduction of CO2 to methanol is predicted to be 59%. This paper unveils the upper efficiency limit of photo catalytic reduction of CO2 and provides the guidance for design of efficient photocatalysts and systems.