Theoretical Efficiency Limits of Photoelectrochemical CO2 Reduction: A Route-Dependent Thermodynamic Analysis.

Solar-fuel formation via photoelectrochemical (PEC) routes using water and CO2 as feedstock has attracted much attention. Most PEC CO2 reduction studies have been focused on the development of novel photoactive materials; however, there is still a lack of understanding of the key limiting factors of this process. In this study, the theoretical limits of Solar-to-Fuel (STF) efficiencies of single- and dual-junction photo-absorbing materials are illustrated for single-step multi-electron CO2 reduction into fuels including HCOO- , CO, CH3 OH and C2 H5 OH. It is also highlighted that STF efficiency depends on the route of two-step PEC CO2 reduction process using CH3 OH as a model fuel. Finally, it is illustrated the beneficial role of alternative strategies such as dual-junction photo-absorbing electrodes, externally applied bias and subsequent reactor chambers on the maximum theoretical efficiencies of PEC CO2 reduction.