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, CH₃OH and C₂H₅OH. It is also highlighted that STF efficiency depends on the route of two-step PEC CO₂ reduction process using CH₃OH 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.