Fundamental Insights into Photoelectrochemical Carbon Dioxide Reduction: Elucidating the Reaction Pathways

The photoelectrochemical (PEC) reduction of carbon dioxide (CO2) to produce solar fuels presents a sustainable strategy to mitigate CO2 emissions and alleviate the global energy crisis. While significant research efforts have been dedicated to optimizing cell system configurations and designing efficient photoelectrocatalysts, there remains a lack of in-depth understanding of the CO2 reduction pathway. This review provides a comprehensive overview of the fundamental insights of PEC CO2 reduction with a focus on CO2 reduction pathways from the perspectives of final products and adsorption modes. First, key challenges are identified and analyzed, including the initial activation of CO2, the competitive hydrogen evolution reaction (HER), and the complex carbon–carbon (C–C) coupling process. The review then examines the fundamental aspects of the reduction process, covering state-of-the-art cell devices, their operational principles, and methodologies for capturing reaction intermediates. The initial activation of CO2 through concerted or sequential proton–electron transfer mechanisms is discussed in detail. Furthermore, potential PEC CO2 reduction pathways are systematically identified and categorized on the basis of the final products and distinct adsorption modes that drive the reduction process, including CO2 insertion, carbon-coordinated and oxygen-coordinated monodentate adsorption, oxygen-coordinated bidentate adsorption, and adsorption on oxygen vacancies. Detailed pathways leading to the formation of C1, C2, and C3 compounds are elucidated, with an emphasis on strategies that enhance selectivity toward C1 and C2+ products. In particular, understanding the CO2 reduction pathways aids in catalyst design. For C1 production, catalyst design focuses on promoting adsorption and activation, as the rate-determining step (RDS) is the initial CO2 activation. In contrast, for C2+ formation, catalyst design strategies aim to increase intermediate concentration, thereby enhancing the lateral interaction of intermediates, which is crucial for C–C coupling. Finally, the review summarizes potential future breakthroughs from electron, interfacial, and ionic pathways, thereby offering insights into the ongoing evolution of PEC reduction technologies.