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

Abstract

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.