Dye-Sensitized Photocathodes Assembly and Tandem Photoelectrochemical Cells for CO₂ Reduction

Increasing concentrations of atmospheric CO2 are a worldwide problem that have triggered considerable environmental concerns, such as global warming, glacier melting, and a loss of biodiversity. Therefore, the conversion and utilization of CO2 have become increasingly urgent. CO2 photoreduction mimics natural photosynthesis and performs CO2 reduction by using solar energy to drive the formation of renewable fuels, which has been documented as a potential solution for energy shortage and global warming. CO2 photoreduction is a promising approach for achieving energy sustainability by forming and utilizing C-based reduction products. The traditional thermocatalytic CO2 reduction technique requires high temperature and high-pressure conditions, which consumes huge energy amounts; by contrast, CO2 photoreduction employs solar energy and electrical energy as the activation energy for catalytic reaction, resulting in faster reaction rates and minimal environmental impact. Although semiconductor-based photocatalysts are advantageous, given their low cost and easy modification, their slow carrier transfer rates and poor photostabilities limit their use in practical applications. Homogenous catalysts consisting of integrated photosensitizers and catalyst units on the surface of semiconductor electrodes can provide more appropriate photocatalytic capabilities for simultaneous CO2 photoreduction and water oxidation. However, differences in reaction conditions for the two half reactions, with the integration of water oxidation and CO2 reduction in single one-catalyst systems, may be inaccessible. To overcome these bottlenecks, a variety of approaches for artificial photosynthesis have been investigated to achieve more highly efficient CO2 photoreduction, and these strategies primarily focused on the optimization of the surface structures of semiconductor electrodes and the development of novel catalysts. When the basic principles of the molecular chemical reaction are combined with surface construction preparations, the photocatalytic activity can be efficiently enhanced. This Account summarizes the mechanisms for CO2 photoreduction in a Dye-Sensitized PhotoElectrochemical Cell (DSPEC), and it outlines the progress made in this area based on the design and assembly of molecular-based DSPEC and the role of chromophore-catalyst assemblies in these applications. By optimizing surface film internal structures, surface molecular assemblies have been prepared that open a door for preparing durable, efficient integrated assemblies for CO2 photoreduction. In addition, this Account also briefly summarizes the research progress of a typical tandem DSPEC cell for coupling CO2 reduction with water oxidation. Based on the research progress and challenges of semiconductor-surface molecular catalyst design, prospects are outlined at the end of the Account, including enhancement of catalytic behaviors and long-term stability, optimization of the surface assembly structures, and novel design of efficient bias-free tandem cells.