Tailoring co-catalysts on Si photocathodes for efficient photoelectrochemical CO₂ reduction: recent progress and prospects of deposition methods

The conversion of carbon dioxide (CO2) into value-added products and fuels is a promising technology to address the climate crisis and meet energy demands. The photoelectrochemical CO2 reduction reaction (PEC CO2RR), which mimics natural photosynthesis, is a sustainable method that uses solar energy to convert CO2 into carbon species such as CO, HCOOH, and C2H4 with high industrial utility. Silicon (Si) is a potential photocathode in the PEC CO2RR because of its narrow bandgap and high carrier mobility. The persistent challenges associated with low efficiency and selectivity in the PEC CO2RR continue to drive ongoing research efforts. These efforts have primarily focused on the development of co-catalysts to enhance the efficiency and selectivity of the resulting products. Depositing an appropriate co-catalyst onto a Si photocathode plays a pivotal role in controlling rapid charge transfer and influencing product selectivity. This review offers a comprehensive analysis of recent advances in co-catalyst design and loading approaches for PEC CO2RR, tailored to the desired CO2 reduction products, with particular emphasis on the typical deposition methods of physical vapor deposition, drop-casting, and photo/electrodeposition. First, the basic principles of PEC CO2RR are introduced. Next, recent Si-based PEC cells for CO2 reduction to different target chemicals are highlighted, and the principles of the co-catalyst design and deposition method are discussed. Finally, the review concludes with a summary on effective strategies to promote selective solar fuel production and a vision for the challenges and opportunities of immobilizing co-catalysts in PEC systems.