Heterogenized homogeneous catalysts for photoelectrochemical carbon dioxide reduction: a path toward ideal hybrid systems

The development of technologies that convert solar or electrical energy into sustainable chemical fuels remains a central challenge in the field of energy research. Among various strategies, photoelectrochemical cells (PECs) that enable the direct conversion of carbon dioxide (CO2) into value-added fuels such as carbon monoxide (CO), formic acid (HCOOH), formaldehyde (HCHO), and methanol (CH3OH) using sunlight have gained considerable attention. While most PEC systems rely on heterogeneous catalysts, the emerging approach of heterogenizing homogeneous molecular catalysts onto electrode surfaces offers a promising pathway that combines the molecular-level tunability of homogeneous systems with the robustness and recyclability of heterogeneous platforms. Anchoring molecular catalysts onto conductive or semiconductive surfaces not only enhances charge transport efficiency from the substrate to the active site, enabling high current densities, but also facilitates integration into device-scale architectures. Among various immobilization strategies, covalent anchoring via functionalized ligands has proven particularly effective in ensuring strong surface binding. However, the impact of such covalent anchoring on the catalytic activity and long-term stability of molecular catalysts remains poorly understood. This review highlights recent advances in hybrid molecular PEC systems for selective CO2 reduction to CO and formate, focusing on the design of modular ligands with surface anchoring functionalities. We summarize current covalent immobilization techniques and discuss the mechanistic implications of catalyst-surface interactions. Finally, we outline key challenges and future directions toward the rational design of robust, selective, and scalable molecular-material hybrid catalysts for solar fuel production.