Covalent Immobilization of Mediators on Photoelectrodes for NADH Regeneration

The reduced nicotinamide adenine dinucleotide (NADH) is a vital biomolecule involved in many biocatalytic processes, and the high cost makes it significant to regenerate NADH in vitro. The photoelectrochemical approach is a promising and environmentally friendly method for sustainable NADH regeneration. However, the free Rh-based mediator ([Cp*Rh (bpy)H₂O]²⁺) in the electrolyte suffers from low efficiency due to the sluggish charge transfer controlled by the diffusion process. Herein, we report an efficient and facile covalent bonding of the Rh-based mediator with the Si-based photocathode for NADH regeneration. The bipyridine-containing covalent organic framework (BpyCOF) layer ensures the even distribution of mediators throughout the surface of the photoelectrode. The graphene interlayer provides a pathway for charge transport and prevents silicon from corrosion. Furthermore, during the synthesis of BpyCOF, it functions as a substrate to promote the growth of the oriented BpyCOF film. The imitated contact between the components of the photocathode favors the charge transfer to the surface to participate in a chemical reaction, thus improving the catalytic performance and the NADH regeneration efficiency, which is four times higher than the reported photocathode modified by the Rh-based mediator. This study offers a new strategy for the construction of photoelectrochemical solar energy conversion devices.