Bio-inspired Catalyst-Modified Photocathode for Bias-Free Photoelectrochemical NADH Regeneration

Cofactors such as nicotinamide adenine dinucleotide (NADH) and its phosphorylated form (NADPH) play a crucial role in natural enzyme-catalyzed reactions for the synthesis of chemicals. However, the stoichiometric supply of NADH for artificial synthetic processes is uneconomical. Here, inspired by the process of cofactor NADPH regeneration in photosystem I (PSI), catalyst-modified photocathodes are constructed on the surface of polythiophene-based semiconductors (PTTH) via self-assembly for photoelectrochemical catalytic NADH regeneration. With the assistance of viologen (vi²⁺) electron transfer mediators (similar function as Ferredoxin in PSI) linked to the [Rh(Cp^*)(bpy)] catalyst, the Rh-vi²⁺@PTTH photocathode exhibits higher photocurrent density (-665 µA cm^-2) with a high apparent turnover frequency (TOF, 168.4 h^-1) under a relatively positive potential (0.0 V vs RHE). In addition, through holistic functional mimics of the photosystem, a tandem photoelectrochemical cell is constructed by assembling a CoPi@BiVO₄ photoanode (artificial photosystem II, PSII) with the Rh-vi²⁺@PTTH photocathode. This system achieves a production rate of 42.5 µm h^-1 cm^-2 and a TOF of 179.3 h^-1 without an externally applied bias for NADH regeneration. The photo-generated NADH is directly employed to assist glutamate dehydrogenase (GDH) in the catalytic conversion of α-ketoglutarate to L-glutamate. This study presents a novel strategic approach for constructing bias-free photoelectrochemical NADH regeneration systems.