Reducing Flavin and Ubiquinone Headgroups with Silicon Nanowire Photocathodes

Photosynthetic biohybridsa structure composed of semiconducting electrodes and carbon dioxide-fixing autotrophs which can be energized by the electrodeoffer a promising platform for selective CO₂ reduction. However, studying the charge-transfer mechanisms from the semiconductor to the cell proves challenging due to a variety of simultaneous processes. Therefore, to deconvolute the system to understand photoelectrochemical performance, we employ model systems composed of a subset of the electron-transfer pathway. Here, we photoelectrochemically reduced ubiquinone-0 (UQ₀) and riboflavin (Rf) (the head groups of ubiquinone-8/10 and flavin mononucleotide/flavin adenine dinucleotide) using Pt-decorated n⁺p-silicon nanowires, a robust catalytic architecture. Under irradiation with 100 mW cm^-2 red light (740 nm), UQ₀ and Rf were reduced with onset potentials of 0.876 V vs the reversible hydrogen electrode (V_RHE) and 0.691 V_RHE, respectively. In addition, UQ₀ achieved a maximum Faradaic efficiency (FE) of 81% with a conversion rate of 1.22 μmol cm^-2 h^-1 at 0.75 V_RHE, while Rf reached its maximum FE and rate at 73% and 0.167 μmol cm^-2 h^-1, respectively, at 0.55 V_RHE. Both redox cofactors were continuously reduced over a 12 h period, demonstrating the robust photosynthetic biohybrid system.