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 (VRHE) and 0.691 VRHE, 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 VRHE, while Rf reached its maximum FE and rate at 73% and 0.167 μmol cm–2 h–1, respectively, at 0.55 VRHE. Both redox cofactors were continuously reduced over a 12 h period, demonstrating the robust photosynthetic biohybrid system.