Solar Energy Conversion through Thylakoid Membranes Wired by Osmium Redox Polymer and Indium Tin Oxide Nanoparticles

For several decades, much attention has been paid to thylakoid membranes (TMs) as photocatalysts for converting solar light to electricity. Despite extensive research, current technology provides only limited photocurrents. Here, a novel method based on TM-composite material was developed for achieving high photocurrent. When a thin film composed of TMs, osmium redox polymer (Os-RP), and indium tin oxide nanoparticles (ITOnp) was formed on a porous graphite surface, appreciable photocurrent as high as 0.5 mA cm^-2 was achieved at 0.4 V vs. Ag/AgCl. Each component plays its own role in transferring electrons from TMs to the anode, resulting in sharp drop in photocurrent with missing any component. Optimization between these three components showed 1 : 0.5 : 30 (TM/Os-RP/ITOnp) was the best ratio. Action spectra confirmed that TMs was the origin of photocurrent. It was inferred from blocking experiments using 3-(3,4-dichlorophenyl)-1,1-dimethylurea as an inhibitor that about 41 % of photocurrent was transferred from Q_A in photosystem II to the electrode via Os-RP and ITOnp. Quantum efficiencies at 430 and 660 nm were 12.2 and 18.5 %, respectively. Turnover frequency for water oxidation depended upon the amount of the composite. A complete cell with Pt/C cathode produced P_max of 122 μW cm^-2 at 758 μA cm^-2 under one sun illumination, which is the highest power density to our knowledge. This study opened a possibility of using TMs as photocatalysts for solar energy conversion.