Preparation of Thylakoid/Polyaniline/Reduced Graphene Oxide/Glassy Carbon Integrated System and Photocurrent Enhancement

Solar energy is enormous, inexhaustible and environmentally friendly. Photosynthesis is a highly developed system that converts solar energy into chemical energy, taking place in thylakoid membrane of plants, algae and cyanobacteria. In the light reaction, solar energy is absorbed by light-harvesting complexes (LHC) containing antenna pigments. Absorbed solar energy is transferred to the reaction center, where the water photolysis into oxygen, protons and electrons occurs by oxygen evolving complex (OEC) attached to photosystem II (PSII). The generated electrons are transferred along the electron transfer pathway called Z-scheme and eventually stored in the form of NADPH. Since quantum yields of photosystem I (PSI) and photosystem II are very high, numerous attempts have been made to integrate these photosynthetic units (PSUs) into photoelectrochemical cells to directly convert light into electricity. Rather than using PSI or PSII, here we present our preliminary results on light energy conversion with thylakoid membrane that is installed on the electrode surface. Thylakoid membrane was isolated from spinach whose oxygen evolving activity was measured by a Clark-type oxygen sensor under the illumination. For the effective light energy conversion, we have developed an integrated system containing thylakoid multilayers on the glassy carbon (GC) electrode. First, 4-aminophenyl (4-AP)-modified GC surfaces were prepared. Isolated thylakoid was mixed with polyaniline (PANI) and graphene oxide (GO), and this composite material was applied onto the 4-AP-modified GC. Since GO has poor electric conductivity, we electrochemically reduced GO to highly conductive reduced graphene oxide (RGO) form. Thus prepared integrated system is denoted as Thyl/PANI/RGO/GC. This integrated system was proven effective in solar energy conversion. To achieve high photocurrent, we prepared the same system with different GO/chlorophyll ratios. After cycling potential to reduce GO to RGO, photocurrent was measured. The ratio of 3:1 (GO:chlorophyll) turned out to give highest photocurrent. The electron transfer pathways have been identified from the blocking experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of Q_B site in PSII. Thylakoid suspension was treated with DCMU before immobilization and then applied to the electrode. Virtually no photocurrent enhancement was observed proving that the electron transfer was blocked. A complete solar cell composed of Thyl/PANI/RGO/GC anode and Pt/C air cathode was constructed. From the polarization curve, the maximum power density of 10.5 μW/cm² with current density of 24.7 μA/cm² under one sun illumination was obtained. The turnover frequency was calculated to be 5.8 water molecule oxidation per PSII unit per second, indicating that the immobilized thylakoid in the form of the multilayer films maintained its activity. This result shows that larger number of thylakoid units are needed for enhanced photocurrent and our method of integrating thylakoid using polyaniline and graphene oxide could be a useful way to this end.