(Keynote) Photocurrent Enhancement through Thylakoid Membranes and Anabaena Variabilis as Biocatalysts for Solar Energy Conversion to Electricity

Abstract

Solar energy conversion to electricity using biological materials has drawn much attention during the past decade as they provide very effective means to harvest and utilize solar light via photosynthesis. Photosynthetic light reactions where light energy is absorbed by light harvesting complexes and transferred to the reaction centers called photosystems I and II (PSI and PSII). Absorbed solar energy induces electron excitation with extreme quantum efficiencies. Excited electrons are transferred along the Z-scheme and stored in the form of NADPH for the carbon fixation. During this process, water molecules are split into oxygen molecules, protons and electrons. In an early development stage, PSI and PSII were isolated and fixed on the electrode surface for solar energy conversion. Many ingenious methods have been suggested to direct electrons to the electrode. However, complex procedures, time consuming process, and relatively low photocurrent density have limited their use for practical applications. Thylakoid membranes (TMs) and cyanobacteria in this sense have advantages that all the necessary components for light utilization are already optimally arranged and thus structures are quite rigid.In this presentation, it will be shown how effectively solar energy can be converted to electricity using TMs and Anabaena variabilis (A. variabilis) as a model cyanobacterium. TMs are easily isolated and purified. TM immobilization on the modified surface resulted in only limited current density of less than microampere because of limited number of TM units and inefficient electron transfer. To achieve enhanced photocurrent density from TMs, we have developed a novel method of loading enriched thylakoid membranes on the electrode in which indium tin oxide nanoparticles (ITO NPs) served as a linker to connect each thylakoid and mediators that transferred electrons to the electrode through ITO NPs. When thionin was used as a mediator, about 43 μA/cm-2 was obtained (Figure 1). About 10 times improvement was made using osmium redox polymer that functions as a molecular wire as well as a mediator. The long polymer chain could have access to the components in the Z-scheme where electrons are taken to the Os complex in the polymer.More photocurrent enhancement has been made when A. variabilis was used. Rather than immobilizing this cyanobacterium on the surface, it was dispersed in the solution to increase its number. We have double mediator system in which the first mediator diffuses in and out of the cell to have access to the TMs located in the cytoplasm. Quinone molecules were found very effective. However, their redox kinetics was rather slow, not producing high photocurrent. With an aid of ferricyanide, the second mediator, great photocurrent enhancement was achieved. Taking electrons from quinone molecules, ferricyanide delivers electrons to the graphene-coated ITO electrode. Depending on the concentrations of 1,4-benzoquinone and ferricyanide, and on the number of A. variabilis, more than 1 mA/cm-2 could be achieved at 0.4 V vs. Ag/AgCl. High photocurrent was maintained for several hours without serious decrease. Study to further enhance photocurrent and to extend lifetime is underway. Our preliminary results demonstrate a possibility of utilizing cyanobacteria that are ubiquitous in environment as alternative energy sources for solar energy conversion. Figure 1