Solar Energy Is Effectively Converted to Electricity and Hydrogen Using Cyanobacteria

Abstract

We present solar energy conversion for electricity generation and hydrogen production from water using Anabaena variabilis (A. variabilis), an oxygenic photosynthetic cyanobacterium. Electrons from water oxidation by the light reactions that take place in thylakoid membranes (TMs) undergo a series of electron transfer pathway called Z-schme. We devised a method in which electrons are directed to the external electrode. In order to extract electrons from TMs, we found that double mediators are very effective in transferring electrons to the electrode from TMs. Dimethylbenzoquinone, the first mediator, can take electrons from the Z-scheme and delivers them to ferricyanide, the second mediator. Thus transferred electrons move to the cathode where oxygen reduction reaction takes place. Figure shows how biosolar cell (BSC) is operating under illumination.Electricity generation was achieved by simply attaching an external load between the anode and the cathode. When A. variabilis was dispersed in solution, the maximum photocurrent up to 2.14 mA cm-2 upon one sun light intensity. A complete cell produced Pmaxof 160 mW cm-2 at 750 mA cm-2 with a quantum efficiency of 4.8% over a whole spectral range and turnover frequency of 17 per PSII for water molecule oxidation. The same concept was also applied to the hydrogen production. When operated in a double-chamber reactor under anaerobic condition, pure hydrogen gas was produced from water with high energy efficiencies and high production rates upon voltage bias. The maximum H2 production rate of 122 mmol H2(mg Chla)-1h-1 at anode potential of 0.4 V vs. Ag/AgCl was achieved. Energy efficiency exceeded 100% at lower anode potentials. We expect that this study opens a new way of utilizing cyanobacteria for appreciable electricity and hydrogen production. The same concept could be extended to other oxygenic cyanobacteria. Figure 1