Cyanobacteria-based double-mediated photo-microbial electrochemical cells are promising future energy sources for electricity generation and hydrogen production

Abstract

Oxygenic photosynthetic microorganisms have been the subject of intensive research for green energy production. Upon irradiation, water molecules are oxidized in photosystem II and the resulting electrons are transferred through the Z-scheme and stored in the form of NADPH. Energy conversion is made possible by redirecting electrons to the electrode. Here we show that double-mediated photo-microbial electrochemical cells (DM-PMECs) can remarkably enhance photocurrent generation from wild type Anabaena variabilis (A. variabilis). 2,6-Dimethyl-1,4-benzoquinone freely penetrates the cell membranes, taking electrons from thylakoid membranes and transferring them to ferricyanide, and finally to the electrode. Dispersed A. variabilis produced photocurrent density as high as 2.14 mA cm⁻² in one sun intensity at a bare ITO electrode without any surface modification. This value far exceeds those reported to date. A complete DM-PMEC with an air cathode produced P_max of 160 μW cm⁻² at 750 μA cm⁻². An incident-photon-to-current conversion efficiency of 4.8% over a whole spectral range and turnover frequency of 17 per PSII for water molecule oxidation are also superior to the reported ones. The same concept has also been applied to hydrogen production. When operated in a double-chamber reactor, pure hydrogen gas was produced from water with high energy efficiencies and high production rates upon applying a voltage bias. The production rate of 122 μmol H₂ (mg_Chla)⁻¹ h⁻¹ at an anode potential of 0.4 V vs. Ag/AgCl is much greater than the reported values which are less than 10 μmol H₂ (mg_Chla)⁻¹ h⁻¹. Energy efficiency was also very high and exceeded 100% at lower anode potentials. We foresee that this study opens a new way of utilizing cyanobacteria for appreciable electricity and hydrogen production employing double mediators. The same concept could be extended to other oxygenic cyanobacteria.