Photocurrent generation by immobilized cyanobacteria via direct electron transport in photo-bioelectrochemical cells

Abstract

Cyanobacteria possess unique and exciting features among photosynthetic microorganisms for energy conversion applications. This study focuses on production of direct electricity using a cyanobacterium called Nostoc sp. (NOS) as a photo-biocatalyst immobilized on carbon nanotubes on the anode of photo-bioelectrochemical cells. By illuminating with light (intensity 76 mW cm⁻²) the NOS immobilized on a carbon nanotube (CNT) modified electrode generated a photocurrent density of 30 mA m⁻² at 0.2 V (vs. Ag/AgCl). The contribution of different photosynthetic pigments in NOS to the light capture was analyzed and chlorophyll-a was found to be the major contributor to light capture followed by phycocyanin. Further investigation using a set of inhibitors revealed that the electrons were redirected predominantly from PSII to the CNT through the plastoquinone pool and quinol oxidase. A rudimentary design photosynthetic electrochemical cell has been constructed using NOS/CNT on the anode and laccase/CNT on the cathode as catalysts. The cell generated a maximum current density of 250 mA m⁻² and a peak power density of 35 mW m⁻² without any mediator. By the addition of 1,4-benzoquinone as a redox mediator, the electricity generation capability was significantly enhanced with a current density of 2300 mA m⁻² and a power density of 100 mW m⁻². The power densities achieved in this work are the highest among ‘non-engineered’ cyanobacteria based electrochemical systems reported to date.