Photo-bioelectrochemical Cells Research Articles

Genetically Engineered Cyanobacteria Enhances Photocurrent Generation in Photo-bioelectrochemical Cell

Cyanobacteria exhibit light dependent exoelectrogenic activity in photo-bioelectrochemical cells (PBEC) generating substantial photocurrent. However compared to the other competing technologies such as photovoltaics, the photocurrent generated by cyanobacteria is lower and is not suitable yet to become a viable alternative technology. Initiatives to understand and enhance the exoelectrogenicity of cyanobacteria are crucial to resolve this caveat. In this perspective, a cyanobacterium named Synechococcus elongatus PCC7942 was genetically engineered to express a heterologous protein called outer membrane cytochrome S for enhancing its exoelectrogenicity. The genetically engineered cyanobacteria exhibited nearly 9 fold higher photocurrent generation than the corresponding wild-type cyanobacterium. Further, power density generated by the genetically engineered cyanobacteria in a rudimentary PBEC was found to be five times higher than that generated by wild-type. The multidisciplinary research work presented here highlights the scope for enhancing photocurrent generation by cyanobacteria thereby benefiting faster advancement of PBEC technology.

Enhanced photo-bioelectrochemical energy conversion by genetically engineered cyanobacteria.

Photosynthetic energy conversion using natural systems is increasingly being investigated in the recent years. Photosynthetic microorganisms, such as cyanobacteria, exhibit light-dependent electrogenic characteristics in photo-bioelectrochemical cells (PBEC) that generate substantial photocurrents, yet the current densities are lower than their photovoltaic counterparts. Recently, we demonstrated that a cyanobacterium named Nostoc sp. employed in PBEC could generate up to 35 mW m(-2) even in a non-engineered PBEC. With the insights obtained from our previous research, a novel and successful attempt has been made in the current study to genetically engineer the cyanobacteria to further enhance its extracellular electron transfer. The cyanobacterium Synechococcus elongatus PCC 7942 was genetically engineered to express a non-native redox protein called outer membrane cytochrome S (OmcS). OmcS is predominantly responsible for metal reducing abilities of exoelectrogens such as Geobacter sp. The engineered S. elongatus exhibited higher extracellular electron transfer ability resulting in approximately ninefold higher photocurrent generation on the anode of a PBEC than the corresponding wild-type cyanobacterium. This work highlights the scope for enhancing photocurrent generation in cyanobacteria, thereby benefiting faster advancement of the photosynthetic microbial fuel cell technology.

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

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.

Photobioelectrochemistry: Solar Energy Conversion and Biofuel Production with Photosynthetic Catalysts

Photobioelectrochemical cells are devices which have been developed over the past few decades and use photosynthetic catalysts for solar energy conversion or biofuel production. In this paper, a critical review of reported photobioelectrochemical systems is presented. The systems discussed include several types of photobioelectrocatalysts: whole cells, organelles, and enzymes. Special attention is paid to power or product generation as well as immobilization and electron transfer strategies used. The issues that need to be addressed in order for such systems to compete with current technologies are also discussed.

Photosynthetic reaction center-functionalized electrodes for photo-bioelectrochemical cells

During the last few years, intensive research efforts have been directed toward the application of several highly efficient light-harvesting photosynthetic proteins, including reaction centers (RCs), photosystem I (PSI), and photosystem II (PSII), as key components in the light-triggered generation of fuels or electrical power. This review highlights recent advances for the nano-engineering of photo-bioelectrochemical cells through the assembly of the photosynthetic proteins on electrode surfaces. Various strategies to immobilize the photosynthetic complexes on conductive surfaces and different methodologies to electrically wire them with the electrode supports are presented. The different photoelectrochemical systems exhibit a wide range of photocurrent intensities and power outputs that sharply depend on the nano-engineering strategy and the electroactive components. Such cells are promising candidates for a future production of biologically-driven solar power.

Cytochrome c-coupled photosystem I and photosystem II (PSI/PSII) photo-bioelectrochemical cells

Photo-bioelectrochemical cells are devices that use biomolecule-modified electrodes for the conversion of solar light to electrical power. We present the construction of a layered assembly of the native photosynthetic reaction centers photosystem I (PSI) and photosystem II (PSII), crosslinked by polyvinyl pyridine/methyl pyridinium and cytochrome c (Cyt. c) that act as an electron transfer mediating layer. Electrostatic interactions and glutaric dialdehyde crosslinking of the protein layers stabilize the biomolecules on the electrodes. The irradiation of the PSII/Cyt. c/PSI-modified electrodes facilitates an electron transfer cascade, where photoexcited PSII leads to O2 evolution and to the reduction of Cyt. c, with the concomitant ejection of electrons from PSI to the electrode, and the reduction of the P700+ sites by the reduced Cyt. c units.

Integration of Photoswitchable Proteins, Photosynthetic Reaction Centers and Semiconductor/Biomolecule Hybrids with Electrode Supports for Optobioelectronic Applications

Light-triggered biological processes provide the principles for the development of man-made optobioelectronic systems. This Review addresses three recently developed topics in the area of optobioelectronics, while addressing the potential applications of these systems. The topics discussed include: (i) the reversible photoswitching of the bioelectrocatalytic functions of redox proteins by the modification of proteins with photoisomerizable units or by the integration of proteins with photoisomerizable environments; (ii) the integration of natural photosynthetic reaction centers with electrodes and the construction of photobioelectrochemical cells and photobiofuel cells; and (iii) the synthesis of biomolecule/semiconductor quantum dots hybrid systems and their immobilization on electrodes to yield photobioelectrochemical and photobiofuel cell elements. The fundamental challenge in the tailoring of optobioelectronic systems is the development of means to electrically contact photoactive biomolecular assemblies with the electrode supports. Different methods to establish electrical communication between the photoactive biomolecular assemblies and electrodes are discussed. These include the nanoscale engineering of the biomolecular nanostructures on surfaces, the development of photoactive molecular wires and the coupling of photoinduced electron transfer reactions with the redox functions of proteins. The different possible applications of optobioelectronic systems are discussed, including their use as photosensors, the design of biosensors, and the construction of solar energy conversion and storage systems.

Integrated photosystem II-based photo-bioelectrochemical cells

Photosynthesis is a sustainable process that converts light energy into chemical energy. Substantial research efforts are directed towards the application of the photosynthetic reaction centres, photosystems I and II, as active components for the light-induced generation of electrical power or fuel products. Nonetheless, no integrated photo-bioelectrochemical device that produces electrical power, upon irradiation of an aqueous solution that includes two inter-connected electrodes is known. Here we report the assembly of photobiofuel cells that generate electricity upon irradiation of biomaterial-functionalized electrodes in aqueous solutions. The cells are composed of electrically contacted photosystem II-functionalized photoanodes and an electrically wired bilirubin oxidase/carbon nanotubes-modified cathode. Illumination of the photoanodes yields the oxidation of water to O(2) and the transfer of electrons through the external circuit to the cathode, where O(2) is re-reduced to water.