ABSTRACT
We know that photosynthesis in plants is based on sunlight splitting water into protons and electrons. The fact that photosynthetic bacteria and unicellular green algae cam divert electrons out of their cells away from routine electron transfer routes in photosynthesis and respiration for power production under our control is a great news. This exoelectrogenesis for BIOPHOTOVOLATAICS (or photosynthetic microbial cell) is light dependent production of external electric current by photosynthetic micro-organisms like autotrophic Cyanobacteria (cyanococcus elongatus Pcc 7942) or unicellular eukaryotic algae. The phenomenon to exploit is extracellular electron transfer away from photosynthesis and respiration taking place simultaneously in the same cell. In theory, we can use such organisms to produce electricity from sunlight that routinely oxidizes water into electrons. How to export electrons to the anode has been a perpetual problem though. Now just in less than two decades there are 1000 published research works available for guidance, specially the works in the last three years. We need to maximize export of electrons to a solid state anode for maximum power per square meter and learn to manage the photocurrent profile. Protons and electrons are consumed at the cathode for reducing oxygen and producing water. Can we immobilize high productivity Cyanobacteria to the anode for fast growth rate (just 1 hour doubling time) and maximum electron harvesting and create an electrode and electron mediators that best interferes with microbial photosynthesis? Their easy genetic construct is a big plus. What we don’t know well is the efficiency of electron export across the periplasmic membrane of the thalkoids in the plant cells
The Basic Quantitative Stoichiometry is
Water + Sunlight produce hydrogen ion + electron + 02
Biophotovolteic cells are associated with the anode. The ideal technology should export electrons to the anode without a mediator or at best a non-toxic mediator sitting on the microbial cell surface immobilized to the anode. The intact cells in a suspension of cyanobacteria with a redox mediator have been found to have at least 35 days long life. The big myytery is “how does the Cyanobacteria donate electrons to its surroundings and how do electrons flow to electrodes”? Actually, the bioelectrochemical process itself is a mystery. These organisms can be genetically modified specially with respect to inactivating electron sinks of respiratory terminal oxidase. A complete exo-electrogenic power system based on Cyanobacteria is not yet available. (https://www.frontiersin.org/articles/10.3389/fpls.2022.955843/full).
A truly ideal technology should deal with simultaneous production of both power and hydrogen using a 1.6 micron diameter iron stressed Synechosystis species PCC6803 or Synechococcus elongatus PCC 7942. Biofilms of the microorganisms immobilized on the anode may expedite charge transfer specially with an electron mediator that sits on the microorganism’s cell surface. It is found that the ten microns larger diameter Chlamydomonas algea may be less productive.
INTRODUCTION
The major source of power on earth is the star of our galaxy, the sun. It is sun’s radiative energy that becomes chemical energy in our plant foods. Also, there are photosynthetic micro-organisms that know how to manage electrons. Oxygenic and photosynthesizing bacteria can extract electrons from water using the photosynthetic system II. A successful BPV system should let as much electrons escape out side the cell away from the photosynthetic system as possible for higher power density. We could harness such electrons free from their atomic prison and produce power. First Biophotovoltaics (BPV) was reported 50 years ago and in 1985 Tanaka worked with Cyanobacteria.
One electrode is catalyzed by photosynthetic bacteria that absorb CO2 from the atmosphere.
Isolated chloroplasts, pure photosystem II, and thalkyoid membranes have been used but with limited success because of short life of cells. Stressed Cyanobacteria have been found to be the best.
BPV is self-renewable, requires less energy, and is very economical to construct for a power density of 1.5 to 7 W per meter square area depending light intensity in Europe verses Riadh respectively. The autotrophs need only minerals to grow and produce electricity by harvesting electrons at the anode. The cathode uses electrons generated at the anode to reduce oxygen. The mechanism remains unknown though. Development of durable and economical electrodes, external circuit to the cathode, and chemical wiring schemes are the immediate needs.
Volteic cells or Microbial Fuel Cells, or microbial solar cells
A Bio-photovoltaic system: Photosynthetic microorganisms (green ovals) absorb light energy (1) and oxidize water (2), liberating molecular oxygen and electrons that would typically be used for carbon dioxide fixation, but in a BPV, they are passed by an electron carrier (EC) to the anode (3). The electrons then pass through an external circuit (4) to the cathode, driven by the difference in redox potential between the electrodes. Finally, they recombine with protons and oxygen (5). Often, the anode and cathode are separated by a proton-permeable membrane. In comparison, in a microbial fuel cell, the electrons are generated by heterotrophic metabolism of organic matter, rather than light-driven water oxidation. Microbial Fuel Cells suffer from mass transfer problems of materials that the organisms live on.
https://www.sciencedirect.com/science/article/pii/S2542435120304396#:~:text=Biophotovoltaics%20(BPV)%20is%20the%20light,to%20power%20small%20electronic%20devices; https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.201900997
Chrisopher J. Howe, Professor at cambride university and his associate Paolo Bombeli are major researchers in this area.
THE TECHNOLOGY
The Organism
The cells of cyanobacteria can be grown in suspension or as a layer on the electrode surface. An extended life beyond 30-35 days will be welcomed.
The Electrode Material
Out of steel, copper, platinum, and tin oxide, tin oxide seems to be the ideal electrode material. Graphine coated steel has been found to work well also. Carbon nanotubes, carbon cloth, and graphite electrodes are less desirable. A two electrode system works best in most applications.
Light Intensity, Wavelength, and Power Density
Biophotovoltaic systems (BPVs) use oxygenic photosynthetic organisms to harvest light energy and deliver electrical outputs. Similar to other light harvesting bioelectrochemical systems (BESs), BPVs have an advantage over photovoltaic systems in that the photo sensitive components are assembled and maintained by living organisms that are capable of self-repair, reproduction, and are able to store energy for power generation in the dark. This review compares the performances of other light harvesting BESs with BPVs and discusses our present understanding of exoelectrogenic activity in cyanobacteria. Current and power outputs for BPVs remain too low to produce energy on a commercial scale. However, here we estimate achievable outputs and conclude that performances of present BPV systems are still far below the theoretical maximum. Let us hope that BPV technology will eventually develop into another valuable tool for the global switch away from carbon-intensive primary energy production.
Microbial biophotovoltaic cells exploit the ability of cyanobacteria and microalgae to convert light energy into electrical current using water as the source of electrons. Such bioelectrochemical systems have a clear advantage over more conventional microbial fuel cells which require the input of organic carbon for microbial growth. However, innovative approaches are needed to address scale-up issues associated with the fabrication of the inorganic (electrodes) and biological (microbe) parts of the biophotovoltaic device. Here we demonstrate the feasibility of using a simple commercial inkjet printer to fabricate a thin-film paper-based biophotovoltaic cell consisting of a layer of cyanobacterial cells on top of a carbon nanotube conducting surface. We show that these printed cyanobacteria are capable of generating a sustained electrical current both in the dark (as a ‘solar bio-battery’) and in response to light (as a ‘bio-solar-panel’) with potential applications in low-power devices. https://europepmc.org/article/med/29109396
https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/celc.201900997
Electron flux pathways of anodic current generation and iron reduction. Red and blue arrows indicate photosynthetic and respiratory electron transfer chain.
In photosynthetic electron transfer chain, electrons are transported from photosystem (PS) II to I via the interchain components, including plastoquinone, the cytochrome b6/f complex, and plastocyanin (or cytochrome c).
In respiratory electron transfer chain, electrons are transported from NAD(P)H dehydrogenase (NDH) or succinate dehydrogenase to terminal oxidases (Tox) via the interchain components. Redox mediators, Type IV pili, and alternate respiratory terminal oxidase (ARTO) are suggested to function at the step of extracellular electron transfer (EET).
https://www.frontiersin.org/articles/10.3389/fmicb.2021.650832/full
HYDOGEN POWER PURE PLAY
Green hydrogen is a hot topic with USA, Europe, and Japan involved with respect to current and planned capacity.
https://www.defianceetfs.com/hydrogen-energy-investment-outlook-for-2023/
There are 680 project with an investment total of US $240 billion. Among the startups gaining momentum are Fuel Cell Energy (FCEL), Bloom Energy (BUENF) Plug Power *PLUG), Ballard Power Systems (BLDP), and Air Products (APD).
Most any nation should prepare for the hydrogen economy.
ABBREVIATIONS
BPV: biophotovoltaic system; CV: cyclic voltammetry; DEET: direct extracellular electron transfer
EET: extracellular electron transfer
FTO: fluorine-doped tin oxide; IEET: indirect extracellular electron transfer; IO: inverse opal
ITO: indium tin oxide; MFC: microbial fuel cell; NADPH: nicotinamide adenine dinucleotide phosphate
PETC: photosynthetic electron transfer chain; PQ: plastoquinone; PSII: photosystem II; SHE: standard hydrogen electrode
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