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BioPhotovoltaic Cell

Biological photovoltaic cells use photosynthetic cells, or parts of cells, to produce electricity. A BPV cell is split into anodic and cathodic half chambers, with the anodic chamber containing the photosynthetic material, which releases electrons. These reduce the anode, allowing a current to flow.

Figure 1.Structure of a BPV

There are many possible structures for a BPV, and the only conserved feature is two regions with electrodes separated by a proton-permeable barrier, one of which contains a photosynthetic structure. Our original BPV was set up in the manner above, with a proton-permeable structure separating the two half-chambers in the shape of a membrane to maximize rate of proton transfer between half-chambers, and carbon electrodes which are shaped in a sheet to maximize contact with bacteria and catalyst. The carbon felt that constitutes the electrodes was chosen because of its high surface area^[1]. Rubber gaskets are added between each layer to make the cell watertight, and four threaded rods hold the structure together. In the cathodic half chamber electrons are released into the solution after travelling around the circuit. The electrons react with protons and Oxygen in solution:

2H⁺ + 2e^- + ½O₂ → H₂O

This reaction occurs slowly under normal conditions, and an expensive platinum catalyst is required to speed the reaction up. Potassium Ferricyanide (also known as Potassium Hexacyanoferrate, K₃[Fe(CN)₆]) acts as an alternative electron acceptor with a much faster rate of uptake^[2].

Anodic Half Cell

At the beginning of the light-dependent stage of photosynthesis, light energy is used to split water into protons and electrons in photolysis:

H₂O → 2H⁺ + 2e^- + ½O₂

These electrons are then transported along a transport chain, and the energy released from their movement used to do work. However some electrons from photolysis escape the cell, and it is these electrons that reduce the anode of the BPV cell, producing a current.

Chloroplasts on their own can be used in BPV cells but these cells often do not possess the required longevity - chloroplasts break down over a period of hours, and require low operating temperatures. As such the most viable BPV cells employ whole photosynthetic organisms - algae or cyanobacteria - as these can repair themselves and as such present a more sustainable option^[3]. The inherent problem with this is that the cell walls/membranes prevent sufficient exoelectrogenesis (electrons leaving the cell). We decided to use Synechocystis PCC sp. 6803, a model cyanobacterium, for our BPV cell prototype - cyanobacteria generally offer higher efficiency as their membrane systems are simpler, and from a synthetic biology perspective cyanobacteria are easier to work with^[3].

Figure 2.Fuel Cell under simulated real-world conditions

Efficiency Improvements

Work has been done on increasing efficiency of these BPV cells by adding an exogenous redox mediator into the anodic chamber. This increases the rate of electron transport into the extracellular space, thus making the system more efficient. Various mediators have been studied and shown to increase efficiency, such as viologens^[4], quinones^[5], and Riboflavin. All of these mediators have been shown to have a positive effect on efficiency, but this approach is not a sustainable one - these mediators are added externally, not produced within the cell. Therefore we have attempted to make the cell produce these and other molecules itself.

Future Improvements

Designing a BioPhotovoltaic Cell that has a much larger surface area to Volume ratio would maximize the exposure of Synechocystis to sunlight. In addition, replacing the carbon felt electrodes with platinum would be beneficial, as Carbon slowly reacts to form Carbon Dioxide and the current electrodes have to be replaced. Also platinum acts as a catalyst for the reaction 2H⁺ + 2e^- + ½O₂ → H₂O, negating the necessity for Potassium Ferricyanide, and allowing extended stability of the cell, as Oxygen acts as a proton acceptor as well as an electron acceptor^[6].

References

• [1] Bielefeld 2013
• [2] Liling Wei, Hongliang Han, Jianquan Shen, Sep 2012
• [3]Photosynthetic biofilms in pure culture harness solar energy in a mediatorless bio-photovoltaic cell (BPV) system Alistair J. McCormick et al.
• [4]Properties of semiconductor electrodes coated with living films of cyanobacteria: Hideo Ochiai, Hitoshi Shibata et al.
• [5] Electrochemical investigation of cyanobacteria Synechococcus sp. PCC7942-catalyzed photoreduction of exogenous quinones and photoelectrochemical oxidation of water: Masaki Torimuraa, Atsushi Mikia et al.
• [6]A High Power-Density Mediator-Free Microfluidic Biophotovoltaic Device for Cyanobacterial Cells: Paolo Bombelli, Thomas Müller et al.

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