Bioelectrochemical systems: Difference between revisions

MFCs harness electrical current from the microbial oxidation of organic matter using a solid electrode as an electron acceptor. The anode surface facilitates microbial attachment and oxidation of organics, therefore generating electrons which are then simultaneously transferred to the cathode compartment through an external circuit containing an external load. Electroneutrality is warranted by ions transport through an ion-permeable medium or a membrane while electricity is produced in the process.

MECs utilize the property of bacteria to convert chemical energy to electrical energy and allow electrolysis of water. External power applied onto the electrical circuit of BES drives electrons from anode to the cathode. This also supports the hydrogen production at the cathode which operates under anaerobic conditions. However the anoxic environment in MECs, along with high concentrations of hydrogen production, can also promote methane production once CO2 and methanogens are available. Methods to mitigate toxic build up includes the aeration of the cathode chamber between batches, lowering of the pH, operation at short retention times, giving a heat shock to the inoculum and adding chemicals that inhibit the growth of methanogens.  

MES (also known as bioelectrosynthesis) uses the reducing power generated from the anodic oxidation to produce value added products at the cathode. Cathodic biocatalysts (with attached cathodic biofilms) reduce the available terminal electron acceptor to produce value added products. Biocathodes are the key components of microbial electrosynthesis, where the electrode oxidizing microorganisms are involved in the formation of reduced value-added product such as acetate, ethanol, butyrate. MES includes the production of chemical compounds in an electrochemical cell by electricity-driven CO2 reduction as well as reduction/oxidation of other organic feedstocks using microbes as biocatalyst<ref name="ref1" />.

EFCs utilize specific enzymes on the electrode surface (or in electrolyte suspension) that facilitate the catalytic oxidation of fuel and drive specific reactions desired for various applications. Different enzymes are used on anode and cathodes, based on their specific redox function. As highly selective enzymes are used at each node, then this eliminates the need of any membrane between the anode and cathode compartments. Enzymatic catalysts usually oxidize the fuel partially and heat generation occurs as a result of side reactions which can impact enzymatic activity. The shelf life of these enzyme-based systems can be extended through encapsulation in micelle polymers that provide a buffering capacity for pH control and a hydrophobic niche to prevent enzymatic degradation<ref name="ref1" />.

MSCs make use of photoautotrophic microbes or higher plants to entrap solar energy which is further utilized by electroactive bacteria to perform electrode-driven reactions. These reactions include generation of electric current or fuels such as hydrogen, methane and ethanol. Primarily photosynthesis leads to the generation of organic compounds, which are subsequently fed into the anode compartment where they are oxidized by electroactive microbes to produce electrons. These electrons are then transferred to the cathodic side where reduction of oxygen leads to the formation of water<ref name="ref1" />.  

PMFCs incorporate the living plant's root system into the MFC anode. The organic products (rhizodeposits) released to the soil by the roots are used as substrate for electricity production by electroactive microbes. So overall PMFCs harness solar radiation by transforming it into green electricity in a clean and efficient manner. The choice of a plant is important as it directly controls the amount of rhizodeposits for bioelectricity generation. PMFCs from Pennisetum setaceum are the most sustainable in terms of power production and S. anglica PMFCs with integrated oxygen reducing biocathodes reached the highest long-term power output. Reed manna grass, rice plants and Spartina anglica have also been used in separate studies<ref name="ref1" />.

[[File:PMFC Diagram.png|400px|thumb|left|Model of a PMFC Producing Electricity and Driving a Light Source. All Rights Reserved.]]

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MDCs use the electric potential difference across the anode and the cathode through MFC technology to operate in situ desalination. They comprise an additional middle compartment for water desalinisation which is partitioned by an anion exchange membrane (AEM) towards the anode and a cation exchange membrane (CEM) towards the cathode. Bacteria on the anode oxidize biodegradable substrates and generate electrons and protons. The electrons are externally transferred to cathode whereas the anions in the desalination compartment migrate to the anode and the cations are transferred to the cathode to maintain charge neutrality and therefore desalination of the middle chamber solution occurs. Also, the migration of ions from the saline water in the middle chamber towards each node increases the conductivity of the anolyte and catholyte and so electrical power production is improved due to higher conductivity and mass transfer<ref name="ref1" />.