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Long-term proposal (or determining what problems need to be solved and researched) as to how we would develop a rechargeable/continuously recharging battery utilizing a rich CO2 environment, photosynthesis, and oxidation/reduction of electrodes. by Tejasvi Niranjan
The idea would require coupling an iron anode (donates electrons to cathode) to a cathode (possibly graphite or something similar). We would need to remove the electron that has flowed to the cathode and use that electron in the production of acetate (acetate, lactate, etc.). The energy to remove the electron from the cathode and using it to make acetate will make use of photosynthesis from algae (Chlamydomonas or Chlorella).
How the anode end works. We will use the facultative anaerobe (i.e.: will respire using oxygen if present, so atmospheric oxygen is not toxic) Rhodoferax ferrireducens. R. ferrireducens oxidizes acetates and derives energy by dropping that electron to an electron acceptor... Fe (III), Fe (III) oxide (the researchers saw rust color turn iron black), Fe (III)-nitrilotriacetic acid, Mn(IV) oxide, nitrate, fumarate, and O2. Assuming Fe (III) is our electron-acceptor, we will develop Fe (II) deposits in the anode, and the electrons from the Fe (II) will flow to the cathode, providing current. Fe (II) will be oxidized back to iron oxide. This system can also be used to not develop current, but rather remove the Fe (II) before it is oxidized back to Fe (III), allowing inhabitants on Mars access to functional iron for tool development, etc.
Problem 1: If R. ferrireducens is in an oxygenated environment, it will use the oxygen, preventing us from obtaining energy from any iron present. If oxygen from the green algae manages to make its way to the R. ferrireducens bacteria, we then need to do one of two things (or both). Somehow prevent the oxygen from getting to the bacteria, or engineer the bacteria to remove the oxygen from their cell bodies. It would be best to have a solution where the bacteria do not use the oxygen for something else, because that oxygen can be later used to support a habitable environment for humans.
Problem 2: If the green algae produce large amounts of oxygen gas, and the R. ferrireducens bacteria continuously remove it from their cell bodies, how can we extract the oxygen so that it can be used to support a habitable environment?
Update: Jaime Liu has suggested using anti-parallel flow in order to remove the oxygen. An example is presented below.
Problem 3: The anode end is pretty straightforward. However, the cathode end is murky. We need a way to extract the electron from the cathode, and pass that electron to either photosystem 1, 2, or both, of the green algae. How might we do that?
Problem 4: We need to engineer the green algae to produce acetate (or similar) using the energy it obtains from photosynthesis. Photosynthetic organisms need the energy they obtain in order to survive. As such, whatever acetate production system we develop needs to not use so much of the algae's energy that we end up killing the cells. At the same time, the algae do not need to proliferate a great deal, so it is a matter of maintaining a careful balance. If we develop a genetic system to produce acetate and it reduces the survival of the algae a certain amount, we need to test how many of these systems we can put into a single algae that will allow it to produce a lot of acetate without dying. The other method would be to have a continuous population of algae that produce a progeny that will use all the energy they obtain to make acetate, killing themselves in the process (but that may be too complicated.)
Problem 5: We need to get the acetate to the R. ferrireducens bacteria. How might we do that?
Update: A possible solution may be to have the algae and the bacteria exist together in solution.
The acetate will be in solution and will not have to move far in order to be used by the bacteria. The reduced iron (assuming it precipitates) will simply fall in the direction of gravity to the anode. This introduces two new problems.
Update: Problem 6: If we provide continuously an outside source of iron, the iron will build up, choking of the space for the bacteria and algae to grow. They may have a simple solution. Simply don't provide more iron, and the oxidized iron in the system will be used, reduced, pass the electron to the cathode, in the process becoming oxidized for further reduction. If we do provide iron, then I'm guessing the system should be designed such that the iron (perhaps the whole anode) can be easily removed.
Update: Problem 7: What does the acetate oxidize into? At what concentration will this be toxic to cells? How doe we remove/make use of this byproduct? Can be it made into something consumable... i.e.: the bio-battery can provide energy, iron, and now carbon-based food.
A few more notes about R. ferrireducens. It is a facultative anaerobic bacterium. It grows between 4-30º C, but at an optimal temperature of 25º C. It grows in a pH environment of 6-7. R. ferrireducens cannot reduce the following: AQDS, chromium (IV), cobalt-EDTA, elemental sulfur, poorly crystalline Fe (III) oxide, Fe (III) citrate, nitrite, 1% oxygen, selenate, selenite, sulfate, sulfite, thiosulfate, or uranium (VI). For electron donors it can utilize acetate, lactate, malate, propionate, pyruvate, benzoate, and succinate. It cannot use butanol, butyrate, capraote, ethanol, formate, glycerol, hydrogen, isobutyrate, methanol, propanol, or valerate.