Microbiological Fuel Cell: Negative Voltage

Hello!

I've been working on a few soil based MFCs and I have been monitoring my voltage and energy output for a week now. I've made the electrodes by covering stainless steel metal meshes with activated carbon using epoxy. I've gotten some funky results as attached.
Untitled.jpg


As you can see... the voltage provided a negative value (and an anomalous positive value for exp 2 on day 1), then after waiting a few more days the results show positive for exp 2 and exp 3.

I have a theory that the voltage I have been generating was not due to microbial action, but rather due to galvanic corrosion of the stainless steel, with the following reactions:

anode (MFC cathode): M -> M+ + e-
cathode (our anode): 2H+ + 2e- -> H2 OR O2(g) + 2H2O(l) + 4e- -> 4OH-(aq).

I am unable to prove this as I cant clearly see corrosion on my top electrode... (perhaps due to it being covered in carbon powder) I do however see water droplet formation which is desired as that indicates my microbes are indeed producing electrons which are being transferred to create H2O:

MFC electrode reactions:

anode: C6H12O6 + 6 H2O → 6 CO2 + 24 H+ + 24e-
cathode: 24 H+ + 24 e- + 6 O2 → 12 H2O

Any thoughts? Does my theory make sense or is there another explanation for the change in current direction? Thanks!
 

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Baluncore

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Welcome to PF. We need more information to understand the experimental setup.
I've made the electrodes by covering stainless steel metal meshes with activated carbon using epoxy.
1. Epoxy is an insulator. What is the conductivity between the mesh and the external soil? Is there electrical connection between the carbon and the mesh or does epoxy cause a problem?
2. Is the soil damp? If so, water may form droplets due to condensation of vapor. If dry there will be poor conductivity.
3. If you measure voltage alone you will have slow reactant consumption. If you draw a current, chemical changes will happen faster. What is the difference between your anode and cathode electrodes?
4. What is the grade and composition of the stainless steel? What is size range of the carbon particles? How was it activated? You may have a circular current on one electrode that runs until the surface materials are consumed or plated with ions from the electrolyte.
5. Do you have a reference or a link to a similar experimental setup?
This is an interesting experiment. Well done so far. Keep up the research.
 

anorlunda

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If you attach a zinc anode to the steel, and the results change, that would be strong evidence for galvanic action.
 
Hey guys! Thanks so much for the feedback... I have some updates:

Welcome to PF. We need more information to understand the experimental setup.

1. Epoxy is an insulator. What is the conductivity between the mesh and the external soil? Is there electrical connection between the carbon and the mesh or does epoxy cause a problem?
2. Is the soil damp? If so, water may form droplets due to condensation of vapor. If dry there will be poor conductivity.
3. If you measure voltage alone you will have slow reactant consumption. If you draw a current, chemical changes will happen faster. What is the difference between your anode and cathode electrodes?
4. What is the grade and composition of the stainless steel? What is size range of the carbon particles? How was it activated? You may have a circular current on one electrode that runs until the surface materials are consumed or plated with ions from the electrolyte.
5. Do you have a reference or a link to a similar experimental setup?
This is an interesting experiment. Well done so far. Keep up the research.
To answer questions 1 and 3, I have tested the resistance of the epoxy and carbon powder that are in use. The epoxy showed high resistance and the carbon had very low resistance. Tested it by putting a voltage (5V) across the materials.

For question 2: Good point. I will start measuring the moisture, temperature and pH of the experiments to have better reliability. (wasn't able to due to lack of equipment previously).

For question 4: The grade is SS 304, Carbon particle size is 0.075mm<20% and activation done using steam at high temperature. - any comments on this?

For question 5: Yes I do! I have mostly based my experiments on this video as I preferred the simple version. There are many other videos that include a salt bridge.

If you attach a zinc anode to the steel, and the results change, that would be strong evidence for galvanic action.
Thanks! Good idea! I am thinking of using magnesium instead as it is easier and cheaper to obtain. Think this is doable? I am unsure if this is the best idea as magnesium is quite reactive.

I was thinking of using electrical glue to hold the carbon instead of my current method with epoxy... any thoughts on that? And I will also increase the size of my experiments from a 5cm diameter to 8cm. Will start measuring the soil properly as mentioned too.

Thanks again guys!
 

Baluncore

Science Advisor
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2,108
To answer questions 1 and 3, I have tested the resistance of the epoxy and carbon powder that are in use. The epoxy showed high resistance and the carbon had very low resistance. Tested it by putting a voltage (5V) across the materials.
I was thinking of using electrical glue to hold the carbon instead of my current method with epoxy... any thoughts on that?
My concern is that the epoxy will coat the electrode and insulate it from the carbon particles. Those carbon particles may become insulated from each other by the epoxy. Can you measure the resistance from the surface particles to the ss electrode to verify conductivity is not blocked by the epoxy?

Conductive electrical glue will add a reactive metal such as silver to the cell.

You might consider using a carbon fibre sheet as the electrode. Products like this have become available recently.
https://www.ebay.com.au/itm/Carbon-Fiber-Welding-Blanket-High-Performance-plumbing-shield-heat-1-4-thick/152224486179?ssPageName=STRK:MEBIDX:IT&var=451413663653&_trksid=p2055119.m1438.l2649

SS 304, composition is 18% chromium, 8% nickel and about 1% manganese, with the balance, iron 73%. That gives a very complex metal electrode.

Some surprising bacteria are found in soils, see; https://en.wikipedia.org/wiki/Iron-oxidizing_bacteria
There are also bacteria that will provide manganese from solution in exchange for iron from an iron pipe.
See also; https://en.wikipedia.org/wiki/Dissimilatory_metal-reducing_microorganisms
 

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