Microbial Fuel Cell

[Cortnerj]

We are doing your microbial fuel cell project but are a little uncertain about how to make the final connection between the wires from cathose to anode. Just twist them together? What about putting an LED between... could we expect to see it light when we've achieved some level of activity?
Thanks

[Craig_Bridge]

You can twist them together using a wire nut. You could use some jumper clip leads to hook them together. Anything that will make a good reliable connection.

An LED is another alternative that might work. Without knowing the electrical properties of the state of your specific MFC at any given time, it would be difficult to predict how an LED would behave and what current limiting resistance it should have.

LED's have a maximum forward current rating and a forward voltage drop. Some of them have an internal current limiting resistor. If your MFC operating state ever reaches an operating point where it is supplying more current than the maximum forward current rating of your LED at the LED's forward voltage drop, you will burn up the LED device (one without a current limiting resistor). If your MFC open circuit operating voltage is always less than the LED's forward voltage drop, the LED will never light. If your MFC never produces enough current to illuminate the LED at the LED's forward voltage drop, the LED will never light.

I would start by just shorting the wires together except when you want to make electrical measurements with your DVM. I would use a DC current scale to measure the current flow (and you could leave the meter attached in this mode if you only are testing one MFC at a time). You should also measure the open circuit voltage occassionally; however, you shouldn't stay in that measurement mode for more than a minute at a time. The MFC operation requires current to flow between the chambers or it can get into a non-functional state in terms of electro/chemical behaviors and being able to produce curent.

To characterize the electrical operating state of the MFC at any point in time, you need to take the open circuit voltage and the short circuit current measurements close together and then either short the leads or leave the DVM in place measuring DC current.

Hope this answers your questions and provides some additional information in terms of being able to characterize the electrical operating state of an MFC.

[Cortnerj]

OK, we twisted the wres together with a nut, the air is bubbling in to the salty side, the mud from the bottom of the river is sealed in to the other side, have a carbon electride in each... its been two days and there is still zero amps. when do we panic???

[kgudger]

In reading over the experiment, I too am confused about what you are supposed to measure. However, as Craig said, you probably do not want to leave your fuel cell with an open circuit (wires not connected together). The experiment says to measure the voltage on the cell - did you do that, and did you get a reading?
With regards to connecting the wires together, did you put the ammeter in series with the wires? That's the only way to get a current measurement. In other words, one electrode wire goes to one input on the meter, the other meter wire goes to the other electrode. Since you may run down the batteries if you leave your multimeter on continuously, you may want to just occasionally open the circuit, insert the meter and take a measurement, then re-short the wires. Please let us know the voltage results and what the circuit is that you are using to measure the current. Thanks.

Keith

[Cortnerj]

OK, here is what we've got: wire from the mud side hooked to the wire on the salt side, except when we take a reading. When we take a reading, we unhook them and put the black lead from the multimeter onto the wire from the mud side, and the red on the salt wire. Multimeter is on milliamps. Then we rehook the wires together after the reading. Does this sound right?? The multimeter works, we tested it on a battery, and the electrodes gave resistance when we tested them as described in the protocol, before we assembled the fuel cell. Electrodes are made from that (expensive!) carbon fabric stuff, connected w/ silver epoxy to the copper wire #8 exacly as in the protocol.

If we are set up correctly, how long should we expect to wait until we get some sort of amperage reading? I just seems like we ought to be getting something after 3 days if it is set up correctly.

Thanks for your time and effort to help with this...

[kgudger]

Did you measure the voltage of the open circuit fuel cell? With the wires disconnected, use the voltage setting on the multimeter.

Multimeters can be either autoranging or not. If you must set the range yourself, then you may not be using a small enough range. The current from a battery may be 100s of milliAmps, but your fuel cell may be producing microamperes. For instance, when I short a 1.5V battery through my multimeter, it produces more current than the 200 milliampere scale can read. Are you using the current reading (Ampere) setting for reading current, and the Voltage setting for reading voltage? The voltage of the fuel cell may be in the millivolt range.

Keith

[Cortnerj]

OK, we have 80 microamps for the mud, zip for the water control! Wow, that was a tense few weeks. How high can we expect this to go? What can we do now that we've got it working? What can we light up or otherwise power with 80 micramps??

[Craig_Bridge]

What can we light up or otherwise power with 80 micramps??

Not much and certainly not anything dramatic or demonstrable.

In helping others with this project, I'm starting to see a pattern of too high an internal resistance in the MFC to be able to provide much power to any external circuit.

Please put your DVM on the resistance scale, open the short between the annode and cathode wires and measure the resistance between the annode and cathode leads, then reverse the DVM leads and measure the resistance again and report those values. Be sure to short the leads back together or put the meter on a DC current scale when you are done with this measurement.

I'm curious what your internal MFC resistance is. Knowing that resistance will help me understand your MFC's operating state better.

[Cortnerj]

OK, I'll do that tonight and let you know. the 8 guage wire seems really really thick!
But the amperage is steadily rising.

[Cortnerj]

OK, we measured the resistance before we assembled the cell, it was 3.5 Ohms. Is that a lot?
(Our amperage is still increasing. Thinking about injecting some acetate to try to feed it.)

[Craig_Bridge]

No, 3.5 ohms is low enough. Some of the others I'm helping that are having difficulty were 2 to 3 orders of magnitude higher resistance. Great job at controlling the internal resistance!

[Cortnerj]

Yes, well, we tried really hard with that! NOT!!! We just followed the darned directions. Used the special carbon cloth, #8 wire and silver epoxy (that stuff is expensive!) I wonder if people are using the wrong cloth? It was hard as heck to find, the supplier listed didn't work but we tracked it down elsewhere.

Next question: We now have three of these fuel cells cranking, all putting out more microamps each day. The first one to start is up to ~150 microamps now. We want to feed one of them. Maybe after a few more days! We are thinking to feed them acetate, since that is sort of the nexus molecule for respiration of all kinds. We saw that Geobacter can breakdown acetate. Lovley used 20 mM. Do you think we could just squirt in some vinegar? It's 5%, or about 0.8 M, so dilute 1:40 and we've got 20mM acetate mud, right?

[donnahardy2]

Hi,

Congratulations on getting power from your MFC’s! Your results are excellent.

Your idea of adding vinegar to feed the anaerobic bacteria in the MFC is a good one, as this has been shown to be one of the best carbon sources for producing electrons in MFC’s.

Here is a reference comparing the power production of MFC’s with different concentrations of acetate. The authors found that there was good power production with a range of 400 to 800 mg/L acetate. Look at the graphs in the results sections to see what I am referring to. If you can’t open this reference, let me know and I’ll upload it; it’s really good:

http://pubs.acs.org/doi/pdf/10.1021/es048927c


From the reference you will see that you want to have a range between 300 to 800 mg acetic acid per liter of mud.

5% acetic acid is 5 grams per 100 ml or 50,000 mg/liter, and you want to dilute the vinegar from 50,000 mg/liter to 500 mg/liter (middle of optimum range), so use the following calculation:

500 mg x 1,000 ml/50,000mg = 10 ml vinegar per liter of mud.

So this is a 1:100 dilution. Your proposed 1:40 dilution would be a little higher than the maximum range found in the reference article, so might inhibit the bacteria.

I hope this helps. Let us know if you have any questions.

Donna Hardy

[Cortnerj]

Update: we added vinegar this AM, 5 mL to half a liter of mud, so a bit more than you suggested... but it was befire your post! Added to one of the cells that seems to have plateued. No big changes in the first 2 hours!

The other two are cranking, the first one is up to 280 microamps. The one we started later but put in a better air-tight container is coming up faster, is at 130 microamps.

We measured volts today. The one at 245 micramps has 250 volts, does that mean 61 milliwatts? volts x amps = watts, right? What is the best way to report the data? Both amps and volts or...? The voltage seems to increase steadily during the measurement but the amperage is more constant, it jumps around a little but it goes up and down around a number and then settles to be almost constant. Is that the behavior others see?

[Cortnerj]

PS we can't open the link unless we pay $30.oo can you post it as a pdf so we can read it?