Microbial Fuel Cell

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SciB
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Re: Microbial Fuel Cell

Post by SciB »

Hi,

Here's my answers:

1. Yes--you want to measure the V across the cell with a resistor connecting them. One thing to make sure of is that you have the resistor connected tightly between the anode and cathode. You are using cables with alligator clips, right? Just make sure that the clips are well attached to the resistor leads. What I do to insure good contact is to hold the jaws of the clip tightly together on the wire and rotate it a little to scrape the teeth on the wire.

2. I know the multimeter instructions can be confusing, but you guessed right--the 200m scale reads in millivolts (mV). One volt equals 1000 mV. So, if you got a reading of 10 mV, that would be 0.010 V. When you calculate the power output of your cell you use the formula P = V^2/R and the voltage has to be in 'volts' not mV. Your results will be in watts, W, but the power of these mud cells is in the microwatt range, uW, so you have to multiply the result by 10^6 in order to convert W to uW.

3. The multimeter probes are red for positive and black for negative. The red should touch the cathode and the black the anode.

4. If the reading jumps around that means something is not making good contact. It could be one of the connections of your wires to the anode or cathode plate but more likely it is intermittent contact between the meter probe and the resistor wire. You need to have a box or some books set up next to the cell so you can support the resistor while you press down hard enough on the wires to make good contact. If you have some extra gator clips you can attach one on the end of each probe so you can clip them onto the wires.

Let me know your power readings from a couple of the resistors. You can get an idea what the readings should be by looking at the graph on the procedure page for a different fuel cell project: https://www.sciencebuddies.org/science- ... #procedure

Good luck!

Sybee
geekalert
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Re: Microbial Fuel Cell

Post by geekalert »

Hello again!
I have just completed my 30 day experiment with this project and I think it went pretty well! [My main problem was that two (out of six) of my fuel cells never produced any electricity but I think a broken circuit is to blame seeing as one was from the control group and one from the experimental- but luckily I used six cells so I can just reject this data.] The measurements on my working fuel cells ranged everywhere from .1 to about 14 which is a pretty wide range so I am not really sure how to compile this data...
I am now working on my research paper and so I wanted to ask some more technical questions if you don't mind:
1) I know that the protons go through the salt-bridge (PEM) into the cathode and that the electrons flow up through the external circuit, but could you explain why/how they separate in this way or what draws them apart?
2) (probably a dumb question...) What happens to the electrons once they go through the external circuit? Similarly, with the protons, once they travel through the salt-bridge to the cathode do they just hang out there or what?
3) Since I took measurements two times a day for 30 days, I have a whole bunch of data- what do you think is the best way to compile this/ what type of graph should I use?
thanks a bunch!
Marg
SciB
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Re: Microbial Fuel Cell

Post by SciB »

Hi Marg,

Congratulations on completing the project. The MFC is a challenging gadget to make work properly as you have seen first hand!

To answer your questions, electrons are produced by the bacteria in the anode container and this causes a negative charge to build up. Since the anode is directly connected to the cathode by a wire, the electrons travel through it to the other chamber where there is a relative deficiency of electrons. There they combine with oxygen (O2) from air dissolved in the salt solution and protons (H+) to make water. This depletes the electrons at the cathode so that more can flow over from the anode and continue to produce power. The salt bridge balances the charges between the two chambers.

As to how to plot your data, here is what the procedure suggests (https://www.sciencebuddies.org/science- ... #procedure)

Analyzing Your Data
1. Plot your data on a scatter plot--power vs day. Make three plots--one for each fuel cell.
2. Did the fuel cells produce electricity? Did the fuel cells start producing electricity right away? Did the electricity production ever peak? How did the electricity production vary over one day?

Let us know if you have more questions.

Sybee
geekalert
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Joined: Sun Oct 25, 2015 11:55 am
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Re: Microbial Fuel Cell

Post by geekalert »

hello
Thanks for the answers! Just two more quick questions
-For my project I measured twice-per-day, but for the scatterplot graph you suggested power vs. day so should I average the two measurements and then use it as a single day's measurement? or do something like: day 1 AM *data*, day 1 PM *data*, day 2 AM *data*, day 2 PM *data*, day 3 AM *data*, day 3 PM *data*
-Since two of my MFCs malfunctioned and I am rejecting their data, should I still make graphs for them or is that unecessary?
Sorry, I am not sure if that made any sense ...
Marg
SciB
Expert
Posts: 2066
Joined: Fri Feb 01, 2013 7:00 am
Occupation: Retired molecular biologist, university researcher and teacher
Project Question: I wish to join Scibuddies to be able to help students achieve the best science project possible and to understand the science behind it.
Project Due Date: n/a
Project Status: Not applicable

Re: Microbial Fuel Cell

Post by SciB »

Hi Marg,

You are making perfect sense! From what you said, one MFC out of each set of three produced no V, so just exclude those values. You should mention this in your report, however, and offer an explanation for the malfunction.

So then, take the average of the AM and PM power values for the two good MFCs and plot them on a scatter graph. I don't know what would be the best way to show the points--all together on one graph or separated:

1. Contr - AM and PM
2. Exptal - AM and PM
3. Contr and Exptal - AM
4. Contr and Exptal - PM

It is better for the sake of comparison to have Cont and Exptal on the same graph, but if the points are not well-separated then it might be confusing.

Do you see much difference between the AM and PM values for each pair of MFCs? If the values are very similar then you could explain in your report that you decided to average the readings and then just plot the average for each set on one scatter graph.

Just try the various plots and decide for yourself what conveys the information best.

Let us know how it comes out.

Sybee
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