Science Buddies: "Ask an Expert"

I am trying to complete the experiment described in the "Turn Mud into Energy with a Microbial Fuel Cell - and a Dash of Salt" project on your website. My mom purchased the Mudwatt kit (Item #SBU-KEE-0001-ITM) from Science Buddies store. We followed the procedure instructions in the project very carefully. We watched the online video on how to set up the fuel cell. Our first attempt we used soil from a nearby creek bed. We set up the fuel cell and took measurements for 8 days. After 4 days our voltmeter registered 170mV for the 1000 ohm resistor, but every day after that it registered 0 for all size resistors. We read all the troubleshooting comments and we didn't find an answer to the drop off in voltage.

We decided to start over. We purchased Scotts Premium Top soil from Home Depot. We cleaned the fuel cell according to instructions on the site including rinsing all parts with rubbing alcohol. We let it dry before setting it up a second time. We followed the directions very carefully. We set it up on the 5th and started taking measurements on Feb. 7th. This time we had 2.4 mV using the 1000 ohm resistor. The next few days the readings have been 1.2, 1.4, 1.1 0.5, 0.3 mV. It appears to be dropping off again. This is not supposed to happen.

Do you have any ideas on why the fuel cell is not working? We are open to any suggestions.

Thanks for your help.

In your first try with the creek mud, you measured a voltage of 170 mV which is a little higher than usual (50-80 mV) for the 1000 ohm resistance but still OK. Then you say the very next day it went to zero. That rapid a decline would make me suspect something was wrong in the circuit. One of the connectors may have come loose. Check all the leads and connectors and try again.

On your second try with the Scott's topsoil, the measured voltage with the 1000 ohm resistor was lower than usual. Again I suspect something may be wrong with the connections. Here's what it says in the troubleshooting section of the Scibuddies project guide:

"Even if the hacker board is set up correctly, some of the wires might be loose or may not be making good electrical contacts in the hacker board. You can try taking the wires out of the microbial fuel cell and then putting them back into the correct positions, and/or gently jiggling the wires around in the slots in the hacker board. Also make sure that none of the exposed parts of the wires are touching each other."

Did you try adding salt to the soil? Are you sure the soil is moist enough? The bacteria need a sufficient amount of water to grow and produce electricity and if they dry out, the fuel cell stops working. Also, these bacteria are anaerobes which means they do not like oxygen. If the soil is not wet enough, it may become aerated which could inhibit them: "If there are air bubbles trapped in the damp topsoil in the microbial fuel cell, this can prevent the bacteria from growing well because they do not want to be exposed to oxygen. When packing the mud in the microbial fuel cell, pat down the mud and electrodes, as described in steps 12–15 in the "Setting Up the Microbial Fuel Cell" section of the Procedure, so that you do not have any trapped air bubbles in the mud. Air bubbles may also be caused by small, hard particles (such as rocks, pebbles, vermiculite, twigs, etc.) being trapped in the mud, and so anything like this should be removed from the topsoil before adding it to the microbial fuel cell."

Finally, the bacteria need to have warmth to grow. If the temperature is below 65F [about 19C] the bacteria may not work very well and not produce enough power: "If the temperature of the room that the microbial fuel cell is in is very cold, the bacteria may not grow well, which would decrease the peak power output. The microbial fuel cell should be kept indoors, at normal room temperatures (about 19–25° C, or 66–77° F), in the same location the entire time after you set it up. Also, if the microbial fuel cell is moved to a different location (particularly if it is at a different temperature), this could disrupt the growth of the bacteria."

I really hope you can find out what is wrong and get the fuel cell working again. Let us know and we will try to help.

Good luck!

Sybee