This phenomenon may be caused by flooding. Since your fuel cell has been running for a long time, I guess water hsa accumulated inside it, which will limit the transmission of hydrogen and may lead to fuel starvation.

Dear Mr. Jing Zhao ,

I am very thankful for your answer.

I forgot to describe MFC. My MFC has used substrate as soil and mediator-less.

I'm sorry I didn't see your problem clearly. I'm engaged in hydrogen and oxygen fuel cells. I don't know much about your problems. I'm sorry

Dear Mr.Jing Zhao ,

Please, don't mind. No problem.

Anyway, I want to thank you because your opinions will help me.

Best regard,

Use different material of anode and cathod..

Sometimes due to contamination that causes reduction at cathode instead of oxidation.

Check your microorganisms.

please explain your MFC configuration. then, i may explain why it happened.

Dear Mr. Sokhee Philemon Jung ,

My MFC has used substrate as soil and mediator-less which the electrodes were made of carbon cloth.

Trung-Hiep Nguyen OK. Then, where are an anode and a cathode located? Please explain the environments for each electrode. How about the wire? What kind of wire did you use?

Dear Mr. Sokhee Philemon Jung ,

Two electrodes are connected to copper wires via resistance 100 Ohm.

Please see the attached picture.

Thank you.

MFC is prone to polarity reversal when substrate material is scarce， especially when the electrodes are the same material.

Thank Prof.Dr. Fei Xu

Trung-Hiep Nguyen What kind of materials are used for anode and cathode? It is good to have oxygen reduction catalysts on the cathode. Anode should be made with something conductive and good for bacterial attachment. Anode should be in the anaerobic condition and cathode should be in a very oxic condition. If they are not in these conditions described above, voltage reversal may occur.

Dear Prof. Sokhee Philemon Jung ,

The anode and cathode were made of carbon cloth.

I will note what conditions you mentioned.

Thank for your recommend

Are this phenomenon related to Exoelectrogen and/or Exoelectrotroph bacteria?

Dear Trung-Hiep Nguyen you can check the individual potentials of each electrode by using a reference electrode and you can figure out what caused that problem either anode or cathode. and one more thing the carbon paper (cathode-my observation) potentials will be very high just right after connection and eventually drop and got settled at some potentials, this may be be due to the difference in carbon paper water absorption or humidifying.

Thank you so much Ramesh Kakarla

Dear Dr. Hiep,

I also study about sediment microbial fuel cell and I also have circumstances like you. I use graphite cloth as cathode electrode and anode electrode. The anode is at the bottom of the sediment and the cathode in the water is in contact with oxygen. Previously, my current was always positive (1.5mA) but there would be times of the day it reversed polarity into negative current in a period of 15-20 minutes. Recently, my current has been continuously negative for several days with a strong odor of H2S.

I aerated oxygen next to the cathode electrode but it didn't work well.

Did you find a solution? Can share with me?

Thanks!

Negative polarity might be due to short circuit. Please make sure your anode and cathode are well separated.