Here are the measures we took. We checked the voltage every 2 or 3 days, so we don’t have a high precision, but it’s enough to have an idea of the evolution.

• MFC 1

The first MFC was giving good results at the beginning but after some time, the voltage totally dropped and even reversed. It’s possible that the copper wire we put directly into the electrode killed the bacteria inside the MFC. Indeed, copper is known for its anti-microbial properties. Or maybe there is a bad connection somewhere, probably at the anode side.

Therefore, while connecting the wire to the electrode, it’s important to assure that the connection is robust enough and can hold for weeks in a soil. It’s also important to isolate carefully the wire to avoid any contact of the copper wire with the soil. An alternative way would be to use stainless steel wire for connecting the electrodes.

• MFC 2

The second MFC, was the one including a mint plant, and gave very good results. The plant was doing fine until we moved down the pot to the basement where there is little light.

• MFC 3

The seal we made at the bottom of the 3^rd MFC didn’t hold, and the MFC start leaking after a few days. The anode probably got oxygenated and the MFC gave very poor results, so we will have to do this test again.

• MFC 4

It’s hard to evaluate the results given by the fourth MFC, as they fluctuate a lot. The expected voltage seems to be around 20-30 mV, but the measurement uncertainty is so high that we cannot use these values. It appears that small MFC are not convenient for running tests, and that small or medium MFC are more adapted.

Common observations

The values we measured are very instable, and can vary a lot according to:

- The level of water in the recipient.

Water increases ionic conductivity but can also block the oxygenation of the cathode if too much water is poured. It is necessary to add water to the MFC to compensate evaporation, but the values are significantly different if measured before or after watering. Therefore it would be necessary to define an amount of water to pour regularly, and a protocol for measuring.

- Electrical connection.

We used a bread board to connect our MFC to a resistor and take measure. But touching the wire while measuring, we realized that this influenced a lot the measure. Moreover, the internal resistance of the MFC is low ( ≈ 100 ohm for the mint MFC), and the resistance of the wires might have a big influence. So, we will have to find a way to avoid bad connection. Maybe soldering the wire would be a solution.