MFC process usually starts with inoculation and acclimatization (known as the start-up phase) under which the cells are run under open-circuit mode. When the open-circuit voltage (OCV) is getting stable, we usually switch cells into the operation mode by closing the circuit. My question is in case the OCV keeps increasing during start-up when is the best time to close the circuit? should we wait until observing less voltage variation (small voltage gradient)?
Of course, you should wait. Different microorganisms involved in microbial fuel production or study present different start-up times. It is a function of their differences in electron generation and capture.
Thank you, Maurice Ekpenyong. I have started my experiment by running the lab-scale constructed wetland-microbial fuel cells (CW-MFCs) under open-circuit condition. The open-circuit voltage has started from 200 mV and it has reached about 800 mV in two months but it is still increasing after 2 months (having a much lower voltage gradient though). I am not sure if I should wait until I see zero gradients in voltage growth or I am good to switch the cell to the closed-circuit now. My other concern is why the voltage has started from 200 mV instead of 0. It should be noted that I have 10 repetitions under the same condition and I am observing the same behavior in all cells.
Dear Atiyeh
In my opinion, frequently shifting between OCV and CCV, could reduce the start-up time in MFC systems. it seems that using the CCV mode could help the exo-electrogenic bacteria in the face of competition among different types of microorganisms.
you can read my relevant researches which were conducted using mixed culture (available in domestic wastewater):
https://www.sciencedirect.com/science/article/abs/pii/S0360319919344398
https://www.sciencedirect.com/science/article/abs/pii/S0013468618314610
https://www.sciencedirect.com/science/article/abs/pii/S0360319915026270
https://scholar.google.com/citations?user=nxPtFxwAAAAJ&hl=fa
As many subject it depends on the final function of the system.
Regarding the acclimatisation, the initial OCV phase has little to do with electroactive microorganisms since there is no electron path between the electrodes, thus no current. During this initial phase, the OCV value reflects the potential difference between a reduced environment (usually where there is the anode) and a more oxidise environment (where there is the cathode), it could be seen as a first evaluation of how well the setup is designed. The enrichment of electroactive microorganisms start when a load is place between the electrodes: the anode is then able to accept electrons and pass them towards the cathode. You can consider than when you initial OCV is above what you would expect as a running voltage under CC conditions, you can then start to apply a low load and then progressively increase the load. Waiting for stabilisation of the OCV is good practice but it has to be put in relation to time, it's relative.
Caricatural example: if you have a fast increase in 2 days (from 50 mV to 600 mV) and then a continuous increase for 1 year (600 mV to 850 mV), you should put the system under load after two days.
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