The good news is that you had observed a current flow upon application of the irradiation. The question is why you expected the current to be positive? A more accurate description is if the current was flowing into or out of the electrode (working electrode) when the light was applied. If the current flows into the electrode, electrons flow away from the electrode, and oxidation occurs at the electrode. If the opposite, reduction occurs. Whether the current is positive or negative depends on if the electrode is regarded as positive or negative electrode. Without such information, it is meaningless to claim the current to be positive or negative.

The good news is that you had observed a current flow upon application of the irradiation. The question is why you expected the current to be positive? A more accurate description is if the current was flowing into or out of the electrode (working electrode) when the light was applied. If the current flows into the electrode, electrons flow away from the electrode, and oxidation occurs at the electrode. If the opposite, reduction occurs. Whether the current is positive or negative depends on if the electrode is regarded as positive or negative electrode. Without such information, it is meaningless to claim the current to be positive or negative.

Thanks Prof. Chen! The electrons are flowing into the working electrode when the light was applied. So I assume the current should be negative? Thanks again!

The actual sign of the current may look different with different instruments, but Prof. Chen is exactly right.  You created a species that is reducible at open circuit potentil so therefore the curren is "cathodic."  Keep in mind that due to Ben Franklin the sign used for current is backwards from the sign used for the electron flow. This is because in that day and in Faraday's day we had no knowledge of the electron which had not yet been discovered.

What will be the possible reasons of getting current vs. time from negative to positive sign in a Chronoamperometry test while working with biocathode microbial fuel cell?

@Chandrima Roy: When you use the word Chronoamperometry, I guess you actually applied a constant positive (+) potential to your biocathode against a reference electrode, aiming to see the current response. I assume the initial cell voltage was cathode (+) || anode (-) in your experiment. If yes, you were initially discharging the cell which would cause a positive current flow from cathode(+) to anode(-). (Note that the flow of electrons is opposite so that the reaction on the cathode is the reduction of an oxidant, e.g. oxygen molecule.) Also, it is important to remember that the reactions leading to the discharge are all sponteneous reduction reactions on the biocathode.

When all reactants of the spontaneous reactions were consumed during the discharging process, the current is supposed to drop to zero if the discharge was achieved via an external load (resistor).

However, if the discharge was achieved through the applied constant positive (+) potential on the biocathode, after all reactants for the spontaneous reduction reactions were consumed, the applied positive potential may then cause electrolysis, i.e. non-sponteneous oxidation reactions from other chemicals in the electrolyte. These non-spontaneous oxidation reactions can in principle cause an opposite current flow.

The word Chronoamperometry could also mean that you applied a constant cell voltage. For discharging, it should be cathode (+) || anode (-). You can then apply the above explanation.

Ah, I just notice that you were askig about the current changing from negative to positive. Would you be able to confirm if the cell voltage was polarised initially as cathode (-) || anode (+)?

(See my further explanation below.)

Yes, the cell voltage was polarised initially as cathode (-)II anode (+).

@Chandrima Roy: Thanks for your feedback. Your initial cell polarization, i.e. cathode (-) II anode (+), means that you were charging the cell (or performing electrolysis). This is unusual because fuel cells are primary power sources which means that they should and can only be discharged via the initial and continuous polarisation of cathode (+) II anode (-). All electrochemical primary power sources are not designed for charinging and recharging, and hence it is the discharging behaviour that should be studied.

By the way, I shall mention that the proper expression for an electrochemical primary cell is (+) cathode | electrolyte | anode (-), i.e. the positive sign is placed before the word cathode. If the electrolyte is not of interest in the study, the expression is simplified as (+) cathode || anode (-). Similarly, for an electrolysis cell, the expression is (-) cathode | electrolyte | anode (+), or simply (-) cathode || anode (+).

Thank you