Hi
I have produced 100's of I-t curve of a comb that is made up of copper e.g. both anode and cathode is copper.
I have fixed the voltage to 1.25v, placed the copper combs in 18.3MR (0.055uS) water and measured the current over 300sec (current is measure of a 10ohm resistor in series with the comb - e.g. volt drop). I have fixed the temp to 25Degc.
The curves I get seem to be very classical of potentiostaic response (according to Bockis - Fundamentals of Electrodics p.1116 MEC 1998). In that, there is a steep current ascent (almost vertically) then decays until the diffusion control sets in (the apx distance of the peak on the time domain is 25sec).
That makes sense!
e.g. the rapid ascent is associated to the charging of the double layer and the decay is part due to the discharge or the "capacitor" through the flowing of current - very leaky capacitor.
However, if I measure the combs capacitance in air they have a capacitance of 10pF therefore in water (e=80 at 20DegC) it would equate to 800pF. Hence, if I only have 800pF at max then the time constant (RC) does not equate to 25secs more like less than 1sec.
Therefore my curves seem to contradict this rationale.
My view is that there is an in-rush of current due to the resistance of the water and then this becomes restricted by the oxidation of the cathode which is shown by a decay. As an additional thought the cell (comb) have now become Galvanic as the Anode has Cu (0.521v) and the cathode will have (Cu+ at 0.153v and Cu2+ at 0.3419v), which, I'm not too sure, have a counter active effect.
I have posted some data in a previous mail titled "Instant drop in current over 60second on two copper electrodes"
Does anyone have any ideas?
WayneL