Re: Instant drop in current over 60second on two copper electrodes

WAYNEL wrote: 
Hi

I have been collecting data of two copper electrodes (current over time
I-t plots) in water.  All of my results are explainable above 1.25v.
However, if you fix the electrodes at 1.25v and measure the current
over time the first 60 second, there is a decay, then it pans out for
at least 5 mins then the current starts to increase.  Data is shown
below.
Can anyone explain why the current drops initially then starts to rise?

At 2.5v the current instantly starts to rise.  Is the drop associated
to the double layer?


One possible explanation is that you start with a Cottrell-like system,
decaying with 1/SQRT(t), and when it gets to a low current, convection
causes an unsteady steady state, so to speak. If you don't control
convection, you get pretty meaningless results.

Double layer charging would come into it, yes, initially, and you can
roughly estimate the time constant for that if you know the electrode
capacitance and the solution resistance. Capacitance will be (very
roughly) about 20 uF/cm^2, and solution resistance you may have to
measure. I think that the time constant RC will come pout rather small
with respect to your observation time.

One easy and perhaps rough way to measure R is to apply an ac voltage
to the cell at a few kHz and measure the ac current. The electrode's
capacitance will then provide practically zero impedance and you are
left with solution resistance R.

--
Dieter Britz,   Kemisk Institut, Aarhus Universitet, Danmark.

.

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