In my case, I helped my colleague to do cyclic voltammetry in aqueous solution. I thought that it was simple, 0~1 V in 0.1M H2SO4 and polymer(carbon)/Ag/ITO provided by him as the electrode, but Pt electrode started to split water as soon as applying voltage. The ref. deviation was less than 0.005 V.
Was that normal? (I am not good at aqueous-based system.)
It is possible to electrolyse water at voltages below 1V. Small electrodes, large distances between electrodes, and inadequate stirring will contribute to the problem.
Pt has low hydrogen and oxygen over volatages
If the CD is very large ( infinity) then water will be decomposed
Yes I would say that is very normal at Pt. On the negative side, Pt has an overpotential of about -0.1V vs SHE and about 1.4V vs SHE on the positive side (in 1M H2SO4). A number of factors (like stated above) can speed the kinetics though.
Yes, it is normal, confirming above statements. In fact if you polarise your Pt cathode the only possible reaction is H2 evolution...
Take a look at the Pourbaix diagram for water. Here very simple explanation: http://en.wikibooks.org/wiki/Introduction_to_Inorganic_Chemistry/Redox_Stability_and_Redox_Reactions
The gray region is where water is thermodynamically stable, below or above that region water electrolyses, depending on the conditions of pH and potential you apply. When using Pt, water electrolysis is not only thermodynamically but kinetically favoured outside the water stability region. Note that the diagram refers to NHE potentials at equilibrium and STP conditions. So you have to correct with respect to your given reference electrode plus consider some overpotentials.
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