Why do we see a limitting current in a linear sweep voltametry and not in a Cyclic voltammetry ?

I don't understand your question. Do you really mean 0.01 mV/s (not mv) or is it 0.01 V/s. Also what do you mean by "0.005 ms" (5 us?) scan rate in LSV. This is not a scan rate. Both LSV and CV are characterized by linear scan rates, not times. If you give the correct parameters, your question may be answered. Also give the experimental conditions (solution, electrode size, etc)

Azzeddine Senadjki if you see a limiting current in your voltammogram whether it is cyclic or linear at a low scan rate, it is because you are having a steady state where the thickness of the diffuse layer does not change on the potential-time scale since the diffuse layer is restored over time. Under hydrostatic conditions, this is achieved at very low scan rates and with micro- or ultra-microelectrodes. On the other hand, when you have peak current it is because the layer thickness changes during the sweep in time because more and more reagent is consumed at the electrochemical interface and diffusion does not make up for the consumption in time so the measured current intensity is not maintained. But as David O Wipf wrote you should provide more details about your experiments and clarify which scan rate you used because in the question the data is confusing.

Regards

the electrolyte is made from three component(NiSO4, NaWO4, NaGluconate) plus a buffer(H3BO3) in order to maintain a stable PH in the solution . the scan rate was 0.005 V/s for CV and 0. 01 V/s for LSV

while doing the test at 400 rpm there where no limitting current in the LSV expriment but going higher in rpm to 1200 rpm a limiting current is noticed at that speed of the rotating electrode, and the formation of the hydrogen gas

but with the CV there where no limitting current in the experience even with a high rotating speed

So my question was why do we see the limiting current at LSV with high rotating speed and not with slow speed and why do we have the formation of the hydrogen gas in the electrolyte? and why the scan rate for CV is smaller than LSV

OK, you have to be careful with your terminology. It seems that you are doing a rotating disk electrode (RDE) experiment. Are you trying to reduce Ni onto the electrode? What type of electrode are you using (Pt, glassy carbon, ...) In RDE experiments, there is usually only a linear potential sweep.

The term cyclic voltammetry is usually confined to the meaning that the solution is unstirred and that mass transfer is by diffusion only.

However, if you do a cyclic voltammetry sweep, the first part of the sweep should be identical to the LSV sweep when you use the same experimental conditions of sweep and rotation rate.

It is likely that you are observing an effect of passivation of the electrode surface during the Ni depostion giving you some peak shaped voltammograms. See these articles for a general idea of the passivation process. It will be different in your case.