Three main factors can influence the capacitive current in a CV: catalyst loading, electrochemical accessible surface area and the scan rate. The capacitive current is proportional to the scan rate. If you keep these factors the same you should have reproducible capacitive currents form cyclic voltammetry.

This may depend on what catalysts you are using? For metal catalyst, it usually generates pesudocapaeitor owing to charge across the interface between the electrolyte and the catalyst. For metal-free catalyst such as nitro-doped graphene, however, it produces EDLCs as long as the electrode contacts with electrolyte. In your case, I don't know where current differeent, it is in a same CV curve? or is different in your multiple measurements?

You can give me a screenshot.

As mentioned by Yongfeng bu above, this will depend on the catalysts you are using, not only the actual metal, but also whether you have it as a supported catalysts, for example nanoparticles on a carbon support or something like that; supports can contribute significantly to the double layer capacitive currents while not necessarily affecting your catalytic currents quite as much. The conditions you are working in to determine the surface area, your electrolyte and the applied potentials, for instance, can also be affecting your double layer. It would be easier to help if you could give us a few more details and what do you mean exactly by getting a different amount of double layer current, is this between different experiments or within the same experiment over time, or what?

In the electrochemical tests, we can not separate from the charging current. Current in both catalytic currents are charging current. In the area of the electrode under the same conditions, the charge current. But the catalytic current will greatly change, because the reactants or solution in the electrode surface changes after the catalytic reaction. So the catalytic current in the second cycle does not repeat. So in many cases the total current does not repeat.

Some electrochemical catalytic reaction will change the electrode area, for example, some products will be deposited or adsorbed on the electrode surface, will not only change the electrode area, also can change the activity of electrode. So the circulating current each will be different.

Three main factors can influence the capacitive current in a CV: catalyst loading, electrochemical accessible surface area and the scan rate. The capacitive current is proportional to the scan rate. If you keep these factors the same you should have reproducible capacitive currents form cyclic voltammetry.

The nature of working electrode can contribute to the double layer capacitance, for example the roughness of the working electrode (acting as electrocatalyst) and the material of the electrode itself, and surface area.

You also must be careful about the potential window used in the scan. If there are redox processes for the materials of the electrode, structural changes can take place and the double layer capacitance can change. You should select a potencial window where the electrode presents a polarized behavior.

But the question is what is the intensity of the difference of double layer current you obtained. It is very normal the physical conditioning of nano structures and their active sites are not uniform throughout the samples.

Any ununiformites and anisotropic nanostructures on the surface will affect on the electrochemical response of the surface so when you see there is no reproducibility in the response pattern, it means the surface is changing with time. Long-term stability of the deposited nanostructures on the electrode surface should be follow up during different scans. The cycles of potential scan should not change the response, if so you can find out the structure on the surface is absolutely unstable. As you may know, the structure of metal-solution interface will be determining factor in DL current because of any change on the structure of this layer with time, you may calculate different capacities for it and finally different currents for it.

The surface is the main factor try to have the smoothest surface of your electrode Good luck

Double layer charging current depends linearly with sweep rate and the peak current from faradeic process depends on the square-root of sweep rate. With this in mind, decrease sweep rate to lower the EDLC and see you faradeic reaction more clearly or keep your sweep rate constant to always have the same amount of DLC (true if no degradation of material occurs).

Well, if you need a equation: ic = A·Cdl·v

Where ic is capacitive current, A is electrode area, v is scan rate and Cdl is double layer capacitance. The last is characteristic of the electrode surface and it depends on the material, structure, roughness....

On the other hand, the faradaic processes are not always depending on square-root of v. For example, the faradaic current for a non diffusive process (adsortion) increases linear with v.

These are all good answers. Better ways to determine capacitance are perhaps pulse techniques such as chronocoulometry or AC methods such as faradaic impedance.

if you are using a small glassy carbon (say smaller than 0,7cm2), your results could be reproducible vastly as long as you make a rigid, constant cleaning (attention: mostly alumina or silica from the polishing slurry is absorbed so use anything feasible, aci, organic, vacuum+heat, uv, ... trial&error) once cleaning of GC is okay, then your result would be better (say you will find 0.5 instead of 0,7 cm2) and then you need to interpret this value for different backgrounds , for ex CNT, or mesh, Gas diff layer or etc...

good luck

If the electrolyte, pH system, temperature, scan rate etc are kept, the apparent surface area of the material (electrode) could have been different. Smooth surface normally gives a better reproducible non faradaic current.  

I agree. The surface capacitance is very dependent on what is on the surface AND the microscopic surface area.  One must be careful to not have too many things changing at the same time.