normally we use a supporting electrolyte to take care of migration currents

run a blank CV

then add your compound  - it gives diffusion current

all currents you measure is diffusion current

see my ppts

Yes that's correct however when you are in transition between getting your electroactive species migrating or just using diffusion a steady state voltammogram will have a different behavior. More accurately there must be a publication where they treat how the voltammetry changes as a supporting electrolyte concentration, Keith Oldham explore this features but he just treated two cases when you have supporting electrolyte and when you don't. I'm interested on see how is the transition.

Thanks