Hello Lithin,

thanks for posting this interesting technical question on RG. A good description of the method for cleaning nickel electrodes for electrochemical measurements is provided in the rather old article which is attached as pdf file. It is outlined here that (citation): "Prior to each measurement, the electrode was mechanically polished to a mirror-like finish. The oxide film on the electrode surface, arising from the polishing process, was removed with dilute HCl solution. The electrode was then rinsed thoroughly with distilled water."

I hope this helps. Good luck with your work and best wishes!