The precipitation reaction of H+ reduced to H2 occurs mainly on the cathode.
The concentration of H+ in the electrolyte has a great influence on the precipitation potential of H2. Acidity control is an important factor in the normal progress of the electrolysis process. When [H+] decreases, Co2+ may be electrochemically reduced to a metallic cobalt sheet. detail find in http://www.wococarbide.com/Library/libraryinfo/id/1469.html