Relationship of Fe2+ concentration in solution and current efficiency in electrodeposition of CoFe filmsa

Our published work on stress evolution in CoFe films [Electrochim. Acta 55 (2010) 9035], indicated that adsorption/desorption of hydrogen is a primary cause of tensile stress which increases with the increase of Fe-content in deposit and decrease of current efficiency, i.e. higher supply of protons to the electrode surface. Using the electroanalytical techniques in this study it has been demonstrated that a higher supply of protons to the electrode surface with increase of Fe2+ concentrations in solution brings about higher Fe-content in deposit and lower current efficiency. First, it was found that the reduction rate of H+ from the solution is faster on CoFe surface with higher Fe-content in CoFe deposit. Second, it was found that higher concentrations of Fe2+ in solution bring about higher surface concentration of H+ through the Fe2+ hydrolysis reaction, i.e. Fe2+ + HOH = FeOH+ + H+. The proton liberated through hydrolysis reaction is reduced as soon as it is formed, together with other electroactive species.