Hydrogen evolution on plasma carburised nickel and effect of iron deposition from the electrolyte in alkaline water electrolysis

• Plasma carburising enhanced catalytic activity of nickel for hydrogen evolution.
• Impurities from KOH electrolyte deposited on cathodes during HER.
• Cathodes with deposited iron underwent activation for HER following anodic polarisation.
• Activating effect of iron can be due to reduction of Fe(OH)42− to reactive Fe.
• Anodic polarisation might enhance HER by producing oxide species reducible to Fe.

Presence of carbon in electrodeposited nanocrystalline Ni-Fe-C cathodes renders a high electroactivity for hydrogen evolution reaction (HER) in hot alkaline solutions. In the present work carbon was introduced into nickel cathodes by plasma treatment in CH4 + H2 gas mixture at 470 °C. Electrochemical measurements were carried out in the solution of 25 wt.% KOH (reagent p.a.) at 80 °C. In some measurements the solution was pre-electrolysed to remove heavy metals. Carburisation resulted in a significant enhancement of catalytic activity of nickel for HER during short cathodic polarisation. Later, differences between the materials almost disappeared, evidently due to deposition of iron and of other heavy metals from the solution. Cathodes with iron deposits underwent an activation following anodic polarisation. It was proposed that the activating effect of iron can be associated with the formation of highly reactive iron during cathodic reduction of oxide species (probably Fe(OH)42−). The activating effect of prior anodic polarisation can be due to the formation of large amounts of oxide species which can undergo the reduction to reactive iron.