Kinetics of the Hydrogen Evolution Reaction on Mild Steel and Nickel Cathodes in Concentrated Sodium Hydroxide Solutions

Abstract

Data are reported on the kinetics of the hydrogen evolution reaction on mild steel and nickel cathodes in sodium hydroxide electrolytes from 4 to 50 weight percent (1–26m) for temperatures of 288–358 K. Measurements were made for both smooth and roughened electrode surfaces up to current densities of about 0.3 A/cm2. Tafel plots for the nickel cathodes are linear over the whole current density range studied. The transfer coefficient, β, has an average value of 0.54 and is unaffected by varying either the caustic concentration, the temperature, or the surface roughness of the electrode. With the mild steel cathodes, the Tafel plots show two linear regions with the inflection point occurring at . In the lower overpotential region, β increases from about 0.5 to at least 1.4 with increasing caustic concentration and increasing temperature. This behavior is interpreted as being caused by a change in reaction mechanism and results from one of the reaction steps becoming reduced in rate. The ability of the caustic concentration or the temperature to affect the reaction mechanism is amplified by roughening the electrode surface. For the linear regions in the Tafel plots at high overpotentials, the slopes are almost invariant to changes in the caustic concentration, the temperature, or the surface roughness. The transition between the two Tafel regions appears to be caused by a change in the electrode surface resulting from the reduction of a surface oxide, possibly . The dependence of the inflection point potential on the experimental variables is analyzed in terms of this potential being equivalent to the equilibrium potential of the inferred surface redox reaction.