Catalytic effects of NH4+ on hydrogen evolution and manganese electrodeposition on stainless steel

Abstract The effects of (NH4)2SO4 concentration (c((NH4)2SO4)) on hydrogen evolution and Mn electrodeposition on stainless steel (SS) in different potential ranges were investigated by linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), potentiostatic polarization, chronoamperometry, scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques. The results show that the NH4+ discharge reaction (NDR) intensifies the overall hydrogen evolution kinetics, and the NDR is catalyzed by increasing c((NH4)2SO4) and over-potential. The electro-crystallization of Mn on SS follows a three-dimensional progressive nucleation and diffusion-limited growth mechanism. Increasing the over-potential could accelerate the nucleation rate and also cause the decline of the nucleation density. The absorbed Mn2+ preferably discharges at low over-potential. Increasing c((NH4)2SO4) at medium over-potential could improve the current efficiency and produce more block-like grains. The nucleation process is suppressed by increasing c((NH4)2SO4) at high over-potential, at which the formation of columnar grains with higher hydrogen contents becomes prevailing.