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.

Key words: ammonium sulfate; manganese electrodeposition; hydrogen evolution; stainless steel cathode