Formation of Ni-O-H-S surface phases on cathodically charged Ni

Highlights: • Solid phases form on a pure Ni surface under cathodic charging in aqueous electrolyte. • The phases have filamentary shapes and cover approximately 30 % of the sample surface with a thickness around 150 nm. • The phases are rich in Ni, O, H, and S, thermodynamically stable in air, and cathodic with respect to the Ni surface. • These phases may be products of a reaction of a metastable nickel hydride at the surface with the aqueous electrolyte. We investigate the formation of surface phases on nickel (Ni) during cathodic charging in 1 M H₂SO₄ solution. Using complementary characterization techniques, we show that these phases are aggregated into filamentary structures with a thickness of ∼150 nm and ∼30 % sample surface coverage. The phases are rich in Ni, O, H, and S, but free of C. Their surface potential is cathodic with respect to the surrounding Ni by ∼70 mV. We propose that these phases may be products of a reaction between the aqueous electrolyte and a metastable nickel surface hydride, assumed to form during cathodic charging. These findings stand to advance understanding of the catalytic activity of Ni cathodes, H uptake and embrittlement in Ni, and the onset of localized corrosion during cathodic charging.