Transition metal sulfides in alkaline hydrogen evolution electrocatalysis: Re-exploring their structure and composition evolution and its correlation with activity

Herein, the activity and stability evolution of transition metal sulfides used as electrocatalysts for alkaline hydrogen evolution reaction (HER) are studied during a prolonged HER period. We have thoroughly characterized and analyzed the composition and structure of NiV₂S₄, NiS, Ni₃S₂, and VS₂ prior to HER and after the HER for 2–20 h at a constant current density of −100 mA cm⁻². It is found that all these metal sulfides in KOH electrolyte are gradually degraded to the corresponding amorphous metal hydroxy salts/oxysulfides (i.e., a-KNi(OH)₃/a-NiO_xS_y and a-KV(OH)₆/a-VO_xS_y) and finally to amorphous metal hydroxy salts/oxides (i.e., a-KNi(OH)₃ and a-KV(OH)₆/a-V₂O₃) from surface to bulk with elongating HER time. Concomitantly, the morphologies of the derived metal hydroxy salts/oxysulfides (oxides) are significantly different from the corresponding metal sulfide precursors, especially those containing metal ions (for example, V³⁺ in NiV₂S₄ and Ni⁺ in Ni₃S₂) in intermediate valence states due to the modification of chemical bonds to an extensive extent invoked by their capability of facilely accepting and donating electrons. This stability and structural evolution of these metal sulfides are substantiated by the calculated Pourbaix diagrams of Ni−S−H₂O and Ni−V−S−H₂O systems. After the HER at −100 mA cm⁻² for 20 h, compared to the corresponding pristine metal sulfides, the apparent HER activities of all the derived metal hydroxy salts/oxide decrease due to the diminution of their ECSAs. On the contrary, their specific activities increase due to the enriched structural defects caused by the amorphous structures and changes in valence state of the metal ions.