Selenium‐Induced Interplay Between Processing, Phase Stability, and Hydrogen Evolution in Multicomponent Pentlandites

Hydrogen evolution reaction (HER) performance of selenium‐rich multicomponent pentlandites was investigated using the Co 9 −x −y Fe x Ni y S 4 Se 4 series prepared as powders, as‐cast ingots, and induction‐hot‐pressed pellets. Structural, electrochemical, spectroscopic, and density functional theory analyses were combined to establish composition‐structure–activity relationships. Selenium extends the single‐phase range after synthesis but promotes partial phase separation during processing, particularly in Fe‐rich compositions. Although the effect of cationic composition on HER performance is comparatively weak within the investigated Se‐rich series, making the catalytic response increasingly dependent on electrode architecture and phase stability, the best HER performance across all material forms is consistently near the equimolar composition but shifted toward the Fe─Co‐rich region, whereas Ni‐rich compositions remain less active. Fe‐rich pellets show the highest activity ( η 20 = 200 mV) together with pronounced surface reconstruction and preferential iron dissolution, while compositions closer to the equimolar region exhibit higher overpotentials ( η 20 = 251–256 mV) with greater stability, revealing a composition‐dependent balance between activity and structural stability. Calculations indicate that selenium shifts the d‐band center toward the Fermi level and strengthens hydrogen adsorption, contributing to sluggish HER kinetics. Overall, selenium suppresses the beneficial activation characteristic of sulfur‐rich systems and fundamentally changes the factors governing HER performance, making it increasingly dependent on phase stability and electrode architecture.