Hexagonal Co₉S₈: Experimental and Mechanistic Study of Enhanced Electrocatalytic Hydrogen Evolution of a New Crystallographic Phase

The crystallographic phase is one of the most important parameters in determining the physicochemical properties of an electrocatalyst. However, existing understanding of phase-performance relationship is still very limited, especially for unconventional phases. Herein, the experimental discovery of the hexagonal close-packed (hcp) phase of Co₉S₈ is presented. This is the first demonstration of the hexagonal phase of Co₉S₈, and through correlated experimental and computational data, the first to elucidate the origin of enhanced catalytic performance from this new phase. The synthesized Fe doped Co₉S₈-hcp (Fe@Co₉S₈-hcp) catalyst, compared to its face-centered cubic (fcc) phase, exhibits small overpotentials of 44.1 and 298 mV at 10 mA and 500 mA cm⁻² for the hydrogen evolution reaction (HER), respectively. Mass activity is enhanced by 64.9 folds compared to the conventional Co₉S₈-fcc at 300 mV, which is the best among all Co₉S₈-based catalysts ever reported. Density functional theory calculations reveal that the enhanced HER of Fe@Co₉S₈-hcp mainly occurs at the Co sites, which synergizes with the doped Fe playing the role of coordination to strengthen H₂O adsorption and dissociation. This study opens a new avenue for designing high-performance electrocatalysts with unconventional phases for energy and environmental applications.