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.