  Hydrogen has been identified as a clean, zero carbon, sustainable, and
promising energy source for the future, and electrochemical water splitting for
hydrogen production is an emission-free, efficient energy conversion
technology. A major limitation of this approach is the unavailability of
efficient, abundant, inexpensive catalysts, which prompts the need for new
catalytic materials. Here, we report the synthesis and electrocatalytic
properties of a novel transition metal-based ternary chalcogenide family,
LaMS$_3$ (M = Mn, Fe, Co, Ni). Powder X-ray diffraction confirms the phase
purity of these materials, while composition analysis using energy dispersive
spectroscopy (EDS) confirms the presence of the stoichiometric ratio of
elements in these compounds. X-ray photoelectron spectroscopy (XPS) and X-ray
absorption spectroscopy (XAS) were used to study the chemical states on the
surface and in bulk, respectively. These materials exhibit bifunctional
catalytic activity towards the two half-reactions of the water-splitting
process, with LaNiS$_3$ being the most active material for both hydrogen
evolution reaction (HER) and oxygen evolution reaction (OER). The LaMS$_3$
compounds show long-term stability with negligible change in the overpotential
at a constant current density of 10 mA cm$^{-2}$ over 18 hours of measurements.
As compared to the corresponding ternary oxides, the LaMS$_3$ materials exhibit
higher activity and significantly lower Tafel slopes. The ability to catalyze
both half-reactions of water electrolysis makes these materials promising
candidates for bifunctional catalysts and presents a new avenue to search for
high-efficiency electrocatalysts for water splitting.
