As an ideal clean energy carrier, hydrogen can be produced through polymer electrolyte membrane (PEM) electrolysis by generating H₂ from H₂O. However, noble platinum group metals (PGM) remain the most common electrocatalysts for hydrogen evolution reaction (HER). The expense and scarcity of these catalytic materials obstruct the wide-spread adoption of electrochemical fuel generation technologies ¹. Sulfur based transition metal dichalcogenides (TMDs) have emerged as a promising alternative to PGM HER electrocatalysts as they are abundant, inexpensive, and exhibit a low HER overpotential in acidic environment ^2,3.

Here we present a systemic assessment of the compositional dependent HER activity and stability for Co-based mixed chalcogen, CoS_xSe_2-x, pyrite TMDs. We observe a decrease in HER activity from the single chalcogen TMDs, CoS₂ and CoSe₂, to mixed chalcogen TMDs, as shown in Figure 1. This observed compositional trend in HER activity can be explained by the unique combination of compositional dependent hydrogen adsorption free energy (ΔG_Had) and bulk resistivity/conductivity of the pyrite TMD. With near thermoneutral ΔG_Had, the highest resistivity among the compositions was observed. The following increase in Se content leads to the decrease in HER activity due to a steady movement away from optimal ΔG_Had. As the composition approaches CoSe₂, however, it is observed that HER activity again increases at higher Se contents, with CoSe₂ exhibiting similar activity to CoS₂. This highlights the convolution of ΔG_Had and material conductivity in determining the HER activity. Furthermore, through stability tests under constant potential HER electrolysis, as shown in Figure 2, Se-rich Co-based pyrite TMDs are found to be more durable than S-rich samples. Therefore, with an HER activity matching that of CoS₂, but with a dramatic improvement in stability, CoSe₂ breaks away from the traditional inverse activity/stability relationship and represents a promising material for non-PGM HER electrocatalysis in acidic based PEM electrolyzers.

1. Wang, J. et al. Mater. 28, (2016) 215-230.
2. Chhowalla, H. et al. Nature Chemistry 5, (2013) 263-275.
3. Jaramillo, T. et al. Science 317, (2007) 100-102.