MoS2/LDH Composites for Efficient Hydrogen Evolution in Alkaline Media

Hydrogen (H 2 ), which has the highest gravimetric energy density and zero carbon content, is widely regarded as a promising energy carrier to fulfill our needs cleanly and sustainably in the future.( 1-3 ) Central to the electrocatalysis are efficient and robust electrocatalysts composed of earth-abundant elements, which are urgently needed for realizing low-cost and high-performance energy conversion devices. Earth-abundant MoS 2 has emerged as a promising hydrogen evolution reaction (HER) catalyst with high activity and durability. However, such a high HER performance is only limited to acidic media, the kinetic becomes rather sluggish in alkaline media. Besides the hydrogen (H ad ) adsorption energy, there should be a second descriptor for gauging the HER catalytic activity in alkaline media—the binding of hydroxyl species, because here H has to be discharged from water instead of from hydronium ions in acidic media. Here, we demonstrate a dramatic enhancement of HER kinetics in base by judiciously hybridizing vertical MoS 2 sheets with another earth-abundant material, layered double hydroxide (LDH) (Figure 1a). The resultant MoS 2 /NiCo-LDH hybrid exhibits an extremely low HER overpotential of 78 mV at 10 mA/cm 2 and a low Tafel slope of 76.6 mV/dec in 1 M KOH solution (Figure 1b). At the current density of 20 mA/cm 2 or even higher, the MoS 2 /NiCo-LDH composite can operate without degradation for 48 hours (Figure 1c). Benefiting from the desirable structural characteristics, the MoS 2 /LDH interfaces synergistically favor the chemisorption of H (on MoS 2 ) and OH (on LDH), and thus can effectively accelerate the water dissociation step and thus the overall HER catalysis (Figure 1a). This work not only brought forth a cost-effective and robust electrocatalyst, but more generally, it opened up new vistas for developing high performance electrocatalysts in unfavorable media recalcitrant to conventional catalyst design. Figure captions Figure 1. (a) TEM images of the MoS 2 /NiCo-LDH composite, showing their interfaces. Inset: Schematic illustration of the HER in MoS 2 /LDH interface in alkaline environment. The synergistic chemisorption of H (on MoS 2 ) and OH (on layered double hydroxide) benefits the water dissociation step. (b) Polarization curves of the carbon fiber paper substrate, bare NiCo-LDH, MoS 2 and MoS 2 /NiCo-LDH composite catalysts in 1 M KOH solution at a scan rate of 5 mV/s. (c) Polarization curves recorded from MoS 2 /NiCo-LDH composite at a scan rate of 5 mV/s before (solid curve) and after (dotted curve) the chronopotentiometry test at -20 mA/cm 2 of 48 hours. Inset: chronopotentiometry responses ( η ~ t ) recorded from MoS 2 /NiCo-LDH composite at high current densities of -20 mA/cm 2 and -50 mA/cm 2 . References D. Strmcnik et al. , Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nat. Chem. 5 , 300-306 (2013). Y. Jiao, Y. Zheng, M. T. Jaroniec, S. Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 44 , 2060-2086 (2015). J. Hu et al. , Increasing stability and activity of core–shell catalysts by preferential segregation of oxide on edges and vertexes: oxygen reduction on Ti–Au@Pt/C. J. Am. Chem. Soc. 138 , 9294-9300 (2016). Figure 1