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Published October 13, 2017 | Supplemental Material
Journal Article Open

Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS_2 and MoSe_2

Abstract

Single crystals of n-type MoS_2 and n-MoSe_2 showed higher electrocatalytic activity for the evolution of H_2(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm^(–2) of current density for MoS^2 samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH(aq) and 1.0 M H_2SO_4(aq), respectively. For MoSe_2 samples, the overpotentials at −10 mA cm^(–2) were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H_2SO_4(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS_2 showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS_2 and MoSe_2 are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role.

Additional Information

© 2017 American Chemical Society. Received: August 4, 2017; Accepted: August 28, 2017; Published: September 6, 2017. J.J. thanks the Camille and Henry Dreyfus Foundation for support through its postdoctoral fellowship program in environmental chemistry. This material is based upon work supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-FG02-03ER15483. J.M.V. acknowledges support through an NRC Ford Foundation Postdoctoral Fellowship. D.A.T. acknowledges support through the NSF Graduate Research Fellowships Program. Single-crystal MoS2 growth and sputtering of Mo was performed at the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. X-ray photoelectron spectroscopy was performed at the Molecular Materials Research Center of the Beckman Institute at Caltech.

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August 19, 2023
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