Nanoengineering of MoS₂ Electrocatalysts for Efficient Hydrogen Evolution Reactions

Abstract

Advanced catalysts with element-abundance, economic-cost and stability are desired for electrochemical hydrogen evolution reaction (HER) to scale-up the promising clean energy of hydrogen.[1] The discoveries in the (de)hydrogenation of petroleum chemicals have inspired several efficient and economic catalysts of metal non-oxides towards HER, such as Mo-based sulfides and carbides.[2] MoS 2 possesses a layered structure exposing Mo- and S- edges, which are highly active for HER due to the moderate hydrogen bonding energy (ΔG H* ).[3] However, its performance is still limited by the poor conductivity and the uncontrolled properties of active-sites. In our recent work, efforts have been made to synergically enhance the conductivity and active-site abundance of MoS 2 , achieving the high current densities and low onset overpotentials for HER.[4-6] For example, we proposed a new route to confine MoS 2 growth within an in-situ formed polysaccharide matrix from glucose condensation during hydrothermal processes, and after the following carbonization, MoS 2 /C nanocomposites evenly integrating ultrathin MoS 2 nanosheets (2 ~ 4 nm) with conducting carbon were successfully harvested (Fig. 1a).[6] The MoS 2 /C exhibited an excellent HER activity characterized by higher current densities and lower onset overpotentials than the conventional MoS 2 . In particular, the MoS 2 /C with a suitable MoS 2 content of 14.8% showed a small onset overpotential of ~80 mV, a high current density of 88 mA cm -2 at η = 200 mV, which are ascribed to the abundant rim-sites on ultrathin MoS 2 nanosheets and the improved conductivity by carbon. On the other hand, a facile microwave-assisted hydrothermal method was further introduced to fabricated active-site enriched MoS 2 nanosheets employing reactant self-shelter, in which the excessive S source (e.g., thiourea) would cover the highly-active Mo-sites during MoS 2 formation, and then be removed by following treatment with H 2 SO 4 , resulting in abundant active sites on MoS 2 . The electrocatalytic test well-confirmed the significantly improved active-sites (~3 orders) and conductivity as compared with the traditionally prepared MoS 2 . Thanks to the facile synthesis and outstanding electrochemical behaviors, our effort is expected to pave the way for earth-abundant, economic and efficient electrocatalysts used in energy conversion and storage. References: [1] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577. [2] C. G. Morales-Guio, L. Stern and X. L. Hu, Chem. Soc. Rev., 2014, 43, 6555. [3] T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science, 2007 317, 100-102. [4] L. C. Yang, S. N. Wang, J. J. Mao, J. W. Deng, Q. S. Gao, Y. Tang, O. G. Schmidt , Adv. Mater., 2013, 25, 1180. [5] Q. S. Gao, N. Liu, S. N. Wang, Y. Tang, Nanoscale, 2014, 6, 14106. [6] N. Liu, L. C. Yang, S. N. Wang, Z. W. Zhong, S. N. He, X. Y. Yang, Q. S. Gao, Y. Tang, J. Power Source, 2015, 275, 588. Figure 1