Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production

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• 作者：Sang Chul Lee ; Jesse D. Benck ; Charlie Tsai ; Joonsuk Park ; Ai Leen Koh ; Frank Abild-Pedersen ; Thomas F. Jaramillo ; Robert Sinclair
• 刊名：ACS Nano
• 出版年：2016
• 出版时间：January 26, 2016
• 年：2016
• 卷：10
• 期：1
• 页码：624-632
• 全文大小：714K
• ISSN：1936-086X

文摘

Amorphous MoS_x is a highly active, earth-abundant catalyst for the electrochemical hydrogen evolution reaction. Previous studies have revealed that this material initially has a composition of MoS₃, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS₂ in composition and chemical state. However, structural changes in the MoS_x catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoS_x catalysts activated under two hydrogen-rich conditions: ex situ in an electrochemical cell and in situ in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoS_x catalyst after both activation procedures as well as spatially localized changes in the chemical state detected via electron energy loss spectroscopy. Using density functional theory calculations, we investigate the mechanisms for this phase transformation and find that the presence of hydrogen is critical for enabling the restructuring process. Our results suggest that the surface of the amorphous MoS_x catalyst is dynamic: while the initial catalyst activation forms the primary active surface of amorphous MoS₂, continued transformation to the crystalline phase during electrochemical operation could contribute to catalyst deactivation. These results have important implications for the application of this highly active electrocatalyst for sustainable H₂ generation.