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The Origin of High Activity of Amorphous MoS₂ in the Hydrogen Evolution Reaction.

ABSTRACT: Molybdenum disulfide (MoS₂ ) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS₂ , as well as an amorphous MoS₂ phase, have been investigated and a detailed comparison is made on Mo-S and Mo-Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS₂ phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS₂ is caused by Li⁺ intercalation and disappears under harsh HER conditions, amorphous MoS₂ maintains its intrinsic short Mo-Mo bond feature and, with that, its high HER activity. Quantum-chemical calculations indicate similar electronic structures of small MoS₂ clusters serving as models for amorphous MoS₂ and the 1T phase MoS₂ , showing similar Gibbs free energies for hydrogen adsorption (?G_H* ) and metallic character.

SUBMITTER: Wu L

PROVIDER: S-EPMC6852468 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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The Origin of High Activity of Amorphous MoS2 in the Hydrogen Evolution Reaction.

Wu Longfei L Longo Alessandro A Dzade Nelson Y NY Sharma Akhil A Hendrix Marco M R M MMRM Bol Ageeth A AA de Leeuw Nora H NH Hensen Emiel J M EJM Hofmann Jan P JP

ChemSusChem 20190808 19

Molybdenum disulfide (MoS2 ) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS2 , as well as an amorphous MoS2 phase, have been investigated and a detailed comparison is made on Mo-S and Mo-Mo bond analysis under operando HER conditions, which reveals a ...[more]

PMID: 31319020

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Project description:Two dimensional (2D) semiconductor materials, such as molybdenum disulfide (MoS2) have attracted considerable interest in a range of chemical and electrochemical applications, for example, as an abundant and low-cost alternative electrocatalyst to platinum for the hydrogen evolution reaction (HER). While it has been proposed that the edge plane of MoS2 possesses high catalytic activity for the HER relative to the "catalytically inert" basal plane, this conclusion has been drawn mainly from macroscale electrochemical (voltammetric) measurements, which reflect the "average" electrocatalytic behavior of complex electrode ensembles. In this work, we report the first spatially-resolved measurements of HER activity on natural crystals of molybdenite, achieved using voltammetric scanning electrochemical cell microscopy (SECCM), whereby pixel-resolved linear-sweep voltammogram (LSV) measurements have allowed the HER to be visualized at multiple different potentials to construct electrochemical flux movies with nanoscale resolution. Key features of the SECCM technique are that characteristic surface sites can be targeted and analyzed in detail and, further, that the electrocatalyst area is known with good precision (in contrast to many macroscale measurements on supported catalysts). Through correlation of the local voltammetric response with information from scanning electron microscopy (SEM) and atomic force microscopy (AFM) in a multi-microscopy approach, it is demonstrated unequivocally that while the basal plane of bulk MoS2 (2H crystal phase) possesses significant activity, the HER is greatly facilitated at the edge plane (e.g., surface defects such as steps, edges or crevices). Semi-quantitative treatment of the voltammetric data reveals that the HER at the basal plane of MoS2 has a Tafel slope and exchange current density (J0) of ∼120 mV per decade and 2.5 × 10-6 A cm-2 (comparable to polycrystalline Co, Ni, Cu and Au), respectively, while the edge plane has a comparable Tafel slope and a J0 that is estimated to be more than an order-of-magnitude larger (∼1 × 10-4 A cm-2). Finally, by tracking the temporal evolution of water contact angle (WCA) after cleavage, it is shown that cathodic polarization has a 'self-cleaning' effect on the surface of MoS2, consistent with the time-independent (i.e., time after cleavage) HER voltammetric response.

Dataset Information

The Origin of High Activity of Amorphous MoS₂ in the Hydrogen Evolution Reaction.

Publications

The Origin of High Activity of Amorphous MoS<sub>2</sub> in the Hydrogen Evolution Reaction.

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