Les premières traces de vie sur Terre : une approche spectroscopique et mimétique du problème

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Les premières traces de Vie sur Terre (une approche spectroscopique et mimétique du problème)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

How Stable Are 2H-MoS2 Edges Under Hydrogen Evolution Reaction Conditions?

MoS2, have emerged as a promising class of electrocatalysts for the production of H2 via the hydrogen evolution reaction (HER) in acidic conditions.The edges of MoS2 are known for their HER activity, but their precise atomistic nature and stability under HER conditions is not yet known. In contrast to other typical uses of MoS2 as a catalyst, under HER there is no external source of sulfur. Therefore, the sulfidation of the edges can only decrease under operating conditions and the thermodynamics of the process are somewhat ill-defined. Our results suggest that the 50%S S-edge may be active for HER via the Volmer-Tafel mechanism and is, despite a high H coverage, stable with respect to H2S release. At the 50%S Mo-edge, the adsorbed hydrogen opens the way for H2S release, leading to the 0%S Mo-edge, which was previously investigated and found to be HER active. HER being a water-based process, we also considered the effect of the presence of H2O and the in-situ formation of OH. For the 50%S Mo-edge, H2O is only very weakly adsorbed and OH formation is unfavorable. Nevertheless, OH assists the loss of sulfur coverage, leading to OH-based HER active sites. In contrast, OH is strongly adsorbed on the 50%S S-edge. By explicitly considering the electrochemical potential using grand-canonical density functional theory, we unveil that the Volmer-Heyrovsky mechanism on sulfur sites is still accessible in the presence of surface OH at the 50%S S-edge. However, the 50%S S-edge is found to be mildly unstable with respect to H2S in the presence of water/OH. Hence, we suggest that the 50%S S-edge evolves over time towards a 0%S S-edge, covered by surface OH that will block permanently the active sites. </div

Revisiting the Active Sites at the MoS 2 /H 2 O Interface via Grand-Canonical DFT: The Role of Water Dissociation

International audienceMoS 2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains debated. By taking the electrochemical potential explicitly into account using grand-canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we herein revisit the active sites of 2H-MoS 2. In addition to the well-known catalytically active edge sites, also specific point-defects on the otherwise inert basal plane provide highly active sites for HER. Given that HER takes place in water, we also assess the reactivity of these active sites with respect to H 2 O. The thermodynamics of proton reduction as a function of the electrochemical potential reveals that four edge sites and three basal plane defects feature thermodynamic over-potentials below 0.2 V. In contrast to current proposals, many of these active sites involve adsorbed OH. The results demonstrate that even though H 2 O and OH block "active" sites, HER can also occur on these "blocked" sites, 1 reducing protons on surface OH/H 2 O entities. As a consequence, our results revise the active sites, highlighting the so far overlooked need to take the liquid component (H 2 O) of the functional interface into account when considering the stability and activity of the various active sites

How to dope the basal plane of 2H-MoS2 to boost the hydrogen evolution reaction?

International audienceMolybdenum disulfide (MoS2) is considered one of the most likely materials that could be turned into low-cost hydrogen evolution reaction (HER) catalysts to replace noble metals in acidic solutions. However, several challenges prevent MoS2 from being truly applicable, including limited number of active sites (typically only the edges are active) and poor conductivity. In this work, we perform an extensive density functional theory (DFT) screening of substitutional doping as a possibility to activate the otherwise inert basal surface. We assess 17 Earth abundant elements for molybdenum doping and 5 elements for sulfur substitution. Systematically determining the preference of the metallic dopants to be located on the edges rather than in the basal plane, we reveal that most dopants are much more likely to be incorporated at the edges, suggesting that advanced synthesis methods are required to obtain basal-plane doped catalysts. The latter may, however, feature many more active sites per MoS2 formula unit, motivating our study on the properties of such substitutionally doped surfaces. For the first time for such a screening study, we explore not only the formation of H*, but also of OH* and to explore the reactivity of the solvent. Two additional phenomena that could hinder the hydrogen production at these sites are investigated, namely H2S release and the (local) segregation/dispersion tendency of the dopants in the basal surface. Moreover, to assess the electrocatalytic activity, we take the electrochemical potential explicitly into account via grand canonical DFT. Compared with pristine MoS2 nanosheets, our results show that most doping elements significantly enhanced the electrocatalytic activity. Considering all assessed factors, we identify the most promising systems: Dimers of Ti, Zr and Hf and the substitution of S by P are predicted to lead to stable active sites on the basal plane with overpotentials of about 0.2 V

Potential and Support-Dependent Hydrogen Evolution Reaction Activation Energies on Sulfur Vacancies of MoS2 from GC-DFT

In this work, we present a detailed mechanistic study of HER at the sulfur vacancy Vs. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, considering the activation energy as a function of electrochemical potential. The results show that the Volmer and Heyrovsky steps depend on the electrochemical value and the activation energies decrease for more negative potential values, while this is not the case for the Tafel step, where the activation energy is essentially constant. From the activation energy values at -0.2 V, it can be concluded that to release H2 at Vs, we follow two Volmer steps and then a Heyrovsky step, since they have the lowest activation energies compared to the others. Heyrovsky is the rate-determining step. In addition, we investigate for the first time the effect of the support on the conductivity of MoS2 and the HER activity of sulfur vacancies. Our results show that copper, gold and graphite supports have no effect on the barrier energies of all steps of the HER mechanism

How to Dope the Basal Plane of 2H-MoS2 to Boost the Hydrogen Evolution Reaction?

Molybdenum disulfide (MoS2) is considered one of the most likely materials that could be turned into low-cost hydrogen evolution reaction (HER) catalysts to replace noble metals in acidic medium. However, several challenges prevent MoS2 from being truly applicable, including limited number of active sites (typically only the edges are active) and poor conductivity. In this work, we perform an extensive density func- tional theory (DFT) screening of substitutional doping as a possibility to activate the otherwise inert basal surface. We assess 17 Earth abundant elements for molybdenum doping and 5 elements (N, O, P, Se and Te) for sulfur substitution. Systematically de- termining the preference of the metallic dopants to be located on the edges rather than in the basal plane, we reveal that most dopants are much more likely to be incorpo- rated at the edges, suggesting that advanced synthesis methods are required to obtain basal-plane doped catalysts. The latter may, however, feature many more active sites per MoS2 formula unit, motivating our study on the properties of such substitutionally doped surfaces. For the first time for such a screening study, we explore not only the adsorption of H, but also of OH and H2O to explore the solvent effect since the reac- tion takes place in an aqueous medium. Two additional phenomena that could hinder the hydrogen production at these sites are investigated, namely H2S release and the (local) segregation/dispersion tendency of the dopants in the basal surface. Moreover, to assess the electrocatalytic activity, we take the electrochemical potential explicitly into account via grand canonical DFT in combinations. Compared with pristine MoS2 nanosheets, our results show that most doping elements significantly enhanced the elec- trocatalytic activity. Considering all assessed factors, we identify the most promising systems: Dimers of Ti, Zr and Hf and the substitution of S by P are predicted to lead to stable active sites on the basal plane with overpotentials of about 0.2 V

The impact of water and the electrochemical potential

International audienc

Potential and support-dependent hydrogen evolution reaction activation energies on sulfur vacancies of MoS2 from GC-DFT

International audienceWe present a detailed mechanistic study of HER at the sulfur vacancy VS of 2H–MoS2. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, determining the activation energy as a function of the electrochemical potential via grand-canonical density functional theory. The results show that the Volmer and Heyrovsky steps depend on the electrochemical potential and the activation energies decrease for more negative potentials, while this is not the case for the Tafel step, for which the activation energy is constant. From the activation energies at −0.2 V vs SHE, it can be concluded that during HER on VS a first hydrogen atom is adsorbed as a spectator via a Volmer step. Then, the catalytic cycle consists of a Volmer and a Heyrovsky step, with the latter being rate determining. In addition, we investigate for the first time the effect of a conductive support on the HER activity of these sulfur vacancies. Our results show that copper, gold and graphite supports have little effects on the activation energies of all steps. Hence, we conclude that cheap, acid-stable, high-surface area carbon supports are well suited for MoS2-based HER catalyst

Revisiting HER on MoS2: The impact of water and the electrochemical potential

International audienceMoS2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains controversial. By explicitly considering the electrochemical potential using grand canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we revisit the active sites of 2H-MoS2. We start with the edges and we assess the influence of the presence of water and the competition between the release of H2S and H2. In our calculations, we consider the 50% S-edge and the 50%S Mo-edge, as they have been shown to be the most stable edge structure. Our calculations reveal that the Mo-edge is very likely to be reconstructed in the presence of water. In particular, H2S formation is only weakly endothermic (0.4 eV). Hence, under HER conditions where there is no sulfur reservoir, sulfur is gradually replaced by oxygen. Fortunately, the 0%S Mo-edge covered by OH is active for HER, as shown in our previous study. For the 50% S-edge, the termination is more stable with respect to H2S release (2.5 eV) and OH is stably adsorbed on the Mo-Mo sites. Our grand canonical DFT computations suggest that this edge is active for HER. The release of H2S at this edge is less likely to take place compared to H2 generation but is still possible as it is endothermic by only 0.21 eV after adsorption of OH. Once this OH substitutions happen, it leads to a 0%S S-edge that is predicted to be inactive for HER. For the basal plane, the thermodynamics of proton reduction as a function of electrochemical potential shows that three basal plane defects exhibit thermodynamic overpotentials below 0.2 V3 and the single sulfur vacancy (Vs) is the most abundant defect. Therefore, we present a detailed mechanistic study of HER on this defect and evaluate the Vomer, Tafel and Heyrovsky transition states for thedifferent possible reaction steps by considering the activation energy as a function of the electrochemical potential