Recent developments of molybdenum and tungsten sulfides as hydrogen evolution catalysts

Recent work shows that nanoparticulate and amorphous molybdenum and tungsten sulfide materials are active catalysts for hydrogen evolution in aqueous solution. These materials hold promise for applications in clean hydrogen production technologies. In this perspective, the syntheses, structures and catalytic activities of nanoparticulate MoS2 and WS2, incomplete cubane-type [Mo3S4]4+ and amorphous MoSx films are summarized, compared, and discussed

Hydrogen evolution catalyzed by MoS3 and MoS2 particles

Amorphous MoS3 particles are prepared using a simple chemical method. Several deposition techniques are developed to fabricate electrodes loaded with MoS3 particles. These electrodes are highly active for hydrogen evolution. The catalytically active species appear to be reduced molybdenum sulfide that contains disulfide ligands. The MoS3 particles are annealed to form polycrystalline and single crystalline MoS3 and MoS2 particles. These particles, as well as commercial MoS2 micro-crystals, show inferior catalytic activity compared to the amorphous MoS3 particles

Amorphous Molybdenum Sulfide Films as Catalysts for Electrochemical Hydrogen Production in Water

Amorphous molybdenum sulfide films are efficient hydrogen evolution catalysts in water. The films are prepared via simple electro-polymerization procedures and are characterized by XPS, electron microscopy and electronic absorption spectroscopy. Whereas the precatalysts could be MoS3 or MoS2, the active form of the catalysts is identified as amorphous MoS2. Significant geometric current densities are achieved at low overpotentials (e.g., 15 mA cm−2 at η = 200 mV) using these catalysts. The catalysis is compatible with a wide range of pHs (e.g., 0 to 13). The current efficiency for hydrogen production is quantitative. A 40 mV Tafel slope is observed, suggesting a rate-determining ion+atom step. The turnover frequency per active site is calculated. The amorphous molybdenum sulfide films are among the most active non-precious hydrogen evolution catalysts

Fe, Co, and Ni Ions Promote the Catalytic Activity of Amorphous Molybdenum Sulfide Films for Hydrogen Evolution

Molybdenum sulfide materials had been shown as promising non-precious catalysts for hydrogen evolution. This paper describes the study of the promotional effect of certain transition metal ions on the activity of amorphous MoS3 films. Ternary metal sulfide films, M-MoS3 (M = Mn, Fe, Co, Ni, Cu, Zn), have been prepared by cyclic voltammetry of aqueous solutions containing MCl2 and (NH4)2[MoS4]. Whereas the Mn-, Cu-, and Zn-MoS3 films show similar or only slightly higher catalytic activity as the MoS3 film, the Fe-, Co-, and Ni-MoS3 films are significantly more active. The promotional effects of Fe, Co, and Ni ions exist under both acidic and neutral conditions, but the effects are more pronounced under neutral conditions. Up to a 12-fold increase in exchange current density and a 10-fold increase in the current density at an overpotential of 150 mV are observed at pH = 7. It is shown that Fe, Co, and Ni ions promote the growth of the MoS3 films, resulting a high surface area and a higher catalyst loading. These changes are the main contributor to the enhanced activity at pH = 0. However, at pH = 7, Fe, Co, and Ni ions appear to also increase the intrisinc activity of the MoS3 film

In search of the authentic nation: landscape and national identity in Canada and Switzerland

While the study of nationalism and national identity has flourished in the last decade, little attention has been devoted to the conditions under which natural environments acquire significance in definitions of nationhood. This article examines the identity-forming role of landscape depictions in two polyethnic nation-states: Canada and Switzerland. Two types of geographical national identity are identified. The first – what we call the ‘nationalisation of nature’– portrays zarticular landscapes as expressions of national authenticity. The second pattern – what we refer to as the ‘naturalisation of the nation’– rests upon a notion of geographical determinism that depicts specific landscapes as forces capable of determining national identity. The authors offer two reasons why the second pattern came to prevail in the cases under consideration: (1) the affinity between wild landscape and the Romantic ideal of pure, rugged nature, and (2) a divergence between the nationalist ideal of ethnic homogeneity and the polyethnic composition of the two societies under consideration

Electrochemical sulfidation of WS2 nanoarrays:strong dependence of hydrogen evolution activity on transition metal sulfide surface composition

The activity of transition metal sulfides for the hydrogen evolution reaction (HER) can be increased by sulfur-enrichment of active metal-sulfide sites. In this report, we investigate the electrochemical sulfidation of atmospherically aged WS2 nanoarrays with respect to enhancing HER activity. In contrast to MoS2, it is found that sulfidation diminishes HER activity. Electrochemical and XPS experiments suggest the involvement of insoluble tungsten oxides in the altered HER and electron transfer properties. This demonstrates the strong dependence of the transition metal dichalcogenide (TMD) composition with the successful sulfur incorporation and subsequent HER activity

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated

Amorphous transition metal sulphide films or solids as efficient electrocatalysts for hydrogen production from water or aqueous solutions

The present invention relates to metal electrocatalysts for hydrogen production from water or aqueous solutions and use thereof in electrodes and electrolysers

Evidence from in Situ X-ray Absorption Spectroscopy for the Involvement of Terminal Disulfide in the Reduction of Protons by an Amorphous Molybdenum Sulfide Electrocatalyst

The reduction of protons into dihydrogen is important because of its potential use in a wide range of energy applications. The preparation of efficient and cheap catalysts for this reaction is one of the issues that need to be tackled to allow the widespread use of hydrogen as an energy carrier. In this paper, we report the study of an amorphous molybdenum sulfide (MoSx) proton reducing electrocatalyst under functional conditions, using in situ X-ray absorption spectroscopy. We probed the local and electronic structures of both the molybdenum and sulfur elements for the as prepared material as well as the precatalytic and catalytic states. The as prepared material is very similar to MoS3 and remains unmodified under functional conditions (pH = 2 aqueous HNO3) in the precatalytic state (+0.3 V vs RHE). In its catalytic state (-0.3 V vs RHE), the film is reduced to an amorphous form of MoS2 and shows spectroscopic features that indicate the presence of terminal disulfide units. These units are formed concomitantly with the release of hydrogen, and we suggest that the rate-limiting step of the HER is the reduction and protonation of these disulfide units. These results show the implication of terminal disulfide chemical motifs into HER driven by transition-metal sulfides and provide insight into their reaction mechanism