Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites ...have been well established for crystalline molybdenum disulfide (c-MoS2) but not for amorphous molybdenum sulfide (a-MoSx), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a-MoSx, prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete Mo3S13(2-) building blocks. Of the three terminal disulfide (S2(2-)) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a-MoSx catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum.
Demand for energy is projected to increase at least twofold by mid-century relative to the present global consumption because of predicted population and economic growth. This demand could be met, in ...principle, from fossil energy resources, particularly coal. However, the cumulative nature of carbon dioxide (CO2) emissions demands that stabilizing the atmospheric CO2 levels to just twice their pre-anthropogenic values by mid-century will be extremely challenging, requiring invention, development and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable and exploitable energy resources, nuclear fusion energy or solar energy are by far the largest. However, in both cases, technological breakthroughs are required with nuclear fusion being very difficult, if not impossible on the scale required. On the other hand, 1 h of sunlight falling on our planet is equivalent to all the energy consumed by humans in an entire year. If solar energy is to be a major primary energy source, then it must be stored and despatched on demand to the end user. An especially attractive approach is to store solar energy in the form of chemical bonds as occurs in natural photosynthesis. However, a technology is needed which has a year-round average conversion efficiency significantly higher than currently available by natural photosynthesis so as to reduce land-area requirements and to be independent of food production. Therefore, the scientific challenge is to construct an ‘artificial leaf’ able to efficiently capture and convert solar energy and then store it in the form of chemical bonds of a high-energy density fuel such as hydrogen while at the same time producing oxygen from water. Realistically, the efficiency target for such a technology must be 10 per cent or better. Here, we review the molecular details of the energy capturing reactions of natural photosynthesis, particularly the water-splitting reaction of photosystem II and the hydrogen-generating reaction of hydrogenases. We then follow on to describe how these two reactions are being mimicked in physico-chemical-based catalytic or electrocatalytic systems with the challenge of creating a large-scale robust and efficient artificial leaf technology.
We report here on a new series of CO2-reducing molecular catalysts based on Earth-abundant elements that are very selective for the production of formic acid in dimethylformamide (DMF)/water mixtures ...(Faradaic efficiency of 90 ± 10%) at moderate overpotentials (500–700 mV in DMF measured at the middle of the catalytic wave). The CpCo(PR 2NR′ 2)I+ compounds contain diphosphine ligands, PR 2NR′ 2, with two pendant amine residues that act as proton relays during CO2-reduction catalysis and tune their activity. Four different PR 2NR′ 2 ligands with cyclohexyl or phenyl substituents on phosphorus and benzyl or phenyl substituents on nitrogen were employed, and the compound with the most electron-donating phosphine ligand and the most basic amine functions performs best among the series, with turnover frequency >1000 s–1. State-of-the-art benchmarking of catalytic performances ranks this new class of cobalt-based complexes among the most promising CO2-to-formic acid reducing catalysts developed to date; addressing the stability issues would allow further improvement. Mechanistic studies and density functional theory simulations confirmed the role of amine groups for stabilizing key intermediates through hydrogen bonding with water molecules during hydride transfer from the Co center to the CO2 molecule.
Converting solar energy into fuel
via
photo-assisted water splitting to generate hydrogen or drive CO
2
reduction is an attractive scientific and technological goal to address the increasing global ...demand for energy and to reduce the impact of energy production on climate change. Engineering an efficient, low-cost photocatalyst is necessary to achieve this technological goal. A photocatalyst combines a photosensitiser and an electrocatalyst to capture light and accelerate the chemical reactions in the same device. In this perspective paper, we first describe the recent developments of some families of semiconductors that are attractive candidates for engineering photocatalysts. We then discuss the use of semiconductors as light harvesting agents, combined with a bio-catalyst, synthetic bio-mimetic molecular catalyst or synthetic all-inorganic catalyst, in photocatalytic hybrid systems for water splitting and CO
2
reduction. To highlight the advantages of semiconductors for engineering efficient and robust photocatalysts, we compare these systems to examples of homogeneous photocatalytic systems constructed from molecular photosensitisers (dyes). We conclude that all-inorganic catalysts coupled to appropriate semiconductors look more promising for the construction of robust photocatalytic hybrid systems for producing solar fuels.
Engineering of an artificial photocatalyst for water splitting and CO
2
reduction applications.
Photoelectrocatalytic cells for water splitting should combine one or two photosensitive units with a water oxidation catalyst at the anode and a hydrogen evolution catalyst at the cathode. In this ...perspective article, we first show how a chemist can take the naturally occurring multi-electron catalysts for these two electro- and photochemical reactions, photosystem II and hydrogenases, as a source of inspiration for the design of original, efficient and robust molecular catalysts. The focus of this article is given to the immobilisation of these natural or bio-inspired catalysts onto conducting surfaces and the design of electrode and photoelectrode materials for hydrogen evolution/uptake and water oxidation.
An alternative approach for the design of photoelectrocatalytic cells for water splitting may include enzymes or bio-inspired catalysts for water oxidation and hydrogen evolution, combined with molecular photosensitizers.
We report herein investigation on crystallization of amorphous molybdenum sulfide a-MoS x induced by electron and laser beam resulting in formation of crystalline molybdenum disulfide c-MoS2. This ...crystallization occurred in situ during transmission electron microscopic and Raman analyses of a-MoS x material. It was also found that a-MoS x to c-MoS2 phase transformation was not fully beneficial for H2-evolving catalytic performance. c-MoS2 showed better robustness but significantly lower catalytic performance. Furthermore, c-MoS2 was less tolerant to oxidation stress, as the one caused by photogenerated holes within the light harvester, compared with a-MoS x catalyst. Thus, a-MoS x is a better candidate for implementation within photocatalysts for overall solar water-splitting application.
A phosphate mediated (photo)electro-reduction process has been developed to activate cuprous oxide (Cu2O) to copper–cuprous oxide (Cu/Cu2O) for high efficient hydrogen evolution reaction (HER) in ...water at neutral pH. The activated copper-based electrode can efficiently catalyze HER with an impressive onset potential of −30 mV vs reversible hydrogen electrode (RHE), low Tafel slope of 60–80 mV·decade–1 and exchange current density j 0 of 3.0 × 10–5 A·cm–2. This represents the first highly active copper-based electrocatalyst that could be used as a low-cost alternative to Pt for water electrolysis.
The spinel MgMn2O4, a cathode material with theoretical capacity of 272 mA h g–1, holds promise for future application in high volumetric magnesium-ion batteries. Atomic-resolution imaging of the ...structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn2O4 for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO2 was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries.
Solar hydrogen generation via water splitting using a monolithic photoelectrochemical cell, also called artificial leaf, could be a powerful technology to accelerate the transition from fossil to ...sustainable energy sources. Identification of scalable methods for the fabrication of monolithic devices and gaining insights into their operating mode to identify solutions to improve performance and stability represent great challenges. Herein, we report on the one-step fabrication of a CoWO|ITO|3jn-a-Si|Steel|CoWS monolithic device via the simple photoinduced deposition of CoWO and CoWS as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalyst layers, respectively, onto an illuminated ITO|3jn-a-Si|Steel solar cell using a single-deposition bath containing the Co(WS4)22– complex. In a pH 7 phosphate buffer solution, the best device achieved a solar-to-hydrogen conversion yield of 1.9%. Evolution of the catalyst layers and that of the 3jn-a-Si light-harvesting core during the operation of the monolithic device are examined by conventional tools such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) together with a bipotentiostat measurement. We demonstrate that the device performance degrades due to the partial dissolution of the catalyst. Still, this degradation is healable by simply adding Co(WS4)22– to the operating solution. However, modifications on the protecting indium-doped tin oxide (ITO) layer are shown to initiate irreversible degradation of the 3jn-a-Si light-harvesting core, resulting in a 10-fold decrease of the performances of the monolithic device.
An electrode made of Au nanoparticles, ca. 13 nm in diameter, displays outstanding catalytic activity for the hydrogen evolution reaction in water. At an overpotential of 200 mV it operates with a ...catalytic rate TOF of 0.3 s-1, which is among the best performances ever achieved for a Pt-free H2-evolving catalyst.
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