Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.

Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs

Graphene-Like Two-Dimensional Materials

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

The production of clean and renewable hydrogen through water splitting using photocatalysts has received much attention due to the increasing global energy crises. In this study, a high efficiency of the photocatalytic H2 production was achieved using graphene nanosheets decorated with CdS clusters as visible-light-driven photocatalysts. The materials were prepared by a solvothermal method in which graphene oxide (GO) served as the support and cadmium acetate (Cd(Ac)2) as the CdS precursor. These nanosized composites reach a high H2-production rate of 1.12 mmol h–1 (about 4.87 times higher than that of pure CdS nanoparticles) at graphene content of 1.0 wt % and Pt 0.5 wt % under visible-light irradiation and an apparent quantum efficiency (QE) of 22.5% at wavelength of 420 nm. This high photocatalytic H2-production activity is attributed predominantly to the presence of graphene, which serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carrier...

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Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene nanosheets.

Iron disulfide for solar energy conversion

Structure and stability of an iron-based catalyst for the oxygen reduction reaction, prepared by heat treatment of carbon-supported iron(III) tetramethoxyphenylporphyrin chloride (FeTMPP−Cl), were investigated. The oxygen reduction in acid electrolyte was examined with the rotating (ring) disk electrode. The measurements confirmed that H2O2 is generated as a byproduct of the oxygen reduction. The structural elucidation of the catalyst showed that the porphyrin decomposes during heat treatment. Nitrogen atoms of the heat-treated porphyrin become bonded at the edge of graphene layers as pyridine- and pyrrole-type nitrogen. Two Fe3+ components as well as metallic, carbidic and oxidic iron were detected by Mossbauer spectroscopy. An electrochemical longevity test and two degradation experiments with sulfuric acid and H2O2 showed that H2O2 causes the degradation of active sites. A 6-fold coordinated Fe3+ compound seems to be responsible for the catalytic activity. Only 8% of the primary iron content is present...

Catalysts for the Oxygen Reduction from Heat-Treated Iron(III) Tetramethoxyphenylporphyrin Chloride: Structure and Stability of Active Sites

Monocrystalline n-FeS2 (pyrite, Eg = 0.95 eV) photoelectrodes with high photocurrent quantum efficiency (> 90%) have been obtained by improvement of the solid-state and interfacial chemistry. During intensive illumination (4-5 W/cm2), photocurrent densities between 1 and 2 A/cm -~ have been observed for single crystals with high electron mobility (/x = 180 cm2-(V-s) -') in presence of the I-/I3- redox couple. Under illumination, a charge of 623.000 C/cm 2 was passed without evidence of photocorrosion. The influence of etching treatments, various redox systems, and organic electro- lytes on the photochemistry of FeS2 was investigated. The formation and the dynamics of a thin oxidation layer that forms at the surface of the electrode in the presence of an acid electrolyte were studied using light reflection techniques and ESCA. FeS2 has a valence energy band with strong d-characterlike Mo- and W-dichalcogenides, that is significant for its stabilization. An unresolved problem with this photoelectrode concerning applications in solar cells is the small photopotential which up to now does not exceed 200 mV (500 mV is theoretically possible). A strong pinning of the Fermi level by surface states is evident from photoelectrochemical measurements. In addition, it is expected that the distance between the conduction band and the Fermi level in our sample will be too large. A low effective carrier den- sity (n = 0.7 x 10 '5 cm -3) was measured, resulting in an extended space-charge layer, which has to be compared with the high absorption coefficient (~ > 6.0 x 10 ~ cm-' for h~ > 1.3 eV). Recently we have shown that FeS~ (pyrite) can be used as a photoactive material for photoelectrochemical and photovoltaic solar cells (1). Several reasons justify the de- velopment, of this material: the semiconductor material consists of cheap, abundant, nontoxic elements. The en- ergy gap of approximately 0.9 eV is sufficient for solar energy applications. The absorption constant was deter- mined to be ~ > 6.0 x 105 cm-' for hv > 1.3 eV, which makes the use of pyrite in thin layer cells attractive (2). Electron mobilities as high as 230 cm~-(V-s) -' have been measured (3). Very important for photoelectrochemical applications is the high contribution of d-states to the val- ence energy band. Therefore, pyrite promises to become similarly favorable as Mo- and W-dichalcogenides in terms of corrosion stability (4, 5). However, the disadvan- tage of a two-dimensional structure, which poses prob- lems in the preparation of photosensitive material (6), can be avoided. FeS2 has the same crystalline and an electronic struc- ture similar to RuS2, which is an efficient and stable photoelectrode for oxygen and chlorine evolution in aqueous electrolytes (7, 8). Pyrite does not evolve oxygen from water under illumination, due to the different coordination chemistry of Fe as compared to Ru. Iron cannot reach high oxidation states or form peroxo- com- plexes with oxygen, which consequently will be trans- fered to sulfur, which forms water-soluble sulfate ions (9). Several authors have studied solid-state properties of pyrite, using both natural and synthetic samples (10-15). A number of publications have also appeared on the electro- chemistry of pyrite (16-20). They all deal with the electro- chemical behavior of natural pyrite obtained in the dark. A larger review was published on the aqueous oxidation of pyrite by molecular oxygen (21). In only one publica- tion from our laboratory small photoeffects were reported Downloaded on 2013-11-04 to IP 134.30.100.205 address. Redistribution subject to ECS license or copyright; see in an electrochemical setup (9). We ecsdl.org/site/terms_usepresent here the first €

Photoelectrochemistry of Highly Quantum Efficient Single‐Crystalline n ‐ FeS2 (Pyrite)

The nature of the sulfur deficit in pyrite, as found before by chemical analysis and density measurements, has been investigated by powder x-ray diffraction (XRD). Structure-factor refinements showed that the sulfur population parameter deviates several percent from unity, although the cubic-unit-cell edge a only varies in the order of ${10}^{\mathrm{\ensuremath{-}}3}$ \AA{}. To do these high-precision a measurements, a determination of the Cu K${\mathrm{\ensuremath{\alpha}}}_{2}$ wavelength is made. Vegard's rule for sulfur-deficient pyrite is presented. The sulfur deficit is explained as a simple Schottky defect, whose vacancy-formation energy is about 0.3 eV. On the basis of ligand-field- and molecular-orbital-theory considerations, the band-structure model of pyrite is modified due to the presence of S vacancies. The most important electronic effect is the formation of defect states in the forbidden zone. This result will be discussed with respect to contradictory measurements concerning the electronic properties of pyrite.

Helmut Tributsch

Papers

Iron disulfide for solar energy conversion

Catalysts for the Oxygen Reduction from Heat-Treated Iron(III) Tetramethoxyphenylporphyrin Chloride: Structure and Stability of Active Sites

Photoelectrochemistry of Highly Quantum Efficient Single‐Crystalline n ‐ FeS2 (Pyrite)

Sulfur deficiency in iron pyrite (FeS 2-x ) and its consequences for band-structure models

Iron sulphide solar cells