Nicolas Alonso-Vante

Iron disulfide for solar energy conversion

Abstract Pyrite ( E g =0.95 eV) is being developed as a solar energy material due to its environmental compatibility and its very high light absorption coefficient. A compilationof material, electronic and interfacial chemical properties is presented, which is considered relevant for quantum energy conversion. In spite of intricate problems existing within material chemistry, high quantum efficiencies for photocurrent generation (>90%) and high photovoltages (≈500 mV) have been observed with single crystal electrodes and thin layers respectively. The most interesting aspect of this study is the use of pyrite as an ultrathin (10–20 nm) layer sandwiched between large gap p-type and n-type materials in a p-i-n like structure. Such a system, in which the pyrite layer only acts as photon absorber and mediates injection of excited electrons can be defined as sensitization solar cell. The peculiar electron transfer properties of pyrite interfaces, facilitating interfacial coordination chemical pathways, may turn out to be very helpful. Significant research challenges are discussed in the hope of attracting interest in the development of solar cells from this abundant material.

Novel low-temperature synthesis of semiconducting transition metal chalcogenide electrocatalyst for multielectron charge transfer: molecular oxygen reduction

Powder and thin layers (< 0.5 μm) of transition metal chalcogenide deposited on various substrates (glass, gc, ito) were synthesized at 140°C. The catalyst compound was obtained when reacting the corresponding transition metal carbonyl compounds and elemental selenium in xylene solvent. Efficient reduction of molecular oxygen to water, in 0.5 M H2SO4 electrolyte, was obtained. This catalyst also showed semiconducting behaviour. XPS-analysis and those results gained by Rutherford backscattering spectrometry (RBS) revealed that oxygen is incorporated in this compound. Transmission electron microscopy (TEM) performed on thin layers deposited on conducting glass or glassy carbon electrodes consisted of amorphous regions and nanocrystal domains with particle sizes ranging from 30 to 40 A. The probable stoichiometry of the semiconducting catalyst compound resulted to be: (Ru1−xMox)ySeOz where 0.02 < x < 0.04; 1 < y < 3 and z ≈ 2y after data analysis obtained with XPS, RBS and TEM.

Oxygen Reduction on Ru1.92Mo0.08SeO4, Ru/Carbon, and Pt/Carbon in Pure and Methanol‐Containing Electrolytes

The oxygen reduction reaction (ORR) activity of a Ru 1.92 Mo 0.08 SeO 4 catalyst, a Vulcan XC72-supported Ru catalyst and, for comparison, a Vulcan XC72-supported Pt catalyst was studied with a rotating ring-disk electrode. The very similar reaction characteristics of the two Ru catalysts in pure and CH 3 OH-containing H 2 SO 4 electrolyte, which differ markedly from those of the Pt catalyst, indicate that the reactive centers in both Ru catalysts must be identical. They are highly selective (>95%) toward reduction to H 2 O (four electron pathway), independent of the presence of methanol. In the latter case, they are 100% selective toward the ORR, i.e., completely methanol tolerant, while the ORR on Pt catalysts is accompanied by significant CH 3 OH oxidation. Based on mass specific current densities, however, the Ru catalysts are significantly less active than the standard Pt catalysts. Only at methanol concentrations above 10-30 mM does their methanol tolerance make them more active than Pt/Vulcan. Implications for their use as cathode catalysts in a direct methanol fuel cell are discussed.

Kinetics studies of oxygen reduction in acid medium on novel semiconducting transition metal chalcogenides

Abstract The aim of this work was to investigate a Ru-cluster material with a low molybdenum concentration. This latter is considered to act as an adsorption site for molecular oxygen. Transition metal chalcogenide materials based on Mo-Ru-Se for electrocatalytic reduction of molecular oxygen in acid medium were chemically prepared by reacting the transition metal carbonyl compounds and selenium powder in xylene solvent (140 °C). The stoichiometry of the semiconducting electrocatalyst corresponds to (Ru 1 − x Mo x ) y SeO z where 0.02 x y z ≈ 2 y . Kinetics parameters were obtained using steady state measurements with the rotating ring disk electrode ( rrde ) with electrocatalyst deposited on glassy carbon disks or with the powder incorporated in a disk of carbon paste matrix. Oxygen reduction proceeds mostly via a four-electron transfer reaction (96%) to water as determined by the rotating ring disk electrode technique. The heterogeneous rate constants, k 1 (water formation from oxygen), k 2 (hydrogen peroxide formation) and k 3 (water production from generated H 2 O 2 ) were evaluated over the potential range of 0.70 to 0V/ nhe . Temperature dependence (296 K to 338 K) and kinetic parameters were evaluated. It is concluded that minimising the molybdenum content leads to improved electrocatalysis.

The structure analysis of the active centers of Ru-containing electrocatalysts for the oxygen reduction. An in situ EXAFS study

Abstract A family of novel catalysts for oxygen electroreduction is presented, based on nanostructured Ru x X y chalcogenide compounds (X=S, Se, Te). EXAFS data suggest that the catalysts have a core of ruthenium atoms, which has triangular co-ordination and a direct metalmetal bond. Depending on the chalcogen, the Ru-cluster consists of two or three metal layers of different size and mutual co-ordination with chalcogen atoms co-ordinated to the periphery of the cluster. Variation of the chalcogen type affects the size of the Ru-cluster and the strength of its interaction with the chalcogen. This influences the interaction of Ru-clusters with oxygen and thus their activity in the reduction of molecular oxygen.

Platinum and non-platinum nanomaterials for the molecular oxygen reduction reaction.

Herein, some chemical approaches to the tailoring of nanodivided materials are highlighted. Transition-metal materials in nanodivided form are essentially devoted to study the electrocatalytic reduction of molecular oxygen (ORR) in acid medium. This Minireview focuses on the physical and chemical structures of platinum-based and non-platinum-based materials. Due to the research dynamism in this field, the discussion is limited to a comprehensive review of ORR cathode materials investigated in the authors laboratory as well as to some relevant work obtained in other laboratories concerning the electrochemical ORR pathway and substrate effects.

Transition metal cluster materials for multi-electron transfer catalysis

Abstract Electrocatalytic studies on oxygen reduction and hydrogen evolution have been performed with Chevrel type cluster compounds in which both the metal within the clusters and the metal in crystal channels between clusters have systematically been changed. The main conclusion is that electronic charge carriers channelled into bimetallic interfacial clusters or cluster-metal associations provide optimal conditions for multi-electron transfer catalysis approaching the performance of platinum in acid electrolyte. The limiting factor in the case of the oxygen reduction is the interfacial instability against electrochemical desintegration of the clusters prior to reaching the thermodynamic redox potential of the reaction to be catalyzed. Molecular oxygen reduction selectivity was found with the mixed cluster in presence of methanol.

Photovoltaic output limitation of n-FeS2 (pyrite) Schottky barriers : a temperature-dependent characterization

Metal/n‐pyrite (metal=Pt, Au, Nb) Schottky barrier type diodes were fabricated on electrochemically reduced either synthetic or natural (100) and (111) surfaces of single crystalline n‐FeS2. The temperature dependence of I‐V curves in darkness were analyzed in the range of 200–350 K on the basis of thermionic emission and recombination models. The calculated effective barrier height was ∼0.60 eV and the activation energy for recombination ∼0.50 eV for all investigated n‐FeS2/Pt samples. The doping density and the extrapolated potential (pseudo flatband situation) from the Mott–Schottky plot, obtained from capacities deduced from potentiostatic complex impedance measurements, were 2.0×1016 cm−3 and 0.25 eV vs Pt for the synthetic n‐pyrite crystal, respectively. From the donor density and barrier height a band bending of 0.5 eV was deduced. Photovoltaic parameters like open‐circuit photovoltage and short‐circuit photocurrent were studied down to temperatures of 200 K. The main phenomenon preventing the gene...

High Methanol Tolerance of Carbon-Supported Pt-Cr Alloy Nanoparticle Electrocatalysts for Oxygen Reduction

The electrocatalytic reduction of oxygen on carbon-supported Pt-Cr (1:1) alloy nanoparticle catalysts with two different metal loadings prepared via a carbonyl route was investigated based on the porous thin-film rotatingdisk-ring electrode technique and compared with that on E-TEK Pt/C catalyst in pure and methanol-containing electrolytes. The as-prepared Pt-Cr alloy nanoparticles, which have single-phase disordered structures, are well dispersed on the surface of carbon with a narrow size distribution even at 40 wt % metal loading. Such catalysts are stable in air up to 600°C. As compared to the Pt/C catalyst, the alloy catalysts showed slightly enhanced activity for the oxygen reduction reaction in pure acid electrolyte and significantly enhanced activity in the presence of methanol, and the ring-current measurements on the homemade catalysts showed a reduction in peroxide yield in pure acid solution. The enhanced activity could be ascribed to the effect of alloying on the initiation and extent of surface oxide formation. Oxygen reduction kinetic analysis indicated a potential dependence of the apparent number of electrons transferred per oxygen molecule during the reduction in methanol-containing solution. High methanol tolerance of Pt-Cr alloy catalysts during the oxygen reduction could be explained well by the lower reactivity of methanol oxidation, which may originate from the composition effect and the disordered structure of the alloy catalysts.

Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction

In this study, low-mass loadings (ca. 5 wt%) Pt/C catalysts were synthesized using the carbonyl chemical route allowing for the heterogeneous deposition of Pt nanoparticles on different carbon-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chemical nature of the carbon-based substrate on the Oxygen Reduction Reaction (ORR) kinetics at Pt nanoparticles surfaces was investigated. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles leads to formation of Pt–N chemical bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the carbon support. It is demonstrated that ca. 25% of the total amount of N atoms were bound to Pt ones. This chemical bond also revealed by the DFT analysis, induces changes in the oxygen adsorption energy at the platinum surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide is used as substrate. In conjunction with the experimental results, DFT calculations describe the interaction between supported platinum clusters and oxygen where the support was modelled accordingly with the carbon-based materials used as substrate. It is demonstrated that the presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O–O distances suggesting facilitated dissociation. Additionally, it is revealed that the strong interaction between Pt clusters and N-containing substrates leads to very slight changes of the cluster–substrate distance even when oxygen is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochemical reactions.