Search

We report on the electrochemical characteristics of pristine and air aged carbon modified titania electrodes in acidic electrolyte, which find application as catalyst supports in electrocatalysis. Films of different chemical composition (including TiO2, TiOx, TiO, TiC, C) are synthesized through a carbothermal treatment of compact anodic TiO2 on Ti substrates at different temperatures, and analyzed with X-ray photoelectron spectroscopy. Their activity towards heterogeneous outer sphere electron transfer and their stability against passivation and corrosion under anodic polarization are investigated by means of electrochemical impedance spectroscopy. It is found that a high fraction of TiC in the pristine carburized films in combination with a thin TiO2 surface layer yields high electron transfer kinetics even after aging for ~9 months, and a high corrosion and passivation resistance until ~0.74VSHE. This finding is essential to the material's application as catalyst support in general.

International audience; Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low-and intermediate-temperature proton-exchange membrane fuel cells. For applications in electro-catalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic hetero-geneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO 2 films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications.

Nanostructuring of electrode materials is a promising approach to enhance the performance of next-generation, high-energy density lithium (Li)-ion batteries. Various experimental and theoretical approaches allow for a detailed understanding of solid-state or surface-controlled reactions that occur in nanoscaled electrode materials. While most techniques which are suitable for nanomaterial investigations are restricted to analysis widths of the order of A to some nm, they do not allow for characterization over the length scales of interest for electrode design, which is typically in the order of mm. In this work, three different self-organized anodic titania nanotube arrays, comprising as-grown amorphous titania nanotubes, carburized anatase titania nanotubes, and silicon coated carburized anatase titania nanotubes, have been synthesized and studied as model composite anodes for use in Li-ion batteries. Their 2D areal Li densities have been successfully reconstructed with a sub-millimeter spatial resolution over lateral electrode dimensions of 20 mm exploiting the 6Li(n,α)3H reaction, in spite of the extremely small areal Li densities (10-20 μg cm-2 Li) in the nanotubular active material. While the average areal Li densities recorded via triton analysis are found to be in good agreement with the electrochemically measured charges during lithiation, triton analysis revealed, for certain nanotube arrays, areas with a significantly higher Li content ('hot spots') compared to the average. In summary, the presented technique is shown to be extremely well suited for analysis of the lithiation behavior of nanostructured electrode materials with very low Li concentrations. Furthermore, identification of lithiation anomalies is easily possible, which allows for fundamental studies and thus for further advancement of nanostructured Li-ion battery electrodes.

Titanium oxide–titanium carbide (TiOxCy) hybrid materials have tunable electronic properties ranging from semiconductive to semimetallic. They can therefore be employed in solar energy conversion applications and as potential substitute for carbon based electrocatalyst supports for use in fuel cells. Understanding of the optical properties of semimetallic TiOxCy is of great importance. In this paper we report on the optical properties of compact TiOxCy in the mid-IR spectral region. TiOxCy reflects the IR light similarly to metals and is therefore suitable as a new substrate for molecular adsorption studies with infrared reflection absorption spectroscopy. For the first time polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) is applied to study quantitatively at the submolecular level the structure and orientation of fatty acid molecules in monolayers adsorbed on the TiOxCy surface. The analysis of the IR band intensities provides information on the structure, packing, and orie...

International audience; The conversion of anodic TiO2 films into TiOxCy in ultrahigh-vacuum (UHV) has been traced by photoemission spectroscopy in order to optimize the process parameters and study the different phase stabilities. In addition, density functional theory (DFT) calculations have been performed in order to elucidate the main questions about TiOxCy composition and stability. The experimental data indicate that the anodic TiO2 film is stable both in UHV and ethylene background up to ca. 600 K, and at this temperature, it starts to reduce leading to suboxide TiOx species. Above ca. 750 K, the formation of TiOxCy starts, since the oxygen vacancies begin to be replaced by carbon atoms. A surface enrichment in TiO2 and elemental carbon has been detected on the converted TiOxCy film at room temperature. Real-time measurements have shown that this phenomenon takes place during the cool down process and DFT calculations suggest a possible explanation: as the temperature decreases below ca. 750 K (temperature at which the formation of TiOxCy starts), the TiOxCy phase is not thermodynamically stable, and it decomposes into TiO2 and elemental carbon. The comparison of the experimental valence band data with DFT results has also allowed to establish that the film surface is not homogeneous and that segregation of TiO and TiC systems may take place. On the other hand, the local compositional study carried out by scanning photoelectron microscopy has shown that the conversion of the film is not homogeneous but depends on the grain orientation, in particular crystallites with an orientation close to 0> and 0> planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiOxCy as an innovative support for electrocatalysis.

Active and stable non-noble metal materials, able to substitute Pt as catalyst or to reduce the Pt amount, are vitally important for the extended commercialization of energy conversion technologies, such as fuel cells and electrolyzers. Here, we report a fundamental study of nonstoichiometric tungsten carbide (WxC) and its interaction with titanium oxycarbide (TiOxCy) under electrochemical working conditions. In particular, the electrochemical activity and stability of the WxC/TiOxCy system toward the ethanol electrooxidation reaction (EOR) and hydrogen evolution reaction (HER) are investigated. The chemical changes caused by the applied potential are established by combining photoemission spectroscopy and electrochemistry. WxC is not active toward the ethanol electrooxidation reaction at room temperature but it is highly stable under these conditions thanks to the formation of a passive thin film on the surface, consisting mainly of WO2 and W2O5, which prevents the full oxidation of WxC. In addition, WxC is able to adsorb ethanol, forming ethoxy groups on the surface, which constitutes the first step for the ethanol oxidation. The interaction between WxC and TiOxCy plays an important role in the electrochemical stability of WxC since specific orientations of the substrate are able to stabilize WxC and prevent its corrosion. The beneficial interaction with the substrate and the specific surface chemistry makes tungsten carbide a good electrocatalyst support or cocatalyst for direct ethanol fuel cells. However, WxC is active toward the HER and chemically stable under hydrogen reduction conditions, since no changes in the chemical composition or dissolution of the film are observed. This makes tungsten carbide a good candidate as electrocatalyst support or cocatalyst for the electrochemical production of hydrogen.

International audience; Composite materials of titania and graphitic carbon, and their optimized synthesis are highly interesting for application in sustainable energy conversion and storage. We report on planar C/TiO2 composite films that are prepared on a polycrystalline titanium substrate by carbothermal treatment of compact anodic TiO2 with acetylene. This thin film material allows for the study of functional properties of C/TiO2 as a function of chemical composition and structure. The chemical and structural properties of the composite on top of individual Ti substrate grains are examined by scanning photoelectron microscopy and micro-Raman spectroscopy. Through comparison of these data with electron backscatter diffraction, it is found that the amount of generated carbon and the grade of anodic film crystallinity correlate with the crystallographic orientation of the Ti substrate grains. On top of Ti grains with ∼(0001) orientations the anodic TiO2 exhibits the highest grade of crystallinity, and the composite contains the highest fraction of graphitic carbon compared to Ti grains with other orientations. This indirect effect of the Ti substrate grain orientation yields new insights into the activity of TiO2 towards the decomposition of carbon precursors.

A detailed investigation of the carbothermal reduction process has been performed by studying different annealing temperatures between 750 and 1050 °C for the conversion of compact TiO 2 films grown anodically on Ti sheets to TiO x C y . The morphology, chemical composition and crystallographic structure of the films before and after the conversion have been studied with scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and grazing incidence diffraction (GID). A thermodynamic model is proposed based on these investigations for the carbothermal conversion mechanism on compact anodic titania films showing that carbon and oxygen contents in the passive film decrease with increasing annealing temperature. The electronic properties have been investigated using electrochemical impedance spectroscopy (EIS) revealing an increase in charge transfer resistance with increasing annealing temperature. First studies of the hydrogen evolution reaction (HER) on TiO x C y indicate that the electrocatalytic properties directly depend on the chemical, structural and electronic properties of the surface since the overpotential of the HER shifts to higher values for increasing annealing temperature. This study is highly important for the production and investigation of conductive and electrochemically stable flat TiO x C y films as model supports for electrocatalysis studies.

In this work, corrosion resistant TiO2@C powder was tested as possible alternative to standard carbon black supports for polymer electrolyte membrane fuel cells. We separately prepared a stock solution of well-defined Pt nanoparticles and different support materials, and thereafter supported the NPs on the respective supports. As result we obtained high Pt dispersion on TiO2@C supports comparable to conventional high surface area Pt/C catalysts. Accelerated stress tests were carried out in accordance with the Fuel Cell Commercialization Conference of Japan recommendations. The tests demonstrated improved corrosion resistance of Pt/TiO2@C (as compared to Pt/C) when probing start/stop or mixed load cycle and start/stop conditions, whereas similar stability was observed when probing load cycle conditions only.

not Available.

not Available.