Cathie Vix-Guterl

Vanadium nitride/carbon nanotube nanocomposites as electrodes for supercapacitors

Nanostructured vanadium nitride/multiwalled carbon nanotubes (VN/CNTs) composites for pseudo-capacitor applications were obtained via the sol–gel synthesis of organic or inorganic vanadium oxide precursors followed by temperature programmed ammonia reduction. Nitrogen adsorption and impedance spectroscopy measurements showed that the incorporation of CNTs during VN synthesis allows VN/CNTs nanocomposites to be obtained with higher porosity, narrower pore size distribution, better conductivity and improved electrochemical properties compared to VN without CNTs. In particular, cyclic voltammetry using three-electrode cells in KOH shows that the contribution of the redox peaks is increased when VN is associated with the carbon nanotubes. As a consequence, a capacitance increase was measured in the two-electrode system. Another important advantage of using VN/CNTs composites is their high capacitance retention (58%) at high current density (30 A g−1) compared with VN (7%), resulting in an enhancement of the energy density at high power. All these positive aspects were significantly more marked when CNTs were incorporated during VN synthesis compared to a material resulting from the physical mixture of VN with CNTs. TEM, XPS and Raman analyses point out that the enhanced electrochemical performance observed with the VN/CNTs composite could be related to an intimate contact between VN and the CNT network, a homogeneous distribution of VN on CNTs and the presence of an open mesoporous texture favouring the access of the electrolyte to the active material surface.

Size-Dependent Hydrogen Sorption in Ultrasmall Pd Clusters Embedded in a Mesoporous Carbon Template

Hydrogen sorption properties of ultrasmall Pd nanoparticles (2.5 nm) embedded in a mesoporous carbon template have been determined and compared to those of the bulk system. Downsizing the Pd particle size introduces significant modifications of the hydrogen sorption properties. The total amount of stored hydrogen is decreased compared to bulk Pd. The hydrogenation of Pd nanoparticles induces a phase transformation from fcc to icosahedral structure, as proven by in situ XRD and EXAFS measurements. This phase transition is not encountered in bulk because the 5-fold symmetry is nontranslational. The kinetics of desorption from hydrogenated Pd nanoparticles is faster than that of bulk, as demonstrated by TDS investigations. Moreover, the presence of Pd nanoparticles embedded in CT strongly affects the desorption from physisorbed hydrogen, which occurs at higher temperature in the hybrid material compared to the pristine carbon template.

Template synthesis of a new type of ordered carbon structure from pitch

New ordered nanoporous carbon materials can be synthesized using pitch and mesoporous silica materials as the carbon precursor and template, respectively. In this case, the silica material (type MCM-48 and SBA-15) is impregnated with pitch and further carbonized. After removal of the silica by acid treatment, a graphitizable carbon material with an ordered micro-mesoporosity is recovered. The preparation route as well as the characteristics of the carbon materials are presented in this paper. It appears that the use of pitch as the carbon precursor presents some advantages compared to carbon obtained by other methods. Moreover, study of the thermal stability of the carbon shows that the structural regularity is preserved upon heat treatment at 1675 K.

Characterization of Carbon Surface Chemistry by Combined Temperature Programmed Desorption with in Situ X-ray Photoelectron Spectrometry and Temperature Programmed Desorption with Mass Spectrometry Analysis

The analysis of the surface chemistry of carbon materials is of prime importance in numerous applications, but it is still a challenge to identify and quantify the surface functional groups which are present on a given carbon. Temperature programmed desorption with mass spectrometry analysis (TPD-MS) and X-ray photoelectron spectroscopy with an in situ heating device (TPD-XPS) were combined in order to improve the characterization of carbon surface chemistry. TPD-MS analysis allowed the quantitative analysis of the released gases as a function of temperature, while the use of a TPD device inside the XPS setup enabled the determination of the functional groups that remain on the surface at the same temperatures. TPD-MS results were then used to add constraints on the deconvolution of the O1s envelope of the XPS spectra. Furthermore, a better knowledge of the evolution of oxygen functional groups with temperature during a thermal treatment could be obtained. Hence, we show here that the combination of these two methods allows to increase the reliability of the analysis of the surface chemistry of carbon materials.

Understanding the mechanism of hydrogen uptake at low pressure in carbon/palladium nanostructured composites

The hydrogen sorption/desorption mechanism below 1 bar at room temperature in porous carbons loaded with nanosized metal particles is not well understood and remains a controversial subject in the literature. The aim of this work is to provide a comprehensive view on the hydrogen sorption/desorption process on C/Pd composites by carefully analysing the phenomena involved during the hydrogen cycles in relation with the material characteristics (amount, size and chemical surface state of the palladium nanoparticles). The C/Pd composites consist of templated microporous carbon in which nanosized palladium particles were homogeneously dispersed. Depending on the synthesis condition, the amount of Pd loaded ranges between 1.4 and 12 wt% and the particle size is ranging from 3 to 15 nm. The presence of a palladium oxide layer on the Pd particle surface is revealed by XPS; the amount of this layer depends on the particle size. The hydrogen sorption/desorption measurements indicate that the total hydrogen amount sorbed on the C/Pd composite exceeds the amount required for the formation of β-palladium hydride. This hydrogen sorption excess is attributed in the literature to the spillover effect. To verify this assumption, we performed a careful exploitation of the hydrogen isotherms along with in situ and ex situ characterizations on the C/Pdx composites and a palladium oxide powder. For the first sorption/desorption cycle, two hydrogen sorption steps were identified and the sorbed hydrogen volume in each step was quantified. The first step which is irreversible is assigned to the reduction of PdO leading to the formation of Pd and water. The second step corresponds to the formation of the palladium hydride (PdHy), a step which is influenced by the presence of water. The processes involved in these two steps are strongly dependent on the Pd particle size. The results presented here clearly demonstrate that the PdO reduction is the predominant phenomenon, explaining the hydrogen uptake excess measured in the first cycle. Although a spillover effect cannot be excluded, the experimental data indicate that its possible contribution would remain significantly much weaker than the contribution due to the PdO reduction. To our knowledge, this is the first time that the effect of the Pd oxide layer on the hydrogen sorption (at low pressure and room temperature) on C/Pdx composites has been experimentally proved and quantified.

Exceptionally highly performing Na-ion battery anode using crystalline SnO2 nanoparticles confined in mesoporous carbon

Confined and unconfined SnO2 nanoparticles in the pores of mesoporous carbon were prepared and tested as anode materials vs. Na. Both composites present small crystalline SnO2 particles (∼3 nm) but different location and dispersion in the carbon matrix. When the particles are homogeneously distributed and confined in the carbon pores, an initial reversible capacity of 780 mA h g−1 is achieved with unprecedented capacity retention of 80 and 54% after 100 and 4000 cycles, respectively, at a high current rate (50 C, 1800 mA g−1). Unexpectedly, over two current rate variation cycles from 1 C to 500 C, the composite recovers 81% and 97%, respectively after returning from the 500 C to the 1 C rate. To our knowledge, no other material with such a long cycling life and superior performance in terms of capacity and rate capability has been reported so far for sodium ion batteries. HRTEM, XRD, N2 adsorption, XPS and galvanostatic cycling results suggest that confined SnO2 particles undergo an enhanced sodium alloying/dealloying process due to their special confinement inside the pores, which increases their conductivity, facilitates the diffusion of Na+ ions and buffers the large volumetric changes during charge/discharge. These high performances cannot be delivered when SnO2 is not confined and not well dispersed in the carbon pores. This work demonstrates that nano-confinement of anode species in carbon is a valuable concept affording the modification of the fundamental properties of guest species along with their electrochemical performances leading to highly stable and performing materials with a long life for Na-ion batteries.

Microporous carbon adsorbents with high CO2 capacities for industrial applications

In this study we attempt to investigate the potential use of two zeolite template carbon (ZTC), EMT-ZTC and FAU-ZTC, to capture CO(2) at room temperature. We report their high pressure CO(2) adsorption isotherms (273 K) that show for FAU-ZTC the highest carbon capture capacity among published carbonaceous materials and competitive data with the best organic and inorganic adsorbing frameworks ever-known (zeolites and mesoporous silicas, COFs and MOFs). The importance of these results is discussed in light of mitigation of CO(2) emissions. In addition to these new experimental CO(2) adsorption data, we also present new insight into the adsorption process of the two structures by Monte Carlo simulations: we propose that two separate effects are responsible for the apparent similarity of the adsorption behaviour of the two structures: (i) pore blocking occurring on EMT-ZTC, and (ii) the change of the carbon polarizability due to the extreme curvature of FAU-ZTC.

Influence of Surface Chemistry on the Adsorption of Oxygenated Hydrocarbons on Activated Carbons

The objective of this work was to study the adsorption of different oxygenated hydrocarbons (methanol, ethanol, 1 and 2-butanol, methyl acetate) on activated carbons from organic mixtures with cyclohexane. Three activated carbons prepared by thermal and chemical treatments of a commercial carbon were employed for this purpose. Their textural properties were found to be similar, whereas their surface chemistries were modified, as shown by temperature-programmed desorption coupled to mass spectrometry (TPD-MS) and X-ray photoelectron spectroscopy (XPS). The adsorption isotherms were obtained by depletion method, and the analysis of adsorbed species was evaluated by TPD-MS to obtain new insight into the interactions between the different hydrocarbons and the carbon surface. Ethanol leads to a high-energy interaction between its hydroxyl function and the oxygenated surface groups and also to a lower energy interaction between the aliphatic part of the molecule and the carbon material. The desorption activation energy for this hydrophilic interaction is high (50 to 105 kJ/mol), and it is related to the nature of the carbon surface groups. The relative importance of these two interactions depend on the size of the alcohol/methanol is similar to ethanol, whereas butanols lead to more dispersive interactions. Methyl-acetate cannot undergo this kind of strong interaction and behaves like cyclohexane, having desorption activation energies ranging between 25 and 45 kJ/mol no matter the molecule and the carbon surface chemistry.

Catalyst-free soft-template synthesis of ordered mesoporous carbon tailored using phloroglucinol/glyoxylic acid environmentally friendly precursors

Carbon porous materials with a periodically ordered pore structure, controlled pore size and geometry and high thermal stability are synthesized using self-assembly of environmentally friendly phloroglucinol/glyoxylic acid precursors with an amphiphilic triblock copolymer template. Glyoxylic acid, a plant-derived compound, is used for the first time as a substituent of carcinogen formaldehyde usually employed in such a synthesis. Thanks to the double functionality, i.e., aldehyde and carboxylic acid, glyoxylic acid plays not only the role of a cross-linker for the formation of the resin but also the role of a catalyst by creation of H-bonding or specific reactions between the precursors. Hence, no extra catalyst such as strong acids (HCl) or bases (NaOH) is any longer required. Carbon films and powders were successfully prepared with high surface areas (up to 800 m2 g−1), high porous volume (up to 1 cm3 g−1), tunable pore size (0.6 nm to 7 nm) and various pore architectures (hexagonal, cubic, and ink-bottle) by tuning the precursor ratio and by applying different manufacturing engineering strategies. Insights on the synthesis mechanism of the phenolic resin and carbon mesostructures were obtained using several analysis techniques, i.e., nuclear magnetic resonance (13C NMR) and FTIR spectroscopy, temperature programmed desorption coupled with mass spectrometry (TPD-MS) and thermo-gravimetric analysis (TGA).

Synthesis of small metallic Mg-based nanoparticles confined in porous carbon materials for hydrogen sorption

MgH2, Mg-Ni-H and Mg-Fe-H nanoparticles inserted into ordered mesoporous carbon templates have been synthesized by decomposition of organometallic precursors under hydrogen atmosphere and mild temperature conditions. The hydrogen desorption properties of the MgH2 nanoparticles are studied by thermo-desorption spectroscopy. The particle size distribution of MgH2, as determined by TEM, is crucial for understanding the desorption properties. The desorption kinetics are significantly improved by downsizing the particle size below 10 nm. Isothermal absorption/desorption cycling of the MgH2 nanoparticles shows a stable capacity over 13 cycles. The absorption kinetics are unchanged though the desorption kinetics are slower on cycling.