Veronica Celorrio

Fast One-Pot Synthesis of MoS2/Crumpled Graphene p–n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS2 nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV-visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS2 nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS2 nanohybrids are seven times more efficient with respect to single MoS2 because of the formation of local p-n MoS2/N-doped graphene nanojunctions, which allow an efficient charge carrier separation.

Tuning CO2 electroreduction efficiency at Pd shells on Au nanocores

The faradaic efficiency of CO2 electroreduction is significantly affected by the thickness of Pd nanoshells on Au cores. The ratio of hydrogen evolution to CO2 reduction was determined by differential electrochemical mass spectrometry. Decreasing the Pd shell thickness from 10 to 1 nm leads to a twofold increase in faradaic efficiency.

Oxygen Reduction Reaction at LaxCa1-xMnO3 Nanostructures: Interplay between A-site Segregation and B-site Valency

The mean activity of surface Mn sites at LaxCa1−xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nanoparticles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1−xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations.

Enhanced MFC power production and struvite recovery by the addition of sea salts to urine.

Urine is an excellent fuel for electricity generation in Microbial Fuel Cells (MFCs), especially with practical implementations in mind. Moreover, urine has a high content in nutrients which can be easily recovered. Struvite (MgNH4PO4·6H2O) crystals naturally precipitate in urine, but this reaction can be enhanced by the introduction of additional magnesium. In this work, the effect of magnesium additives on the power output of the MFCs and on the catholyte generation is evaluated. Several magnesium sources including MgCl2, artificial sea water and a commercially available sea salts mixture for seawater preparation (SeaMix) were mixed with real fresh human urine in order to enhance struvite precipitation. The supernatant of each mixture was tested as a feedstock for the MFCs and it was evaluated in terms of power output and catholyte generation. The commercial SeaMix showed the best performance in terms of struvite precipitation, increasing the amount of struvite in the solid collected from 21% to 94%. Moreover, the SeaMix increased the maximum power performance of the MFCs by over 10% and it also changed the properties of the catholyte collected by increasing the pH, conductivity and the concentration of chloride ions. These results demonstrate that the addition of sea-salts to real urine is beneficial for both struvite recovery and electricity generation in MFCs.

Electrochemical Reduction of Carbon Dioxide at Gold-Palladium Core–Shell Nanoparticles: Product Distribution versus Shell Thickness

The electrocatalytic reduction of CO2 at carbon‐supported Au‐Pd core–shell nanoparticles is investigated systematically as a function of the Pd shell thickness. Liquid‐ and gas‐phase products were determined by off‐line 1H NMR spectroscopy and on‐line electrochemical mass spectrometry. Our results uncover the relationship between the nature of the products generated and the Pd shell thickness. CO and H2 are the only products generated at 1 nm thick shells, whereas shells of 5 and 10 nm produced HCOO−, CH4 and C2H6. The concentration of HCOO− detected in the electrolyte was dependent on the applied potential and reached a maximum Faradaic efficiency of 27 % at −0.5 V versus the reversible hydrogen electrode for 10 nm thick shells. We conclude that collisions between absorbed hydrogen at relaxed Pd lattices and strongly bound “CO‐like” intermediates promote the complete hydrogenation to C1 and C2 alkanes without the generation of other products, such as alcohols and aldehydes.

Surface Activation of Pt Nanoparticles Synthesised by “Hot Injection” in the Presence of Oleylamine

Oleylamine (OA) based “hot injection” colloidal synthesis offers a versatile approach to the synthesis of highly monodisperse metallic and multi-metallic alloyed nanostructures in the absence of potentially toxic and unstable phosphine compounds. For application in heterogeneous catalysis and electrocatalysis, the adsorbed OA species at the metal surfaces should be effectively removed without compromising the structure and composition of the nanostructures. Herein, we investigate the removal of OA from colloidal Pt nanoparticles through 1) “chemical methods” such as washing in acetic acid or ethanol, and ligand exchange with pyridine; and 2) thermal pre-treatment between 185 and 400 °C in air, H2 or Ar atmospheres. The electrochemical reactivity of Pt nanoparticles is acutely affected by the presence of surface organic impurities, making this material ideal for monitoring the effectiveness of OA removal. The results showed that thermal treatment in Ar at temperatures above 400 °C provides highly active particles, with reactivity comparable to the benchmark commercial catalyst, Pt/ETEK. The mechanism involved in thermal desorption of OA was also investigated by thermogravimetric analysis coupled to mass spectrometry (TGA-MS). Oxidation of HCOOH and adsorbed CO in acidic solution were used as test reactions to assess the Pt electrocatalytic activity.

Charge Transport Across High Surface Area Metal/Diamond Nanostructured Composites

High surface area composites featuring metal nanostructures and diamond particles have generated a lot of interest in the fields of heterogeneous catalysis, electrocatalysis, and sensors. Diamond surfaces provide a chemically robust framework for active nanostructures in comparison with sp(2) carbon supports. The present paper investigates the charge transport properties of high surface area films of high-pressure, high-temperature diamond particles in the presence and absence of metal nanostructures, employing electrochemical field-effect transistors. Oxygen- and hydrogen-terminated surfaces were generated on 500 nm diamond powders. Homogeneously distributed metal nanostructures, with metal volume fractions between ca. 5 and 20%, were either nucleated at the diamond particles by impregnation or incorporated from colloidal solution. Electrochemical field-effect transistor measurements, employing interdigitated electrodes, allowed the determination of composite conductivity as a function of electrode potential, as well as in air. In the absence of metal nanostructures, the lateral conductivity of the diamond assemblies in air is increased by over one order of magnitude upon hydrogenation of the particle surface. This observation is consistent with studies at diamond single crystals, although the somewhat modest change in conductivity suggests that charge transport is not only determined by the intrinsic surface conductivity of individual diamond particles but also by particle-to-particle charge transfer. Interestingly, the latter contribution effectively controls the assembly conductivity in the presence of an electrolyte solution as the difference between hydrogenated and oxygenated particles vanishes. The conductivity in the presence of metal nanoparticles is mainly determined by the metal volume fraction, while diamond surface termination and the presence of electrolyte solutions exert only minor effects. The experimental trends are discussed in terms of the electrochemical formation of charge carriers in the diamond particles, percolation theory, and charge screening at the double layer.

Insights into the durability of Co–Fe spinel oxygen evolution electrocatalysts via operando studies of the catalyst structure

Elemental reorganisation and oxidation state changes of key active sites in Co–Fe spinels are investigated by in situ X-ray photoemission spectroscopy (XPS) and operando X-ray absorption spectroscopy (XAS) under oxygen evolution operating conditions. The combination of the two techniques allows identifying both the surface and bulk modifications on the oxides and relating them to the activity loss during extended cycling. The results show that Co–Fe spinels experience a surface irreversible phase evolution under oxygen evolution reaction (OER) conditions, resulting in the formation of an amorphous layer composed of new stable Co(III) and Fe(III) species. Accelerated ageing tests show that the durability, intended as the performance loss during cycling treatments, is not directly related to the structural/chemical stability of the spinels but to the new species formed at the surface due to the electrochemical work. Thus, the material that experienced more significant changes was also the most durable one, demonstrating that the understanding of the chemical and/or structural evolution of the materials during the catalytic process can be the key for the design of highly active and stable catalysts.

Effect of Ba content on the activity of La1-x Ba x MnO3 towards the oxygen reduction reaction

Abstract The electrocatalytic activity of La1‐xBaxMnO3 nanoparticles towards the oxygen reduction reaction (ORR) is investigated as a function of the A‐site composition. Phase‐pure oxide nanoparticles with a diameter in the range of 40 to 70 nm were prepared by using an ionic liquid route and deposited onto mesoporous carbon films. The structure and surface composition of the nanoparticles are probed by XRD, TEM, EDX, and XPS. Electrochemical studies carried out under alkaline conditions show a strong correlation between the activity of La1‐xBaxMnO3 and the effective number of reducible Mn sites at the catalysts layer. Our analysis demonstrates that, beyond controlling particle size and surface elemental segregation, understanding and controlling Mn coordination at the first atomic layer is crucial for increasing the performance of these materials.