Mechanism of Oxygen Reduction in Aprotic Li–Air Batteries: The Role of Carbon Electrode Surface Structure

Electrochemical oxygen reduction in aprotic media is a key process that determines the operation of advanced metal–oxygen power sources, e.g., Li–O₂ batteries. In such systems oxygen reduction on carbon-based positive electrodes proceeds through a complicated mechanism that comprises several chemical and electrochemical steps involving either dissolved or adsorbed species, and as well side reactions with carbon itself. Here, cyclic voltammetry was used to reveal the effects of imperfections in the planar sp² surface structure of carbon on the Li oxygen reduction reaction (Li-ORR) mechanism by means of different model carbon electrodes (highly oriented pyrolytic graphite (HOPG), glassy carbon, basal, and edge planes of pyrolytic graphite), in dimethyl sulfoxide (DMSO)-based electrolyte. We show that the first electron transfer step O₂ + e^– ⇆ O₂^– (followed by ion-coupling Li⁺ + O₂^– ⇆ LiO₂) does not involve oxygen chemisorption on carbon as evidenced by the independence of its rate on the carbon electrode surface morphology. The second electron transfer leading to Li₂O₂ (Li⁺ + LiO₂ + e^– ⇆ Li₂O₂) is strongly affected by the electrode surface even in highly solvating DMSO. Formation of Li₂O₂ via the electrochemical reaction could be observed only on the nearly ideal basal plane of graphite. In contrast, for more disordered electrode surfaces, (and/or bulk) the only reduction peak revealed on cyclic voltammograms corresponds to LiO₂ formation, supporting that solution-mediated mechanism for Li₂O₂ growth is more favorable in that case. We also show that increased defect concentrations on the carbon electrode surface promote the formation of Li₂CO₃ during ORR, albeit relatively slower than Li₂O₂ formation

Lithium Ion Coupled Electron-Transfer Rates in Superconcentrated Electrolytes: Exploring the Bottlenecks for Fast Charge-Transfer Rates with LiMn₂O₄ Cathode Materials

The charge-transfer kinetics of lithium ion intercalation into Li_<i>x</i>Mn₂O₄ cathode materials was examined in dilute and concentrated aqueous and carbonate LiTFSI solutions using electrochemical methods. Distinctive trends in ion intercalation rates were observed between water-based and ethylene carbonate/diethyl carbonate solutions. The influence of the solution concentration on the rate of lithium ion transfer in aqueous media can be tentatively attributed to the process associated with Mn dissolution, whereas in carbonate solutions the rate is influenced by the formation of a concentration-dependent solid electrolyte interface (SEI). Some indications of SEI layer formation at electrode surfaces in carbonate solutions after cycling are detected by X-ray photoelectron spectroscopy. The general consequences related to the application of superconcentrated electrolytes for use in advanced energy storage cathodes are outlined and discussed

WS2 nanotubes dressed in gold and silver: synthesis, optoelectronic properties, and NO2 sensing

This conference contribution is focused on decoration of WS2 nanotubes (NT-WS2) with gold and silver nanoparticles via facile routes implying direct reaction of tungsten disulfide with water-soluble AuIII and AgI species at 100oC. The underlying mechanism of these interactions will be discussed in details based on extensive studies of reaction mixtures and resulting metal–NT-WS2 nanocomposites, including thorough X-ray photoelectron spectroscopy (XPS) analysis. Surprising features in optical spectra of the designed nanocomposites would be reported, including suppression of plasmon resonance in tiny noble metal nanoparticles (< 10 nm in diameter) grown onto NT-WS2. The plasmonic features of individual gold nanoparticles on the surface of disulfide nanotube were also characterized by electron energy loss spectroscopy in scanning transmission electron microscopy mode (STEM-EELS). Photoresistive NO2-sensing response of NT-WS2 under green light illumination (Ȝmax = 530 nm) and its enhancement by plasmonic gold “nanoantennas” will be reported as well

Experimental and Computational Insight into the Chemical Bonding and Electronic Structure of Clathrate Compounds in the Sn–In–As–I System

Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, waste-heat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn_{24–<i>x</i>–δ}In_<i>x</i>As_{22–<i>y</i>}I₈ clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling

The Chemistry of Imperfections in N‑Graphene

Many propositions have been already put forth for the practical use of N-graphene in various devices, such as batteries, sensors, ultracapacitors, and next generation electronics. However, the chemistry of nitrogen imperfections in this material still remains an enigma. Here we demonstrate a method to handle N-impurities in graphene, which allows efficient conversion of pyridinic N to graphitic N and therefore precise tuning of the charge carrier concentration. By applying photoemission spectroscopy and density functional calculations, we show that the electron doping effect of graphitic N is strongly suppressed by pyridinic N. As the latter is converted into the graphitic configuration, the efficiency of doping rises up to half of electron charge per N atom

Size-Dependent Structure Relations between Nanotubes and Encapsulated Nanocrystals

The structural organization of compounds in a confined space of nanometer-scale cavities is of fundamental importance for understanding the basic principles for atomic structure design at the nanolevel. Here, we explore size-dependent structure relations between one-dimensional PbTe nanocrystals and carbon nanotube containers in the diameter range of 2.0–1.25 nm using high-resolution transmission electron microscopy and ab initio calculations. Upon decrease of the confining volume, one-dimensional crystals reveal gradual thinning, with the structure being cut from the bulk in either a <110> or a <100> growth direction until a certain limit of ∼1.3 nm. This corresponds to the situation when a stoichiometric (uncharged) crystal does not fit into the cavity dimensions. As a result of the in-tube charge compensation, one-dimensional superstructures with nanometer-scale atomic density modulations are formed by a periodic addition of peripheral extra atoms to the main motif. Structural changes in the crystallographic configuration of the composites entail the redistribution of charge density on single-walled carbon nanotube walls and the possible appearance of the electron density wave. The variation of the potential attains 0.4 eV, corresponding to charge density fluctuations of 0.14 e/atom

Role of PdO_<i>x</i> and RuO_<i>y</i> Clusters in Oxygen Exchange between Nanocrystalline Tin Dioxide and the Gas Phase

The effect of palladium- and ruthenium-based clusters on nanocrystalline tin dioxide interaction with oxygen was studied by temperature-programmed oxygen isotopic exchange with mass-spectrometry detection. The modification of aqueous sol–gel prepared SnO₂ by palladium and, to a larger extent, by ruthenium, increases surface oxygen concentration on the materials. The revealed effects on oxygen exchangelowering the threshold temperature, separation of surface oxygen contribution to the process, increase of heteroexchange rate and oxygen diffusion coefficient, decrease of activation energies of exchange and diffusionwere more intensive for Ru-modified SnO₂ than in the case of SnO₂/Pd. The superior promoting activity of ruthenium on tin dioxide interaction with oxygen was interpreted by favoring the dissociative O₂ adsorption and increasing the oxygen mobility, taking into account the structure and chemical composition of the modifier clusters

Role of PdO_<i>x</i> and RuO_<i>y</i> Clusters in Oxygen Exchange between Nanocrystalline Tin Dioxide and the Gas Phase

The effect of palladium- and ruthenium-based clusters on nanocrystalline tin dioxide interaction with oxygen was studied by temperature-programmed oxygen isotopic exchange with mass-spectrometry detection. The modification of aqueous sol–gel prepared SnO₂ by palladium and, to a larger extent, by ruthenium, increases surface oxygen concentration on the materials. The revealed effects on oxygen exchangelowering the threshold temperature, separation of surface oxygen contribution to the process, increase of heteroexchange rate and oxygen diffusion coefficient, decrease of activation energies of exchange and diffusionwere more intensive for Ru-modified SnO₂ than in the case of SnO₂/Pd. The superior promoting activity of ruthenium on tin dioxide interaction with oxygen was interpreted by favoring the dissociative O₂ adsorption and increasing the oxygen mobility, taking into account the structure and chemical composition of the modifier clusters

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)₂Te₃ Crystal Doped with Mn

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone

Negligible Surface Reactivity of Topological Insulators Bi₂Se₃ and Bi₂Te₃ towards Oxygen and Water

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported to be the main sources of surface deterioration by chemical reactions. In the present work, we investigate the behavior of the topological surface states on Bi₂X₃ (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both <i>in situ</i> (from 10^–8 to 0.1 mbar) and <i>ex situ</i> (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi₂Se₃ and Bi₂Te₃ surfaces and only leads to slight <i>p</i>-doping. In dry air, the oxidation of the Bi₂Te₃ surface occurs on the time scale of months, in the case of Bi₂Se₃ surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi₂X₃-based devices