Vibrational and electronic entropy of β-cerium and γ-cerium measured by inelastic neutron scattering

Time-of-flight (TOF) inelastic neutron-scattering spectra were measured on β-cerium (double hcp) and γ-cerium (fcc) near the phase-transition temperature. Phonon densities of states (DOS) and crystal-field levels were extracted from the TOF spectra. A softening of the phonon DOS occurs in the transition from β- to γ-cerium, accounting for an increase in vibrational entropy of ΔSvibγ-β=(0.09±0.05)kB/atom. The entropy calculated from the crystal-field levels and a fit to calorimetry data from the literature were significantly larger in β-cerium than in γ-cerium below room temperature, but the difference was found to be negligible at the experimental phase-transition temperature. A contribution to the specific heat from Kondo spin fluctuations was consistent with the quasielastic magnetic scattering, but the difference between phases was negligible. To be consistent with the latent heat of the β-γ transition, the increase in vibrational entropy at the phase transition may be accompanied by a decrease in electronic entropy not associated with the crystal-field splitting or spin fluctuations. At least three sources of entropy need to be considered for the β-γ transition in cerium

Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”

in situ tracking of redox transitions and mode of catalysis

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoint potentials of about 1.0 V (CoII0.4CoIII0.6 ↔ all-CoIII) and 1.2 V (all-CoIII ↔ CoIII0.8CoIV0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveals that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in an amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo CoII ↔ CoIII ↔ CoIV oxidation-state changes coupled to structural modification and deprotonation of Co-oxo bridges. By the encounter of two (or more) CoIV ions, an active site is formed at which the O–O bond-formation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward the development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity

Nature of Nitrogen Incorporation in BiVO4 Photoanodes through Chemical and Physical Methods

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated: modification of the precursors for the spray pyrolysis recipe and post-deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N-modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X-ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen (i.e., N-2). Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N-2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of approximate to 0.1 eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower-bandgap absorber materials for photoelectrochemical energy conversion applications

Protection Mechanism against Photocorrosion of GaN Photoanodes Provided by NiO Thin Layers

The photoelectrochemical properties of n-type Ga-polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well-defined thickness and high crystal quality, NiO is grown by plasma-assisted molecular-beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole-injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion. © 2020 The Authors. Solar RRL published by Wiley-VCH Gmb

Structure of the catalytic sites in Fe/N/C-catalysts for O-2-reduction in PEM fuel cells

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x ≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials

The Phonon Entropy of Metals and Alloys: The Effects of Thermal and Chemical Disorder

Vibrational entropy is important for the thermodynamics of alloying, alloy formation, phase transitions and phase stability at high temperature. Vibrational entropies of alloying and alloy formation were calculated for 32 binary alloys and intermetallic compounds using phonon DOS curves taken from the literature. The vibrational entropies of formation span a wide range from -0.64 to +0.55 kB/atom, and the vibrational entropies of alloying ranged from -0.39 to +1.0 kB/atom. This range exceeds the range of configurational entropy of a binary alloy, which reaches a maximum value of +0.69 kB/atom and a minimum value of 0 kB/atom. The vibrational entropy of the NiTi martensitic transition was measured using low-temperature calorimetry and inelastic neutron-scattering. The high-temperature B2 phase of NiTi has a vibrational entropy 0.5 kB/atom larger than the low-temperature martensitic phase. The difference in vibrational entropy accounts for the total entropy of the austenitic-martensitic phase transition. Inelastic neutron scattering was used to show that the phonon DOS of V is unchanged between 20 and 1000°C, inconsistent with the phonon softening expected from thermal expansion. It is found that the effects of volume expansion and rising temperature exert equal and opposite shifts on the phonon DOS. The pure temperature dependence of the phonon DOS is due to strong phonon-phonon scattering, which in turn leads to a large anharmonic vibrational entropy contribution at high temperature. The vibrational entropy of eight chemically disordered Cu-Au alloys was measured using inelastic neutron scattering. The analysis of the phonon entropy of a disordered alloy was performed in a novel way by modeling the partial vibrational entropies of Cu and Au. The partial vibrational entropies of Cu and Au were shown to be slowly varying and smooth functions of composition. The vibrational entropy of disordering in Cu3Au is calculated as 0.24 ± 0.02 kB/atom, substantially larger than results predicted from recent theoretical work.</p

Photoelectrochemical Properties of In,Ga N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy

We investigated the photoelectrochemical properties of both n and p type In,Ga N nanowires NWs for water splitting by in situ electrochemical mass spectroscopy EMS . All NWs were prepared by plasma assisted molecular beam epitaxy. Under illumination, the n In,Ga N NWs exhibited an anodic photocurrent, however, no O2 but only N2 evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p In,Ga N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H2. After photodeposition of Pt on such NWs, the photo current density was significantly enhanced to 5 mA cm2 at a potential of 0.5 V NHE under visible light irradiation of 40 mW cm2. Also, incident photon to current conversion efficiencies of around 40 were obtained at 0.45 V NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p In,Ga N NWs are a promising basis for solar hydrogen productio

Novel environmental monitoring detector for discriminating fallout and airborne radioactivity

Abstract Early warning networks are used for detecting abnormal radioactivity levels in the environment. State-of-the-art networks are equipped with both dose rate detectors and spectrometric stations. Current networks don’t automatically discriminate between radioactivity on the ground and in the air. A novel directional sensing gamma radiation detector utilizing a collimated phoswich scintillator was developed. The signals from the two scintillator materials are separated using a pulse shape discrimination. The separated signals are employed to determine the radioactivity concentrations on the ground and in the air assuming specific concentration distributions. Limitations related to imperfect directional sensing and dead time are discussed