Growth and Characterization of Ce- Substituted Nd2Fe14B Single Crystals

Single crystals of (Nd1-xCex)2Fe14B are grown out of Fe-(Nd,Ce) flux. Chemical and structural analysis of the crystals indicates that (Nd1-xCex)2Fe14B forms a solid solution until at least x = 0.38 with a Vegard-like variation of the lattice constants with x. Refinements of single crystal neutron diffraction data indicate that Ce has a slight site preference (7:3) for the 4g rare earth site over the 4f site. Magnetization measurements show that for x = 0.38 the saturation magnetization at 400 K, a temperature important to applications, falls from 29.8 for the parent Nd2Fe14B to 27.6 (mu)B/f.u., the anisotropy field decreases from 5.5 T to 4.7 T, and the Curie temperature decreases from 586 to 543 K. First principles calculations carried out within density functional theory are used to explain the decrease in magnetic properties due to Ce substitution. Though the presence of the lower-cost and more abundant Ce slightly affects these important magnetic characteristics, this decrease is not large enough to affect a multitude of applications. Ce-substituted Nd2Fe14B is therefore a potential high-performance permanent magnet material with substantially reduced Nd content.Comment: 11 Pages, 8 figures, 5 table

The partial oxidation of methane over Pd/Al2O3 catalyst nanoparticles studied in-situ by near ambient-pressure x-ray photoelectron spectroscopy

Near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to study the chemical state of methane oxidation catalysts in-situ. Al2O3{supported Pd catalysts are prepared with different particle sizes ranging from 4 nm to 10 nm. These catalysts were exposed to conditions similar to those used in the partial oxidation of methane (POM) to syn-gas and simultaneously monitored by NAP-XPS and mass spectrometry. NAP-XPS data show changes in the oxidation state of the palladium as the temperature in- creases, from metallic Pd0 to PdO, and back to Pd0. Mass spectrometry shows an increase in CO production whilst the Pd is in the oxide phase, and the metal is reduced back under presence of newly formed H2. A particle size effect is observed, such that CH4 conversion starts at lower temperatures with larger sized particles from 6 nm to 10 nm. We find that all nanoparticles begin CH4 conversion at lower temperatures than polycrystalline Pd foil

The Chemical Evolution of the La0.6Sr0.4CoO3−δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

© The Author(s) 2018Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.Fonds zur Förderung der wissenschaftlichen Forschung (FWF)212921411

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient Pressure X ray Photoelectron Spectroscopy

Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water-splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co3O4/Co(OH)2 biphasic electrocatalyst on Si by means of operando ambient-pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH)2 and partial conversion of the spinel Co3O4 phases to CoO(OH) under precatalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core-level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co3O4 catalyst supports this interpretation and reveals that the presence of Co(OH)2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design

Probing the Surface of Platinum during the Hydrogen Evolution Reaction in Alkaline Electrolyte

Understanding the surface chemistry of electrocatalysts in operando can bring insight into the reaction mechanism, and ultimately the design of more efficient materials for sustainable energy storage and conversion. Recent progress in synchrotron based X-ray spectroscopies for in operando characterization allows us to probe the solid/liquid interface directly while applying an external potential, applied here to the model system of Pt in alkaline electrolyte for the hydrogen evolution reaction (HER). We employ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify the oxidation and reduction of Pt-oxides and hydroxides on the surface as a function of applied potential, and further assess the potential for hydrogen adsorption and absorption (hydride formation) during and after the HER. This new window into the surface chemistry of Pt in alkaline electrolyte brings insight into the nature of the rate limiting step, the extent of H ad/absorption, and its persistence at more anodic potentials

Revealing in Situ Li Metal Anode Surface Evolution upon Exposure to CO2 Using Ambient Pressure X-Ray Photoelectron Spectroscopy

Because they deliver outstanding energy density, next-generation lithium metal batteries (LMBs) are essential to the advancement of both electric mobility and portable electronic devices. However, the high reactivity of metallic lithium surfaces leads to the low electrochemical performance of many secondary batteries. Besides, Li deposition is not uniform, which has been attributed to the low ionic conductivity of the anode surface. In particular, lithium exposure to CO2 gas is considered detrimental due to the formation of carbonate on the solid electrolyte interphase (SEI). In this work, we explored the interaction of Li metal with CO2 gas as a function of time using ambient pressure X-ray photoelectron spectroscopy to clarify the reaction pathway and main intermediates involved in the process during which oxalate formation has been detected. Furthermore, when O2 gas is part of the surrounding environment with CO2 gas, the reaction pathway is bypassed to directly promote carbonate as a single product

The Mechanism of SEI Formation on a Single Crystal Si(100) Electrode

A fundamental study of interfacial phenomena on a Si(100) single crystal electrode in organic carbonate-based electrolytes was carried out. The SEI formation on the Si(100) single crystal electrode was investigated as a function of the electrolyte composition, electrode potential and LixSi lithiation degree. Fourier transform infrared spectroscopy (FTIR) and X-ray photon spectroscopy (XPS) studies of the SEI layer during early stages of SEI formation indicate a strong dependence of the SEI composition on the electrolyte composition. However, the influence of the electrolyte composition becomes negligible at low potentials, when lithium alloys with Si and forms amorphous LixSi. The effect of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) electrolyte additives on the composition of the SEI layer was evaluated

Regulating oxygen activity of perovskites to promote NO_x oxidation and reduction kinetics

Understanding the adsorption and oxidation of NO on metal oxides is of immense interest to environmental and atmospheric (bio)chemistry. Here, we show that the surface oxygen activity, defined as the oxygen 2p-band centre relative to the Fermi level, dictates the adsorption and surface coverage of NOx and the kinetics of NO oxidation for La1−xSrxCoO3 perovskites. Density functional theory and ambient-pressure X-ray photoelectron spectroscopy revealed favourable NO adsorption on surface oxygen sites. Increasing the surface oxygen activity by increasing the strontium substitution led to stronger adsorption and greater storage of NO2, which resulted in more adsorbed nitrogen-like species and molecular nitrogen formed upon exposure to CO. The NO oxidation kinetics exhibited a volcano trend with surface oxygen activity, centred at La0.8Sr0.2CoO3 and with an intrinsic activity comparable to state-of-the-art catalysts. We rationalize the volcano trend by showing that increasing the NO adsorption enhances the oxidation kinetics, although NO adsorption that is too strong poisons the surface oxygen sites with adsorbed NO2 to impede the kinetics