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Transition metal carbides, such as molybdenum carbides, are promising substitutes for noble metals as low-cost, durable electrocatalysts. Under ambient conditions, however, these carbides are subject to oxidation due to their oxophilic nature. The partially oxidized surface may possess both oxygen-modulated metallic-like hydrogen adsorption sites and Brønsted-acidic hydroxyl sites. However, the impact of surface oxidation on electrochemical processes such as the hydrogen evolution reaction (HER) has rarely been studied. Here, we synthesized β-Mo2C catalysts and oxidized their surfaces electrochemically to varying extents to study the effects of surface oxidation on HER activity. The degree of surface oxidation was controlled by applying different potential windows to metal carbide catalysts. The samples with varying degrees of surface oxidation were tested for their HER activity. Experimental data indicate that the Tafel slope for the HER and double-layer capacitance were negatively affected by surface oxidation, particularly due to the loss of carbon and the formation of electrochemically less active surface oxides. The surface oxidation was studied experimentally by X-ray photoelectron spectroscopy and simulated using density functional theory (DFT), ab initio thermodynamics, and charge transfer estimates. Our DFT calculation results suggest that the model β-Mo2C (011) surface favors the adsorption of O* from water during the electrochemical oxidation, giving rise to the anodic current. Oxygen atoms preferentially interact with surface C sites, forming stable –C＝O species and oxycarbide-like surfaces. Highly oxidized surfaces become kinetically unstable and undergo a deeper, substitutional oxidation through –C＝O replacement by O*, forming a thermodynamically stable Mo(IV) surface oxide, where each Mo atom coordinates with up to six O atoms. The rate-limiting step switches from CO desorption, occuring within the potential window from 0.28 to 0.51 V, to water dissociation above 0.51 V. The computational results agree well with experimental observations, such as the onset potential (0.6 V) for rapid surface oxidation. This work helps to unravel the details of evolution of surface sites during an oxidation process of molybdenum carbides along with their effects on catalytic properties and lays the foundation for their practical use in various applications. [ABSTRACT FROM AUTHOR]

The description of the design, manufacturing technology, and test results of thin-film solid-state lithium-ion batteries with a nanocomposite negative electrode Si@O@Al is given herein. This electrochemical system features the hike on the charging curve plateau, which is interpreted as the change from I–V of the Ti-Si@O@Al contact. The latter is due to the change in the type of silicon conductivity during lithiation, as a result of which the ohmic metal-semiconductor contact proves to be biased in the reverse direction, and the charging current is maintained by minority charge carriers. It is shown that the current-conducting component Si@O@Al is formed by a solid solution a-Si(Al), which has a p-type conductivity. The change in the type of conductivity occurs as a result of silicon compensation through lithiation. It was found that Si@O@Al is nonlinear conductor, which can be considered as a percolation cluster formed by amorphous silicon nanoparticles and molecular clusters of silicon dioxide. The height of the Schottky barrier of the Ti|a-Si(Al) contact and the electron affinity of the a-Si(Al) solid solution were estimated. [ABSTRACT FROM AUTHOR]

Along with the soaring demands for all-solid-state thin-film lithium-ion batteries, the problem of their energy density rise becomes very acute. The solution to this problem can be found in development of 3D batteries. The present work deals with the development of a technology for a 3D solid-state lithium-ion battery (3D SSLIB) manufacturing by plasma-chemical etching and magnetron sputtering technique. The results on testing of experimental samples of 3D SSLIB are presented. It was found that submicron-scale steps appearing on the surface of a 3D structure formed on Si substrate by the Bosch process radically change the crystal structure of the upper functional layers. Such changes can lead to disruption of the layers' continuity, especially that of the down conductors. It is shown that surface polishing by liquid etching of the SiO2 layer and silicon reoxidation leads to surface smoothing, the replacement of the dendrite structure of functional layers by a block structure, and a significant improvement in the capacitive characteristics of the battery. [ABSTRACT FROM AUTHOR]

A structural model of LiPON solid electrolyte, containing elements that simulate drift conductivity, diffusion conductivity, and leakage current was proposed. The dependence of the impedance of the structural model on frequency was calculated, and the parameters of the model at which the theoretical curve best approximates the experimental Nyquist diagrams were determined. Based on these data, the ion diffusion coefficient and conductivity of LiPON were calculated, which are D1 = 1.5 × 10-11 cm²/s and σ = 1.9 × 10-6 S/cm, respectively. [ABSTRACT FROM AUTHOR]

The increase of thickness of silicon films as negative electrodes of lithium-ion batteries is known to result in notable worsening their cyclability. Some recently, the authors have shown that the layered composites based on the Si–O–Al system possess much better cyclability. The present work declares that cyclic stability depends also on parameters of galvanostatic cycling. The composite films of thickness 3 μm, composed of 14 wt% of oxygen, 17 wt% of aluminum, and 67 wt% of silicon, deposited in the form of multi-layered structures, are shown to demonstrate a sufficiently high stability over more than 200 charge–discharge cycles with the capacity fading less than 0.09% per cycle if the cycling is performed within the potential range of 0.1–0.7 V relative to the lithium reference electrode. [ABSTRACT FROM AUTHOR]

Adsorbate vibrational excitations are an important fingerprint of molecule/surface interactions, affecting temperature contributions to the free energy and impacting reaction rate and equilibrium constants. Furthermore, vibrational spectra aid in identifying species and adsorption sites present in experimental studies. Despite their importance, knowledge of how adsorbate frequencies scale across materials is lacking. Here, by combining previously reported experimental data and our own density-functional theory calculations, we reveal linear correlations between vibrational frequencies of adsorbates on transition metal surfaces. Through effective-medium theory, linear muffin-tin orbital theory, and the d-band model, we rationalize the squares of the frequencies to be fundamentally linear in their scaling across transition metal surfaces. We identify the adsorbate-binding energy as a descriptor for certain molecular vibrations and rigorously relate errors in frequencies to errors in adsorption energies. We also discuss the impact of scaling on surface thermochemistry and adsorbate coverage. [ABSTRACT FROM AUTHOR]

The selective ring-opening of cellulose-derived furanic molecules is a promising pathway for the production of industrially relevant linear oxygenates, such as 1,6-hexanediol. 2,5-Dimethylfuran (DMF) is employed as a model compound in a combined experimental and computational investigation to provide insights into the metal-catalyzed ring-opening. Ring-opening to 2-hexanol and 2-hexanone and ring-saturation to 2,5-dimethyltetrahydrofuran (DMTHF) are identified as two main parallel pathways. DFT calculations and microkinetic modeling indicate that DMF adsorbs on Ru in an open-ring configuration, which is potentially a common surface intermediate that leads to both ring-opening and ring-saturation products. Although the activation barriers for the two pathways are comparable, the formation of DMTHF is more thermodynamically favorable. In addition, steric interactions with co-adsorbed 2-propoxyl, derived from the solvent, and the oxophilic nature of Ru play key roles to determine the product distribution: the former favors less bulky, that is, ring-closed, intermediates, and the latter retards O−H bond formation. Finally, we show that the hydrodeoxygenation of oxygenated furanics, such as 5-methylfurfural and (5-methyl-2-furyl)methanol, on Ru occurs preferentially at oxygen-containing side groups to form DMF, followed by either ring-opening or ring-saturation. [ABSTRACT FROM AUTHOR]

We propose non-parametric methods for both local and global sensitivity analysis of chemical reaction models with correlated parameter dependencies. The developed mathematical and statistical tools are applied to a benchmark Langmuir competitive adsorption model on a close packed platinum surface, whose parameters, estimated from quantum-scale computations, are correlated and are limited in size (small data). The proposed mathematical methodology employs gradient-based methods to compute sensitivity indices. We observe that ranking influential parameters depends critically on whether or not correlations between parameters are taken into account. The impact of uncertainty in the correlation and the necessity of the proposed non-parametric perspective are demonstrated. [ABSTRACT FROM AUTHOR]