Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of ...low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose 'surface distortion' as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalize the electrocatalytic properties of state-of-the-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under a simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and highlights strategies to design more durable ORR nanocatalysts.
Abstract
With sufficiently high cooling rates, liquids will cross their equilibrium melting temperatures and can be maintained in a metastable undercooled state before solidifying. Studies of ...undercooled liquids reveal several intriguing dynamic phenomena and because explicit connections between liquid structure and liquids dynamics are difficult to identify, it remains a major challenge to capture the underlying structural link to these phenomena. Ab initio molecular dynamics (AIMD) simulations are yet especially powerful in providing atomic-scale details otherwise not accessible in experiments. Through the AIMD-based study of Cr additions in Al-based liquids, we evidence for the first time a close relationship between the decoupling of component diffusion and the emergence of dynamic heterogeneities in the undercooling regime. In addition, we demonstrate that the origin of both phenomena is related to a structural heterogeneity caused by a strong interplay between chemical short-range order (CSRO) and local fivefold topology (ISRO) at the short-range scale in the liquid phase that develops into an icosahedral-based medium-range order (IMRO) upon undercooling. Finally, our findings reveal that this structural signature is also captured in the temperature dependence of partial pair-distribution functions which opens up the route to more elaborated experimental studies.
The total energies of formation of Cu–Ni, Ag–Ni and Au–Ni superstructures based on a fcc lattice have been calculated using the linear-muffin-tin-orbital (LMTO) method in the full potential approach. ...Both unrelaxed and relaxed structures have been included in the calculations. The energy of formation is decomposed into three terms, a volume deformation contribution, a chemical contribution and a relaxation contribution. The enthalpies of mixing of the disordered solid solutions have been calculated using an Ising-like cluster expansion for both the chemical and relaxation effects. In the Gibbs energy of mixing, a configurational entropy of mixing calculated with the cluster variation method (CVM) has been introduced and the thermal excitations due to the electronic and vibrational effects have been taken into account. The miscibility gaps displayed in the three considered binary systems have been calculated.
•The partial heat of dissolution of solid Nb in liquid Al was determined at 1126 K.•Heats of formation of NbFe2 and NbNi3 were measured by Al dissolution calorimetry.•The solubility of Nb in liquid ...Al is ∼10 times higher than accepted values.•Neumann-Kopp’s rule is overestimating the Cp of NbAl3.
The heat of dissolution at infinite dilution of solid Nb in liquid Al was determined indirectly by dissolving small pieces of solid NbAl3 in liquid aluminum in an isothermal calorimeter at 1126 ± 0.5 K. A value of ∞Qdiss(NbAl3,298 K,1126 K) = 75.3 ± 3.0 kJ mol−1 was measured. The derived partial enthalpy of mixing at infinite dilution for Nb in Al is ΔmixH∞ = -243.2 kJ mol−1. A lower limit for the solubility of solid Nb in liquid Al is also obtained from these measurements. The observed limit is 10 times higher than the accepted experimental value in the literature. In addition, heat content measurements for NbAl3 were performed at three different temperatures using drop calorimetry with empty crucibles. The measured heat contents are compared to values obtained from phonon calculations in the quasi-harmonic approximation.
The quality of the determined heat of dissolution was checked by measuring the enthalpy of formation of solid NbFe2 and NbNi3. Values of -17.5 ± 2.9 kJ (mol of atoms)−1 and -30.7 ± 3.5 kJ (mol of atoms)−1 were found. These values agree with available experimental and theoretical data from the literature which confirms the validity of our experimental approach. A new, unknown and metastable magnetic configuration was discovered for NbFe2 in ab-initio simulations and will be discussed.
We present a study of dynamic properties of liquid aluminum using density-functional theory within the local-density (LDA) and generalized gradient (GGA) approximations. We determine the temperature ...dependence of the self-diffusion coefficient as well the viscosity using direct methods. Comparisons with experimental data favor the LDA approximation to compute dynamic properties of liquid aluminum. We show that the GGA approximation induce more important backscattering effects due to an enhancement of the icosahedral short range order (ISRO) that impact directly dynamic properties like the self-diffusion coefficient. All these results are then used to test the Stokes-Einstein relation and the universal scaling law relating the diffusion coefficient and the excess entropy of a liquid.
It has been suggested that icosahedral short-range order (SRO) occurs in deeply undercooled melts of pure metallic elements. We report results of first-principles molecular dynamics simulations for ...stable and undercooled zirconium liquids. Our results emphasize the occurrence of a local order more complex than the icosahedral one. For stable liquid, the local order is interpreted on the basis of a competition between a polytetrahedral SRO and a bcc-type SRO. We also demonstrate that a bcc-type SRO increases with the degree of undercooling.
Bulk and Surface Properties of Liquid Al-Li and Li-Zn Alloys Trybula, Marcela; Gancarz, Tomasz; Gasior, Wladyslaw ...
Metallurgical and materials transactions. A, Physical metallurgy and materials science,
11/2014, Letnik:
45, Številka:
12
Journal Article
Recenzirano
Odprti dostop
Physicochemical properties like density, surface tension, and viscosity of liquid binary Al-Li and Li-Zn alloys have been measured using draining crucible method. The experimentally measured ...surface-tension values have been compared to theoretical results based either on the Butler model or the compound formation model assuming the existence of the most favored
A
1
B
2
and
A
2
B
3
clusters. Several models for viscosity calculation have been also applied and discussed in confrontation with measured data. Finally, the clustering effects in the liquid Al-Li and Li-Zn alloys have been examined using two microscopic functions,
i.e.
, the concentration fluctuation function in the long-wavelength limit and the Warren-Cowley short-range order parameter.
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