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.
Although they exhibit huge versatility, coordination complexes have been rarely investigated in the field of cathode materials for batteries. Despite their relatively high molecular mass, according ...to the nature of the metallic center and that of the ligand, the E ° value and the electron transfer kinetics can be adjusted to develop a performant material compatible with the electrolyte. Here, we propose to investigate Fe II poly-bipyridine complexes with a view to check the impact of the nature of the electrolyte as well as the influence of the distance between two redox centers when polymerized on the electrochemical response in battery conditions. To understand these changes, three lithium salts have been studied: LiClO 4 , LiPF 6 and LiTFSI (TFSI = bis(trifluoromethane)sulfonimide). In order to mimic these impacts, monomer complexes (mono- and binuclear) have been electrochemically studied, whereas, thanks to ab initio calculations, their redox behavior has been correlated to the ligand environment of the metallic center. Finally, despite their expected low mass capacity, these polymeric coordination complexes have been involved in battery conditions.
We present a series of ab initio molecular dynamics simulations (MD) for liquid Ni, Zr and Ta to investigate the local structure and its evolution upon undercooling, and we analyze our results with ...respect to recent neutron scattering experiments. By means of the common-neighbor analysis, which gives a three-dimensional image of the inherent structures, we have found an evolution upon undercooling that depends on the system under consideration. The short-range order (SRO) of Ni is characterized by icosahedral and distorted icosahedral structures, and undercooling is accompanied by the occurrence of a complex polytetrahedral SRO that consists of more numerous Frank and Kasper polyhedra larger than the simple icosahedron. The same holds for Zr but we show that a strong competition exists with the bcc-like SRO which increases with the degree of undercooling. The SRO of Ta shows a complex polytetrahedral type order, very close to that of the A15 phase upon undercooling. The time evolution of the configurations is also examined in terms of the mean-square displacement.
A series of First Principles calculations is undertaken to characterize and explain the enhancement of the catalytic activity of oxygen on top of very disordered nanomaterials of Pt. As the ...adsorption of OH fragment on top of the surfaces is known as the limiting factor in the Oxygen Reduction Reactions (ORR) process in these systems, our calculations propose to determine the influence of the local geometry of the various sites on the adsorption energy of OH in order to discover a simple descriptor allowing to predict the reactivity at these surfaces as a function of their morphology and strain. For this purpose, the geometry of Pt slabs with various thickness (3, 5 and 7 atomic layers) including a large number of point defects are optimized in order to generate a very rich catalog of inequivalent sites of reactivity on both surfaces of the slabs. Given the very large distortion of the geometry of the sites, these latter had to be categorized into several classes for which the behavior with respect to catalytic activity is determined. A new descriptor taking into account the distortion of the geometry of the sites is introduced, allowing to recover the linear dependence of the adsorption energy of OH with respect to the effective coordination number of the sites, as observed in highly symmetric and planar surfaces of Pt.
Although they exhibit huge versatility, coordination complexes have been rarely investigated in the field of cathode materials for batteries. Despite their relatively high molecular mass, according ...to the nature of the metallic center and that of the ligand, the
E
° value and the electron transfer kinetics can be adjusted to develop a performant material compatible with the electrolyte. Here, we propose to investigate Fe
II
poly-bipyridine complexes with a view to check the impact of the nature of the electrolyte as well as the influence of the distance between two redox centers when polymerized on the electrochemical response in battery conditions. To understand these changes, three lithium salts have been studied: LiClO
4
, LiPF
6
and LiTFSI (TFSI = bis(trifluoromethane)sulfonimide). In order to mimic these impacts, monomer complexes (mono- and binuclear) have been electrochemically studied, whereas, thanks to
ab initio
calculations, their redox behavior has been correlated to the ligand environment of the metallic center. Finally, despite their expected low mass capacity, these polymeric coordination complexes have been involved in battery conditions.
Iron trisbipyridine coordination complexes can be used as redox storage unit. Their abilities to be used as cathode material according to the nature of the electrolyte, is checked by electrochemical investigations and supported by
ab initio
calculation.
We present a joint experimental and density functional theory (DFT) study on the effect of atomic vacancies on the restructuring of platinum—transition metal alloy nanocatalysts and the associated ...changes in electrocatalytic activity. Atomic vacancies were introduced into slabs composed of pure Pt monolayers, and the structures were relaxed using the Vienna ab initio simulation package code. Effects of i) the concentration and ii) the spatial distribution of atomic vacancies in the slabs on surface and bulk restructuring were investigated. Highly disordered nanostructures featuring large variations of the in‐plane and out‐of‐plane nearest‐neighbour distances around the mean were observed upon relaxation. These findings were confirmed experimentally by using hollow PtNi/C nanoparticles synthesized by a combination of galvanic replacement and the nanoscale Kirkendall effect (a vacancy‐mediated interdiffusion mechanism). The experimental results also show that hollow PtNi/C nanoparticles feature a combination of oxophilic and oxophobic catalytic sites on their surface and are thus highly active both for electrochemical oxidation and reduction reactions.
Active vacancies: DFT calculations predict surface restructuring of pure Pt elementary slabs in platinum—transition metal alloy nanocatalysts by the introduction of atomic vacancies. Restructuring is experimentally confirmed by the enhanced electrochemical activity of the nanoparticles.
Hollow nanoparticles made of a noble metal and a non-noble metal can be synthesized by a combination of galvanic replacement and the nanoscale Kirkendall effect. In these nanostructures, a metallic ...shell composed of individual nanocrystallites interconnected via grain boundaries surrounds a central cavity that is accessible to molecules through nanometre-sized pores. Moreover, because atomic vacancies enter the metal lattice during their formation, the surface of hollow metal nanoparticles is jagged both at the atomic and the nanometre scale. That gives them specific electrocatalytic properties, which are illustrated with reactions of increasing complexity (COads monolayer electrooxidation, oxygen reduction reaction, methanol and ethanol electrooxidation) and rationalized with Density Functional Theory calculations.
First principles calculations have been made to theoretically interpret the structural stability of SrTiO3 as a function of pressure or strain. We show that the orthorhombic Pbnm structure is more ...stable than the ideal perovskite one due to rotations of TiO6 octahedra at high pressures or deformations of TiO6 octahedra at low pressures. Such a study is extended to the structural stability of the Ruddlesden-Popper Srn+1TinO3n+1 phases for n=1, 2 and 3, in which the lattice constraints are induced by the intercalation of SrO layers in the perovskite structure.
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