Search

The reported study was devoted to the investigation of viscoelastic behavior for solid and porous ultra-high molecular weight polyethylene (UHMWPE) under compression. The obtained experimental stress curves were interpreted using a two-term Prony series to represent the superposition of two coexisting activation processes corresponding to long molecular (~160 s) and short structural (~20 s) time scales, respectively, leading to good statistical correlation with the observations. In the case of porous polymer, the internal strain redistribution during relaxation was quantified using digital image correlation (DIC) analysis. The strongly inhomogeneous deformation of the porous polymer was found not to affect the relaxation times. To illustrate the possibility of generalizing the results to three dimensions, X-ray tomography was used to examine the porous structure relaxation at the macro- and micro-scale levels. DIC analysis revealed positive correlation between the applied force and relative density. The apparent stiffness variation for UHMWPE foams with mixed open and closed cells was described using a newly proposed three-term expression. Furthermore, in situ tensile loading and X-ray scattering study was applied for isotropic solid UHMWPE specimens to investigate the evolution of internal structure and orientation during drawing and stress relaxation in another loading mode.

The copper-zinc-alumina (CZA) catalyst is one of the most important catalysts. Nevertheless, the still limited understanding of the complex CZA structure hampers further optimization. Critical to the production of a highly active and stable catalyst are optimal start-up procedures in hydrogen. Here, by employing operando X-ray absorption spectroscopy and X-ray diffraction, we follow how the industrial CZA precursor evolves into the working catalyst. Two major events in the activation drastically alter the copper- and zinc-containing components in the CZA catalyst and define the final working catalyst structure: the reduction of the starting copper(II) oxide and the switch to catalytic conditions. These drastic structural changes are accompanied by the gradual structural change. Understanding what happens during these events is key to develop tailored start-up protocols that are aimed at maximal longevity and activity of the catalysts., Angewandte Chemie. International Edition, 61 (15), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

Starvation, flooding, and dry‐out phenomena occur in polymer electrolyte membrane fuel cells (PEMFCs), due to heterogeneous local conditions, material inhomogeneity, and uneven flow distribution across the single cell active area and in between the individual cells. The impact of the load level and air feed conditions on the performance was identified for individual single cells within a 10‐cell stack. Analysis of the current density distribution across the active area at the cell level was correlated with electrochemical impedance spectroscopy to enable operando fault diagnostic without any impact of the applied analytical tools on the single cell behavior. Moreover, the combination of both technologies allows in‐depth analysis of fault mechanisms in fuel cell single cells with improved sensitivity. Current density distribution and the quantitative assessment of the performance homogeneity demonstrated high sensitivity to small humidity changes and allow the detection of critical events, such as dry‐out in single cells. Impedance analysis is more sensitive regarding polarization and diffusion limitations and allows detection of cell flooding. The combination of both techniques is required for reliable identification of air starvation faults.

This work presents an original approach to preparing pure and Ni-doped CeO2 nanoparticles (NPs) that can be directly drop-casted on a substrate or calcined to form powders. The reduction of the NPs in H-2 is very different than the one usually anticipated for supported Ni-CeO2 catalysts. In situ soft X-ray absorption and infrared spectroscopies revealed that the reduction of Ce4+ into Ce3+ in H-2 proceeds via simultaneous oxidation of Ni2+ ions into Ni delta+(2

Due to their diversity in the composition, lattice structures and physical/chemical properties, and various oxidation states (2+, 3+, 4+, and 5+), (V x O y ) nanomaterials have attached much attention for developing new rechargeable batteries, including lithium‐ion batteries (LIBs), sodium‐ion batteries (NIBs), zinc‐ion batteries (ZIBs), and magnesium‐ion batteries (MIBs) as well as new energy storage concepts such as light‐rechargeable batteries. However, to further improve the electrochemical performance of V x O y ‐based batteries, it is crucial to understand the various electrochemical mechanisms taking place in these materials for LIBs, NIBs, ZIBs, and MIBs. This review covers a systematical discussion of in situ and operando analysis methods carried out on V2O5, VO2, Li x V y O z , Na x V y O z , Zn x V y O z , and Mg x V y O z for LIBs, NIBs, ZIBs, and MIBs and the fundamental insights they have provided in the energy storage mechanisms in these batteries.

Renewable technologies, and in particular the electric vehicle revolution, have generated tremendous pressure for the improvement of lithium ion battery performance. To meet the increasingly high market demand, challenges include improving the energy density, extending cycle life and enhancing safety. In order to address these issues, a deep understanding of both the physical and chemical changes of battery materials under working conditions is crucial for linking degradation processes to their origins in material properties and their electrochemical signatures. In situ and operando synchrotron-based X-ray techniques provide powerful tools for battery materials research, allowing a deep understanding of structural evolution, redox processes and transport properties during cycling. In this review, in situ synchrotron-based X-ray diffraction methods are discussed in detail with an emphasis on recent advancements in improving the spatial and temporal resolution. The experimental approaches reviewed here include cell designs and materials, as well as beamline experimental setup details. Finally, future challenges and opportunities for battery technologies are discussed.

Structural dynamics of a Mn‐Na2WO4/SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2WO4 with Mn7SiO12. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied., Give and take: Operando Raman spectroscopy in combination with in situ XRD and thermal analysis was applied to study a Mn‐Na2WO4/SiO2 catalyst at work during the oxidative coupling of methane. Melting of Na2WO4 and Mn7SiO12‐MnWO4 redox chemistry occur simultaneously, thereby enabling the transient release and storage of active sodium oxide species. Coupling of these processes is essential to avoid catalyst deactivation.

International audience; The concept of pore hierarchization for enhancing zeolite catalytic performance has been the main focus during the past decade; the main research activities have widely emphasized the preparation of single-component hierarchical zeolite catalysts. Here we report on the critical aspect of interconnectivity of multicomponents in hierarchical zeolite catalysts using a combined gravimetric and infrared spectroscopy (AGIR) analysis. The variances of macroscopic diffusion behaviors between the global multicomponent catalyst (indexed as global diffusion time constant) and its zeolitic moiety (indexed as micropore diffusion time constant) are differentiated via simultaneous gravimetric and infrared analysis, respectively. The effectiveness factors correlated with the diffusion time constants show that the nonideal interconnectivity between the zeolitic and nonzeolitic components weakens the full potential of the multicomponent catalyst.

Hydrogen gas (H2) has been considered as one of the most promising energy carriers for its high energy density (283 KJ•mol-1) and clean combustion product (H2O). Electrochemical water splitting is one of the most efficient approaches to produce hydrogen. However, the anodic oxygen evolution reaction (OER) involves a sluggish four-electron process, which heavily impedes the industrial application. Therefore, it is of great challenge to design the high-active OER catalysts with lower cost. In this thesis, we developed a series of Cr containing nonprecious-metal-based hydroxide materials as advanced OER catalysts and investigated the synergistic effect and regulation effect on the intrinsic OER catalysis and crystallization process, respectively. (i) The ternary NiFeCr hydroxide electrode with nanoporous sheet structure has been achieved via facile electrodeposition method and demonstrates enhanced OER catalytic performance over the benchmark NiFe hydroxide electrode. (ii) Then spectroelectrochemical operando Raman and X-ray absorption spectroscopy (XAS) reveal the synergistic effect from Cr incorporation that Cr and Fe substitute for the Ni’s lattice position in pristine Ni(OH)2 phase, which in turn is converted into the active Cr, Fe-substituted-β-NiOOH intermediate with the valence states of Cr6+, Fe3+ and Ni2+, whose shorter Cr-O may promote the OER process. (iii) Thereafter, the analogous catalyst of CoFeCr hydroxide is also developed and shows the improved OER catalytic performance as well, due to the similar property of Co to Ni. (iv) Last, the regulation effect from the Cr on crystallization and phase homogenization of NiFe, CoFe oxide/hydroxides is also investigated to reveal that Cr significantly induces the multi-phase with large particle size into homogeneously amorphous nanodots with the sharply size reduction to 2~3 nm, and the obtained amorphous Cr incorporated Ni(Co)Fe nanodots also demonstrate enhanced OER performance, which is attributed to the enlarged surface area and formation of active intermediates during reaction. This thesis fundamentally investigates the trimetallic oxyhydroxide compounds containing Cr and contributes to the rational design of advanced OER catalysts with enhanced catalytic activity.

Current Li-ion batteries dominate the market but face great challenges with respect to safety, cost and the higher energy and power densit requirements of electrical vehicles and stationary energy storage. Relevant for mobile electrical transport, Li-O2 batteries in theory offer the highest specific energy among all the lithium electrochemical energy storage systems. Research efforts have been made to address the challenges that impede the functioning of this battery, which include low round trip efficiency, low specific capacity and poor cycling stability. To understand the origin of these issues, attaining a deeper understanding of the mechanism behind the electrochemical reactions is of vital importance. This also forms the foundation for exploring ideal oxygen cathodes and better electrolytes. The aqueous zinc batteries are another potential candidate for large scale electric energy storage owing to its low-cost, high operational safety, and environmental benignity. However, it is not easy to find a suitable insertion cathode for ZIBs, because the electrostatic interaction between divalent Zn ions. The development of host materials for ZIBs is still in its infancy, and in-depth understanding of the electrochemical processes involved is paramount at this early stage. The focus of this thesis is on attaining mechanistic insight into the electrochemical processes occurring in working Li-O2 and aqueous zinc batteries, which guides the choice/design of proper electrode materials for these next generation battery systems.

International audience; It is generally believed that CO2 electroreduction to multi‐carbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu‐N‐C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN4 coordination environment, atomically dispersed in a nitrogen‐doped conductive carbon matrix. This material achieves aqueous CO2 electroreduction to ethanol at a Faradaic yield of 55 % under optimized conditions (electrolyte: 0.1 m CsHCO3, potential: −1.2 V vs. RHE and gas‐phase recycling set up), as well as CO electroreduction to C2‐products (ethanol and ethylene) with a Faradaic yield of 80 %. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.

Converting solar energy into hydrogen through photoelectrochemical (PEC) water splitting offers a promising route towards a fully renewable energy economy. A fundamental understanding of photogenerated charge recombination processes in semiconducting photoelectrodes is a key consideration in optimizing solar water splitting cells and intensity-modulated photovoltage spectroscopy (IMVS) has recently emerged as a promising technique for gaining new insight. However, the interpretation of IMVS data under various conditions (that is, when photoelectrodes are in sacrificial electrolytes or employ catalytic overlayers) is not complete. Using IMVS data we present herein an analysis of charge recombination processes under open circuit conditions on a model photoanode system: nanostructured hematite. Employing two sacrificial oxidation conditions compared to standard water oxidation conditions, our IMVS results establish a direct correlation between surface recombination processes and the low frequency IMVS response. We found that surface intermediate states for water oxidation under open circuit condition exhibit a lifetime of 229 ms under standard illumination conditions. Applying a nickel iron oxide overlayer also gives insight into surface passivation effects through IMVS analysis of photoanode system.