Thermal Runaway Mechanism in Ni-Rich Cathode Full Cells of Lithium-Ion Batteries: The Role of Multidirectional Crosstalk.

Crosstalk, the exchange of chemical species between battery electrodes, significantly accelerates thermal runaway (TR) of lithium-ion batteries. To date, the understanding of their main mechanisms has centered on single-directional crosstalk of oxygen (O2) gas from the cathode to the anode, underestimating the exothermic reactions during TR. However, the role of multidirectional crosstalk in steering additional exothermic reactions is yet to be elucidated due to the difficulties of correlative in situ analyses of full cells. Herein, the way in which such crosstalk triggers self-amplifying feedback is elucidated that dramatically exacerbates TR within enclosed full cells, by employing synchrotron-based high-temperature X-ray diffraction, mass spectrometry, and calorimetry. These findings reveal that ethylene (C2H4) gas generated at the anode promotes O2 evolution at the cathode. This O2 then returns to the anode, further promoting additional C2H4 formation and creating a self-amplifying loop, thereby intensifying TR. Furthermore, CO2, traditionally viewed as an extinguishing gas, engages in the crosstalk by interacting with lithium at the anode to form Li2CO3, thereby accelerating TR beyond prior expectations. These insights have led to develop an anode coating that impedes the formation of C2H4 and O2, to effectively mitigate TR.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation.

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.

Plasma-assisted oxidation of Cu(100) and Cu(111).

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO2. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO2RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O2-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu2O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate's orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu2O is formed during the initial growth phase, both Cu2O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu2O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO2 Hydrogenation.

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO2 hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H2, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO2 hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts.

Operando NRIXS and XAFS Investigation of Segregation Phenomena in Fe-Cu and Fe-Ag Nanoparticle Catalysts during CO2 Electroreduction.

Operando nuclear resonant inelastic X-ray scattering (NRIXS) and X-ray absorption fine-structure spectroscopy (XAFS) measurements were used to gain insight into the structure and surface composition of FeCu and FeAg nanoparticles (NPs) during the electrochemical CO2 reduction (CO2 RR) and to extract correlations with their catalytic activity and selectivity. The formation of a core-shell structure during CO2 RR for FeAg NPs was inferred from the analysis of the operando NRIXS data (phonon density of states, PDOS) and XAFS measurements. Electrochemical analysis of the FeAg NPs revealed a faradaic selectivity of 36 % for CO in 0.1 M KHCO3 at -1.1 V vs. RHE, similar to that of pure Ag NPs. In contrast, a predominant selectivity towards H2 evolution is obtained in the case of the FeCu NPs, analogous to the results obtained for pure Fe NPs, although small Cu NPs have also been shown to favor H2 production.

In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution.

NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation.

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts.

Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction.

Entraining in trout: a behavioural and hydrodynamic analysis.

Rheophilic fish commonly experience unsteady flows and hydrodynamic perturbations. Instead of avoiding turbulent zones though, rheophilic fish often seek out these zones for station holding. A behaviour associated with station holding in running water is called entraining. We investigated the entraining behaviour of rainbow trout swimming in the wake of a D-shaped cylinder or sideways of a semi-infinite flat plate displaying a rounded leading edge. Entraining trout moved into specific positions close to and sideways of the submerged objects, where they often maintained their position without corrective body and/or fin motions. To identify the hydrodynamic mechanism of entraining, the flow characteristics around an artificial trout placed at the position preferred by entraining trout were analysed. Numerical simulations of the 3-D unsteady flow field were performed to obtain the unsteady pressure forces. Our results suggest that entraining trout minimise their energy expenditure during station holding by tilting their body into the mean flow direction at an angle, where the resulting lift force and wake suction force cancel out the drag. Small motions of the caudal and/or pectoral fins provide an efficient way to correct the angle, such that an equilibrium is even reached in case of unsteadiness imposed by the wake of an object.