Distribution of Sulfur in Carbon/Sulfur Nanocomposites Analyzed by Small-Angle X-ray Scattering

Insights into the CO2 Capture Characteristics within the Hierarchical Pores of Carbon Nanospheres Using Small-Angle Neutron Scattering

Understanding adsorption processes at the molecular level has transformed the discovery of engineered materials for maximizing gas storage capacity and kinetics in adsorption-based carbon capture applications. In this work, we studied the molecular mechanism of gas (CO₂, H₂, methane, and ethane) adsorption inside an interconnected porous network of carbon. This was achieved by synthesizing novel macro-meso-microporous carbon (M³C) nanospheres with interconnected pore structures. The M³Cs showed a CO₂ capture capacity of 5.3 mmol/g at atmospheric CO₂ pressure, with excellent kinetics. This was due to fast CO₂ adsorption within the interconnected hierarchical macro-meso-microporous M³C. In situ small-angle neutron scattering (SANS) under various CO₂ pressures indicated that the macro- and mesopores of M³C enable fast diffusion of CO₂ molecules inside the micropores, where adsorbed CO₂ molecules densify into a liquid-like state. This strong densification of CO₂ molecules causes fast CO₂ diffusion in the macro- and mesopores of M³C, restarting the adsorption cycle for fresh CO₂ molecules until all pores are completely filled. Notably, M³C also showed good capture capacities for hydrogen and various hydrocarbons, with excellent selectivity toward ethane over methane.

Bimetallic Exsolved Heterostructures of Controlled Composition with Tunable Catalytic Properties

In this paper, we show how the composition of bimetallic Fe-Ni exsolution can be controlled by the nature and concentration of oxygen vacancies in the parental matrix and how this is used to modify the performance of CO₂-assisted ethane conversion. Mesoporous A-site-deficient La_0.4Sr_0.6-αTi_0.6Fe_0.35Ni_0.05O_3±δ (0 ≤ α ≤ 0.2) perovskites with substantial specific surface area (>40 m²/g) enabled fast exsolution kinetics (T < 500 °C, t < 1 h) of bimetallic Fe-Ni nanoparticles of increasing size (3-10 nm). Through the application of a multitechnique approach we found that the A-site deficiency determined the concentration of oxygen vacancies associated with iron, which controlled the Fe reduction. Instead of homogeneous bimetallic nanoparticles, the increasing Fe fraction from 37 to 57% led to the emergence of bimodal Fe/Ni₃Fe systems. Catalytic tests showed superior stability of our catalysts with respect to commercial Ni/Al₂O₃. Ethane reforming was found to be the favored pathway, but an increase in selectivity toward ethane dehydrogenation occurred for the systems with a low metallic Fe fraction. The chance to control the reduction and growth processes of bimetallic exsolution offers interesting prospects for the design of advanced catalysts based on bimodal nanoparticle heterostructures.

Unveiling the role of micropores in porous carbon for Li-S batteries using operando SAXS

The movement of the sulfur species of a lithium-sulfur battery cathode was directly observed through pioneering operando SAXS analysis. Micropore is a prior repository for sulfur before and after the electrochemical reaction. Mesopore is actual reaction site for sulfur species. The separate properties of the pores were established, adding critical insight to advanced carbon cathode material design.

Methods of green synthesis of Au NCs with emphasis on their morphology: A mini-review

Greener synthetic methods are becoming more popular as a means of reducing environmental pollution caused by reaction byproducts. Another important advantage of green methods is their low cost and the abundance of raw materials. Herein, we investigate the green Au nanoclusters (NCs) using microorganisms (bacteria and fungi) and plant extraction with various shapes and development routes. Natural products derived from plants, tea, coffee, banana, simple amino acids, enzyme, sugar, and glucose have been used as reductants and as capping agents during synthesis in literature. The synthesis techniques are generally chemical, physical and green methods. Green synthesis of Au NCs using bacteria and fungi can be divided into intracellular and extracellular. In an intracellular manner, bacterial cells are implanted in a culture medium containing salt and heated under suitable growth conditions. However, in an extracellular manner, the Au ions are directed from the outside into the cell. Thus, these methods are considered as a better alternative to chemical and physical synthesis. The research on green synthesis of Au nanoparticles (NPs) and its influence on their size and morphology are summarized in this review.

Direct Observation of the Xenon Physisorption Process in Mesopores by Combining In Situ Anomalous Small-Angle X-ray Scattering and X-ray Absorption Spectroscopy

The morphology and structural changes of confined matter are still far from being understood. This report deals with the development of a novel in situ method based on the combination of anomalous small-angle X-ray scattering (ASAXS) and X-ray absorption near edge structure (XANES) spectroscopy to directly probe the evolution of the xenon adsorbate phase in mesoporous silicon during gas adsorption at 165 K. The interface area and size evolution of the confined xenon phase were determined via ASAXS demonstrating that filling and emptying the pores follow two distinct mechanisms. The mass density of the confined xenon was found to decrease prior to pore emptying. XANES analyses showed that Xe exists in two different states when confined in mesopores. This combination of methods provides a smart new tool for the study of nanoconfined matter for catalysis, gas, and energy storage applications.

Detailed and Direct Observation of Sulfur Crystal Evolution During Operando Analysis of a Li-S Cell with Synchrotron Imaging

Herein, we present a detailed investigation of the electrochemically triggered formation and dissolution processes of α- and β-sulfur crystals on a monolithic carbon cathode using operando high-resolution synchrotron radiography (438 nm/pixel). The combination of visual monitoring with the electrical current response during cyclic voltammetry provides valuable insights into the sulfur formation and dissolution mechanism. Our observations show that the crystal growth process is mainly dictated by a rapid equilibrium between long-chain polysulfides on one side and solid sulfur/short-chain polysulfides on the other side, which is consistent with previous studies in this field. The high temporal and spatial resolution of synchrotron imaging enables the observation of different regimes during the sulfur formation and dissolution process. The appearance of short-chain polysulfides after the first anodic CV peak initiates a rapid dissolution process of α-sulfur crystals on the cathode. The increase in the long-chain lithium polysulfide concentration at the cathode surface during charge results in an increased crystal growth rate, which in turn produces imperfections in α- and β-sulfur crystals. There are strong indications that these defects are fluid inclusions, which may trap dissolved polysulfides and therefore reduce the electrochemical cell capacity.

Operando Analysis of a Lithium/Sulfur Battery by Small-Angle Neutron Scattering

This study reports the use of operando small-angle neutron scattering to investigate processes in an operating Li/S battery. The combination with impedance spectroscopy yields valuable insights into the precipitation and dissolution of lithium sulfide during 10 cycles of galvanostatic cycling. The use of a deuterated electrolyte increases strongly the sensitivity to detect the sulfur and Li₂S precipitates at the carbon host electrode and allows us to observe the time-dependent initial wetting of the system. No correlation of the scattering signal of the micropores with either lithium sulfide or sulfur is observable during the whole course of the experiment. Hence both reaction products do not precipitate inside the microporous structure but on the outer surface of the micrometer-sized carbon fibers used in this study. The excellent scattering contrast allows a detailed analysis of the formation and dissolution process of nanoscopic Li₂S structures. While lithium sulfide particles grow homogeneously during the precipitation period, smaller Li₂S particles dissolve first followed by a sudden dissolution of the larger Li₂S particles.

Investigation of porous materials with large surface heterogeneity using the generalized Porod's scattering law method

Surface heterogeneity is ubiquitous in both natural and man-made materials, and can significantly influences material properties. However, it is very challenging to noninvasively probe the variation of surface properties in porous materials. Recently, we have proposed a method, i.e., the generalized Porod's scattering law method (GPSLM), to obtain the surface heterogeneity information in bulk porous materials by extending the classic Porod's scattering method. However, it was not clear if the GPSLM can be applied to other more complex materials, such as porous materials with dead pores, i.e., pores that guest fluid molecules cannot access or porous materials whose solid matrix can adsorb small guest molecules. In this paper, we theoretically extend the GPSLM to study those more complex situations. For all five cases with different levels of complexity discussed in this work, the scattering intensity at the Porod's law region always follows a parabolic function of scattering length density (SLD) of the guest fluid. Moreover, the minimum value of the scattering intensity is all related to the surface heterogeneity of the porous materials. The SLD of the guest fluid at which the minimum intensity is reached is always related to the surface-averaged SLD of materials. We also discuss the potential limitations and possible future applications of the GPSLM. As the GPSLM is based on the contrast variation method commonly used for a wide range of materials, such as geological materials, biomaterials, and colloidal suspensions, the theoretical development here is potentially useful for researchers who would like to apply the GPSLM to more complicated materials besides porous materials.

Sulfur Diffusion within Nitrogen-Doped Ordered Mesoporous Carbons Determined by in Situ X-ray Scattering

The low intrinsic conductivity of sulfur necessitates conductive additives, such as mesoporous carbons, to the cathode to enable high-performance metal-sulfur batteries. Simultaneous efforts to address polysulfide shuttling have introduced nitrogen-doped carbons to provide both conductivity and suppressed shuttling because of their strong interaction with sulfur. The strength of this interaction will impact the ability to fill the mesopores with sulfur via melt infusion. Here, we systematically investigate how nitrogen doping influences the rate that molten sulfur can infiltrate the mesopores and the overall extent of pore filling of highly ordered mesoporous doped carbons using in situ small angle X-ray scattering (SAXS). The similarity in electron density between molten sulfur and the soft carbon framework of the mesoporous material leads to a precipitous decrease in the scattered intensity associated with the ordered structure as voids are filled with sulfur. As the nitrogen doping increases from 1 to 20 at. %, the effective diffusivity of sulfur in the mesopores decreases by an order of magnitude (2.7 × 10^-8 to 2.3 × 10^-9 cm/s). The scattering becomes nearly invariant within 20 min of melt infiltration at 155 °C for all but the most doped carbon, which indicates that submicron-sized mesoporous carbon particles can be filled rapidly. Additionally, the nitrogen doping decreases the sulfur content that can be accommodated within the mesopores from 95% of the mesopores filled without doping to only 64% filled with 20 at. % N as determined by the residual scattering intensity. Sulfur does not crystallize within the mesopores of the nitrogen-doped carbons, which is further indicative of the strong interactions between the nitrogen species and sulfur that can inhibit polysulfide shuttling. In situ SAXS provides insights into the diffusion of sulfur in mesopores and how the surface chemistry of nitrogen-doped carbon appears to significantly hinder the infiltration by sulfur.

Pore geometry effect on the synthesis of silica supported perovskite oxides

The formation of perovskite oxide nanoparticles supported on ordered mesoporous silica with different pore geometry is here presented. Systematic study was performed varying both pore shape (gyroidal, cylindrical, spherical) and size (7.5, 12, 17nm) of the hosts. LaFeO₃, PrFeO₃ and LaCoO₃ were chosen as target guest structures. The distribution of the oxide nanoparticles on silica was comprehensively assessed using a multi-technique approach. It could be shown that the pore geometry plays a determining role in the conversion of the infiltrated metal nitrates to metal oxide. In particular, slow degradation kinetic was observed in highly curved pores, which fostered nucleation and crystallization of the guest species. In spherical pore systems the enhancement of pore size caused a remarkable delay of the decomposition of the metal salts, but at the same time improved the homogeneous distribution of the oxide particles in the matrix.

Liquid Sulfur Impregnation of Microporous Carbon Accelerated by Nanoscale Interfacial Effects

Impregnation of porous carbon matrices with liquid sulfur has been exploited to fabricate composite cathodes for lithium-sulfur batteries, aimed at confining soluble sulfur species near conducting carbon to prevent both loss of active material into the electrolyte and parasitic reactions at the lithium metal anode. Here, through extensive computer simulations, we uncover the strongly favorable interfacial free energy between liquid sulfur and graphitic surfaces that underlies this phenomenon. Previously unexplored curvature-dependent enhancements are shown to favor the filling of smaller pores first and effect a quasi-liquid sulfur phase in microporous domains (diameters <2 nm) that persists ∼30° below the expected freezing point. Evidence of interfacial sulfur on carbon is shown to be a 0.3 eV red shift in the simulated and measured interfacial X-ray absorption spectra. Our results elucidate the critical morphology and thermodynamic properties necessary for future cathode design and highlight the importance of molecular-scale details in defining emergent properties of functional nanoscale interfaces.