Advanced Structural Analysis of Nanoporous Materials by Thermal Response Measurements

Thermal response measurements based on optical adsorption calorimetry are presented as a versatile tool for the time-saving and profound characterization of the pore structure of porous carbon-based materials. This technique measures the time-resolved temperature change of an adsorbent during adsorption of a test gas. Six carbide and carbon materials with well-defined nanopore architecture including micro- and/or mesopores are characterized by thermal response measurements based on <i>n</i>-butane and carbon dioxide as the test gases. With this tool, the pore systems of the model materials can be clearly distinguished and accurately analyzed. The obtained calorimetric data are correlated with the adsorption/desorption isotherms of the materials. The pore structures can be estimated from a single experiment due to different adsorption enthalpies/temperature increases in micro- and mesopores. Adsorption/desorption cycling of <i>n</i>-butane at 298 K/1 bar with increasing desorption time allows to determine the pore structure of the materials in more detail due to different equilibration times. Adsorption of the organic test gas at selected relative pressures reveals specific contributions of particular pore systems to the increase of the temperature of the samples and different adsorption mechanisms. The use of carbon dioxide as the test gas at 298 K/1 bar provides detailed insights into the ultramicropore structure of the materials because under these conditions the adsorption of this test gas is very sensitive to the presence of pores smaller than 0.7 nm

Textural Characterization of Micro- and Mesoporous Carbons Using Combined Gas Adsorption and <i>n</i>‑Nonane Preadsorption

Porous carbon and carbide materials with different structures were characterized using adsorption of nitrogen at 77.4 K before and after preadsorption of <i>n</i>-nonane. The selective blocking of the microporosity with <i>n</i>-nonane shows that ordered mesoporous silicon carbide material (OM-SiC) is almost exclusively mesoporous whereas the ordered mesoporous carbon CMK-3 contains a significant amount of micropores (∼25%). The insertion of micropores into OM-SiC using selective extraction of silicon by hot chlorine gas leads to the formation of ordered mesoporous carbide-derived carbon (OM-CDC) with a hierarchical pore structure and significantly higher micropore volume as compared to CMK-3, whereas a CDC material from a nonporous precursor is exclusively microporous. Volumes of narrow micropores, calculated by adsorption of carbon dioxide at 273 K, are in linear correlation with the volumes blocked by <i>n</i>-nonane. Argon adsorption measurements at 87.3 K allow for precise and reliable calculation of the pore size distribution of the materials using density functional theory (DFT) methods

In-Depth Investigation of the Carbon Microstructure of Silicon Carbide-Derived Carbons by Wide-Angle X‑ray Scattering

Polymer-based silicon carbide-derived carbons (Si-CDCs) synthesized at temperatures from 600 to 1500 °C using different templating methods were characterized by wide-angle x-ray scattering (WAXS), Raman spectroscopy, and transmission electron microscopy (TEM). A recently developed advanced algorithm for fitting the whole WAXS data curve of non-graphitic carbons, that is, carbons with a polyaromatic sp² structure, revealed fine details about the CDC microstructure on the level of the graphene layers. In particular, this approach allowed the quantification of disorder effects in the graphene stacks and the clarification of the peculiarity of CDCs. It is seen that, contrary to other types of carbons, almost no stacking of the sp² layers occurs; that is, the stack height <i>L</i>_c is rather small (8 Å) and increases only slightly with higher synthesis temperatures, whereas the graphene layer extent <i>L</i>_a grows significantly, from 16 to 29 Å. Additionally, the microstructures of various types of CDCs were investigated: a hexagonal CDC and a cubic ordered mesoporous CDC, as well as a macroporous CDC. The WAXS analysis reveals that soft-templated CDCs, featuring macroporosity with an average wall thickness of hundreds of nm, show a more uniform stacking order than mesoporous CDCs obtained by hard-templating, with an average wall thickness of a few nanometers

Imine-Linked Polymer-Derived Nitrogen-Doped Microporous Carbons with Excellent CO₂ Capture Properties

A series of nitrogen-doped microporous carbons (NCs) was successfully prepared by direct pyrolysis of high-surface-area microporous imine-linked polymer (ILP, 744 m²/g) which was formed using commercial starting materials based on the Schiff base condensation under catalyst-free conditions. These NCs have moderate specific surface areas of up to 366 m²/g, pore volumes of 0.43 cm³/g, narrow micropore size distributions, and a high density of nitrogen functional groups (5.58–8.74%). The resulting NCs are highly suitable for CO₂ capture adsorbents because of their microporous textural properties and large amount of Lewis basic sites. At 1 bar, NC-800 prepared by the pyrolysis of ILP at 800 °C showed the highest CO₂ uptakes of 1.95 and 2.65 mmol/g at 25 and 0 °C, respectively. The calculated adsorption capacity for CO₂ per m² (μmol of CO₂/m²) of NC-800 is 7.41 μmol of CO₂/m² at 1 bar and 25 °C, the highest ever reported for porous carbon adsorbents. The isosteric heats of CO₂ adsorption (<i>Q</i>_st) for these NCs are as high as 49 kJ/mol at low CO₂ surface coverage, and still ∼25 kJ/mol even at high CO₂ uptake (2.0 mmol/g), respectively. Furthermore, these NCs also exhibit high stability, excellent adsorption selectivity for CO₂ over N₂, and easy regeneration and reuse without any evident loss of CO₂ adsorption capacity

Stretchable and Semitransparent Conductive Hybrid Hydrogels for Flexible Supercapacitors

Conductive polymers showing stretchable and transparent properties have received extensive attention due to their enormous potential in flexible electronic devices. Here, we demonstrate a facile and smart strategy for the preparation of structurally stretchable, electrically conductive, and optically semitransparent polyaniline-containing hybrid hydrogel networks as electrode, which show high-performances in supercapacitor application. Remarkably, the stability can extend up to 35 000 cycles at a high current density of 8 A/g, because of the combined structural advantages in terms of flexible polymer chains, highly interconnected pores, and excellent contact between the host and guest functional polymer phase

Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB₂ Nanoparticles

Recent years have seen an increasing research effort focused on nanoscaling of metal borides, a class of compounds characterized by a variety of crystal structures and bonding interactions. Despite being subject to an increasing number of studies in the application field, comprehensive studies of the size-dependent structural changes of metal borides are limited. In this work, size-dependent microstructural analysis of the VB₂ nanocrystals prepared by means of a size-controlled colloidal solution synthesis is carried out using X-ray powder diffraction. The contributions of crystallite size and strain to X-ray line broadening is separated by introducing a modified Williamson–Hall method taking into account different reflection profile shapes. For average crystallite sizes smaller than ca. 20 nm, a remarkable increase of lattice strain is observed together with a significant contraction of the hexagonal lattice decreasing primarily the cell parameter <i>c</i>. Exemplary density-functional theory calculations support this trend. The size-dependent lattice contraction of VB₂ nanoparticles is associated with the decrease of the interatomic boron distances along the <i>c</i>-axis. The larger fraction of constituent atoms at the surface is formed by boron atoms. Accordingly, lattice contraction is considered to be a surface effect. The anisotropy of the size-dependent lattice contraction in VB₂ nanocrystals is in line with the higher compressibility of its macroscopic bulk structure along the <i>c</i>-axis revealed by theoretical calculations of the respective elastic properties. Transmission electron microscopy indicates that the VB₂ nanocrystals are embedded in an amorphous matrix. X-ray photoelectron spectroscopy analysis reveals that this matrix is mainly composed of boric acid, boron oxides, and vanadium oxides. VB₂ nanocrystals coated with these oxygen containing amorphous species are stable up to 789 °C as evidenced by thermal analysis and temperature dependent X-ray diffraction measurements carried out under Ar atmosphere. Electrokinetic measurement indicates that the aqueous suspension of VB₂ nanoparticles with hydroxyl groups on the surface region has a good stability at neutral and basic pH arising from electrostatic stabilizatio

Structural Characterization of Micro- and Mesoporous Carbon Materials Using In Situ High Pressure ¹²⁹Xe NMR Spectroscopy

In situ high pressure ¹²⁹Xe NMR spectroscopy in combination with volumetric adsorption measurements were used for the textural characterization of different carbon materials with well-defined porosity including microporous carbide-derived carbons, ordered mesoporous carbide-derived carbon, and ordered mesoporous CMK-3. Adsorption/desorption isotherms were measured also by NMR up to relative pressures close to <i>p</i>/<i>p</i>₀ = 1 at 237 K. The ¹²⁹Xe NMR chemical shift of xenon adsorbed in porous carbons is found to be correlated with the pore size in analogy to other materials such as zeolites. In addition, these measurements were performed loading the samples with <i>n</i>-nonane. Nonane molecules preferentially block the micropores. However, ¹²⁹Xe NMR spectroscopy proves that the nonane also influences the mesopores, thus providing information about the pore system in hierarchically structured materials

Self-Supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels as Highly Active and Durable Electrocatalysts

Developing electrocatalysts with low cost, high activity, and good durability is urgently demanded for the wide commercialization of fuel cells. By taking advantage of nanostructure engineering, we fabricated PtAg nanotubular aerogels (NTAGs) with high electrocatalytic activity and good durability via a simple galvanic replacement reaction between the in situ spontaneously gelated Ag hydrogel and the Pt precursor. The PtAg NTAGs have hierarchical porous network features with primary networks and pores from the interconnected nanotubes of the aerogel and secondary networks and pores from the interconnected thin nanowires on the nanotube surface, and they show very high porosities and large specific surface areas. Due to the unique structure, the PtAg NTAGs exhibit greatly enhanced electrocatalytic activity toward formic acid oxidation, reaching 19 times higher metal-based mass current density as compared to the commercial Pt black. Furthermore, the PtAg NTAGs show outstanding structural stability and electrochemical durability during the electrocatalysis. Noble metal-based NTAGs are promising candidates for applications in electrocatalysis not only for fuel cells, but also for other energy-related systems