8 research outputs found
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
sp<sup>2</sup> 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 sp<sup>2</sup> layers occurs; that is, the stack height <i>L</i><sub>c</sub> is rather small (8 Å) and increases only slightly
with higher synthesis temperatures, whereas the graphene layer extent <i>L</i><sub>a</sub> 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 CO<sub>2</sub> 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 m<sup>2</sup>/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 m<sup>2</sup>/g, pore volumes of 0.43 cm<sup>3</sup>/g, narrow
micropore size distributions, and a high density of nitrogen functional
groups (5.58–8.74%). The resulting NCs are highly suitable
for CO<sub>2</sub> 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 CO<sub>2</sub> uptakes of 1.95 and 2.65 mmol/g at 25 and 0
°C, respectively. The calculated adsorption capacity for CO<sub>2</sub> per m<sup>2</sup> (μmol of CO<sub>2</sub>/m<sup>2</sup>) of NC-800 is 7.41 μmol of CO<sub>2</sub>/m<sup>2</sup> at
1 bar and 25 °C, the highest ever reported for porous carbon
adsorbents. The isosteric heats of CO<sub>2</sub> adsorption (<i>Q</i><sub>st</sub>) for these NCs are as high as 49 kJ/mol at
low CO<sub>2</sub> surface coverage, and still ∼25 kJ/mol even
at high CO<sub>2</sub> uptake (2.0 mmol/g), respectively. Furthermore,
these NCs also exhibit high stability, excellent adsorption selectivity
for CO<sub>2</sub> over N<sub>2</sub>, and easy regeneration and reuse
without any evident loss of CO<sub>2</sub> 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 VB<sub>2</sub> 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 VB<sub>2</sub> 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 VB<sub>2</sub> 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 VB<sub>2</sub> 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 VB<sub>2</sub> 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. VB<sub>2</sub> 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 VB<sub>2</sub> 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 <sup>129</sup>Xe NMR Spectroscopy
In
situ high pressure <sup>129</sup>Xe 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><sub>0</sub> = 1 at 237 K.
The <sup>129</sup>Xe 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, <sup>129</sup>Xe 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