Abstract: A detailed study is given on the synthesis of a hierarchical porous carbon, possessing both meso- and macropores, using a mesophase pitch (MP) as the carbon precursor. This carbon material is prepared by the nanocasting approach involving the replication of a porous silica monolith (hard templating). While this carbon material has already been tested in energy storage applications, various detailed aspects of its formation and structure are addressed in this study. Scanning electron microscopy (SEM), Hg porosimetry and N2 physisorption are used to characterize the morphology and porosity of the carbon replica. A novel approach for the detailed analysis of wide-angle x-ray scattering (WAXS) from non-graphitic carbons is applied to quantitatively compare the graphene microstructures of carbons prepared using MP and furfuryl alcohol (FA). This WAXS analysis underlines the importance of the carbon precursor in the synthesis of templated porous carbon materials via the nanocasting route. Our study demonstrates that a mesophase pitch is a superior precursor whenever a high-purity, low-micropore-content and well-developed graphene structure is desired.
Abstract: Carbon materials with defined porosity are prepared using the nanocasting approach. The structural properties of the prepared carbon materials are examined by SEM, XRD, XPS, elemental analysis, nitrogen physisorption and Hg porosimetry. The materials exhibit an interconnected porous network with spherical pores in the macropore range, being a replica of spherical SiO2 particles. The average macropore size (300 nm – 700 nm) and surface area (35 m2g−1 – 470 m2g−1) can be tailored by choice of template and carbon precursor. More importantly, based on silica templates prepared by flame pyrolysis, the whole process, including HF etching of the template, can be easily industrialized. Lithium storage measurements are used to demonstrate the beneficial transport properties of the porous carbon materials which are referenced against non-porous carbons. The porous carbon materials exhibit high capacity (550 mAhg−1 at C/5) and excellent rate capability (90 mAhg−1 at 60C). Surprisingly, the excellent lithium storage properties are related to the macroporous framework rather than high surface area and/or micro- and mesoporosity.
Abstract: The safe and efficient storage of hydrogen is still one of the remaining challenges towards fuel cell powered cars. Metal hydrides are a promising class of materials as they allow the storage of large amounts of hydrogen in a small volume at room temperature and low pressures. However, usually the kinetics of hydrogen release and uptake and the thermodynamic properties do not satisfy the requirements for practical applications. Therefore current research focuses on catalysis and the thermodynamic tailoring of metal hydride systems. Surprisingly, carbon materials used as additive or support are very effective to improve the hydrogen storage properties of metal hydrides allowing fast kinetics and even a change in the thermodynamic properties. Even though the underlying mechanisms are not always well understood, the beneficial effect is probably related to the peculiar structure of the carbon materials. This feature article gives an introduction to the different carbon materials, an overview of the preparation strategies to synthesize carbon/hydride nanocomposites, and highlights the beneficial effect of carbon by discussing two important hydrides: MgH2 and NaAlH4.
Abstract: Current kinetic limitations of carbon anode materials in sodium-ion batteries can be effectively tackled by using tailor-made carbon materials with hierarchical porosity prepared via the nanocasting route. Capacities exceeding 100 mA h g−1 at C/5 are found while exhibiting excellent rate capability and reasonable cycle life.
Abstract: Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH4 is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H2. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH4 particle size and the presence of a carbon support. We focused on NaAlH4/porous carbon nanocomposites prepared by melt infiltration. The NaAlH4 was confined in the mainly 2−3 nm pores of the carbon, resulting in a lack of long-range order in the NaAlH4 structure. The hydrogen release profile was modified by contact with the carbon; even for 10 nm NaAlH4 on a nonporous carbon material the decomposition of NaAlH4 to NaH, Al, and H2 now led to hydrogen release in a single step. This was a kinetic effect, with the temperature at which the hydrogen was released depending on the NaAlH4 feature size. However, confinement in a nanoporous carbon material was essential to not only achieve low H2 release temperatures, but also rehydrogenation at mild conditions (e.g., 24 bar H2 at 150 °C). Not only had the kinetics of hydrogen sorption improved, but the thermodynamics had also changed. When hydrogenating at conditions at which Na3AlH6 would be expected to be the stable phase (e.g., 40 bar H2 at 160 °C), instead nanoconfined NaAlH4 was formed, indicating a shift of the NaAlH4↔Na3AlH6 thermodynamic equilibrium in these nanocomposites compared to bulk materials.
Abstract: Lithium borohydride (LiBH4) is a promising material for hydrogen storage, with a gravimetric hydrogen content of 18.5%. However, the thermodynamics and kinetics of its hydrogen release and uptake need to be improved before it can meet the requirements for mobile applications. In this study, we investigate the confinement of LiBH4 in ordered mesoporous SiO2 and its effect on the hydrogen sorption properties. We demonstrate that, only under hydrogen pressure, melt infiltration is an effective method for the synthesis of LiBH4/SBA-15 nanocomposites. Our work clearly shows that formation of lithium silicates from LiBH4 and SiO2 can effectively be suppressed by hydrogen. Thus, under hydrogen pressure, LiBH4 can fully fill the mesopores of SBA-15 while the long-range order of the mesopores is maintained. The confined LiBH4 has enhanced hydrogen desorption properties, with desorption starting at 150 °C. However, upon dehydrogenation, SiO2 and decomposition products of LiBH4 react to form Li2SiO3 and Li4SiO4, leading to irreversible hydrogen loss.
Abstract: Structural properties of NaAlH4/C nanocomposites were studied using 23Na and 27Al solid-state NMR. The samples were synthesized by melt infiltration of a highly porous carbon support, with typical pore sizes of 2−3 nm. Physical mixtures of high surface carbon with alanates in different stages of hydrogen desorption show somewhat broadened resonances and a small negative chemical shift compared to pure alanates. This is most likely caused by a susceptibility effect of the carbon support material, which shields and distorts the applied magnetic field. After melt infiltration, 23Na and 27Al spectra are broadened with a small downfield average shift, which is mainly caused by a chemical shift distribution and is explained by a larger disorder in the nanoconfined materials and a possible charge transfer to the carbon. Our measurements show that the local structure of the nanoconfined alanate is the similar to bulk alanate because a comparable chemical shift and average quadrupolar coupling constant is found. In contrast to bulk alanates, in partly desorbed nanocomposite samples no Na3AlH6 is detected. Together with a single release peak observed by dehydrogenation experiments, this points toward a desorption in one single step. 23Na spectra of completely desorbed NaAlH4/C and NaH/C nanocomposites confirm the formation of metallic sodium at lower temperatures than those observed for bulk alanates. The structural properties observed with solid-state NMR of the nanoconfined alanate are restored after a rehydrogenation cycle. This demonstrates that the dehydrogenation of the NaAlH4/C nanocomposite is reversible, even without a Ti-based catalyst.
Abstract: Hydrogen is expected to play an important role as an energy carrier in a future, more sustainable society. However, its compact, efficient, and safe storage is an unresolved issue. One of the main options is solid-state storage in hydrides. Unfortunately, no binary metal hydride satisfies all requirements regarding storage density and hydrogen release and uptake. Increasingly complex hydride systems are investigated, but high thermodynamic stabilities as well as slow kinetics and poor reversibility are important barriers for practical application. Nanostructuring by ball-milling is an established method to reduce crystallite sizes and increase reaction rates. Since five years attention has also turned to alternative preparation techniques that enable particle sizes below 10 nanometers and are often
used in conjunction with porous supports or scaffolds. In this Review we discuss the large impact of nanosizing and -confinement on the hydrogen sorption properties of metal hydrides. We illustrate possible preparation strategies, provide insight into the reasons for changes in kinetics, reversibility and thermodynamics, and highlight important progress in this field. All in all we provide the reader with a clear view of how nanosizing and -confinement can beneficially affect the hydrogen sorption properties of the most prominent materials that
are currently considered for solid-state hydrogen storage.
Abstract: In the search for suitable solid state hydrogen storage systems, NaAlH4 (7.4 wt % H2) holds great promise due to its suitable thermodynamical properties. However, hydrogen release and uptake are hampered by high activation energies, most likely due to solid state mass transfer limitations. A recent strategy to improve the hydrogen de- and rehydrogenation properties of NaAlH4 is to reduce the particle size to the nanometer scale. We prepared high loadings of nanosized NaAlH4 confined in the pores of a carbon support by melt infiltration. XRD, nitrogen physisorption, high pressure DSC and solid-state NMR are used to evidence that the molten NaAlH4 infiltrates the carbon support, and forms a nanosized NaAlH4 phase lacking long-range order. The confined NaAlH4 shows enhanced hydrogen dehydrogenation properties and rehydrogenation under mild conditions that is attributed to the nanosize and close contact to the carbon matrix.
Abstract: A porous carbon/silicon nanocomposite was synthesized in a one-step procedure based on a soft-templating methodology, taking advantage of phase separation between mesophase-pitch and organic polymers as soft templates. The resulting nanocomposite exhibits a highly stable reversible capacity of 450 mA h g-1 in a vinylene carbonate-containing electrolyte.
Abstract: Magnesium (hydride) is a promising system for the reversible on-board storage of hydrogen, but suffers from slow sorption kinetics and a high thermodynamic stability of the hydride. We explored a combined approach to tackle these problems: nanosizing and carbon-supporting the magnesium, and doping it with nickel. Samples were prepared by melt infiltration with magnesium of nanoporous carbon onto which 1-12 wt% nickel nanoparticles had been predeposited. For loadings up to 15 wt% MgH2, 10-30 nm crystallites with different compositions were formed inside the porous carbon, each giving a specific H-2 desorption signature. Surprisingly, higher Mg loadings resulted in more homogeneously mixed samples, which was due to the facilitated wetting of the carbon with the magnesium due to the presence of nickel. Hydrogen release temperatures close to that of Mg2NiH4 were observed for high MgH2 loadings (50 wt%) and small amounts of Ni (Mg0.95Ni0.05). The favourable H-2 desorption properties could mainly be attributed to excellent kinetics due to the efficient mixing of magnesium, nickel and carbon on the nanoscale.
Abstract: Hollow carbon fibers with a defined tubular cross-section were obtained by simply heating poly(styrene) in the presence of iron salts. The structures possess a high content of mesoporosity and macroporosity. The carbon itself is characterized by a well-developed sp(2)-graphene structure. When used as anode material in lithium ion batteries, a remarkably high reversible capacity as high as 860 mAh/g was observed, which is attributed to both the morphology of the carbon and the incorporation of iron species.
Abstract: The kinetics and thermodynamics for the reversible hydrogen desorption from NaH (and NaH derived from NaAlH4) are greatly improved by nanosizing and providing close contact to a porous carbon matrix.
Abstract: A novel method for the preparation of hierarchically porous LiFePO4 electrode materials for lithium ion batteries has been investigated. A meso/macroporous carbon monolith, a conductive framework, was prepared and infiltrated with the LiFePO4 precursors to increase the electrode/electrolyte interface and improve the rate capability of the battery. The final LiFePO4/carbon monoliths feature a meso/macroporous hierarchical structure. The monoliths were calcined at increasing temperatures, from 650 to 800 °C, to determine the structural and sintering effects on the electrochemical properties of the materials. The samples were characterized using SEM, TEM, nitrogen sorption, and XRD analysis prior to electrochemical testing. The results showed that the capacity of the LiFePO4/carbon electrodes achieved 82% of the theoretical capacity at 0.1C discharge rate.
Abstract: A high-performance polyaniline electrode was prepared by potentiostatic deposition of aniline on a hierarchically porous carbon monolith (HPCM), which was carbonized from the mesophase pitch. A capacitance value as high as 2200 F g(-1) (per weight of polyaniline) is obtained at a power density of 0.47 kW kg(-1) and an energy density of 300 W h kg(-1). This active material deposited on HPCM also has the advantageous of high stability. These properties can be essentially attributed to the backbone role of HPCM. The method also has the advantage of a topology that is favorable for kinetics at high power densities, thus, contributing to the increase of ionic conductivity and power density. There is also no need for a binder, which not only lowers the preparation costs but also offers advantages in terms of stability and performance.
Abstract: In this paper, we report on Li storage in hierarchically porous carbon monoliths with a relatively higher graphite-like ordered carbon structure. Macroscopic carbon monoliths with both mesopores and macropores were successfully prepared by using meso-/macroporous silica as a template and using mesophase pitch as a precursor. Owing to the high porosity (providing ionic transport channels) and high electronic conductivity (ca. 0.1 S cm(-1)), this porous carbon monolith with a mixed conducting 3D network shows a superior high-rate performance if used as anode material in electrochemical lithium cells. A challenge for future research as to its applicability in batteries is the lowering of the irreversible capacity.
Abstract: A novel strategy for the synthesis of hierarchical porous carbon is presented here based on the spinodal phase separation of mesophase pitch, which acts as a carbon precursor, and a soft polymer template. A continuous macropore structure and mesopores are obtained upon carbonization, as shown in the figure. The material exhibits remarkable performance as the anode material in lithium-ion batteries.
Abstract: Porous silicon carbide monoliths were obtained using the infiltration of preformed SiO2 frameworks with appropriate carbon precursors such as mesophase pitch. The initial SiO2 monoliths possessed a hierarchical pore system, composed of an interpenetrating bicontinuous macropore structure and 13 nm mesopores confined in the macropore walls. After carbonization, further heat treatment at ca. 1400 degrees C resulted in the formation of a SiC-SiO2 composite, which was converted into a porous SiC monolith by post-treatment with ammonium fluoride solution. The resulting porous SiC featured high crystallinity, high chemical purity and showed a surface area of 280 m(2) g(-1) and a pore volume of 0.8 ml g(-1).
Abstract: Owing to the limited resources of fossil fuels, hydrogen is proposed as an alternative and environment-friendly energy carrier. However, its potential is limited by storage problems, especially for mobile applications. Current technologies, as compressed gas or liquefied hydrogen, comprise severe disadvantages and the storage of hydrogen in light-weight solids could be the solution to this problem.
Since the optimal storage mechanism and optimal material have yet to be identified, this first handbook on the topic provides an excellent overview of the most probable candidates, highlighting both their advantages as well as drawbacks.
From the contents:
* Physisorption
* Clathrates
* Metal hydrides
* Complex hydrides
* Amides, imides, and mixtures
* Destabilized systems
* Borazan
* Aluminum hydride
* Nanoparticles
A one-stop reference on all questions concerning hydrogen storage for physical and solid state chemists, materials scientists, chemical engineers, and physicists.
Abstract: The aim of this work was the generation of carbon materials with high surface area, exhibiting a hierarchical pore system in the macro- and mesorange. Such a pore system facilitates the transport through the material and enhances the interaction with the carbon matrix (macropores are pores with diameters > 50 nm, mesopores between 2 – 50 nm).
Thereto, new strategies for the synthesis of novel carbon materials with designed porosity were developed that are in particular useful for the storage of energy.
Besides the porosity, it is the graphene structure itself that determines the properties of a carbon material. Non-graphitic carbon materials usually exhibit a quite large degree of disorder with many defects in the graphene structure, and thus exhibit inherent microporosity (d < 2nm). These pores are traps and oppose reversible interaction with the carbon matrix. Furthermore they reduce the stability and conductivity of the carbon material, which was undesired for the proposed applications.
As one part of this work, the graphene structures of different non-graphitic carbon materials were studied in detail using a novel wide-angle x-ray scattering model that allowed precise information about the nature of the carbon building units (graphene stacks). Different carbon precursors were evaluated regarding their potential use for the synthesis shown in this work, whereas mesophase pitch proved to be advantageous when a less disordered carbon microstructure is desired.
By using mesophase pitch as carbon precursor, two templating strategies were developed using the nanocasting approach. The synthesized (monolithic) materials combined for the first time the advantages of a hierarchical interconnected pore system in the macro- and mesorange with the advantages of mesophase pitch as carbon precursor.
In the first case, hierarchical macro- / mesoporous carbon monoliths were synthesized by replication of hard (silica) templates. Thus, a suitable synthesis procedure was developed that allowed the infiltration of the template with the hardly soluble carbon precursor.
In the second case, hierarchical macro- / mesoporous carbon materials were synthesized by a novel soft-templating technique, taking advantage of the phase separation (spinodal decomposition) between mesophase pitch and polystyrene. The synthesis also allowed the generation of monolithic samples and incorporation of functional nanoparticles into the material.
The synthesized materials showed excellent properties as an anode material in lithium batteries and support material for supercapacitors.
Abstract: Mesoporous particles are a novel, promising material for new applications mainly due to their
tuneable and well defined, i.e. ordered, pore structure. Since a substantial fraction of the
mesopores is accessible from the outside, these particles can be functionalised and used as a
carrier for other materials such as polymers, proteins or nanocrystals.
There is a great interest in the development of simple and flexible techniques to build up larger 2D or 3D structures of mesoporous particles. Ink jet printing is an attractive way to print any kind of two-dimensional structures containing these particles on any kind of substrate. The technique is economical, fast and versatile and enhances the industrial applicability of this new class of material.
We developed a manufacturing process to produce inks containing spherical mesoporous silica particles in order to print 2D structures.
Starting from rheological measurements on the inks, we found a maximum solids content of
77vol% for the used particles in water. Surface tension and colloidal stability were also examined.
A filter process was developed in order to remove agglomerates and oversized particles.
Stroboscopy technique was used to examine the drop formation and printing performance. The
inks were printed with a piezo drop-on-demand (DOD) ink jet printer.
We developed ink formulations which allow us to print structures smaller than 200μm consisting of mesoporous particles alone or the mesoporous particles and a binder which firmly holds the particles on the substrate. Electron microscopy was used to examine the printed structures.
Abstract: The present invention relates to a method based on phase separation for the production of porous carbon monoliths, the monoliths produced according to the invention and the use thereof.
Abstract: An extremely high-performance polyaniline electrode was prepared by potentiostatic deposition of aniline on hierarchically porous carbon monolith (HPCM), which was carbonized from mesophase pitch. A capacitance value of 2200 F g-1 of polyaniline was obtained at a power density of 0.47 kW kg-1 and an energy density of 300 Wh kg-1. This active material deposited on HPCM also has the advantageous of high stability. These superior advantages can be attributed to the backbone role of HPCM. This method also has the advantages of not introducing any binder, thus contributing to the increase of ionic conductivity and power density. High specific capacitance, high power and energy density, high stability, and low cost of active material make it very promising for supercapacitors.
Abstract: This disclosure relates to a porous electrically conductive carbon material having interconnected pores in first and second size ranges from 10µm to 100nm and from less than 100nm to 3nm and a graphene structure and to diverse uses of the material such as an electrode in a lithium-ion battery and a catalyst support, e.g. for the oxidation of methanol in a fuel cell. The carbon material has been heat treated to effect conversion to non-graphitic carbon with the required degree of order at a temperature in the range from 600°C to 1000°C. A lithium-ion battery and an electrode for a lithium-ion battery are also claimed.
