Silicon Carbide-derived Carbon Research Articles

Silicon Carbide-Derived Carbon Prepared by Fused Salt Electrolysis and Electrochemical Performance

A number of carbide-derived carbon (CDC) samples were successfully synthesized by the electrolysis of SiC powder in molten CaCl2. The electrolysis was conducted at different temperatures (850, 900, and 950°C) for 48 h in argon at an applied constant voltage of 3.1 V. The structure of the resulting carbon is characterized by X-ray diffraction, Raman spectroscopy, and transmission electron microscope techniques. Cyclic voltammetry and galvanostatic charge/discharge measurements are applied to investigate electrochemical performances of the SiC-CDC material. It can be seen that the degree of order of the SiC-CDC increases monotonically along with elevation of reaction temperature, while the highest specific surface area 1137.74 m2/g together with a specific capacitance of 161.27 F/g at a current density 300 mA/g was achieved from sample synthesized at 900°C.

High-power supercapacitors based on hierarchical porous nanometer-sized silicon carbide-derived carbon

The nanoscale microporous carbide-derived carbon (nano-CDC) is synthesized by chlorination of silicon carbide nano-powder with a particle diameter around 60nm and further pore-tuned by KOH activation with different KOH/nano-CDC ratios. Based on the higher specific surface area (SSA), a hierarchical micro- and meso-pore structure (especially for the greatly produced mesopores), and the shorter inherent ion transport distance within porous nano-carbons, the KOH-activated nano-CDC exhibits superior supercapacitive performances. Its specific capacitance is up to 141Fg−1, 156% increase compared with that of pristine nano-CDC (54Fg−1). Most interestingly, the cyclic voltammogram curve of the activated nano-CDC can keep a rectangular-like shape even at a scan rate of 5000mVs−1, exhibiting significantly better power performance. This work confirms that constructing favorable pore structure in nanometer-sized porous carbons is an effective strategy for fabricating high-power supercapacitors.

Effect of fluorine doping on structure and CO2 adsorption in silicon carbide-derived carbon

We investigate the effect of fluorine-functionalisation of nanoporous silicon carbide-derived carbon (SiCDC), on its structural as well as adsorption properties, with the aim of assessing its suitability for CO2 capture. The morphology and structure of samples fluorinated to three different F/C ratios are characterized by several analysis techniques, as well as gas adsorption. Fluorination is seen to have a stabilising effect on SiCDC, and stronger C–F bonds are formed at high levels of fluorination. Further, increasing fluorination level leads to a decrease of specific surface area and total pore volume, consistent with recent simulation results from this laboratory. Fluorination has little effect on the ultra-microporosity at low levels of fluorination, but leads to significant decrease at high levels of fluorination. Sub-atmospheric pressure CO2 adsorption kinetics is interpreted using a bidisperse pore structure model, considering particle scale diffusion in large micropores and local grain scale diffusion with an interfacial barrier in ultra-micropores. The comparison of the CO2 uptake-time curves for the fluorinated and non-fluorinated samples shows slightly slower uptake with increasing fluorination level, largely due to decrease in pore volume and surface area. The isosteric heat of adsorption and activation energy barriers both decrease mildly with increased level of fluorination.

Fluorinated carbide-derived carbon: more hydrophilic, yet apparently more hydrophobic.

We explore the effect of fluorine doping on hydrophobicity of nanoporous silicon carbide-derived carbon (SiCDC), and investigate the underlying barriers for adsorption and diffusion of water vapor and CO2 in the fluorinated and nonfluorinated structures. We develop atomistic models of fluorine-doped SiCDC at three different levels of fluorination, based on a hybrid reverse Monte Carlo constructed model of SiCDC, and develop a novel first-principles force field for the simulation of adsorption and transport of water and CO2 in the fluorine-doped carbon materials. We demonstrate an apparent dual effect of fluorination, showing that while fluorination generates more hydrophilic carbon surfaces, they actually act as more hydrophobic structures due to enhanced energy barriers in the disordered network of microporous carbon. While an increase in adsorption energy and in water uptake is seen for fluorine-doped carbon, large internal free energy barriers as well as the results of MD simulations demonstrate that the increased adsorption is kinetically limited and not experimentally observable on practical time scales. We show that an increase in apparent hydrophobicity due to fluorination is mediated by larger free energy barriers arising from stronger binding of fluid molecules inside the pore network, as opposed to repulsion or steric hindrance to the diffusion of molecules through narrow pore entries. For carbon dioxide, adsorption enthalpies and activation energy barriers are both decreased on fluorination, indicating weakened solid-fluid binding energies in the fluorinated systems.

Barriers to diffusion of CO2 in microporous carbon derived from silicon carbide

We investigate the adsorption equilibrium and uptake kinetics of CO2 in nanoporous silicon carbide-derived carbon (SiC-DC) experimentally, as well as by simulation through grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) simulations using a hybrid reverse Monte Carlo (HRMC) simulation-based atomistic structural model of the carbon. The kinetics is best explained when a grain surface barrier resistance mechanism is incorporated in a bidisperse pore structure model, considering particle scale diffusion in large micropores and a local grain scale diffusion in ultra-micropores. Good agreement is found between simulated and experimental isotherms; however, experimental particle scale diffusivities are almost two orders of magnitude smaller than those obtained from EMD, suggesting the presence of long range barriers for CO2 not captured by the HRMC model structure. These barriers also lead to about two orders of magnitude reduction in the particle-scale diffusivity of CO2 compared to CH4. On the other hand activation barriers for grain scale diffusion of CO2 are comparable to those for CH4 in the SiC-DC, and in carbon molecular sieves, consistent with the ultra-microporous nature of the grains. The activation barrier for interfacial mass transfer coefficient at the grain surface is also consistent with values for carbon molecular sieves.

Preparation and Structure Regulation of Silicon Carbide-Derived Carbon/ Spherical Natural Graphite Composites

To meet the requirements of the energy storage materials for high energy density and high power density, porous silicon carbide/derived carbon-spherical natural graphite (SiC-CDCs@NG) composites were prepared. The composites were composed of tailored porous carbide-derived carbon and graphitized carbon with excellent conductivity and a high energy storage capacity. The composition, structure, morphology, pore structure, and specific surface area of the composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and N2 adsorption/ desorption analysis. The composites exhibited a high and adjustable specific surface area and micro-pore volume, with a micro-pore size of between 0.5 and 0.7 nm. Varying the molar ratio of NG and Si allowed optimization of the micro-pore volume, pore size distribution, specific surface area, and composition and content of the CDCs shell and NG core.

Impact of H2O on CO2 Separation from Natural Gas: Comparison of Carbon Nanotubes and Disordered Carbon

The adsorption of pure H2O, CO2/CH4, and H2O/CO2/CH4 mixtures in carbon nanotubes (CNTs) ranging from (6,6) to (15,15), and in a reconstructed model of a silicon carbide derived carbon (SiC-DC), was studied using Monte Carlo simulations, at 300 K. We also investigated the effect of preadsorbed water and the choice of water model including SPC, SPC/E, TIP3P, TIP4P, and TIP5P, on the adsorption of CO2/CH4 and H2O/CO2/CH4 mixtures in the CNTs and SiC-DC. Using the SPC model, our simulations reveal that, below saturation pressure, the adsorption of water is negligible in CNTs with diameters ranging from 0.81 to 2.03 nm, and in the SiC-DC. Water fills the CNTs and the SiC-DC model suddenly when the pressure reaches a critical value that is above the saturation pressure, and exhibits nonmonotonic variation with CNT diameter. It was also found that the small amount of water adsorbed from the saturated H2O/CO2/CH4 mixture has little impact on the adsorption of the components CO2 and CH4. When water is preadsorbed, it is found to be present in the form of hydrogen-bonded clusters, and the average water cluster size is reduced when the amount of adsorbed CO2 and CH4 is increased. For the adsorption of a CO2/CH4 mixture, the CO2 selectivities of the (7,7) CNT and SiC-DC increase almost linearly with the density of preadsorbed water, while the capacities of CH4 and CO2 in the narrow (7,7) CNT are reduced, and the capacity of CO2 in the SiC-DC is slightly enhanced by the preadsorbed water. It was found that the choice of water model has little impact on the adsorption of the CO2/CH4 mixtures in either carbon. Noticeable adsorption of H2O was observed for the H2O/CO2/CH4 mixture in the (7,7) CNT and SiC-DC having 0.05 g/cm3 preadsorbed water, as a result of strong hydrogen bonding between the preadsorbed water and water molecules from the gas phase. For the H2O/CO2/CH4 mixture, the CO2 selectivities of the (7,7) CNT and SiC-DC are subsequently enhanced by the additional cooperative water–adsorbate interactions. We conclude that, among the CNTs investigated, the (7,7) CNT shows the best performance for separating CO2 from natural gas at atmospheric pressure and is superior to the amorphous carbon, SiC-DC.

Differences in the adsorption and diffusion behaviour of water and non-polar gases in nanoporous carbon: role of cooperative effects of pore confinement and hydrogen bonding

We investigate the effect of pore confinement and molecular geometry on the adsorption and self-diffusion of H2O, CO2, Ar, CH4, C3H6, SF6 and C5H12, in a realistic model of nanoporous silicon carbide derived carbon (SiC-DC), constructed using hybrid reverse Monte Carlo simulation. Adsorption isotherms, adsorbate–adsorbate and adsorbate–adsorbent contributions to the isosteric heat of adsorption are determined to study the effect of pore confinement, microporosity and molecular geometry on adsorption of these gases. We describe the cooperative effect of pore confinement and hydrogen bonding on the formation of water clusters and anomalous adsorption behaviour of water compared with non-polar gases. We find that, in contrast to literature results based on the slit-pore model, pore-filling does not occur below the saturation pressure in hydrophobic amorphous carbon materials such as SiC-DC and activated carbon fibre. We also compare self-diffusivities and activation energy barriers of water and non-polar gases in the microporous structure of SiC-DC to identify underlying correlations with molecular properties. We demonstrate that the self-diffusivity of water deviates considerably from the correlation between diffusivity and molecular kinetic diameter observed for non-polar gases. This is attributed to the reduced diffusivity of water, and its relatively large energy barrier at high loadings despite its small kinetic diameter, which is due to the blocking effect of water clusters at pore entries.

Fabrication and CO2 capture performance of silicon carbide derived carbons from polysiloxane

Microporous silicon carbide derived carbons (SiC-DCs) were obtained by chlorination with post treatment in ammonia (NH3) using a commercially available polymethyl(phenyl)siloxane resin. In this paper, attentions were focused on the compositional and structural evolutions as well as the CO2 capture performances of SiC-DCs as a function of chlorination temperature or affected by post-treatment. Microscopy analysis showed that the geometry and macrostructure, as well as the observed graphite ribbons were finely inherited from the as-received silicon carbide. Post-treatment under NH3 could introduce a few nitrogen-contained groups to SiC-DCs, and has an advantage on increasing the ultra-microporosity with little effect on the carbon crystallinity, both of which finally help increasing the CO2 adsorption capacity. Increasing etching temperature (900–1100°C) would lead to a tiny increase of carbon crystallinity and porosity (specific surface area and total pore volume). However, the ultra-micropore volume decreased instead, thus leading to a decrease of CO2 adsorption capacity. The final SiC-DCs samples exhibit excellent CO2 capture performances at 0°C under the ambient pressure, with a maximum value of 5.75mmol/g. Under the same condition, the N2 adsorption is as low as 0.46mmol/g, suggesting a potential selective adsorbent for CO2 and N2 separation.

Micro- and Mesoporous Carbon - Selenium Composite Cathode for Lithium / Selenium Battery

The lithium/sulfur (Li/S) cells have demonstrated promising energy density values, exceeding that of the high energy density Li ion cells.[1-3] However, the majority of Li/S cells exhibit poor cycling performance, which is the key limiting factor for the commercialization of such a technology. One of the challenges is the dissolution of intermediate lithium polysulfide species formed during cycling, which induces a shuttle phenomenon. Low intrinsic electric conductivity of S and largely insoluble low order lithium polysulfides (such as Li2S, Li2S2) also deteriorate cycle stability. The somewhat similar lithium-selenium (Li/Se) system was recently found to offer higher rate performance even for the bulk Se.[4] Selenium is an environmentally friendly material, with high theoretical volumetric capacity of ~3260 mAh/cm3 (not considering expansion upon lithiation) and moderately high theoretical gravimetric capacity of 678 mAh/g based on molecular weight of 78.97 g/mol and density of 4.81 g/cm3. High volumetric capacity is particularly attractive for both EV and electronic device applications. Challenges with Li/Se systems are somewhat similar to that of Li/S systems, including polyselenide dissolution during the cell operation and limited (although an order of magnitude higher than in S) electronic conductivity. Several groups utilized porous carbon structure to entrap Se into conductive framework and showed promising results.[5, 6] This approach might be able to both reduce polyselenide dissolution and increase the electrical conductivity of selenium for higher power density.In this study, we have successfully prepared Se infiltrated ordered meso- and microporous silicon carbide derived carbon (CDC) composites with high Se contents (~60 wt. %). 2016 coin cells were assembled using a Li metal foil anode and Se-CDC cathodes with various electrolytes. The cells were tested at different C-rates of C/10, C/5, C/2, and 1C. The durability test was continued at C/5 for 150 cycles.The rate performance of Se-CDC cathodes showed a significant impact of electrolyte composition (Figure 1). The Se-CDC composite exhibits a stabilized discharge capacity of 457 mAh/gse within 3 cycles at 0.1 C charge/discharge rate. However, the cell with electrolytes 1 was slowly missing active material and stabilized only after ~15 cycles. Se utilization in Se-CDC cells was also affected by the electrolyte composition. However, the overall performance of all Li/Se-CDC cells in all electrolytes was rather stable compared to that of the similar Li/S-CDC cells. Very stable performance up to 150 cycles was demonstrated (Figure 2). After 150 cycles, the discharge capacity of the Se-CDC in electrolyte 3 retained approximately 72% of its theoretical capacity, while the same electrode in electrolyte 1 and electrolyte 2 showed relatively poor capacity retention of 39% and 33% respectively.In this talk we will discuss the key performance differences between Se and S cathode chemistries and offer our explanation to the improved Li-Se cell stability.ACKNOWLEDGEMENTSDifferent aspects of this work were supported by the Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korea government Ministry of Knowledge Economy (grant 20118510010030) and by the US Army Research Office (grant W911NF-12-1-0259). REFERENCES1. Ji, X., K.T. Lee, and L.F. Nazar, A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nature Materials, 2009. 8(6): p. 500-506.2. Lee, J.T., et al., Sulfur-Infiltrated Micro- and Mesoporous Silicon Carbide-Derived Carbon Cathode for High-Performance Lithium Sulfur Batteries. Advanced Materials, 2013. 25(33): p. 4573-4579.3. Song, M.-K., Y. Zhang, and E.J. Cairns, A Long-Life, High-Rate Lithium/Sulfur Cell: A Multifaceted Approach to Enhancing Cell Performance. Nano letters, 2013. 13(12): p. 5891-5899.4. Abouimrane, A., et al., A New Class of Lithium and Sodium Rechargeable Batteries Based on Selenium and Selenium–Sulfur as a Positive Electrode. Journal of the American Chemical Society, 2012. 134(10): p. 4505-4508.5. Luo, C., et al., Selenium@ Mesoporous Carbon Composite with Superior Lithium and Sodium Storage Capacity. ACS nano, 2013. 7(9): p. 8003-8010.6. Yang, C.P., et al., An Advanced Selenium–Carbon Cathode for Rechargeable Lithium–Selenium Batteries. Angewandte Chemie International Edition, 2013. 52(32): p. 8363-8367.

Adsorption of CH4 and CH4/CO2 mixtures in carbon nanotubes and disordered carbons: A molecular simulation study

We report a comparison of the adsorption of CH4 and CO2/CH4 mixtures of different composition in three different types of nanoporous carbons including carbon nanotubes, and activated carbon fibre (ACF-15) and silicon carbide derived carbon (SiC-DC) having distinctly different disordered structures, using Monte Carlo simulation. CO2 is represented as a linear molecule, and both the united-atom and full-atom models are investigated for CH4. It is found that the united-atom model of CH4 overestimates the adsorption of CH4 in all these adsorbents compared to the 5-site model, as a consequence of the enhanced 1-site CH4-adsorbent potential energy. Moreover, the selectivities of the nanoporous carbons for CO2 relative to CH4 calculated using the 1-site CH4 model are underestimated compared to those from the 5-site model, at pressures up to 3.0MPa. However, differences in the structural disorder of porous carbon models have little impact on CO2 selectivity. Our simulations reveal that the selectivity of an adsorbent for a particular species is strongly dependant on adsorbate–adsorbate interaction effects, comprising the adsorbate–adsorbate potential interactions and an adsorbate sieving effect. As a balance between the confinement and adsorbate–adsorbate effects, it is found that increasing the concentration of CO2 in the gas phase increases the selectivity of (10, 10) CNT dramatically, while having negligible impact on the selectivities in amorphous carbons. Further, it is shown that increasing the temperature reduces the performance of all the carbons in separating CO2, and that an isolated (7, 7) CNT has the best performance for CO2/CH4 separation in comparison to the disordered nanoporous carbons investigated.

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 Lc is rather small (8 Å) and increases only slightly with higher synthesis temperatures, whereas the graphene layer extent La 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.

Etching process of silicon carbide from polysiloxane by chlorine

The etching process of silicon carbide by dry chlorine was investigated as a function of etching temperature using a commercial available polymethyl(phenyl)siloxane resin. Results from etching rate show increasing etching temperature could lead to a change for etching mechanism from diffusion-controlling to interface reaction-controlling. The chlorination of β-SiC in this study should be managed at above 600°C and can complete at 900°C for 3h. A pronounced core–shell structure was observed owing to the partial conversion. Silicon carbide derived carbons (SiC-DCs) are in highly microporosity, with single-modal pore size distributions at around 0.9nm.

Slow diffusion of methane in ultra-micropores of silicon carbide-derived carbon

We investigate macroscopic uptake kinetics of CH4 in silicon carbide-derived carbon (SiC-DC). Ultra-microprosity in SiC-DC is found based on CO2 adsorption at 273K, but which has poor accessibility to Ar at 87K. The adsorption kinetics of CH4 is found to follow a bidisperse pore structure model, considering relatively rapid particle scale diffusion in large micropores, and a much slower local grain (or microparticle) scale diffusion in ultra-micropores. The grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The particle scale diffusivities compare well with the results of equilibrium molecular dynamics simulations using a hybrid reverse Monte Carlo simulation constructed model of SiC-DC, with similar activation energy. On the other hand microscopic quasi-elastic neutron scattering measurements are found to probe only short-range barriers with lower activation energy. It is anticipated that ultra-micropores will not make a significant contribution to the transport in any membrane or adsorption-based process based on SiC-DC, due to the extremely slow transport in these ultra-micropores and their small pore volume.

Influence of Structural Heterogeneity on Diffusion of CH4 and CO2 in Silicon Carbide-Derived Nanoporous Carbon.

We investigate the influence of structural heterogeneity on the transport properties of simple gases in a Hybrid Reverse Monte Carlo (HRMC) constructed model of silicon carbide-derived carbon (SiC-DC). The energy landscape of the system is determined based on free energy analysis of the atomistic model. The overall energy barriers of the system for different gases are computed along with important properties, such as Henry constant and differential enthalpy of adsorption at infinite dilution, and indicate hydrophobicity of the SiC-DC structure and its affinity for CO2 and CH4 adsorption. We also study the effect of molecular geometry, pore structure and energy heterogeneity considering different hopping scenarios for diffusion of CO2 and CH4 through ultramicropores using the Nudged Elastic Band (NEB) method. It is shown that the energy barrier of a hopping molecule is very sensitive to the shape of the pore entry. We provide evidence for the influence of structural heterogeneity on self-diffusivity of methane and carbon dioxide using molecular dynamics simulation, based on a maximum in the variation of self-diffusivity with loading. A comparison of the MD simulation results with self-diffusivities from quasi-elastic neutron scattering (QENS) measurements and, with macroscopic uptake-based low-density transport coefficients, reveals the existence of internal barriers not captured in MD simulation and QENS experiments. Nevertheless, the simulation and macroscopic uptake-based diffusion coefficients agree within a factor of 2–3, indicating that our HRMC model structure captures most of the important energy barriers affecting the transport of CH4 in the nanostructure of SiC-DC.

Silicon carbide-derived carbon nanocomposite as a substitute for mercury in the catalytic hydrochlorination of acetylene.

Acetylene hydrochlorination is an important coal-based technology for the industrial production of vinyl chloride, however it is plagued by the toxicity of the mercury chloride catalyst. Therefore extensive efforts have been made to explore alternative catalysts with various metals. Here we report that a nanocomposite of nitrogen-doped carbon derived from silicon carbide activates acetylene directly for hydrochlorination in the absence of additional metal species. The catalyst delivers stable performance during a 150 hour test with acetylene conversion reaching 80% and vinyl chloride selectivity over 98% at 200 °C. Experimental studies and theoretical simulations reveal that the carbon atoms bonded with pyrrolic nitrogen atoms are the active sites. This proof-of-concept study demonstrates that such a nanocomposite is a potential substitute for mercury while further work is still necessary to bring this to the industrial stage. Furthermore, the finding also provides guidance for design of carbon-based catalysts for activation of other alkynes.

Effects of structural properties of silicon carbide-derived carbons on their electrochemical double-layer capacitance in aqueous and organic electrolytes

High surface area silicon carbide-derived carbons (Si-CDCs) synthesized by chlorination of beta silicon carbide (βSiC) with two different particle sizes (6 μm and 50 nm) show different porosities with graphitic structure. Transmission electron microscopy, Raman spectroscopy and argon (Ar) and carbon dioxide (CO2) sorption analyses are used to examine the textural properties of the Si-CDCs. The results show that the particle size of the precursor affects the surface area and porosity of carbons. Furthermore, an additional heat treatment of the Si-CDC with 50-nm particle size for 24 h at 1,000 °C results in a collapse of the pore structure and reduces the surface area. The capacitive behaviours are investigated in H2SO4 and in tetraethyl ammonium tetrafluoroborate (TEABF4)/acetonitrile (AN). The electrochemical performance of the Si-CDCs is influenced by the particle size, surface area, pore volume and pore size distribution. The Si-CDCs exhibit capacitances in 1 M H2SO4 of up to 179 F g−1 and very stable charge–discharge performance over 5,000 cycles. This study shows the crucial importance of ultramicropores less than 1 nm combined with nanosized particles for achieving high capacitance in aqueous electrolyte. Moreover, the graphitic degree at the surface of the Si-CDCs enhances considerably the rate capability and stability in both electrolytes.

Ordered mesoporous silicon carbide-derived carbon for high-power supercapacitors

Micro/mesoporous carbon was prepared by chlorination of ordered mesoporous silicon carbide derived from magnesio-thermal reduction of templated carbon-silica precursors. These materials were then used as active materials for electrochemical capacitors and characterized in 1.5M NEt4BF4/AN. The electrodes showed outstanding rate capability (90% of capacity retention at 1V/s and time constant of 1s) with high specific areal capacitance (0.5F/cm2 of electrode), that makes such hierarchical porous carbons promising for high power and energy density supercapacitors.

Direct prediction of the desalination performance of porous carbon electrodes for capacitive deionization

Desalination by capacitive deionization (CDI) is an emerging technology for the energy- and cost-efficient removal of ions from water by electrosorption in charged porous carbon electrodes. A variety of carbon materials, including activated carbons, templated carbons, carbon aerogels, and carbon nanotubes, have been studied as electrode materials for CDI. Using carbide-derived carbons (CDCs) with precisely tailored pore size distributions (PSD) of micro- and mesopores, we studied experimentally and theoretically the effect of pore architecture on salt electrosorption capacity and salt removal rate. Of the reported CDC-materials, ordered mesoporous silicon carbide-derived carbon (OM SiC-CDC), with a bimodal distribution of pore sizes at 1 and 4 nm, shows the highest salt electrosorption capacity per unit mass, namely 15.0 mg of NaCl per 1 g of porous carbon in both electrodes at a cell voltage of 1.2 V (12.8 mg per 1 g of total electrode mass). We present a method to quantify the influence of each pore size increment on desalination performance in CDI by correlating the PSD with desalination performance. We obtain a high correlation when assuming the ion adsorption capacity to increase sharply for pore sizes below one nanometer, in line with previous observations for CDI and for electrical double layer capacitors, but in contrast to the commonly held view about CDI that mesopores are required to avoid electrical double layer overlap. To quantify the dynamics of CDI, we develop a two-dimensional porous electrode modified Donnan model. For two of the tested materials, both containing a fair degree of mesopores (while the total electrode porosity is ∼95 vol%), the model describes data for the accumulation rate of charge (current) and salt accumulation very well, and also accurately reproduces the effect of an increase in electrode thickness. However, for TiC-CDC with hardly any mesopores, and with a lower total porosity, the current is underestimated. Calculation results show that a material with higher electrode porosity is not necessarily responding faster, as more porosity also implies longer transport pathways across the electrode. Our work highlights that a direct prediction of CDI performance both for equilibrium and dynamics can be achieved based on the PSD and knowledge of the geometrical structure of the electrodes.

Transition metal loaded silicon carbide-derived carbons with enhanced catalytic properties

Carbide-derived carbons (CDC) with incorporated transition metal nanoparticles (∼2.5nm) were prepared using a microemulsion approach. Time-consuming post synthesis functionalization of the carbon support material can thus be avoided and nanoparticle sizes can be controlled by changing the microemulsion composition. This synthesis strategy is a technique for the preparation of highly porous carbon materials with a catalytically active component. In particular we investigated the integration of ruthenium, palladium, and platinum in a concentration ranging from 4.45 to 12wt.%. It was found that the transition metal has a considerable influence on sorption properties of resulting nanoparticle-CDC composite materials. Depending on the used metal salt additive the surface area and the pore volume ranges from 1480m2/g and 1.25cm3/g for Pt to 2480m2/g and 2.0cm3/g for Ru doped carbons. Moreover, members of this material class show impressive properties as heterogeneous catalysts. The liquid phase oxidation of tetralin and the partial oxidation of methane were studied, and electrochemical applications were also investigated. Primarily Pt doped CDCs are highly active in the oxygen reduction reaction, which is of great importance in present day fuel cell research.