Nanoporous Carbon Membranes Research Articles

Recent progress in the synthesis and applications of nanoporous carbon films

The synthesis and applications of thin nanoporous carbon films or membranes (NPCFs) are outlined with a focus on recent literature examples. NPCFs have large specific surface area, high electrical and thermal conductivity and both chemical and mechanical stability properties that facilitate increasing conventional uses in purification and separation, adsorption, and catalysis. They have potential applications in nanodevices, supercapacitors, lithium ion batteries, and chemical and bio-sensors. We summarize the various NPCF fabrication techniques, including solid- or soft-templating, direct pyrolysis on porous organic polymer precursors, chemical or physical vapor deposition and electrochemical deposition, and highlight the future design trends in the fabrication of NPCFs with ordered nanopore structures.

An experimental evaluation and molecular simulation of high temperature gas adsorption on nanoporous carbon

A combination of experiments and molecular simulations has been used to further understand the contribution of gas adsorption to the carbon dioxide (CO 2) selectivity of nanoporous carbon (NPC) membranes as a function of temperature and under mixed gas conditions. Whilst there have been various publications on the adsorption of gases onto carbon materials, this study aims to benchmark a simulation model with experimental results using pure gases. The simulation model is then used to predict mixed gas behaviour. These mixed gas results can be used in the assessment of NPC membranes as a suitable technology for both carbon dioxide separations from air-blown syngas and from natural gas. The gas adsorption experiments and molecular simulations have confirmed that CO 2 is more readily adsorbed on nanoporous carbon than methane (CH 4) and nitrogen (N 2). Increasing the temperature reduces the extent of adsorption and the CO 2 selectivity. However, the difference between the CO 2 and N 2 heats of adsorption is significant resulting in good CO 2/N 2 separation even at higher temperatures.

ChemInform Abstract: Characterization of Supported Nanoporous Carbon Membranes (SNPCMs).

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Diffusion of H 2, CO, N 2, O 2 and CH 4 Through Nanoporous Carbon Membranes

Diffusion of pure H 2, CO, N 2, O 2 and CH 4 gases through nanoporous carbon membrane is investigated by carrying out non-equilibrium molecular dynamics (NEMD) simulations. The flux, transport diffusivity and activation energy for the pure gases diffusing through carbon membranes with various pore widths were investigated. The simulation results reveal that transport diffusivity increases with temperature and pore width, and its values have a magnitude of 10 −7 m 2·s −1 for pore widths of about 0.80 to 1.21 nm at 273 to 300 K. The activation energies for the gases diffusion through the membrane with various pore widths are about 1–5 kJ·mol −1. The results of transport diffusivities are comparable with that of Rao and Sircar (J. Membr. Sci., 1996), indicating the NEMD simulation method is a good tool for predicting the transport diffusivities for gases in porous materials, which is always difficult to be accurately measured by experiments.

Non-equilibrium molecular dynamics simulation on permeation and separation of H 2/CO in nanoporous carbon membranes

Permeation and separation of H 2/CO binary mixtures in nanoporous carbon membranes are investigated by non-equilibrium molecular dynamics simulations. The carbon membrane pores are modeled as slit-like pores with entrance and exit. The buffer regions between the control volumes and membrane pores are employed to take into account the effects of the entrance and exit of the membrane pores. The effects of pore width, separation temperature, feed gas pressure, the molar fraction of hydrogen, and membrane thickness on flux and dynamic separation factor are discussed. The simulation results indicate that the pore width strongly affects the flux and dynamic separation factor. In addition, molecular sieving dominates the separation of H 2/CO mixtures, when the pore width is smaller by about 0.64 nm, and, in this case, the dynamic separation factor reaches 52.88 at 0.5 MPa and 300 K. The dynamic separation factor increases with the separation temperature and the decrease of feed gas pressure, while changes slightly with the molar fraction of H 2 in the feed gas. Moreover, the dynamic separation factor increases with membrane thickness at the pore width of 0.64 nm, while decreases at the pore width of 1.01 nm due to different separation mechanisms.

Effect of pyrolysis temperature and operating temperature on the performance of nanoporous carbon membranes

Technology designed to capture and store carbon dioxide (CO 2) will play a significant role in the near-term reduction of CO 2 emissions and is considered necessary to slow global warming. Nanoporous carbon (NPC) membranes show promise as a new generation of gas separation membranes suitable for CO 2 capture. We have made supported NPC membranes from polyfurfuryl alcohol (PFA) at various pyrolysis temperatures. Positron annihilation lifetime spectrometry (PALS) and wide angle X-ray diffraction (WAXD) results indicate that the pore size decreases whilst the porosity increases with increasing pyrolysis temperature. The membrane performance results support these findings with a significant increase in permeance being seen with increasing pyrolysis temperature, which relates to the increase in porosity. Mixed gas performance measurements also show an increase in CH 4 permeance as the operating temperature is increased from 35 to 200 °C, which can be related to an increase in the rate of diffusion. However, the selectivity decreases with increasing operating temperature due to the smaller changes in the CO 2 permeance. These smaller changes in CO 2 permeance can be related to the stronger adsorption of this gas on the carbon surface at lower operating temperatures. Interestingly, regardless of the original pyrolysis temperature, the selectivity at higher operating temperatures is similar, whereas the permeance remains related to this pyrolysis temperature.

Separation Performance of Nanoporous Carbon Membranes Fabricated by Catalytic Decomposition of CH4 Using Ni/Polyamideimide Templates

Using a Ni/polyamideimide sol as a template, a nanoporous carbon membrane was prepared in a single coating by means of a novel technique: the carbonization of a nickel-containing polyamideimide precursor combined with the catalytic decomposition of methane in a nitrogen atmosphere. The separation performance of the membrane was significantly better than that fabricated by a single coating and conventional pyrolysis, even as good as or better than those for three-coating carbon membranes. This procedure significantly reduced the cost of the membrane production and provided a promising technical way for the preparation of carbon membrane. The surface and cross-sectional morphologies, the composition, and the microstructure of the carbon membranes were characterized by means of SEM-EDX, AFM, and XRD techniques. The properties of the precursor and membranes were investigated by FT-IR absorption spectra and H2-TPD.

Characterization of Nanoporous Structures of Polyphenylene Oxide Derived Carbon Membranes by Means of 129Xe NMR

The 129Xe NMR spectroscopy has become a powerful technique of materials characterization because the xenon atom has a very large polarizability. It is well known that the signal of xenon sorbed in porous media is sensitively affected by the surrounding environments such as the chemistry of material surface. In this study, the pore properties of nanoporous PPO (polyphenylene oxide) derived carbon membranes were characterized by means of the variable temperature (VT)-hyperpolarized Xe NMR. The Xe NMR results showed good agreements with the adsorption results of CO2 for the PPO derived nanoporous carbon membranes. It was clearly shown that the 129Xe NMR could be used as one of the promising characterization methods of nanoporous materials with low surface area and small pore volume.

Performance Characteristics of Nanoporous Carbon Membranes for Protein Ultrafiltration

Nanoporous carbon membranes could be very attractive for applications of ultrafiltration in the biotechnology industry because of their greater mechanical strength and longer membrane life. The objective of this study was to obtain quantitative data on the performance characteristics of nanoporous carbon membranes formed within a stainless steel support that was first modified by deposition of silica particles within the macroporous support. The nanoporous carbon membrane effectively removed small solutes from a protein solution using diafiltration, with performance comparable to that of commercial polymeric membranes. Protein fouling was evident, although the nanoporous carbon membranes were easily regenerated; cleaning with 0.5 N NaOH at 50 degrees C completely restored the water permeability for multiple cycles. The nanoporous carbon membranes were also compatible with steam sterilization. Significant increases in process flux could be obtained using periodic back-pulsing, with no evidence of any structural alterations in the membrane. These results clearly demonstrate the potential benefits and opportunities for using nanoporous carbon membranes for protein ultrafiltration.

Nanoporous carbon membranes for separation of nitrogen and oxygen: Insight from molecular simulations

Here we summarize some of our recent work on using molecular simulations to understand the key mechanism that result in large differences in the reported permeation rates of O 2 and N 2 in nanoporous carbon membranes. Two different representation of the amorphous nanoporous membrane structure are used; the hypothetical C 168 Schwarzite and a single wall carbon nanotube with a constriction. By comparing the results obtained from empirical planar graphite potential and an ab initio-based potential, the effect of carbon curvature and the presence of non-hexagonal carbon rings in C 168 Schwarzite is also investigated. It is found using either force field, that the energetic effect alone cannot explain the experimental observations. However, simulations performed using carbon nanotube with a constriction show that the size or entropic effect can be dominant. In particular, it is shown that an appropriate size constriction can result in large transport resistance to nitrogen while letting oxygen to pass through at a much higher rate, even though these gases have very similar molecular sizes and interaction energetics.

Development and characterization of nanoporous carbon membranes for protein ultrafiltration

Nanoporous carbon ultrafiltration membranes could be very attractive for bioprocessing applications, but existing carbon membranes tend to have very low hydraulic permeability due to the large thickness of the carbon layer. We have developed a new method for producing nanoporous carbon membranes using a stainless steel support that has first been modified by slip-casting silica particles into the macropores of the support. The nanoporous carbon membrane is then formed by pyrolysis of polyfurfuryl alcohol with polyethylene glycol used as the pore forming agent. The sub-micron-sized silica particles allow thin integral membranes to be formed after only two or three coats of the polyfurfuryl alcohol. Dextran sieving curves for the nanoporous carbon were similar to those of a commercial 100 kDa polyethersulfone ultrafiltration membrane, with a slightly broader pore size distribution. Performance characteristics for the nanoporous carbon were only slightly below those of commercial polymeric ultrafiltration membranes, but the nanoporous carbon was stable even after prolonged exposure to 3N NaOH. These results demonstrate that high performance nanoporous carbon ultrafiltration membranes can be made using silica-modified stainless steel supports.

High performance nanoporous carbon membranes for air separation

The preparation of porous stainless steel supports was found to have a significant impact on the properties of nanoporous carbon membranes fabricated upon them. Nanofillers were incorporated into porous stainless steel supports to modify the pore structure by reducing the average pore size and porosity. Carbon membrane properties were examined as a function of support variables such as filler content, shape, size and nature of the particles. Optimum performances, in terms of the ideal selectivity ratio for oxygen to nitrogen permeances ( S O 2 / N 2 ∼ 3 – 6 ) and the oxygen permeance (10 −8 mol m −2 s −1 Pa −1), were obtained when the filler completely saturated the support. This represents about a two order of magnitude improvement in oxygen permeance when compared to carbon membranes prepared on unmodified porous stainless steel supports. The origin of the improvement in the permeance is due to the formation of carbon membranes which are on average two orders of magnitude thinner than those formed on unmodified supports, i.e., the carbon membranes exists as very thin layers around and between the silica nanoparticles. A simple geometric model based on the packing of silica particles inside the porous stainless steel support is proposed to visualize and quantify this effect. The generality of the support modification concept is also demonstrated by the ability to employ different types of nanofillers and support geometries to obtain carbon membranes with high flux. Air separation experiments show that these membranes can produce both oxygen rich streams enriched to as much as 48% by volume and nitrogen rich streams enriched to over 90% by volume at reasonable operating conditions.

Molecular Sieving Using Single Wall Carbon Nanotubes

By using molecular dynamics and grand canonical Monte Carlo simulations, we find that a nanotube with a constriction results in high transport resistance to nitrogen while allowing oxygen to pass at a much higher rate even though these gases have very similar sizes and energetics. This provides an understanding of the reported high permeation rates of oxygen relative to nitrogen in nanoporous carbon membranes and a basis for designing nanotubes with constrictions using available technologies for membrane-based separations.

Modification of macroporous stainless steel supports with silica nanoparticles for size selective carbon membranes with improved flux

Silica nanoparticles were slip cast into porous stainless steel supports, which were then coated with polyfurfuryl alcohol and pyrolyzed to make nanoporous carbon membranes. The single gas permeances of the membranes formed on modified stainless steel supports were found to be between two and three orders of magnitude larger than the permeances of nanoporous carbon membranes (<10 −11 mol m −2 s −1 Pa −1) synthesized on unmodified supports. Importantly, these high permeances (10 −8–10 −9 mol m −2 s −1 Pa −1) were achieved within the same range of O 2/N 2 selectivities (3–5) that we have observed for single gases permeating at much lower fluxes through the nanoporous carbon membranes on unmodified supports. The nanoporous carbon membranes also were formed by combining the silica nanoparticles with polyfurfuryl alcohol resin and applying the mixture directly onto an unmodified support. This simpler process was as effective in producing selective-high permeance membranes. In both cases the significant increase in permeance without loss of selectivity is attributed to the silica nanoparticles filling the macropores of the stainless steel supports, thereby leading to the formation of very thin but selective carbon layers.

Hierarchical Modeling O2 and N2 Adsorption in C168 Schwarzite: From Quantum Mechanics to Molecular Simulation

The adsorption of pure O2 and N2 and their mixture in C168 schwarzite, as a model for a nanoporous carbon adsorbent, has been studied with use of grand canonical Monte Carlo simulations. The gas−carbon interaction is modeled by a pairwise additive atom−atom Lennard-Jones potential with parameters determined from ab initio quantum mechanical computations. For pure O2, the adsorption simulated with the ab initio potential is similar to that with the empirical Steele potential derived from gas adsorption on planar graphite in the limit of zero loading; however, for pure N2, the adsorption simulated with the ab initio potential is much larger. Consequently, a large adsorption selectivity of N2 over O2 from their mixture is found with the ab initio potential. With both potentials the adsorbed gas molecules are found to preferentially align along the channel intersection of the C168 schwarzite structure, and a localized nonuniform density distribution of the adsorbed gas molecules is observed, which is more evident with the ab initio potential. The predictions of mixture adsorption using the ideal-adsorbed-solution theory based on the adsorption of only the pure gases agree well with the simulations. This work demonstrates the importance of an accurate adsorbate−adsorbent interaction potential in the determination of gas adsorption behavior, and suggests that nanoporous carbon membranes might be useful for air separation.

Modeling ideal selectivity variation in nanoporous membranes

A parallel resistance transport model has been developed to describe variations in separation ability (quality) of nanoporous carbon membranes. The model considers transport through high-selectivity, high-resistance nanopores in combination with low-selectivity, low-resistance defect pores. Although few in number or small as a percentage of membrane area, these low-selectivity pores exert a disproportionate influence on permeation. The predictive qualities of the model are demonstrated using permeation data from the literature. Using flux ratios of oxygen to nitrogen, we can predict the area fraction ( α) of defect pores. At an area fraction of only 10 −9 defect pores, the oxygen-to-nitrogen ratio falls to near unity—indicating no selectivity for oxygen. Although developed for nanoporous carbon membranes, the approach will work for zeolitic and other forms of nanoporous membranes.

Using nanoporous carbon membranes in fuel cells

ABSTRACTWe have synthesized nanoporous carbon membranes that have monodisperse pores of 4–5 Å. These membranes have excellent size and shape selectivity that makes them an ideal candidate for use as separators in fuel cells. The selectivity of these membranes to gases such as N2, O2 and water gas [carbon monoxide and hydrogen] were measured using a permeation testing unit. These membranes were then tested as separators in fuel cells.

Temperature- and pressure-dependent transient analysis of single component permeation through nanoporous carbon membranes

Using a transient analysis of permeation, i.e. the time lag method, as a function of temperature and pressure, it is shown that each of the parameters needed to fully evaluate adsorption and diffusion can be obtained in situ. These experiments were conducted on a supported tubular nanoporous carbon membrane, 5.1 cm 2 in area and prepared by ultrasonic deposition of poly(furfuryl alcohol) onto a porous stainless steel support. The permeation experiments were conducted at temperatures ranging from 25 to 225°C and over a range of pressures from 100 to 700 kPa. Under these conditions the fluxes ranged from 10 −7 to 10 −4 mol/m 2/s with permeances ranging between 10 −12 and 10 −9 mol/m 2/s/Pa. Heats of adsorption were found to be 2.5, 2.21, 3.05 and 2.52 kcal/mol for N 2, O 2, Ar and CO 2, respectively, and generally lower than those reported for granular nanoporous carbons. The apparent activation barriers to diffusion were also found to be low at 2.06, 5.87, 4.12, 5.89 and 2.19 kcal/mol for He, N 2, O 2, Ar and CO 2. These results point to the presence of two parallel pathways for transport — the major one through the nanopores but a second through a few defect pores. Assumed to be on the order of 50 nm in diameter, these defects were calculated to represent a total area fraction of 3.43×10 −9.

Study of the effect of morphology of nanoporous carbon membranes on permselectivity

ABSTRACTThe present work is aimed at developing uniform nanoporous carbon membranes on porous stainless steel supports. Thin layers of polyfurfuryl alcohol are applied on the stainless steel supports using spin coating method. Nanoporous carbon is obtained by pyrolyzing polyfurfuryl alcohol films under inert atmosphere. Uniform nanoporous carbon membranes were obtained by repeated spin coating and pyrolysis of polyfurfuryl alcohol on the stainless steel supports. The morphology of the membranes was examined using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). It was shown that there is direct correlation between the morphology and the selectivity of the membranes. SEM and AFM show the presence of globular domains during the formation of carbon membranes. The selectivity increased with the decrease in the size of the globules. It was also shown that the annealing of the membrane affected both the morphology and the selectivity of the membrane.

Monte Carlo simulation of transport diffusion in nanoporous carbon membranes

A dynamic Monte Carlo (MC) simulation method was applied to calculate the transport diffusion of hydrogen and hydrocarbons in nanoporous carbon membranes with slit-like pores. Molecular movements were characterised by surface diffusion in which fluxes were strong close to the location of the pore wall. The pore size was shown to be an important factor in determining the selectivity of hydrocarbons in the membranes. The calculated diffusivities were in semi-quantitative agreement with published experimental data.