Dual-layer hollow carbon fiber membranes for gas separation consisting of carbon and mixed matrix layers

Hollow carbon fiber membranes for gas separation have been successfully fabricated for the first time by a special type of precursor. This precursor is dual-layer hollow fiber composed of a polysulfone-beta zeolite (PSF-beta) mixed matrix outer layer and a Matrimid inner layer. Pure gas permeation measurements show that the resultant hollow carbon fiber has O₂/N₂ and CO₂/CH₄ selectivities of 9.3 and 150, respectively; this performance is much better than that of the hollow carbon fiber derived from single-layer Matrimid hollow fiber. Mixed gas measurements show the CO₂/CH₄ selectivity of 128. After pyrolysis, the PSF-beta layer in the dual-layer precursor evolves into a continuous structure of closely packed zeolite particles embedded in the PSF carbon residue. TGA spectra suggest that the possible reason for the above observation is that the PSF-beta outer layer and Matrimid inner layer has significantly changed each other’s pyrolysis dynamics and thermal degradation process.     

Fabrication of polysulfone asymmetric hollow‐fiber membranes by coextrusion through a triple‐orifice spinneret

Polysulfone (PSf) asymmetric hollow‐fiber membranes, which have a dense outer layer but a loose inner layer, were tentatively fabricated by coextrusion through a triple‐orifice spinneret and a dry/wet‐phase inversion process. Two simple polymer dopes were tailored, respectively, for the dense outer layer and the porous inner layer according to the principles of the phase‐inversion process. By adjusting the ratio of the inner/outer extrusion rate, the hollow‐fiber membranes with various thicknesses of outer layers were achieved. The morphology of the hollow‐fiber membranes was exhibited and the processing conditions and the water permeability of the membrane were investigated. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 259–266, 2004     

Optimization and modification of PVDF dual-layer hollow fiber membrane for direct contact membrane distillation; application of response surface methodology and morphology study

RSM methodology was applied to present mathematical models for the fabrication of polyvinylidene fluoride (PVDF) dual-layer hollow fibers in membrane distillation process. The design of experiments was used to investigate three main parameters in terms of polymer concentration in both outer and inner layers and the flow rate of dope solutions by the Box-Behnken method. According to obtained results, the optimization was done to present the proper membrane with desirable properties. The characteristics of the optimized membrane (named HF-O) suggested by the Box-Behnken (at the predicted point) showed that the proposed models are strongly valid. Then, a morphology study was done to modify the fiber by a combination of three types of a structure such as macro-void, sponge-like and sharp finger-like. It also improved the hydrophobicity of outer surface from 87 to 113° and the mean pore size of the inner surface from 108.12 to 560.14 nm. The DCMD flux of modified fiber (named HF-M) enhanced 62% more than HF-O when it was fabricated by considering both of RSM and morphology study results. Finally, HF-M was conducted for long-term desalination process up to 100 hr and showed stable flux and wetting resistance during the test. These stepwise approaches are proposed to easily predict the main properties of PVDF dual-layer hollow fibers by valid models and to effectively modify its structure.     

Improved microstructure of asymmetric niobium pentoxide hollow fiber membranes

Asymmetric niobium pentoxide (Nb₂O₅) hollow fiber membranes were prepared by the phase inversion and sintering process at temperatures ranging from 1000 to 1350°C. The effects of extrusion parameters on the morphology and properties of the produced membranes were systematically explored. Asymmetric hollow fibers with regular inner contour were obtained at extrusion flow rates of 15 and 25 ml min⁻¹ of ceramic suspension and internal coagulant, respectively. Hollow fibers sintered at temperatures greater than 1200°C presented modifications in the morphology of Nb₂O₅ grains, which were also evidenced by X-ray diffraction and Raman analyses. Hollow fibers produced with an air gap of 50 mm presented a dense outer sponge-like layer and micro-voids formed from the inner surface. These hollow fibers sintered at 1200°C presented suitable bending resistance and water permeability (24.2 ± 0.60 MPa and 3.00 ± 0.01 L h^-1 m^-2 kPa^-1, respectively). The outer sponge like layer was mitigated when the fibers were produced without air-gap.     

Dual-layer hollow fiber of polyaniline–cellulose acetate prepared with simple wet technique of chemical polymerization of aniline

Polyaniline–cellulose acetate hollow fibers prepared from cellulose acetate (CA) solution of aniline (ANI) were founded to show dual layers structure, when wet technique was applied for using syringe injection of the solution to acidic water for ANI polymerization. In this technique, the polymerization of ANI occurred simultaneously by the monomer injection in coagulated CA fibers. Then, the PANI–CA composite fibers were obtained. The formation of the PANI–CA composite fiber was dependent upon HCl concentration and initiator of ammonium persulfate. Especially, when the coagulation time was 1 min, the obtained PANI–CA fibers showed hollowed dual-layer structure having outer layer of PANI and inner layer of CA. Evidence was presented that the dual structure fiber had 40 μm outer layer and 60 μm inner porous layer in their thicknesses. Cyclic voltammograms of the PANI–CA fiber were indicated that the outer layer was composed of PANI layer showing electrochemical properties with electrical capacity of about 0.003 C.     

A novel dual-layer forward osmosis membrane for protein enrichment and concentration

We have demonstrated the prospect of dual-layer polybenzimidazole-polyethersulfone/polyvinylpyrrolidone (PBI–PES/PVP) hollow fiber nanofiltration (NF) membranes in the forward osmosis (FO) process for the enrichment and concentration of pharmaceutical products without denaturing the component of interests. The dual-layer hollow fiber membrane via coextrusion technology has an ultra-thin selective skin around 10 μm, fully open-cell water channels underneath and a microporous sponge-like support structure. The self-charged PBI selective skin has an average pore radius at 0.4 nm with a sharp pore size distribution. Experimental results show that the newly developed dual-layer hollow fiber nanofiltration membrane can achieve a high throughput for lysozyme enrichment and less protein fouling when using it as a FO membrane. In addition, the high divalent salt rejection towards Mg²⁺ at around 90% of this dual-layer membrane ensures the enriched lysozyme product with high purity and without change and denaturing.     

Piezoelectric Fibers with Uniform Internal Electrode by Co-Extrusion Process

Piezoelectric fibers with internal electrodes were fabricated by the co-extrusion process. The initial feedrods, which were composed of an outer piezoelectric PZN–PZT layer, a thin conducting PZN–PZT/Ag layer inside, and fugitive carbon black at the center, were co-extruded through a reduction die (1 mm) to form a continuous fiber. After thermal treatment and sintering, the PZN–PZT/Ag layer became the inner electrode, while the carbon black at the center was removed by oxidation to form an empty space. Three different types of fibers were produced: (i) solid fiber filled with an inner electrode, (ii) hollow fiber clad with a uniform inner electrode, and (iii) hollow fiber clad with a partial inner electrode. The piezoelectric properties of the fibers were evaluated in terms of their longitudinal strain (s₃₁) or transverse displacement. When the dimensions of the fiber were 840 μm (outer diameter) × 420 μm (inner diameter) × 40 mm (length), the longitudinal strains of the solid fiber with the inner electrode and hollow fiber clad with the uniform inner electrode were 5.25 × 10⁻⁵ and 8.5 × 10⁻⁵ m/m, respectively, under an applied voltage of 100 V (0.48 kV/mm) at a frequency of 100 Hz. For the hollow fiber clad with a partial inner electrode with the same dimensions, the transverse displacement was 80 μm under the same applied electric field.     

High-flux CO2-stable oxygen transport hollow fiber membranes through surface engineering

The influences of bulk diffusion and surface exchange on oxygen transport of (La_0.6Ca_0.4)(Co_0.8Fe_0.2)O_3-δ (LCCF) hollow fiber membranes were investigated. As an outcome, two strategies for increasing the oxygen permeation were pursued. First, porous LCCF hollow fibers as support were coated with a 22 μm dense LCCF separation layer through dip coating and co-sintering. The oxygen permeation of the porous fiber with dense layer reached up to 5.10 mL min^?1 cm^-2 at 1000 °C in a 50 % CO₂ atmosphere. Second, surface etching of dense LCCF hollow fibers with H₂SO₄ was applied. The surface etching of both inner and outer surfaces leads to a permeation improvement up to 86.0 %. This finding implies that the surface exchange reaction plays a key role in oxygen transport through LCCF hollow fibers. A good long-term (>250 h) stability of the asymmetric hollow fiber in a 50 % CO₂ atmosphere was found at 900 °C.     

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation

An ultrathin La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm³/cm² s (5.77 cm³/cm² min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm³/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9-11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).     

Ultrathin polymeric interpenetration network with separation performance approaching ceramic membranes for biofuel

Biofuel has emerged as one of the most strategically important sustainable fuel sources. The success of biofuel development is not only dependent on the advances in genetic transformation of biomass into biofuel, but also on the breakthroughs in separation of biofuel from biomass. The “separation” alone currently accounts for 60–80% of the biofuel production cost. Ceramic membranes made of sophisticated processes have shown separation performance far superior to polymeric membranes, but suffers fragility and high fabrication cost. We report the discovery of novel molecular engineering and membrane fabrication that can synergistically produce polymeric membranes exhibiting separation performance approaching ceramic membranes. The newly discovered Polysulfone/Matrimid composite membranes are fabricated by dual‐layer coextrusion technology in just one step through phase inversion. An ultrathin dense‐selective layer made of an interpenetration network of the two materials with a targeted and stable interstitial space is formed at the interface of two layers for biofuel separation. The combined molecular engineering and membrane fabrication approach may revolutionize future membrane research and development for purification and separation in energy, environment, and pharmaceuticals. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Fabrication and characterization of dual-layer hollow-fiber ultrafiltration membranes

In this study, poly(vinylidene fluoride) (PVDF) dual-layer hollow-fiber UF membranes were prepared via phase inversion in one step. Laboratory-synthesized amphiphilic poly(vinylidene fluoride)-g-poly(ethylene glycol) methyl ether methacrylate (PVDF-g-POEM) was incorporated as a hydrophilic modifier of the outer layer by blending. The effects of the dope formulation and membrane formation conditions on membrane structure and UF performance were investigated. The parameters investigated included the PVDF-g-POEM loading in the outer layer, the PEG additive content in the outer layer, the external coagulant composition, and the polymer concentration in the inner layer. The effects of adding PVDF-g-POEM and PEG were found to depend on the external coagulant composition; when a water/ethanol mixture was used as coagulant, the fibers formed in the presence of PEG exhibited larger pores, as confirmed by both SEM characterization and a solute rejection method. The porosity of the inner layer was observed to increase with decreasing inner-layer dope concentration and upon weakening the external coagulant. A more porous inner layer led to a higher transmembrane pure water flux. An antifouling test confirmed that both membrane hydrophobicity and surface pore size affected the membrane fouling pattern and the final FRR. The highest FRR of 83.3% was obtained with the hollow fiber M5A, which was characterized by a compact surface and contained PVDF-g-POEM in its polymer matrix.     

Effects of lanthanum strontium cobalt ferrite (LSCF) cathode properties on hollow fibre micro-tubular SOFC performances

Single layer La_0.6Sr_0.4Co_0.2Fe_0.8O₃ hollow fibre (HF) precursors (<1 mm ID) produced by phase inversion (PI) were sintered at 1,200, 1,350 and 1,400 °C. The increase in sintering temperature resulted in microstructural changes in the LSCF fibres, reflected in their electrical conductivities. LSCF-based cathodes with different designs were brushed onto co-extruded nickel–gadolinium-doped ceria (CGO) anode/CGO electrolyte dual-layer HFs (<1 mm ID) fabricated by PI. The effect of cathode layers on the overall performance of the fuel cells (FCs) was assessed using nearly identical anode and electrolyte compositions, thicknesses, and microstructures. Cathode microstructure design caused cells to perform differently producing peak power densities of 0.35–0.7 W cm⁻² at 600 °C. Impedance spectroscopy analysis at 600 °C on the FCs produced 0.12–0.24 Ω cm² confirming the cathode’s structural effect on the overall area-specific resistance of the FCs. The best performing FC with a brush-deposited cathode was compared to a similar FC where cathode was deposited by dip coating; at 600 °C the first produced 0.6 W cm⁻² while the second cell 0.7 W cm⁻². Co-extruding anodes and electrolytes by using PI and combining dip coating for cathode deposition could lead to the fabrication of FCs with enhanced microstructures and improved performances.     

A gradient composite coating to protect SiC-coated C/C composites against oxidation at mid and high temperature for long-life service

To improve the oxidation resistance of carbon/carbon (C/C) composites at mid and high temperature, a gradient composite coating was designed and prepared on SiC-coated C/C composites by in situ formed-SiO₂ densifying the porous SiC-ZrSi₂ pre-coating. SiO₂ gradient distribution was conducive to inhibiting the cracking of the coating. A dual-layer structure with the outer dense layer and the inner microporous layer was formed in the coating during densifying. The dense layer had excellent oxygen diffusion resistance and the microporous layer alleviated CTE mismatch between SiC inner coating and dense layer. Moreover, ZrSiO₄ particles inhibited crack propagation and stabilized SiO₂ glass. Therefore, the coating can protect the C/C composites from oxidation at 1473 K, 1573 K and 1773 K for 810 h, 815 h and 901 h, respectively. The coated samples underwent 30 thermal cycles between room temperature and 1773 K without mass loss, exhibiting good thermal shock resistance.     

Polybenzimidazole/poly(tetrafluoro ethylene) composite membranes for high temperature proton exchange membrane fuel cells

A porous poly(tetrafluoro ethylene) (PTFE) thin film (thickness 16 ± 2 μm) is used as a supporting material for polybenzimidazole (PBI) to prepare the PBI/PTFE composite membrane (thickness 38 ± 2 μm). The perfluorosulfonic acid resin (Nafion) is used as a coupling agent at the interface between PTFE and PBI to improve the bonding between PBI and PTFE. The composite membrane, after doping with phosphoric acid, is used to prepare membrane electrode assemblies (MEAs). A 450 h continuous fuel cell life test at 160 °C with a fixed current density i = 200 mA cm⁻² and a 20 cycles cell on/off test, in which the fuel cell is operated at 160 °C with i = 200 mA cm⁻² for 12 h and then switched off at room temperature in an ambient environment for 12 h per cycle, are performed. Both tests show good fuel cell performances.     

An excellent ablative composite based on PBO reinforced EPDM

Short poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers were first used to reinforce ethylene–propylene–diene terpolymer (EPDM) as thermal insulation materials. The effects of PBO fiber length and content on the mechanical and ablative properties of the composites were investigated in detail. Comparing with the severe breakage occurred in short aramid fibers as fillers, only some necking deformation is observed in PBO fibers filled EPDM after processed. After ablated by oxyacetylene flame, the carbonized PBO fibers still keep solid fibrous structure instead of hollow one of carbonized aramid fibers in the char layer. As a result, the PBO fibers/composites show significantly higher tensile strength and ablation resistant abilities than the aramid fibers/composites. Moreover, with the length and content of PBO fibers increasing, both the tensile strength and the ablation resistance of the composites increase gradually though the break elongation reduces sharply. Considering the properties requirement of thermal insulator, PBO fibers with 3.42–5.56 wt% in content and 3–4 mm in length are preferred. The mass loss rate and the erosion rate as low as 0.05 g s⁻¹ and 0.10 mm s⁻¹ are observed in the optimal samples, respectively, which is evidently lower than that of the best aramid fibers/EPDM-based insulations reported so far.     

Enhanced hollow fiber membrane performance via semi-dynamic layer-by-layer polyelectrolyte inner surface deposition for nanofiltration and forward osmosis applications

The layer-by-layer (LBL) polyelectrolyte deposited membranes have drawn increasing attention in various applications due to the ease of selective layer formation and their stability and versatility. In this study, the LBL deposition was performed at the inner surface of the polyethersulfone (PES) hollow fiber substrate to form composite nanofiltration (NF) membrane. The semi-dynamic deposition procedure was adopted with the aid of syringes. The newly developed inner deposited (id-LBL) membranes were then tested in NF and forward osmosis (FO) applications and the performance were compared with outer surface deposition as well as some literature data. The id-LBL membranes could not only withstand higher operating pressure but also possess superior hardness rejection especially in high concentration mixed salt solutions (more than 95% rejection to Mg²⁺ and Ca²⁺ in a 5000 ppm total dissolved salt (TDS) mixture under 4.8 bar). As for the FO process, with only two layer deposition, the id-LBL membranes also demonstrated significant performance improvement with increased water flux (up to 70 L/m² h using 0.5 M MgCl₂ as draw solution in active layer facing draw solution configuration) and reduced salt leakage (around 0.5 g/m² h using 1 M MgCl₂ draw solution in active layer facing feed water configuration). This study suggests that for hollow fiber substrate, the inner surface is more suitable for the formation of the selective layer via LBL deposition than the outer surface.     

Experimental study of CO2 absorption/stripping via PVDF hollow fiber membrane contactor

Gas–liquid hollow fiber membrane contactor can be a promising alternative for the CO₂ absorption/stripping due to the advantages over traditional contacting devices. In this study, the structurally developed hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via a wet spinning method. The membranes were characterized in terms of morphology, permeability, wetting resistance, overall porosity and mass transfer resistance. From the morphology analysis, the membranes demonstrated a thin outer finger-like layer with ultra thin skin and a thick inner sponge-like layer without skin. The characterization results indicated that the membranes possess a mean pore size of 9.6 nm with high permeability and wetting resistance and low mass transfer resistance (1.2 × 10⁴ s/m). Physical CO₂ absorption/stripping were conducted through the fabricated gas–liquid membrane contactor modules, where distilled water was used as the liquid absorbent. The liquid phase resistance was dominant due to significant change in the absorption/stripping flux with the liquid velocity. The CO₂ absorption flux was approximately 10 times higher than the CO₂ stripping flux at the same operating condition due to high solubility of CO₂ in water as confirmed with the effect of liquid phase pressure and temperature on the absorption/stripping flux.     

Microporous polypropylene hollow fibers with double layers

Microporous polypropylene hollow fibers with double layers were prepared by stretching double layered polypropylene microtubes containing polymethylsilsesquioxane fillers: the relatively smaller filler in the inner layer and relatively larger filler in the outer layer. The resultant hollow fibers have a finely interconnected fibrous structure parallel to fiber axis. Their N₂ gas permeabilities were measured to estimate the fibrous structure: tortuosity factor, effective porosity, and pore size. © 1995 John Wiley & Sons, Inc.     

Microtubes made of carbon nanotubes

Microtubes made of multi-walled carbon nanotubes were prepared via infiltration of CNT-suspension through a microfiltration hollow fiber membrane. Shrinking of the entangled CNT network during the drying allows withdrawal of CNT-microtubes from the hollow fiber. Currently, microtubes have a length of ∼50 cm, outer diameter of ∼1.7 mm and scalable inner diameter by varying the infiltration time resulting in wall thicknesses of 130–320 μm. The BET surface area is 200 m²/g with a porosity of 48–67% and an electrical conductivity ∼20 S/cm. We propose to use such novel CNT-microtubes for the fabrication of tubular electrochemical cells and membrane filtration processes.     

Pushing the limits of high performance dual‐layer hollow fiber fabricated via I2PS process in dehydration of ethanol

The immiscibility induced phase separation (I²PS) process was introduced as a novel method to fabricate hollow fibers with exceptionally high water permeance and reasonably high water/ethanol selectivity in dehydration of ethanol by pervaporation. As a continuation of the previous work, this study discloses the mechanisms to enhance the performance of hollow fibers spun via I²PS by elucidating the material selection at the inner‐layer. Moreover, it revealed the methods to reduce mass‐transport resistance by enhancing surface porosity for both inner and outer surfaces to further improve the permeation flux of the membranes. The continuous performance test demonstrates that the fibers spun from the I²PS possess a stable dehydration performance throughout the monitored period of 300 h. A comparison with pervaporation membranes in the literatures verifies the superiority of the membranes spun via I²PS process with the highest water permeation flux of 9.5 kg/m² h and the permeate water purity of 95.8 wt % at 80°C. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3006–3018, 2013