a b s t r a c tHydrophobic polyvinylidene fluoride (PVDF) membranes have been used in membrane distillation, however its performance in terms of permeation flux and salt rejection still needs to be improved. In this study, polyvinylidenedifluoride-co-chlorotrifluoroethylene (PVDF-CTFE), a PVDF based copolymer, was employed to fabricate both flat sheet and hollow fiber membranes. Two pore forming additives (PEG and LiCl) were used in order to examine their effects on the membrane micro-structure and consequently the distillation performance. The crystalline property of the polymers was studied via X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) to reveal its role in the membrane formation. The membrane morphology (SEM), surface property (AFM) and membrane properties (contact angle, pore size, porosity and pore size distribution) were systematically examined and compared among PVDF, PVDF-CTFE membranes as well as their counterparts with pore forming additives. The results revealed that PVDF-CTFE membrane exhibited optimal membrane morphology, pore structure and hydrophobicity, thus delivering better distillation performance (permeation flux up to 62.09 kg/m 2 h, and distillate conductivity as low as 5 μS/cm) in comparison with their PVDF membrane counterparts.
a b s t r a c tHydrophobic flat-sheet membranes were prepared by poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) via the non-solvent induced phase separation (NIPS) process for membrane distillation (MD) application. Different types of LiCl-based mixed additives were applied to investigate their effects on membrane properties and MD performance. The membranes were evaluated in terms of membrane morphology, pore size and distribution, porosity, surface roughness, hydrophobicity, as well as the direct contact membrane distillation (DCMD) performance. Clear evidences were obtained that the mixed additives altered the phase inversion process, which resulted in the variation in membrane morphology, structure, properties and performance. The mass ratio of PEG/LiCl ranging from 5:0 to 0:5 was comprehensively investigated to study the synergistic effects of two additives on PVDF-CTFE membranes. The membrane M5 (PEG:LiCl ¼1:1, total content 8 wt% ) showed optimal properties and MD performance for combining the structure of higher hydrophobicity and pore interconnectivity, small pore size and narrow pore size distribution. The addition of LiCl to PEG-containing solutions benefited the solid-liquid demixing process, which increased the hydrophobicity, porosity, pore interconnectivity, and MD performance of the resultant membranes. While added PEG to LiCl-containing solutions showed slightly influences. Furthermore, it was also found that PVP/LiCl and glycerol/LiCl were not suitable for hydrophobic membrane preparation as they strikingly reduced membrane hydrophobicity. While H 3 PO 4 and H 2 O were confirmed suitable to mix with LiCl for hydrophobic PVDF-CTFE membrane preparation.
Hard carbon is the most attractive anode material for electrochemical sodium/potassium-ion storage. The preparation of hard carbon spheres directly from the broad sources of biomass is of great interest but barely reported. Herein, we developed a simple two-step hydrothermal method to construct porous carbon microspheres directly from the original waste biomass of camellia shells. The porous carbon microspheres have high specific capacities of 250 mAh g−1 and 264.5 mAh g−1 at a current density of 100 mA g−1 for sodium-ion batteries and potassium-ion batteries, respectively. And it has excellent cycle stability for sodium ions and potassium ions outperforming most reported hard carbons, which is mainly attributed to the microporous structure and spherical morphology. The work paves a way to prepare porous hard carbon spheres directly from biomass for alkali metal-ion batteries.
In this work, poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) was used for hydrophobic membrane preparation by the non-solvent induced phase inversion (NIPS) technique. The effects of poly(ethylene glycol) (PEG) molecular weight and dosage were investigated in terms of the membrane morphology, contact angle, surface free energy, and membrane pore structure for both surface pores and overall pores. All membranes possessed a typical liquid-liquid demixing asymmetric structure and the contact angles were higher than 85. Furthermore, increasing the PEG molecular weight and dosage significantly altered the membrane pore structure and surface roughness as a result of the variation of the phase inversion process. The solid-liquid demixing was responsible for the variation of membrane morphology, pore structure, hydrophobicity, and DCMD performance as PEGs with higher molecular weight or dosage were added. The PVDF-CTFE membranes were suitable for MD application owing to their high hydrophobicity, small pore size with narrow pore distribution, high DCMD performance, especially the interconnected pore structure. The membrane containing 5 wt% PEG-400 was evidenced to be the optimal one for the MD process, mainly according to the high interconnected pore structure which provide more passages for vapour transfer. The permeate flux was 17.98 kg (m À2 h À1) with a conductivity as low as 7 mS cm À1 at the temperature difference of 30 C. In addition, an excellent performance sustainability was observed including a relatively steady permeate flux and conductivity during the 360 h continuous DCMD operation.
Four common types of additives for polymer membrane preparation including organic macromolecule and micromolecule additives, inorganic salts and acids, and the strong non-solvent H 2 O were used to prepare poly (vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) hydrophobic flat-sheet membranes. Membrane properties including morphology, porosity, hydrophobicity, pore size and pore distribution were investigated, and the permeability was evaluated via direct contact membrane distillation (DCMD) of 3.5 g/L NaCl solution in a DCMD configuration. Both inorganic and organic micromolecule additives were found to slightly influence membrane hydrophobicity. Polyethylene glycol (PEG), organic acids, LiCl, MgCl 2 , and LiCl/H 2 O mixtures were proved to be effective additives to PVDF-CTFE membranes due to their pore-controlling effects and the capacity to improve the properties and performance of the resultant membranes. The occurrence of a pre-gelation process showed that when organic and inorganic micromolecules were added to PVDF-CTFE solution, the resultant membranes presented a high interconnectivity structure. The membrane prepared with dibutyl phthalate (DBP) showed a nonporous surface and symmetrical cross-section. When H 2 O and LiCl/H 2 O mixtures were also used as additives, they were beneficial for solid-liquid demixing, especially when LiCl/H 2 O mixed additives were used. The membrane prepared with 5% LiCl + 2% H 2 O achieved a flux of 24.53 kg/(m 2 ·hr) with 99.98% salt rejection. This study is expected to offer a reference not only for PVDF-CTFE membrane preparation but also for other polymer membranes.
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