Ai Mei Zhu

Enhancement of hydroxide conductivity by grafting flexible pendant imidazolium groups into poly(arylene ether sulfone) as anion exchange membranes

Anion exchange membranes (AEMs) have been recognized as one of the most prospective polyelectrolytes for fuel cells due to their faster electrode reaction kinetics and the potential of adopting cheaper metal catalysts against proton exchange membranes (PEMs). Herein, a series of poly(arylene ether sulfone)s containing a flexible pendant imidazolium cation were synthesized by grafting bromine-bearing imidazolium-based ionic liquids into a hydroxyl-bearing poly(ether sulfone) matrix. 1H NMR spectroscopy was used to confirm the as-synthesized copolymers. Atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) were used to characterize the morphology of the membranes. The incorporation of the flexible side-chain imidazolium groups is beneficial to the aggregation of the ionic clusters leading to the formation of hydrophilic/hydrophobic phase-separated morphology and nano-channels. As a result, an enhancement in the ionic conductivity can be achieved. Therefore, the as-prepared AEMs possess higher ionic conductivity than traditional benzyl-type AEMs. The weight-based ion exchange capacity (IECw) of the membranes was in the range of 1.01–1.90 meq. g−1. Correspondingly, their ionic conductivity was in the range of 22.13–59.19 and 51.66–108.53 mS cm−1 at 30 and 80 °C, respectively. Moreover, the membranes also exhibit good alkaline stability and interesting single cell performance. This work presents a facile and universal route for the synthesis of AEMs with superior performance.

Phenolphthalein-based Poly(arylene ether sulfone nitrile)s Multiblock Copolymers As Anion Exchange Membranes for Alkaline Fuel Cells

A series of phenolphthalein-based poly(arylene ether sulfone nitrile)s (PESN) multiblock copolymers containing 1-methylimidazole groups (ImPESN) were synthesized to prepare anion exchange membranes (AEMs) for alkaline fuel cells. The ion groups were introduced selectively and densely on the unit of phenolphthalein as the hydrophilic segments, allowing for the formation of ion clusters. Strong polar nitrile groups were introduced into the hydrophobic segments with the intention of improving the dimensional stability of the AEMs. A well-controlled multiblock structure was responsible for the well-defined hydrophobic/hydrophilic phase separation and interconnected ion-transport channels, as confirmed by atomic force microscopy and small angle X-ray scattering. The ImPESN membranes with low swelling showed a relatively high water uptake, high hydroxide ion conductivity together with good mechanical, thermal and alkaline stability. The ionic conductivity of the membranes was in the range of 3.85-14.67×10(-2) S·cm(-1) from 30 to 80 °C. Moreover, a single H2/O2 fuel cell with the ImPESN membrane showed an open circuit voltage of 0.92 V and a maximum power density of 66.4 mW cm(-2) at 60 °C.

Molecular Simulation of Water/Alcohol Mixtures' Adsorption and Diffusion in Zeolite 4A Membranes

The COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field with two sets of partial atomic charges of water was used to simulate adsorption and diffusion behavior of water/methanol and water/ethanol mixtures in zeolite 4A at 298 K. The adsorption of alcohol first increased and then decreased with increasing pressure, whereas the adsorption of water increased progressively until an adsorption equilibrium was reached. Both the adsorbed molecules and the zeolite framework were treated as a fully flexible model in MD simulations. The simulation results show that the effects of the size and steric hindrance of the diffusing molecules on diffusivity are significant. The diffusivity of water, methanol, and ethanol molecules decreases by 1 order of magnitude in the order of water > methanol > ethanol. The diffusivity of water molecules depends on the mass fraction and the partial charges of water in zeolite 4A. The ethanol and methanol molecules have restricted motion through the alpha-cages, whereas the water molecules can easily pass through the alpha-cages window at low feed alcohol concentrations. And the extent of hydrogen bonding increased with increasing water concentration.

Imidazolium-Functionalized Poly(arylene ether sulfone) Anion-Exchange Membranes Densely Grafted with Flexible Side Chains for Fuel Cells

With the intention of optimizing the performance of anion-exchange membranes (AEMs), a set of imidazolium-functionalized poly(arylene ether sulfone)s with densely distributed long flexible aliphatic side chains were synthesized. The membranes made from the as-synthesized polymers are robust, transparent, and endowed with microphase segregation capability. The ionic exchange capacity (IEC), hydroxide conductivity, water uptake, thermal stability, and alkaline resistance of the AEMs were evaluated in detail for fuel cell applications. Morphological observation with the use of atomic force microscopy and small-angle X-ray scattering reveals that the combination of high-local-density-type and side-chain-type architectures induces distinguished nanophase separation in the AEMs. The as-prepared membranes have advantages in effective water management and ionic conductivity over traditional main-chain polymers. Typically, the conductivity and IEC were in the ranges of 57.3-112.5 mS cm(-1) and 1.35-1.84 mequiv g(-1) at 80 °C, respectively. Furthermore, the membranes exhibit good thermal and alkaline stability and achieve a peak power density of 114.5 mW cm(-2) at a current density of 250.1 mA cm(-2). Therefore, the present polymers containing clustered flexible pendent aliphatic imidazolium promise to be attractive AEM materials for fuel cells.

Side-chain-type anion exchange membranes bearing pendant quaternary ammonium groups via flexible spacers for fuel cells

To realize high performance anion exchange membranes (AEMs) for alkaline fuel cells (AFCs), a series of quaternized poly(ether sulfone)s (PESs) with different lengths of flexible spacers linking cationic groups and the backbone was synthesized via nucleophilic polycondensation, demethylation and Williamson reactions. Atomic force microscopy (AFM) phase images show clear hydrophilic/hydrophobic phase separation for all the side-chain-type AEMs. The PES-n-QA membrane with hexyleneoxy spacers (n = 6) between the cationic groups and backbone (benzene ring) exhibited the maximum conductivity of 62.7 mS cm−1 (IEC = 1.48 meq. g−1) at 80 °C. The AEM materials are found to have an improved long-term alkaline stability by extending the length of the flexible spacer (n ≥ 4). The PES-12-QA membrane with a flexible dodeceneoxy spacer demonstrated the highest alkaline stability, where the conductivity and IEC only decreased by 8.1% and 6.9% after immersing in a 1 M aqueous KOH solution at 60 °C for 720 h. Furthermore, the single fuel cell performance test using PES-6-QA as an AEM showed a maximum power density of 108.3 mW cm−2 at a current density of 250 mA cm−2 at 60 °C.

Characterization and Permeation Performance of Novel Organic-Inorganic Hybrid Membranes of Poly(vinyl Alcohol)/1,2-Bis(triethoxysilyl)ethane

Bis(trialkylsilyl) precursor was used to modify polymer membranes for the first time. Novel organic-inorganic hybrid membranes of poly(vinyl alcohol) (PVA)/1,2-bis(triethoxysilyl)ethane (BTEE) were prepared through a sol-gel method for pervaporation dehydration of ethanol. The permeability and selectivity of the membranes were improved simultaneously. The physicochemical properties of the hybrid membranes were investigated. With increasing BTEE content, the amorphous region in the hybrid membranes increased and became more compact. Phase separation took place in the hybrid membranes containing abundant BTEE, and silica particles distributed in the PVA matrix homogeneously. Compared to PVA membranes, the hybrid membranes exhibit high thermal stability and improved separation performances. Silica-hybrids reduced significantly the swelling of PVA membranes in an aqueous solution. The Flory-Huggins interaction parameter of water with membranes chi13 increased with increasing BTEE content, whereas that of ethanol with membranes chi23 decreased. Diffusion behavior of water and ethanol through the membranes were analyzed using the Maxwell-Stefan equation. When BTEE content was below 6 wt%, diffusion coefficient of water D13 increased remarkably and that of ethanol D23 decreased slightly.

A Fully Flexible Potential Model for Carbon Dioxide

National Natural Science Foundation of China [50573063]; Program for New Century Excellent Talents in University of the State Ministry of Education [NCET-05-0566]; Specialized Research Fund for the Doctoral Program of Higher Education of China [2005038401]

UV-crosslinked chitosan/polyvinylpyrrolidone blended membranes for pervaporation

Chitosan is an important biomacromolecule and polyvinylpyrrolidone (PVP) is a biocompatible synthetic polymer. The crosslinked chitosan/PVP blended membranes were prepared via UV irradiation. The as-prepared membranes have a highly crosslinked chitosan/PVP network structure originated from self-crosslinking of PVP and branching of chitosan onto PVP chains during UV irradiation. UV-Crosslinking significantly enhanced the mechanical strength and thermal stability of the blended membranes. Their highest tensile strength is twice as much as that of the pristine chitosan membrane. Maintaining the same swelling degree of the pristine chitosan membrane, the blended membranes have high sorption selectivity towards methanol and water. The as-prepared membranes exhibit an excellent performance in pervaporation separation of methanol/ethylene glycol and water/ethanol and show great potential as new biomedical materials and in the removal of alcohol and water from organics.

Comb-shaped phenolphthalein-based poly(ether sulfone)s as anion exchange membranes for alkaline fuel cells

A series of novel comb-shaped phenolphthalein-based poly(ether sulfone)s was synthesized for preparing anion exchange membranes (AEMs). Hexadecyldimethylamine with a long alkyl chain was used as the quaternization reagent to form a comb-shaped architecture of the copolymers. Due to the presence of a long alkyl side chain with hydrophobicity, the as-synthesized comb-shaped AEMs possess a self-anti-swelling property resulting in a low water uptake and swelling ratio. The PES-B100-C16 membrane exhibits excellent alkaline stability due to the presence of large volumetric β-alkyl chains linking to the cationic group that resist the attack of OH−, and retain available ionic conductivity in a 2 M KOH solution at 60 °C for 360 h. An open circuit voltage of the single cell reached 0.67 V, and the maximum power density was 43 mW cm2 at a current density of 125 mA cm−2 without optimization in a single H2/O2 alkaline fuel cell at 50 °C.

Microstructure-related performances of poly(vinyl alcohol)-silica hybrid membranes: a molecular dynamics simulation study

Molecular dynamics (MD) simulations were used to reveal the relationship between the microstructure and performance of PVA–silica hybrid membranes from the hybridization of silanols, R–Si(OH)3. We first studied the PVA membranes hybridized by silanols with the linear alkyl group of –CnH2n+1 to investigate the effect of their size on the microstructure and properties of the hybrid membranes, then studied hybridization of H2N(CH2)3–Si(OH)3 (APTS) from the hydrolysis of APTEOS. Silica hybridization reduced the mobility of PVA chains remarkably, raised the amorphous region in the PVA matrix, and adjusted the membrane microstructure. Group R in the silanol R–Si(OH)3 has a prodigious effect on the microstructure and performances of the hybrid membranes. Small free volume cavities decreased, and the interchain spacing of PVA chains and big cavities increased with increasing size of group R. Furthermore, MD simulations revealed a relationship between the microstructure and performances of the PVA/APTS hybrid membranes. The results could provide guidance for designing novel functional silica-based hybrid membranes.