Xuemei Wu

Enhancement of hydroxide conductivity by the di-quaternization strategy for poly(ether ether ketone) based anion exchange membranes

Poly(ether ether ketone) (PEEK) with multiple quaternary ammonium groups on pendent side chains is synthesized through the chloromethylation di-quaternization route, using bi-functional 1,4-diazabicyclo[2,2,2]octane (DABCO) as quaternization reagent. The materials are made into tough and transparent anion exchange membranes (AEMs) by solvent casting. The purpose is to promote efficient hydroxide ion conductive channels, which are particularly important and challenging to improve hydroxide conductivity of AEMs due to the inherently low mobility of hydroxide ions. Transmission electron microscopy (TEM) images show ionic clusters of bigger size scattering in the di-quaternized membranes compared with the mono-quaternized membranes. Given similar ion exchange capacities (IECs), the di-quaternized membranes exhibit much higher values of effective hydroxide ion mobility (∼3.5 × 10−4 cm2 s−1 V−1) than the mono-quaternized membranes (∼1.7 × 10−4 cm2 s−1 V−1). The structure of multiple quaternary ammonium groups on the pendent side chain promotes hydrophilic–hydrophobic micro-phase separation and efficient hydroxide ion conductive channels in the membranes. As a result, hydroxide conductivity of the di-quaternized membranes is about 2 to 3 fold higher than that of the mono-quaternized membranes with similar IEC, exhibiting a high value of about 35.3 mS cm−1 at 25 °C. At a certain IEC, the di-quaternized membranes have fewer pendent side chains on the polymer backbone, which also benefits the mechanical and chemical stabilities of the AEMs.

Magnetic titania-silica composite–Polypyrrole core–shell spheres and their high sensitivity toward hydrogen peroxide as electrochemical sensor

A novel core-shell sphere with controlled shell thickness was synthesized by in situ chemical oxidative polymerization of pyrrole on FTS (Fe(2)O(3)/TiO(2)/SiO(2) composite) surface. The dual porosity of 2-3 nm and 40-50 nm in FTS core particle provides the hybrids with a high surface area to volume ratio, which enormously facilitates the molecule diffusion process. Furthermore, the porous FTS particle encapsulate Fe(2)O(3) and TiO(2) leading to its synergetic interaction with the PPy coating based on FTIR analysis. The unique structure and composition of the hybrid spheres result in new sensing property that is not available from their single counterparts. Cyclic voltammetry results demonstrate that the spheres with appropriate concentration of PPy exhibit enhanced electrocatalytic activity toward the reduction of H(2)O(2) in 0.1 M phosphate buffer solution. The sensing performance tests show that the hybrids possess good linear response in wide H(2)O(2) concentration range (10-4000 μM) and high sensitivity to H(2)O(2) (0.653 AM(-1) cm(-2)) at room temperature. The formation mechanism of the spheres was proposed based on the fact that the FTS core was coated firstly by a smooth PPy layer and then PPy nanoparticles. The work reported here provides an alternative concept for preparation of functional materials with new nanostructures and properties.

A H3PO4 preswelling strategy to enhance the proton conductivity of a H2SO4-doped polybenzimidazole membrane for vanadium flow batteries

A H3PO4 preswelling strategy is proposed to prepare H2SO4-doped polybenzimidazole (PBI) membranes for vanadium flow batteries (VFB). Before being immersed in 3.0 M H2SO4, PBI membranes are preswelled by immersion in concentrated H3PO4, which leads to a higher H2SO4 doping level, thereby dramatically reducing the area resistance of the PBI membrane to 0.43 Ω cm2, which is close to that of Nafion 212 (0.35 Ω cm2) and much lower than that of Fumasep®FAP-450 (0.64 Ω cm2). Meanwhile, the substantially high selectivity is maintained. The VFB assembled with the H3PO4 preswelled PBI membrane displays high energy efficiencies (EE: 80.9–89.2%) over a current density range of 20–80 mA cm−2, much higher than those of the non-preswelled PBI membrane (EE: 66.8–84.5%), Nafion 212 (EE: 63.1–75.6%) and Fumasep®FAP-450 (EE: 75.5–82.6%). The stable performance over 50 charge–discharge cycles demonstrates the good physicochemical stability of the preswelled PBI membrane. Considering the above results, the H3PO4 preswelling strategy proposed herein is facile and efficient for fabricating high-performance PBI membranes for VFB.

Thin skinned asymmetric polybenzimidazole membranes with readily tunable morphologies for high-performance vanadium flow batteries

A series of thin skinned asymmetric polybenzimidazole (PBI) membranes with readily tunable morphologies are fabricated by leaching out the porogen dibutyl phthalate (DBP), for vanadium flow batteries (VFBs). The ultrathin defect-free skin layer fully guarantees high ion selectivity of the membrane. Meanwhile, the area resistance (AR) of the asymmetric PBI membrane is dramatically reduced compared to that of the dense one because of interconnected macro-pores in the sublayer. The membrane morphologies and properties are readily adjusted by varying the porogen content (0–300 wt%), thus managing well the trade-off between AR and ion selectivity. The membrane prepared by adding 200 wt% porogen has a high porosity of 74.9 vol% and an appropriately dense skin thickness of 4.9 μm, and yields the best balance between AR and ion selectivity, assembled with which the flow battery achieves excellent cell performances (coulombic efficiency, CE: 99.0%; energy efficiency, EE: 82.3%) as well as a moderate capacity decay rate (CDR, 0.4% per cycle) at 80 mA cm−2 over cycling. The thin skinned asymmetric PBI membranes prepared here surpass the commercial Nafion 211 membrane (CE: 84.6%; EE: 68.1%; CDR: 1.3% per cycle) in terms of cell performances and cost, becoming a promising candidate for VFBs.

Electrospun nanofiber enhanced imidazolium-functionalized polysulfone composite anion exchange membranes

A novel method of improving interfacial compatibility in electrospun anion exchange membranes (AEMs) is developed by using imidazolium-functionalized polysulfone (IMPSF) as both electrospun fiber mats and interfiber voids filler. Scanning electron microscopy (SEM) illustrates the defect-free and fiber-retained morphology of the IMPSF electrospun AEMs. Transmission electron microscopy (TEM) shows better aggregation of ion clusters in the IMPSF electrospun AEMs. As a result, the electrospun AEMs prepared in the present work exhibit much higher hydroxide conductivity increment than most reported electrospun AEMs (around 1.7 folds in 20 °C water and 100 folds at 60 °C, relative humidity (RH) 40% as compared with the corresponding cast AEMs). Excellent interfacial compatibility and microphase separation morphology also promote swelling resistance and mechanical and alkaline stabilities of the IMPSF electrospun composite AEMs. Compared with the cast AEMs, tensile strength increment is up to 22%, alkaline stability increases more than one fold after immersing in 1 M KOH at 60 °C for 24 h. Results in the present work are helpful to componential design of the electrospun AEMs.

Polybenzimidazole membranes with nanophase-separated structure induced by non-ionic hydrophilic side chains for vanadium flow batteries

Polybenzimidazole (PBI) membranes with nanophase-separated structure induced by non-ionic hydrophilic side chain are designed and fabricated for vanadium flow batteries (VFBs). The designed PBI membranes are prepared by grafting non-ionic hydrophilic side chains via an N-substitution reaction. This molecular modification induces nanophase separation and formation of hydrophilic clusters, which act as effective proton transfer pathways, hence dramatically improving the proton conductivity. Meanwhile, the vanadium permeability is inappreciable due to the appropriate size of hydrophilic clusters and Donnan exclusion of protonated grafted-PBI membranes (GPBI). Free from ion exchange groups, GPBI membranes maintain the good chemical stability of the pristine PBI membrane. As a result, the designed membrane exhibits an impressive performance, combining ultrahigh proton conductivity, ion selectivity and chemical stability. The GPBI-based VFB exhibits a coulombic efficiency of over 99% and an energy efficiency of 84% at 120 mA cm−2, the highest reported for dense PBI membranes for VFB applications. The decent stability of GPBI membranes is demonstrated by the stable performance over 200 charge–discharge cycles and the ex situ immersion test. This work provides a new insight into the design of high-performance PBI membranes for VFB applications.

Dimensionally stable hexamethylenetetramine functionalized polysulfone anion exchange membranes

A novel alkaline group, hexamethylenetetramine (HMTA) with four tertiary amine groups and β-hydrogen-absent structure, has been employed as mono-quaternization reagent to prepare HMTA mono-quaternized polysulfone anion exchange membranes (PSF-QuOH AEMs). Analyzed by molecular dynamics simulations, mono-quaternized HMTA shows a superior aggregating ability by strong electrostatic interaction with hydroxide to suppress water swelling even at high IECs. In particular, the PSF-QuOH membrane with a high IEC of 2.23 mmol g−1 exhibits a low swelling ratio of 21% even at 60 °C. The resulting high concentration of cationic groups and the interactions between multiple hydrogen atoms in HMTA and hydroxide/water are helpful to induce the formation of the continuous and efficient hydrogen-bond networks, promoting ionic transport. High hydroxide conductivity of 35 mS cm−1 is achieved at 20 °C. Excellent swelling resistance also benefits the mechanical and chemical stabilities of the PSF-QuOH membranes. A considerable mechanical strength of 17.7 MPa is observed in the fully hydrated membrane. The hydroxide conductivity is stable at around 86% of the initial value after 1 M KOH immersion at 60 °C for 168 h.

A bilateral electrochemical hydrogen pump reactor for 2-propanol dehydrogenation and phenol hydrogenation

A bilateral electrochemical hydrogen pump reactor is proposed for the first time. In one electrochemical hydrogen pump (EHP) configuration, in situ adsorbed hydrogen atoms for phenol hydrogenation at the cathode are donated by the dehydrogenation of 2-propanol instead of a conventional H2 or H2O anode feedstock. For the anodic 2-propanol dehydrogenation EHP reactor, by increasing Pt–Ru/C catalyst loading and applying a pulse current operation, the applied potential can be controlled below 0.2 V, which is much lower than the thermodynamic dissociation potential of water (1.23 V). For the cathodic cyclohexanone hydrogenation EHP reactor, the hydrogenation rate reaches 73.9 mmol h−1 g−1Pd, nearly three times of that in aqueous-phase selective hydrogenation reactors. Pd/C and Pt/C catalysts have high catalytic selectivity to cyclohexanone (95.5%) and cyclohexanol (95.4%), respectively. In the bilateral EHP reactor, 2-propanol dehydrogenation and phenol hydrogenation are completed simultaneously, exhibiting a comparable hydrogenation rate, selectivity and conversion to that in the individual EHP reactors. The feasibility of the bilateral EHP reactor provides a novel idea to efficiently integrate multiple reactors into one configuration, which greatly simplifies hydrogen production, storage and transportation, as well as reactor equipment.

Effects of Hydrophobicity of Diffusion Layer on the Electroreduction of Biomass Derivatives in Polymer Electrolyte Membrane Reactors

For the first time, the hydrophobicity design of a diffusion layer based on the volatility of hydrogenation reactants in aqueous solutions is reported. The hydrophobicity of the diffusion layer greatly influences the hydrogenation performance of two model biomass derivatives, namely, butanone and maleic acid, in polymer electrolyte membrane reactors operated at atmospheric pressure. Hydrophobic carbon paper repels aqueous solutions, but highly volatile butanone can permeate in vapor form and achieve a high hydrogenation rate, whereas, for nonvolatile maleic acid, great mass transfer resistance prevents hydrogenation. With a hydrophilic stainless-steel welded mesh diffusion layer, aqueous solutions of both butanone and maleic acid permeate in liquid form. Hydrogenation of maleic acid reaches a similar level as that of butanone. The maximum reaction rate is 340 nmol cm(-2)  s(-1) for both hydrogenation systems and the current efficiency reaches 70 %. These results are better than those reported in the literature.

Tailored Robust Hydrogel Composite Membranes for Continuous Protein Crystallization with Ultrahigh Morphology Selectivity

The tailored and robust hydrogel composite membranes (HCMs) with diverse ion adsorption and interfacial nucleation property are prepared and successfully used in the continuous lysozyme crystallization. Beyond the heterogeneous supporter, the HCMs functioning as an interface ion concentration controller and nucleation generator are demonstrated. By constructing accurately controlled nucleation and growth circumstances in the HCM-equipped membrane crystallizer, the target desired morphology (hexagon cube) and brand-new morphology (multiple flower shape) that differ from the ones created in the conventional crystallizer are continuously and repetitively generated with ultrahigh morphology selectivity. These tailored robust HCMs show great potential for improving current approaches to continuous protein crystallization with specific crystal targets from laboratorial research to actual engineering applications.