Zhengjin Yang

A strategy to construct alkali-stable anion exchange membranes bearing ammonium groups via flexible spacers

A novel strategy is provided to construct alkali-stable poly(phenylene oxide) based anion exchange membranes to alleviate cation-induced degradation. The facile and safe Suzuki-Miyaura coupling reaction was employed in grafting phenylpropyl bromide followed by quaternization and alkalization. Enhanced hydroxide conductivity and suppressed water swelling were obtained.

Highly Conductive Anion‐Exchange Membranes from Microporous Tröger's Base Polymers

The development of polymeric anion-exchange membranes (AEMs) combining high ion conductivity and long-term stability is a major challenge for materials chemistry. AEMs with regularly distributed fixed cationic groups, based on the formation of microporous polymers containing the V-shape rigid Trögers base units, are reported for the first time. Despite their simple preparation, which involves only two synthetic steps using commercially available precursors, the polymers provide AEMs with exceptional hydroxide conductivity at relatively low ion-exchange capacity, as well as a high swelling resistance and chemical stability. An unprecedented hydroxide conductivity of 164.4 mS cm(-1) is obtained at a relatively a low ion-exchange capacity of 0.82 mmol g(-1) under optimal operating conditions. The exceptional anion conductivity appears related to the intrinsic microporosity of the charged polymer matrix, which facilitates rapid anion transport.

A Novel Methodology to Synthesize Highly Conductive Anion Exchange Membranes.

Alkaline polyelectrolyte fuel cell now receives growing attention as a promising candidate to serve as the next generation energy-generating device by enabling the use of non-precious metal catalysts (silver, cobalt, nickel et al.). However, the development and application of alkaline polyelectrolyte fuel cell is still blocked by the poor hydroxide conductivity of anion exchange membranes. In order to solve this problem, we demonstrate a methodology for the preparation of highly OH− conductive anion exchange polyelectrolytes with good alkaline tolerance and excellent dimensional stability. Polymer backbones were grafted with flexible aliphatic chains containing two or three quaternized ammonium groups. The highly flexible and hydrophilic multi-functionalized side chains prefer to aggregate together to facilitate the formation of well-defined hydrophilic-hydrophobic microphase separation, which is crucial for the superior OH− conductivity of 69 mS/cm at room temperature. Besides, the as-prepared AEMs also exhibit excellent alkaline tolerance as well as improved dimensional stability due to their carefully designed polymer architecture, which provide new directions to pursue high performance AEMs and are promising to serve as a candidate for fuel cell technology.

Click Chemistry Finds Its Way in Constructing an Ionic Highway in Anion-Exchange Membrane

To find the way to construct an ionic highway in anion-exchange membranes (AEMs), a series of side-chain-type alkaline polymer electrolytes (APEs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymer backbones were synthesized via Cu(I)-catalyzed click chemistry. The resulting triazole groups and quaternary ammonium (QA) groups facilitate the formation of a continuous hydrogen bond network, which will lead to high hydroxide conductivity according to Grotthuss-type mechanism. Microphase separation induced by long alkyl side chains contributes at the same time to further improving the hydroxide conductivity of the resultant AEMs. Hydroxide conductivity as high as 52.8 mS/cm is obtained for membrane TA-14C-1.21 (IEC = 1.21 mmol/g) with the longest pendant chain at 30 °C, and the conductivity can be increased to 140 mS/cm when the temperature was increased to 80 °C. Moreover, the corresponding water uptake is only 8.6 wt % at 30 °C. In the meantime, the membrane properties can be tuned by precisely regulating the hydrophilic/hydrophobic ratio in the cationic head groups. Compared with AEMs containing triazole and quaternized trimethylammonium head groups, enhanced dimensional stability and mechanical properties are obtained by tuning side-chain chemistry. However, the alkaline stability of the membrane is not as stable as anticipated, probably because of the existence of the triazole ring. Further study will be focused on increasing the alkali stability of the membrane. We envisage that the side-chain-type APEs meditated by click chemistry bearing long hydrophobic side chains pendant to the cationic head groups hold promise as a novel AEMs material.

Alkaline Anion-Exchange Membranes Containing Mobile Ion Shuttles.

A new class of alkaline anion-exchange membranes containing mobile ion shuttles is developed. It is achieved by threading ionic linear guests into poly(crown ether) hosts via host-guest molecular interaction. The thermal- and pH-triggered shuttling of ionic linear guests remarkably increases the solvation-shell fluctuations in inactive hydrated hydroxide ion complexes (OH(-) (H2 O)4 ) and accelerates the OH(-) transport.

Cationic metal–organic framework porous membranes with high hydroxide conductivity and alkaline resistance for fuel cells

Hydroxide conductivity and alkaline stability are challenging issues for anion exchange membrane fuel cells (AEMFCs). Here, a novel sandwiched anion exchange membrane (AEM) was prepared from porous bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) entrapped cationic metal–organic frameworks (MOFs) with a polyvinyl alcohol (PVA) coating on the two sides. The porous BPPO matrix prepared by a non-solvent induced phase separation (NIPS) method provides numerous interconnected nanopores and sponge-like walls and enhanced uptake of alkali. The entrapped cationic MOFs can work as OH− conductive channels while the PVA coating can block the crossover of fuels such as methanol. The final membranes can reach an OH− conductivity of 145 mS cm−1 at 80 °C and a methanol permeability of 3.68 × 10−7 cm2 s−1, the performance of which is much higher than that in the existing literature.

Highly Water Resistant Anion Exchange Membrane for Fuel Cells.

For anion exchange membranes (AEMs), achieving efficient hydroxide conductivity without excessive hydrophilicity presents a challenge. Hence, new strategies for constructing mechanically strengthened and hydroxide conductive (especially at controlled humidity) membranes are critical for developing better AEMs. Macromolecular modification involving ylide chemistry (Wittig reaction) for the fabrication of novel AEMs with an interpenetrating polymer network structure is reported. The macromolecular modification is cost effective, facile, and based on a one-pot synthesis. AEM water uptake is reduced to 3.6 wt% and a high hydroxide conductivity (69.7 mS cm(-1) , 90 °C) is achieved simultaneously. More importantly, the membrane exhibits similar tensile strength (>35 MPa) and comparable flexibility in both dry and wet states. These AEMs could find further applications within anion exchange membrane fuel cells with low humidity or photoelectric assemblies.

Anion exchange membranes with branched ionic clusters for fuel cells

Highly conductive anion exchange membranes (AEMs) are urgently desired for various electro-chemical technologies like fuel cells, flow batteries and electro-dialysis. Available strategies for enhancing the hydroxide conductivity of AEMs commonly focus on increasing the concentration of cationic sites, which, in turn, causes undesirable, excessive dimensional swelling and poor alkaline stability. To overcome this problem, a novel AEM with flexible branched ionic clusters was developed in this study. The improved cation density and cation mobility in the branched ionic clusters have resulted in highly ordered nano-scale channels for hydroxide ion transport and an excellent fuel cell performance of 266 mW cm−2 at 60 °C. Low water uptake and restricted swelling ratio as well as good alkaline stability were also observed benifiting from its unique polymer architecture.

Ion exchange membranes from poly(2,6-dimethyl-1,4-phenylene oxide) and related applications

Ion exchange membranes (IEMs) play a significant role in fields of energy and environment, for instance fuel cells, diffusion dialysis, electrodialysis, etc. The limited choice of commercially available IEMs has produced a strong demand of fabricating IEMs with improved properties via facile synthetic strategies over the past two decades. Poly(phenylene oxide) (PPO) is considered as a promising polymeric material for constructing practical IEMs, due to its advantages of good physicochemical properties, low manufacturing cost and easy post functionalization. In this review, we present the accumulated efforts in synthetic strategies towards diverse types of PPO-based IEMs. Relation between polymer structures and the resulted features is discussed in detail. Besides, applying IEMs from PPO and its derivatives in fuel cell, diffusion dialysis and electrodialysis is summarized and commented.

Thermally triggered polyrotaxane translational motion helps proton transfer

Synthetic polyelectrolytes, capable of fast transporting protons, represent a challenging target for membrane engineering in so many fields, for example, fuel cells, redox flow batteries, etc. Inspired by the fast advance in molecular machines, here we report a rotaxane based polymer entity assembled via host–guest interaction and prove that by exploiting the thermally triggered translational motion (although not in a controlled manner) of mechanically bonded rotaxane, exceptionally fast proton transfer can be fulfilled at an external thermal input. The relative motion of the sulfonated axle to the ring in rotaxane happens at ~60 °C in our cases and because of that a proton conductivity (indicating proton transfer rate) of 260.2 mS cm−1, which is much higher than that in the state-of-the-art Nafion, is obtained at a relatively low ion-exchange capacity (representing the amount of proton transfer groups) of 0.73 mmol g−1.Proton exchange is critical in many applications, such as in conductive proton exchange membranes, but achieving fast proton exchange still remains a challenge. Here the authors report fast proton exchange in a rotaxane based polymer by exploiting thermally triggered translational motion of the mechanically bonded rotaxane.