Nanwen Li

Highly stable, anion conductive, comb-shaped copolymers for alkaline fuel cells.

To produce an anion-conductive and durable polymer electrolyte for alkaline fuel cell applications, a series of quaternized poly(2,6-dimethyl phenylene oxide)s containing long alkyl side chains pendant to the nitrogen-centered cation were synthesized using a Menshutkin reaction to form comb-shaped structures. The pendant alkyl chains were responsible for the development of highly conductive ionic domains, as confirmed by small-angle X-ray scattering (SAXS). The comb-shaped polymers having one alkyl side chain showed higher hydroxide conductivities than those with benzyltrimethyl ammonium moieties or structures with more than one alkyl side chain per cationic site. The highest conductivity was observed for comb-shaped polymers with benzyldimethylhexadecyl ammonium cations. The chemical stabilities of the comb-shaped membranes were evaluated under severe, accelerated-aging conditions, and degradation was observed by measuring IEC and ion conductivity changes during aging. The comb-shaped membranes retained their high ion conductivity in 1 M NaOH at 80 °C for 2000 h. These cationic polymers were employed as ionomers in catalyst layers for alkaline fuel cells. The results indicated that the C-16 alkyl side chain ionomer had a slightly better initial performance, despite its low IEC value, but very poor durability in the fuel cell. In contrast, 90% of the initial performance was retained for the alkaline fuel cell with electrodes containing the C-6 side chain after 60 h of fuel cell operation.

Comb-shaped polymers to enhance hydroxide transport in anion exchange membranes

Comb-shaped poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymers with quaternary ammonium (QA) groups have been synthesized organizing into well-defined micro-morphology for efficient anion (hydroxide) transport. These molecular comb structures show a dramatic enhancement in conductivity and water resistance compared with non-comb-shaped PPOs.

Enhancement of proton transport by nanochannels in comb-shaped copoly(arylene ether sulfone)s.

We gratefully acknowledge support of this research by the WCU(World Class University) program, National Research Foundation(NRF) of the Korean Ministry of Science and Technology (no. R31-2008-000-10092-0). NRCC publication 52850.

Click-chemistry for nanoparticle-modification

Both function and use of nanoparticles (NPs) to a great extent are dominated by interfacial energies, which in turn can be addressed by chemical modifications. This article focuses exclusively on the use of ‘click’-chemistry for NP-surface modification, also putting a major focus on the application of the resulting NPs in bio- and nanoscience. As ‘click’-chemistry is a universal method to link reaction partners in high efficiency, solvent insensitivity and at moderate reaction conditions, its use for engineering NP surfaces has become widespread. The basic approach of Cu(I)-catalyzed azide/alkyne-‘click’ (CuAAC) chemistry for NP-science is elucidated in this article, together with the applications of the resulting surface modified NPs in medicine, nanotechnology and bioassay-science.

A new class of highly-conducting polymer electrolyte membranes: Aromatic ABA triblock copolymers

Highly proton-conducting polymer electrolyte membrane (PEMs) materials are presented as alternatives to state-of-the-art perfluorinated polymers such as Nafion®. To achieve stable PEMs with efficient ionic nanochannels, novel fully aromatic ABA triblock copolymers (SP3O-b-PAES-b-SP3O) based on sulfonated poly(2,6-diphenyl-1,4-phenylene oxide)s (A, SP3O) and poly(arylene ether sulfone)s (B, PAES) were synthesized. This molecular design for a PEM was implemented to promote the nanophase separation between the hydrophobic polymer chain and hydrophilic ionic groups, and thus to form well-connected hydrophilic nanochannels that are responsible for the water uptake and proton conduction. Relative to other hydrocarbon-based PEMs, the triblock copolymer membranes showed a dramatic enhancement in proton conductivity under partially hydrated conditions, and superior thermal, oxidative and hydrolytic stabilities, suggesting that they have the potential to be utilized as alternative materials in applications operating under partly hydrated environments.

Highly stable anion exchange membranes based on quaternized polypropylene

A series of novel quaternized polypropylene (PP) membranes with ‘side-chain-type’ architecture was prepared by heterogeneous Ziegler–Natta catalyst mediated polymerization and subsequent quaternization. Tough and flexible anion exchange membranes were prepared by melt-pressing of bromoalkyl-functionalized PP (PP-CH2Br) at 160 °C, followed by post-functionalization with trimethylamine (TMA) or N,N-dimethyl-1-hexadecylamine (DMHDA) and ion exchange. By simple incorporation of a thermally crosslinkable styrenic diene monomer during polymerization, crosslinkable PP-AEMs were also prepared at 220 °C. PP-AEM properties such as ion exchange capacity, thermal stability, water and methanol uptake, methanol permeability, hydroxide conductivity and alkaline stability of uncrosslinked and crosslinked membranes were investigated. Hydroxide conductivities of above 14 mS cm−1 were achieved at room temperature. The crosslinked membranes maintained their high hydroxide conductivities in spite of their extremely low water uptake (up to 56.5 mS cm−1 at 80 °C, water uptake = 21.1 wt%). The unusually low water uptake and good hydroxide conductivity may be attributed to the “side-chain-type” structures of pendent cation groups, which probably facilitate ion transport. The membranes retained more than 85% of their high hydroxide conductivity in 5 M or 10 M NaOH aqueous solution at 80 °C for 700 h, suggesting their excellent alkaline stability. It is assumed that the long alkyl spacer in the ‘side-chain-type’ of 9 carbon atoms between the polymer backbone and cation groups reduces the nucleophilic attack of water or hydroxide at the cationic centre. Thus, PP-based AEMs with long “side-chain-type” cations appear to be very promising candidates with good stability for use in anion exchange membrane fuel cells (AEMFCs).

Morphological transformation during cross-linking of a highly sulfonated poly(phenylene sulfide nitrile) random copolymer

We present a new approach of morphological transformation for effective proton transport within ionomers, even at partially hydrated states. Highly sulfonated poly(phenylene sulfide nitrile) (XESPSN) random network copolymers were synthesized as alternatives to state-of-the-art perfluorinated polymers such as Nafion®. A combination of thermal annealing and cross-linking, which was conducted at 250 °C by simple trimerisation of ethynyl groups at the chain termini, results in a morphological transformation. The resulting nanophase separation between the hydrophilic and hydrophobic domains forms well-connected hydrophilic nanochannels for dramatically enhanced proton conduction, even at partially hydrated conditions. For instance, the proton conductivity of XESPSN60 was 160% higher than that of Nafion® 212 at 80 °C and 50% relative humidity. The water uptake and dimensional swelling were also reduced and mechanical properties and oxidative stability were improved after three-dimensional network formation. The fuel cell performance of XESPSN membranes exhibited a significantly higher maximum power density than that of Nafion® 212 under partially hydrated environments.

Crosslinking of comb-shaped polymer anion exchange membranes via thiol–ene click chemistry

To produce anion conductive and durable polymer electrolytes for alkaline fuel cell applications, a series of cross-linked quaternary ammonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide)s with mass-based ion exchange capacities (IEC) ranging from 1.80 to 2.55 mmol g−1 were synthesized via thiol–ene click chemistry. 1H nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FTIR) were used to confirm the chemical structure of the samples. From small angle X-ray scattering (SAXS), it was found that the cross-linked membranes developed microphase separation between the hydrophilic PPO backbone and the hydrophobic alkyl side chains. The ion conductivity, dimensional stability, and alkaline durability of the cross-linked membranes were evaluated. The hydroxide ion conductivity of the cross-linked samples reached 60 mS cm−1 in liquid water at room temperature. The chemical stabilities of the membranes were evaluated under severe, accelerated aging conditions and degradation was quantified by measuring the ionic conductivity changes during aging. The cross-linked membranes retained their relatively high ion conductivity and good mechanical properties in both 1 M and 4 M NaOH at 80 °C after 500 h. Attenuated total reflection (ATR) spectra were used to study the degradation pathways of the membranes, and it was determined that β-hydrogen (Hofmann) elimination was likely to be the major pathway for degradation in these membranes.

Controlled functionalization of poly(4-methyl-1-pentene) films for high energy storage applications

A new family of poly(4-methyl-1-pentene) ionomer [PMP-(NH3)xA-y] (x = 1, 2, 3 and A = Cl−, SO42−, PO43−, y = NH3 content) modified (NH3+)xAx− ionic groups has been synthesized. The ionomers were synthesised using either a traditional Ziegler–Natta or a metallocene catalyst for the copolymerisation of 4-methyl-1-pentene and bis(trimethylsilyl)amino-1-hexene. A systematic study was conducted on the effect of the subsequent work-up procedures that can prevent undesirable side reactions during the synthesis of the [PMP-(NH3)xA-y] ionomers. The resulting PMP-based copolymers were carefully monitored by a combination of nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), mechanical properties, dielectric properties, and electric displacement–electric field (D–E) hysteresis loop measurements. Our results reveal that the [PMP-(NH3)xA-y] ionomer films show a significantly enhanced dielectric constant (∼5) and higher breakdown field (∼612 MV m−1) as compared with pure PMP films. Additionally, these PMP-based films show good frequency and temperature stabilities (up to 160 °C). A reliable energy storage capacity above 7 J cm−3 can be obtained, and is twice the energy storage capacity of state-of-the-art biaxially oriented polypropylene films, which can be attractive for technological applications for energy storage devices.

Facilitating Anion Transport in Polyolefin-Based Anion Exchange Membranes via Bulky Side Chains.

Highly anion-conductive polymer electrolyte membranes with excellent alkaline stabilities for fuel cell applications were prepared. Thus, a series of polyolefin copolymers with poly(4-methyl-1-pentene) (PMP) moieties containing bulky side chains and side-chain quaternary ammonium (QA) groups were prepared through copolymerization with a Ziegler-Natta catalyst and subsequent quaternization. The separation of hydrophilic microphase and hydrophobic microphase was induced by PMP bulky side chains, and then well-connected ionic domains were formed. This result was confirmed by AFM (atomic force microscopy) and SAXS (small-angle X-ray scattering) analyses. It was discovered that well-defined ionic domains of the PMP-TMA-x (TMA, trimethylamine) membranes depended on the content of PMP moieties. The well-defined ionic domains enhanced the hydroxide conductivity of the PMP-TMA-x membranes despite their lower water uptake (WU) as compared to polypropylene (PP)-containing membranes (PP-TMA-x). The PMP-TMA-41 membrane showed the highest ionic conductivity value (43 mS/cm) while maintaining low WU (29.2 wt %) at room temperature. The membranes mostly preserved (>93.0%) their initial hydroxide conductivity after alkaline treatment (10 M aqueous NaOH, 80 °C, 700 h), thereby revealing desirable alkali stability characteristics. Presumably, the nucleophilic attack from hydroxide or water in the cationic center is inhibited by long alkyl spacers (-CH2-)n (n = 9) which are located between the cation groups and the polymer backbone.