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The process of crosslinking is widely employed to increase the physicochemical stability of anion exchange membranes and, in some cases, improve ion conductivity. For a general case in which a polymer is crosslinked by a crosslinking agent, the physicochemical properties of the polymer can be greatly altered, depending on the type of crosslinking agent. In this study, we induced crosslinking without a crosslinking agent to intentionally maximise various physical properties (i.e., mechanical properties, swelling ratios, and so forth) of two commercially-available polymers. A triazole was incorporated into the conducting group to maximise the ion conductivity, especially under room humidity (RH) conditions. The crosslinked PPO-SEBS membranes prepared through this approach were not only capable of forming very thin membranes (10 μm thickness) with excellent physical properties (34.3 MPa of tensile strength and 91.6% of elongation at break) but also exhibited high hydroxide ion conductivity under 95% RH, and conductivity plays an important role in achieving good fuel cell performance. When the membrane electrode assembly (MEA), as fabricated utilising a crosslinked PPO-SEBS membrane and a platinum on carbon (Pt/C) catalyst on each electrode, was operated in conditions with a H2/O2 gas flow and a 60 °C temperature, a stable fuel cell performance was obtained for a long period of time (300 hours) at a maximum power density of 405 mW cm−2. This result surpasses the performance of commercialized AEMs and is comparable with the performance levels of cutting-edge AEMs when operated under similar conditions.

The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve.

We chemically modify a commercially available elastomeric triblock copolymer, poly(styrene-b-ethylene-co-butylene-b-styrene) SEBS, to produce quaternary ammonium-functionalized hexyl bis(quaternary ammonium)-mediated partially-crosslinked SEBSs with different grafting degree of the conducting head groups as highly conductive and stable anion exchange membranes (AEMs). In an attempt to achieve a high ion exchange capacity and hence conductivity of the corresponding membranes without causing ‘gelation’, which these types of SEBS polymers typically experience, we use a SEBS with a high content (57%) of styrene, and graft the quaternary ammonium (QA) as both a crosslinker and the conducting head group on the side chain of the SEBS. The partial crosslinking approach, in combination with introducing these QA-conducting head groups as the side chain of the SEBS, helps to form larger ion clusters, and also to form a nano-phase morphology with long-range connecting channels. This induced the highest ion conductivity, 174.8 mS cm−1 at 80 °C, reported to date among these types of SEBS series. In addition, we obtain excellent cell performance with a current density of 450 mA cm−2 at 0.6 V and a peak power density of 315 mW cm−2 at 95% RH and 60 °C using the corresponding AEM.

In this study, spirobiindane-based poly(arylene ether sulfone)s with quaternary ammonium-functionalized side chains were synthesized as alkaline anion exchange membrane fuel cell (AEMFC) electrode binding materials. A series of novel AEMFC electrode ionomers with different main-chain structures were prepared. Three-dimensional spirobiindane structures were introduced to improve the gas permeability of the binding material. The ionomers were characterized by NMR and thermogravimetric analysis. Single-cell performance tests using the ionomers were also carried out. The ionomer sample with a spirobiindane unit in the polymer backbone and quaternary ammonium-functionalized hexyloxy side chain showed good potential for AEMFC applications. A single-cell using this ionomer as a binder exhibited a peak power density of 140 mW/cm2. The modification of main-chain is considered to be a suitable approach for the synthesis of AEMFC electrode ionomers.

Quaternary ammonium-functionalized poly(ether sulfone) (QA-PES) block copolymers with different block chain lengths were prepared as anion exchange membranes. Copolymers with similar hydrophilic/hydrophobic block ratios and hence similar IEC values, but with various oligomer block chain lengths were synthesized to investigate, for the first time, how the length of each oligomer included in the block copolymers affected the chemophysical and electrical properties of the obtained QA-PES anion exchange membranes. The copolymer with the optimal hydrophilic:hydrophobic block chain length (QA-PES-16-30) showed good phase separation. This led to the optimal formation of ionic clusters, and the highest ion conductivity of 81.01 mS cm−1 at 80 °C, as well as excellent physicochemical stability even after alkaline treatment in 1 M NaOH at 60 °C for 500 h, due to strengthening of the hydrophobic regions, strongly suggesting that the block chain length of each of the hydrophilic and hydrophobic blocks can affect the physicochemical properties of the polymer membranes. The H2/O2 single cell performance using the QA-PES-16-30 membrane showed a maximum power density of 260 mW cm−2, much higher than that obtained from the Tokuyama A201.

The recent development of ultrathin anion exchange membranes and optimization of their operating conditions have significantly enhanced the performance of alkaline-membrane fuel cells (AMFCs); however, the effects of the membrane/electrode interface structure on the AMFC performance have not been seriously investigated thus far. Herein, we report on a high-performance AMFC system with a membrane/electrode interface of novel design. Commercially available membranes are modified in the form of well-aligned line arrays of both the anode and cathode sides by means of a solvent-assisted molding technique and sandwich-like assembly of the membrane and polydimethylsiloxane molds. Upon incorporating the patterned membranes into a single-cell system, we observe a significantly enhanced performance of up to ∼35% compared with that of the reference membrane. The enlarged interface area and reduced membrane thickness from the line-patterned membrane/electrode interface result in improved water management, reduced ohmic resistance, and effective utilization of the catalyst. We believe that our findings can significantly contribute further advancements in AMFCs.

Anion exchange membranes based on quaternary ammonium-functionalized poly(ether sulfone ketone) block copolymers (QA-PESK) with various hydrophilic–hydrophobic oligomer block ratios (10:7, 10:18, and 10:26) were synthesized, and the block length effect on the membranes' physicochemical and electrical properties were systematically investigated. The QA-PESK-10-18 membrane, prepared using a hydrophilic and hydrophobic block ratio of 10:18, displayed well-balanced hydrophilic/hydrophobic phase separation, the highest conductivity of 23.19 mS cm−1 at 20 °C and 57.84 mS cm−1 at 80 °C, and the highest alkaline stability among the three block ratios tested, indicating that the membranes' properties were closely related to their morphologies, which were determined by the hydrophilic/hydrophobic ratio of the block copolymer. The H2/O2 single cell performance using the QA-PESK-10-18 revealed a maximum power density of 235 mW cm−2.

An additive-induced physically crosslinked system for use as anion exchange membranes was developed by blending an additive (DHBQA) containing both long alkyl (C12, dodecyl) and quaternary ammonium groups into a dodecyl-substituted and quaternary ammonium-functionalized PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) (C12-PPO-QA). The additive, whose diammonium groups act as ion conductors, and whose long alkyl chains bind to the alkyl chains of the C12-PPO-QA polymer through van der Waals interactions, induced the formation of ion clusters as well as physical crosslinking of the C12-PPO-QA polymer. Controlling the amount of additive to maximize its intermolecular interactions with the polymer resulted in a PPO-10.8 membrane (10.8 wt% of additive relative to polymer) with very high ion conductivities of 49.2 mS cm−1 at 20 °C and 107.6 mS cm−1 at 80 °C and low swelling ratios of 18.2% at 20 °C and 33.3% at 80 °C, as well as excellent mechanical and alkaline stability. The H2/O2 single cell performance using PPO-10.8 reached a maximum power density about 60 mW cm−2, higher than that of the PPO-0 sample.

In this report, we introduced spirobiindane group to poly(arylene ether sulfone) (PES) to build the structure of polymers with intrinsic microporosity (PIMs). A novel PESs (QOH-SBIs), which have spirobiindane and tetra(quaternary ammonium) hydroxide pendant moieties, were synthesized for anion-conducting binder material in membrane electrode assembly (MEA) of solid alkaline exchange membrane fuel cell (SAEMFC). The time-lag method was used to check the high gas permeability of the polymers. The high permeability is due to the micro-pores at the molecular level that is formed by the difference in chain thicknesses between two alternating units, thick spirobiindane group and thin arylene ether sulfone group. QOH-SBIs shows a semi-rigid chain conformation in a solution. The inter-chain spacing and chain conformation were measured with wide angle X-ray diffraction (WAXD) and small angle neutron scattering (SANS), respectively. High gas permeability directly affected the performance of SAEMFC. The MEA with spirobiindane-modified PES shows much higher maximum power density than that of spirobiindane-free PESs.

Poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs)-based anion exchange membranes (AEMs) with four of the most widely investigated head groups were prepared. Through a combination of experimental and simulation approaches, the effects of the different types of head groups on the properties of the AEMs, including hydroxide conductivity, water content, physicochemical stability, and fuel cell device performance were fully explored. Unlike other studies, in which the conductivity was mostly investigated in liquid water, the conductivity of the PPO-based AEMs in 95% relative humidity (RH) conditions as well as in liquid water was investigated. The conductivity trend in 95% RH condition was different from that in liquid water but corresponded well with the actual cell performance trend observed, suggesting that the AEM fuel cell performance under in situ cell conditions (95% RH, 60 °C, H2/O2) is more closely related to the conductivity measured ex situ under 95% RH conditions (60 °C) than in liquid water. On the basis of the conductivity data and molecular simulation results, it was concluded that the predominant hydroxide ion-conducting mechanism in liquid water differs from that in the operating fuel cell environment, where the ionomers become hydrated only as a result of water vapor transported into the cells.

Electrode properties determine the membrane-electrode assembly (MEA) performance of fuel cells and are influenced by the materials and processing. A high-performance electrode requires good adhesion between the membrane and catalyst, excellent gas permeability, and good ion conduction. Here, sulfonated poly(arylene ether sulfone) (SPAES) containing a bulky spirobiindane (Spiro) group is proposed as a new hydrocarbon electrode binder for MEAs. The effect of the Spiro group on MEA performance was compared to that of the common biphenylsulfone. The structural differences between the two binders affected the mass transport region of the current-voltage polarization (related to gas permeation). The spiro-polyethersulfone (PES) had 115 times higher O2 permeability (30 Barrer) than that of PES. From 1H NMR spectra, the degree of sulfonation (DS) of each SPAES was confirmed, where the proton conductivity of Spiro 30, Spiro 20, and BPS 30 were 28.2, 10.1, and 26.9 mS cm−1, respectively. In addition, the DS of the electrode binder contributed to the Ohmic region of the i–V curve. The Ohmic resistance of these hydrocarbon binders was similar to that of commercial Nafion binder (0.13–0.15 Ω cm2). However, the charge transfer resistance of our binders was higher than that of Nafion due to insufficient gas permeability and low proton conductivity.

Poly(dibenzylated benzimidazolium) bromides (Bz-PBI-Br) were converted successfully to OH− ion conducting poly(dibenzylated benzimidazolium) hydroxides (Bz-PBI-hydroxides) by the treatment of KOH. The Bz-PBI-hydroxides obtained in this study showed an excellent alkali tolerance compared to previously synthesized poly(dimethylated benzimidazolium) hydroxides (Me-PBI-hydroxides). According to 1H-NMR analysis, Me-PBI-hydroxides were decomposed during KOH treatment. In order to find out the reason, density functional theory (DFT) calculations of two benzimidazolium structures, e.g., dimethylated benzimidazolium (Me-BI+) and dibenzylated benzimidazolium (Bz-BI+), were performed. Bz-BI+ showed lower electron affinity and OH−-binding energies at the C2 position of the benzimidazolium ring than Me-BI+. These DFT results strongly confirm that Bz-BI+ is less vulnerable to an OH− attack than Me-BI+; this contributes to the enhanced stability and OH− ion conductivity of the Bz-PBI-hydroxides.

Crosslinked poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) using hydrophilic crosslinkers that contain ethylene oxide (EO) were prepared as novel anion exchange membranes (AEMs), and the effect of crosslinker EO length was systematically investigated. The physicochemical, morphological and electrochemical properties of the corresponding AEMs were also compared against the membrane prepared from a typical hydrophobic alkyl-type crosslinker. The ethylene oxide crosslinker helped to form ion clusters and to enhance the water-holding capacity of the corresponding AEMs, which promoted high conductivity both in water and in 95% room humidity conditions, whilst maintaining high physicochemical stability. However, it was also found that the presence of a long ethylene oxide as a crosslinker may induce polymer crystallinity, which reduces both conductivity and the alkaline stability of the corresponding crosslinked membranes. The effect of ethylene oxide chain length on the morphology, electrochemical and physicochemical properties, were also investigated. The xBEO-PPO membrane, having bis(ethylene oxide) (BEO) as a crosslinker, showed the highest conductivity of 131.96 mS/cm in water at 80 °C, and 55.21 mS/cm in 95% RH at 80 °C; this was owing to its crystalline nature. The single-cell performance of 444 mW/cm2 peak power density, was also obtained from the xBEO-PPO membrane.

Poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) triblock copolymers are prepared with 50% hexyl quaternary ammonium (HQA) and 20% alkyl chains of different chain lengths: C4 (50HQA-20C4-SEBS) and C12 (50HQA-20C12-SEBS). Their physicochemical and electrical properties are investigated for use as novel alkaline anion exchange membranes. In most other comb-type polymers, the alkyl chain is introduced as a hydrophilic spacer unit between the polymer backbone and conducting head group. In the current study, additional alkyl chains are grafted onto the SEBS polymer as hydrophobic side chains as well as typical hydrophilic side chains. Introduction of this extra hydrophobic alkyl spacer group is done to obtain well-connected morphology, increased free volume, enhanced water uptake and related conductivity, together with improved physicochemical properties of the membranes obtained. The results are compared with HQA-functionalized SEBS without the hydrophobic alkyl spacer group (50HQA-20C0-SEBS), and the effects of the alkyl chain lengths on the structure of polymers and physicochemical properties of the corresponding membranes are investigated.

To realize effective polymer electrolyte fuel cell (PEFC) operation under non-humidified conditions, a new self-humidifying dual exchange membrane fuel cell (DEMFC) was designed and evaluated. The DEMFC was fabricated using sequentially aligned membrane electrode assemblies (MEAs) consisting of an anion exchange membrane (AEM) and a cation exchange membrane (CEM). In this system, water simultaneously generated by the half-cell reactions at both the anode and the cathode is then supplied to the other MEA through the gas diffusion medium and the bipolar plate in the DEMFC under completely dry conditions, resulting in high cell performance. Also, durability of a DEMFC five-cell stack was tested by accelerated on/off operation under fully dry conditions. No cell degradation occurred over 50 cycles (200 h), indicating that this DEMFC design provides an effective approach for constructing practical miniaturized PEFCs that do not require external humidification systems.