Chengji Zhao

From metal–organic framework (MOF) to MOF–polymer composite membrane: enhancement of low-humidity proton conductivity

A chiral two-dimensional MOF, {[Ca(D-Hpmpc)(H2O)2]·2HO0.5}n (1, D-H3pmpc = D-1-(phosphonomethyl) piperidine-3-carboxylic acid), with intrinsic proton conductivity has been synthesized and characterized. Structure analysis shows that compound 1 possesses protonated tertiary amines as proton carriers and hydrogen-bonding chains served as proton-conducting pathways. Further, MOF–polymer composite membranes have been fabricated via assembling polymer PVP with different contents of rod-like 1 submicrometer crystals. Interestingly, the proton conductivity of this composite membrane containing 50 wt% 1 is rapidly increased, compared with that of pure submicrometer crystals at 298 K and ∼53% RH. Therefore, it is feasible to introduce humidification of PVP into composite membranes to enhance low-humidity proton conductivity; and humidified PVP with adsorbed water molecules plays an important role in proton conduction indicated by the results of water physical sorption and TG/DTG analyses. This study may offer a facile strategy to prepare a variety of solid electrolyte materials with distinctive proton-conducting properties under a low humidity.

Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells

Silane-cross-linked polybenzimidazole (PBI) membranes with high proton conductivity and excellent mechanical properties were successfully prepared by using a silane monomer, γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560), as a cross-linker. Fourier transform infrared spectroscopy and solubility tests were used to characterize and confirm the cross-linked structure in the membranes. The silane-cross-linked membranes displayed excellent chemical stability and improved mechanical strength. Especially at high temperature (130 °C), where the tensile strength value was in the range of 68.6 to 99.3 MPa, while that of the pristine PBI was 61.7 MPa. Moreover, the proton conductivity was significantly enhanced because the silane-cross-linked structure in the membranes could absorb more phosphoric acid. Considering the tradeoff of mechanical properties and proton conductivity, 3% KH560 in weight was demonstrated to be the optimum content in the membranes, for instance, the SCPBI-3/7.95 PA (the cross-linker content was 3 wt% and the PA doping level was 7.95) had a proton conductivity of 0.081 S cm−1 and that of the SCPBI-3/9.07 PA was 0.114 S cm−1 at 200 °C, while that of pristine PBI was 0.015 S cm−1 at 200 °C.

Cross-linked hydroxide conductive membranes with side chains for direct methanol fuel cell applications

A series of novel poly(ether ether ketone) copolymers containing methyl groups on the side chain were prepared based on a new monomer (3,4-dimethyl)phenylhydroquinone. Then a series of hydroxide exchange membranes with different IEC values were obtained through bromination and quaternary amination of the copolymers. By adjusting the contents of methyl groups in the copolymers, we could control the final structures of the membranes. The chemical structures of the monomers and copolymers were analyzed by 1H NMR spectroscopy. After that, for the purpose of enhancing the dimensional stability and methanol resistance of the membrane, we prepared cross-linked membranes through a Friedel–Crafts reaction between bromomethyl groups and aromatic rings. The properties of the membranes related to fuel cell application were evaluated in detail. All the membranes showed good thermal and mechanical stabilities and conductivities. Moreover, the cross-linked membranes exhibit better dimensional stabilities and selectivities. Among those membranes, xPEEK–Q-100 showed a high conductivity (0.036 S cm−1 at 80 °C), a low swelling ratio of 6.6% and a methanol permeation coefficient of 2.9 × 10−7 cm2 s−1. The outstanding properties indicated that the application of PEEK–Q-xx membranes in fuel cells was promising.

Cross-linked aromatic cationic polymer electrolytes with enhanced stability for high temperature fuel cell applications

Diamine-cross-linked membranes were prepared from cross-linkable poly(arylene ether ketone) containing pendant cationic quaternary ammonium group (QPAEK) solution by a facile and general thermal curing method using 4,4′-diaminodiphenylmethane with rigid framework and 1,6-diaminohexane with flexible framework as cross-linker, respectively. Self-cross-linked cationic polymer electrolytes membranes were also prepared for comparison. The diamines were advantageously distributed within the polymeric matrix and its amine function groups interacted with the benzyl bromide of QPAEK, resulting in a double anchoring of the molecule. Combining the excellent thermal stability, the addition of a small amount of diamines enhanced both the chemical and mechanical stability and the phosphoric acid doping (PA) ability of membranes. Fuel cell performance based on impregnated cross-linked membranes have been successfully operated at temperatures up to 120 °C and 180 °C with unhumidified hydrogen and air under ambient pressure, the maximum performance of diamine-cross-linked membrane is observed at 180 °C with a current density of 1.06 A cm−2 and the peak power density of 323 mW cm−2. The results also indicate that the diamine-cross-linked membranes using the rigid cross-linker show much improved properties than that using the flexible cross-linker. More properties relating to the feasibility in high temperature proton exchange membrane fuel cell applications were investigated in detail.

Macromolecular covalently cross-linked quaternary ammonium poly(ether ether ketone) with polybenzimidazole for anhydrous high temperature proton exchange membranes

Poly(ether ether ketone) bearing benzyl bromide groups (Br–PEEK) was synthesized and a series of cross-linked membranes (Br–PEEK–x%PBI) based on Br–PEEK with polybenzimidazole (PBI) as a macromolecular cross-linker was prepared to improve the dimensional stability and tensile strength without reducing proton conductivity. X-ray photoelectron spectroscopy (XPS) confirmed the success of the cross-linking reaction. After being ammoniated, the quaternary ammonium PEEK membranes were immersed in phosphoric acid and anhydrous phosphoric acid doped membranes were obtained. The phosphoric acid doped membranes without PBI as the cross-linker had excess volume swelling and could not remain integrated. The other cross-linked membranes had good dimensional stabilities. Because PBI could absorb phosphoric acid, the proton conductivities of cross-linked membranes first increased and then decreased with the content of PBI increasing. The highest proton conductivity was 0.081 S cm−1 at 200 °C for the PA–PEEK–20%PBI membrane. The dimensional stabilities, oxidative stabilities and tensile strength of PA–PEEK–x%PBI membranes improved. The PA–PEEK–30%PBI membrane could last for 7.5 h in 3 wt.% H2O2, 4 ppm Fe2+ Fenton solution at 80 °C before breaking into pieces. Energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) were used for detailed research.

Quaternized poly (ether ether ketone)s doped with phosphoric acid for high-temperature polymer electrolyte membrane fuel cells

Quaternized poly(ether ether ketone)s (QPEEKs), which were aminated by trimethylamine (TMeA), triethylamine (TEtA), tripropylamine (TPrA) and 1-methylimidazole (MeIm), were prepared and used as phosphoric acid (PA)-doped high-temperature proton exchange membranes. These QPEEK membranes showed high glass transition temperature (Tg was higher than 483 K) and high thermal stability (T5% was higher than 486 K). The tensile strengths of these QPEEK membranes were higher than 60 MPa. The PA-doped im-QPEEK, which was aminated by MeIm, had the highest Wdoping (159 wt%) and proton conductivity (0.05 S cm−1 at 473 K). For the other three PA-doped QPEEK membranes, the Wdoping and proton conductivity decreased with the increase of the length of trialkyl side chains on quaternary ammonium groups. According to our study, the PA absorbing ability was subjected to the structures of quaternary ammonium groups instead of the basicities of quaternary aminating reagents. All PA-doped membranes had great oxidative stability and could last for more than 5 h in 3 wt% H2O2, 4 ppm Fe2+ Fenton solution at 353 K.

Crosslinked hybrid membranes based on sulfonated poly(ether ether ketone)/γ-methacryloxypropyltrimethoxysilane/phosphotungstic acid by an in situ sol–gel process for direct methanol fuel cells

Proton exchange membranes with high dimensional stabilities and low water uptakes were constructed by incorporating phosphotungstic acid (PWA) into a cross-linked network composed of a crosslinkable sulfonated poly(ether ether ketone) containing dipropenyl groups (SDPEEK) and γ-methacryloxypropyltrimethoxysilane (KH570). The chemical structures of the hybrid membranes were confirmed by FT-IR spectroscopy and scanning electron microscopy (SEM). The results indicated that PWA particles were well dispersed in these membranes. The influences of the dispersed PWA on the properties of membranes such as thermal stability, water uptake, swelling ratio, proton conductivity, methanol permeability and mechanical property were researched. The addition of KH570-5/PWA in the hybrid membranes contributed to the improvement of the dimensional stabilities. And the hybrid membranes with 10–40wt% PWA showed higher proton conductivities than Nafion 117 at 80 °C, while the methanol permeabilities of these membranes were much lower than that of Nafion 117. The membranes also exhibited excellent mechanical properties. These results imply that the SDPEEK/KH570-5/PWA-x membranes are promising materials in the direct methanol fuel cells (DMFC) applications.

Novel side-chain-type sulfonated diphenyl-based poly(arylene ether sulfone)s with a hydrogen-bonded network as proton exchange membranes

A new bisphenol monomer, 3,3′,5,5′-tetramethoxy-4,4′-dihydroxybiphenyl, was synthesized and copolymerized to prepare diphenyl-based poly(arylene ether sulfone) copolymers containing tetra-methoxy groups (MOPAES). After converting the methoxy group to the reactive hydroxyl group, the resulting side-chain-type sulfonated copolymers (SOPAES) with a hydrogen bonded network were obtained by a sulfobutylation reaction. The copolymers were characterized and confirmed by 1H NMR, FT-IR, thermogravimetric analysis (TGA) and small-angle X-ray scattering. The water uptake, proton and methanol transport properties of the resulting membranes were also determined for fuel cell applications. These SOPAES series membranes showed high proton conductivity in the range of 0.032–0.054 and 0.084–0.142 S cm−1 at 25 and 80 °C under hydrated conditions, respectively. SOPAES-40 (IEC = 1.38 mequiv. g−1) showed comparable proton conductivity with Nafion 117 in the hydrated state. The methanol permeability of these membranes was in the range of 1.58–4.29 × 10−7 cm2 s−1, which is much lower than Nafion (1.55 × 10−6 cm2 s−1). It should be noted that the intra/inter hydrogen bonds formed between sulfonic acid and hydroxyl groups or between hydroxyl and hydroxyl groups improved the mechanical properties and reduced the methanol permeability of the membranes effectively. A combination of suitable proton conductivity, low water uptake, and low methanol crossover for selected SOPAES indicates that they are good candidates as proton exchange membrane materials for fuel cells.

Considerations of the Effects of Naphthalene Moieties on the Design of Proton-Conductive Poly(arylene ether ketone) Membranes for Direct Methanol Fuel Cells

Novel sulfonated poly(arylene ether ketones) (SDN-PAEK-x), consisting of dual naphthalene and flexible sulfoalkyl groups, were prepared via polycondensation, demethylation, and sulfobutylation grafting reaction. Among them, SDN-PAEK-1.94 membrane with the highest ion exchange capacity (IEC = 2.46 mequiv·g(-1)) exhibited the highest proton conductivity, which was 0.147 S· cm(-1) at 25 °C and 0.271 S·cm(-1) at 80 °C, respectively. The introduction of dual naphthalene moieties is expected to achieve much enhanced properties compared to those of sulfonated poly(arylene ether ketones) (SNPAEK-x), consisting of single naphthalene and flexible sulfoalkyl groups. Compared with SNPAEK-1.60 with a similar IEC, SDN-PAEK-1.74 membrane showed higher proton conductivity, higher IEC normalized conductivity, and higher effective proton mobility, although it had lower analytical acid concentration. The SDN-PAEK-x membranes with IECs higher than 1.96 mequiv·g(-1) also exhibited higher proton conductivity than that of recast Nafion membrane. Furthermore, SDN-PAEK-1.94 displayed a better single cell performance with a maximum power density of 60 mW·cm(-2) at 80 °C. Considering its high proton conductivity, excellent single cell performance, good mechanical stabilities, low membrane swelling, and methanol permeability, SDN-PAEK-x membranes are promising candidates as alternative polymer electrolyte membranes to Nafion for direct methanol fuel cell applications.

Fully aromatic naphthalene-based sulfonated poly(arylene ether ketone)s with flexible sulfoalkyl groups as polymer electrolyte membranes

A series of sulfonated naphthalene-based poly (arylene ether ketone)s (SNPAEK-xx) with pendant sulfoalkyl groups were prepared by polycondensation of 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene and o-methylhydroquinone, followed by a demethylation and sulfobutylation reaction. The sulfonate degree of SNPAEK-xx could be controlled easily by adjusting the ratio of 1,4-butane sultone to the hydroxyl content in the demethylated polymers. Flexible and tough membranes with reasonably high mechanical strength were prepared. SNPAEK-xx membranes showed a high ionic exchange capacity (IEC) in the range of 1.13 to 2.27 mequiv. g−1, and the highest proton conductivity of 0.191 S cm−1 at 80 °C. They exhibited low methanol permeability in the range of 1.25–10.22 × 10−7 cm2 s−1, which was much lower than that of Nafion 117. Transmission electron microscopy analysis of SNPAEK-xx revealed that they had a more obvious phase separated structure between the hydrophilic side chain and hydrophobic fully aromatic domains at a higher IEC. Combining their high thermal and mechanical stability, high selectivity, lower water swelling ratio, SNPAEK-xx membranes could be promising materials as alternative to Nafion membranes for direct methanol fuel cell applications.