Improving anion exchange membranes for DMAFCs by inter-crosslinking CPPO/BPPO blends

New anion exchange membranes are prepared by heat-treating the blend base membranes of chloroacetylated poly(2,6-dimethyl-1,4-phenyleneoxide)(CPPO)/bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO). A partially inter-crosslinked structure can be formed therein via a Friedel–Crafts reaction without adding any crosslinking reagent or catalyst. FT-IR and NMR analyses are used to confirm this inter-crosslinking structure and quantify the crosslinking distribution. Physical properties, such as toughness and thermal stability, are enhanced remarkably due to this heat treatment. Furthermore, the final membranes exhibit a high hydroxyl conductivity (up to 0.032 S cm⁻¹ at 25 °C) and extremely low methanol permeability (1.26–1.04 × 10⁻⁷ cm² s⁻¹), which match the requirements for application in low temperature direct methanol alkaline fuel cells (DMAFCs).     

Electro chemical properties of porous PVdF-HFP membranes prepared with different nonsolvents

Porous poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) membranes were prepared by solvent–nonsolvent evaporation technique. Morphology and porosity of the membranes were varied with different nonsolvents and had an effect on electrochemical properties. The porous membranes were functionalized with different liquid electrolyte solutions such as p-toluene sulfonic acid/phosphoric acid/sulfuric acid. Maximum electrolyte uptake and minimal electrolyte leakage were tailored by the optimized porosity of the membranes. Thermal behavior obtained in this study ensures the complete evaporation of nonsolvents and ensures its thermal stability. The pTSA-activated PVdF-HFP/THF membrane exhibited high ionic conductivity of about 27.27 mS/cm and a lower methanol permeability in the range of 9.7 × 10⁻⁸ cm²/s. High compatibility between pTSA solution and porous PVdF-HFP polymer electrolyte membrane enhances its electro chemical behavior than that of conventional liquid electrolytes.     

Preparation of PVDF/PEO-PPO-PEO blend microporous membranes for lithium ion batteries via thermally induced phase separation process

Microporous poly(vinylidene fluoride)/polyethylene oxide-co-polypropylene oxide-co-polyethylene oxide (PVDF/PEO-PPO-PEO, or PVDF/F127) blend membranes were prepared via thermally induced phase separation (TIPS) process using sulfolane as the diluent. Then they were soaked in a liquid electrolyte to form polymer electrolytes. The effects of F127 weight fraction on the morphology, crystallinity and porosity of the blend membranes were studied. It was found that both electrolyte uptake of blend membranes and ionic conductivity of corresponding polymer electrolytes increased with the increase of F127 weight fraction. The maximum ionic conductivity was found to reach 2.94 ± 0.02 × 10⁻³ S/cm at 20 °C. Electrochemical stability window was stable up to 4.7 V (vs. Li⁺/Li). The testing results indicated that the PVDF/F127 blend membranes prepared via TIPS process can be used as the polymer microporous matrices of polymer electrolytes for lithium ion batteries.     

A study of linear versus angled rigid rod polymers for proton conducting membranes using sulfonated polyimides

Linear and angled monomers were incorporated into the main chain of a polyimide in order to investigate the effect of kinked versus linear polymers on membrane properties such as water uptake and proton conductivity. Polymers prepared using linear 4,4′-sulfonyldianiline, SPI1, and using angled 3,4′-sulfonyldianiline, SPI2, were cast into membranes possessing ion exchange capacities that varied from 0.79 to 2.75 meq g⁻¹. Membranes are thermally stable up to 300 °C under air. Proton conductivity of both membranes increases with temperature to values of 0.1-0.2 S cm⁻¹. The conductivity of angled, SPI2 membranes is greater than those prepared from SPI1 for a given IEC but water uptakes are lower. These differences are attributed to increased entanglements of the angled polymers, which limits the degree of swelling and increases the proton concentration. These results may be important in the design of proton conducting membranes from other rigid polyarylenes.     

Preparation of alkaline anion exchange membranes based on functional poly(ether-imide) polymers for potential fuel cell applications

A novel poly(ether-imide)-based alkaline anion exchange membrane with no free base has been prepared and characterized for its ionic conductivity in water, which is a critical metric of its applicability in a liquid-fed direct methanol fuel cell. The poly(ether-imide)-based membranes were prepared by chloromethylation, quaternization and alkalization of commercial poly(ether-imide) and the derivatives were characterized by NMR. The chemical and thermal stabilities were investigated by measuring changes of ionic conductivities when the membranes were placed in various alkaline concentrations and temperatures for 24 h. The membranes were stable at all concentrations of KOH at room temperature, but not at elevated temperatures. The membranes were stable in 1.0 M KOH solution up to 80 °C without losing membrane integrity. The measured conductivity of the formed membrane ranged from 2.28 to 3.51 × 10⁻³ S/cm at room temperature. This preliminary study indicates that functionalized poly(ether-imide) has suitable conductivity suggesting that it can be used as an alkaline anion exchange membrane in fuel cell applications.     

Self-standing single lithium ion conductor polymer network with pendant trifluoromethanesulfonylimide groups: Li diffusion coefficients from PFGSTE NMR

Solid electrolyte materials have the potential to improve performance and safety characteristics of batteries by replacing conventional solvent-based electrolytes. For this purpose, new candidate single ion conductor self-standing networks were synthesized with trifluoromethane-sulfonylimide (TFSI) lithium salt based monomer using poly(ethyleneglycol) dimethacrylate (PEGDM 750) as crosslinker. The highest ionic conductivity was 3.4 × 10⁻⁷ S cm⁻¹ at 30 °C in the dry state. Thermal and mechanical analyses showed good thermal stability up to 190 °C and rubbery-like properties at ambient temperature. A direct relationship between ionic conductivity and glassy or rubbery state of the membranes was found. Vogel–Tammann–Fulcher behavior was observed in the dry state which is consistent with a lithium conductivity correlated with polymer chain mobility. By swelling the network in propylene carbonate, a self-standing electrolyte gel could be obtained with an ionic conductivity as high as 1 × 10⁻⁴ S cm⁻¹ at 30 °C. The individual diffusion coefficients of mobile species in the material (¹⁹F and ⁷Li) were measured and quantified using pulsed-field gradient nuclear magnetic resonance (PFG-NMR). Diffusion coefficients for the most mobile components of the lithium cations and fluorinated anions at 100 °C in dry membranes have been found to be 3.4 × 10⁻⁸ cm² s⁻¹ and 2.1 × 10⁻⁸ cm² s⁻¹ respectively.     

Compositional effect of PVdF-PEMA blend gel polymer electrolytes for lithium polymer batteries

Owing to their improved mechanical properties and good polymer miscibility, the blend gel polymer electrolytes of poly (vinylidene fluoride) (PVdF)-poly(ethyl methacrylate) (PEMA) have been prepared using solvent casting technique and characterized for their electrochemical performances. The electrolyte shows a maximum ionic conductivity of 1.5 × 10⁻⁴ S cm⁻¹ at 301 K for the 90:10 blend ratio of PVdF:PEMA system with good transport property. The ionic conductivity is enhanced, in accompany with improved microstructural homogeneity, at low PEMA contents, while the decreased conductivity at high contents has been attributed to increasing crystalline PEMA domains. With the optimum PVdF:PEMA ratio, the complex system was found to facile reasonable ionic transference number and exhibit superior interfacial stability with Li electrode.     

Preparation and properties of Nafion/hollow silica spheres composite membranes

In present work, hollow silica spheres (HSS)/Nafion^® composite membranes were prepared by solution casting. The thermal properties, water retention, swelling behavior and proton conductivity of the composite membranes were explored. It was found that HSS dispersed well at micrometer scale in the obtained composite membranes by SEM and TEM observation. Thermal properties of composite membranes were improved than that of recast Nafion^® membrane. Compared with the recast Nafion^® membrane, the composite membranes showed higher water uptake and lower swelling degree at the temperature range from 40 to 100 °C. At the same HSS loading, the smaller the diameter of HSS in composite membranes, the more the water uptake, however, the swelling degree of composite membranes was increased. The proton conductivity of the composite membrane with 3–5 wt.% HSS (120 and 250 nm) increased distinctively at above 60 °C, reached the optimal value at 100 °C, and decreased slowly when the temperature exceeded 100 °C.     

Self-assembled polyelectrolyte multilayered films on Nafion with lowered methanol cross-over for DMFC applications

The paper is concerned with the deposition of self-assembled polyelectrolyte multilayer on Nafion membrane by layer-by-layer (LbL) technique with lowered methanol cross-over for direct methanol fuel cell (DMFC) applications. The formation of self-assembled multilayered film on Nafion was characterized by UV–vis spectroscopy and it was found that the polyelectrolyte layers growth on the Nafion surface regularly. Furthermore, the proton conductivity and methanol cross-over measurements were carried out for characterization of the LbL self-assembled composite membranes. The results showed that the concentration and pH of the polyelectrolytes significantly affect the proton conductivity and methanol barrier properties of the composite membranes. 10⁻¹ monomol polyelectrolyte concentration and pH 1.8 was found to be optimum deposition conditions considering proton conductivity and methanol permeation properties of the LbL self-assembled composite membranes. The methanol permeability of the 10 bi-layers of PAH1.8/PSS1.8 deposited LbL self-assembly composite membrane was significantly suppressed and found to be 4.41 × 10⁻⁷ cm²/s while the proton conductivity value is in acceptable range for fuel cell applications.     

In situ formation of platinum nanoparticles in Nafion recast film for catalyst-incorporated ion-exchange membrane in fuel cell applications

Platinum (Pt) nanoparticles were synthesized inside a Nafion polyelectrolyte membrane for use as a catalyst membrane integrated layer in fuel cells. The integrated membrane was prepared by making use of the cation exchange between the tetraammineplatinum (II) cations ([Pt(NH₃)₄]²⁺) and sulfonic groups in the Nafion molecules, followed by film casting and chemical reduction. The synthesized Pt nanoparticles, which had a cubic shape with diameters of 11.5–14.5 nm, dispersed in the recast Nafion film, increased its proton conductivity and open circuit voltage compared with the pristine Nafion membrane. The Pt-incorporated membrane provided a 29% increment of the maximum power density, seemingly by oxidizing the crossover methanol passing through the proton-exchange membrane. At a high loading of Pt (over 3 wt.% in this study), the Nafion clusters were likely squeezed by the synthesized Pt nanoparticles so as to decrease the water uptake and proton conductivity. This hypothesis was also supported by the increased Ohmic resistance in the I–V polarization curve.     

Fluorene-containing block sulfonated poly(ether ether ketone) as proton-exchange membrane for PEM fuel cell application

A series of sulfonated block poly(ether ether ketone)s with different sulfonic acid group clusters were successfully synthesized by nucleophilic displacement condensation. Membranes were accordingly cast from their DMSO solutions, and fully characterized by determining the ion-exchange capacity, water uptake, proton conductivity, dimensional stabilities and mechanical properties. The experimental results showed that the main properties of the membrane can be tailored by changing the cluster size of sulfonic acid groups. The membrane of block-7c(40) has good mechanical, oxidative and dimensional stabilities together with high proton conductivity (5.09 × 10⁻² S cm⁻¹) at 80 °C under 100% relative humidity. The membranes also possess excellent thermal and dimensional stabilities. These polymers are potential and promising proton conducting membrane material for PEM full cell applications.     

PPO-based acid–base polymer blend membranes for direct methanol fuel cells

New acid–base polymer blend membranes for direct methanol fuel cells (DMFC) have been designed using a very accessible commercial polymer, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). The preparation begins with the sulfonation and bromination of PPO to sulfonated PPO (SPPO) and bromomethylated PPO (BrPPO), respectively. Blend membranes are formed by mixing n-propylamine(PrNH₂)-neutralized SPPO and PrNH₂-aminated BrPPO solutions in N-methyl-2-pyrrolidone (NMP), and casting the mixed solution on glass petri dishes followed by acidification with aqueous hydrochloric acid. The compatibility between the acid and base components of the blend is assured by using acidic and basic polymers deriving from the same parent polymer (PPO). Ionic crosslinking is established between the sulfonic groups of SPPO and the amine groups of aminated BrPPO. The ionic crosslinking strengthens the membrane dimensional stability by reducing water uptake and membrane swelling up to temperatures as high as 80 °C. The membranes fabricated as such display good resistance to methanol crossover amidst some, but acceptable loss of proton conductivity. The characteristic factor (i.e. the ratio of proton conductivity to methanol permeability) increases noticeably with the BrPPO content, with the sample containing 30 wt.% BrPPO showing a 16-fold improvement over Nafion 117. The mechanical properties and oxidative stability of the blend membranes also satisfy the requirements for fuel cell assembly and operation.     

Characterization and performance of proton exchange membranes for direct methanol fuel cell: Blending of sulfonated poly(ether ether ketone) with charged surface modifying macromolecule

Modification of sulfonated poly(ether ether ketone) (SPEEK) membrane was attempted by blending charged surface modifying macromolecule (cSMM). The modified membrane was tested for direct methanol fuel cell (DMFC) application; i.e. a SPEEK/cSMM blend membrane was compared to a SPEEK membrane and a Nafion 112 membrane for the thermal and mechanical stability, methanol permeability, and proton conductivity. Thermal and mechanical stability of the blended membrane were slightly reduced from the SPEEK membrane but still higher than the Nafion 112 membrane. The blend membrane was found to be promising for DMFC applications because of its lower methanol diffusivity (2.75 × 10⁻⁷ cm² s⁻¹) and higher proton conductivity (6.4 × 10⁻³ S cm⁻¹), than the SPEEK membrane. A plausible explanation was given for the favorable effect of cSMM blending.     

A composite microporous gel polymer electrolyte prepared by ultra-violet cross-linking

A new type of composite microporous gel polymer electrolyte was prepared by directly coating the hydrolyzed prepolymers onto PVdF microporous membrane, and then polymerizing and cross-linking with ultra-violet (UV). Their chemical, thermal, surface microscopic configuration, swelling ability and electrochemical properties have been investigated for various prepolymer’s solution concentrations. The swelling ability and ionic conductivity of the membrane supporting hybrid gel electrolyte (MSHGE) could reach an extreme point at 0.15 g/ml of the prepolymer’s solution. It is thought that their performance can be affected by the surface microscopic configuration and the quality of coated copolymer. The Arrhenius-type relationship was observed in the temperature dependence of ionic conductivity. The ionic conductivity of MSHGE (PVdF-15) at room temperature can reach 6.18 × 10⁻³ S cm⁻¹, and its electrochemical stability window is about 4.9 V.     

Preparation and properties of UV irradiation-induced crosslinked sulfonated poly(ether ether ketone) proton exchange membranes

The crosslinkable sulfonated poly(ether ether ketone)s (SPEEKs) were synthesized by nucleophilic substitution reaction of diallyl bisphenol A, tert-butylhydroquinone, 4,4′-difluorobenzophenone and sodium 5,5′-carbonylbis(2-fluorobenzene-sulfonate). The SPEEKs with high intrinsic viscosity showed good solubility and could be cast into flexible and transparent membranes. The SPEEK membranes containing benzophenone (BP) and triethylamine (TEA) photo-initiator system were treated by UV light to promote crosslinking. The experimental results revealed that the crosslinked membrane with 10 min irradiation time showed the most potential as proton exchange membrane for direct methanol fuel cell applications. When the irradiation time increased from 0 to 10 min, the water uptake decreased from 29.1 to 26.1%, and the tensile modulus and the tensile strength enhanced sharply from 0.80 to 1.44 GPa and from 40.3 to 63.4 MPa, respectively. In addition, the methanol diffusion coefficient reduced sharply from 1.70 × 10⁻⁶ to 7.42 × 10⁻⁷ cm²/s with only slight sacrifice in the proton conductivity, which made the crosslinked membrane with 10 min irradiation time possess the highest selectivity.     

Synthesis and characterization of poly(aryl ether ketone) ionomers with sulfonic acid groups on pendant aliphatic chains for proton-exchange membrane fuel cells

A series of parent poly(aryl ether ketone)s bearing different content of unsaturated pendant propenyl groups were synthesized via nucleophilic substitution polymerization from 3,3′-diallyl-4,4′-dihydroxybiphenyl, 9,9′-bis(4-hydroxyphenyl) fluorene and 4,4′-difluorobenzophenone. The polymers with pendant aliphatic sulfonic acid groups were further synthesized by free radical thiol-ene coupling reactions between 3-mercapto-1-propanesulfonic sodium and the parent propenyl functional copolymers. The resulting sulfonated polymers with high inherent viscosity (1.83-4.69 dL/g) were soluble in polar organic solvents and can form flexible and transparent membranes by casting from their solutions. The copolymers with different ion exchange capacity could be conveniently synthesized by varying the monomers ratios. Transmission electron microscopy (TEM) was used to examine the microstructures of the membrane and the results revealed that significant hydrophilic/hydrophobic microphase separation with spherical, uniform-sized (5-10 nm) and well-dispersed hydrophilic domains was afforded. The proton conductivities of the as-prepared membranes and the state-of-the-art Nafion 117 membrane in fully hydrated state were investigated. The results revealed that the proton conductivity of the synthesized membranes increased more remarkably than that of Nafion 117 membrane with increasing temperature. The membrane with 1.69 mequiv/g of IEC had a conductivity of 2.5 × 10⁻² Scm⁻¹ at 100 °C. The membranes also possessed excellent mechanical properties, good thermal, oxidative, hydrolytic and dimensional stabilities.     

Preparation of Nafion/sulfonated poly(phenylsilsesquioxane) nanocomposite as high temperature proton exchange membranes

Nafion/sulfonated poly(phenylmethyl silsesquioxane) (sPPSQ) composite membranes are fabricated using homogeneous dispersive mixing and a solvent casting method for direct dimethyl ether fuel cell (DDMEFC) applications operated above 100 °C. The inorganic conducting filler, sPPSQ significantly affects the characteristics in the nanocomposite membranes by functionalization with an organic sulfonic acid to PPSQ. Moreover, sPPSQ content plays an important role in membrane properties such as microstructure, proton conductivity, fuel crossover, and single cell performance test. With increasing sPPSQ content in the nanocomposite membrane, the proton conductivity increased and fuel crossover decreased. However, in a higher temperature range above 110 °C, Nafion/sPPSQ 5 wt.% composite membrane has the highest proton conductivity. Also, the DME permeability for the composite membrane with higher sPPSQ content increased sharply. The excessive sPPSQ content caused a large aggregation of inorganic fillers, leading to the deterioration of membrane properties. In this study, the optimal sPPSQ content for maximizing the DDMEFC performance was 5 wt.%. Our nanocomposite membranes demonstrated proton conductivities as high as 1.57 × 10⁻¹ S/cm at 120 °C, which is higher than that of Nafion. The cell performances were compared to Nafion/sPPSQ composite membrane with Nafion 115, and the composite membrane with sPPSQ yielded better cell performance than Nafion 115 at temperatures ranging from 100 to 120 °C and at pressures from 1 to 2 bar.     

A novel sandwiched membrane as polymer electrolyte for application in lithium-ion battery

A novel kind of sandwiched polymer membrane was prepared, which consists of two outer layers of electrospun poly(vinyl difluoride) (PVDF) fibrous films and one inner layer of poly(methyl methacrylate) (PMMA) film. Its characteristics were investigated by scanning electron microscopy and X-ray diffraction. The membrane can easily absorb non-aqueous electrolyte to form gelled polymer electrolytes (GPEs). The resulting gelled polymer electrolytes had a high ionic conductivity up to 1.93 × 10⁻³ S cm⁻¹ at room temperature, and exhibited a high electrochemical stability potential of 4.5 V (vs. Li/Li⁺). It is of great potential application in polymer lithium-ion batteries.     

Sulfonated poly(fluorenyl ether) membranes containing perfluorocyclobutane groups for fuel cell applications

This paper describes the preparation and electrochemical properties of new proton conducting polymer membranes, sulfonated poly(fluorenyl ether) membrane-containing perfluorocyclobutane (PFCB) moieties for fuel cell applications. The sulfonated polymers were prepared via thermal cyclodimerization of 9,9-bis(4-trifluorovinyloxyphenyl)fluorene and subsequent post-sulfonation using chlorosulfonic acid (CSA) as a sulfonating agent. The post-sulfonation reaction was carried out by changing the molar ratio of CSA/repeating unit of the polymer at room temperature for 5 h and the resulting sulfonated polymers showed different degrees of sulfonation (DS) and ion exchange capacities (IEC). With the increment of CSA content, the DS, IEC and water uptake of the sulfonated polymer membranes increased. Their proton conductivity was investigated as a function of temperature. The polymer membrane with an IEC value of 1.86 mmol/g showed a water content of 25% similar to Nafion-115's but showed higher proton conductivity than Nafion-115 over the temperature 25–80 °C. The polymer membrane with lower water uptake and higher IEC showed similar proton conductivity and methanol permeability to Nafion-115. These results confirmed that the sulfonated poly(fluorenyl ether)-containing PFCB groups could be a promising material for fuel cell membranes.     

Systematic investigation on new SrCo1−yNbyO3−δ ceramic membranes with high oxygen semi-permeability

SrCo_1−yNb_yO_3−δ (y = 0.025–0.4) were synthesized for oxygen separation application. The crystal structure, phase stability, oxygen nonstoichiometry, electrical conductivity, and oxygen permeability of the oxides were systematically investigated. Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y = 0.025–0.2. However, the further increase in niobium concentration led to the formation of impurity phase. The niobium doping concentration also had a significant effect on electrical conductivity and oxygen permeability of the membranes. SrCo_0.9Nb_0.1O_3−δ exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of ∼2.80 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C for a 1.0-mm membrane under an air/helium oxygen gradient. The further investigation demonstrated the oxygen permeation process was mainly rate-limited by the oxygen bulk diffusion process.