Proton conducting properties of ionically cross-linked poly(1-vinyl-1,2,4 triazole) and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) electrolytes

The synthesis and thermal as well as proton conducting properties of complex polymer electrolytes based on poly(2-acrylamido-2-methyl-1-propanesulfonic acid) PAMPS and poly(1-vinyl-1,2,4-triazole) PVTri were investigated. The materials were produced by complexation of PAMPS with PVTri at various compositions to get PVTriP(AMPS) _x where x is the molar ratio of the polymer repeating units and varied from 0.25 to 4. The structure of the materials was confirmed by FT-IR spectroscopy. The TGA results verified that the polymer electrolytes are thermally stable up to approximately 200 °C. The DSC and SEM results demonstrated the homogeneity of the materials. The electrochemical stability of the materials was studied by cyclic voltammeter (CV). Proton conductivity, activation energy, and water/methanol uptake of these membranes were also measured. After humidification (RH = 50%), PVTriP(AMPS)₂ and PVTriP(AMPS)₄ showed proton conductivities of 0.30 and 0.06 S/cm at 100 °C, respectively.     

Proton‐conducting blend membranes of crosslinked poly(vinyl alcohol)–sulfosuccinic acid ester and poly(1‐vinyl‐1,2,4‐triazole) for high temperature fuel cells

New type of composite membranes were synthesized by crosslinking of poly(vinyl alcohol) (PVA) with sulfosuccinic acid (SSA) and intercalating poly(1‐vinyl‐1,2,4‐triazole) (PVTri) into the resulting matrix. The complexed structure of the membranes was confirmed by Fourier transform infrared (FTIR) spectroscopy. The resulting hybrid membranes were transparent, flexible, and showed good thermal stability up to ～200°C. The proton conductivities of the membranes were investigated as a function of PVTri and SSA and operating temperature. The water/methanol uptake was measured and the results showed that solvent absorption of the materials increased with increasing PVTri content in the matrix. The proton conductivity of the membranes continuously increased with increasing SO₃H content, PVTri content, and the temperature. In the anhydrous state, the maximum proton conductivity is 7.7 × 10^?5 S/cm for PVA–SSA–PVTri‐1 and for PVA–SSA–PVTri‐3 is 1.6 × 10^?5 S/cm at 150°C. After humidification (RH = 100%), PVA–SSA–PVTri‐4 showed a maximum proton conductivity of 0.0028 S/cm at 60°C. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Anhydrous proton conducting polymer electrolytes based on poly(vinylphosphonic acid)-heterocycle composite material

The development of anhydrous proton conducting membrane is important for the operation of polymer electrolyte membrane fuel cell (PEMFC) at intermediate temperature (100-200 °C). In this study, we have investigated the acid-base hybrid materials by mixing of strong phosphonic acid polymer of poly(vinylphosphonic acid) (PVPA) with the high proton-exchange capacity and organic base of heterocycle, such as imidazole (Im), pyrazole (Py), or 1-methylimidazole (MeIm). As a result, PVPA-heterocycle composite material showed the high proton conductivity of approximately 10⁻³ S cm⁻¹ at 150 °C under anhydrous condition. In particular, PVPA-89 mol% Im composite material showed the highest proton conductivity of 7×10⁻³ S cm⁻¹ at 150 °C under anhydrous condition. Additionally, the fuel cell test of PVPA-89 mol% Im composite material using a dry H₂/O₂ showed the power density of approximately 10 mW cm⁻² at 80 °C under anhydrous conditions. These acid-base anhydrous proton conducting materials without the existence of water molecules might be possibly used for a polymer electrolyte membrane at intermediate temperature operations under anhydrous or extremely low humidity conditions.     

Synthesis and proton conductivity studies of doped azole functional polymer electrolyte membranes

The development of anhydrous proton-conducting membranes is important for the operation of polymer electrolyte membrane fuel cell (PEMFC) at intermediate temperature (100-200 °C). In this work, poly(vinylbenzylchloride), PVBC was produced by free radical polymerization of 4-vinylbenzylchloride and then it was modified with 5-aminotetrazole (ATET) to obtain poly(vinylbenzylaminotetrazole), PVBC-ATET. The composition of the polymer was verified by elemental analysis (EA) and the structure was characterized by FT-IR and ¹³C NMR spectra. According to the elemental analysis result, PVBC was modified by ATET with 80% yield. The polymer was doped with trifluoromethanesulfonic acid (TA) at various molar ratios, x = 1.25, 2.5, 3.75 with respect to tetrazole unit. The proton transfer from TA to the tetrazole rings was proved with FT-IR spectroscopy. Thermogravimetry (TG) analysis showed that the samples are thermally stable up to approximately 200 °C. Differential scanning calorimetry (DSC) results illustrated the homogeneity of the materials. Cyclic voltammetry (CV) study illustrated that the electrochemical stability domain for PVBC-ATET-TA_2.5 extends over 3.0 V. The proton conductivity of these materials increased with dopant concentration and the temperature. Maximum proton conductivity of PVBC-ATET-TA_2.5 was found to be 0.01 S/cm at 150 °C in the anhydrous state.     

Novel triazole functional sol-gel derived inorganic-organic hybrid networks as anhydrous proton conducting membranes

In this work, novel inorganic-organic hybrid networks were prepared to obtain anhydrous proton conducting membranes for fuel cells. 3-glycidoxypropyl trimethoxy silane (GPTMS) was functionalized with 1H-1,2,4-triazole (Tri) and 3-aminotriazole (ATri) via ring opening of the epoxide ring and then sol-gel polymerization was performed to produce triazole containing silane networks abbreviated as Si-Tri and Si-ATri. In addition during sol-gel process trifluoromethane sulfonic acid (TA) was introduced into the matrix with several stoichometric ratios. Fourier transform infrared spectroscopy (FT-IR) confirmed the tethering of the Tri and ATri into the silane compound and the sol-gel reaction. Thermogravimetry analysis (TGA) showed that the membranes are thermally stable up to 200 °C. Differential scanning calorimeter (DSC) verified the softening effect of the dopant. The morphology of the membranes was analyzed with SEM images. The proton conductivity of these novel silane networks were studied by dielectric-impedance spectroscopy. Although proton conductivity of these membrane electrolytes depends on the acid ratio, the membrane without dopant produced a proton conductivity of 8.7 × 10⁻⁵ S/cm at 150 °C in dry state. The conductivity isotherms show Vogel-Tamman-Fulcher (VTF) behavior which implies the coupling of the charge carriers with the segmental motion of the polymer chains. A maximum proton conductivity of 8.9 × 10⁻⁴ S/cm was obtained for the sample Si-TriTA1 in the anhydrous condition.     

Proton conducting polydimethylsiloxane/zirconium oxide hybrid membranes added with phosphotungstic acid

Inorganic polymer based hybrid membranes consisting of zirconium oxide and polydimethylsiloxane (PDMS) have been synthesized by sol-gel processes. The organic/inorganic polymeric hybrid membranes showed thermal stability and flexibility up to 300 °C. The membrane becomes proton conducting polymer electrolyte when added with 12-phosphotungstic acid (PWA). The conductivity of the membranes was measured in the temperature range from room temperature to 150 °C under saturated humidity and a maximum conductivity of 5×10⁻⁵ S cm⁻¹ was obtained at 150 °C.     

Polysulfone ionomers for proton-conducting fuel cell membranes: 2. Sulfophenylated polysulfones and polyphenylsulfones

Polysulfones and polyphenylsulfones having pendant phenyl groups with sulfonic acid units have been prepared by lithiation of the respective polymer, followed by reaction with 2-sulfobenzoic acid cyclic anhydride. The resulting ionomers were cast into membranes and properties such as thermal stability, ion-exchange capacity, water sorption and proton conductivity were evaluated. These membranes proved to have a high thermal stability, with a decomposition temperature between 300 and 350 °C, and a high proton conductivity, 60 mS/cm at 70 °C for a polyphenylsulfone with 0.9 sulfonic acid group per repeating unit measured at 100% relative humidity. Moreover, some of the membranes endured immersion in water at temperatures ranging from 20 to 150 °C without swelling extensively, and therefore kept their mechanical stability under these conditions. It was also shown that these membranes retained a high conductivity up to 150 °C under humidifying conditions. The combination of properties make these membranes potential candidates for fuel cells operating at temperatures above 100 °C.     

New highly proton-conducting membrane poly(vinylpyrrolidone)(PVP) modified poly(vinyl alcohol)/2-acrylamido-2-methyl-1-propanesulfonic acid (PVA-PAMPS) for low temperature direct methanol fuel cells (DMFCs)

A new type of chemically cross-linked polymer blend membranes consisting of poly(vinyl alcohol) (PVA), 2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) and poly(vinylpyrrolidone) (PVP) have been prepared and evaluated as proton conducting polymer electrolytes. The proton conductivity (σ) of the membranes was investigated as a function of cross-linking time, blending composition, water content and ion exchange capacity (IEC). Membranes were also characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), and the differential scanning calorimetry (DSC). Membrane swelling decreased with cross-linking time, accompanied by an improvement in mechanical properties and a small decrease in proton conductivity due to the reduced water absorption. The membranes attained 0.088 S cm⁻¹ of the proton conductivity and 1.63 mequiv g⁻¹ of IEC at 25±2 °C for a polymer composition PVA-PAMPS-PVP being 1:1:0.5 in mass, and a methanol permeability of 6.1×10⁻⁷ cm² s⁻¹, which showed a comparable proton conductivity to Nafion 117, but only one third of Nafion 117 methanol permeability under the same measuring conditions. The membranes displayed a relatively high oxidative durability without weight loss of the membranes (e.g. 100 h in 3% H₂O₂ solution and 20 h in 10% H₂O₂ solution at 60 °C). PVP, as a modifier, was found to play a crucial role in improving the above membrane performances.     

Study of the incorporation of protic ionic liquids into hydrophilic and hydrophobic rigid-rod elastomeric polymers

A series of polymers was synthesized that contain a rigid aromatic backbone connected through triazine linkages that are cross-linked by flexible diamine-terminated poly(ethylene oxide) oligomers. Polymers were made that contained both hydrophilic sulfonated aromatic and hydrophobic pyridinium triflate backbones. Thermal and mechanical properties of the resulting polymer films were studied, as well as uptake of water and protic ionic liquids. Ionic liquid uptake varied from 41 to 440%, depending upon the nature of the polymer. The ionic liquid-doped films were analyzed for proton conductivity at high temperatures (>150 °C) under non-humidified conditions. Conductivities as high as 5×10⁻² S/cm were observed at 150 °C.     

Synthesis and NMR studies of the polymer membranes based on poly(4-vinylbenzylboronic acid) and phosphoric acid

Proton conduction in novel anhydrous membranes based on host polymer, poly(4-vinylbenzylboronic acid), (P4VBBA) and phosphoric acid, (H₃PO₄) as proton solvent was studied. The materials were prepared by the insertion of the proton solvent into P4VBBA at different stoichiometric ratios to get P4VBBA·xH₃PO₄ composite electrolytes. Homopolymer and the composite materials were characterized by FT-IR, ¹¹B MAS NMR and ³¹P MAS NMR. ¹¹B MAS NMR results suggested that acid doping favors or leads to a four-coordinated boron arrangement. ³¹P MAS NMR results illustrated the immobilization of phosphoric acid to the polymer through condensation with boron functional groups (B-O-P and/or B-O-P-O-B). Thermogravimetric analysis (TGA) showed that the condensation of composite materials starts approximately at 140 °C. An exponential weight loss above this temperature was attributed to intermolecular condensation of acidic units forming cross-linked polymer. The insertion of phosphoric acid into the matrix softened the materials shifting T_g to lower temperatures. The temperature dependence of the proton conductivity was modeled with Arrhenius relation. P4VBBA·2H₃PO₄ has a maximum proton conductivity of 0.0013 S/cm at RT and 0.005 S/cm at 80 °C.     

Synthesis and characterization of poly(1-vinyl-3-alkylimidazolium) iodide polymers for quasi-solid electrolytes in dye sensitized solar cells

A series of new poly(1-vinyl-3-alkylimidazolium) iodide polymers with different alkyl derivatives such as methyl, propyl and perflurodecyl have been synthesized. The alkyl substituent influenced some properties such as solubility, thermal stability, glass transition and crystallinity of the polymers. For instance, polymer having the propyl substituent was soluble in solvents of intermediate polarity such as acetonitrile, chloroform and THF, the one with the methyl substituent was only soluble in very polar solvents such as water and methanol and the fluorinated polymer was only soluble in DMF. The alkyl substituent also influenced the thermal stability in the order methyl > propyl > perflurodecyl and all the polymers thermally decomposed between 250 and 400 °C in nitrogen. The poly(1-vinyl-3-alkyl-imidazolium) iodide polymers having propyl and methyl substituents were amorphous polymers showing a glass transition temperature of 43 and 21 °C, respectively; and perflurodecyl polymers were semi-crystalline with a Tm at 153 °C and a Tg at 20 °C, as indicated by differential scanning calorimetry.Polymer electrolytes were formulated as mixtures of the ionic liquid 1-methyl-3-propylimidazolium iodide and the poly(1-vinyl-3-alkylimidazolium) iodide polymers. These polymer electrolytes showed ionic conductivities in the range of 10⁻³ to 10⁻⁷ S/cm at room temperature which strongly depended on the ionic liquid content. Finally, poly(1-vinyl-3-propyl-imidazolium) iodide was used to obtain gel electrolytes by adding it to a typical acetonitrile electrolyte used in dye sensitized solar cells (DSSCs). Solar cells with 1 cm² area prepared using the polymer gel electrolyte yielded a maximum light-to-electricity conversion efficiency of 3.73%.     

Synthesis and characterization of proton conducting inorganic-organic hybrid nanocomposite membranes based on tetraethoxysilane/trimethylphosphate/3-glycidoxypropyltrimethoxysilane/heteropoly acids

A series of novel proton conductive inorganic-organic nanocomposite hybrid membranes doped with phosphotungstic acid (PWA)/phosphomolybdic acid (PMA) and trimethylphosphate PO(OCH₃)₃ have been prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), and tetraethoxysilane (TEOS) as precursors. The hybrid membranes were studied with respect to their structural and thermal properties, elastic moduli and proton conductivity. Thermal analysis including TG and DTA confirmed that the membranes were thermally stable up to 200 °C. Thermal stability of membranes was significantly enhanced by the presence of SiO₂ framework. Proton conductivity of 1.59 × 10⁻² S/cm with composition of 50TEOS-5PO(OCH₃)₃-35GPTMS-10PWA was obtained (1.15 × 10⁻² S/cm for 10 mol% PMA) at 90 °C under 90% relative humidity. The proton conductivity of the nanocomposite membranes is due to the proton-conducting path through the GPTMS-derived “pseudopolyethylene oxide (pseudo-PEO)” networks in which the trapped solid acid (PWA/PMA) as a proton donor is contained. The molecular water absorbed in the polymer matrix is also presumed to provide high proton mobility, resulting in an increase of proton conductivity with increasing relative humidity.     

Proton conducting membranes based on semi-interpenetrating polymer network of Nafion and polybenzimidazole

A new strategy to prepare the reinforced composite membranes for polymer electrolyte membrane fuel cells (PEMFCs), which can work both in humidified and anhydrous state, was proposed via constructing semi-interpenetrating polymer network (semi-IPN) structure from polybenzimidazole (PBI) and Nafion^®212, with N-vinylimidazole as the crosslinker. The crosslinkable PBI was synthesized from poly(2,2′-(m-phenylene)-5,5′-bibenzimidazole) and p-vinylbenzyl chloride. The semi-IPN structure was formed during the membrane preparation. The composite membranes exhibit excellent thermal stability, high-dimensional stability, and significantly improved mechanical properties compared with Nafion^®212. The proton transport in the hydrated composite membranes is mainly contributed by the vehicle mechanism, with proton conductivity from ∼10⁻² S/cm to ∼10⁻¹ S/cm. When the temperature exceeds 100 °C, the proton conductivity of the semi-IPN membranes decreases quickly due to the dehydration of the membranes. Under anhydrous condition, the proton conductivity of the membranes will drop to ∼10⁻⁴ S/cm, which is also useful for intermediate temperature (100-200 °C) PEMFCs. The benzimidazole structure of PBI and the acidic component of Nafion^® provide the possibility for the proton mobility via structure diffusion involving proton transfer between the heterocycles with a corresponding reorganization of the hydrogen bonded network.     

Polybenzimidazole containing benzimidazole side groups for high-temperature fuel cell applications

A polybenzimidazole (PBI) containing bulky basic benzimidazole side groups, poly[2,2′-(2-benzimidazole-p-phenylene)-5,5′-bibenzimidazole] (BIpPBI), was prepared via the condensation polymerization of 3,3′-diaminobenzidine tetrahydrochloride dihydrate with 2-benzimidazole terephthalic acid in PPA. BIpPBI was found to be soluble in aprotic polar solvents without the addition of inorganic salts, such as lithium chloride, and the BIpPBI film also showed very good acid retention capability as well as very high proton conductivity. The maximum acid content of the BIpPBI film was approximately 81 wt.% and the proton conductivity value of the acid-doped BIpPBI membrane was 0.16 S cm⁻¹ at 180 °C and a 0% relative humidity. For comparison, the maximum proton conductivity of the most commonly used polymer for the high-temperature fuel cell membrane, poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (mPBI) membrane, is approximately 0.06 S·cm⁻¹ at 180 °C under anhydrous conditions at a 65 wt.% acid content, which is the maximum acid content that a mPBI membrane can have.     

Proton conducting acid-base mixed materials under water-free condition

In recent years, a lot of attentions have been paid for a development of water-free polymer electrolyte membranes fuel cells (PEMFC) at intermediate temperatures (above 100 °C) because of many technological advantages of higher temperature operation. However, the proton conductivity of conventional polymer membranes under water-free condition is usually very low and the polymeric membranes are not stable at higher temperatures. So, the development of non-hydrous proton conducting membrane under water-free condition has been a state of the art issue in the advanced PEMFC technology. In this study, non-hydrous protonic conducting material was prepared by the mixing of acidic surfactant of mono-dodecylphosphate (MDP) and organic base of benzimidazole (BnIm). The proton conductivity and thermal stability of MDP-BnIm mixed material increased with the mixing ratio of BnIm. Maximum proton conductivity of MDP-BnIm mixed material (BnIm mixing ratio of 200 wt.%. vs. MDP) was found to be 1×10⁻³ S cm⁻¹ at 150 °C under water-free condition.     

Hybrid proton-conducting membranes for polymer electrolyte fuel cells: Phosphomolybdic acid doped poly(2,5-benzimidazole)—(ABPBI-H3PMo12O40)

The synthesis and characterization of a novel hybrid organic-inorganic material formed by phosphomolybdic acid H₃PMo₁₂O₄₀ (PMo₁₂) and poly(2,5-benzimidazole) (ABPBI) is reported. This material, composed of two proton-conducting components, can be cast in the form of membranes from methanesulfonic acid (MSA) solutions. Upon impregnation with phosphoric acid, the hybrid membranes present higher conductivity than the best ABPBI polymer membranes impregnated in the same conditions. These electrolyte membranes are stable up to 200 °C, and have a proton conductivity of 3 × 10⁻² S cm⁻¹ at 185 °C without humidification. These properties make them very good candidates as membranes for polymer electrolyte membrane fuel cells (PEMFC) at temperatures of 100-200 °C.     

Agar-based films for application as polymer electrolytes

New types of polymer electrolytes based on agar have been prepared and characterized by impedance spectroscopy, X-ray diffraction measurements, UV-vis spectroscopy and scanning electronic microscopy (SEM). The best ionic conductivity has been obtained for the samples containing a concentration of 50 wt.% of acetic acid. As a function of the temperature the ionic conductivity exhibits an Arrhenius behavior increasing from 1.1 × 10⁻⁴ S/cm at room temperature to 9.6 × 10⁻⁴ S/cm at 80 °C. All the samples showed more than 70% of transparency in the visible region of the electromagnetic spectrum, a very homogeneous surface and a predominantly amorphous structure. All these characteristics imply that these polymer electrolytes can be applied in electrochromic devices.     

Acetic acid-doped poly(ethylene oxide)-modified poly(methacrylate): a new proton conducting polymeric gel electrolyte

A proton conducting polymeric gel membrane was first developed from poly(ethylene oxide)-modified poly(methacrylate) (PEO-PMA) containing poly(ethylene glycol) dimethylether (PEGDE). Acetic acid (HAc) was doped by immersing the polymeric film directly in the aqueous solution of HAc. Characterization by FT-IR, XRD and AC conductivity measurements were carried out on the film electrolytes consisting of different gel compositions. The ionic conductivity of the membrane showed a sensitive variation with the immersion time and concentration of the acid in the doping solution through the changes in the contents of acid and water in the gel. The ionic conductivity also depended on the PEGDE content in the polymer. The proton conductivity was 6.2×10⁻⁴ S cm⁻¹ at 20 °C and 1.0×10⁻³ S cm⁻¹ at 80 °C for the gel prepared from HAc concentration of 3.0 mol l⁻¹. The temperature dependence of the conductivity was found to be consistent with Arrhenius-type relationship at a temperature range from 20 to 80 °C, except for the films with low PEGDE contents. The apparent activation energy for the proton conduction was in the range of 5-30 kJ mol⁻¹, depending on the HAc concentration and the polymer matrix composition. The FT-IR spectra of the polymeric membranes showed that HAc does not protonate the carbonyl or ester groups of the polymer matrix, but interacts with them by the hydrogen bonding interaction or weak molecular interactions.     

Synthesis and properties of new sulfonated poly(p-phenylene) derivatives for proton exchange membranes. I

Several high molecular weight poly(2,5-benzophenone) derivatives were synthesized by high yield nickel-catalyzed coupling polymerization of 2,5-dichloro-4′-substituted benzophenones. The monomers were prepared by Friedel-Crafts catalyzed reaction of 2,5-dichlorobenzoyl chloride and several aromatic compounds. The resulting polymers are organosoluble and show no evidence of crystallinity by differential scanning calorimetry (DSC). The temperatures of 5% weight loss of the polymers via dynamic thermogravimetric analysis in air were above 480 °C. Sulfonation of selected polymers utilizing concentrated or fuming sulfuric acid at room temperature introduced sulfonic acid moieties to the aromatic side group. Activated fluoro aryl groups were also used to generate pendent sulfonated functionalities. The sulfonated polymers were examined for ion exchange capacities, water absorption capacities and proton conductivities. The sulfonated polymers were not good film formers, but could be demonstrated to show high values of proton conductivity in the range of 0.06-0.11 S/cm when supported on glass fabrics or via polymer blending strategies.     

Proton conducting polydimethylsiloxane/metal oxide hybrid membranes added with phosphotungstic acid(II)

Flexible polymer electrolyte membranes consisting of zirconia (titania) and polydimethylsiloxane (PDMS) with the different molecular mass of 4500 and 600 have been synthesized by sol-gel processes. The polymeric membranes showed thermal stability and flexibility up to 300 °C due to the presence of cross-linkable inorganic nano-phase in the hybrid macromolecular matrix. The membrane becomes proton conducting polymer electrolyte by addition of 12-phosphotungstic acid (PWA). The conductivity increased up to 7.7×10⁻² S/cm through ultrasonic treatment on the membrane, which is in the application level to polymer electrolyte fuel cells. The hybrid materials can be recognized as new, low cost and environment friendly electrolyte membranes.