Proton-conducting Hybrid Membranes Research Articles

Using Metal-Organic Framework HKUST-1 for the Preparation of High-Conductive Hybrid Membranes Based on Multiblock Copolymers for Fuel Cells.

Novel proton-conducting hybrid membranes consisting of sulfonated multiblock copolymer of polysulfone and polyphenylsulfone (SPES) reinforced with a HKUST-1 metal-organic framework (MOF) (5, 10, and 20 wt. %) were prepared and characterized for fuel cell applications. The presence of the MOF in the copolymer was confirmed by means of FE-SEM and EDS. The hybrid membranes show a lower contact angle value than the pure SPES, in agreement with the water uptake (WU%), i.e., by adding 5 wt. % of the MOF, this parameter increases by 20% and 40% at 30 °C and 60 °C, respectively. Additionally, the presence of the MOF increases the ion exchange capacity (IEC) from 1.62 to 1.93 mequivH+ g−1. Thermogravimetric analysis reveals that the hybrid membranes demonstrate high thermal stability in the fuel cell operation temperature range (<100 °C). The addition of the MOF maintains the mechanical stability of the membranes (TS > 85 MPa in the Na+ form). Proton conductivity was analyzed using EIS, achieving the highest value with a 5 wt. % load of the HKUST-1. This value is lower than that observed for the HKUST-1/Nafion system. However, polarization and power density curves show a remarkably better performance of the hybrid membranes in comparison to both the pure SPES and the pure Nafion membranes.

Enhanced proton conductivity assisted by sodium ions in the proton conductive hybrid membranes

Development of new proton exchange membranes for fuel-cells draw considerable attentions. A family of proton conductive membranes based on poly(sodium 4-styrene sulfonate) (PSSNa) and polyvinylidene fluoride (PVDF) combining with various additives oxalic acid (OX), phosphate (PA) and phosphomolybdic acid H3PMo12O40·XH2O (PMo12) have been fabricated through a slurry casting approach, and are defined as PSSNa-PVDF-OX, PSSNa-PVDF-PMo12 and PSSNa-PVDF-PA respectively, whose proton conductivities have been systematically investigated. Among them, PSSNa-PVDF-OX shows the superior proton conductivity, compared to PSSNa-PVDF-PA and PSSNa-PVDF-PMo12, and the best proton conductive value can reach up to 5.0 × 10−3 S cm−1 at 60 °C and 95% relative humidity, proving the good compatibility of OX with PSSNa and PVDF, giving birth to an effective proton transport channel. In order to demonstrate the role of Na+, poly(4-styrene sulfonic acid) (PSSA) is employed to replace PSSNa, generating another series of proton conductive membranes PSSA-PVDF-OX. The proton conductivity of PSSNa-PVDF-OX is greater than those of PSSA-PVDF-OX, which is attributed to the strong interactions between Na+ and water molecules, accelerating proton movement. Summarily, the enhanced conductivity is attributed to the generated efficient conduction pathways constructed from Na+ ions, water molecules, abundant -COOH from OX and -SO3- groups from PSSNa as well as rich F groups from PVDF.

Synergy between Isomorphous Acid and Basic Metal-Organic Frameworks for Anhydrous Proton Conduction of Low-Cost Hybrid Membranes at High Temperatures.

Metal-organic frameworks (MOFs) embedded in polymer have showed efficiency in improving proton conduction of hybrid membranes under hydrated conditions. However, anhydrous proton conduction of such hybrid membranes over 100 °C remains great challenge. Here, proton conductive hybrid membranes combined acid group (-SO3H)- and basic group (-NH2)-modified isomorphous MOFs, namely UiO-66(SO3H) (abbreviated as A, the initial of acid) and UiO-66(NH2) (abbreviated as B, the initial of basic) and a low-cost polymer (chitosan, CS) were prepared. The proton conductivity of the optimum dual MOF-cofilled hybrid membranes (CS/A + B) reached 3.78 × 10-3 S/cm at 120 °C and under anhydrous conditions, under which each component, that is MOF A, MOF B and CS, and single MOF-filled hybrid membranes (CS/A and CS/B) nearly lost proton conduction without exception, producing unprecedented results of one plus one more greater than two. The synergistic effects among UiO-66(SO3H), UiO-66(NH2), and CS on improving conductivity are also observed under hydrated conditions, the highest proton conductivity of CS/A + B reached 5.2 × 10-2 S/cm, which is 1.86, compared to that of the pure CS membrane at 100 °C and 98% relative humidity. The anhydrous proton conductivity of CS/A + B over 100 °C is one of the highest for MOF-based hybrid membranes. MOFs and hybrid membranes were extensively characterized and the proton conductive mechanism was revealed. The achievements open a new avenue for MOF-based anhydrous proton-conducting membranes and would advance the exploration of future application of these MOFs in fuel cells.

Nanostructured Polyelectrolytes Based on SPEEK/TiO2 for Direct Ethanol Fuel Cells (DEFCs)

Proton-conducting hybrid membranes consisting of poly(ether ether ketone) sulfonated (SPEEK) and titanium oxide (TiO 2 ) were prepared using the sol-gel technique for application in direct ethanol fuel cells. The effect from TiO 2 incorporation on membrane properties such as ethanol uptake, pervaporation and proton conductivity was investigated. The uptake and permeated flux decreased with increasing content of TiO 2 . The ethanol permeability was about one order of magnitude smaller than Nafion  117. FTIR spectra indicated that PEEK was sulfonated and the second degradation temperature of SPEEK58 samples confirmed the titanium oxide incorporation. The proton conductivity in ethanol solution was of the order of 10 -3 S cm -1 when 4 or 8 wt% TiO 2 were added, and generally increased with addition of TiO 2 .

Novel proton conductive hybrid membranes based on sulfonated polysulfone and benzotriazole

Abstract

Enhanced conductivity and electrochemical properties for class of hybrid systems via sol–gel techniques

Enhanced high proton-conducting hybrid membranes were prepared via the sol–gel technique using phosphotungsticacid (PWA) mixed with polyvinylalcohol (PVA) and phosphosilicate (P2O5/SiO2). These new class of membranes showed excellent proton conductivities and were confirmed to be promising electrolytes for fuel cells. Moreover, their mechanical, thermal, swelling and structural properties were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis, and swelling measurements. Also proton conductivity and electrochemical studies were performed. The materials were thermally stable up to 300°C, and the swelling capability was found to be increased by addition of the SiO2 function. The phosphosilicate mixed with poly(vinylalcohol) (PVA) and heteropolyacid thus gave rise to the creation of a strong PVA/PWA/P2O5/SiO2 hybrid membrane network through Keggin ions.

Hybrid membranes of PVA for direct ethanol fuel cells (DEFCs) applications

Proton-conducting hybrid membranes composed of poly(vinyl alcohol) (PVA), phosphotungstic acid (HPW) and (diethylenetriamine)pentaacetic acid (DTPA) where prepared. The effect of HPW doping and crosslink with DTPA on the membranes properties such as uptake, pervaporation and proton conductivity was investigated. Uptake and permeated flux decreases with increasing content of HPW and DTPA. Ethanol permeabilities obtained was about two orders of magnitude smaller than Nafion® 117. FTIR spectra indicated that HPW was incorporated into the polymer matrix and DTPA acted as crosslink agent. The proton conductivity was in the order of 10−3 S cm−1 with added 4 wt.% of DTPA and generally increases with the addition of HPW.

High proton conductivity of poly (methyl methacrylate)-based hybrid membrane for PEMFCs

A high proton-conducting PMMA–SiO 2–P 2O 5 (75–20–5 wt.%) membrane with a conductivity of 1.35 × 10 −1 S cm −1 at 90 °C for value of relative humidity of 50% was prepared and an excellent thermal stability was found by hybridizing phosphosilicate with PMMA using the sol–gel technique. This is one of the most promising ways to obtain high performance proton-conducting hybrid membranes.

Lamellar crystals as proton conductors to enhance the performance of proton exchange membrane for direct methanol fuel cell

Zirconium glyphosate (ZrG) is a solid proton conductor with layered crystal structure. The inorganic veneer sheets of ZrG are covalently intercalated by glyphosate molecules with carboxylic acid end groups (–COOH). The existence of abundant –COOH groups both inside and on the surface of ZrG provides additional proton-conducting channels facilitating the proton conduction through and around the inorganic crystals. ZrG is incorporated into the sulfonated polyether ether ketone (SPEEK) matrices to prepare proton-conducting hybrid membranes. The conductivity of the hybrid membranes is higher than the pristine SPEEK membrane, and increases with increasing ZrG content. Furthermore, the enhancement of the proton conductivity is more obvious at elevated temperatures. At 25°C, the proton conductivity of the hybrid membrane with 16wt% ZrG is 1.4 times higher than that of the pristine membrane. When the temperature increases to 55°C, the conductivity of the hybrid membrane with 8wt% ZrG is more than twice that of the pristine SPEEK membrane. The prolonged and tortuous pathways originated from the incorporation of inorganic crystals lead to reduced methanol permeability. The selectivity of the hybrid membrane is increased by as much as 72% compared to the pristine SPEEK membrane.

Anhydrous Proton Conducting Hybrid Membrane Electrolytes for High Temperature (>100°C) Proton Exchange Membrane Fuel Cells

Anhydrous proton conducting hybrid membranes were prepared by the sol-gel process with trimethoxysilane (TMOS)/trimethoxypropylsilane (TMPS)/triethyl phosphate (TEP) and BMIMBF4 ionic liquid as precursors. The Fourier transform infrared and NMR measurements have shown good chemical stability of these hybrid membranes. Average pore sizes and specific surface areas were investigated by the Brunauer–Emmett–Teller method. Thermogravimetric analysis and differential thermal analysis measurements confirmed that the hybrid membranes were thermally stable up to 250°C. Maximum anhydrous conductivity of 6.4 × 10−3 S/cm was obtained for 40 wt % ionic liquid based hybrid membrane, at 150°C, with a hydrogen permeability value of 0.8 × 10−12 mol/cm s Pa.

Synthesis and characterization of new proton conducting hybrid membranes for PEM fuel cells based on poly(vinyl alcohol) and nanoporous silica containing phenyl sulfonic acid

Organic–inorganic hybrid proton exchange membranes were prepared from poly(vinyl alcohol) (PVA) and various amounts of nanoporous silica containing phenyl sulfonic acid groups. These hybrid membranes were prepared via co-condensation of functionalized nanoporous SBA-15 (SBA-ph-SO 3H) as hydrophilic inorganic modifier, glutaraldehyde (GLA) as cross-linking agent in a PVA matrix. These membranes were characterized for their morphology, thermal stability, electrochemical and physicochemical properties using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and water uptake studies. The SBA-ph-SO 3H/PVA composite membranes have a higher water retention and thermal stability than that of Nafion 117, perhaps because of responsibility of both acidic groups and nanoporous structure of silica additive. This work demonstrates the promising potential of new composite membranes for the development of high-performance and high-stability PEM fuel cells with improved proton conductivity.

Preparation, characterization and evaluation of proton-conducting hybrid membranes based on sulfonated hydrogenated styrene–butadiene and polysiloxanes for fuel cell applications

This paper describes the preparation of proton-conducting hybrid membranes (HMs) obtained by a solvent casting procedure using a solution containing sulfonated hydrogenated styrene–butadiene (HSBS-S) and an inorganic–organic mixture (polysiloxanes) previously prepared by a sol–gel route. HSBS-S copolymers with different sulfonation degrees were obtained and characterized by means of elemental analysis (EA), chemical titration and electrochemical impedance spectroscopy (EIS). HSBS-S with the best properties in terms of proton conductivity and solubility for the casting procedure was selected to prepare the HMs. The solvent casting procedure permitted the two phases to be homogeneously distributed while maintaining a relatively high proton conductivity in the membrane. HMs with different blend ratios were characterized using structural (Fourier transform infrared–attenuated total reflectance (FTIR–ATR), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC)), electrical (EIS), physicochemical (water uptake, ion-exchange capacity) and thermal (TGA–MS) methods. Finally, the optimized HSBS-S membrane and HMs were tested in hydrogen single fuel cells to obtain the polarization and power curves at different cell temperatures and gas pressures. Results indicate that HMs show a considerable improvement in performance compared to the optimized HSBS-S membrane denoting the benefit of incorporating the inorganic–organic network in the hydrogenated styrene–butadiene matrix. A Nafion membrane was used as reference material throughout this work.

Protic Ionic Liquid-Based Hybrid Proton-Conducting Membranes for Anhydrous Proton Exchange Membrane Application

A new type of proton-conducting hybrid membranes were prepared by in situ cross-linking of a mixture of polymerizable oils containing protic ionic liquids (PILs) and silica nanoparticles or mesoporous silica nanospheres. The resultant hybrid membranes are semitransparent, flexible, and show good thermal stability, good and tunable mechanical properties. Incorporation of proper amount of silica fillers significantly increased the proton conductivity of the membranes, probably due to the ion transport channel or network structures formed in the membranes. However, further addition of silica fillers might block the formed ion transport channels and decrease the conductivity of hybrid membranes. Compared with silica nanoparticles, mesoporous silica nanospheres is more effective in enhancing the conductivity and in preventing the release of ionic liquid component from the composite membranes. Under anhydrous conditions, the produced hybrid membranes show proton conductivity up to the order of 1 × 10−2 S/cm at ...

Proton Conducting Hybrid Membranes Based on Aromatic Polymers Blends for Direct Methanol Fuel Cell Applications

AbstractComposite membranes made‐up of sulphonated polyetheretherketone (SPEEK, DS = 0.5) and of a sulphonated and silylated hybrid organic–inorganic derivative of polyphenylsulphone (SiSPPSU, DS = 2) were prepared and characterised. The water and methanol uptake, as well as thermal properties, of blend membranes containing 5 and 10 wt.‐% SiSPPSU were evaluated. The electrochemical performance of the membranes was evaluated by electrochemical impedance spectroscopy (EIS) and tested in a direct methanol fuel cell (DMFC) station. The presence of SiSPPSU modified the properties of SPEEK, allowing to increase its performance in terms of proton conductivity, thermal and hydrolytic stability. These blends appeared to be suitable electrolytes for DMFC applications.

Novel Organic−Inorganic Hybrids with Increased Water Retention for Elevated Temperature Proton Exchange Membrane Application

A new class of proton-conducting hybrid membranes have been developed using a combination of a solvent-directed infiltration method and sol−gel chemistry with a range of organofunctional silane and phosphate precursors. The phase-separated morphology of Nafion is used as a structure-directing template, which drives the inorganic component into the ionic clusters of the Nafion membrane. The kinetics of the sol−gel reactions were monitored using spectroscopic techniques. Photoacoustic Fourier transform infrared spectroscopy (PA-FTIR) confirms formation of Si−O−Si and Si−O−P bridges in the hybrid membranes, indicating silicate and phosphosilicate structures. The presence of the silicate/phosphosilicate network in the hybrid membranes enhances their thermal stability, thermomechanical properties, water retention at elevated temperatures, and relaxation temperature Tc. Scanning electron microscopy (SEM) and small angle neutron scattering were used to determine the morphology and microstructure of these membranes. A structural model of the hybrids is proposed to describe the size and shape of the inorganic particles, which is consistent with the SEM observations. Proton conductivity measurements were made from 30 to 80 °C and at relative humidities ranging from 30% to 90%. The presence of inorganics in the polymer membrane has improved the water management in these new organic−inorganic hybrids at elevated temperatures above 100 °C, which is a key parameter when designing proton-exchange membranes for medium-temperature fuel cell application.

New proton conducting hybrid membranes for HT-PEMFC systems based on polysiloxanes and SO 3H-functionalized mesoporous Si-MCM-41 particles

For increased efficiency of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), new types of membranes have to be developed. This approach has been realized by preparing hybrid membranes containing SO 3H-functionalized mesoporous Si-MCM-41 as hydrophilic inorganic modifier in a polysiloxane matrix exhibiting sulfonic acid groups and basic heterocyclic groups like benzimidazole. The proton conductivity of sulfonated particles was modelled on the atomic scale in order to understand the influence of the density of sulfonic acid groups and of the presence of water molecules. The different hybrid membranes are characterized concerning their thermal stability, water uptake, and proton conductivity. Whereas the proton conductivity of well-established, but expensive and at >120 °C not long-time stable Nafion membranes continuously decreases with increasing temperature, the polysiloxane membranes, which suffer from a low-proton conductivity at around 100 °C, recover at about 120 °C due to intrinsic proton transport. At 180 °C the pure polysiloxane shows a proton conductivity which is only one order of magnitude lower than that of Nafion. Moreover, if the polysiloxane membrane contains additionally 10 wt.% of an SO 3H-modified Si-MCM-41, the proton conductivity of such hybrid membrane at temperatures >180 °C and low relative humidity <10% is higher than that of Nafion membranes by a factor of 10.

Preparation and Characterization of Proton Conductive Phosphosilicate Membranes Based on Inorganic-Organic Hybrid Materials

A series of proton conductive inorganic-organic hybrid membranes doped with phosphoric acid (<TEX>$H_3PO_4$</TEX>) and/or triethylphosphate (PO(OEt)<TEX>$_3$</TEX>) have been prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) as precursors. High proton conductivity of 3.0 <TEX>${\times}$</TEX> <TEX>$10^{-3}$</TEX> S/cm with composition of 50TEOS-30GPTMS-20APTES-50<TEX>$H_3PO_4$</TEX> was obtained at 120 <TEX>${^{\circ}C}$</TEX> under 50% relative humidity. Thermal stability of membrane was significantly enhanced by the presence of SiO<TEX>$_2$</TEX> framework up to 250 <TEX>${^{\circ}C}$</TEX>. XRD revealed that the gels are amorphous. IR spectra showed a good complexation of <TEX>$H_3PO_4$</TEX> in the matrix. The conductivity under 75% relative humidity was significantly improved by addition of APTES due to the increase in concentration of defected site in hybrid matrix. The effect of PO(OEt)<TEX>$_3$</TEX>, humidifying time, and heat-treatment were also investigated. PO(OEt)<TEX>$_3$</TEX> had no improvement on conductivity and conductivity increased with humidifying time, however, decreased with heating temperature.

PWA-doped PEG/SiO 2 proton-conducting hybrid membranes for fuel cell applications

A series of hybrid membranes were prepared through acid-catalyzed sol–gel process from alkoxysilane-endcapped poly(ethylene glycol) (PEG) precursor, which was further modified with monophenyl trimethoxysilane (MPh) during the course of sol–gel reaction. Phosphotungstic acid (PWA) was doped into hybrid matrix at different levels to provide the resulting membrane with proton conducting behavior. Water uptake, proton conducting property, methanol permeability and microstructure of these hybrid membranes were investigated. The 29Si solid-state NMR analysis of hybrid membranes showed that doping of PWA into hybrid matrix led to different stereo-chemical structures. PWA doping indirectly promotes formation of T P 3 (Ph Si (O Si) 3) structure during sol–gel reaction and the relative contribution of each structure was calculated quantitatively. The water uptake was decreased as both MPh and PWA content increased. The proton conductivity of water-swollen membranes was in the order of 10 −3 to 10 −5 S cm −1. The methanol permeability of hybrid membranes reduced markedly beyond 50 phr PWA content and which is about more than one order lower than that of Nafion 115. Finally, the feasibility of these hybrid membranes as electrolyte in proton conducting membrane fuel cells (PEMFC) was investigated and the preliminary results were made into comparison with similar hybrid membranes and Nafion 115. A maximum current density of 32 mA cm −2 obtained with PWA-doped PEG/SiO 2 hybrid membrane in hydrogen/oxygen (H 2/O 2) mode at ambient conditions is comparable with that of similar hybrid membrane which was evaluated at high temperature and pressure.

Proton-conducting membranes with high selectivity from phosphotungstic acid-doped poly(vinyl alcohol) for DMFC applications

Proton-conducting hybrid membranes composed of poly(vinyl alcohol) (PVA) and phosphotungstic acid (PWA) were prepared by solution-blending. The effect of PWA doping on the membrane properties such as water uptake, ion-exchange capacity (IEC), proton conductivity and methanol permeability was investigated. Up to 20 wt.% PWA content, both water uptake and methanol permeability slightly increased then decreased continuously until 90 wt.%. Fourier transform infrared spectra indicate that significant amount of PWA was maintained in the polymer matrix even after several hours of immersion in water. Combining FTIR results and the similarity in water uptake and methanol permeability of PVA/PWA hybrid membranes suggests that the variations in associated properties are ascribed to intermolecular hydrogen bonding interaction between hydroxyl groups of PVA and PWA. Methanol permeabilities of hybrid membranes were significantly lower than that of Nafion115. The proton conductivity of hybrid membranes was in the order of 10 −4 S cm −1 and generally increases with PWA content. Finally, the possibility of PVA/PWA hybrid membranes for use in PEMFC was explored. In H 2/O 2 mode, the performance increases with PWA content and a maximum current density of 46 mA cm −2 was obtained with PVA 20 wt.% and PWA 80 wt.% (PVA20PWA80) sample. Performance of DMFC with PVA20PWA80 increases with temperature and reached to 80 mA cm −2 at 80 °C.

DBSA-doped PEG/SiO 2 proton-conducting hybrid membranes for low-temperature fuel cell applications

Hybrid membranes were made through a sol–gel process from 3-(triethoxysilyl)propyl isocyanate functionalized by polyethylene glycol (PEG). 4-Dodecylbenzene sulfonic acid (DBSA) was incorporated into the matrix as a proton source. DBSA-doped PEG/SiO 2 hybrid membranes exhibit reasonable proton conductivity, and some of them show extremely low methanol permeability, which resulted in higher value of the ratio of proton conductivity to methanol permeability than that of Nafion membrane. Therefore, DBSA-doped PEG/SiO 2 hybrid membranes can reduce the problem associated with methanol cross-over in direct methanol fuel cells (DMFC). The feasibility of these materials as a polymer electrolyte in hydrogen/oxygen fuel cells (H 2/O 2 FC) as well as DMFC is investigated with the aim of developing a suitable membrane electrode assembly (MEA) fabrication method for hybrid membranes utilizing the membrane consisting of 80 wt.% monophenyl trimethoxysilane (MPh) as silica phase modifier and 20 wt.% DBSA (MPh80DBSA20) as a model material. Fuel cell measurements were carried out at low temperature and atmospheric pressure. The preliminary results are presented and made into comparison with the performance of similar hybrid membranes and Nafion.