Composite proton-conducting polymer membranes for clean hydrogen production with solar light in a simple photoelectrochemical compartment cell

Photocatalytic hydrogen production under sunlight was demonstrated using a two-compartment cell without any electrical or chemical bias. New composite polymer membranes act as compartment separator as well as support for coated electrodes and photocatalyst. Commercial P25 was used as model photocatalyst and was spin-coated on different types of membranes. With these high amounts of hydrogen were produced from pure water in a separated half-cell. Higher proton concentrations in low concentrated hydrochloric acid enhance the photocatalytic hydrogen generation.     

Composite S-PEEK membranes for medium temperature polymer electrolyte fuel cells

Sulphonated-PEEK polymers with two different sulphonation degrees (DS) were obtained by varying the sulphonation parameters. Ionomeric membranes were prepared as a reference. Composite membranes were obtained by mixing different percentage of 3-aminopropyl functionalised silica to the polymers dissolved in DMAc. The resulting membranes were characterised in terms of water uptake, IEC and proton conductivity in different conditions of temperature and relative humidity.     

Sulfonated poly(ether ether ketone)/polybenzimidazole oligomer/epoxy resin composite membranes in situ polymerization for direct methanol fuel cell usages

A diamine-terminated polybenzimidazole oligomer (o-PBI) has been synthesized for introducing the benzimidazole groups (BI) into sulfonated poly(ether ether ketone) (SPEEK) membranes. SPEEK/o-PBI/4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) composite membranes in situ polymerization has been prepared for the purpose of improving the performance of SPEEK with high ion-exchange capacities (IEC) for the usage in the direct methanol fuel cells (DMFCs). The composite membranes with three-dimensional network structure are obtained through a cross-linking reaction between PBI oligomer and TMBP and the acid-base interaction between sulfonic acid groups and benzimidazole groups. Resulting membranes show a significantly increasing of all of the properties, such as high proton conductivity (0.14 S cm⁻¹ at 80 °C), low methanol permeability (2.38 × 10⁻⁸ cm² s⁻¹), low water uptake (25.66% at 80 °C) and swelling ratio (4.11% at 80 °C), strong thermal and oxidative stability, and mechanical properties. Higher selectivity has been found for the composite membranes in comparison with SPEEK. Therefore, the SPEEK/o-PBI/TMBP composite membranes show a good potential in DMFCs usages.     

Comprehensive performance enhancement of polybenzimidazole based high temperature proton exchange membranes by doping with a novel intercalated proton conductor

On the study of high temperature proton exchange membrane (HTPEM), the trade-off between proton conductivity and physico-chemical property (such as mechanical strength, dimensional stability and methanol resistance) remained a main obstacle for comprehensive performance enhancement. To address this issue, novel HTPEM was prepared by doping phosphotungstic acid intercalated ferric sulfophenyl phosphate (FeSPP-PWA) into polybenzimidazole (PBI) via hot pressed method. Intense hydrogen bonding network was built between PBI and FeSPP-PWA, rendering construction of proton channels and reinforcement of physico-chemical property. As a novel proton conductor, FeSPP-PWA facilitated formation of efficient proton transfer pathway. The layered morphology and inorganic intrinsity of FeSPP-PWA also improved the mechanical and dimensional stability while reducing the methanol permeability of the PBI/FeSPP-PWA membranes. The composite membrane exhibited good thermal stability up to 200 °C. The proton conductivity of PBI/FeSPP-PWA (30 wt%) reached 110 mS cm^?1 at 170 °C and 100% RH, and was 69.3 mS cm^?1 at 180 °C and 50% RH. The PBI/FeSPP-PWA also showed low methanol permeability and high membrane selectivity for application in direct methanol fuel cells.     

New membranes based on ionic liquids for PEM fuel cells at elevated temperatures

Proton exchange membrane (PEM) fuel cells operating at elevated temperature, above 120 °C, will yield significant benefits but face big challenges for the development of suitable PEMs. The objectives of this research are to demonstrate the feasibility of the concept and realize [acid/ionic liquid/polymer] composite gel-type membranes as such PEMs. Novel membranes consisting of anhydrous proton solvent H₃PO₄, the protic ionic liquid PMIH₂PO₄, and polybenzimidazole (PBI) as a matrix have been prepared and characterized for PEM fuel cells intended for operation at elevated temperature (120–150 °C). Physical and electrochemical analyses have demonstrated promising characteristics of these H₃PO₄/PMIH₂PO₄/PBI membranes at elevated temperature. The proton transport mechanism in these new membranes has been investigated by Fourier transform infrared and nuclear magnetic resonance spectroscopic methods.     

Anhydrous proton conducting membranes for PEM fuel cells based on Nafion/Azole composites

Proton conducting membranes are the most crucial part of energy generating electrochemical systems such as polymer electrolyte membrane fuel cells (PEMFCs). In this work, Nafion based proton conducting anhydrous composite membranes were prepared via two different approaches. In the first, commercial Nafion115 and Nafion112 were swelled in the concentrated solution of azoles such as 1H-1,2,4-triazole (Tri), 3-amino-1,2,4-triazole (ATri) and 5-aminotetrazole (ATet) as heterocyclic protogenic solvents. In the second, the proton conducting films were cast from the Nafion/Azole solutions. The partial protonation of azoles in the anhydrous membranes were studied by Fourier transform infrared (FT-IR) spectroscopy. Thermal properties were investigated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA results showed that Nafion/ATri and Nafion/ATet electrolytes are thermally stable at least up to 200 °C. Methanol permeability measurements showed that the composite membranes have lower methanol permeability compared to Nafion112. Nafion115/ATri system has better conductivity at 180 °C, exceeding 10⁻³ S/cm compared to other Nafion/heterocycle systems under anhydrous conditions.     

Ionomers based on multisulfonated perylene dianhydride: Synthesis and properties of water resistant sulfonated polyimides

A novel locally and densely sulfonated dianhydride with four sulfonic acid groups, 1,6,7,12-tetra[4-(sulfonic acid)phenoxy]perylene-3,4,9,10-tetracarboxylic dianhydride (SPTDA), was successfully synthesized by direct sulfonation of the parent dianhydride, 1,6,7,12-tetraphenoxyperylene-3,4,9,10-tetracarboxylic dianhydride (PTDA). Sulfonated copolyimides were prepared from SPTDA, nonsulfonated dianhydride 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianydride, 4,4′-diaminodiphenyl ether (a) or dodecane-1,12-diamine (b). The synthesized copolymers, with the -SO₃H group on the polymer side chain, possess high molecular weights and high viscosities, and they form tough, flexible membranes. The copolymer membrane with an ion exchange capacity of 2.69 mequiv. g⁻¹ had a proton conductivity of 0.126 S cm⁻¹ at 20 °C and 0.292 S cm⁻¹ at 100 °C; the latter is much higher than that of Nafion^® 117 under the same conditions. The mechanical properties of the copolymer membranes were almost unchanged after accelerated water stability testing at 140 °C for 100 h; this indicates excellent hydrolytic stability of the synthesized copolyimides.     

The limits of proton conductivity in polymeric sulfonated membranes: A modelling study

In this work, the conductivity limits of sulfonated membranes are investigated through a model analysis. A recent analytical conductivity model has been modified by reducing the number of variables to only three parameters, representing the hydration level, the ion exchange capacity and the morphology of the membrane. The effects of these parameters on the conductivity are investigated through a parametric analysis, showing significant trends.Particular values of the morphology parameter define ideal conditions, in which the model conductivities constitute upper limits for real membranes. In particular, the model conditions of “ideal isotropic membrane” and “ideal non-tortuous membrane” are compared with the experimental proton conductivity of a number of polymeric membranes in the literature. It appears that membranes such as Nafion and Dow are close to the condition of “ideal isotropic membrane”, and their conductivity can be improved only by decreasing their tortuosity. On the other hand, the conductivity of other sulfonated polymers as SPEEK is well below the limit and can be enhanced by improving the membrane percolation properties.     

Hybrid ion-exchange membranes for fuel cells and separation processes

This work reports the preparation and characterization of hybrid membranes cast from dispersions of inorganic fillers in sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene solutions. Silica gel, SBA-15 and sepiolite, all of them functionalized with phenylsulfonic acid groups, were used as fillers. For comparative purposes, the performance of composite membranes cast from dispersions of functionalized inorganic fillers in Nafion^® solutions was investigated. Inspection of the texture of the membranes by using SEM techniques shows that the fillers are better dispersed in sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene than in Nafion^®. The value of the water uptake for the membranes prepared from the former polyelectrolyte is in most cases at least three times that measured for hybrid Nafion^® membranes. The conductivity of the membranes was measured at 80 °C by impedance spectroscopy obtaining values of 3.44, 6.90 and 3.54 S m⁻¹ for the hybrid membranes based on the triblock copolymer containing functionalized silica gel, SBA-15 and sepiolite fillers, respectively. These results compare very favourably with those obtained at 80 °C for Nafion^® hybrid membranes containing silica gel, SBA-15 and sepiolite, all of them fuctionalized with phenylsulfonic acid groups, whose conductivities are, 2.84, 6.75 and 3.31 S m⁻¹, respectively. Resistance measurements carried out under controlled humidity conditions show that the conductivity of sulfonated triblock copolymer membranes containing functionalized SBA-15 filler undergoes a rather sharp increase when they are conditioned under an atmosphere of 75%, or larger, relative humidity.     

Fabrication of novel proton exchange membranes for DMFC via UV curing

The radiation hardening of various UV curable resins provides a simple but powerful method to fabricate thin films or membranes with desirable physical and chemical properties. In this study, we proposed to use this method to fabricate a novel proton exchange membrane (PEM) for direct methanol fuel cells (DMFC) with good mechanical, transport and stability properties. The PEM was prepared by crosslinking a mixture of a photoinitiator, a bifunctional aliphatic urethane acrylate resin (UAR), a trifunctional triallyl isocyanate (TAIC) crosslinker and tertrabutylammonium styrenesulfonate (SSTBA) to form a uniform network structure for proton transport. Key PEM parameters such as ion exchange capacity (IEC), water uptake, proton conductivity, and methanol permeability were controlled by adjusting the chemical composition of the membranes. The IEC value of the membrane was found to be an important parameter in affecting water uptake, conductivity as well as the permeability of the resulting membrane. Plots of the water uptake, conductivity, and methanol permeability vs. IEC of the membranes show a distinct change in the slope of their curves at roughly the same IEC value which suggests a transition of structural changes in the network. It is demonstrated that below the critical IEC value, the membrane exhibits a closed structure where hydrophilic segments form isolated domains while above the critical IEC value, it shows an open structure where hydrophilic segments are interconnected and form channels in the membrane. The transition from a closed to an open proton conduction network was verified by the measurement of the activation energy of membrane conductivity. The activation energy in the closed structure regime was found to be around 16.5 kJ mol⁻¹ which is higher than that of the open structure region of 9.6 kJ mol⁻¹. The membranes also display an excellent oxidative stability, which suggests a good lifetime usage of the membranes. The proton conductivities and the methanol permeabilities of all membranes are in the range of 10⁻⁴ to 10⁻² S cm⁻¹ and 10⁻⁸ to 10⁻⁷ cm² s⁻¹, respectively, depending on their crosslinking density. The membranes show great selectivity compared with those of Nafion^®. The possibility of using this PEM for DMFC devices is suggested.     

Membranes based on phosphotungstic acid and polybenzimidazole for fuel cell application

Composite membranes based on phosphotungstic acid (PWA) adsorbed on silica (SiO₂) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied. The membranes with high tensile strength and thickness of less than 30 μm can be cast. They are chemically stable in boiling water and thermally stable in air up to 400°C. Proton conductivity is influenced by the temperature (range: 30–100°C), relative humidity and PWA loading in the membrane. Maximum conductivity of 3.0×10⁻³ S/cm is obtained at 100% relative humidity and 100°C with membrane containing 60 wt.% PWA/SiO₂ in PBI. Conductivity measurements performed at higher temperatures, in the range from 90°C to 150°C, give almost stable values of 1.4–1.5×10⁻³ S/cm at 100% relative humidity.     

Sulfonated SBA-15 mesoporous silica-incorporated sulfonated poly(phenylsulfone) composite membranes for low-humidity proton exchange membrane fuel cells: Anomalous behavior of humidity-dependent proton conductivity

Sulfonated SBA-15 mesoporous silica (SM-SiO₂)-incorporated sulfonated poly(phenylsulfone) (SPPSU) composite membranes are fabricated for potential application in low-humidity proton exchange membrane fuel cells (PEMFCs). The SM-SiO₂ particles are synthesized using tetraethoxy silane (TEOS) as a mechanical framework precursor, Pluronic 123 triblock copolymer as a mesopore-forming template, and mercaptopropyl trimethoxysilane (MPTMS) as a sulfonation agent. A distinctive feature of the SM-SiO₂ particles is the long-range ordered 1-D skeleton of hexagonally aligned mesoporous cylindrical channels bearing sulfonic acid groups. Based on a comprehensive characterization of the SM-SiO₂ particles, the effect of SM-SiO₂ (as a functional filler) addition on the proton conductivity of the SPPSU composite membrane is examined as a function of temperature and relative humidity. An intriguing finding is that the proton conductivity of the SPPSU composite membrane exhibits a strong dependence on the relative humidity of measurement conditions. This anomalous behavior is further discussed with an in-depth consideration of the characteristics and dispersion state of SM-SiO₂ particles, which affect the tortuous path for proton movement, water uptake, and state of water. Notably, at low-humidity conditions, the SM-SiO₂ particles in the SPPSU composite membrane serve as an effective water reservoir to tightly retain water molecules and also as a supplementary proton conductor, whereas they behave as a barrier to proton transport at fully hydrated conditions.     

Proton-conducting composite membranes based on polybenzimidazole and sulfonated mesoporous organosilicate

Sulfonated mesoporous organosilicate (s-MPOs) was synthesized by the one-step sol–gel method as a novel inorganic additive derived for use in the fuel cell. TEM observations revealed that the s-MPOs has well-ordered structure and many SO₃H groups on the inner surface of the mesopores. The s-MPOs was added to the proton-conductive polymer matrix, polybenzimidazole (PBI) in the presence of H₃PO₄, and the proton conductivities were measured at 60–100 °C under controlled humidity. The PBI composites filled with only 1 wt% of s-MPOs gave proton conductivity more than 10-times higher than the original PBI/H₃PO₄ membrane. The s-MPOs possessing many SO₃H groups were able to form effective proton conductive pathways via its periodic structure and to improve the conductivity. The greatest conductivity was estimated to be 0.21 S cm⁻¹ at 80 °C and 98 %RH in case of a PBI/s-MPOs20 (incl. approx. 20 mol% of the SO₃H units in MPS) composite.     

Properties and fuel cell performance of a Nafion-based,sulfated zirconia-added,composite membrane

The effect of an acidic inorganic additive, i.e. sulfated zirconia, on Nafion-based polymer electrolytes is evaluated by comparing the properties in terms of conductivity and fuel cell performance of a composite sulfated zirconia-added Nafion membrane with those of an additive-free Nafion membrane. The peculiar surface properties of the selected filler promote a higher hydration level and a higher conductivity for the composite membrane under unsaturated conditions, i.e. at 20% RH. Tests on H₂-air fully humidified cells, monitored at 70 °C and at atmospheric pressure, reveal small differences when passing from a plain Nafion to a composite Nafion/sulfated zirconia membrane as electrolyte. However, remarkably great improvements are observed for the composite membrane-based cell when the comparison tests are run at low relative humidity and high temperature, this outlining the beneficial role of the sulfated zirconia additive.     

Hydrogen permeation through the Pd-Nb-Pd composite membrane: Surface effects and thermal degradation

The composite membranes based on Group 5 metals are capable of H₂ separation with the high speed and infinite selectivity. The chemical and thermal stability are critical issues for the application of such membranes in the field of hydrogen energy. In order to understand the degradation mechanisms, the H₂ permeation through composite Pd_2μm-Nb_100μm-Pd_2μm membranes was investigated in a very wide pressure range: (10⁻⁵-10⁴) Pa. At higher pressures the surface contaminations only moderately decreased the permeation. However the permeation experiments at lower pressures demonstrated that an orders of magnitude change in the probability of H₂ molecule dissociative sticking is actually hidden behind this relatively moderate effect. The membranes poisoned by the surface contaminations could be recovered by their exposure to O₂ at (300-400) °C. Heating at temperature higher than 500 °C resulted in the irreversible decrease of permeation and in the pronounced change of permeation behavior at the variation of H₂ pressure. An extremely high permeation was observed at lower pressures at the clean surface of Pd coating. That allows developing an effective membrane pump for hydrogen isotopes.