New modified Nafion-bisphosphonic acid composite membranes for enhanced proton conductivity and PEMFC performance

Proton exchange membranes remain a crucial material and a key challenge to fuel cell science and technology. In this work, new Nafion membranes are prepared by a casting method using aryl- or azaheteroaromatic bisphosphonate compounds as dopants. The incorporation of the dopant, considered at 1 wt% loading after previous selection, produces enhanced proton conductivity properties in the new membranes, at different temperature and relative humidity conditions, in comparison with values obtained with commercial Nafion. Water uptake and ionic exchange capacity (IEC) are also assessed due to their associated impact on transport properties, resulting in superior values than Nafion when tested in the same experimental conditions. These improvements by doped membranes prompted the evaluation of their potential application in fuel cells, at different temperatures. The new membranes, in membrane-electrode assemblies (MEAs), show an increased fuel cell maximum power output with temperature until 60 °C or 70 °C, followed by a decrease above these temperatures, a Nafion-like behaviour when measured in the same conditions. The membrane doped with [1,4-phenylenebis(hydroxymethanetriyl)]tetrakis(phosphonic acid) (BP2) presents better results than Nafion N-115 membrane at all studied temperatures, with a maximum power output performance of ～383 mW cm^?2 at 70 °C. Open circuit potentials of the fuel cell were always higher than values obtained for Nafion MEAs in all studied conditions, indicating the possibility of advantageous restrain to gas crossover in the new doped membranes.     

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.     

Sulfonated MWNT and imidazole functionalized MWNT/polybenzimidazole composite membranes for high-temperature proton exchange membrane fuel cells

Composite membranes used for proton exchange membrane fuel cells comprising of polybenzimidazole (PBI) and carbon nanotubes with certain functional groups were studied, because they could enhance both the mechanical property and fuel cell performance at the same time. In this study, sodium poly(4-styrene sulfonate) functionalized multiwalled carbon nanotubes (MWNT-poly(NaSS))/PBI and imidazole functionalized multiwalled carbon nanotubes (MWNT-imidazole)/PBI composite membranes were prepared. The functionalization of carbon nanotubes involving non-covalent modification and covalent modification were confirmed by FITR, XPS, Raman spectroscopy, and TGA. Compared to unmodified MWNTs and MWNT-poly(NaSS), MWNT-imidazole provided more significant mechanical reinforcement due to its better compatibility with PBI. For MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes at their saturated doping levels, the proton conductivities were up to 5.1 × 10⁻² and 4.3 × 10⁻² S/cm at 160 °C under anhydrous condition respectively, which were slightly higher than pristine PBI (2.8 × 10⁻² S/cm). Also, MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes showed relatively improved fuel cell performance at 170 °C compared to pristine PBI.     

Branched polybenzimidazole / polymeric ionic liquid cross-linked membranes with high proton conductivity and mechanical properties for HT-PEM applications

Branched polymers have unique three-dimensional dendritic structures, so they have received a lot of attention in the application of high temperature proton exchange membranes. In this work, we synthesize BOPBI-X (X = 3%, 6%, 9%) membranes with different branched ratios based on the synthesis of OPBI, due to the introduction of a rigid triazine structure with a larger free volume, the membranes could absorb more phosphoric acid (PA) while maintaining sufficient mechanical strength. Among them, the BOPBI-6% membrane obtains splendid comprehensive performance, when the PA doping level (ADL) is 9.55, it achieves a proton conductivity of 99.2 mS cm^?1 at 180 °C, which is 1.8 times higher than OPBI (54.8 mS cm^?1), and it performs well in long-term oxidation stability test after 144 h, it still has a mass retention rate of 90%. For the sake of further boost the performance of the membrane, cross-linkable polymeric ionic liquid (PIL) is introduced to the system. Among them, BOPBI-PIL-30% membrane has sufficient mechanical properties (5.50 ± 0.8 MPa), and the proton conductivity (146.9 mS cm^?1) at 180 °C is also excellent, so BOPBI-PIL-30% membrane is expected to be a promising candidate as HT-PEMs.     

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.     

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.     

The relationship between proton conductivity and water permeability in composite carboxylate/sulfonate perfluorinated ionomer membranes

Composite carboxylate/sulfonate Nafion^® films were prepared with different ratios of anion equivalents as a test of the relationship between proton conductivity and water permeability in perfluorinated ionomers. Substitution of carboxylate groups for sulfonate groups dramatically reduced water permeability and modestly reduced proton conductivity in the films. Small angle X-ray scattering (SAXS) was used to probe the polymer microstructure and showed that with up to 20% carboxylate equivalents, the size of hydrated ionic domains within the ionomer remains constant. Water sorption measurements showed little decrease in water uptake with increased carboxylate functionality, supporting the SAXS data. Methanol/water permeability experiments showed that the carboxylate moieties slow the diffusion of both water and methanol, with no permselectivity of water. Results indicate that the proton transport mechanism is altered by addition of carboxylic acid. As such, the composites may show interesting behavior in H₂/O₂ and direct methanol fuel cells. It is proposed that permeability and conductivity measurements may provide an inexpensive and simple screening tool for early evaluation of new experimental proton exchange membranes.     

Sulfonated graphene oxide as an inorganic filler in promoting the properties of a polybenzimidazole membrane as a high temperature proton exchange membrane

A commercial perfluorinated sulfonic acid (PFSA) membrane, Nafion, shows outstanding conductivity under conditions of a fully humidified surrounding. Nevertheless, the use of Nafion membranes that operate only at low temperature (<100 °C) can lead to some disadvantages in PEMFC systems, such as a low impurity tolerance and slow kinetics. To overcome the above problems, this study introduces a highly durable composite membrane with an inorganic filler for a high-temperature proton exchange membrane fuel cell (HT-PEMFC) applications under anhydrous conditions. In this work, polybenzimidazole (PBI) is used as a polymer electrolyte membrane with the addition of a sulfonated graphene oxide (SGO) inorganic filler. The amount of SGO filler was varied (0.5–6 wt.%) to study its influence on proton conductivity at elevated temperature, mechanical stability as well as phosphoric acid doping level. In particular, PBI-SGO composite membranes exhibited higher the level of acid dopant and proton conductivities than those of the pure PBI membranes. The PBI-SGO 2 wt.% composite membrane displayed the highest proton conductivity, with a value of 9.142 mS cm⁻¹ at 25 °C, and it increased to 29.30 mS cm⁻¹ at 150 °C. The PBI-SGO 2 wt.% also displayed the maximum values in the acid doping level (11.63 mol of PA/PBI repeat unit) and mechanical stability (48.86 MPa) analyses. In the HT-PEMFC test, compared with a pristine PBI membrane, the maximum power density was increased by 40% with the use of a PBI composite membrane with 2 wt.% SGO. These results show that the PBI-SGO membrane has a great potential to be applied as an alternative membrane in HT-PEMFC applications, offering the possibility of improving impurity tolerance and kinetic reactions.     

In-situ preparation of PSSA functionalized ZWP/sulfonated PVDF composite electrolyte as proton exchange membrane for DMFC applications

A series of proton exchange electrolytes were synthesized by blending polystyrene sulfonic acid (PSSA) functionalized ZWP ion exchanger and sulfonated poly(vinylidene fluoride) (SPVDF) as base by using solution casting method. The poly(vinylidene fluoride) was sulfonated by employing a direct sulfonation technique demonstrated in the literature. Surface modification of the ZWP was done to obtain the PSSA-ZWP ion exchanger. The membranes were synthesized by using ZWP and PSSA-ZWP as ion exchangers in the SPVDF polymer matrix. The physicochemical characterization of the membranes was performed by using FT-IR and XRD. The scanning electron microscope (SEM) was used to investigate the surface morphology of the fabricated membranes for any possible defects. Important membrane parameters, such as water uptake (up to 26%), methanol uptake (up to 22%), chemical stability (7.4%) and mechanical stability (tensile strength of up to 44 MPa), were measured and are reported. The ion exchange capacity (max 0.62 meq g^?1) and electrochemical characterization of the membranes was conducted and parameters such as transport number (max 0.84) indicating good ion selectivity of the membranes and proton conductivity (max 3.89 mS/cm) were also determined. The single cell DMFC performance of the SPVDF-ZWP-PSSA membrane was evaluated at three different operating temperatures of 30 °C, 60 °C and 90 °C, out of which the synthesized membrane performed best at 60 °C with maximum current density and power density of 49.8 mAcm^?2 and 20.1 mWcm^?2 respectively.     

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.     

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.     

High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation,morphology and physical and proton conductivity properties

Porous polybenzimidazole (PBI) based blend membranes were prepared by adding different amounts of lignosulfonate (LS) in the presence of LiCl salt. The morphology characteristics of the PBI/LS blends were investigated by FT-IR, atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses. The relation between the membrane morphology and membrane proton conductivity was studied. Results showed that LS content has a significant influence on the membrane morphology. High amount of LS in the blend created micro-pores within the membrane where increase in the LS content up to 20 wt% resulted in membranes containing pores with a mean diameter of about 0.8 μm. The resulting PBI/LS (0–20 wt%) membranes indicated high PA doping levels, ranging from 3 to 16 mol of PA per mole of PBI repeat units, which contributed to their unprecedented high proton conductivities of 4–96 mS cm⁻¹, respectively, at 25 °C. The effect of temperature on the proton conductivity of blends was also investigated. The results showed that by rising the temperature, the proton conductivity increases in PBI/LS blends. In the blend containing 20 wt% LS, proton conductivity increased from 98 mS cm⁻¹ at 25 °C to 187 mS.cm⁻¹at 160 °C which can be considered as an excellent candidate for use in both high and low temperature proton exchange membrane fuel cells.     

Polybenzimidazole composite membranes containing imidazole functionalized graphene oxide showing high proton conductivity and improved physicochemical properties

Polymer composite membranes are fabricated using poly[2,2'-(m-phenylene)-5,5′-bibenzimidazole] (PBI) as a polymer matrix and imidazole functionalized graphene oxide (ImGO) as a filler material for high temperature proton exchange membrane fuel cell applications. ImGO is prepared by the reaction of o-phenylenediamine with graphene oxide (GO). The compatibility of ImGO with PBI matrix is found to be better than that of GO, and as a result PBI composite membrane having ImGO exhibits improved physicochemical properties and larger proton conductivity compared with pure PBI and PBI composite membrane having GO. For example, PBI composite membrane having 0.5 wt% of ImGO shows enhanced tensile strength (219.2 MPa) with minimal decrease of elongation at break value (28.8%) compared with PBI composite membrane having 0.5 wt% of GO (215.5 MPa, 15.4%) and pure PBI membrane without any filler (181.0 MPa, 34.8%). The proton conductivity of this membrane, at 150 °C under anhydrous condition, is 77.52 mS cm^?1.     

On the proton conductivity of Nafion-Faujasite composite membranes for low temperature direct methanol fuel cells

Although zeolites are introduced to decrease methanol crossover of Nafion membranes for direct methanol fuel cells (DMFCs), little is known about the effect of their intrinsic properties and the interaction with the ionomer. In this work, Nafion-Faujasite composite membranes prepared by solution casting were characterized by extensive physicochemical and electrochemical techniques. Faujasite was found to undergo severe dealumination during the membrane activation, but its structure remained intact. The zeolite interacts with Nafion probably through hydrogen bonding between Si-OH and SO₃H groups, which combined with the increase of the water uptake and the water mobility, and the addition of a less conductive phase (the zeolite) leads to an optimum proton conductivity between 0.98 and 2 wt% of zeolite. Hot pressing the membranes before their assembling with the electrodes enhanced the DMFC performance by reducing the methanol crossover and the serial resistance.     

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.     

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.     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.     

High performance ZIF-8/PBI nano-composite membranes for high temperature hydrogen separation consisting of carbon monoxide and water vapor

Two types of advanced nano-composite materials have been formed by incorporating as-synthesized wet-state zeolitic imidazolate frameworks-8 (ZIF-8) nano-particles into a polybenzimidazole (PBI) polymer. The loadings of ZIF-8 particles in the two membranes (i.e., 30/70 (w/w) ZIF-8/PBI and 60/40 (w/w) ZIF-8/PBI) are 38.2 vol % and 63.6 vol %, respectively. Due to different ZIF-8 loadings, variations in particle dispersion, membrane morphology and gas separation properties are observed. Gas permeation results suggest that intercalation occurs when the ZIF-8 loading reaches 63.6 vol %. The incorporation of ZIF-8 particles significantly enhances both solubility and diffusion coefficients but the enhancement in diffusion coefficient is much greater. Mixed gas tests for H₂/CO₂ separation were conducted from 35 to 230 °C, and both membranes exhibit remarkably high H₂ permeability and H₂/CO₂ selectivity. The 30/70 (w/w) ZIF-8/PBI membrane has an H₂/CO₂ selectivity of 26.3 with an H₂ permeability of 470.5 Barrer, while the 60/40 (w/w) ZIF-8/PBI membrane has an H₂/CO₂ selectivity of 12.3 with an H₂ permeability of 2014.8 Barrer. Mixed gas data show that the presence of CO or water vapor impurity in the feed gas stream does not significantly influence the membrane performance at 230 °C. Thus, the newly developed H₂-selective membranes may have bright prospects for hydrogen purification and CO₂ capture in realistic industrial applications such as syngas processing, integrated gasification combined cycle (IGCC) power plant and hydrogen recovery.