Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

Sulfonic acid functionalized graphene oxide paper sandwiched in sulfonated poly(ether ether ketone): A proton exchange membrane with high performance for semi-passive direct methanol fuel cells

Sulfonated poly(ether ether ketone) (SPEEK) membranes have been deposited on the both sides of a sulfonic acid functionalized graphene oxide (SGO) paper to form a proton exchange membrane (PEM) with a sandwiched structure. The obtained SPEEK/SGO/SPEEK membrane could exhibit proton conductivity close to Nafion^® 112 and lower methanol permeability. The use of this SPEEK/SGO/SPEEK membrane greatly improves the performance of the semi-passive direct methanol fuel cell (DMFC). The semi-passive DMFC with the SPEEK/SGO/SPEEK membrane is found to be capable of delivering the peak power density 60% higher than that with the commercial Nafion^® 112. This, along with its comparable durability to Nafion^® 112, strongly suggests the great promise of using the SPEEK/SGO/SPEEK membrane as the PEM.     

Investigation of the effects of AMPS-modified nanoclay on fuel cell performance of sulfonated aromatic proton exchange membranes

Here we describe preparation and characterization of a series of nanocomposite polyelectrolytes based on partially sulfonated poly (ether ether ketone) (SPEEK) and organically modified montmorillonite (MMT). Optimum degree of sulfonation for SPEEK is selected based on its transport properties. MMT is modified via ion exchange reaction using a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as a functional modifier. AMPS-MMT at different loadings is introduced into the SPEEK matrices via the solution intercalation technique. Also, the nanocomposite membranes are fabricated using SPEEK and commercially available nanoclays like Cloisite Na (Na-MMT) and Cloisite 15A. Transport properties, proton conductivity and methanol permeability of the fabricated composite membranes are evaluated. Presence of AMPS-MMT significantly decreases the activation energy needed for proton conductivity. A membrane based on SPEEK/AMPS-MMT-3 wt% is selected as an optimum formulation which exhibits a high selectivity and power density at the elevated methanol concentrations. Moreover, it is found that the optimum nanocomposite membrane not only provides higher power output compared to the neat SPEEK and Nafion^®117 membranes, but also exhibits a higher open circuit voltage (OCV) in comparison with pristine SPEEK and commercial Nafion^® 117 membranes. Owing to the desirable transport and electrochemical properties SPEEK/AMPS-MMT nanocomposite can be considered as an alternative membrane for direct methanol fuel cell applications.     

Enhancement of proton conductivity of polymer electrolyte membrane enabled by sulfonated nanotubes

Proton exchange membrane (PEM) with high proton conductivity is crucial to the commercial application of PEM fuel cell. Herein, sulfonated halloysite nanotubes (SHNTs) with tunable sulfonic acid group loading were synthesized and incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to prepare nanocomposite membranes. Physicochemical characterization suggests that the well-dispersed SHNTs enhance the thermal and mechanical stabilities of nanocomposite membranes. The results of water uptake, ionic exchange capacity, and proton conductivity corroborate that the embedded SHNTs interconnect the ionic channels in SPEEK matrix and donate more continuous ionic networks. These networks then serve as proton pathways and allow efficient proton transfer with low resistance, affording enhanced proton conductivity. Particularly, incorporating 10% SHNTs affords the membrane a 61% increase in conductivity from 0.0152 to 0.0245 S cm⁻¹. This study may provide new insights into the structure-properties relationships of nanotube-embedded conducting membranes for PEM fuel cell.     

Increases in the proton conductivity and selectivity of proton exchange membranes for direct methanol fuel cells by formation of nanocomposites having proton conducting channels

We explore an approach to effectively enhance the properties of cost-effective hydrocarbon proton-exchange membranes for application in the direct methanol fuel cell (DMFC). This approach utilizes sulfonated silica nanoparticles (SA-SNP) as additives to modify sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK). The interaction between the sulfonic acid groups of SA-SNP and those of SPAEEKK combined with hydrophilic-hydrophobic phase separation induce the formation of proton conducting channels, as evidenced by TEM images, which contribute to increases in the proton conductivity of the SPAEEKK/SA-SNP nanocomposite membrane. The presence of SA-SNP nanoparticles also reduces methanol crossover in the membrane. Therefore, the SPAEEKK/SA-SNP nanocomposite membrane shows a high selectivity, which is 2.79-fold the selectivity of Nafion^®117. The improved selectivity of the SPAEEKK/SNP nanocomposite membrane demonstrates potential of this approach in providing hydrocarbon-based PEMs as alternatives to Nafion in direct methanol fuel cells.     

A study on fabrication of sulfonated poly(ether ether ketone)-based membrane-electrode assemblies for polymer electrolyte membrane fuel cells

The porosity effect of catalyst electrodes in membrane-electrode assemblies (MEAs) using a hydrocarbon-based polymer as electrolyte and ionomer was investigated on physical and electrochemical properties by varying the content of ionomer binder (dry condition) in the catalyst electrodes. The MEAs were compared with the Nafion^®-based MEA using Nafion^® 112 and 5 wt.% ionomer solution (EW = 1100) in terms of porosity values, scanning electron microscopic images, Nyquist plots, dielectric spectra and I–V polarization curves. In this study, sulfonated poly(ether ether ketone) (SPEEK) membranes with 25 ± 5 μm of thickness and 5 wt.% ionomer solutions have been prepared. The prepared membranes were characterized in terms of FT-IR, DSC and proton conductivity. Proton conductivity of the SPEEK membranes was compared with one of the Nafion^® membranes with relative humidity. The porosity of the catalyst electrodes was calculated using the properties of catalyst, ionomer solution and solvent. As a result, the performance of the new type polymer (i.e., SPEEK in this study)-based MEA with the similar membrane conductivity and porosity of the catalyst electrode in the Nafion^® MEA was similar to that of the Nafion^® MEA.     

Modification of sulfonated poly(ether ether ketone) proton exchange membrane for reducing methanol crossover

A drawback of sulfonated aromatic main-chain polymers such as sulfonated poly(ether ether ketone)s (SPEEKs) is their high methanol crossover when the proton conductivity is sufficient for direct methanol fuel cell (DMFC) applications. To overcome this disadvantage, in this paper, the SPEEK substrate was coated with the crosslinked chitosan (CS) barrier layer to form the two-layer composite membranes. Scanning electron microscope (SEM) micrographs showed that the CS layer was tightly adhered on the SPEEK substrate and the thickness of CS layer could be adjusted by varying the concentration of CS solution. It was noticed that with the increment of thickness of CS layer, the methanol diffusion coefficient of the composite membranes significantly dropped from 3.15 × 10⁻⁶ to 2.81 × 10⁻⁷ cm² s⁻¹ at 25 °C which was about one order of magnitude lower than those of the pure SPEEK and Nafion^® 117 membranes. In addition to the effective methanol barrier, the composite membranes possessed adequate thermal stability (the 5% weight lose temperature exceeded 240 °C) and good proton conductivity. The proton conductivity of all composite membranes was in the order of 10⁻² S cm⁻¹ and increased with the elevation of temperature. Furthermore, the composite membranes exhibited much higher selectivity (conductivity/methanol diffusion coefficient) compared with the pure SPEEK and Nafion^® 117 membranes. These results indicated that introducing the crosslinked CS layer onto the SPEEK surface was an effective method for improving the performance of the SPEEK membrane, especially for reducing the methanol crossover.     

Improved proton conduction of sulfonated poly (ether ether ketone) membrane by sulfonated covalent organic framework nanosheets

A type of sulfonated covalent organic framework nanosheets (TpPa-SO₃H) was synthesized via interfacial polymerization and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to prepare proton exchange membranes (PEMs). The densely and orderly arranged sulfonic acid groups in the rigid skeleton of the TpPa-SO₃H nanosheets, together with their high-aspect-ratio and well-defined porous structure provide proton-conducting highways in the membrane. The doping of TpPa-SO₃H nanosheets led to an increased ion exchange capacity up to 2.34 mmol g^?1 but a 2-folds reduced swelling ratio, remarkably mitigating the trade-off between high IEC and excessive swelling ratio. Based on the high IEC and orderly arranged proton-conducting sites, the SPEEK/TpPa–SO₃H–5 membrane exhibited the maximum proton conductivity of 0.346 S cm^?1 at 80 °C, 1.91-folds higher than the pristine SPEEK membrane. The mechanical strength of the composite membrane was also improved by 2.05-folds–74.5 MPa. The single H₂/O₂ fuel cell using the SPEEK/TpPa–SO₃H–5 membrane presented favorable performance with an open voltage of 1.01 V and a power density of 86.54 mW cm^?2.     

Improving the proton conductivity of proton exchange membranes via incorporation of HPW-functionalized mesoporous silica nanospheres into SPEEK

A highly stable composite proton exchange membrane (PEM) was developed by loading phosphotungstic acid in mesoporous silica nanospheres (HPW@MSNs) and blending with sulfonated poly (ether ether ketone) (SPEEK). The SPEEK/HPW@MSNs-0.5 membrane exhibits enhanced comprehensive performance, such as improved and stable proton conductivity and increased methanol barrier property. The proton conductivity decreased by 15.10% after 240 h at 60 °C and was 1.9 times lower than that of the SPEEK/HPW membrane. The selectivity of the SPEEK/HPW@MSNs-0.5 membrane was about 2.0 times that of the pure SPEEK membrane and 3.4 times that of the SPEEK/HPW membrane.     

Crosslinked SPEEK/AMPS blend membranes with high proton conductivity and low methanol diffusion coefficient for DMFC applications

The crosslinked sulfonated poly (ether ether ketone)/2-acrylamido-2-methyl-1-propanesulfonic acid (SPEEK/AMPS) blend membranres were prepared and evaluated as proton exchange membranes for direct methanol fuel cell (DMFC) applications. The structure and morphology of SPEEK/AMPS membranes were characterized by FTIR and SEM, respectively. The effects of crosslinking and AMPS content on the performance of membranes were studied and discussed in detail. The proton conductivity and methanol diffusion coefficient of SPEEK/AMPS membranes increased gradually with the increase of AMPS content. Most SPEEK/AMPS membranes exhibited higher proton conductivity than Nafion^® 117 (0.05 S cm⁻¹ at 25 °C). However, all the membranes possessed much lower methanol diffusion coefficient compared with Nafion^® 117 (2.38 × 10⁻⁶ cm² s⁻¹) under the same measuring conditions. Even the methanol diffusion coefficient (8.89 × 10⁻⁷ cm² s⁻¹) of SPEEK/AMPS 30 sample with the highest proton conductivity (0.084 S cm⁻¹ at 25 °C) was only about one third of that of Nafion^® 117. The selectivity of all the SPEEK/AMPS membranes was much higher in comparison with Nafion^® 117 (2.8 × 10⁴ S s cm⁻³). In addition, the SPEEK/AMPS membranes possessed relatively good thermal and hydrolytic stability. These results suggested that the SPEEK/AMPS membranes were particularly promising to be used as proton exchange membranes in DMFCs, and the high proton conductivity, low methanol diffusion coefficient and high selectivity were their primary advantages for DMFC applications.     

Sulfonated bisphenol-A-polysulfone based composite PEMs containing tungstophosphoric acid and modified by electron beam irradiation

Composite sulfonated bisphenol-A-polysulfone (SPSF) based PEMs, containing tungstophosphoric acid (TPA) and modified by electron beam (EB) irradiation with doses of 50 and 100 kGy, were prepared and characterized by a number of physico-chemical methods. The probability of the cross-linking was increased by post treatment of the irradiated membranes at 180 °C for 8 h into a vacuum oven. The cross-linking strongly affected membrane properties, reducing their swelling, but at the same time decreasing their proton conductivity. The proton conductivity of the membranes was measured by a two electrode ac impedance technique using a frequency response analyzer. The PEMs performance was tested in H₂/O₂ fuel cell (FC) and found to approach that of Nafion^® 117 commercial PEM at the same conditions. The PEMs composite stability was adequate at least for 2 days, during which no degradation of the performance was observed.     

Effect of cation contamination and hydrated pressure loading on the mechanical properties of proton exchange membranes

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as electrolyte thin films to transport protons in proton exchange membrane (PEM) fuel cells. The mechanical degradation of the membrane represents a common failure mode that limits the operational life of the fuel cells. In the present work, effect of contamination related to cation exchange on the mechanical reliability of PEMs was investigated. We applied the bulge test technique to assess the mechanical properties of Nafion^® PFSA membranes simulating pressure loading on hydrated PEMs in fuel cells. The corresponding elastic moduli of Nafion^® before and after cation exchange were analyzed and compared with the results measured by uniaxial tension experiments at selected humidity conditions, showing increasing stiffness with the increase of cation radius. We also used the out-of-plane tearing test method to characterize the fracture behaviors of PEMs. The effects of cation exchange and water absorption on mechanical and fracture properties of PEMs at different temperatures are discussed in terms of cation and water interactions with the molecular structure of PFSA polymers.     

Direct methanol fuel cell performance of sulfonated poly (2,6-dimethyl-1,4-phenylene oxide)-polybenzimidazole blend proton exchange membranes

Various sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polybenzimidazole (PBI) blend membranes were prepared and investigated as proton exchange membranes (PEMs) for direct methanol fuel cell (DMFC) applications. With increasing PBI content water swelling, ion exchange capacity, proton conductivity and methanol permeability of SPPO-PBI membranes were found to be decreased due to acid-base interactions between sulfonate and the amine groups of the blended components. Among various SPPO-PBI blend membranes, 80:20 wt% was found as the optimum composition, which showed the highest membrane selectivity parameter. Direct methanol-air single fuel cell tests revealed a higher cell efficiency of 11.6% for SPPO80-PBI20 than 10.9% for Nafion^®117 at 5 M methanol feed, and also a higher power density of 57.6 mW.cm⁻² compared to 39.4 mW.cm⁻² for Nafion^®117. Transport properties as well as DMFC performance results of SPPO-PBI blend PEMs converge to indicate their potential for DMFC applications.     

Study on improvement of the selectivity of proton exchange membrane via incorporation of silicotungstic acid-doped silica into SPEEK

Nafion based proton exchange membrane (PEM) has long been used as conventional PEM in direct methanol fuel cell (DMFC) industry. However, the high cost of Nafion membrane and other drawbacks like high methanol crossover hinder the advancement of this industry. This study aims to develop a low cost membrane using sulfonated poly ether ether ketone (SPEEK) polymer. Silica and silicotungstic acid (SiWA) were incorporated into the membrane matrix using solution casting method. The optimum loading of the additives was tuned and it is discovered that the SPEEK membrane containing 10 wt% of silica and 5 wt% of SiWA has the best performance due to its high proton conductivity and moderately low methanol permeability. The performance of the membrane can further be enhanced by adding (3-aminopropyl)triethoxysilane (APTES) and carbonyldiimidazole (CDI) as coupling agents. Inclusion of APTES and CDI in SPEEK could not only improve the compatibility between organic SPEEK and inorganic additives, but also improve the homogeneity and dispersity of the additives. As a result, the resultant membrane with a better dimensional stability achieves high selectivity (10.60 × 10⁴ S.s/cm³) up to 6.5 times more than pristine SPEEK membrane and 1.3 times higher than the commercial Nafion 117 membrane.     

Enhancement in proton conductivity and methanol cross-over resistance by sulfonated boron nitride composite sulfonated poly (ether ether ketone) proton exchange membrane

Fuel cells are the promising new non-conventional power source for vehicles as well as portable devices. Direct methanol fuel cell (DMFC) is especially attractive since it uses low cost liquid methanol as a fuel. Proton exchange membrane is one of the most crucial part of DMFC. Herein, we synthesized the sulfonated boron nitride (SBN) based SPEEK composite membranes for the DMFC application. SBN was synthesized by covalent functionalization of hydroxylated BN by using 3-mercaptopropyl trimethoxysilane and sulfonated by subsequent oxidation of mercapto group. Sulfonated poly (ether ether ketone) is used as a polymer matrix for SBN. With well controlled content of SBN into SPEEK matrix exhibit high proton conductivity, IEC and water content along with excellent mechanical strength. Composite membranes show low methanol cross over and high selectivity, which makes them attractive candidate for proton exchange membrane for direct methanol fuel cells.     

Multi-functionalized acid-base double-shell nanotubes are incorporated into the proton exchange membrane to cope with low humidity conditions

Proton exchange membranes (PEM) with high proton conductivity and water retention are critical to the commercial application of proton exchange membrane fuel cells (PEMFC). In this study, acid-base double-shell nanotubes with carboxylate inner shell and an imidazole outer shell (DSNT-A@B) are synthesized via continuous distillation-precipitation polymerization using halloysite nanotubes (HNTs) as seeds. Then, it is incorporated into sulfonated poly (ether ether ketone) matrix to prepare composite membranes. The carboxylic inner shell can increase the content of combined water, thereby giving the composite membrane higher water retention. The imidazole shell acts as basic shell to create acid-base pairs with the membrane and inner shell to promote proton conductivity following the Grotthuss mechanism. The results show that when the blending amount is 5 wt%, the proton conductivity of the composite membrane reaches 0.336 S/cm at 80 °C and 100% relative humidity (RH), which is twice as high as that of the original membrane. In particular, the water loss of SPEEK/DSNT-A@B-10 composite membrane is only 54.55% at 40 °C and 20% RH, which is 32.77% lower than the SPEEK membrane. Therefore, this DSNT-A@B/SPEEK composite membrane can be used as a potential candidate for high temperature and low humidity fuel cells.     

Syntheses of sulfonated star-hyperbranched polyimides and their proton exchange membrane properties

We synthesized novel sulfonated star-hyperbranched polyimides composed of a hydrophobic hyperbranched polymer for polymer stability and a hydrophilic sulfonated polyimide as the proton-transport site in the core-shell structure. The proton conductivities of the star-hyperbranched polyimide membranes were measured as functions of the relative humidity and temperature using electrochemical impedance spectroscopy. Although the water uptake and IEC value for the sulfonated star-hyperbranched polyimide membranes were almost constant, the proton conductivity of the membrane strongly depended on the molecular weight of the hydrophilic sulfonated polyimide as the shell. Especially, the conductivity of the high molecular weight star-hyperbranched polyimide membranes was significantly superior to that determined in Nafion^® at all temperatures and was 0.51 S cm⁻¹ at 80 °C and 98% RH, which may suggest that a good proton-transport pathway in the core-shell structure is formed. Consequently, this material proved to be promising as a proton exchange membrane and may have potential applications for use in fuel cells.     

Crosslinked sulfonated poly(ether ether ketone) proton exchange membranes for direct methanol fuel cell applications

In the present study, a series of the crosslinked sulfonated poly(ether ether ketone) (SPEEK) proton exchange membranes were prepared. The photochemical crosslinking of the SPEEK membranes was carried out by dissolving benzophenone and triethylamine photo-initiator system in the membrane casting solution and then exposing the resulting membranes after solvent evaporation to UV light. The physical and transport properties of crosslinked membranes were investigated. The membrane performance can be controlled by adjusting the photoirradiation time. The experimental results showed that the crosslinked SPEEK membranes with photoirradiation 10 min had the optimum performance for proton exchange membranes (PEMs). Compared with the non-crosslinked SPEEK membranes, the crosslinked SPEEK membranes with photoirradiation 10 min markedly improved thermal stabilities and mechanical properties as well as hydrolytic and oxidative stabilities, greatly reduced water uptake and methanol diffusion coefficients with only slight sacrifice in proton conductivities. Therefore, the crosslinked SPEEK membranes with photoirradiation 10 min were particularly promising as proton exchange membranes for direct methanol fuel cell (DMFC) applications.     

Blended Nafion/SPEEK direct methanol fuel cell membranes for reduced methanol permeability

Sulfonated poly(ether ether ketone)s (SPEEKs) were substituted on a polymer main chain that had previously been prepared by sulfonation of poly(ether ether ketone)s in concentrated sulfuric acid for a specified time. The product was then blended with Nafion^® to create composite membranes. The blended SPEEK-containing membranes featured flaky domains dispersed in the Nafion^® matrix. These blends possessed a high thermal decomposition temperature. Additionally, owing to the more crystalline, the blended membranes had a lower water uptake compared to recast Nafion^®, the methanol permeability was reduced to 1.70 × 10⁻⁶ to 9.09 × 10⁻⁷ cm² s⁻¹ for various SPEEK concentrations, and a maximum proton conductivity of ∼0.050 S cm⁻¹ was observed at 30 °C. The single-cell performances of the Nafion^®/SPEEK membranes, with various SPEEK concentrations and a certain degree of sulfonation, were 15–25 mW cm⁻² for SPEEK53 and 19–27 mW cm⁻² for SPEEK63, at 80 °C. The power density and open circuit voltage were higher than those of Nafion^® 115 (power density = 22 mW cm⁻²). The blended membranes satisfy the requirements of proton exchange membranes for direct methanol fuel cell (DMFC) applications.     

Polymer electrolyte membrane with magnetic nanoparticles containing benzimidazole terminals: An approach to induce proton transfer species on membrane surface

Proton conductive species incorporated in polymer electrolyte membrane (PEM), in general, are randomly existed in the membrane, and this limits the proton transfer efficiency. The present work proposes an approach to align the proton conductive species on the PEM surface so that the first contact of proton species from anode effectively initiates proton transfer efficiently. By simply conjugating benzimidazole (Bz) with magnetic nanoparticles (MagNPs) followed by applying magnetic field, Bz can be induced and aligned on the surface of sulfonated poly (ether ether ketone) (SPEEK) membrane and this results in 10% increment of the proton conductivity. The present work demonstrates a simple way to create a pathway on the membrane surface for proton hopping to improve the proton conductivity.