Carbon nanotubes reinforced proton exchange membranes in fuel cells: An overview

The proton exchange membrane (PEM) is the core component in a fuel cell. In this review, recent progress and developments on per-fluorinated and non-fluorinated membranes with carbon nanotubes (CNTs) as reinforced fillers have been summarized on many key topics. Topics reviewed stem from correlating the mechanical stability, thermal stability, water retention capacity and proton conductivity of various membranes across different functionalized CNTs. In addition, topics such as the preparation strategies of membrane matrix and CNTs filler, the reinforced mechanism of CNTs in membrane are presented. Throughout, the impact of interactions between CNTs and various types of PEM is also discussed to present a deeper perspective. Finally, the strategy for improving the performance of PEM and the challenges of CNTs-based membranes are analyzed for prospects.     

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.     

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.     

Mechanical durability of proton exchange membranes with catalyst platinum dispersion

The durability of proton exchange membrane (PEM) fuel cells remains a challenging issue for their long term operational use. Degradation of the PEM related to dissolution of the adjacent catalyst and re-deposition into the PEM significantly reduces cell efficiency. We investigate the effects of platinum (Pt) dispersions intended to simulate the re-deposited catalyst on the mechanical durability of the PEM. The bulge technique was applied to characterize the mechanical properties of PEMs simulating pressure loading on fully hydrated membranes in fuel cells. The results showed that with increasing Pt dispersion concentration the stiffness of the PEMs increased, and the membranes became less ductile and inclined to fracture at lower stresses under pressure loading. We also used the out-of-plane tearing test to characterize membrane fracture behavior which revealed the harmful effects of Pt dispersion on the fracture resistance under different environmental conditions. Deterioration in fracture resistance was explained in terms of the Pt distribution and aggregation as defects inside the membranes as characterized by electron microscopy. Fracture was shown to initiate preferentially at the interface of Pt particles and the polymer matrix, and propagate through the defect regions in polymer with lower energy, thus reducing the overall fracture resistance of the PEM.     

Characterization of polyethyleneterephthalate (PET) based proton exchange membranes prepared by UV-radiation-induced graft copolymerization of styrene

Polymer electrolyte membranes (PEMs) were successfully prepared by simultaneous ultraviolet (UV) radiation-induced graft copolymerization of styrene (35 vol.% concentration) onto poly(ethyleneterephthalate) (PET) film, followed by sulfonation on the styrene monomer units in the grafting chain using 0.05 M chlorosulfonic acid (ClSO₃H). The radiation grafting and the sulfonation have been confirmed by titrimetric and gravimetric analyses as well as Fourier Transform Infrared (FTIR) spectroscopy. The maximum ion-exchange capacity (IEC) of the PEM was measured to be 0.04385 mmol g⁻¹ at its highest level of grafting and sulfonation. They exhibited high thermal and mechanical properties as well as oxidative stability. They are highly stable in H₂SO₄ solutions and can be used in the acidic fuel cells. The membranes showed low water uptake as well as low proton conductivity than Nafion. In this study, the preparation of PEMs from commodity-type polymers is found to be very inexpensive and is a suitable candidate for applications in fuel cells.     

Sulfonated poly(arylene ether sulfone) nanocomposite electrolyte membrane for fuel cell applications: A review

Polymer electrolyte membrane (PEM) fuel cells are considered a promising technology for generating power with water as a byproduct. Recently, sulfonated poly(arylene ether sulfone) (SPAES) has emerged as a most suitable alternative for PEM applications because of its high proton conductivity, high CO tolerance, and low fuel crossover. However, the existing SPAES polymeric membrane materials have poor chemical reactivity, mechanical processability, and thermal usability. Thus, the effects of mixing inorganic nanomaterials with SPAES polymers on proton conductivity, power density, fuel crossover, thermal and chemical stability, and durability are discussed in this review. Further, the progress in preparation methods and fuel cell characteristics by the addition of silica, clay, heteropolyacids (HPA), and carbon nanotubes (CNTs) in polymer membrane materials for PEM applications is also discussed.     

New non-fluoridated hybrid proton exchange membranes based on commercial precursors

New hybrid proton conducting membranes based on sulfonated copolymers of styrene and allyl glycidyl ether using tetraethyl orthosilicate were syntheses. The composition and structure of the copolymers and membranes has been proven by elemental analysis, IR and NMR spectroscopy. Based on quantum chemical calculations a sulfonation mechanism of copolymers was proposed. The characteristics of membranes were evaluated by thermal analysis, dynamic mechanical analysis, electrochemical impedance spectroscopy, water uptake, swelling and ion exchange capacity tests. The hybrid membranes are characterized by high proton conductivity of 4.21 10⁻² S cm⁻¹ (70 °C, 75% RH), activation energy of proton transport (24.5 kJ mol⁻¹), ion-exchange capacity (2.1 mmol g⁻¹), and thermal stability up to 260°С. The hybrid membranes showed water uptake of 6 and 51% at 30 °C and 100 °C, respectively. The suitability of the hybrid membranes toward fuel cell applications was tested through a single cell analysis.     

Silica-related membranes in fuel cell applications: An overview

Silica is the most common inorganic filler used in fuel cells, especially for proton exchange membrane fuel cell and direct alcohol fuel cell applications. Silica has played an important role in improving the performance of fuel cells by enhancing their membrane properties. Recently, silica has been widely implemented in different types of membranes, such as fluorinated membranes (Nafion), sulfonated membranes (SPEEK, SPS, SPAES, SPI) and other organic polymer matrixes. The incorporation of silica into membrane matrices has improved the thermal stability, mechanical strength, water retention capacity and proton conductivity of the membrane. This review describes the interactions between silica and different types of polymer matrices in fuel cells and how they boost fuel cell performance. In addition, this review also discusses the current challenges of silica-related membrane-based fuel cells and predicts the future prospects of silica in membrane-based fuel cell applications.     

Cross‐linked polymer electrolyte membrane based on a highly branched sulfonated polyimide with improved electrochemical properties for fuel cell applications

Sulfonated polyimides (SPIs) are extremely suitable as polymer electrolyte membranes (PEMs) for fuel cell applications, except for their poor water stability. Cross‐linking is a method that is commonly used to improve the weak hydrolytic stability of SPI membranes. However, this strategy significantly decreases the proton conductivity of the membrane, which leads to a lower fuel cell power density. In this work, a cross‐linked SPI membrane containing a highly branched polymer main chain was fabricated as a PEM. With a similar ion‐exchange capacity value, the cross‐linked membrane containing branched main chains showed an improved proton conductivity. Also, this membrane remained 92.3% of pristine weight after a hydrolytic stability test about 120 hours. In a single direct methanol fuel cell, the cross‐linked membrane containing a branched structure showed a higher power density (53.4 mW cm^?2) than the common cross‐linked membrane (43.0 mW cm^?2), indicating that branching is effective for improving the electrochemical properties of PEM‐based cross‐linked SPIs.     

Properties and fuel cell performance of proton exchange membranes prepared from disulfonated poly(sulfide sulfone)

A series of disulfonated poly(sulfide sulfone)s (SPSSF)s copolymers are synthesized via direct aromatic nucleophilic substitution polycondensation of 4,4′-dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS) and 4,4′-thiobisbenzenethiol at various molar ratios. Tough and flexible membranes with 30 mol% (SPSSF30) to 50 mol% (SPSSF50) SDCDPS monomers are obtained by casting from DMAc solution. Their physicochemical properties including thermal properties, mechanical properties, water uptake, swelling ratio and oxidative stability are fully investigated. And the fuel cell performance of SPSSF membranes at different temperature and relative humidity is evaluated comprehensively for the first time. It is found that the SPSSF40 membrane exhibited low dimensional change in the temperature range of 20–100 °C, good mechanical properties, high oxidative stability and comparable fuel cell performance to Nafion 212 membrane. Besides, the H₂ crossover density of the SPSSF40 membrane is only 50% of that of Nafion 212 membrane. Consequently, SPSSF40 membranes prove to be promising candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells operated at medium temperatures.     

Preparation and characterization of a modified montmorillonite/sulfonated polyphenylether sulfone/PTFE composite membrane

Organically modified montmorillonites are valuable materials that have been used to improve the permeability, water retention, and proton conductivity of proton exchange membrane for fuel cells. A sulfonated montmorillonite/sulfonated poly (biphenyl ether sulfone)/Polytetrafluoroethylene (SMMT/SPSU-BP/PTFE) composite membrane was prepared for fuel cells. The thermal stability of the SMMT was tested by the thermogravimetry-mass spectrometry (TGA-MS) and its structure in the composite membrane was characterized by X-ray diffraction (XRD). It was found that SMMT was stable up to 205 °C and the interlayer distance of the nanoclay expanded from 1.43 nm to 1.76 nm after the organic sulfonic modification. The SMMT was completely exfoliated in the composite membranes. The properties of ion-exchange capacity, water uptake, swelling ratio, proton conductivity, and mechanical strength of the composite membranes were investigated as well. The good water retention of SMMT made the SMMT/SPSU-BP and SMMT/SPSU-BP/PTFE composite membranes have about 20% more bound water than the SPSU-BP membrane. Due to the reinforce effect of the PTFE porous film, the SMMT/SPSU-BP/PTFE composite membrane presented low swelling even at elevated temperature and high stress strength. All of the properties indicate that the SMMT/SPSU-BP/PTFE composite membrane is very promising as the PEM for medium temperature PEMFCs.     

Neoteric advancements in polybenzimidazole based polymer electrolytes for high-temperature proton exchange membrane fuel cells - A versatile review

The world's dependence on hydrocarbon fuel to generate power has proven to be the primary source of energy production. The emission of dangerous toxic and effluent gases during the process of hydrocarbon extraction and utilisation poses a massive threat to the environment and human life. This has driven the research into green energy technology; polymer electrolyte membrane fuel cells (PEMFCs) is one of the future century's bright green and clean energy producers. The most critical factor in the fuel cell is the polymer electrolyte membrane (PEM), which is the heart of the fuel cell assembly. Recently, polybenzimidazole (PBI)-based high-temperature polymer electrolytes have attracted researchers because they have high chemical and thermal stability combined with fillers that can control proton mobility. There are several limitations to the usage of PBI in high-temperature PEMFCs and this review summarizes the various structural modification: phase inversion, semi-interpenetrating IPNs, branched blocks and physical modification methods like crosslinking, blending, doping, progress done by various researchers to tackle the drawbacks in PBI-based polymer electrolyte membranes.     

Conductivity of aromatic-based proton exchange membranes at subzero temperatures

Stable proton exchange membrane (PEM) with good proton conductivity at subzero temperatures is important for the development of PEM fuel cell cold start. In this work, subfreezing conductivity was reported for several aromatic-based PEMs including sulfonated polyimides (SPIs) with three values of ion-exchange capacity (IEC), sulfonated poly(ether ether ketone) (SPEEK) and disulfonated poly(arylene ether sulfone) copolymer (SPSU) as well as Nafion^® 212. Measurements were performed using the electrochemical impedance spectroscopy (EIS) technique. The results showed that only fully hydrated SPEEK (IEC, 1.75) and SPSU (IEC, 2.08) had comparable conductivities with Nafion^® 212 at subzero temperatures. Considering implement of gas purge before subzero storage of PEM fuel cell, the conductivity for those PEMs humidified by water vapor at activity of 0.75 was also investigated. The state of water in aromatic-based PEMs was quantified by differential scanning calorimetry (DSC), and its correlation with conductivity of the membrane was also discussed.     

Characterization of highly sulfonated PEEK based membrane for the fuel cell application

In this study, poly(bisphenol-A-ether ketone) (PBAEK) is synthesized via nucleophilic aromatic substitution poly condensation between bisphenol A and 4,4-difluorobenzophenone, and the synthesized polymers are sulfonated using chlorosulfuric acid and suitable synthesis conditions for the temperature and sulfonating reagent content. The sulfonation degree of polymer is calculated using element analyses. The prepared sulfonated polymers are characterized for potential fuel cell applications through determining their water uptake, proton conductivity, and thermal stability. The significant advantage of the synthesized sulfonated PBAEK (sPBAEK) is its better solubility relative to commercial PEEK in various solvents, because sPBAEK backbones contain bisphenol A. The water uptake of the membrane increases with increases in the sulfonation degree. The sPBAEK membrane exhibits increased proton conductivity compared with the PBAEK membrane at 100% relative humidity conditions. As the sulfonation degree increases, the proton conductivity increases due to the increasing content in the hydrophilic domain. This property allows the prepared membranes to be potential candidates for proton exchange membrane fuel cells.     

Chemical and radiation crosslinked polymer electrolyte membranes prepared from radiation-grafted ETFE films for DMFC applications

To develop a highly chemically stable polymer electrolyte membrane for application in a direct methanol fuel cell (DMFC), doubly crosslinked membranes were prepared by chemical crosslinking using bifunctional monomers, such as divinylbenzene (DVB) and bis(p,p-vinyl phenyl) ethane (BVPE), and by radiation crosslinking. The membranes were prepared by grafting of m,p-methylstyrene (MeSt) and p-tert-butylstyrene (tBuSt) into poly(ethylene-co-tetrafluoroethylene) (ETFE) films and subsequent sulfonation. The effects of the DVB and BVPE crosslinkers on the grafting kinetics and the properties of the prepared membranes, such as water uptake, proton conductivity and chemical stability were investigated. Radiation crosslinking was introduced by irradiation of the ETFE base film, the grafted film or the sulfonated membrane. The membrane crosslinked by DVB and BVPE crosslinkers and post-crosslinked by γ-ray irradiation of the corresponding grafted film possessed the highest chemical stability among the prepared membranes, a significantly lower methanol permeability compared to Nafion^® membranes, and a better DMFC performance for high methanol feed concentration. Therefore, this doubly crosslinked membrane was promising for application in a DMFC where relatively high methanol concentration could be fed.     

Sulfonation of PEEK-WC polymer via chloro-sulfonic acid for potential PEM fuel cell applications

The preparation and characterization of thin dense sulfonated poly-ether-ether-ketone with cardo group (PEEK-WC) membranes for proton exchange membrane fuel cell (PEMFC) applications are described. The sulfonation of PEEK-WC polymer was realized via chloro-sulfonic acid and different kinds of membrane samples were prepared with a sulfonation degree ranging from 67 to 99%. The degree of sulfonation, homogeneity and thickness significantly affect both the membrane transport properties and the electrochemical performances. The dense character of the membranes was confirmed by SEM analysis. Proton conductivity measurements were carried out in a temperature range from 30 to 80 °C and at 100% of relative humidity, reaching 5.40×10⁻³ S/cm⁻¹ as best value at 80 °C and with a sulfonation degree (DS) of 99%. At the same conditions, a water uptake of 17% was achieved. DSC and TGA characterizations were used in order to determine the thermal stability of the membranes, confirming a T_g ranging between 206 and 216 °C depending on the DS, whereas FT-IR yielded indication about intermolecular interactions and water uptake at various sulfonation degrees.     

Fluorinated sulfonated poly (arylene ether)s bearing semi-crystalline structures for highly conducting and stable proton exchange membranes

An aromatic nucleophilic substitution reaction-based direct polycondensation of 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 1,4-bis(4-fluorobenzoyl)benzene (1,4-FBB), and 3,3′-disulfonate-4,4′-difluorodiphenylsulfone is used to synthesize 1,4-FBB in fluorinated sulfonated semi-crystalline poly (arylene ether) copolymers. The ion exchange capacity, mechanical stability, thermal stability, and oxidative stability of membranes with various degrees of sulfonation and water absorption are studied. ¹H nuclear magnetic resonance and atomic force microscopy verify the chemical structure of the synthesized ion exchange membrane. The semi-crystalline 6 F-membrane shows reliable chemical, mechanical, thermal, and electrochemical stability. Compared to Nafion 212®, which is a commercial ion exchange membrane, the semi-crystalline 6 F-membrane shows excellent cell performance as current density is increased. All the results mentioned above indicate that this semi-crystalline 6 F-membrane is a good candidate to replace the commercial membranes.     

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.     

Self-humidifying nanocomposite membranes based on sulfonated poly(ether ether ketone) and heteropolyacid supported Pt catalyst for fuel cells

In the present study, the self-humidifying nanocomposite membranes based on sPEEK and Cs_2.5H_0.5PW₁₂O₄₀ supported Pt catalyst (Pt-Cs_2.5H_0.5PW₁₂O₄₀ catalyst or Pt-Cs_2.5) and their performance in proton exchange membrane fuel cells with dry reactants has been investigated. The XRD, FTIR, SEM-EDXA and TEM analysis were conducted to characterize the catalyst and membrane structure. The ion exchange capacity (IEC), water uptake and proton conductivity measurements indicated that the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes have higher water absorption, acid and proton-conductive properties compared to the plain sPEEK membrane and Nafion-117 membrane due to the highly hygroscopic and acidy properties of Pt-Cs_2.5 catalyst. The single cells employing the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes exhibited higher cell OCV values and cell performances than those of plain sPEEK membrane and Nafion-117 membrane under dry or wet conditions. Furthermore, the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes showed good water stability in aqueous medium. After investigation of several membranes such as sPEEK and sPEEK/Pt-Cs_2.5 membranes, the self-humidifying nanocomposite membrane with sulfonation degree of 65.12% for its sPEEK and 15 wt.% of catalyst with 1.25 wt.% Pt within catalyst was found to be the best proton exchange membrane for fuel cell applications. This self-humidifying nanocomposite membrane has a higher single cell performance than the Nafion-117 which was frequently used as a proton exchange membrane for fuel cell applications.