Sulfonated Polyimide Research Articles

Effects of polyimide sequence and monomer structures on CO2 permeation and mechanical properties of sulfonated polyimide/ionic liquid composite membranes

The effects of polymer structure on CO2 separation and mechanical properties of ion gel membranes composed of ionic liquids (ILs) and sulfonated polyimides (SPIs) were investigated. SPIs with different sequential distributions of ionic groups (multiblock and random) were synthesized. The multiblock copolymer exhibited higher IL uptake (∼80 wt.%) than the random copolymer, resulting in higher CO2 permeability (∼480 Barrer). The multiblock copolymer exhibited a clearer phase-separated structure than the random copolymer. However, the strain at the break of the multiblock copolymer was lower owing to the brittleness of the non-ionic phase. To improve the mechanical properties, an SPI containing fluorinated groups as a non-ionic part was also synthesized. Compared with the random SPI ion gel membranes, the multiblock SPI ion gel membranes containing fluorinated groups exhibited good CO2 permeability (∼500 Barrer) and simultaneously ductile properties with higher strain at the break due to the plasticization of the non-ionic phase, enabling a thin and tough membrane with excellent CO2 separation properties. These results indicate that the polyimide sequence in addition to the chemical structure of monomers affects CO2 permeation and mechanical properties of the sulfonated polyimide/ionic liquid composite membranes.

A Sulfonated Polyimide/Nafion Blend Membrane with High Proton Selectivity and Remarkable Stability for Vanadium Redox Flow Battery.

A sulfonated polyimide (SPI)/Nafion blend membrane composed of a designed and synthesized SPI polymer and the commercial Nafion polymer is prepared by a facile solution casting method for vanadium redox flow battery (VRFB). Similar molecular structures of both SPI and Nafion provide good compatibility and complementarity of the blend membrane. ATR-FTIR, 1H-NMR, AFM, and SEM are used to gain insights on the chemical structure and morphology of the blend membrane. Fortunately, the chemical stability of the SPI/Nafion blend membrane is effectively improved compared with reported SPI-based membranes for VRFB applications. In cycling charge-discharge tests, the VRFB with the as-prepared SPI/Nafion blend membrane shows excellent battery efficiencies and operational stability. Above results indicate that the SPI/Nafion blend membrane is a promising candidate for VRFB application. This work opens up a new possibility for fabricating high-performance SPI-based blend membrane by introduction of a polymer with a similar molecular structure and special functional groups into the SPI polymer.

Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10−7 cm2 min−1) and higher proton selectivity (1.06 × 105 S min cm−3) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm−2. The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm−2, whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.

Clariant expands its capacity for emission control catalysts to meet growing global demand

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10−7 cm2 min−1) and higher proton selectivity (1.06 × 105 S min cm−3) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm−2. The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm−2, whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.

Synthesis of clay‐armored coatable sulfonated polyimide nanocomposites as robust polyelectrolyte membranes

AbstractHerein, coatable sulfonated polyimide (SPI) and clay‐reinforced SPI membranes SPI‐clay 3%, SPI‐clay 5%, and SPI‐clay 7% were successfully fabricated by one‐step high temperature via direct imidization method. The membranes were cast as a coatable thin film using a solution casting method and grafted vermiculite clay nanoparticle were incorporated into the neat SPI as reinforcement by the sonication method. Three different formulated nanocomposite membranes were investigated using different characterization techniques such as Fourier transform infrared spectroscopy as peaks at 1166 and 1227 cm−1 confirmed successful sulfonation. In Proton (1H) NMR synthesis of SPI confirmed as aromatic proton at 7.3–8.8 ppm depicts successful sulfonation and X‐ray diffraction results confirmed the crystalline structure of clay, as its content increased (7%) clear diffraction peak arises at 6 and 25°. Scanning electron microscopy (SEM) provides information about surface morphology of clay reinforced SPI membranes, and SEM micrographs shown uniform dispersion of clay nanofillers and developed easy transfer of electron. Thermogravimetric analysis was performed to investigate the thermal stability of synthesized films, results of thermographs shown degradation in the range of 510–600°C. Different physicochemical parameters employed and their results show the effectiveness of synthesized clay reinforced SPI membranes. Water uptake (WU%) about 0.96%, hydrolytic about 98 h and oxidative stability up to 80°C, ions exchange capacity about 3.16 mmol/g for synthesized clay reinforced SPI membranes. Measurement regarding Dimensional changes was also investigated and dimensional changes (1.557 ∆t/∆l). All these results reveal that the clay‐reinforced coatable SPI membranes are a promising material for polymer electrolyte membranes to be used in fuel cell energy applications.

High conductivity membrane containing polyphosphazene derivatives for vanadium redox flow battery

The primary task to improve the performance of vanadium redox flow battery (VRFB) is to develop the membranes with high proton conductivity. Herein, a string of sulfonated polyimide (SPI) blend with poly [bis (4,4′-diaminobenzidine-2,2′-disulfonic acid) phosphazene] (PDAP) membranes were designed and prepared. The introduction of PDAP containing amphoteric functional groups not only provides more proton transport groups (–SO3H), but also reach the purpose to prevent the migration of vanadium ions, which is caused by the Donnan repulsion to vanadium ions from N-protonation. Therefore, the prepared membrane exhibits high proton conductivity (up to 1.33 × 10−1 S cm−1 at room temperature) and low vanadium permeability (1.37 × 10−8 cm2 s−1). The performance of VRFB with SPI/0.5% PDAP membrane exhibits a voltage efficiency (VE) of 87.02% and an energy efficiency (EE) of 84.04% at 100 mA cm−2, which were better than that of Nafion 212 (VE: 82.04%, EE: 76.99%). In addition, the VRFB with SPI/0.5% PDAP membrane can be cycled continuously and perfectly for 500 times at 100 mA cm−2 with stable efficiencies. After cycling, the membrane was intact without any defects, and the FTIR spectra and morphology of the membrane did not change significantly. These results prove that the SPI/0.5% PDAP membrane is an ideal membrane in VRFB applications.

A novel double branched sulfonated polyimide membrane with ultra-high proton selectivity for vanadium redox flow battery

The development of a vanadium redox flow battery (VRFB) urgently requires proton conductive membranes with high proton selectivity. Herein, we synthesized branched anhydride monomer 1,3,5-tris(3,4-anhydride phenoxy)benzene using a three-step method. Furthermore, we prepared successfully novel double branched sulfonated polyimide (dbSPI) membranes with ultra-low vanadium ion permeability (0.4–2.5×10−8 cm2 min−1), one or two orders of magnitude lower than that of the Nafion 212 membrane (7.25×10−7 cm2 min−1). Among all membranes, the dbSPI-50 membrane with the highest proton selectivity (23.25×105 S min cm−3), approximately 68 times higher than that of the Nafion 212 membrane, was chosen for assembling of a VRFB single cell. Because of its ultra-low vanadium ion permeability, the dbSPI-50 membrane exhibited much longer self-discharge time (97 h) and superior capacity retention (82.6–91.3%) compared with the Nafion 212 membrane. Moreover, both coulomb efficiency (99.6–98.5%) and energy efficiency (69.3–80.1%) of the dbSPI-50 membrane are higher than those of the Nafion 212 membrane after 300 times charge-discharge tests at a current density from 200 to 100 mA cm−2. All these results demonstrate that the optimized dbSPI-50 membrane is a promising candidate for applications in VRFBs.

Polyelectrolyte proton exchange membranes: synthesis and characterization of sulfonated polyimide membranes using novel stilbene-containing diamine

A new unsymmetrical diamine, 4-(2-aminophenoxy)-4′-aminostilbene was successfully synthesized and used to synthesize stilbene-containing sulfonated polyimides. 2,2′-Benzidine-disulfonic acid, 1,4,5,8-naphthalene tetracarboxylic dianhydride and different mole ratios of 4-(2-aminophenoxy)-4′-aminostilbene and 2-bis(4-(4-aminophenoxy)phenyl) hexafluoropropane were used for the synthesis of stilbene-containing polyimides with sulfonic acid groups. The synthesized polymers were characterized using various physicochemical characterization techniques such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and electrochemical impedance spectroscopy. Furthermore, the viscosity, water uptake, ion exchange capacity, swelling ratio, hydrolytic stability, oxidative stability, mechanical properties and proton conductivity of the synthesized membranes were also analyzed. Sulfonated polyimide membranes showed a water uptake capacity of 18.93–28.89%, proton conductivity of 0.028–0.039 S cm−1 at 30 °C, ion exchange capacity of 1.469 meq g−1 and hydrolytic stability of 40–44 h at 80 °C. The solubility of the sulfonated polyimide membranes was improved when unsymmetrical diamine was used to prepare sulfonated polyimides.

Comparing the Ion-Conducting Polymers with Sulfonate and Ether Moieties as Cathode Binders for High-Power Lithium-Ion Batteries.

Two types of ion-conducting polyimides with sulfonate or ether functional groups were synthesized as ion-type or coordination-type cathode binders for lithium-ion batteries (LIBs), respectively. Although superior ion transport abilities have been reported for both types of ion-conducting polymers, their electrochemical performances are significantly different and the corresponding transport mechanisms at the electrolyte/electrode interface remain elusive. Here, we combine experimental and computational techniques to investigate the cathode interface in the presence of both functional polymer binders in comparison with the poly(vinylidene fluoride) (PVDF) binder as reference. A broad shoulder in the cyclic voltammogram accompanied by a poor rate performance of battery tests was observed for a LiFePO4 cathode with coordination-type ether-based polyimide (EPI) binder. In contrast, a LiFePO4 cathode with ion-type sulfonated polyimide (SPI) binder exhibits smaller concentration polarization, achieving satisfactory capacity at high current density. Simulations show that the ether-based binder strongly coordinates Li ions and thus slows the diffusion of Li ions. This leads to the reduction of the LIB electrochemical performance at a high C-rate. In contrast, the negative moiety of the SPI binder leads to less localization of Li ions, allowing a slightly higher Li-ion mobility. Conventional PVDF shows no affinity to Li ions, leading to less Li-ion accumulation at the electrode/electrolyte interface. Yet, the cathode surface covered with PVDF shows the lowest Li-ion diffusivity compared to those with the two types of Li-ion-conducting binders. Therefore, cathodes with SPI and PVDF binders show less polarization at the electrode interface and allow higher C-rate performance of LIBs. The combined results provide a comprehensive understanding of the mechanism of ion conduction in ion- and coordination-type Li-ion-conducting polymer binders. This gives valuable insight into the design of next-generation polymer materials for high-power LIBs.

Fabrication, morphological, structural and electrochemical characterization of sulfonated polyimide/clay-based hybrid nanocomposite membranes for energy application

Sulfonated polyimide (SPI) using 1, 4, 5, 8-Naphthalenetetracaboxylic Dianhydride was fabricated by a one-step high-temperature direct imidization method in the presence of organo-modified clay (sepiolite) as reinforcement in the polymer matrix. Different sets of SPI and clay sepiolite (in different content 1–7 wt. %) in the form of thin films were fabricated by the solution casting method. These films were cast at a temperature of 80 °C for 12 h followed by curing at 150 °C and 180 °C for 1 and 6 h, respectively. Several physicochemical parameters and analytical techniques were performed to characterize the newly fabricated nanocomposite membranes. Ion exchange capacity and hydrolytic study results showed the impact of clay content on the hydrolytic stability and morphology of hybrid membranes. Lowering in ion exchange capacity with higher hydrolytic stability was observed by increasing the clay contents in the polymeric matrix. The as-synthesized membranes were tested for its oxidative stability by Fenton reagent and observed that membrane with 5-wt (%) clay presented the maximum oxidative stability. The SPI-clay hybrid membrane with 7 wt.% of clay exhibited a water-uptake of 3.03% at 40 °C and proton conductivity of 2.45 mmol/g at 25 °C, while the pure SPI showed the values of 3.62% and 3.12 mmol/g in the same experimental conditions. FT-IR analysis investigated the structural aspects of neat SPI and all other synthesized nanocomposite membranes. The thermogravimetric analysis examined the presence of inorganic materials in the synthesized membranes along with their thermal stability profile. X-ray diffraction patterns were used to scrutinize the crystalline nature of neat sulfonated polyimide and the SPI-Clay composites membrane. In conclusion, these nanocomposite membranes can be applied as proton exchange films in fuel cells for energy application due to their interesting physicochemical, morphological, structural, and electrochemical properties.

4-(4-Aminophenoxy)benzene-1,3-diamine-based covalent cross-linked sulfonated polyimide as proton exchange membranes in fuel cell

Hydrolytic and oxidative stabilities of sulfonated polyimide membranes are of significant concern when they are used as fuel cell polyelectrolyte membranes. To increase the hydrolytic and oxidative stabilities of sulfonated polyimide membranes, atriamine crosslinker, 4-(4-aminophenoxy)benzene-1,3-diamine, was synthesized, characterized, and successfully used to synthesize covalently cross-linked sulfonated polyimide membranes. 1,4,5,8-Naphthalenetetracarboxylic dianhydride, 2,2’-benzidine-disulfonic acid, 4-(4-aminophenoxy)benzene-1,3-diamine and 2-bis(4-(4-aminophenoxy)phenyl) hexafluoropropane-based cross-linked sulfonated polyimide membranes have been synthesized and compared with linear sulfonated polyimide membrane in terms of various fuel cell application parameters. Covalent cross-linking is a simple and efficient method to increase the lifetime of the polyelectrolyte membranes. The covalently cross-linked sulfonated polyimidemembranes show an ion exchange capacity of 1.426 1.441 meq.g-1, proton conduction of 0.026 – 0.031 S cm-1, and water uptake capacity 10.7 – 17.07%. As the triamine concentration is increases, both hydrolytic and oxidative stabilities of the membranes also increased. The linear sulfonated polyimide showed hydrolytic stability of 43 h, whereas hydrolytic stability of the cross-linked sulfonated polyimide membranes is increased up to 70 h.

Modification and Optimization of Rice Husk Ash Bio-Filler in Sulfonated Polyimide Membrane for Direct Methanol Fuel Cell

Direct methanol fuel cell (DMFC) is a very high potential renewable and sustainable energy for portable devices as it has very high energy density and more environmentally friendly. Adding filler into the membrane matrix had reported in improved performance of the DMFC. However, addition of filler will lead to the decrease of the ion exchange capacity of the membrane, which will in turn reduce the fuel cell performance. Hence, this work studies the modification of rice husk ash via sulfonation process and the effect of the synthesis parameter of novel sulfonated polyimides/sulfonated rice husk ash (SPI/sRHA) composite membrane towards fuel cell performance where filler sRHA had been synthesized via the mixing reaction between rice husk ash (RHA) with chloroform and chlorosulfonic acid. The composition of filler in membranes was optimized with different synthesis parameters such as different types of filler used, loading of filler in membrane and membrane annealing temperature. Investigation of the effects for these three parameters on the physicochemical properties of the composite membranes produced was carried out using response surface methodology (RSM) approach to optimize the synthesis parameters of the composite membrane. The fillers and composite membranes were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller (BET) analysis and transmission electron microscopy. Different synthesis parameters had shown different influences on the physicochemical properties of the composite membranes and consequently, gave different passive fuel cell performances of DMFCs too. The optimized membrane (S-12-sR) showed enhanced physicochemical performance as well as the passive single cell performance as compared to pure SPI and Nafion 117 membranes. The water uptake, proton conductivity and IEC that had been achieved by the optimized membrane were 90.97%, 0.1891 S cm−1 and 0.2608 mmol g−1 respectively. The maximum power density of the passive DMFC was improved from 8.1 mW cm−2 to 16.4 mW cm−2 under ambient conditions.

Ionic Liquid/Sulfonated Polyimide Composite Membranes: Effect of Polyimide Sequence on CO2 Transport Properties

The carbon dioxide capture and storage (CCS) plays an important role in reducing greenhouse gases, but its high energy cost is a critical issue. The CO2 separation is recognized as a key process for reducing cost, and a membrane separation method is one of the promising candidates with a low energy cost. Ion gels comprising polymer networks and ionic liquids (IL), which exhibit unique properties such as non-volatility and CO2 absorption selectivity, have attracted attention as environmentally friendly separation membranes. Previous work in our group has shown that sulfonated polyimide (SPI) with an imidazolium cation exhibits good compatibility with an IL. SPI can hold high content of IL (~75 wt%) and form a uniform, flexible, tough (~10 MPa of Young’s modulus) and thin composite membrane.[1-3] This SPI was synthesized by random copolymerization of a sulfonated ionic monomer and a non-ionic monomer. The composite membranes separated into bicontinuous two phases of ionic and non-ionic domains, which contribute to the formation of gas diffusion path and mechanically tough network, respectively. By using multiblock copolymers of SPI, it is expected that the size and connectivity of each domain are improved, which could lead to the increase of the CO2 permeability and mechanical toughness of the membrane. In this study, we fabricated an 1-buty-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][NTf2])/SPI composite membranes composed of multiblock-type SPI (mb-SPI) and compared their property with those of random-type SPI (r-SPI).75 wt%[C4mim][NTf2]/mb-SPI composite membranes exhibited higher CO2 permeability (372 barrer) than the corresponding r-SPI (235 barrer) while the gas selectivity (CO2/N2) is 26, which is of the same magnitude as that of r-SPI (CO2/N2 is 28). The mb-SPI can hold higher IL content up to 80 wt% than r-SPI, resulting in higher CO2 permeability (480 barrer; CO2/N2 is 27). The higher ionic conductivity also suggested the higher connectivity of ionic domains in mb-SPI system. On the other hand, from tensile tests, it was found that the mb-SPI membrane was less ductile than the r-SPI membrane. The result of dynamic mechanical analysis (DMA) indicates that the difference of internal structure between the membranes composed of mb-SPI and r-SPI is one of the reasons of the poor mechanical property.To fabricate more strong membrane, we used SPI with more rigid polymer backbone. Rigid mb-SPI exhibits high CO2 permeability (450 barrer) and gas selectivity (CO2/N2 is 24), comparable with the conventional one, and the mechanical property was superior to the conventional one. From these results, we concluded that the mb-SPI membrane having high CO2 permeability and toughness was successfully fabricated.

Heat-Treated Charge-Transfer Polymer Blend Membrane for Application in Polymer Electrolyte Fuel Cells

IntroductionWe developed polymer blend type electrolyte membranes (PEMs) using charge-transfer (CT) interactions for polymer electrolyte fuel cells (PEFCs) and evaluate effect of heat treatment to characteristics and performance of the CT polymer blend membranes.CT interaction is one of molecular interaction, and CT complex can be formed by electron accepting units and donating units which have optimal molecular structures. CT complex formation was applied to control the position of proton conductive groups in PEMs. By applying this characteristic, we have developed CT complex polymer hybrid membranes. Especially, CT polymer blend consisting of two polymers, electron-accepting sulfonated polyimide (SPI) and -donating polyether (Poly-DAN), showed high mechanical strength and gas barrier property.[1] During this study, it was confirmed that CT complex concentration increased by heat treatment. In this study, to further understand the effect of CT complex formation on PEM performance, heat treatment was performed to enhance the extent of CT complex formation in the membrane.ExperimentalIn this work, we used two model polymers with simple chemical structure. Sulfonated polyimide homopolymer (SPI) was used as the electron-accepting polymer, while polyether-containing electron-rich dialkoxynaphthalene (Poly-DAN) was used as the electron-donating polymer.[2]SPI/Poly-DAN blend membranes were prepared with or without a heat treatment step.SPI and Poly-DAN can form a CT complex in the blend membrane during solvent evaporation at 60 °C in vacuo, and CT blend membranes without heat treatment were thus obtained. Heat-treated CT blend membranes were obtained by heating at 150 °C in a N2 atmosphere for several hours. The heat-treated membranes were then immersed in water to remove thermally decomposed sulfuric acid moieties.Visible spectroscopy was used to confirm CT complex formation and the amount. Thermal property, mechanical strength and structural analysis of the CT blend membranes were evaluated by TGA, tensile test, 1H NMR etc. IEC, proton conductivity and fuel cell performance at 80˚C and 110 ˚C were also evaluated.Results and discussionA characteristic absorbance is observed in the visible spectra of the untreated and the heat-treated membranes at 530 nm. After heat treatment at 150 °C for 50 h, the concentration of CT complex in the membrane was significantly enhanced by about 13 times. The 1H NMR and FT-IR results suggest that the main chain structure of SPI does not significantly change during heat treatment. From DSC and XRD results, we suggested that Poly-DAN unit with low glass transition temperature inserted between two SPI main chains due to CT complex formation by heat treatment.After heat treatment, SPI/Poly-DAN blend membranes did not completely dissolve in DMSO, and the weight loss after water immersion of the heat-treated SPI/Poly-DAN blend membranes was smaller than that of the untreated membrane. These results indicate that a cross-linked structure was not only formed by CT complex formation but also due to chemical cross-linking during heat treatment. Because of chemical cross-linking of sulfonic acid by heat treatment, experimental IEC of SPI/Poly-DAN (1:0.05 mol) was smaller (2.51) than theoretical value (3.41).Heat-treated SPI/Poly-DAN blend membranes showed higher mechanical strength (50.8 MPa) than Nafion 212 (15.5 MPa) and highly chemical durability compared to the untreated membrane by the synergetic effect of enhanced CT complex formation and chemical cross-linking. Heat-treated SPI/Poly-DAN membranes showed comparable proton conductivity (32.3 mS cm−1, 80 °C, and 90% RH), although some cross-linking occurred between sulfonic acid units due to the heat treatment process.In fuel cell tests, heat-treated SPI/Poly-DAN membranes had maximum power densities of 255 mW cm−2 at 80 °C and 95% RH and 59.0 mW cm−2 at 110 °C and 31% RH, indicating that these heat-treated CT complex membranes could be used for fuel cell applications.References S. Feng et al., Journal of Membrane Science, 548 (2018).S. Feng et al., ACS Applied Energy Materials, 2, 8715–8723 (2019). Figure 1

Novel sulfonated polyimide membrane blended with flexible poly[bis(4-methylphenoxy) phosphazene] chains for all vanadium redox flow battery

To improve SPI stability, poly[bis(4-methylphenoxy) phosphazene] (PMPP) was designed and synthesized using polyphosphazene's unique and strong flexibility. A novel sulfonated polyimide (SPI) composite membrane embedded with flexible PMPP chains was prepared for all vanadium redox flow battery applications. SPI/PMPP composite membrane characterizations, such as proton conductivity, vanadium ion permeability, stability, and single cell performance, were thoroughly investigated. PMPP's unique properties not only allow the membrane to exhibit excellent anti-hydrolysis and anti-oxidation stability, but also maintain high proton conductivity and low vanadium ion permeability (as low as 4.81 × 10−7 cm2 min−1). It can also form a rigidity-flexibility network with SPI to improve membrane mechanical properties. It can be deduced that introducing PMPP has achieved many things in one stroke. Results show that the hydrolysis and oxidation stabilities of SPI/PMPP membrane were significantly improved. The SPI/2% PMPP membrane exhibits higher columbic efficiency (CE: 97.52%), voltage efficiency (VE: 84.88%), and energy efficiency (EE: 82.21%) than that of the N212 membrane (CE: 94.97%, VE: 82.04%, EE: 76.99%) at a high current density of 100 mA cm−2. In addition, the charging capacity loss of a battery with a SPI/2% PMPP membrane has also significantly decreased by about 206.70 mAh after 110 cycles at 100 mA cm−2 with the same electrolyte volume compared to a pristine SPI. Meanwhile, the high temperature resistance of SPI/PMPP membrane also shows the possibility of being applied with fuel cells.

Quinoxaline-based semi-interpenetrating polymer network of sulfonated poly(arylene ether)s and sulfonated polyimides as proton exchange membranes

Semi-interpenetrating polymer network (sIPN) proton exchange membranes are prepared from sulfonated polyimides (SPIs) and cross-linkable sulfonated poly(arylene ether)s (SPAEs) with 3,3′-diaminobenzidine (DAB) as cross-linker. The cyclocondensation between DAB and benzyl moiety in SPAEs results in a covalently cross-linked SPAE networks where SPIs can be immobilized by interlocking; meanwhile, the interlocked structure is also strengthened by acid–base ionic interactions between the sulfonic acid groups of sulfonated polymers (SPAEs and SPIs) and quinoxaline moieties formed in cross-linking. For comparison, the corresponding SPAE/SPI blend membranes are also prepared and evaluated. The proton conductivity, dimensional changes, mechanical properties, oxidative stability, water stability as well as methanol permeability are investigated for the membranes. The results show that the sIPN membranes have obviously higher performance than the pristine SPAE, SPI and SPAE/SPI blend membranes due to the interlocked structures. The sIPN membrane sIPN-SPAEK(2/1)-SPI with ion exchange capacity of 2.00 mequiv g−1 shows good mechanical properties, high stability, reasonably high proton conductivity of 128 mS cm−1 and low methanol permeability of 0.58 × 10–6 cm2 s−1.

Preparation of sulfonated polyimide/polyvinyl alcohol composite membrane for vanadium redox flow battery applications

A novel series of sulfonated polyimide (SPI)/polyvinyl alcohol (PVA) composite membranes applied in vanadium redox flow battery (VRFB) is prepared and characterized in this work. With the hydrophilicity and good barrier of PVA, the composite membranes exhibit better hydrolysis stability and vanadium ion resistance than normal SPIs with five-membered rings. In VRFB single-cell tests, because of the highest proton selectivity, the SPI5/PVA5 composite membrane shows lower self-discharge rate, higher coulombic efficiency, and energy efficiency (EE) (79.5% vs 77.8% at 30 mA cm−2) than that of Nafion117. Besides, the composite membranes present stable performances up to 100 cycles without significant decline in EE.

High temperature membranes based on PBI/sulfonated polyimide and doped-perovskite nanoparticles for PEM fuel cells

A new sulfonated aromatic diamine monomer containing nitrogen heterocycles was synthesized and employed to prepare a novel sulfonated polyimide (SPI). To develop proton exchange membranes, new nanocomposite blend membranes consist of the prepared SPI and polybenzimidazole (PBI) were fabricated with incorporation of SrCe0.9Yb0.1O3-δ (SCYb) doped-perovskite nanoparticles with a solution-casting method. The goal of this work is to study the effect of SPI and SCYb doped-perovskite nanoparticles on the important parameters of the PBI membrane specially proton conductivity and fuel cell performance. The proton conductivity and phosphoric acid doping level of the PBI-SPI-SCYb nanocomposite blend membranes improved due to an interaction of –SO3H group and thiazole rings of SPI and N–H groups of PBI in the oxygen vacancies of SCYb doped-perovskite nanoparticles. Substitution of Ce4+ by Yb3+ in the SCYb doped-perovskite nanoparticles produce oxygen vacancies and decrease the columbic repulsion between protons and positive ions. Furthermore at highest phosphoric acid doping level of 14 mol phosphoric acid per monomer unit, the nanocomposite blend membranes displayed proton conductivity of 131 mS/cm at 180 °C and 8% relative humidity. The increase in power density from 0.31 W/cm2 in PBI-SPI blend membranes (SPI/PBI: 25 wt%) to 0.59 W/cm2 in PBI-SPI-SCYb nanocomposite blend membranes (SPI/PBI: 25 wt% and 7 wt% of SCYb) was achieved at 0.5 V, 8% RH and 180 °C, which demonstrates that these developed nanocomposite blend membranes have a high potential to be regarded as the most promising candidates for high-temperature fuel cell with improved proton conductivity.

Development and Characterization of Functionalized Titanium Dioxide-Reinforced Sulfonated Copolyimide (SPI/TiO2) Nanocomposite Membranes with Improved Mechanical, Thermal, and Electrochemical Properties

Sulfonated polyimide (SPI) membranes are regarded as the potential alternative to Nafion as a polymer electrolyte membrane (PEM) for fuel cells. Herein, sulfonated copolyimide is synthesized through a direct polymerization method using 1,4,5,8-naphthalene tetra carboxylic dianhydride (NDTA), and sulfonated diamine 4,4′-diamino diphenyl-2,2′-disulfonic acid (BDSA). Sulfonated copolyimide is re-dissolved in dimethyl sulfoxide for the fabrication of thin films. The nanocomposites of both 1.0 and 2.0% of the functionalized TiO2-reinforced (SPI/TiO2) are fabricated by blending and casting method. TiO2 nanoparticles are surface-functionalized using (3-aminopropyl)triethoxysilane (APTES). Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared FTIR), and Brunauer-Emmitt-Teller (BET) techniques are used to reveal the distinctive characteristics of the newly synthesized SPI/TiO2 nanocomposite membranes. The as-synthesized SPI/TiO2 membranes are used to investigate their physicochemical and electrochemical parameters, including water uptake, ion exchange capacity (IEC), swelling property, oxidation stability, and dimensional change stability. The SPI and SPI/TiO2 composite membranes exhibit an ion-exchange value of 1.412 to 1.286 mmol/g. The SPI/TiO2 composite membranes show excellent H2O stability at room temperature. Overall, the results indicate that the SPI/TiO2 membranes exhibit the potential as PEM for direct methanol fuel cell applications. These nanocomposite membranes are also applicable in clean energy production from environment-friendly sources.

Effects of sulfonate incorporation and structural isomerism on physical and gas transport properties of soluble sulfonated polyimides

This study presents the effects of sulfonate incorporation and structural isomerism of soluble sulfonated polyimides (SPIs) for potential membrane applications. The fluorinated dianhydride (6FDA) and three commercially sulfonated diamines, 2,4,6-trimethyl-3,5-diaminobenzenesulfonic acid (tDSA), 2,5-diaminobenzenesulfonic acid (pDSA), and 2,4-diaminobenzenesulfonic acid (mDSA), were used to synthesize SPIs in a one-step polycondensation. The SPIs are soluble in common organic solvents and have good mechanical properties as represented by tensile strength of 88.4–115.8 MPa and elongation at break of 10.3–15.2%. The resulting SPI membranes exhibit low dielectric constants (~2.61) and linear thermal expansion coefficients (CTE) ranging from 54.6 to 71.7 ppm/K. These SPI membranes have fractional free volume (FFV) of 0.119–0.148. The sulfonation incorporation induced closer inter-chain packing, smaller FFV, larger CTE, higher average refractive indices (nav), better transparency and higher ideal selectivities for almost all gas pairs for their membranes. The para-linked TEA-pSPI membrane exhibited a larger inter-chain d-spacing, larger FFV, larger CTE, smaller nav and slightly higher permeability coefficients than the corresponding meta-linked TEA-mSPI membrane. This study guides molecular architecture for improving particular membrane performance by introducing sulfonation groups as well as adjusting either para- or meta-linkages into polymeric main chains.