Guanidinium cationic covalent organic nanosheets-based anion exchange composite membrane for fuel cells

Covalent organic frameworks (COFs) used for anion exchange membrane fuel cells (AEMFCs) are commonly endowed with ion conductivity by post-synthesis modification. However, this method usually results in uneven distribution of functional groups, low functionalization and severe ion capacity fade. Limited by hydrophobic skeleton and relatively large particle size of COFs, the COFs doping amount of the composite membrane is not high. Here we design and synthesize a series of guanidinium cationic covalent organic nanosheets-based anion exchange composite membranes. The positively charged guanidinium group as a building block can induce COF-DhaTG_Cl self-exfoliation into a few layered nanosheets through strong interlayer repulsion. Then, the nanosheets were introduced into quaternary ammonium-functionalized poly(2,6-dimethyl-1,4-phenyl ether) (QPPO). A series of COF-DhaTG_Cl/PPO composite AEMs was prepared with the highest doping amount of 30 wt% by casting method. The porous structure and repeat cationic guanidinium units on the skeleton will expose ion sites to the target ones, providing faster OH⁻ diffusion kinetics in one-dimensional channels. The OH⁻ conductivity of COF-DhaTG_Cl/PPO-20 composite membrane can reach 148.65 mS/cm at 80 °C. Meanwhile, the composite membrane also exhibits enhanced mechanical strength and alkaline stability with the maximum stress strength of 37.3 MPa and the residual conductivity of 96.29% after immersion in 2 M NaOH solution at 60 °C for two weeks.     

N-spirocyclic ammonium-functionalized graphene oxide-based anion exchange membrane for fuel cells

Graphene oxide (GO) is a potential material in the electrode and membrane of polymer electrolyte membrane fuel cells due to its unique structure and various oxygen-containing functional groups. A class of three-layered GO/poly (phenylene oxide) for AEMs was prepared in this work. GO was functionalized with highly stable 6-azonia-spiro [5.5]undecane groups and used as a fast hydroxide conductor, named ASU-GO. Functionalized by N-spirocyclic cations, poly (phenylene oxide) (PIPPO) was then combined with ASU-GO and GO to fabricate the ASU-GO/PIPPO and GO/PIPPO. Notably, the maximum hydroxide conductivity of the ASU-GO/PIPPO was 73.7 mS cm⁻¹ at 80 °C, which was 3 times higher than that of the GO/PIPPO. The enhancement in hydroxide conductivity was due to the changes in the hydroxide transport mechanism and the poor stacked structure of the ASU-GO layer. Only 10.8% drops in hydroxide conductivity of ASU-GP/PIPPO after the alkaline test (1 M KOH at 80 °C for 700 h). Furthermore, the ASU-GO/PIPPO-50 membrane showed a maximum peak power density of 102 mW cm⁻², demonstrating the prepared membrane was promising in the AEM applications.     

Profile of extended chemical stability and mechanical integrity and high hydroxide ion conductivity of poly(ether imide) based membranes for anion exchange membrane fuel cells

We designed and synthesized a poly(ether imide) (PEI) membrane that has good chemical and mechanical stabilities. Alkalized PEI (A-PEI) membrane was fabricated by solution casting of chloromethylated PEI (CM-PEI) followed by quaternization and alkalization. The chemical structure of the synthesized polymers was verified by proton nuclear magnetic resonance (¹H NMR) and Fourier-transform infrared spectroscopy (FT-IR). Physiochemical properties of the membrane such as ion exchange capacity, water uptake, and swelling ratio were investigated. The membranes with a high degree of chloromethylation (DC) exhibited elevated hydroxide ion conductivity in range of 6.7–44.2 mS/cm at 90 °C under 100% relative humidity (RH). The hydrophilic-hydrophobic phase separation was verified by atomic force microscope (AFM) and small angle X-ray scattering (SAXS) measurements. Chemical stability was evaluated by measuring the durability of membranes while they were soaked in oxidative and alkaline solutions at 60 °C for 200 h.     

High chemical stability anion exchange membrane based on poly(aryl piperidinium): Effect of monomer configuration on membrane properties

In recent years, ether-free polyaryl polymers prepared by superacid-catalyzed Friedel-Crafts polymerization have attracted great research interest in the development of anion exchange membranes(AEMs) due to their high alkali resistance and simple synthesis methods. However, the selection of monomers for high-performance polymer backbone and the relationship between polymer structure construction and properties need further investigated. Herein, a series of free-ether poly(aryl piperidinium) (PAP) with different polymer backbone steric construction were synthesized as stable anion exchange membranes. Meta-terphenyl, p-terphenyl and diphenyl-terphenyl copolymer were chosen as monomers to regulate the spatial arrangement of the polymer backbone, which tethered with stable piperidinium cation to improve the chemical stability. In addition, a multi-cation crosslinking strategy has been applied to improve ion conductivity and mechanical stability of AEMs, and further compared with the performance of uncrosslinked AEMs. The properties of the resulting AEMs were investigated and correlated with their polymer structure. In particular, m-terphenyl based AEMs exhibited better dimensional stability and the highest hydroxide conductivity of 144.2 mS/cm at 80 °C than other membranes, which can be attributed to their advantages of polymer backbone arrangement. Furthermore, the hydroxide conductivity of the prepared AEMs remains 80%–90% after treated by 2 M NaOH for 1600 h, exhibiting excellent alkaline stability. The single cell test of m-PTP-20Q4 exhibits a maximum power density of 239 mW/cm² at 80 °C. Hence, the results may guide the selection of polymer monomers to improve performance and alkaline durability for anion exchange membranes.     

Poly(arylene ether ketone) with pendant pyridinium groups for alkaline fuel cell membranes

Using the step-growth polycondensation reaction, poly(arylene ether ketone) (PAEK) and activated poly(arylene ether ketone)-NHS intermediates (PAEK-N) were synthesized. PAEK-NHS intermediates with pyridinium groups (PAEK-PYR) were obtained by adding different amounts of PYR groups. The successful syntheses of PAEK, PAEK-N, and PAEK-PYR were confirmed by nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy. Several important membrane properties (e.g., ionic exchange capacity (IEC), water uptake, anion conductivity, and thermal and mechanical stability) were investigated for their applications in alkaline fuel cells. Water uptake, swelling ratio, anion conductivity, and IEC increased with increasing PYR contents, while the mechanical properties decreased. Among a series of prepared membranes, the PAEK-PYR100 and PAEK-PYR125 membranes showed IEC and anion conductivity values that were higher than those of a commercial AHA membrane. Also, all of the prepared membranes were thermally stable up to 255 °C and show excellent chemical stability in alkaline conditions.     

Effect of hydroxide and carbonate alkaline media on anion exchange membranes

The effect of hydroxide and carbonate alkaline environments on the chemical stability and ionic conductivity of five commercially available anion exchange membranes was investigated. Exposure of the membranes to concentrated hydroxide environments (1 M) had a detrimental effect on ionic conductivity with time. Over a 30-day period, decreases in conductivity ranged from 27% to 6%, depending on the membrane. The decrease in ionic conductivity is attributed to the loss of stationary cationic sites due to the Hofmann elimination and nucleophilic displacement mechanisms. Exposure of the membranes to low concentration hydroxide (10⁻⁴ M) or carbonate/bicarbonate (0.5 M Na₂CO₃/0.5 M NaHCO₃) environments had no measurable effect on the ionic conductivity over a 30-day period. ATR-FTIR spectroscopy confirmed degradation of membranes soaked in 1 M KOH. Apparition of a doublet peak in the region between 1600 cm⁻¹ and 1675 cm⁻¹ confirms formation of carbon-carbon double bonds due to Hofmann elimination. Membranes soaked in mild alkaline environments did not show formation of carbon-carbon double bonds.     

Anion conductive poly(tetraphenyl phthalazine ether sulfone) containing tetra quaternary ammonium hydroxide for alkaline fuel cell application

The poly(tetraphenyl ether ketone sulfone)s (PTPEKSs) were synthesized from 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenyl benzene (BFBTPB) and bis(4-fluorophenyl) sulfone with bis(4-hydroxydiphenyl) sulfone in sulfolane. The synthesis of poly(tetraphenyl phthalazine ether sulfone)s (PTPPESs) was carried out via an intramolecular ring-closure reaction of dibenzoylbenzene moiety with hydrazine monohydrate. The PTPPES-QAHs [poly(tetraphenyl phthalazine ether sulfone-quaternary ammonium hydroxide)]s were synthesized via chloromethylation of PTPPES, quaternization with trimethylamine, and followed by an anion exchange of tetra-quaternary ammonium chloride polymers with KOH. Different contents of quaternized unit in PTPPES-QAH (15, 20, 25 mol% of BFBTPB) were studied by FT-IR, ¹H NMR spectroscopy, and thermogravimetric analysis (TGA). Sorption experiments were conducted to observe the interaction of quaternized polymers with water. The ion exchange capacity (IEC), ion conductivity and cell performance of PTPPES-QAH were evaluated with increasing the degree of quaternization.     

Phosphoric acid doped sulfonated poly(tetra phenyl isoquinoline ether sulfone) copolymers for high temperature proton exchange membrane potential application

Phosphoric acid-doped sulfonated poly(tetra phenyl isoquinoline ether sulfone)s (PA-SPTPIESs) were successfully synthesized for high temperature proton exchange membrane. Poly(tetra phenyl ether ketone sulfone)s (PTPEKS) were prepared from 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenyl benzene (BFBTPB) and bis(4-fluorohenyl) sulfone with bis(4-hydroxyphenyl) sulfone. The synthesis of the poly(tetra phenyl isoquinoline ether sulfone)s (PTPIESs), was carried out via an intramolecular ring-closure reaction of dibenzoylbenzene of PTPEKS with benzylamine. The sulfonated poly(tetra phenyl isoquinoline ether sulfone)s (SPTPIESs) were obtained by following sulfonation with concentrated sulfuric acid and doped by phosphoric acid. Different contents of sulfonated unit on PTPIESs (8, 12, 16 mol% of BFBTPB) and PA-SPTPIESs were studied by FT-IR, ¹H NMR spectroscopy, and thermogravimetric analysis (TGA). Strong acid–base interaction effect between poly benzisoquinoline (PBI) and sulfonic acid groups formed ionic crosslinking network between polymer chains. The ion exchange capacity (IEC) and proton conductivity of PA-SPTPIESs were evaluated with degree of sulfonation and doping of phosphoric acid.     

Simultaneously enhanced hydroxide conductivity and mechanical properties of quaternized chitosan/functionalized carbon nanotubes composite anion exchange membranes

Low-cost biopolymer chitosan has received considerable attention in the field of anion exchange membranes (AEMs) because it can be easily quaternized and avoids the carcinogenic chloromethylation step. Simultaneously increasing the ionic conductivity and improving mechanical properties of quaternized chitosan (QCS) is key for its high-performance application. In this study, new composite AEMs consisting of QCS and functionalized carbon nanotubes (CNTs) were prepared. CNTs were coated with a thick silica layer onto which high-density quaternary ammonium groups were then grafted. The insulator silica coating effectively prohibits electron conduction among nanotubes and the grafted –NR₃⁺ provides new OH⁻ conductive sites. Incorporating 5 wt% functionalized CNTs into the matrix enhanced ionic conductivity to 42.7 mS cm⁻¹ (80 °C) which was approximately 2 times higher than that of pure QCS. The effective dispersion of CNTs and appropriate interfacial bonding between nanofiller and QCS improved the mechanical properties of AEMs, including both the strength and toughness of the composite membranes. An alkaline direct methanol fuel cell equipped with the composite membrane (5% functionalized CNTs loading) produced an maximum power density of 80.8 mW cm⁻² (60 °C), which was 57% higher than that of pure QCS (51.5 mW cm⁻²). This study broadens the application of natural polymers and provides a new way to design and fabricate composite AEMs with both improved mechanical properties and electrochemical performance.     

Hydroxide exchange composite membrane based on soluble quaternized polyetherimide for potential applications in fuel cells

A series of soluble quaternized polyetherimides (QAPEIs) have been successfully synthesized by homogeneous quaternization in trimethylamine aqueous solution. ¹H NMR spectra confirm the successful synthesis of QAPEI. The QAPEIs exhibit good solubility in membrane-preparation solvents, making it possible to prepare the QAPEI composite membrane. Novel composite hydroxide exchange membranes have been prepared by incorporating QAPEIs with polytetrafluoroethylene (PTFE) membranes. The SEM images, gas permeation measurements and FTIR spectra show that the QAPEI is successfully filled in PTFE membrane and the resulted composite membrane is dense and smooth. The ion exchange capacity of composite membranes ranges from 0.35 to 0.58 mmol g⁻¹. The composite membranes have appropriate water uptake (≤154%) and moderate swelling ratio (≤42%) even at 60 °C. The hydroxide conductivity of the composite membrane reaches 11.9 mS cm⁻¹ at 20 °C that increases to 35.2 mS cm⁻¹ at 60 °C. TGA curve shows that the composite membrane possesses high thermal stability (T_OD: 210 °C). All these properties indicate that the QAPEI/PTFE composite membranes are good candidates for use as HEMs in HEM fuel cells.     

Nafion/polyaniline composite membranes specifically designed to allow proton exchange membrane fuel cells operation at low humidity

A Nafion and polyaniline composite membrane (designated Nafion/PANI) was fabricated using an in situ chemical polymerization method. The composite membrane showed a proton conductivity that was superior to that obtained with Nafion^® 112 at low humidity (e.g. RH = 60%). Water uptake measurements revealed similarities between the Nafion^® 112 and Nafion/PANI membranes at different humidities. The high conductivity of the Nafion/PANI membrane at low humidity is hypothesized to be due to the existence of the extended conjugated bonds in the polyaniline; proton transfer is facilitated via the conjugated bonds in lower humidity environments allowing retention of the relatively high conductivity. Correspondingly, the performance of a single cell fuel cell containing the Nafion/PANI composite membrane is improved compared to a Nafion^® 112-containing cell under low humidity conditions. This is important for portable fuel cells, which are required to operate without external humidification.     

Imidazolium-functionalized polysulfone hydroxide exchange membranes for potential applications in alkaline membrane direct alcohol fuel cells

A series of imidazolium-functionalized polysulfones were successfully synthesized by chloromethylation-Menshutkin two-step method. PSf-ImOHs show the desired selective solubility: insoluble in alcohols (e.g., methanol and ethanol), and soluble in 50 vol.% aqueous solutions of acetone or tetrahydrofuran, implying their potential applications for both the alcohol-resistant membranes themselves and the ionomer solutions in low-boiling-point water-soluble solvents. PSf-ImOH also possesses very high thermal stability (T_OD: 258 °C), higher than quaternary ammonium and quaternary phosphonium functionalized polysulfones (T_OD: 120 °C and 186 °C, repsectively). Ion exchange capacity (IEC) of PSf-ImOH membranes ranges from 0.78 to 2.19 mmol g⁻¹ with degree of chloromethylation from 42% to 132% of original chloromethylated polysulfone. As expected, water uptake, swelling ratio, and hydroxide conductivity increase with IEC and temperatures. With 2.19 mmol g⁻¹ of IEC, the PSf-ImOH 132% membrane exhibits the highest hydroxide conductivity (53 mS cm⁻¹ at 20 °C), higher than those of all other reported polysulfone-based HEMs (1.6–45 mS cm⁻¹) and other imidazolium-functionalized HEMs (19.6–38.8 mS cm⁻¹). In addition, PSf-ImOH membranes have low methanol permeability of 0.8–4.7 × 10⁻⁷ cm² s⁻¹, one order of magnitude smaller than that of Nafion212 membrane. All these properties indicate imidazolium-functionalized polysulfone is very promising for potential applications in alkaline membrane direct alcohol fuel cells.     

Synthesis and characterization of crosslinked poly (vinyl alcohol)/layered double hydroxide composite polymer membranes for alkaline direct ethanol fuel cells

The low ionic conductivity and low thermal stability of conventional quaternary ammonium group functionalized anion-exchange membranes (AEM) are two key parameters that limit the performance of AEM direct ethanol fuel cells (AEM DEFCs). The present work is to address these issues by synthesizing crosslinked poly (vinyl alcohol)/layered double hydroxide (PVA/LDH) hybrid membranes with solution casting method. The experimental results indicate that incorporating 20 wt.% LDH into the PVA resulted in not only a higher ionic conductivity, but also a lower ethanol permeability. The performance test of the DEFC using the PVA/LDH hybrid membrane shows that the fuel cell can yield a power density of 82 mW cm⁻² at 80 °C, which is much higher than that of the AEM DEFC employing the quaternary ammonium group functionalized membrane. A constant current discharge test shows that the PVA/20LDH membrane can be operated stably at relatively high temperatures.     

Comparison of different types of membrane in alkaline direct ethanol fuel cells

It has been understood that the use of cation-exchange membranes (CEM) and alkali-doped polybenizimidazole membranes (APM) in alkaline direct ethanol fuel cells (DEFC) with an added base in the fuel exhibits performance similar to the use of anion-exchange membranes (AEM). The present work is to assess the suitability of the three types of membrane to alkaline DEFCs by measuring and comparing the membrane properties including the ionic conductivity, the species permeability, as well as the thermal and mechanical properties. The comparison shows that: (i) the AEM is still the most promising membrane for the alkaline DEFC, although the thermal stability needs to be further enhanced; (ii) before solving the problem of the poor thermal stability of AEMs, the CEM is another choice for the alkaline DEFC running at high temperatures (<90 °C); and (iii) the APM can also be applied to the alkaline DEFC operating at high temperatures, but its mechanical property needs to be substantially enhanced and the species permeability needs to be dramatically decreased.     

Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm⁻¹ at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H₂/O₂ fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm⁻² at 260 mA cm⁻².     

Preparation and properties of phosphoric acid doped sulfonated poly(tetra phenyl phthalazine ether sulfone) copolymers for high temperature proton exchange membrane application

Phosphoric acid-doped sulfonated poly(tetra phenyl phthalazine ether sulfone) (PA-SPTPPES) copolymers were successfully synthesized by the 4,4′-dihydroxydiphenylsulfone with 1,2-bis(4-fluorobenzoyl)-3,4,5,6-tetraphenylbenzene (BFBTPB) and 4,4′-difluorodiphenylsulfone in sulfolane. Poly(tetra phenyl phthalazine ether sulfone)s (PTPPESs) were prepared via an intramolecular ring-closure reaction of dibenzoylbenzene of precursor and hydrazine. The sulfonated poly(tetra phenyl phthalazine ether sulfone) (SPTPPES) membranes were obtained by sulfonation under concentrated sulfuric acid, and followed phosphoric acid-doped by immersion in phosphoric acid. Different contents of doped and sulfonated unit of PA-SPTPPES (10, 15, 20 mol% of BFBTPB) were studied by FT-IR, ¹H NMR spectroscopy, and thermo gravimetric analysis (TGA). The ion exchange capacity (IEC) and proton conductivity of SPTPPESs and PA-SPTPPESs were evaluated with increase of degree of sulfonation and doping level. The PA-SPTPPESs membranes exhibit proton conductivities (80 °C, relative humidity 30%) of 41.3 ∼ 74.1 mS/cm and the maximum power densities of PA-SPTPPES 10, 15, and 20 were about 294, 350, and 403 mW/cm².     

Stability enhancement of polymer electrolyte membrane fuel cells based on a sulfonated poly(ether ether ketone)/poly(vinylidene fluoride) composite membrane

Most proton-conducting membranes based on sulfonated aromatic polymers exhibit significant dimensional change by hydration, and this leads to degradation of fuel cell performance on prolonged operation. In this study, as a means of improving the stability of a polymer electrolyte membrane fuel cell, composite membranes employing a porous poly(vinylidene fluoride) (PVdF) substrate and sulfonated poly(ether ether ketone) (sPEEK) electrolyte are prepared and their hydration behaviours, including water uptake and dimensional change, are examined. The electrochemical characteristics of membrane/electrode assemblies using the sPEEK/PVdF composite membrane are also analyzed. The initial cell performance is comparable with that of a cell based on a pure sPEEK membrane. Furthermore, the stability of the cell using the sPEEK/PVdF composite membrane is considerably improved during a humidity cycle test wherein hydration and dehydration are periodically repeated.     

Double cross-linked polyetheretherketone proton exchange membrane for fuel cell

The proton exchange membrane based on polyetheretherketone was prepared via two steps of cross-linking. The properties of the double cross-linked membrane (water uptake, proton conductivity, methanol permeability and thermal stability) have been investigated for fuel cell applications. The prepared membrane exhibited relatively high proton conductivity, 3.2 × 10⁻² S cm⁻¹ at room temperature and 5.8 × 10⁻² S cm⁻¹ at 80 °C. The second cross-linking significantly decreased the water uptake of the membrane. The performance of direct methanol fuel cell was slightly improved as compared to Nafion^® 117 due to its low methanol permeability. The results indicated that the double cross-linked membrane is a promising candidate for the polymer electrolyte membrane fuel cell, especially for the direct methanol fuel cell due to its low methanol permeability and high stability in a methanol solution.     

Incorporation of H3PO4 into three-dimensional polyacrylamide-graft-starch hydrogel frameworks for robust high-temperature proton exchange membrane fuel cells

To enhance the anhydrous proton conductivities of proton exchange membranes, we report here the incorporation of H₃PO₄ into three-dimensional (3D) framework of polyacrylamide-graft-starch (PAAm-g-starch) hydrogel materials using extraordinary absorption of hydrogels to H₃PO₄ aqueous solution. Intrinsic microporous structure can close to seal H₃PO₄ molecules in the interconnected 3D frameworks of PAAm-g-starch after suffering from dehydration. The hydrogel membranes are thoroughly characterized by morphology observation, thermal stability, swelling kinetics, proton-conducting performances as well as electrochemical behaviors. The results show that the H₃PO₄ loadings and therefore the proton conductivities of the hydrogel membranes are dramatically enhanced by employing PAAm-g-starch matrix. H₃PO₄ loading of 88.68 wt% and an anhydrous proton conductivity as high as 0.046 S cm⁻¹ at 180 °C are recorded. A fuel cell using a thick membrane shows a peak power density of 517 mW cm⁻² at 180 °C by feeding with H₂/O₂ streams. The high H₃PO₄ loading, reasonable proton conductivity in combination with simple preparation, low cost and scalable matrix demonstrates the potential use of PAAm-g-starch hydrogel membranes in high-temperature proton exchange membrane fuel cells.     

Preparation of polytetrafluoroethylene porous membrane based composite alkaline exchange membrane with improved tensile strength and its fuel cell test

A strategy to improve the alkaline exchange membrane tensile strength was introduced based on porous PTFE and quaternized polyvinyl benzyl chloride (qPVBz/OH⁻). A thin PTFE film was used as the support to prepare the composite AAEM to improve the mechanical strength. Tensile stress tests showed 51.1 MPa which was twice that of the pure anion exchange membrane qPVBz/OH⁻. SEM observations showed that the pores of the PTFE were successfully filled with qPVBz/OH⁻ polymer and there was no structure defects in the cross-section of the resultant membrane. The synthesized composite membrane had very good thermal stability. The through plane ionic conductivity was in the range of 1.65 × 10⁻² to 2.2 × 10⁻² S cm⁻¹ in the temperature range of 25 °C–60 °C. The power density of H₂ and O₂ fuel cell gave 162 mW cm⁻².