Cross-linked Polybenzimidazole Membranes Research Articles

Mechanically strengthened polybenzimidazole membrane via a two-step crosslinking strategy for high-temperature proton exchange membrane fuel cell

To simultaneously increase the mechanical strength and proton conductivity of Phosphoric acid-doped polybenzimidazole (PA-PBI), a facile two-step crosslinking approach is proposed to prepare the imidazole-rich crosslinked polybenzimidazole membranes for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) in this work. Unlike the classic crosslinking approaches that usually enhance mechanical strength of membrane but lead to a decrease of proton conductivity, the mechanical strength of two-step crosslinked membrane improves from 3.93 MPa to 9.62 MPa and proton conductivity increases from 91 mS cm−1 to 183 mS cm−1 at 200 °C compared with pristine OPBI membrane. The promotion should be ascribed to the introduction of PA-affinity sites on crosslinkers and the formation of homogeneous crosslinking networks. Meanwhile, due to the crosslinked networks, the oxidative and dimensional stability of crosslinked membranes are also promoted. In addition, the two-step cross-linking method can avoid the self-reaction between cross-linking agents, thereby obtaining a more uniform cross-linking network, which is of great significance for the use of fuel cells. When the membranes are used in a fuel cells, they exhibited a maximum power density of 448.8 mW cm−2 at 160 °C (H2/Air, without humidification.).

Improving the ohmic polarization of high-temperature proton exchange membrane fuel cells using crosslinked polybenzimidazole membranes containing acidophilic quaternary ammonium groups synthesized by one-step strategy

Ingenious crosslinked network structure in phosphoric-acid-doped polybenzimidazole membranes can mitigate the mutual restriction of proton conductivity and mechanical properties. However, the complicated synthesis of tailored macromolecular crosslinker and the time-consuming post-treatment hinder their practical application as high-temperature proton exchange membranes. Herein, crosslinked polybenzimidazole membranes are synthesized using small molecular crosslinkers containing acidophilic quaternary ammonium groups through a one-step crosslinking strategy. After doping with phosphoric acid, the quaternary ammonium-biphosphate ion-pair coordination and the crosslinked structure result in the improved anhydrous proton conductivity, oxidation stability, and mechanical strength of the formed membranes compared to the sample without the crosslinking structure. A membrane with the optimized degree of crosslinking exhibits an anhydrous conductivity of 72.27 mS/cm at 160 °C, with a tensile strength of 12.14 MPa. Benefiting from the crosslinked structure and high proton conductivity, the accordingly formed membrane electrode assembly possesses a high open-circuit voltage of 1.01 V and animproved ohmic polarization, delivering a peak power density of 0.66 W/cm2 using hydrogen as fuel and air as oxidant.

Cross-Linked Polybenzimidazoles Containing Porous Organic Cages for High-Temperature Proton Conduction

We prepared a series of porous organic cages (POCs) with cross-linked polybenzimidazole membranes. Because of the introduction of the POC with numerous amine groups and ultramicroporous structures in the polymers, which enhanced the PA interactions and increased free volume, the POC-PBI membranes exhibited better proton conductivity and higher acid uptake. 20%-POC-OPBI showed the highest conductivity at 47.2 mS cm−1, followed by OPBI at 27.2 mS cm−1.

Construction of Highly Conductive Cross-Linked Polybenzimidazole-Based Networks for High-Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are of great interest to researchers in industry and academia because of their wide range of applications. This review lists some creative cross-linked polybenzimidazole-based membranes that have been prepared in recent years. Based on the investigation into their chemical structure, the properties of cross-linked polybenzimidazole-based membranes and the prospect of their future applications are discussed. The focus is on the construction of cross-linked structure of various types of polybenzimidazole-based membranes and their effect on proton conductivity. This review expresses the outlook and good expectation of the future direction of cross-linked polybenzimidazole membranes.

Finely tuning the microporosity in phosphoric acid doped triptycene-containing polybenzimidazole membranes for highly permselective helium and hydrogen recovery

High-performance polymer membranes with well-defined microporosity and size-sieving ability are especially attractive for helium and hydrogen recovery. Here we report novel macromolecular engineering of polybenzimidazole (PBI) membranes that integrate hierarchical triptycene units for high permeability and polyprotic acid doping for size sieving via controllable manipulation of microporous architecture. The triptycene moieties disrupt chain packing and introduce additional configurational free volume, leading to significantly boosted He and H2 permeabilities compared to previously reported PBI membranes. The acid doping resulted in crosslinked PBI membranes via hydrogen bonding and proton transfer with dramatically enhanced gas selectivities. Via adjusting the H3PO4-doping level, triptycene-based polybenzimidazole (TPBI) composite membranes (TPBI-(H3PO4)x) exhibit the highest gas selectivities for He enrichment (i.e., α(He/CH4) = 7052 ± 156) and H2 purification (i.e., α(H2/CH4) = 5128 ± 110) among existing polymeric gas separation membranes. Additionally, under mixed-gas conditions at 150 °C, the TPBI-(H3PO4)0.98 membrane displays a H2 permeability of 46.7 Barrer and a H2/CO2 selectivity of 16, far beyond the Robeson's 2008 upper bound for H2/CO2 separation. The facile and diverse tunability and excellent gas separation performance make TPBI-(H3PO4)x membranes highly attractive for helium and hydrogen separation.

Highly stable epoxy-crosslinked polybenzimidazole membranes for organic solvent nanofiltration under strongly basic conditions

Organic solvent nanofiltration (OSN) often needs to be conducted under extreme conditions in the chemical and food industries. In this work, we fabricated polybenzimidazole (PBI)-based OSN membranes that possess excellent stability under strongly basic conditions at pH 13. Two crosslinked PBI membranes were prepared by treating pristine PBI membranes with two epoxy-containing crosslinkers. Various permeation and characterization tests were performed to investigate the resistance of the membranes to solvents and basic environments. Both crosslinked membranes displayed reasonable permeance in pure solvents at room temperature under 10 bar operating pressure, along with a sharp molecular discrimination performance in ethanol environments. The crosslinked PBI membranes were found to be stable and did not completely lose their separation properties at high pH, although the effective pore size of the membranes increased presumably due to swelling. The separation performance of the membranes was found to be reversible between filtrations of neutral and basic aqueous solutions. Therefore, our crosslinked PBI membranes, which display exceptional stability, hold great potential for applications in OSN under basic conditions.

Facile preparation of polybenzimidazole membrane crosslinked with three-dimensional polyaniline for high-temperature proton exchange membrane

Phosphoric acid (PA) doped polybenzimidazole (PBI) membrane has been regarded as one of most effective high temperature proton exchange membrane (HT-PEM). However, it has to face poor mechanical properties at high acid doping levels (ADLs) and significant acid loss at high temperature during the operation of fuel cell. Herein, we prepare cross-linked PBI membranes containing different contents of the cross-linker three-dimensional polyaniline (PANI) via a facile N-substitution reaction. The obtained cross-linked PBI membrane exhibits superior tensile strength, stability and proton conductivity. More importantly, robust mechanical strength and good acid retention of the PA-doped cross-linked PBI membrane are demonstrated even at high ADL (above 20). The cross-linked PANI-30%-g-OPBI membrane exhibits a high ADL of 22.9, high proton conductivity of 122.8 mS cm−1 at 160 °C. Furthermore, the peak power density reaches 447 mW cm−2 at 160 °C, which is 2.5 times than that based on the corresponding PBI membrane. Molecular dynamics simulation results indicate that the decreased mobility of polymer chains improve the mechanical properties of the cross-linked membranes, and the high intermolecular interaction energy of the aniline-biphosphate ion pair significantly decrease PA leaching in the membranes due to the introduction of 3D molecular structure of PANI.

Cross-linked polybenzimidazoles containing hyperbranched cross-linkers and quaternary ammoniums as high-temperature proton exchange membranes: Enhanced stability and conductivity

High proton conductivity with sufficient stability for phosphoric acid (PA) doped polybenzimidazole membranes is critical for applications in fuel cells. Macromolecular cross-linkers with hyperbranched structures have large volumes and many functional groups, which can not only react with polymer backbones to form multiple cross-linked sites, but also provide favorable conditions for subsequent functional group modifications to regulate the interaction between polymers and PA; however, such cross-linkers are rarely exploited in fuel cells. Herein, a series of cross-linked polybenzimidazole membranes were successfully prepared based on a novel hyperbranched cross-linker and the incorporation of numerous quaternary ammonium groups. These cross-linked polybenzimidazole membranes containing a hyperbranched cross-linker and quaternary ammonium groups showed superior performance, in terms of mechanical properties, oxidative resistance and proton conductivity. The tensile strength of the PA doped cross-linked membranes was more than 20.0 MPa. These cross-linked membranes showed only a slight weight loss and no cracks after immersion in Fenton's reagent for 200 h, while the linear membrane was broken into pieces after immersion in Fenton's reagent for 100 h. With low acid loading, the membranes containing cross-linked polybenzimidazole with quaternary ammonium groups still exhibited good conductivity. As a result of the excellent comprehensive properties, the obtained fuel cell based on the membrane with 15% of the hyperbranched cross-linker showed a great power density of 260 mW cm−2, which was 36.8% higher than that of the fuel cell based on the corresponding linear membrane.

Crosslinked polybenzimidazoles containing branching structure as membrane materials with excellent cell performance and durability for fuel cell applications

Crosslinking is an effective method to improve the properties of high temperature proton exchange membranes based on polybenzimidazole. However, the compact structure of crosslinked polybenzimidazole hinders the phosphoric acid absorption of the membranes, resulting in a relatively poor fuel cell performance. Recently, we find that branched polymers can absorb more phosphoric acid with a larger free volume, but suffer from deteriorated mechanical strength. In this work, a new method is proposed to obtain excellent over-all properties of high temperature proton exchange membranes. A series of crosslinked polybenzimidazoles containing branching structure as membrane materials are successfully prepared for the first time. Compared with conventional crosslinked membranes, these crosslinked polybenzimidazole membranes containing branching structure exhibit a higher phosphoric acid doping level and proton conductivity, improved durability, lower swelling rate and comparable mechanical strength. In particular, the fuel cell base on the crosslinked and branched membrane with a 10% ratio of crosslinker in non-humidified hydrogen/air at 160 °C achieves a power density of 404 mW cm−2. The results indicate that the combination of crosslinking and branching is an effective approach to improve the properties of polybenzimidazole membrane materials.

Fabrication of crosslinked polybenzimidazole membranes by trifunctional crosslinkers for high temperature proton exchange membrane fuel cells

Two trifunctional bromomethyls containing crosslinkers, 1,3,5-tris(bromomethyl)benzene (B3Br) and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (Be3Br), are employed to covalently crosslink polybenzimidazole (PBI) membranes for the high temperature proton exchange membrane fuel cell. The presence of three bromomethyl groups in each crosslinker molecule is expected to create more free volume for acid doping while enhancing the adhesive strength of the PBI chains. In addition, the influence of the two crosslinker structures on the property of the crosslinked membranes is compared and analyzed. All the crosslinked PBI membranes exhibit longer morphology durability over the pristine PBI membrane toward the radical oxidation. Moreover, the crosslinked PBI membranes with the crosslinker Be3Br containing three ethyl groups display superior acid doping level, high conductivity and excellent mechanical strength simultaneously, over those with the crosslinker B3Br and the pristine PBI membrane. Single cell measurements based on the acid doped membrane with a crosslinking degree of 7.5% by Be3Br demonstrate the technical feasibility of the prepared membranes for high temperature proton exchange membrane fuel cells.

Strengthening Phosphoric Acid Doped Polybenzimidazole Membranes with Siloxane Networks for Using as High Temperature Proton Exchange Membranes

Two silane compounds, i.e., ((chloromethyl)phenylethyl)trimethoxysilane (Ph‐Si) and 3‐chloropropyltriethoxysilane (Pr‐Si), are employed as covalent crosslinkers to fabricate high temperature proton exchange membranes based on polybenzimidazole (PBI) in order to better understand the correlation between the structure and the property of the crosslinked membranes. All the crosslinked PBI membranes display longer morphology durability over the neat PBI membrane toward the radical oxidation. The crosslinked membranes with the crosslinker containing a rigid group of phenylene (PBI‐Ph) show superior acid doping level, high conductivity, and excellent mechanical strength simultaneously, comparing to those with the crosslinker having a flexible alkyl chain (PBI‐Pr) and the pristine PBI based membrane. The technical feasibility for using as high temperature proton exchange membranes is demonstrated by the acid doped crosslinked PBI‐Ph membranes via fuel cell tests. image

Long-term durability of HT-PEM fuel cells based on thermally cross-linked polybenzimidazole

Abstract Long-term durability of high temperature polymer electrolyte membrane fuel cells based on thermally cross-linked polybenzimidazole membranes was studied and compared with reference membranes based on linear polybenzimidazole. The test was conducted at 160 °C under constant load currents of 200 mA cm −2 for periods of 1000, 4400, and 13,000 h. Extensive beginning-of-life (BoL) and end-of-test (EoT) characterisation was carried out, and disturbance of the steady state operated cells was minimised by limiting in-line diagnostics to the low-invasive technique of electrochemical impedance spectroscopy (EIS). Up until the operating time of 9200 h, the cell equipped with the cross-linked membrane showed an average degradation rate of 0.5 μV h −1 , compared to 2.6 μV h −1 for the reference membrane, though parallel tests for a shorter period of time showed deviations, likely due to malfunctioning contact between layers or cell components. For the full test period of 13,000 h, the average voltage decay rate was about 1.4 and 4.6 μV h −1 for cells equipped with cross-linked and linear polybenzimidazole membranes, respectively. EIS and post-test analysis revealed that the cross-linked membrane showed better stability in terms of area specific resistance due to improved acid retention characteristics.

Crosslinked polybenzimidazole membranes for organic solvent nanofiltration (OSN): Analysis of crosslinking reaction mechanism and effects of reaction parameters

Recently, polybenzimidazole (PBI) membranes crosslinked with dibromoxylene (DBX) were shown to retain their molecular separation performance in the harsh conditions characteristic of organic solvent nanofiltration (OSN). This work is focused on better understanding of the crosslinking reaction between PBI and DBX, and finding the parameters important for achieving higher degrees of crosslinking. A statistical approach based on Design of Experiments was used to identify the most significant parameters and interactions affecting the crosslinking reaction. High gain in weight and high bromine content after the reaction are expected to be indirectly related to membranes with high crosslinking degrees. Hence, these two responses were measured as a function of reaction temperature, reaction time, excess of DBX, concentration of DBX and reaction solvent (acetonitrile and toluene). All parameters were found to have a positive effect on both responses, and the reaction was found to be faster in acetonitrile than in toluene. All obtained results were statistically evaluated using Analysis of Variance, and a physical interpretation of the statistical models was attempted.

Hydrophilically surface-modified and crosslinked polybenzimidazole membranes for pervaporation dehydration on tetrahydrofuran aqueous solutions

Chemical modification has been carried out on polybenzimidazole (PBI) membranes for pervaporation dehydration on tetrahydrofuran (THF)/water mixtures. The PBI membrane crosslinked with 30wt% of a polybenzoxazine (CR-PBI-30) shows good stability and high separation performance for pervaporation dehydration on the THF aqueous solutions in a wide range of concentrations above 50wt%. Incorporation of poly(styrene sulfonic acid) (PSSA) chains to CR-PBI-30 membrane surface increases the surface hydrophilicity and water dissolution/absorption of the resulting membrane (CR-PBI-30-PSSA), so as to increase its water permeability without sacrificing membrane selectivity. The CR-PBI-30-PSSA membrane shows extremely high separation factors (the water concentrations at the permeate side (Cw)>99.99wt%) and moderate permeation fluxes (about 180gh−1m−2) for pervaporation dehydration on the THF aqueous solutions in concentrations above 80wt%. The membrane also shows a Cw>97.5wt% and a water flux of 220gh−1m−2 for a 70wt% THF aqueous solution. This work demonstrates an effective approach to prepare membranes for pervaporation dehydration on aqueous solutions of high polar solvents, especially for the feed solutions possessing high water contents.

Beyond polyimide: Crosslinked polybenzimidazole membranes for organic solvent nanofiltration (OSN) in harsh environments

In this work, we report a new class of organic solvent nanofiltration (OSN) membranes fabricated from polybenzimidazole (PBI) which exhibit superior chemical stability compared to other well-known polymeric membranes such as polyimide. Integrally skinned asymmetric PBI membranes were prepared and crosslinked using either an aliphatic or an aromatic bifunctional crosslinker. Three batches of membranes with the same composition were prepared and crosslinked with each crosslinker. The membrane performance showed excellent reproducibility in organic solvents and water in terms of flux and retention profile. Also, both membranes showed good tolerance towards extreme pH conditions. To critically assess their chemical stability the membranes were tested in realistic chemical process conditions that employ different types of acids and bases, e.g. concentrated dichloroacetic acid in acetonitrile, piperidine in N,N-dimethylformamide (DMF) and monoethanolamine in water. The membranes modified with aliphatic crosslinker could not retain their properties when DMF was used as the organic solvent. This was found to be due to dissolution of PBI in DMF rather than degradation due to pH exposure. On the other hand, it was shown that the membrane modified with the aromatic crosslinker exhibits superior stability and higher permeances in comparison to the membrane crosslinked with the aliphatic crosslinker. The results obtained show that membranes fabricated from crosslinked PBI were stable and have the potential for applications in chemically harsh conditions found in processes ranging from pharmaceutical to petrochemical industries.

Synthesis of novel cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells

Polybenzimidazole (PBI) copolymer containing hydroxyl functional groups was synthesized in this study. Two series of cross-linked PBI membranes have been prepared based on the copolymer with two different crosslinkers reacted with the hydroxyl groups on the PBI main chain. The resultant membranes were investigated in terms of oxidative stability, thermal properties, mechanical properties, and proton conductivity. The mechanical and thermal properties were enhanced in the cross-linked membranes compared to the non-cross-linked membrane. In addition, the thermal degradation temperature of the membranes increased with increasing degree of cross-linking. The Fenton's test was used as the reference of the oxidative stability of the membranes. It was found that cross-linking led to a significant improvement in oxidative stability of the membranes. The proton conductivity of phosphoric acid doped cross-linked PBI membranes could reach 8×10−3Scm−1 at 160°C.

Novel epoxy-based cross-linked polybenzimidazole for high temperature proton exchange membrane fuel cells

An approach has been proposed to prepare the reinforced phosphoric acid (PA) doped cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells (HT-PEMFCs), using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker. FT-IR measurement and solubility test showed the successful completion of the crosslinking reaction. The resulting cross-linked membranes exhibited improved mechanical strength, making it possible to obtain higher phosphoric acid doping levels and therefore relatively high proton conductivity. Moreover, the oxidative stability of the cross-linked membranes was significantly enhanced. For instance, in Fenton’s reagent (3% H 2O 2 solution, 4 ppm Fe 2+, 70 °C), the cross-linked PBI-NGDE-20% membrane did not break into pieces and kept its shape for more than 480 h and its remaining weight percent was approximately 65%. In addition, the thermal stability was sufficient enough within the operation temperature of PBI-based fuel cells. The cross-linked PBI-NGDE-X% (X is the weight percent of epoxy resin in the cross-linked membranes) membranes displayed relatively high proton conductivity under anhydrous conditions. For instance, PBI-NGDE-5% membrane with acid uptake of 193% exhibited a proton conductivity of 0.017 S cm −1 at 200 °C. All the results indicated that it may be a suitable candidate for applications in HT-PEMFCs.

Cross-linked polybenzimidazole with enhanced stability for high temperature proton exchange membrane fuel cells

Cross-linked polybenzimidazole membranes were obtained by heating at 160 °C, using 4,4′-diglycidyl(3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP) as the cross-linker. The cross-linking reaction temperature was determined by DSC and the successful completion of the cross-linking reaction was shown by FTIR and solubility tests. The cross-linked membranes showed high proton conductivity and strong mechanical properties, as well as low swelling after immersion in 85% phosphoric acid at 90 °C. For instance, the membrane with a cross-linker content weight percent of 20% (PBI-TMBP 20%) with a PA doping level of 4.1 exhibited a proton conductivity of 0.010 S cm−1 and a low swelling volume of 50%. Moreover, the cross-linked membranes showed excellent oxidative stability. The PBI-TMBP 20% cross-linked membrane tested in Fenton's reagent (3% H2O2 solution, 4 ppm Fe2+, 70 °C) kept its shape for more than 480 h and did not break. In particular, the proton conductivity of the PA-PBI-TMBP 20% membrane after Fenton's test (30% H2O2, 20 ppm Fe2+, 85 °C) remained at a high level of 0.009 S cm−1. This investigation proved that cross-linking is a very effective approach for improving the performance of proton exchange membranes.