Polybenzimidazole Membranes Research Articles

Aqueous Redox Flow Cells Utilizing Verdazyl Cations enabled by Polybenzimidazole Membranes.

Non-aqueous organic redox flow batteries (RFB) utilizing verdazyl radicals are increasingly explored as energy storage technology. Verdazyl cations in RFBs with acidic aqueous electrolytes, however, have not been investigated yet. To advance the application in aqueous RFBs it is crucial to examine the interaction with the utilized membranes. Herein, the interactions between the 1,3,5-triphenylverdazyl cation and commercial Nafion 211 and self-casted polybenzimidazole (PBI) membranes are systematically investigated to improve the performance in RFBs. The impact of polymer backbones is studied by using mPBI and OPBI as well as different pre-treatments with KOH and H3PO4. Nafion 211 shows substantial absorption of the 1,3,5-triphenylverdazylium cation resulting in loss of conductivity. In contrast, mPBI and OPBI are chemically stable against the verdazylium cation without noticeable absorption. Pre-treatment with KOH leads to a significant increase in ionic conductivity as well as low absorption and permeation of the verdazylium cation. Symmetrical RFB cell tests on lab-scale highlight the beneficial impact of PBI membranes in terms of capacity retention and I-V curves over Nafion 211. With only 2 % d-1 capacity fading 1,3,5-triphenylverdazyl cations in acidic electrolytes with low-cost PBI based membranes exhibit a higher cycling stability compared to state-of-the-art batteries using verdazyl derivatives in non-aqueous electrolytes.

Theoretical Studies on the Chemical Degradation and Proton Dissociation Property of PBI used in High-Temperature Polymer Electrolyte Membrane Fuel Cells.

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are gaining more and more attention due to their higher efficiency than low-temperature ones. Polybenzimidazole (PBI) membranes are the most popular membranes used in HT-PEMFCs. However, their chemical stability and chemical degradation mechanisms, which directly affect the lifetime of fuel cells, have been hardly reported. We applied the density functional theory and used ABPBI as an example membrane to investigate the chemical degradation mechanisms of PBI membranes. The possible degradation mechanisms that occurred on eight sites have been proposed, where sites 2 and 3 located on the phenyl ring are determined as two weak sites toward OH radical and oxygen molecule attack. When the terminal is the H atom at site 7, it is also weak under OH radical attack. Regarding these, the substituent effect on the chemical stability of polymers has been studied. By introducing four -C2F5 or -CN groups, the barrier heights of the corresponding degradation reactions are increased; thus, the chemical stabilities of related membranes are improved. The selection of terminal atoms was also explored for alleviating the chemical degradation of the membrane. The investigated proton transfer properties of nine model compounds revealed that introducing four -C2F5 or -CN groups improves the proton dissociation properties occurring at the cathode. The increase of phosphoric acid concentration is helpful for the proton transfer at both the membrane and the cathode. This work may hopefully help the design and synthesis of HT-PEMFCs with good stability and high efficiency.

Phosphoric Acid Electrolyte Uptake and Retention Analysis on UiO‐66‐NH2 Polybenzimidazole Nanocomposite Membranes

ABSTRACTHigh‐temperature proton exchange membrane fuel cells (HT‐PEMFCs) have a major advantage over low‐temperature fuel cells due to their better tolerance to higher carbon monoxide content in the hydrogen feed, simpler fuel processing, and better heat management. However, a key challenge in the development of HT‐PEMFCs is the potential for acid leaching from phosphoric acid‐doped polybenzimidazole membranes, which can reduce overall fuel cell performance. This study investigates the effect of post‐synthetic modification of the UiO‐66‐NH2 metal–organic framework (MOF) on the acid electrolyte uptake and retention of MOF/poly(4,4ʹ‐diphenylether‐5,5ʹ‐bibenzimidazole) (OPBI) nanocomposite membranes. Thermogravimetric analysis (TGA) was used to correlate the membrane properties with acid uptake. This work revealed that the presence of MOF with functional groups that can form hydrogen bonds with phosphoric acid molecules was able to alleviate the acid retention in the OPBI membrane with lower acid uptake. TGA demonstrated that the lower bound moisture content in the nanocomposite membranes was correlated to the lower acid uptake. In addition, the thermal stability of the nanocomposite membranes was found to improve.

Effects of different ratios of flexible links and rigid structures in side chains on membrane properties for HT-PEM applications

A series of grafted polybenzimidazoles with ratios of flexible links and rigid structures in side chains based on amino polybenzimidazole (AmPBI) was successfully prepared by simple N-substitution reaction. Due to the adequate number of amino groups on the AmPBI backbone, sufficient N–H sites reduce consumption of imidazole group during grafting reactions, while simultaneously elevating the number of proton transfer site. The grafted polybenzimidazole membranes with ratios of flexible links and rigid structures in side chains have excellent properties especially in the phosphoric acid (PA) uptake, pretty swelling ratio and proton conductivity. Importantly, PA uptake, swelling ratio, mechanical property, proton conductivity and power density can be controlled by changing the ratios of flexible links and rigid structures. The grafted membrane (AmPBI-P3) with the longer flexible link in the side chain shows a low swelling ratio of 128.62%, which is lower than that of other AmPBI-PX and AmPBI. Moreover, AmPBI-P3 shows a high proton conductivity of 126.9 mS cm−1 which is about 1.74 times higher than pure AmPBI (73.1 mS cm−1) and still reaches 91.4 mS cm−1 at 180 °C after the PA retention capacity test under 160 °C/0%RH for 240 h. A great power density of 738.5 mW/cm2 at 180 °C with dry H2–O2 and no backpressure is impressive among many reported membranes.

Enhancing Performance and Stability of High-Temperature Proton Exchange Membranes through Multiwalled Carbon Nanotube Incorporation into Self-Cross-Linked Fluorenone-Containing Polybenzimidazole.

Addressing critical challenges in enhancing the oxidative stability and proton conductivity of high-temperature proton exchange membranes (HT-PEMs) is pivotal for their commercial viability. This study uncovers the significant capacity of multiwalled carbon nanotubes (MWNTs) to absorb a substantial amount of phosphoric acid (PA). The investigation focuses on incorporating long-range ordered hollow MWNTs into self-cross-linked fluorenone-containing polybenzimidazole (FPBI) membranes. The absorbed PA within MWNTs and FPBI forms dense PA networks within the membrane, effectively enhancing the proton conductivity. Moreover, the exceptional inertness of MWNTs plays a vital role in reinforcing the oxidation resistance of the composite membranes. The proton conductivity of the 1.5% CNT-FPBI membrane is measured at 0.0817 S cm-1 at 160 °C. Under anhydrous conditions at the same temperature, the power density of the 1.5% CNT-FPBI membrane reaches 831.3 mW cm-2. Notably, the power density remains stable even after 200 h of oxidation testing and 250 h of operational stability in a single cell. The achieved power density and long-term stability of the 1.5% CNT-FPBI membrane surpass the recently reported results. This study introduces a straightforward approach for the systematic design of high-performance and robust composite HT-PEMs for fuel cells.

Double cross-linked 3D layered PBI proton exchange membranes for stable fuel cell performance above 200 °C

Phosphoric acid doped proton exchange membranes often experience performance degradation above 200 °C due to membrane creeping and phosphoric acid evaporation, migration, dehydration, and condensation. To address these issues, here we present gel-state polybenzimidazole membranes with double cross-linked three-dimensional layered structures via a polyphosphoric acid sol-gel process, enabling stable operation above 200 °C. These membranes, featuring proton-conducting cross-linking phosphate bridges and branched polybenzimidazole networks, effectively anchor and retain phosphoric acid molecules, prevent 96% of its dehydration and condensation, improve creep resistance, and maintain excellent proton conductivity stability. The resulting membrane, with superior through-plane proton conductivity of 0.348 S cm−1, delivers outstanding peak power densities ranging from 1.20–1.48 W cm−2 in fuel cells operated at 200-240 °C and a low voltage decay rate of only 0.27 mV h−1 over a 250-hour period at 220 °C, opening up possibilities for their direct integration with methanol steam reforming systems.

Carbon dioxide hydrogenation to methanol by flame-deposited CuO/ZrO2-polymer membrane reactors

Carbon dioxide hydrogenation for methanol synthesis, within the framework of carbon dioxide utilization represents a promising strategy. Nevertheless, high conversion of carbon dioxide into methanol presents significant challenges, primarily arising from thermodynamic constraints and catalyst deactivation due to the formation of water as a by-product. In response to these challenges, this investigation focused on the development of an innovative hybrid catalytic membrane reactor utilizing flexible polymeric membranes with flame-made catalysts. The nanostructured CuO/ZrO2 thin layers were synthesized by flame spray pyrolysis and directly deposited on two polyimide polymers and polybenzimidazole membranes. The CuO/ZrO2-polymer membranes are reduced by H2 at 300 °C resulting in Cu/ZrO2-polymer nanocomposites. This novel hybrid membrane design facilitated the efficient separation of by-product water from the reaction environment, effectively promoting carbon dioxide conversion and ultimately leading to enhanced methanol production rate. At 200 °C and 20 bar, these catalytic membrane reactors demonstrate exceptional performance, achieving 113 % improvement in carbon dioxide conversion and a remarkable 106 % increase in methanol production rate compared to conventional catalytic fixed bed reactors. Overall, the catalytic membrane, synthesized using flame spray pyrolysis, has shown long-term stability and ability to suppress catalyst deactivation caused by water, ensuring sustained catalyst stability throughout the reaction.

Poly(phenylene oxide) cross-linked with polybenzimidazole for the applications of high-temperature proton-exchange membrane fuel cells

A series of imidazolium-functionalized poly(phenylene oxide) (QPPO) cross-linked with polybenzimidazole (PBI) (cQPPOx-PBI) membranes were prepared. Compared to the pure PBI membrane without a cross-linked structure nor imidazolium groups, the cQPPOx-PBI membranes showed enhanced toughness, phosphoric acid (PA) retention, and chemical stability. The cQPPOx-PBI membranes showed a enhanced PA uptake than that of PBI membranes, and the cQPPO35-PBI membrane achieves a high conductivity of 139 mS cm−1 at 160 °C, which is much higher than that of PBI membrane under the same conditions (26 mS cm−1). The performance of the single fuel cell assembled with the obtained membranes showed a similar trend as their conductivity, and the cell with cQPPO35-PBI showed a maximum power density of 344 mW cm−2 at 140 °C, while the value for PBI membrane was only 175 mW cm−2 under the same conditions. Herein, a straightforward method was developed for introducing imidazolium groups and cross-linked structures, which can be employed to prepare PEMs with enhanced performance for HT-PEMFCs application.

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.).

Experimental investigation and comparison of PBI/MWCNT and PSF/MWCNT membranes for recovering water from RO reject of brackish water by FO

The performances of polybenzimidazole (PBI) and polysulfone (PSF) membranes for recovering water from reverse osmosis (RO) reject of brackish water through forward osmosis (FO) were assessed and compared. Non-functionalised multi-walled carbon nanotubes (MWCNT) were added to the membrane casting solutions, with concentrations ranging from 0 to 3 wt%. The experiment was conducted for eight samples using RO reject of brackish water as the feed solution (FS) and 2 M analytical grade MgCl2 as the draw solution (DS). The hydrophilicity, water permeability, salt rejection rate (Rs), water flux (WF) and porosity of the membranes improved with increasing MWCNT content up to 2 wt%. Also, the structural parameter, salt permeability and reverse solute flux decreased. PBI/MWCNT2 wt% exhibited the best performance among the membranes tested compared with porosity of 70 ± 4 %, structural parameter of 0.36 ± 0.2 μm, and Rs of 93.5 %. In contrast with the pristine PBI membrane, an average water flux enhancement of 15 % and 49 % was observed for the FS and DS sides, respectively, for PBI/MWCNT2 wt%. It is evident from the results that including MWCNT improves the performance of both membranes, with better relative performance for PBI membranes than PSF membranes.

Hydrophilic-hydrophobic Janus polybenzimidazole membrane electrode assemblies regulate hydrogen evolution for high efficient electrochemical CO2 reduction

The membrane Electrode Assemblies (MEAs) electrolyzers are the most attractive systems for the electrolytic conversion of CO2 into commodity chemicals and fuels at commercially relevant current densities. Suppressing the hydrogen evolution reaction (HER) is vital to the highly efficient electrochemical CO2 reduction reaction (CO2RR). However, little attention has been paid to the HER. Herein, polybenzimidazole (PBI) Janus membranes with significant hydrophobic/hydrophilic asymmetric surface wettability were constructed for regulating the HER in CO2RR. The HER was modulated by adjusting the microstructure and surface hydrophobicity of the membrane as well as the CO2 feeding method. The p-PBI-HCF MEA showed a significant suppression of HER and a 6-fold improvement in CO selectivity over p-PBI MEA. The p-PBI-HCF exhibited superior CO2RR performance in MEA compared to commercial membranes (FAA-3-50 and Nafion115). At 2.0–3.0 V, the CO Faraday efficiency (FECO) of the p-PBI-HCF MEA electrolyzer remained above 92%. The optimal energy efficiency range was between 2.0 and 2.4 V when the FECO was close to 100%. The p-PBI-HCF provided 92% FECO and a current density of 225 mA cm−2 at 3 V. This work provides guidelines for the development of viable PBI membranes and MEAs for CO2RR.

A simple approach to balancing conductivity and capacity fade in vanadium redox flow batteries by the tunable pretreatment of polybenzimidazole membranes

The membrane, a critical component of a vanadium redox flow battery (VRFB) cell, largely governs the battery performance through its barrier properties and ohmic resistance, as well as contributes substantially to cell stack cost in commercial systems. Polybenzimidazole (PBI) is emerging as a promising commercially available material for VRFB membranes with properties that can be tailored through suitable low-cost pretreatments. Herein, we demonstrate how pre-swelling in blended alkaline solutions can be a highly effective and accessible strategy to improve PBI conductivity for application as a VRFB membrane, whilst enabling the tunable balancing of trade–off relations with electrolyte crossover that lead to undesirable capacity fade. A figure of merit is determined, showing that the inclusion of fractions of both NaOH and KOH in the pretreatment mixture is deemed necessary to ensure that both vanadium permeability and area-specific resistance are sufficiently minimized, respectively, in the membrane by achieving optimal, intermediate levels of swelling. By applying blended pretreatments, PBI membranes could be tailored to result in high performance cell tests, exhibiting stable and high energy efficiencies of ∼85 % with minimal capacity fade over 100 cycles at 120 mA·cm−2.

Building water molecule chains in polybenzimidazole membrane toward superior vanadium redox flow battery

Optimizing the proton conductivity of polybenzimidazole (PBI) membrane is of great significance to realize vanadium redox flow battery (VRFB) with the superior performance and low cost for commercialization. However, the fundamental reason of the poor proton conductivity of PBI, which severely hinders its application in VRFB, is not addressed yet. To tackle this issue, hydrogen bonds between PBI chains are scissored through a deprotonation strategy by using KOH/ethanol as the casting solvent in this study, weakening the interaction between PBI chains and enlarging the free volume to build water molecule chains inside. The water molecule chains effectively promote the proton transfer under Grotthuss mechanism, increasing the proton conductivity by over 1400 %. The energy efficiency of the VRFB assembled with the engineered membrane is improved from 71.5 % to 83.3 % under 80 mA cm−2, in addition to preserving the membrane’s ability to inhibit the permeability of vanadium ions as well as maintaining the structural stability. This study innovates a facile strategy for membrane optimization, and the profound analysis of structure–property relationship of PBI membrane is highly critical for guiding the development of membrane.

Achieving 1060mWcm-2 with 0.6mgcm-2 Pt Loading Based on Imidazole-Riched Semi-Interpenetrating Proton Exchange Membrane at High-Temperature Fuel Cells.

Enhancing phosphoric acid (PA) doping in polybenzimidazole (PBI) membranes is crucial for improving the performance of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, excessive PA uptake often leads to drawbacks such as PA loss and compromised mechanical properties when surpassing PA capacity of PBI basic functionality. Herein, a new strategy that integrates high PA uptake, mechanical strength, and acid retention is proposed by embedding linear PBI chains into a crosslinked poly(N-vinylimidazole) (PVIm) backbone via in-situ polymerization. The imidazole (Im)-riched semi-interpenetrating polymer network (sIPN) membrane with high-density nitrogen moieties, significantly enhancing the PA doping degree to 380% shows an excellent conductivity (0.108Scm-1). Meanwhile, the crosslinking structure in the sIPN membrane ensures adequate mechanical properties, low hydrogen permeability, and a relatively low swelling ratio. As a result, the single cell based on the membrane achieves the highest power density of 1060mWcm-2 with a low Pt loading (0.6mgcm-2) up to now and exhibits excellent fuel cell stability.

A polybenzimidazole-covalent organic framework hybrid membrane with highly efficient proton-selective transport channels for vanadium redox flow battery

Polybenzimidazole (PBI) is considered as the one of the most promising materials for proton exchange membranes (PEMs) owing to its negligible crossover and outstanding chemical stability. However, the low proton conductivity of PBI membrane restricts its commercial application in vanadium flow battery (VFB). Herein, the PBI hybrid membranes with high proton conductivity were prepared by introducing crystalline covalent organic frameworks (TpPa and TpPa-SO3H (STpPa)). Owing to the highly ordered sub-nanometer pores within TpPa and STpPa, faster proton highway was constructed in the hybrid membrane. Meanwhile, the sulfonic acid groups introduced to STpPa further provided additional proton transport sites and vanadium permeation barriers to achieve high ion selectivity. Accordingly, the cell assembled with PBI hybrid membrane with 3% STpPa achieved superior voltage efficiency (95.22%–74.84%) and energy efficiency (91.38%–74.38%) at 40–240 mA cm−2, and exhibited superb electrochemical stability during 800 cycles at 160 mA cm−2. This work offers a facile and efficient strategy to construct high-performance PBI-based PEMs by incorporating functional porous materials, which has incredible prospect for use in VFBs.

Solvent-resistant crosslinked polybenzimidazole membrane for use in enhanced molecular separation

In molecular separation using membranes, enhanced parameters, such as an increased flux and selectivity, are required for efficiency. Additionally, in molecular separation using solvents, solvent resistance is critical in preventing solvent-induced damage. However, most membranes lack resistances to specific harsh solvents, and simultaneously enhancing solvent flux and selectivity performance, which exhibit a trade-off relationship, is challenging. In this study, a high-performance solvent-resistant polybenzimidazole (PBI) membrane is fabricated via chemical crosslinking with divinyl sulfone (DVS). The membrane consists of porous finger-like support and dense skin layers, and the uniform DVS crosslinking of the PBI membrane is confirmed via chemical analysis. The DVS-crosslinked PBI membrane exhibits a superior solvent stability and separation performance, with an ethanol permeance and molecular weight cut-off of 5.88 L m−2 h−1 bar−1 and 483 g mol−1, respectively. These values represent an enhanced permeance and rejection performance compared to those of the pristine membrane. The DVS-crosslinked PBI membrane also displays a long-term stability in mepenzolate bromide separation over 72 h.

Stability and performance of in-situ formed phosphosilicate nanoparticles in phosphoric acid-doped polybenzimidazole composite membrane fuel cells at elevated temperatures

One of the effective strategies to pursue the highly durable high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) is to introduce inorganic fillers to the phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. Among the inorganic fillers, phosphates such as phosphosilicate are effective in mitigating acid loss at elevated temperatures (200–300 °C). In this paper, the effect of in situ formed phosphosilicate on the performance and stability of SiO2/PA/PBI composite membranes is studied in detail. The mechanical properties and electrochemical performances of the in situ formed SiO2/PA/PBI membranes depend strongly on the content of in situ formed Si5P6O25 fillers and its distribution and microstructure in the membrane. Such in situ formed SiO2/PA/PBI composite membranes show a high conductivity of 53.5 mS cm−1 at 220 °C. The assembled single cell shows a maximum peak power density (PPD) of 530.6 mW cm−2 and excellent stability at elevated temperature of 220 °C for over 130 h. The exceptional stability at 220 °C is most likely due to the existence of predominant amorphous phosphosilicate phases in the in situ formed SiO2/PA/PBI composite membranes, which inhibits the evaporation and leaching of PA at elevated temperatures. The results indicate the practical application of in situ formed SiO2/PA/PBI composite membranes for HT-PEMFCs.

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.

Silane-crosslinked polybenzimidazole with different hydroxyl content for high-temperature proton exchange membrane

High temperature proton exchange membrane fuel cells (HT-PEMFCs) can operate over 100 °C without humidification, which avoids the catalyst poisoning and enhances the efficiency of the power conversion. However, the classic phosphoric acid (PA) doped polybenzimidazole (PBI) still remains the challenge in the trade-off between mechanical strength and proton conductivity. The γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560), as a crosslinking agent, is widely used to improve the mechanical properties of membranes. Although it can enhance the mechanical strength of PBI membranes, it sacrifices the hydrogen of imidazole groups, resulting in a decrease in proton conductivity. In this present work, silane-crosslinked PBI proton exchange membranes (PEMs) were synthesized by the reaction between the pyridine-bridged PBI containing different hydroxyl (-OH) content with KH560. Through comprehensive structural characterization of the polymer and model compound, it has been determined that this crosslinking method does not compromise the N–H acidic sites on the imidazole ring. The crosslinking agent KH560 provides additional hydroxyl groups to PBI and participates in the construction of hydrogen bonding networks after the ring-opening reaction. It was found that the addition of pyridine and hydroxyl groups could promote the phosphoric acid uptake, and the appropriate degree of silane crosslinking could maintain the PBI membranes with reasonable mechanical properties. Proton conductivity and HT-PEMFCs performance of the obtained crosslinked films were further enhanced with the dimensional stability, thermal stability and mechanical properties. Among them, CPyOPBI–OH–20 presented the best overall performance, exhibiting the highest proton conductivity of 0.095 S cm−1 and the maximum power density of 767 mW cm−2 at 180 °C under the anhydrous condition, which was 4.2 times that of OPBI (182 mW cm−2).

Cucurbit[6]uril builds a water channel for improving the water/salt selectivity of polybenzimidazole

Polybenzimidazole (PBI) has garnered significant interest in desalination due to its exceptional chemical stability and mechanical strength, but suffers from poor water permeability. In this work, different contents of hydroxylated Cucurbit [6]uril (CB [6] were added to PBI to build a “water transport channel”. The effects of CB [6] addition on the chemical structure and surface morphology were analyzed by FTIR, SEM, and Zeta potential, respectively. Based on the solution-diffusion model, the effect of CB [6] on the water/salt transport properties of PBI membranes was thoroughly investigated. The results show that appropriate CB [6] doping can enhance water sorption of PBI through the special “outer hydrophilic and inner hydrophobic cavity” structure, and boost water diffusion with the channeling effect of the hydrophobic CB [6] cavity. While the complexation between the carbonyl group in CB [6] and Na+ ions slightly increased salt sorption, the specific cavity structure of CB [6] can significantly decrease salt diffusion by blocking salt ions and increasing transport path tortuosity. The cavity effect of CB [6] was further confirmed by hybridizing CB [6] with biphenylsulfonyl-substituted PBI when the cavity can be occupied. Therefore, the introduction of CB [6] improves the water/salt diffusion selectivity and then the permeability selectivity of PBI. Notably, the PBI@CB [6] membrane with a 0.25 wt% CB [6] loading exhibited a 40-fold increase in water permeability coefficient compared to pure PBI, while experiencing only a slight decrease in salt permeability coefficient, water/salt selectivity almost exceeding the empirical upper bound line of polymers. This work provides a theoretical foundation for the development of high-performance desalination membranes based on macrocyclic molecules such as CB [6].