1. Introduction

Polybenzimidazoles (PBIs) polymers are among the heterocyclic polymers which contains benzimidazole moiety in the polymeric backbone. The first time in literature in 1961, Vogel and Marvel have synthesized the aromatic heterocyclic polybenzimidazoles (PBIs) polymers using a melt polycondensation polymerization method containing different aromatic tetraamines monomers and aromatic dicarboxylic acids[1]. Since then, PBI based polymers have received extensive attention worldwide due to their remarkable physiochemical properties, which makes them superior candidates for various potential applications in the past two decades. Many researchers all over the world have analysed and reviewed the synthetic development and modification of various PBI polymer structures for their potential electrochemical applications[2-4]. Among several structures, the first ever commercialized aromatic PBI structure is poly[2,2-(m-phenylene)-5,5-bibenzimidazole], which is popularly known as meta-PBI (m-PBI) was commercialized by Hoechst Celanese in the year 1983. In the recent years, PBI Performance Products Inc. becomes the bulk supplier of high performance PBI commercially named as Celazole^®. Chung et al. in 1997 have given the historical developments and future research & development scopes of high performance PBIs[5]. Cong et al. have reviewed recent advancement of PBI based membranes in 2021[6], while the nanocomposite PBI-based nanocomposites for various advanced potential applications have been evaluated and reviewed by Kausar in the year 2018[7].

In the beginning, in a replacement of asbestos materials PBI polymers have been evolved as a highly thermally stable non-flammable textile fiber[8] and an elevated temperature stable matrix resin materials for various defence and aerospace related applications[3]. Since the aromatic heterocyclic amphoteric benzimidazole moieties in the PBI backbone can also be effectively modified by strong acids (phosphoric acid, sulfuric acid) to form proton conducting membranes beside retaining their high thermal and thermomechanical properties[9], hence, PBI based polymer materials have been utilized extensively as polymer electrolyte membranes (PEMs) for the fuel cells over the last 2 decades[ 10-12]. Specifically, while doping with sulfuric acid and most preferably phosphoric acid, the PBI/acid complex used to exhibit very high proton conduction properties, lower gas permeability, good mechanical and tensile properties and durable operation under harsh fuel cell environment at elevated temperature[ 13-17]. Wainright et al. have developed a PBI-phosphoric acid (PA) complex for the first time in order to utilized these materials in high-temperature fuel cells[13]. Thus, extensive research and development have been performed to synthesize various PBI structures, backbone modifications, membrane formation and their processing as low cost and high performance PEMs for high temperature fuel cells[18-21].

Beside their potential applications in the fuel cell, PBI has been utilized in various other diverse applications due to their unique properties. Since the asymmetric reverse osmosis (RO) membranes were developed by Loeb and Sourirajan in 1950 from cellulose acetate via a phase inversion[22], since then polymeric membranes were found to be an attractive alternative choice of materials for many separation processes, such as adsorption, distillation, extraction, and chromatography. Systems associated with pressure-driven polymer membranes become adapted and widely utilized in many industrial separations related applications due to their good performance with low energy consumption, scaled up synthesis, cost-effective operation, environmental benignity, compared with existing thermal based systems[ 23-25]. Because of the distinguished thermal, mechanical stabilities and robust properties, PBI membranes have been widely fabricated for various separation driven applications such as pervaporation (PV), ultrafiltration (UF), microfiltration (MF) organic solvent resistant nanofiltration (OSN), and gas separation studies, under harsh operational environments.

This review aims to summarize (1) the recent developments of PBI based flat sheet membrane materials including various chemical structure modifications of PBI backbone structures, variation of main chain backbone structure during synthesis, fabrication of PBI nanocomposite membranes with inorganic filler materials in order to augment their physicochemical properties, ion conduction properties, mechanical and tensile properties for their superior utilization in the high temperature PEM fuel cell. (2) the recent research trends and developments of flat sheet PBI membranes for separation related applications, more specifically in organic solvent nanofiltration (OSN) membranes and H₂/ CO₂ gas separation membranes. Finally, the prospects and challenges associated with developing next-generation PBI materials and membranes will be analysed and discussed.

2. Proton Exchange Membrane Fuel Cell (PEMFC)

Fuel cells is a devices developed for electrochemical energy conversion from the chemical energy of Hydrogen (H₂) fuel to the electrical energy via chemical reaction[26-28]. Fuel cell (FC) contains of three major components: the cathode, the anode and the electrolyte. The fuel is oxidized at anode and oxygen becomes reduced in the cathode. Moving of ions through the electrolyte produced current in the external circuit which is used to power a device. Under operational FC polymer electrolyte membranes remains sandwiched between the anode and the cathode which forms the membrane electrode assembly (MEA)[29-31]. The anode operates as an interface between the fuel (e.g. H₂) and electrolyte and can catalyses the oxidation reaction which generates H⁺ and free electrons. Via an external circuit the electrons conduct to the load. Whereas, the cathode acts as an interface between the oxygen and the electrolytes, which catalyzes oxygen reduction reaction, and provides path of electron flow from the load to the electrode via an external circuit (see schematic representation of a FC in Fig. 1). The electrolyte layer operates as a physical barrier between the hydrogen and the oxygen, which restricts the direct mixing of H₂ and O₂ but enables conduction of ionic charges between the cathode and anode and the complete cell electric circuit. Among the various types of fuel cells the proton exchange membrane fuel cells (PEMFCs) are significantly considered as the most attractive fuel cell technology due to their affordable application in automobile industries, portable power generation and stationary applications[32-35]. The PEMFC was first ever developed for the Gemini space vehicle. PEMFCs contains some of the important features, such as: less corrosion, less fuel crossover, pollution free operation etc. The PEMFC is considered as the most cost effective and efficient compared to other types of FCs. Depending upon the nature of polymer electrolytes, PEMFCs can operate ranges from lower to elevated temperature (30 to 180°C) and generates high power density in comparison to other type of fuel cells[35,36].

2.1. Desired properties of efficient PEM

Generally, a superior PEM working under operational fuel cell should consists (1) efficient proton conduction with zero electronic conductivity, (2) low permeation of oxygen and fuel, (3) good oxidative and hydrolytic stability, (4) excellent mechanical robustness in both dry and hydrated states, (5) cost effective synthesis, and (6) membrane durability for the membrane electrode assembly (MEA) fabrication and testing[32]. In the modern years, significant efforts have been made by the researchers worldwide to develop efficient PEM and alternatives to replace costly Nafion based membrane which operates up to 180°C without humidification.

2.2. Phosphoric acid (PA) doped polybenzimidazoles as PEM

In general, perfluorinated membrane Nafion has been widely utilized in literature to develop efficient PEM[37]. Nafion membranes displays thermal and chemical stability hence can be utilized safely as efficient membrane for PEMFC use[38] and that is the reason for the wide use of the perfluorinated based membrane as PEM in the last one-two decade. But the perfluorinated based polymers are consisting of significantly lower glass transition temperature (T_g) around 120°C, which causes deterioration of mechanical stability beyond 120°C[39], Perfluorinated membranes are highly costly and can conducts protons efficiently only up to 80°C under 100% humidification. Temperature beyond 100°C and less humidification, Nafion or any other perfluorinated types membranes are unable to conduct. There are additional drawbacks also like high methanol crossover which decays membranes performances[40,41]. In order to mitigate these problems associated with Nafion, extensive research have been performed by synthesizing non-perfluorinated based polymers structure[35]. Phosphoric acid (PA) doped PEM has emerged as a most efficient membrane towards this. The first ever synthesis of polybenzimidazole (PBI) structure was developed by Marvel and co-workers which was mechanically strong even at elevated temperature with good chemical stability[ 1,42]. PBI membranes were also used by the U.S. Air force due to its extraordinary properties like thermal, chemical and mechanical durability etc.[43]. The PA doped PBI based membranes are currently being evolved and popular in literature as efficient PEM, as it conducts proton at elevated temperature (150~200°C) without any humidification[12,44,45]. PBI is an amorphous aromatic heterocyclic polymer where the polymeric repeat unit consists of both the proton donor (-NH-) and proton acceptor (-N=) H-bonding sites through which PBI structure generates specific interactions with both the polar protic and aprotic solvents[ 46]. Most of the PBI based polymers are separated from reaction mixture as noodles or strong threads having good film forming properties[47-49]. Savinell et al.[13,45] have first ever introduced acid doped PBI to improve the qualities of PEM. This work has been the path breaking development for the fabrication of cost effective PBI membrane with high performances. Several authors reported that PA loaded PBI membrane exhibits very high proton conductivity, low gas permeability/crossover, decent thermal and oxidative stability and lower water drag coefficient. PA doped PBI membrane produces low vapour pressure even at high temperature because of the development of 3D networking of phosphoric acid molecules[50].

3. Thermal Stability of Polybenzimidazoles

PBI possess highly superior thermal stability due to their aromatic heterocyclic rigid backbone structure. Two distinct weight losses are observed for PBIs: an initial weight loss can be observed in the temperature range between 100~150°C and the second weight loss can be observed around > 550°C[51]. The first weight loss is attributed to the loss of loosely bound absorbed water molecules from the PBI matrix. The second stage thermal degradation observed at around > 550°C because of the decomposition of polymeric benzimidazole groups and backbone structure. The thermal stability results of the PBIs also depends on the type of tetramine monomers and the dicarboxylic acids used in the polycondensation polymerization to obtain the PBI structure[51]. This observation obtained from various literature findings of different PBI structures resembles that, PBI polymer exhibits superior thermal stability which enables their vast application in the HTPEMFCs and other diverse potential applications.

4. Mechanical Properties of Polybenzimidazoles

It is obvious that, the superior thermal stability of PBI membranes also accounts for their significant mechanical robustness which is a very important parameter for a superior quality polymer. The thermo-mechanical behaviour of the PBI membranes was investigated in literature reports using dynamic mechanical analysis. The dynamic storage modulus corresponds to the energy storing capacity in the elastic portion of the membrane[ 52]. In other words, it is a measure of the stiffness of the material. The mechanical robustness of PBI membranes is determined in terms of its storage modulus (E'), loss modulus (E''), and tan δ as a function of temperature. The E' values of PBIs differ from each other, depending on the used tetramine monomers and the DCA structure. The E' values of various types of synthetically prepared PBI backbone structures are reported in literature[51-54], which resembles superior mechanical robustness. The E' decreases with increasing temperature for the PBI structures indicates that mechanical property becomes poorer at higher temperature. However, the change is not very significant which proves the fact that PBIs have good mechanical strength even at very high temperature, which is beneficial for their high temperature applications. It is also clear cut observation from literature reports that all the PBIs obtained from para dicarboxylic acid structures shows higher E' at all temperatures than meta substituted PBIs, suggesting that the meta structure has inferior mechanical property compare to para structure oriented PBI polymers. The higher modulus of para structure polymer may be due to the different structural packing of para owing to their symmetrical nature. The glass transition temperature (T_g) of PBI polymers are generally very high and varies from 300 to 450°C depending on the PBI backbone structure[51-54]. Generally, the para connected PBI displays lower T_g than the meta PBI[51]. The T_g values differ from polymer to polymer depending on the tetramine monomers and DCA structure used for making the various structures of PBI. Due to the existence of such high T_g (300 to 450°C), PBIs have been extensively utilized in literature in the past two decades for various potential applications even at harsh experimental condition.

5. Oxidative Stability of Polybenzimidazoles

The oxidative stability of PBI membranes have been performed extensively in literature by Fenton’s test. In the test, the hydroxyl (OH•) and hydroperoxyl (OOH•) radicals attack PBI backbone and oxidative degradation takes place. In various literature reports scientists have measured the oxidative stability of all PBI polymers as a function of time. It has been found that, synthetically modified PBI structures exhibited improved oxidative stability compared to conventional PBIs[51]. For example, it was observed that only 60% polymeric weight remains after 120 h in case of conventional PBI obtained from IPA, whereas ~80% polymer weight remains in case of synthetically prepared Py-PBIs made from IPA. This may be due to the presence of bulky pyridine groups in the main chain of Py-PBI polymers. Therefore, these new Py-PBIs have better usability than the conventional PBI in an oxidative environment. Hence, overall literature reports support the fact that PBI holds 60~80 wt% of initial weight even after Fenton’s test, and synthetically obtained PBIs often remains not brittle after the Fenton’s test; it proves that the PBI membranes are stable in a drastic chemical environment[ 54,55].

6. Synthesis of Various PBI Structures for PEMFCs

In the late 20th century, many research groups have started synthesized PBI for diverse applications. The PBI structure developed by Marvel et al.[1,42] has exhibited high temperature and chemical resistance. Various synthetic methodology has been evolved in literature which consists of utilization of different tetramine monomers and dicarboxylic acids to synthesize various structural variation of PBI (Table 1). PBI can be synthesized by polycondensation reaction of tetraamines (TAB) and di-acids (Fig. 2). Literature reports consists various types of polycondensations: 1) melt condensation polymerization[56] 2) solution condensation polymerization[57] 3) catalytic condensation polymerization[58].

For solution induced polycondensation, effects of several solvents have been investigated. Various high boiling solvents such as N, N-dimethylformamide (DMF), N, N-dimethyl acetamide (DMAc) have been utilized[59,60]. But the drawback associated with these solvents are the generation of polymers with very low molecular weight. This drawback was resolved by Iwakura et al.[61] in which all compounds[equimolar mixture of dicarboxylic acids and tetramine monomers were taken into polyphosphoric acid (PPA) medium] were taken in a three neck round bottom flask connected to N₂ flow for 24 h -26 h reaction at 180~210°C. Here PPA acted as catalyst cum solvent. Various tetra ammine and di-acid monomers were synthesized and varied to developed alternative types of polybenzimidazoles (Table 1).

Since 1961, after the first ever synthesis of PBI by Marvell et al., several research groups all over the globe have been evolved in developing and modifying new verities structure of PBI for imply them in a wide range of areas and diverse fields. PBI possesses some of the unique features and properties along with some structural and physical limitations, so as per the requirements and to improve their properties; researchers have synthesized verities structures of PBI. Some of the structures include poly(4,4ʹ-diphenylether-5,5ʹ -bibenzimidazole) (OPBI)[62], poly[2,2ʹ-(1,4-phenylene)- 5,5ʹ-benzimidazole] (p-PBI)[63], poly(2,5- benzimidazole) (AB-PBI)[64], sulfonated PBI[65], pyridine based PBI (Py-PBI)[51,54], naphthalene based PBI[66], crosslinked and hyperbranched PBI[67,68], fluorinated PBI[69], N-substituted PBI (N-PBI)[70], PBI with sulfone or sulfonic acid groups in the backbone[71] meta-para random PBI copolymer[72], and many more (Fig. 3). All these PBI structures have been widely utilized in for the development of efficient PEMs for HT-PEMFC.

PBI possesses strong intramolecular and intermolecular hydrogen bonding along with a rigid network structure which results in reduced solubility in of these materials in common organic solvents. In order to improve the stability scientists have performed several strategies such as: N-alky chain substitution in the PBI backbone[73], introduction of the hetero atoms in the backbone structures[74], post polymerization with sulfonic acid functionalities to prepare sulfonated PBI etc. All the process improves the phosphoric acid uptake capacity followed by improved acid doping level and proton conduction for superior application in fuel cells. Flexibility of the PBI backbone was increased either by performing modification in the main chain or side chain with flexible groups, alkyl spacer linkages, or bulky group substituted hetero atoms. The presence of sulfonic acid functionalities in PBI structures increases the water and PA uptake capacity of the membranes. Some of the reported structural modification for improved membrane performances is discussed here: Maity et al. have developed synthetically prepared flexible Tetraamine monomer 2,6-bis(3´,4-diaminophenyl)- 4-phenylpyridine (Py-TAB) for the synthesis of various pyridine bridge polybenzimidazoles (Py-PBIs) by altering the di-acids. The resulting Py-PBI polymers remain more flexible with improved solubility and the resulted proton conductivity from the obtained proton exchange membranes also have been improved from conventional PBI[51]. Harilal et al. have synthetically developed a crosslinked proton-exchange membranes (PEMs) containing PyOPBI polymer and brominated polyphenylene oxide (Br-PPO). The resulted crosslinked membrane performed excellent proton conductivity (0.123 S cm^–1) along with superior FC efficiencies. The generated power density was 290 mW cm^–2 along with the current density of 848.7 mA cm^–2 at 0.3 V whereas, under identical operational condition, MEA developed from pristine PyOPBI membrane exhibits power density of 96.4 mW cm^–2 and a current density of 321.5 mA cm^–2[75]. In another work, they have developed three new polymeric structures of Py-PBIs utilizing various aryl di acids and PyTAB. The single cell obtained from MEA of these bulky substituted PyOPBI membranes resulted power density between 144~240 mW cm^–2 under H₂/O₂ at 160°C, which is remarkably higher than pristine PyOPBI (90.4 mW cm^–2)[76]. In their next work, they have designed three-dimensional (3D) iptycene-based porous PBI structures (IPyPBIs) as highly flexible PEM a proton conductivity of 0.24 S cm⁻¹ at 180°C (0% humidity) along with a peak power density of 0.28 W cm⁻² at 160°C in H₂/O₂[77]. Recently, pyridine-functionalized fluorophenylene-containing pyridine-bridged PBI based copolymers were designed with a proton conductivity of 0.17 S cm⁻¹ at 180°C and a peak power density of 332 mW cm⁻² 160°C[78]. Overall, several groups all over the globe have contributed significantly in order to develop various structural entity of PBIs with improved electrochemical performances, which makes them superior candidates for the PEMFC.

7. Various Casting Methods for PBI Based Membranes

By analysing the nature and the types of PBI based polymers, various types of membrane casting strategies have been evolved in the literature. Three main casting methods have been utilized extensively for preparation of efficient PEMs. Benicewiz et al. have first ever reported the sol-gel method for development of PBI membranes[12]. In this process, after completion of the polycondensation polymerization reaction in presence of polyphosphoric acid (PPA) medium, the viscous solution was directly casted onto a glass petri dishes. Hence the resulted membranes will exhibit a sol-gel transformation or transition through surrounding moisture absorption. These membranes displayed high proton conducting behaviours in nature, in addition to their efficient thermal and mechanical durability. Another method was developed by Savinell et al.[80] is the imbibing process using DMAc and LiCl solutions, where the LiCl acts as a stabilizer. After evaporation of the DMAc solvent, the PBI membrane was peeled out from the glass petri dishes followed by washing with water to remove trace amount of LiCl and DMAc as the DMAc solvent is poisonous to platinum catalyst which is used in MEA. In this process the PA holding level of PBI were 5 to 16 moles per repeating unit[81]. In addition, utilization of porogen also evolved in literature in order to fabricated porous PBI based PEMs. In this approach, after porogen treatment during membrane casting, the low molecular weight porogen were removed by dipping those membranes in suitable solvents (MeOH, hydrazine and water) (Fig. 4)[82]. The generation of pores all over the membranes interfaces resulted superior PA uptake in the membranes followed by improved proton conductivity and excellent fuel cell electrochemical performances[ 82-84].

U.S. Pat. No. 5,525,436 described a strategy of doping the PBI film in presence of a strong acid (ex. phosphoric acid or sulfuric acid) to develop a single phase system[85]. In another U.S. Pat. No. 5,945,233 Onorato et al. mentioned that PBI based paste or gel can be formed by mixing PBI polymer with a suitable acid solution, which allows to generate a gel-like or pastelike consistency at room temperature[86]. Sannigrahi et al. have reported the phosphoric acid mediated thermoreversible gelation of PBI by studying the thermodynamics of gelation, the gel morphology, gelation kinetics, test tube tilting method and UV-Vis spectroscopic techniques[87]. Also several synthetic strategies have been utilized in literature to develop super quality PEM by applying modification in the polymer backbone and in the polymeric side chains[73,88,89]. The major problem is to develop a membrane by maintaining the high PA doping level along with decent mechanical stability. Sometime, the acid content present in the membrane found to be too high to process the membrane due to its very poor mechanical stability [48]. Hence, it is important to maintain a balance between the acid uptake level and the mechanical robustness of the membranes to obtain a PEM with improved quality. Therefore, to mitigate these issues PBI based nanocomposite membranes have been evolved in literature as superior quality PEMs for potential application in the HT-PEMFCs because of their significantly improved physical properties due to the incorporation of surface functionalized inorganic nanofillers.

8. PBI Based Polymer Nanocomposite Mixed Matrix Membranes as PEMs

Nanofillers can play a pivotal role in order to improve the physical properties (ion conducting properties, mechanical and dimensional properties) of the membranes by their incorporation into the membrane matrix[90]. The nanofillers are basically inorganic solid nanoparticles comprise of inorganic materials which differs from the polymer backbone matrix in terms of structure and composition. The impregnation of suitable nanoparticles as nanofillers into the PBI matrix can develop mixed matrix PEM with improved PA doping level, proton conductivity and mechanical durability [79,91]. Polymer nanocomposite comprises of a combination of two main components: (a) the nanofiller, (b) the membrane forming polymer matrix in which the nanofillers are incorporated. Nanocomposite membranes have gained vast attention due to their diverse application in various industrial sectors[92-95]. A verity of nanofillers have reported in literature such as: graphene oxide (GO)[95], carbon nanotube (CNT)[96], silane based inorganic nanofillers[97], surface modified silica nanoparticles[98-102], clay[52,103,104], etc. Incorporation of post synthetically modified hydrophilic metal organic framework (MOF) and covalent organic framework (COF) nanofillers into PBI backbone results significantly improved physical properties in the mixed matrix PEMs[79,105,106]. The incorporation of even a very less loading of nanofillers also brings remarkable changes in the polymer properties without affecting the polymeric backbone structure. The obtained mixed matrix membranes (MMMs) results decreased gas permeability, improved mechanical strength and thermal resistance, enhanced chemical and oxidative stability, acid loading capacity and proton conductivity. The nanofillers have much impact on the polymeric membrane network due to their unique properties such as: (i) presence of hydrophilic functionalities in the nanofillers, (ii) high surface area, high porosity obtained from the inorganic filler materials (like MOFs, COFs), (iii) proton conducting functional groups present in the inorganic nanofiller materials etc. The classification of nanofillers can be done into three groups, on the basis of dimensions; (i) one dimensional (e.g., clay)[104] (ii) two dimension[e.g., graphene oxide (GO), carbon nanotube (CNT), covalent organic framework (COFs), metal organic framework (MOFs) etc.][107-109] and (iii) three dimension (e.g., silica nanoparticles (SiNP), MOFs, COFs) etc.[110,111]. In the recent times, various modifications have been performed on PBI matrix utilizing different types of nano fillers[112-115]. The phosphoric acid (PA) doping level, proton conductivity and the mechanical robustness have been significantly impacted using MOFs potential nanofillers. Mukhopadhyay and Das et al. have developed two post synthetically modified UiO-66-NH₂ MOFs (PSM1 and PSM2) and have incorporated these MOF materials in oxypolybenzimidazole (OPBI) matrix in order to fabricate efficient proton conducting mixed matrix polymer-MOF PEMs. The resulting membrane (PSM2-10%) exhibit proton conductivity as high as 0.308 S/cm at 160°C (anhydrous environment), when doped with PA[79]. Recently, a post synthetically modified MIL based flexible MOFs (53-S and 88B-S) have been incorporated into the OPBI matrix to generate MOF loaded MMMs. The resulting PA doped 88B-S-7.5% membrane exhibit a proton conductivity of 0.304 S/cm at 160°C along with significant mechanical robustness (Fig. 5)[116]. Das et al. have incorporated melamine based Schiff base network type P@MCOF materials into the m-PBI matrix and fabricated mixed matrix composite membranes which resulted high proton conductivity of 0.309 S/cm at 180°C under PA doped condition (5 fold increased proton conductivity with respect to m-PBI)[53]. Carbon nanotube and surface modified GO are also capable of increasing the several properties of the PBI[115,117]. Shao et.al. have reported multiwall carbon nanotubes (MWCNTs) containing 0.1~1 wt% nanofillers in OPBI nanocomposites with a proton conductivity of 5.8 × 10^-4 S/cm and good mechanical toughness[115]. Mukherjee et al. have recently developed SI-RAFT initiated block-co-polymers grafted on the surface of MWCNT in order to utilize those materials as potential nanofillers with OPBI, which exhibits superior proton conductivity of 0.164 S/cm at 180°C along with very high mechanical and tensile properties[118]. Das et al. have synthesized polymer-g-GO (GOP) nanomaterials through RAFT polymerization method and utilized these GOP nanofillers in order to prepare OPBI matrix supported membrane with excellent proton conductivity of 0.327 S/cm at 160°C along with remarkably improved physical properties with respect to pristine OPBI[55]. In recent times, scientists have synthesized the amine modified silica nanoparticle[98] and the montmorillonite and cloisite clay nanosheets to influence the properties of the PBI membranes[104,119]. Maity et al.[102] have developed ionic liquid modified silica/OPBI MMMs with high proton conductivity and less PA leaching. Singha et al.[98] have synthesized long chain amine modified SiNP and utilized them to fabricate OPBI matrix supported MMMs for PEMFC applications. Gorre at al. have developed ionic liquid modified Silica (ImILSi) loaded OPBI membranes with proton conductivity 0.219 S/cm at 180°C[120]. Recently, Kutcherlapati et al.[101] have reported the synthesis of poly (N-vinylimidazole) grafted SiNP as nanofillers with OPBI to fabricate efficient mixed matrix PEMs. Mukherjee et al. have developed block co-polymer grafted on SiNP (pNVI-b-pNVT-g-SiNP, pNVT-b-pNVIg-SiNP) for their potential utilization as nanofillers into OPBI matrix[99]. To solve the PA leaching problem of PEMs scientists have also developed organically modified cloisite nanoclay impregnated OPBI as super proton conducting PEMs[52]. Several other literature reports containing silica, MOF, titania and other nanofiller loaded PBI based PEMs with improved physical properties and proton conductivities[121-127]. All these research findings have driven the vast progress of the efficient proton conducing PEMs for various PEMFCs application.

9. Chemical Modification in the PBI Backbone for Development of Flat Sheet OSN Membranes

Solvent resistance is an important criterion for development of organic solvent nanofiltration (OSN) membranes with long durability, lifespan and good separation performance. Due to its excellent thermal, mechanical robustness, and resistance towards various organic solvents, PBIs has been widely utilized for fabrication of OSN membranes. Being the group member of organic heterocyclic polymers, PBI remains insoluble in various organic solvents. But PBI exhibits excessive swelling and plasticizing effect in presence of strong polar solvents, more specifically in polar aprotic solvents[128]. Chemical crosslinking of PBIs can be performed by various organic compounds, where covalent bonding of PBI imidazole rings ‘N–H’ groups occurs with the active functional groups of the crosslinkers[129]. Among the various strategies, crosslinking found to be highly efficient strategy in order to improve the chemical durability and separation performances of PBI membranes. Chen et al. have fabricated a cross-linked PBI membrane in a reaction with aqueous potassium persulphate (K₂S₂O₈)[130]. The fabricated crosslinked PBI membrane found to be stable in acetone, alcohols, dimethylformamide (DMF), dimethylacetamide (DMAc) along with a molecular weight cut off (MWCO) of 1000 g mol^-1 in DMF and resulted to be a superior performing OSN membrane. Without utilizing covalent crosslinking technique, Ignacz et al. developed a unique strategy to ion-stabilize PBI and polymer of intrinsic microporosity (PIM-1) blended membranes through simple treatment with HCl[131]. The resulted PBI membranes found to be resistant towards polar aprotic solvents. Impregnation of PIM-1 into the PBI network enhance the permeance of the membranes up to 4 times along with decrement in the MWCO. Lee et al. have synthesized PBI based high pH stable OSN membrane. To improve the membrane chemical stability, a crosslinked reaction was performed with PBI membrane from an epoxy group containing silane precursor. The development of organic- inorganic crosslinked network within the PBI structure was performed following a hydrolysis and condensation reaction of methoxysilane in the 3-glycidyloxypropyltrimethoxysilane (GPTMS). The crosslinked PBI membrane showed improved stability performances when exposed to organic solvents and no decomposition was noticed even at high basicity (pH 13). The membrane showed an ethanol permeance of 27.74 LMHbar^-1 along with an eosin Y rejection of > 90% at 10 bar operational pressure at ambient condition[132].

The imidazole ‘N-H’ groups PBI can be reacted with bi-functional alkyl halides such as 1,4-dibromobutane (DBB), α,α´-dibromo-p-xylene (DBX) and α,α´-dichloro-p-xylene (DCX) which can form crosslinked PBI structure. The crosslinked PBI based OSN membranes was developed by Nam and co-workers for the recycling of alcoholic solvents using non-solvent induced phase separation with different dope solution concentration and coagulant composition of water/ethanol mixtures. The PBI membrane crosslinked with DBX produced excellent mechanical robustness and solvent resistance and utilized as potential OSN membranes. The crosslinked PBI membrane prepared by ˃ 20 wt% dope concentration coagulated in water showed a rejection of > 90% to Congo Red (MW of 696.66 g/mol) while pure ethanol permeances was more than 22.5 LMH/bar at 5 bar. Investigation on coagulant composition indicated that ethanol permeance through crosslinked PBI OSN membrane increased with increasing of ethanol concentration in water/ ethanol mixture coagulants[133]. In another study, PBI based crosslinked OSN membrane have been developed by Nam and co-workers. The solvent permeance in the presence or absence of cross-linking was investigated and the stability was also confirmed through long operation. The permeance test was carried out with five different solvents: water, ethanol, benzene, N, N-dimethylacetamide (DMAc) and n-methyl- 2-pyrrolidone (NMP); each of the initial flux was 6500 L/m²h (water, 2 bar), 720 L/m²h (DMAc, 5 bar), 185 L/m²h (benzene, 5 bar), 132 L/m²h (NMP, 5 bar), 65 L/m²h (ethanol, 5 bar) and the pressure between 2 and 5 bar was applied depending on the type of membrane[ 134]. Wang et al. tightened the PBI membrane structure using DCX and improved its salt rejections to separate cephalexin from aqueous solutions in NF and FO applications[67,135]. The modified PBI membranes resulted decrement in the effective mean pore size along with a narrow pore size distribution, and demonstrate superior ion rejection performance for liquid based separation, especially for the fractionation of multivalent cations and anions from monovalent ions and acted as superior membranes for NF and FO. Chen et al. have developed PBI crosslinked asymmetric nanofiltration (NF) membranes using p-xylene dichloride. The solvent stability of PBI membranes improved significantly after crosslinking. The modified crosslinked PBI structures were extensively utilized in the solvent resisted nanofiltration (SRNF) applications. Dyes of altered molecular weights (MW) (in the range of 400~1100) and charges were selected as solutes to perform filtration experiments. The modified crosslinked PBI membranes showed very high rejection up to 99.9% for the selected dyes in various solvents. In comparison to the pristine PBI membranes, the newly developed crosslinked membranes exhibits remarkable retention of dyes of different molecular weights in different tested solvents[136]. Livingston and co-workers have developed OSN membranes from PBI which exhibits superior chemical durability with respect to polyimide and other conventional polymer membranes. Development of asymmetric PBI membranes were performed through crosslinking technique using either an aromatic bi-functional crosslinker or with an aliphatic crosslinker. Both the membranes exhibit high resistance and tolerance towards severe basic condition (high pH). The membrane crosslinked with aromatic DBX crosslinker exhibits excellent stability with very high permeance compared with crosslinked PBI with aliphatic crosslinker (DBB) and performs to be suitable candidate for OSN[137,138].

Development of highly robust PBI based OSN membranes were performed by Kim et al. The active layer of the membranes was reinforced with a coating of ultrathin polydopamine (pDA) followed by a simultaneous co-crosslinking of the pDA-coated PBI membrane. Due to the presence of pDA coating layer, the rejection properties of the PBI membrane have been remarkably enhanced through the overall reduction of pore size and defects fillings in the surface of the membrane. The modified PBI membranes exhibits very high rejection properties and very high solvent stability even in polar aprotic solvents. The pDA-coated co-crosslinked PBI membranes resulted remarkably high rejection property to low molecular weight solutes (99.9% for 308 Da PPG and 97.6% for 236 Da styrene dimer) with very stable solvent permeance[139]. Tashvigh et al. used DBX and hyperbranched polyethylenimine (HPEI) to double crosslink PBI/sulfonated polyphenylsulfone (sPPSU) blend membranes. The addition of sPPSU and the double crosslinking process resulted in membranes with a higher solvent flux. The membranes had smaller pores but no notable changes were observed in the permeance properties[140]. Tashvigh et al. have developed a facile approach to fabricate thin film composite (TFC) membranes with high solvent resistivity for potential application as OSN membrane. The fabrication process involves two steps: (1) chemically crosslinking of PBI membrane with DBX and (2) exposing the membrane top surface with polyethylenimine (PEI) and formation of selective layer of ultrathin PEI-DBX on the top of PBI substrate. The fabricated membranes possess a MWCO of ~350 g mol⁻¹ with pure acetone, ethanol, toluene and tetrahydrofuran permeances of 14, 4.5, 4, 14, 4 and 1 Lm⁻²h⁻¹bar⁻¹, respectively along with maintaining remarkable chemical stability in DMAc at 50°C[141]. Chung and co-workers have developed high-flux PBI membranes for OSN. Asymmetric PBI membranes have been crosslinked using trimesoyl chloride (TMC) solution and 2-methyl tetrahydrofuran (2-MeTHF). The resultant membrane exhibits a remarkable rejection of 99.6% to remazol brilliant blue R (MW of 627 g/mol) while maintained acetone, acetonitrile, isopropanol and ethanol and isopropanol permeances of 29.0, 40.7, 5.8, and 13.8 LMH/bar at 10 bar, respectively[142].

Recently Szekely and co-workers have performed ionically crosslinking of PBIs with difunctional organic acids (oxalic and squaric acids) in water medium at room temperature to improve membrane properties. The reversible nature of the PBI-based membranes enabled the successful recovery of the pristine PBI by treatment under mild basic conditions. The resulted MWCO and solvent flux of the PBI membranes crosslinked squaric and oxalic acids were 844 and 779 g mol⁻¹ and 139.7 and 153. L m⁻² h⁻¹ bar⁻¹ in N,N-dimethylacetamide respectively, at 30 bar[144]. Blanford et al. have developed high-flux PBI/graphene oxide (GO) mixed matrix membranes (MMM) for OSN. N-benzylation of polybenzimidazole was performed with 4-(chloromethyl)benzyl alcohol in order to prepare hydroxylated polybenzimidazole (Fig. 6). Hydroxylation reaction and incorporation of GO nanosheets in the polymer matrix have significantly increased the acetone permeance up to 45.2 ± 1.6 L m⁻² h⁻¹ bar⁻¹, which is approximately 5 fold higher than that of pristine benchmark membrane[143]. Zhao et al. have developed high-performance nanofiltration membranes containing an interpenetrating polymer networks (IPN) incorporating PDA and polybenzimidazole (PBI). The in situ polymerization of PDA have created an IPN, which has provided a green alternative to covalently crosslinked membranes. In the resulted membrane, due to the formation of IPN, a permeance up to 12 L m⁻² h⁻¹ bar⁻¹ was resulted in DMF with a MWCO of 190~850 g mol^-1[145]. Fei et al. have reported a pioneering research by utilizing an oxidant-promoted biophenol coating as a reproducible, versatile, highly scalable method to nicely tailor the separation performance of PBI based OSN membranes[146].

10. Chemically Modified Flat Sheet PBI Membranes for Gas Separation Studies

Despite of PBI membranes attractive H₂/CO₂ selectivity even at elevated temperatures, its low H₂ permeability (about 1~2 barrer at 35°C) hinders potential application for H₂ separation[147-149]. Therefore, development of an ultrathin dense-selective layer can increase the gas separation properties of the membranes. The lower permeability of PBI membranes is resulted from its limited free volume of the effective chain packing of PBI membranes due to the presence of intense H-bonding and π–π stacking present in the aromatic moiety[150-153]. Therefore, in order to maintain high gas permeability along with very high H₂/CO₂ selectivity, many research works have been performed in order to do chemical modification in the main polymer backbone. Various structural designs and modifications such as N-substituted modification, main chain backbone modification as well as aforementioned chemical crosslinking modifications have been performed in literature to improve the gas permeability and selectivity performances.

Kharul and co-workers have developed PBI from 3,3´-diaminobenzidene and 4,4´-(hexafluoroisopropylidene) bis(benzoic acid) or 5-tert-butyl Isophthalic acid, which showed excellent gas separation and permeation properties with respect to PBI developed from isophthalic acid[151]. The polymer backbone structure developed with hexafluoroisopropylidene or tert-butyl group in the PBI structure resulted lower chain packing density, slightly decreased thermal stability and along with slightly enhanced solvent solubility. PBI-HFA or PBI-BuI have been compared with PBI-I in terms of their permeation properties, which resulted that sorption was increased for CO₂ and CH₄ compared to other gases; and the diffusivity selectivity performances of H₂ and O₂ gases along with diffusivity of various gases were increased[151]. The modified PBI shows changes in permeability without notable loss in selectivity which makes these membranes to lie near the Robeson’s upper bound[154] shown in Fig. 7.

In another work, Kharul and co-workers have developed N-Substituted PBIs high degree of substitution and quantitative yield. With respect to normal methyl or n-butyl substituted PBI, substitution with 4-tert-butyl benzyl group resulted increased fractional free volume in the membrane, which causes improved gas permeability for the 4-tert-butyl benzyl substituted PBI, whereas, the ideal selectivity followed a reverse order. A high P_He/P_Ar selectivity of 345 was shown by DMPBI-I, along with that permeation characteristics of other N-substituted PBIs also have been significantly enhanced, which indicates a research direction that the N-substitution of PBI can be a promising strategy to fabricate high performance PBI membranes[155]. In another work, they have developed a series of PBI structures from 3,3´-diaminobenzidine (DAB) and various substituted aromatic dicarboxylic acids. Among the membranes the 4,4-(hexafluoroisopropylidene)bis(benzoic acid) and the 5-tert-Butylisophthalic acid developed PBI showed better H₂ and O₂ permeability with respect to other PBIs. The developed PBI membranes O₂/H₂ ideal gas selectivity found to be remarkably higher than various conventional membranes. The addition of bulky substituents for the development of PBI– BuI and PBI–HFA structures increased the permeability approximately 16.9~40 times with respect to any unsubstituted conventional PBI. Despite of the reduced selectivity of the developed PBI membranes than that of PBI-I, these membranes selectivity were 2~3 times higher compared to various conventional gas separation polymers like polysulphons or polycarbonate[156]. Gas transport properties of a series of polybenzimidazoles based on a tetraaminodiphenylsulfone (TADPS) monomer have been characterized at temperatures from 35 to 190°C by Stevens et al. They have observed increased permeability with increasing temperature for all gases, and activation energies of permeation increase with increasing gas size with the exception of CO₂. CO₂ exhibitted a lower activation energy of permeation than that of H₂ or He, due to the existing strong sorption effects. Observations concluded that gas separations with TADPS-based PBIs were strongly size selective, with CO₂/N₂, CO₂/CH₄, and N₂/CH₄ selectivities observed decrement with increasing temperature. However, H₂/CO₂ selectivities increase with increasing temperature due to a lower activation energy of permeation for CO₂ than for H₂. All PBIs tested move toward the upper right on the H₂/CO₂ upper bound (Fig. 7) as temperature increases[149]. Kumbharkar et al. have developed two different PBI structures by varying the acid moiety. PBI-BuI (developed from 5-tert-butyl isophthalic acid) and PBI-I (developed from isophthalic acid) were selected for performing N-substitution reaction to introduce alkyl groups in order to monitor and vary the bulkiness and the flexibility of the synthesized PBIs. Modified PBI structures significantly increased the gas permeability for various different gases by 1.2~129 times compared to pristine conventional PBIs. Along with that, the permselectivity of PO₂ /PN₂ was also increased (up to 237%), whereas they observed some decrement in permselectivity for other gas pairs[157]. Modification of PBI backbone with p-xylene dichloride was performed by Hosseini et al. which resulted slight enhancements in the selectivity performance of H₂/CO₂ and H₂/N₂ and other gas pairs. The selectivity of the membranes was found to be remarkably improved after performing cross-linking of Matrimid with p-xylene diamine. The cross-linked PBI/Matrimid (75/25 wt%) membrane with XDA exhibited a better H₂/CO₂ selectivity of around 26 owing to the reduced segment mobility of polymer chains and the increased chain packing density. Results summarized from this study reveal the astonishing features of these developed membranes for ideal gas separation applications with a great potential for H₂ gas separation and purification on industrial grade[158]. Han et al. developed a new synthesis strategy to synthesize microporous polybenzimidazole (TR-PBI) membranes, which exhibits exceptional high permeable properties for small gas molecules along with exhibiting remarkable molecular sieving properties. The TR-PBI membrane showed a remarkable transport performance for H₂/CO₂ at 120°C which suppresses the 2008 Robeson’s upper bound of H₂/CO₂ with promising H₂ permeabilities[154,159].

Li et al. synthesized four PBI derivatives (see Fig. 8) containing bent and rigid configurations that possessed bulky side groups and hampers close chain packing[150]. Comparing with commercial m-PBI, the structurally modified PBIs had higher molecular weights, slightly decreased thermal stabilities, and enhanced organo-solubilities. The introduction of bulky, flexible functional groups into the PBI backbone effectively produced new PBIs with much higher H₂ permeability. However, these PBI derivatives showed a lower H₂/CO₂ selectivity range of 5~7 than that of m-PBI because of the trade-off relationship between gas permeability and selectivity. Among these polymers, the 6F–PBI membrane had an interesting relationship between CO₂ permeability and temperature because its permeability remained nearly constant from near-ambient temperature to 250°C[150]. These developed membranes suppressed the Robeson’s upper bound[154]. Borjigin et al. developed a sulfonyl group containing series of PBIs in Eaton's reagent for application as high temperature H₂/CO₂ gas separation membranes. These sulfonyl-containing PBI membranes exhibits superior gas separation properties for H₂/CO₂. Among the polymers, the polymers developed from terephthalic or isophthalic acid and tetraaminodiphenylsulfone crossed the Robeson's upper bound (Fig. 7) for H₂/CO₂[160]. A new class of melamine/PBI-based polymer thermoset has been fabricated for production of flat-sheet films. It was found that the PBI-PMF film is capable of applications at elevated temperature (up to 300°C) and for operation at 250°C for prolong time. A comparison of parent PBI and the other polyimides reveals that PBI-PMF has remarkably enhanced the selectivity of H₂/CO₂ and CO₂/CH₄ gases[153]. Moon et al. have synthesized blends of Celazole^®PBI and ortho- functional HAB-6FDA-CI polyimide, they have varied the composition of polyimide ranging from 20 to 80 wt%. Compared with pure Celazole^® in the 20 to 33 wt% polyimide compiled blends, the H₂/CO₂ selectivities at 35°C found to be increased after a heat treatment due to a decrease in free volume of the PBI matrix phase. H₂ permeability found to be increased after a heat treatment for the 33% polyimide loaded blend, which have resulted an improved membrane transport property by exceeding the 2008 upper bound for both 20 and 33% polyimide heat treated compatibilized blend membranes[161]. Synthesis and characterization of novel PBI random copolymers for application as elevated temperature H₂/CO₂ membranes by Singh et al. The co-polymerization route resulted tailorable permeability and selectivity by combining the high H₂ permeability aspect of the highly disrupted loosely packed hexafluoroisopropylidene diphenyl group containing PBI segments (6FPBI) with the highly selective tightly packed phenylene group containing PBI segments (m-PBI). 6F/m-PBI copolymers with varying the ratios of 6F-PBI and m-PBI were synthesized. The structure and ratio of the 6F-PBI and m-PBI fractions were confirmed using FTIR and NMR spectroscopy analysis. The gas transport properties of the copolymer thin films were measured as a function of the operating conditions. The H₂ permeability increased while H₂/CO₂ selectivity decreased as the 6F-PBI copolymer fraction was increased[162]. The H₂/CO₂ separation performance of these copolymers surpassed the 2008 Robeson's upper bound at 250°C[155].

Zhu et al. have demonstrated that, chemical crosslinking of PBI in solid state can enhance the H₂/CO₂ selectivity significantly, which is in contrast to the literature where crosslinking of PBI resulted decreased H₂/CO₂ selectivity. They have prepared a series of crosslinked PBIs by immersion of PBI thin membranes in terephthaloyl chloride various time periods in order to obtain various degrees of crosslinking (Fig. 9). Crosslinking of PBI resulted decreased CO₂ sorption along with significantly enhanced H₂/CO₂ selectivity with a slight decrement in the H₂ permeability. For example, the H₂/CO₂ selectivity resulted enhancement from 15 to 23 whereas the H₂ permeability resulted decrement from 45 to 39 Barrers at 200°C after crosslinking of PBI. The separation performance of the crosslinked PBI surpasses the Robeson's upper bound estimated at 200°C, which proves superiority of these membranes in CO₂ capture and H₂ purification[147]. Naderi et al. have prepared chemically crosslinked PBI thin films using 1,3,5-Tris(bromomethyl)benzene (TBB) in order to vary the microstructure of polymer chains for achieving a very high sieving ability for H₂/CO₂ separation. The gas separation experiment of H₂/CO₂ was performed at 150°C. The membrane with the lowest functional free volume and with the highest crosslinking density exhibits the best performance for H₂/CO₂ separation with a remarkable H₂/CO₂ selectivity of 24 and a H₂ permeability of 9.6 Barrer. The membranes surpass the Robeson's upper bound and some of the other conventional membranes, and therefore act as a promising candidate for and CO₂ capture and H₂ purification at elevated temperatures [163]. Jin et al. have synthesized a crosslinked PBI-triglycidylisocyanurate (TGIC) and sulfonated graphene (SG) composite membrane. By adding SG, the membrane became more compact due to the smaller interlayer spacing and swelling ratio. The PBI–TGIC/SG membrane performed separation of H₂ from the H₂/CO₂ mixture (50 : 50 v/v) and proved to be a superior candidate for gas separation. For H₂/CO₂ separation, H₂ permeated through the membrane with 99.99% selectivity and a high permeability of up to 1676 barrer with very high flux of up to 0.22 mL min⁻¹ cm⁻² at 300°C[164]. Szekely and co-workers have developed gas separation materials by manipulating a PBI backbone possessing kinked moieties. The selection of PBI occurred due to the presence of imidazole NH functionalities which can increase CO₂ affinity along with enhancement in the sorption capacity, and therefore favours CO₂ over other gasses. They have designed an intrinsically microporous PBI (iPBI) containing a spiro- bisindane structure. Presence of a kinked moiety in association with crosslinking contributed significant enhancement in the polymer properties, which resulted increased gas separation performance. The BET surface area of PBI also found to be increased 30-fold by just introducing kinked structure by just replacing a flat benzene ring. iPBI membrane resulted a good CO₂ uptake of 1.4 mmol g⁻¹ at 1 bar and 3.6 mmol g⁻¹ at 10 bar. Gas sorption uptake and breakthrough experiments were also performed with gas mixtures of CO₂ /CH₄ (50%/50%) and CO₂ /N₂ (50%/50%), which proves the high selectivity of CO₂ over both CH₄ and N₂[152].

11. Conclusion

Polybenzimidazole membranes have achieved significant attention in the past two decades for various potential applications for its unique properties. This review summarizes the development of superior proton conducting PEMs and nanocomposite PEMs developed from highly thermally and mechanically robust flat sheet PBI membranes for high temperature PEMFC applications. At presently, the development of anion exchange membranes (AEM) membranes from PBI and PBI nanocomposites are the focus of attraction, therefore the development of PBI based AEMs are growing in literature reports and there are various synthetic scopes to develop superior PBI based AEM materials with improved physical properties. So, this area could be explored more in the near future to develop high performance FCs. Due to its excellent thermal, mechanical robustness, and resistance towards various organic solvents, PBIs has been widely utilized for fabrication of OSN membranes in the last 10~15 years. This review comprises various synthetically modified flat sheet PBI based membranes for OSN applications. Along with that, literature findings of chemically modified flat sheet PBI based membranes for gas separation and permeation studies also have been compiled in this review article. Therefore, PBI membranes have been evolved in literature for various potential applications and been a promising polymer backbone structure to be evaluated further by the modern and future scientists for various potential applications for academics, industry and mankind.