Expansion of the operational temperature range for polymer-electrolyte membrane fuel cells (PEMFCs) above 200 °C significantly reduces hydrogen purification requirements. Here, we report a hybrid composite of poly(2,5-benzimidazole) (ABPBI) and CsH2PO4, doped with H3PO4, as a PEM for PEMFC operation at >200 °C up to 250 °C and beyond. The optimal ratio of ABPBI repeating units to CsH2PO4 is 1:1 (mol/mol). Materials are extensively characterized by elemental analysis, scanning electron microscopy, HAADF STEM, elemental mapping, electrochemical impedance spectroscopy, proton conductivity, mechanical testing, and Fourier transform infrared spectroscopy. It is suggested that PEMFCs with the extended operational temperature range (>220 °C) might be categorized as ultrahigh-temperature polymer-electrolyte membrane fuel cells (UT-PEMFCs).

Developing effective approaches for the synthesis of nanomaterials with enhanced properties for applications in high-temperature hydrogen recovery and gas separation technology is a challenging task. Nanoporous polymer films hold significant potential for a diverse range of applications owing to their distinctive characteristics, such as high surface area, adjustable pore size, and selectivity for chemical interactions. The study presents a two-stage method (enabling less toxic solvents) for the production of nanoporous films of heat-resistant and highly permeable fluorine-containing polynaphthoylenebenzimidazole (or polybenzimidazobenzophenanthroline) (PNBI-6F), produced from polymer solutions in DMSO and N-MP. Nanoporosity of the samples is revealed by the CO2 adsorption method. It has been determined that the selection of the solvent can influence the characteristics and structure of the nanoporous polymer matrix. The gas transport properties of the films in the temperature range 20-250 degrees C have also been examined. All of the obtained nanoporous PNBI-6F films retain their mechanical properties at the maximum temperature for an extended period of time. The XRD and DMA methods and gas transport properties unexpectedly reveal a unique two-state behavior with distinct activation energies for gas permeability for each state. The initial state is characterized by lower gas permeability and free volume upon initial heating below 150 degrees C. A second state, which is metastable and characterized by an increase in gas permeability and free volume, occurs above 150 degrees C and persists in the sample over a prolonged period of time after cooling. Overcoming high-temperature gas separation challenges for H2-CO2 mixtures is essential for improving current hydrogen recovery processes and for better purification of reformed hydrogen. Therefore, it is important that the obtained gas transport characteristics significantly exceed the upper bound of the 2008 H2-CO2 Robeson diagram. The obtained results suggest the application of the nanoporous material in high-temperature hydrogen recovery technology.

Interphase boundary interactions are essential for high-temperature polymer electrolyte membrane fuel cell membrane electrode assembly (MEA) operation. Interactions between the self-phosphorylating polybenzimidazole (PBI)-6F coating on a carbon nanofiber electrode and the self-phosphorylating proton-conducting membrane during MEA operation would improve the cell performance. The presented approach represents a novel path for the development of a PBI membrane-based MEA.

Two-stage original synthesis was used to obtain homogeneous films of polynaphthoylenebenzimidazole with an ester bridge group (PNBI-O): prepolymer (PANI-O) films were first synthesized, and then 100% thermal cyclization at 350 degrees C was performed to prepare PNBI-O films. The presence of micropores in PNBI-O films is shown by the CO2 sorption method; the distinct peak is observed at similar to 0.55 nm and the vast majority of micropores in PNBI-O are in the range 0.5-0.65 nm. It was established by the DMA method that the glass transition of PNBI-O takes place at 450 degrees C. A differential gas chromatographic method was utilized to measure gas permeability coefficients (P) of six gases (He, H2, CO2, CH4, O2, and N2) at 150-250 degrees C. Temperature coefficients (the apparent activation energies) of permeability were determined. P(H2) = 162 Barrer and ideal selectivity (alpha) = 16 for H2/CO2 were attained. These values are higher than the 2008 Robeson upper bound. The analysis of permeability of the components of H2-CO2 gas mixtures of various compositions at 250 degrees C showed that the H2/CO2 separation factor can be higher by 3.5 times than ideal selectivity. Thus, the capabilities of PNBI-O as the polymer for high-temperature separation membranes for the separation of industrial hydrogen-containing mixtures were demonstrated.

A new type of polybenzimidazole-based proton exchange composite membrane, poly(2,5-benzimidazole) (ABPBI)–CsH2PO4, was obtained. The possibility of application of the composite membrane in a hydrogen–air fuel cell operating at 160–250 °C was demonstrated. In the fuel cell tests, the maximum power density reached ~400 mW/cm2 at 250 °C. The resulting data along with high open circuit voltage (0.92–0.95 V) indicate high quality and low hydrogen crossover of the membrane.

High-temperature polymer-electrolyte membrane fuel cells (HT-PEMFCs) operate at 150-200 degrees C, making it possible to use hydrogen contaminated with carbon monoxide. However, the main drawback for their distribution is the need to improve stability of membrane-electrode assembly components, especially cathodes. The cathodes based on carbon nanofiber (CNF) mat are free-standing (self-supporting), and are prepared using the method of electrospinning from a polyacrylonitrile solution containing Zr and Ni salts, followed by the stabilization and pyrolysis steps. The composite Zr, Ni-containing Pt/CNF cathodes are obtained after Pt nanoparticle deposition on the CNF surface. A novel self-phosphorylating polybenzimidazole (PBI-6F) was deposited to the surface of Pt/CNF to improve the triple-phase boundary. Substitution of OMe-groups and hydrophobic nature of CF3-groups lead to improvements in proton conductivity and gas transport of the cathode as well as proton-conducting contacts between cathode and membrane. The materials are studied using N-2 and CO2 gas adsorption, TEM, HAADF STEM. The operation of the H-2/air HT-PEMFC shows that the application of the PBI-6F-covered PBI-6F/Pt/CNF cathode results in an enhancement of HT-PEMFC performance, compared with the uncovered Pt/CNF cathode, providing similar to 20% increase in maximum power density.

The synthesis of porous organic polymers (POPs) based on a polymer of intrinsic microporosity (PIM), which represent a new class of promising materials, has been studied in detail. These porous systems are formed by precise joining of organic building blocks through covalent bonds in order to create predefined assemblies and can possess a three-dimensional (3D) or 2D-layered structure. The model reactions are considered and porosimetry studies are performed for the resulting POPs.

This review focuses on the synthesis and investigation of fluorine-free heterocyclic polyheteroarylenes (PHAs) which can be used as proton exchange membranes (PEMs) in hydrogen–air fuel cells at 60–200 °C. Below 100 °C, sulfonated PHAs are typically employed as PEMs. Among them, polynaphthoyleneimides with SO3H substituents have received significant attention. At higher temperatures (>120 °C), polybenzimidazole-based PEMs doped with phosphoric acid exhibit superior performance.

Homogeneous films of polynaftoylenebenzimidazole (PNBI-sigma) were obtained by two-stage synthesis from pre- polymer films by thermal cyclization at 350 degrees C. The presence of micropores in PNBI-sigma films is shown by the method of CO2 2 sorption, the micropore specific surface area is 285 m2/g. 2 /g. Gas permeability of He, H2, 2 , CO2, 2 , CH4, 4 , O2 2 and N2 2 was studied by differential gas chromatographic method in the temperature range of 50-250 degrees C, and the activation energies of permeability were determined. At 250 degrees C, hydrogen permeability coefficient of 220 Barrers and ideal selectivity of 17 for H2/CO2 2 /CO 2 were achieved, which significantly exceeds the values for the upper bound of 2008 Robeson diagram. The analysis of permeability of the components of mixtures of gases H2/CO2 2 /CO 2 and H2/CH4 2 /CH 4 of various compositions at 250 degrees C showed that with a decrease in the concentration of the sorbed component (CO2 2 or CH4) 4 ) a sharp increase in the separation factor is observed. When the composition of the H2/ 2 / CO2 2 mixture is 70/30, the H2/CO2 2 /CO 2 separation factor increases by more than an order of magnitude compared to the selectivity for individual gases while the hydrogen flow is maintained. The obtained experimental data demonstrate the capabilities of PNBI-sigma for high-temperature separation processes, including the separation of industrial hydrogen-containing mixtures.

High-temperature polymer-electrolyte membrane fuel cells (HT-PEMFCs) are a very important type of fuel cells since they operate at 150–200 °C, making it possible to use hydrogen contaminated with CO. However, the need to improve the stability and other properties of gas-diffusion electrodes still impedes their distribution. Self-supporting anodes based on carbon nanofibers (CNF) are prepared using the electrospinning method from a polyacrylonitrile solution containing zirconium salt, followed by pyrolysis. After the deposition of Pt nanoparticles on the CNF surface, the composite anodes are obtained. A new self-phosphorylating polybenzimidazole of the 6F family is applied to the Pt/CNF surface to improve the triple-phase boundary, gas transport, and proton conductivity of the anode. This polymer coating ensures a continuous interface between the anode and proton-conducting membrane. The polymer is investigated using CO2 adsorption, TGA, DTA, FTIR, GPC, and gas permeability measurements. The anodes are studied using SEM, HAADF STEM, and CV. The operation of the membrane–electrode assembly in the H2/air HT-PEMFC shows that the application of the new PBI of the 6F family with good gas permeability as a coating for the CNF anodes results in an enhancement of HT-PEMFC performance, reaching 500 mW/cm2 at 1.3 A/cm2 (at 180 °C), compared with the previously studied PBI-O-PhT-P polymer.

Polyacrylonitrile and polyheteroarylenes, such as polybenzimidazole (PBI) and a polymer of intrinsic microporosity (PIM-1), have been employed to prepare nanoporous electrospun carbon nanofiber (CNF)-based materials for high-temperature proton-exchange (or polymer-electrolyte) membrane (HT-PEM) fuel cells. The nanoporous CNF mats are obtained by Nanospider (needle-free) electrospinning method from polymer solution followed by pyrolysis at 1500 degrees C to form nanoporous electrospun polymer nanofiber self-supporting mats with micropores (D < 2 nm) and mesopores (D 2-50 nm). The nanoporous CNF samples are extensively characterized by N-2 and CO2 adsorption applying the BET, BJH, Dubinin-Radushkevich (DR), NLDFT, and GCMC methods, CO2 uptake, Raman spectroscopy, elemental analysis, electrical conductivity, electron microscopy, and XPS. The role of the polymer precursor on the obtained values of specific surface area (SSA) and volume for micro- and mesopores is presented and discussed. The PBI-based CNF material reaches a micropore SSA of 919 m(2) g(-1) and CO2 uptake of 4.0 mmol g(-1) derived from CO2 adsorption (273 K) data, and a micropore SSA of 873 m(2) g(-1) according to the t-method derived from N-2 adsorption data. Close values confirm higher accessibility of micropores compared with the case of PIM-based CNF, where the micropore SSA values derived from CO2 and N-2 adsorption data are different and indicate the partial inaccessibility of micropores for low-temperature nitrogen adsorption (77 K). Platinum-decorated CNF mats are successfully tested as electrodes for HT-PEM fuel cells, showing the feasibility of using the mats as cathodes; nevertheless, further optimization is required. For CNF anodes, the HT-PEM fuel cell performance reaches 0.69 V at 0.2 A cm(-2) and 0.53 W cm(-2) at 1.4 A cm(-2) which permits the use of the Pt/CNF mats as anodes.

Fuel cells on polybenzimidazole (PBI) membrane belong to high-temperature polymer-electrolyte membrane fuel cells (HT-PEMFC). When a polymer-electrolyte complex of PBI with o-phosphoric acid (PA) is applied as a proton-conducting membrane, the proton conductivity is provided without humidification above 120°C. Hydrogen-air HT-PEMFCs are able of operating effectively at 150–200°C, which allows application of technical hydrogen contaminated with CO as fuel. However, it should be noted that application of conventional “thin-film” Pt/C electrodes based on electrically conductive carbon black with Pt nanoparticles in aggressive PA environment results in electrochemical corrosion of carbon, which leads to the loss of Pt electrocatalyst particles and their aggregation (Ostwald ripening). Evidently, there is a need to replace the carbon black with more stable carbon nanostructured materials. It has been shown that self-supporting mats (essentially “felt”) based on carbon nanofibers (CNF) can be used as HT-PEMFC anodes. The CNF mats were obtained in three stages. At the first stage, the nanofiber precursor material was obtained by electrospinning of solution of a copolymer of acrylonitrile with methyl acrylate (with addition of ZrCl4). Then, the mats were stabilized by thermal oxidation (350°C, air). Afterwards, the CNFs were kept in Zn(NO3)2 (porogen) solution and pyrolyzed (1000 ^∘ C, vacuum). To improve the proton conductivity, N-phosphonoethylated cardo poly(benzimidazole) (PBI-PhT-P) was deposited on the CNF surface which leads to an improvement in the performance characteristics of HT-PEMFC. The obtained materials were examined by electron microscopy. Their specific surface area and specific volume were investigated by the N2 and CO2 adsorption methods (up to 597 m2/g and 0.170 cm3/g). After Pt deposition, the CNFs were successfully tested as anodes (up to 0.4 mA/cm2 at 625 mV) in hydrogen-air HT-PEMFC.

Carbon nanofibers are currently used in many applications including electrochemical power sources, particularly, fuel cells. Their properties are highly dependent on the micro- and mesoporous structure. Here we provide a porosimetric analysis of the polyacrylonitrile-based electrospun composite Zr- and Ni-containing carbon nanofiber mats by N2 and CO2 adsorption methods for the first time. It was found that pyrolysis temperature affects specific surface area and volume: the values increase for the sample pyrolyzed at 900 °C compared with the initial stabilized nanofibers (300 °C, air) according to the Dubinin --- Radushkevich, non-local density functional theory (NLDFT) and grand canonical Monte-Carlo methods (GCMC). For higher pyrolysis temperatures (1000 and 1200 °C), the porosimetric parameters decrease compared with the one pyrolyzed at 900 °C. According to the NLDFT and GCMC pore size distribution, the difference for pyrolyzed samples is mostly related to a sharp decrease in the specific surface area for pores with a size of ~ 0.5 nm and an increase for pores at 0.55--0.8 nm compared with the initial stabilized sample. The study demonstrates a way to adjust porosimetric parameters depending on the pyrolysis conditions of the nanofiber mats, since it can improve characteristics of such type of carbon materials in electrochemical devices

Development of new microporous organic polymers attracts significant attention due to a wide scope of promising applications. In addition, the synthesis of soluble, non-crosslinking polymers of high surface area and uniform microporosity is very challenging, and the methods for soluble microporous polymers formation are rather limited. In this work, we report a new approach to construct porous polyphenylenes which employs the Diels–Alder polycondensation of multifunctional ethynyl-containing monomers of different spatial architecture with bis(cyclopentadienone)s. The resulting polymers were soluble in common organic solvents, and their structure and properties were assessed by NMR, TGA, DSC, and SEC studies. The polymers demonstrated a specific surface area up to 751 m2·g−1 and ultramicroporous (pore size ≤ 0.6 nm) structure. N2 and CO2 adsorption–desorption data revealed that porosity parameters, e.g., specific surface area and pore sizes, can be tuned selectively by varying the type of monomers and reaction conditions.

High-temperature polymer-electrolyte membrane fuel cells (HT-PEM FC) are a very important type of fuel cell since they operate at 150–200 °C, allowing the use of hydrogen contaminated with CO. However, the need to improve stability and other properties of gas diffusion electrodes still hinders their distribution. Anodes based on a mat (self-supporting entire non-woven nanofiber material) of carbon nanofibers (CNF) were prepared by the electrospinning method from a polyacrylonitrile solution followed by thermal stabilization and pyrolysis of the mat. To improve their proton conductivity, Zr salt was introduced into the electrospinning solution. As a result, after subsequent deposition of Pt-nanoparticles, Zr-containing composite anodes were obtained. To improve the proton conductivity of the nanofiber surface of the composite anode and reach HT-PEMFC better performance, dilute solutions of Nafion®, a polymer of intrinsic microporosity (PIM-1) and N-ethyl phosphonated polybenzimidazole (PBI-OPhT-P) were used to coat the CNF surface for the first time. These anodes were studied by electron microscopy and tested in membrane-electrode assembly for H2/air HT-PEMFC. The use of CNF anodes coated with PBI-OPhT-P has been shown to improve the HT-PEMFC performance.

The development of phosphorylated polybenzimidazoles (PBI) for high-temperature polymer–electrolyte membrane (HT-PEM) fuel cells is a challenge and can lead to a significant increase in the efficiency and long-term operability of fuel cells of this type. In this work, high molecular weight film-forming pre-polymers based on N1,N5-bis(3-methoxyphenyl)-1,2,4,5-benzenetetramine and [1,1′-biphenyl]-4,4′-dicarbonyl dichloride were obtained by polyamidation at room temperature for the first time. During thermal cyclization at 330–370 °C, such polyamides form N-methoxyphenyl substituted polybenzimidazoles for use as a proton-conducting membrane after doping by phosphoric acid for H2/air HT-PEM fuel cells. During operation in a membrane electrode assembly at 160–180 °C, PBI self-phosphorylation occurs due to the substitution of methoxy-groups. As a result, proton conductivity increases sharply, reaching 100 mS/cm. At the same time, the current-voltage characteristics of the fuel cell significantly exceed the power indicators of the commercial BASF Celtec® P1000 MEA. The achieved peak power is 680 mW/cm2 at 180 °C. The developed approach to the creation of effective self-phosphorylating PBI membranes can significantly reduce their cost and ensure the environmental friendliness of their production.

Polymer of intrinsic microporosity (PIM-1) is characterized by a developed microporous structure. Needle-free NanospiderTM electrospinning of the PIM-1 solution with subsequent pyrolysis either under vacuum or in argon-hydrogen medium (7 vol% H2) at 900-1200 degrees C results in PIM-1 based carbon nanofibers (CNF) self-supporting mats. The samples were thoroughly characterized by N2 and CO2 adsorption using the BET, BJH, Dubinin-Radushkevich (DR), NLDFT and GCMC methods, CO2 uptake, and by Raman spectroscopy, elemental analysis, electrical conductivity, SEM. Since the N2 adsorption is found to be inapplicable, the presence of microporosity in the CNF is confirmed by CO2 adsorption (273 K) by the DR, NLDFT and GCMC methods. The CNF reach micropore specific surface area of 919 m2 g-1, micropore volume of 0.257 cm3 g-1 and CO2 uptake of 4.01 mmol g-1, which is higher than for initial PIM-1. Pt/PIM-1 CNF self-supporting mat, was successfully tested as anode for HT-PEM fuel cell.

High strength flame-resistant semi-ladder fibers of the "Lola " family (PNBI) have been obtained by an acid-free and more environmentally friendly method with controlled heating of precursor fibers in an inert atmosphere. The suggested two-step method consists in preparation of a prepolymer (PANI-O) in N-methylpyrrolidone. The prepared PANI-O dopes were additionally modified by introducing a certain part of a non-solvent (ethanol) to smooth the interdiffusion mass-transfer and prepare defectless precursor fibers. A detailed study of the thermal behavior of the PANI-O fibers by DSC and TGA shows that the maximum exothermic effect is observed at 320-370 degrees C, associated with formation of a ladder structure, i.e., transformation of the PANI-O into the PNBI-O. As a result, after the heat treatment, the flame-resistant PNBI "Lola " family fibers were obtained with an oxygen index of up to 83, tensile strength of similar to 500 MPa, relative elongation of-5% and elastic modulus of similar to 12 GPa.

Polybenzimidazoles (PBI) doped with phosphoric acid (PA) are promising electrolytes for medium temperature fuel cells. Their significant disadvantage is a partial or complete loss of mechanical properties and an increase in hydrogen permeability at elevated temperatures. Covalent silanol crosslinking is one possible way to stabilize PBI membranes in the presence of PA. Three organo-substituted silanes, namely (3-Bromopropyl)trimethoxysilane (SiBr), trimethoxy [2-(7-oxabicyclo [4.1.0]hept-3-yl)ethyl]silane (Si-biC) and (3-Glycidyloxypropyl)trimethoxysilane (KH 560), were used as covalent crosslinkers of PBI-O-PhT in order to determine the effect of the silane structure and crosslinking degree on membrane properties. The crosslinking degree was 1–50%. All crosslinked membranes were characterized by impedance and IR-spectroscopy. The mechanical properties, morphology, stability and hydrogen permeability of the membranes were determined. In the case of silanes with linear substituents (SiBr, KH 560), a denser structure is formed, which is characterized by greater oxidative stability and lower hydrogen permeability in comparison to the silane with a bulk group. All the crosslinked membranes have a higher mechanical strength compared with the initial PBI-O-PhT membrane both before and after doping with PA. Despite the hardening of the polymer matrix of the membranes, their proton conductivity changes insignificantly. It was shown that cross-linked membranes can be used in fuel cells.