Epoxides cross-linked hexafluoropropylidene polybenzimidazole membranes for application as high temperature proton exchange membranes

Strategies for Mitigating Phosphoric Acid Leaching in High-Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have become one of the important development directions of PEMFCs because of their outstanding features, including fast reaction kinetics, high tolerance against impurities in fuel, and easy heat and water management. The proton exchange membrane (PEM), as the core component of HT-PEMFCs, plays the most critical role in the performance of fuel cells. Phosphoric acid (PA)-doped membranes have showed satisfied proton conductivity at high-temperature and anhydrous conditions, and significant advancements have been achieved in the design and development of HT-PEMFCs based on PA-doped membranes. However, the persistent issue of HT-PEMFCs caused by PA leaching remains a challenge that cannot be ignored. This paper provides a concise overview of the proton conduction mechanism in HT-PEMs and the underlying causes of PA leaching in HT-PEMFCs and highlights the strategies aimed at mitigating PA leaching, such as designing crosslinked structures, incorporation of hygroscopic nanoparticles, improving the alkalinity of polymers, covalently linking acidic groups, preparation of multilayer membranes, constructing microporous structures, and formation of micro-phase separation. This review will offer a guidance for further research and development of HT-PEMFCs with high performance and longevity.

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.

A Novel High Temperature Fuel Cell Proton Exchange Membrane with Nanoscale Phase Separation Structure Based on Crosslinked Polybenzimidazole with Poly(vinylbenzyl chloride)

A semi-aromatic polybenzimidazole (DPBI) is synthesized via polycondensation of decanedioic acid (DCDA) and 3,3-diaminobenzidine (DAB) in a mixed phosphorus pentoxide/methanesulfonic acid (PPMA) solvent. Ascribing to in-situ macromolecular crosslinker of ploly((vinylbenzyl chloride) (PVBC), a robust crosslinked DPBI membrane (DPBI-xPVBC, x refers to the weight percentage of PVBC in the membrane) can be obtained. Comprehensive properties of the DPBI and DPBI-xPVBC membranes are investigated, including chemical structure, antioxidant stability, mechanical strength, PA uptake and electrochemical performances. Compared with pristine DPBI membrane, the PA doped DPBI-xPVBC membranes exhibit excellent antioxidative stability, high proton conductivity and enhanced mechanical strength. The PA doped DPBI-10PVBC membrane shows a proton conductivity of 49 mS cm^-1 at 160 °C without humidification. Particularly, it reveals an enhanced H₂/O₂ single cell performance with the maximum peak power density of 405 mW cm^-2, which is 29% higher than that of pristine DPBI membrane (314 mW cm^-2). In addition, the cell is very stable in 50 h, indicating the in-situ crosslinked DPBI with a macromolecular crosslinker of PVBC is an efficient way to improve the overall performance of HT-PEMs for high performance HT-PEMFCs.

Effect of Organo-Silanes Structure on the Properties of Silane-Crosslinked Membranes Based on Cardo Polybenzimidazole PBI-O-PhT

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.

Enhancement of Proton Conductivity Performance in High Temperature Polymer Electrolyte Membrane, Processed the Adding of Pyridobismidazole

A pyridobisimidazole unit was introduced into a polymer backbone to obtain an increased doping level, a high number of interacting sites with phosphoric acid and simple processibility. The acid uptake of poly(pyridobisimidazole) (PPI) membrane could reach more than 550% (ADL = 22), resulting in high conductivity (0.23 S·cm⁻¹ at 180 °C). Along with 550% acid uptake, the membrane strength still held 10 MPa, meeting the requirement of Proton Exchange Membrane (PEM). In the Fenton Test, the PPI membrane only lost around 7% weight after 156 h, demonstrating excellent oxidative stability. Besides, PPI possessed thermal stability with decomposition temperature at 570 °C and mechanical stability with a glass transition temperature of 330 °C.

Nanocomposite membranes of polybenzimidazole and amine-functionalized carbon nanofibers for high temperature proton exchange membrane fuel cells

Carbon nanofibers functionalized with aminobenzoyl groups (CNF–aminobenzoyl) were prepared via direct Friedel–Crafts acylation in polyphosphoric acid. The functionalization of CNFs was characterized using XPS, FTIR, TGA, and Raman analyses. Hexafluoroisopropylidene-containing polybenzimidazole (6FPBI) composite membranes containing pristine CNFs or CNF–aminobenzoyl were prepared using solvent-assisted dispersion and solvent-casting methods. In this work, the influence of the incorporation of functionalized CNFs on several physicochemical properties of the 6FPBI nanocomposite membranes, including their thermal stability, mechanical strength, and acid doping level, was studied. The results showed that CNF–aminobenzoyl provided better mechanical reinforcement for the nanocomposite membrane, compared to pristine CNF. The SEM observation confirmed the good compatibility between the CNF–aminobenzoyl fillers and the 6FPBI matrix. For the 0.3 wt% CNF–aminobenzoyl/6FPBI composite membrane, the tensile stress was increased by 12% to be 78.9 MPa (as compared to the 6FPBI membrane), the acid doping level was improved to 12.0, and the proton conductivity at 160 °C was measured above 0.2 S cm⁻¹. Furthermore, the fuel cell performance of the membrane electrolyte assembly (MEA) for each nanocomposite membrane was evaluated. The maximum power density at 160 °C was found up to 461 mW cm⁻² for the MEA based on the 0.3 wt% CNF–aminobenzoyl/6FPBI composite membrane.

Carbon nanofibers functionalized with aminobenzoyl groups (CNF–aminobenzoyl) were prepared via direct Friedel–Crafts acylation in polyphosphoric acid.

Crosslinked Pore-Filling Anion Exchange Membrane Using the Cylindrical Centrifugal Force for Anion Exchange Membrane Fuel Cell System

In this study, novel crosslinked pore-filling membranes were fabricated by using a centrifugal force from the cylindrical centrifugal machine. For preparing these crosslinked pore-filling membranes, the poly(phenylene oxide) containing long side chains to improve the water management (hydrophilic), porous polyethylene support (hydrophobic) and crosslinker based on the diamine were used. The resulting membranes showed a uniform thickness, flexible and transparent because it is well filled. Among them, PF-XAc-PPO70_25 showed good mechanical properties (56.1 MPa of tensile strength and 781.0 MPa of Young’s modulus) and dimensional stability due to the support. In addition, it has a high hydroxide conductivity (87.1 mS/cm at 80 °C) and low area specific resistance (0.040 Ω·cm²), at the same time showing stable alkaline stability. These data outperformed the commercial FAA-3-50 membrane sold by Fumatech in Germany. Based on the optimized properties, membrane electrode assembly using XAc-PPO70_25 revealed excellent cell performance (maximum power density: 239 mW/cm² at 0.49 V) than those of commercial FAA-3-50 Fumatech anion exchange membrane (maximum power density: 212 mW/cm² at 0.54 V) under the operating condition of 60 °C and 100% RH as well. It was expected that PF-XAc-PPO70_25 could be an excellent candidate based on the results superior to those of commercial membranes in these essential characteristics of fuel cells.

Recent Progress in the Development of Composite Membranes Based on Polybenzimidazole for High Temperature Proton Exchange Membrane (PEM) Fuel Cell Applications

The rapid increasing of the population in combination with the emergence of new energy-consuming technologies has risen worldwide total energy consumption towards unprecedent values. Furthermore, fossil fuel reserves are running out very quickly and the polluting greenhouse gases emitted during their utilization need to be reduced. In this scenario, a few alternative energy sources have been proposed and, among these, proton exchange membrane (PEM) fuel cells are promising. Recently, polybenzimidazole-based polymers, featuring high chemical and thermal stability, in combination with fillers that can regulate the proton mobility, have attracted tremendous attention for their roles as PEMs in fuel cells. Recent advances in composite membranes based on polybenzimidazole (PBI) for high temperature PEM fuel cell applications are summarized and highlighted in this review. In addition, the challenges, future trends, and prospects of composite membranes based on PBI for solid electrolytes are also discussed.

3D Network Structural Poly (Aryl Ether Ketone)-Polybenzimidazole Polymer for High-Temperature Proton Exchange Membrane Fuel Cells

Poor mechanical property is a critical problem for phosphoric acid-doped high-temperature proton exchange membranes (HT-PEMs). In order to address this concern, in this work, a 3D network structural poly (aryl ether ketone)-polybenzimidazole (PAEK-cr-PBI) polymer electrolyte membrane was successfully synthesized through crosslinking reaction between poly (aryl ether ketone) with the pendant carboxyl group (PAEK-COOH) and amino-terminated polybenzimidazole (PBI-4NH2). PAEK-COOH with a poly (aryl ether ketone) backbone endows superior thermal, mechanical, and chemical stability, while PBI-4NH2 serves as both a proton conductor and a crosslinker with basic imidazole groups to absorb phosphoric acid. Moreover, the composite membrane of PAEK-cr-PBI blended with linear PBI (PAEK-cr-PBI@PBI) was also prepared. Both membranes with a proper phosphoric acid (PA) uptake exhibit an excellent proton conductivity of around 50 mS cm-1 at 170°C, which is comparable to that of the well-documented PA-doped PBI membrane. Furthermore, the PA-doped PAEK-cr-PBI membrane shows superior mechanical properties of 17 MPa compared with common PA-doped PBI. Based upon these encouraging results, the as-synthesized PAEK-cr-PBI gives a highly practical promise for its application in high-temperature proton exchange membrane fuel cells (HT-PEMFCs).

Construction of High-Performance, High-Temperature Proton Exchange Membranes through Incorporating SiO2 Nanoparticles into Novel Cross-linked Polybenzimidazole Networks

The practical applications of phosphoric acid-doped polybenzimidazole (PA-PBI) as high-temperature proton exchange membranes (HT-PEMs) are mainly limited by their poor dimensional-mechanical stability at high acid doping levels (ADLs) and the leaching of PA from membranes during fuel cell operation. In this work, to overcome these issues, we fabricated novel cross-linked PBI networks with additional imidazole groups by employing a newly synthesized bibenzimidazole-containing dichloro compound as cross-linker and an arylether-type Ph-PBI as matrix. Ph-PBI featured by good solubility under high molecular weight offers satisfactory film-forming ability and mechanical strength using for the matrix. Importantly, the additional imidazole moieties in BIM-2Cl endow the cross-linked PBI membranes improved dimensional-mechanical stability with simultaneously enhanced ADLs and proton conductivity. Furthermore, superior acid retention capability is obtained by incorporating porous polyhydroxy SiO₂ nanoparticles into these cross-linked networks. As a result, the SiO₂/cross-linked PBI composite membranes are suitable to manufacture membrane electrode assemblies (MEAs), and an excellent H₂/O₂ cell performance with a peak power density of 497 mW cm^-2 at 160 °C under anhydrous conditions can be achieved.

Non-Fluorinated Polymer Composite Proton Exchange Membranes for Fuel Cell Applications - A Review

The critical component of a proton exchange membrane fuel cell (PEMFC) system is the proton exchange membrane (PEM). Perfluorosulfonic acid membranes such as Nafion are currently used for PEMFCs in industry, despite suffering from reduced proton conductivity due to dehydration at higher temperatures. However, operating at temperatures below 100 °C leads to cathode flooding, catalyst poisoning by CO, and complex system design with higher cost. Research has concentrated on the membrane material and on preparation methods to achieve high proton conductivity, thermal, mechanical and chemical stability, low fuel crossover and lower cost at high temperatures. Non-fluorinated polymers are a promising alternative. However, improving the efficiency at higher temperatures has necessitated modifications and the inclusion of inorganic materials in a polymer matrix to form a composite membrane can be an approach to reach the target performance, while still reducing costs. This review focuses on recent research in composite PEMs based on non-fluorinated polymers. Various inorganic fillers incorporated in the PEM structure are reviewed in terms of their properties and the effect on PEM fuel cell performance. The most reliable polymers and fillers with potential for high temperature proton exchange membranes (HTPEMs) are also discussed.

Poly(vinylbenzylchloride) Based Anion-Exchange Blend Membranes (AEBMs): Influence of PEG Additive on Conductivity and Stability

In view of the many possible applications such as fuel cells and electrolysers, recent interest in novel anion exchange membranes (AEMs) has increased significantly. However, their low conductivity and chemical stability limits their current suitability. In this study, the synthesis and characterization of several three- and four-component anion exchange blend membranes (AEBMs) is described, where the compositions have been systematically varied to study the influence of the AEBM's composition on the anion conductivities as well as chemical and thermal stabilities under strongly alkaline conditions. It was shown that the epoxide-functionalized poly(ethylene glycol)s that were introduced into the four-component AEBMs resulted in increased conductivity as well as a marked improvement in the stability of the AEBMs in an alkaline environment. In addition, the thermal stability of the novel AEBMs was excellent showing the suitability of these membranes for several electrochemical applications.