Phosphoric acid doped polysulfone membranes with aminopyridine pendant groups and imidazole cross-links

Udel polysulfone based membranes with 4-aminopyridine pendant groups and cross-linking imidazole units are synthesized in a simple two step reaction. The ratio of 4-aminopyridine and imidazole is varied and the materials are extensively characterized. The average phosphoric acid uptake (in 85 wt% PA) ranges between 90 wt% and 452 wt% depending on the ratio of 4-aminopyridine and imidazole and the membranes show good proton conductivity of up to 65 mS cm-1 at 160 °C under non-humidified conditions. The applicability of these materials as a polymer electrolyte membrane was proven by single cell tests at 130 °C. The relationships between PA uptake, chemical composition and mechanical stability are reported. Proton conductivity and mechanical properties only depend on the phosphoric acid content, which, however is a function of the chemical composition

Layered composite membranes based on porous PVDF coated with a thin, dense PBI layer for vanadium redox flow batteries

A commercial porous polyvinylidene fluoride membrane (pore size 0.65 μm, nominally 125 μm thick) is spray coated with 1.2–4 μm thick layers of polybenzimidazole. The area resistance of the porous support is 36.4 mΩ cm2 in 2 M sulfuric acid, in comparison to 540 mΩ cm2 for a 27 μm thick acid doped polybenzimidazole membrane, and 124 mΩ cm2 for PVDF-P20 (4 μm thick blocking layer). Addition of vanadium ions to the supporting electrolyte increases the resistance, but less than for Nafion. The expected reason is a change in the osmotic pressure when the ionic strength of the electrolyte is increased, reducing the water contents in the membrane. The orientation of the composite membranes has a strong impact. Lower permeability values are found when the blocking layer is oriented towards the vanadium-lean side in ex-situ measurements. Cells with the blocking layer on the positive side have significantly lower capacity fade, also much lower than cells using Nafion 212. The coulombic efficiency of cells with PVDF-PBI membranes (98.4%) is higher than that of cells using Nafion 212 (93.6%), whereas the voltage efficiency is just slightly lower, resulting in energy efficiencies of 85.1 and 83.3%, respectively, at 80 mA/cm2

From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides

The polybenzimidazoles represent a large family of high-performance polymers containing benzimidazole groups as part of the structural repeat unit. New application areas in electrochemical cells and separation processes have emerged during the last two decades, which has been a major driver for the tremendous development of new polybenzimidazole chemistries and materials in recent years. This comprehensive treatise is devoted to an investigation of the structural scope of polybenzimidazole derivatives, polybenzimidazole modifications and the acid-base behavior of the resulting materials. Advantages and limitations of different synthetic procedures and pathways are analyzed, with focus on homogeneous solution polymerization. The discussion extends to the solution properties of the obtained polybenzimidazoles and the challenges that are faced in connection to molecular weight determination and processing. Methods for polybenzimidazole grafting or crosslinking, in particular by N-coupling, are reviewed and successful polymer blend strategies are identified. The amphoteric nature of the benzimidazole groups further enriches the chemistry of the polybenzimidazoles, as cationic or anionic ionenes are obtained depending on the pH. In the presence of protic acids, such as phosphoric acid, cationic ionenes in the form of protic polybenzimidazoliums are obtained. The acid sorption dramatically changes the physicochemical properties of the material, which is discussed and analyzed in detail. Cationic ionenes are also obtained by full N-alkylation of a polybenzimidazole to the corresponding poly(dialkyl benzimidazolium), which has been intensively explored as a new direction in the field of anion exchange membranes recently. In the higher end of the pH scale in aqueous hydroxide solutions, anionic ionenes in the form of polybenzimidazolides are obtained as a result of the deprotonation of the benzimidazole groups. The ionization of the polymer results in dramatically changed physicochemical properties as compared with the pristine material, which is described and discussed. From a technological point of view, performance and stability targets continue to motivate further research and development of new polybenzimidazole chemistries and energy materials The overall aim of this review is therefore to identify challenges and opportunities in this area from synthetic chemistry and materials science perspectives to serve as a solid basis for further development prospects

Nanostructure-property relationship of two perfluorinated sulfonic acid (PFSA) membranes

The physicochemical properties of perfluorinated sulfonic acid (PFSA) polymers are closely correlated with their nanostructure. However, their real nano-structural morphology is still controversial because it is difficult to observe their accurate morphology at the nanoscale. Moreover, studies on the nanostructures of the PFSA membranes have been mainly focused on the ionic domain. On this basis, here we describe the crystalline domain of two PFSA membranes as well as their ionic domain based on small-angle X-ray scattering results. Both ionic and crystalline domains showed significant alterations during hydration, and the different behaviors based on the side-chain length of the two PFSA membranes are also described. The short side chain-tethered PFSA membrane (higher ion exchange capacity (IEC)) showed a widespread ionic domain and lacking crystalline domain with their relatively temperature-dependent tendency compared to the flexible long side chain-tethered PFSA membrane (lower IEC). On this basis, the correlation between nanostructure and membrane properties is described from various perspectives

Properties of Anion Exchange Membranes with a Focus on Water Electrolysis

Recently, alkaline membrane water electrolysis, in which membranes are in direct contact with water or alkaline solutions, has gained attention. This necessitates new approaches to membrane characterization. We show how the mechanical properties of FAA3, PiperION, Nafion 212 and reinforced FAA3-PK-75 and PiperION PI-15 change when stress–strain curves are measured in temperature-controlled water. Since membranes show dimensional changes when the temperature changes and, therefore, may experience stresses in the application, we investigated seven different membrane types to determine if they follow the expected spring-like behavior or show hysteresis. By using a very simple setup which can be implemented in most laboratories, we measured the “true hydroxide conductivity” of membranes in temperature-controlled water and found that PI-15 and mTPN had higher conductivity at 60 °C than Nafion 212. The same setup was used to monitor the alkaline stability of membranes, and it was found that stability decreased in the order mTPN > PiperION > FAA3. XPS analysis showed that FAA3 was degraded by the attack of hydroxide ions on the benzylic position. Water permeability was analyzed, and mTPN had approximately two times higher permeability than PiperION and 50% higher permeability than FAA3

Vanadium Redox Flow Batteries Using <i>meta</i>-Polybenzimidazole-Based Membranes of Different Thicknesses

15, 25, and 35 μm thick <i>meta</i>-polybenzimidazole (PBI) membranes are doped with H₂SO₄ and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H₂SO₄, PBI absorbs about 2 mol of H₂SO₄ per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, <i>meta</i>-PBI shows a negligible crossover of V³⁺ and V⁴⁺ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBI’s nanosized pores. It turns out that charge efficiency (CE) of VRFBs using <i>meta</i>-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick <i>meta</i>-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin <i>meta</i>-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H₂SO₄-doped <i>meta</i>-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm^–2