Cross-Linked Alkaline Anion Exchange Membrane from N-Spirocyclic Quaternary Ammonium and Polybenzimidazole

Abstract

Two major challenges, namely, hydroxide conductivity and alkaline stability of the polymer membrane, are yet to be resolved adequately in spite of significant research outcomes on alkaline anion exchange membrane (AAEM) in the recent past. To address these challenges, in this work, the development of ionically cross-linked AAEMs has been achieved by blending pyridine-bridged polybenzimidazole (PyPBI) and N-spirocyclic quaternary ammonium spiro ionene polymer (SP). Further, membranes were converted to porous membranes by adding different weight percentages of porogen in the membrane matrix. Membranes were converted to hydroxide-conducting AAEMs by dipping into 1 M KOH solution, and under this condition, a part of the −NH– groups of PyPBI was deprotonated to form ammonium–imidazolate complexes with SP, which resulted in ionic cross-linking in the AAEM. Hydroxide ion conductivity of 129 mS/cm at 90 °C was obtained in the case of the S70P30-OH membrane, which was a hydroxide-form membrane obtained from the blend of 70 wt % SP and 30 wt % PyPBI, and this membrane showed the highest KOH uptake among all other AAEMs prepared in this study. On the other hand, among the porous ionically cross-linked membranes studied here, the S50P50-P25-OH (blends of 50 wt % SP and 50 wt % PyPBI with 25% porogen) membrane showed the highest hydroxide ion conductivity (117 mS/cm at 90 °C). All the ionically cross-linked AAEMs displayed excellent alkaline stability and remained unaffected during alkaline stability test in 1 M KOH at 80 °C for as long as the test was carried out (960 h). Observing the exceptional stability in 1 M KOH of S50P50-OH and S50P50-P25-OH membranes, OH– conductivity analysis and alkaline stability tests of these samples were carried out even in 2 M KOH, and we found that these membranes retained ∼80% of their OH– conductivity value even after 500 h of alkaline treatment in 2 M KOH at 60 °C. Furthermore, membranes were found to be useful in alkaline water electrolysis, and the best performance was shown by the S70P30-OH membrane, which displayed a current density of 100 mA cm–2 at 2.6 V. Overall, these recently developed membranes retained hydroxide conductivity, structural and thermal stability even after harsh alkaline treatment for a longer period of time.

SEBS-Based Anion Exchange Membrane Grafted with N-Spirocyclic Quaternary Ammonium Cations for Fuel Cells

“…It can be noted that the peak power density is about 286 mW cm –2 at a current density of 400 mA cm –2 at 80 °C. As shown in Figure b, the single cell performances of the reported AEMs grafted with cations with a N -spirocyclic structure are compared, and the peak power densities for SEBS-p-ASU-TMA-40 reach the general level. ,,,,− This result indicates that the SEBS-p-ASU-TMA-40 membrane is a potential material in the application of AEMFCs. To further evaluate the in situ durability of the AEMFC, the MEA with the SEBS-p-ASU-TMA-40 anion exchange membrane is operated at a constant current of 200 mA cm –2 at 60 °C, and the result is shown in Figure c.…”

Cross-Linked Polybenzimidazoles as Alkaline Stable Anion Exchange Membranes

“…The FT-IR spectra of cross-linked poly­(butylated pyridinium benzimidazolium) iodide polymer membranes are presented in Figure . In the IR spectra, the peak at 3060 cm –1 is attributed to the stretching vibration of the aromatic C–H bond, and another peak observed in the range of 2982–2880 cm –1 represents the vibration of the aliphatic C–H bond, − which remains absent in the pristine PyPBI polymers and appears with intense peak intensity due to the resulting alkylation in the CPBPBI polymer backbone. The aromatic pyridine ring C–H stretching frequency is observed at 830 cm –1 .…”

“…The aromatic pyridine ring C–H stretching frequency is observed at 830 cm –1 . The other desired peaks at 1600 and 1477 cm –1 are ascribed to the stretching vibration of CC/CN and the in-plane benzimidazole ring deformation, respectively. − …”

Strong and Flexible High-Performance Anion Exchange Membranes with Long-Distance Interconnected Ion Transport Channels for Alkaline Fuel Cells

“…Moreover, the chemical stability of AEMs is also critical to the performance and lifetime of AEMFCs, in which the alkaline durability of the polymeric skeleton is an important factor. Currently, there are various AEM skeletons, including polysulfone, polyphenylene ether (PPO), polyaryl ether, polystyrene, polyolefin, poly­(norbornene) and other aromatic and aliphatic copolymer skeletons. − Among them, AEMs containing sulfone and ether bonds in the matrix are unstable under alkaline conditions because of the higher risk of degradation of these two chemical bonds under the attack of OH – , which would compromise the alkaline stability of AEMs. − In contrast, AEMs without any other heteroatomic bonds in the polymer skeleton and with only C–C covalent bonds in the main chain have been reported to have long-term alkaline stability. , For example, the cross-linked polyvinylbenzyl chloride (PVBC)/ hexamethylenetetramine (HMTA) AEMs prepared by Vasudevan et al are quite stable under alkaline conditions (93% IEC retention after immersion in 5 M KOH at 30 °C for 240 h) . Liu et al prepared AEMs with a fully-IPN structure by adding polyvinylbenzyl chloride (PVBC) and poly-1-vinylimidazole (PVIM) to poly­(vinyl alcohol) (PVA) solution (the residual rate of hydroxide conductivity is around 74.6% after immersion in 4 M KOH solution at 60 °C for 1320 h) .…”