Polyfluorenes Bearing N,N-Dimethylpiperidinium Cations on Short Spacers for Durable Anion Exchange Membranes

Abstract

Alicyclic quaternary ammonium cations having all the β-protons in a strain-free ring structure are in general highly base-resistant and are thus very attractive to employ for anion exchange membrane (AEM) applications. However, tethering cations such as N,N-dimethylpiperidinium (DMP) to polymer backbones without introducing any weak links is quite challenging. In the present study, we have attached pairs of piperidine rings in their 4-position to fluorene and 2,7-diphenylfluorene via methylene bridges using straightforward S N 2 reactions. These fluorenes were subsequently utilized as monomers in polyhydroxyalkylations to prepare poly(fluorene alkylene)s with different contents of the piperidine groups. AEMs were cast after quaternizing the piperidine groups to introduce DMP and spirocyclic 6-azonia-spiro-[5,5]undecane-6-ium (ASU) cations, respectively. The AEMs reached very high hydroxide ion conductivities, 100−156 mS cm −1 at 80 °C, in the ion exchange capacity range 1.8−2.4 mequiv g −1 . X-ray scattering showed ionomer peaks indicating ionic clustering with a characteristic distance d = 2.0−2.9 nm depending on the ion exchange capacity. The AEMs displayed high thermal stability, up to ∼250 °C, and 1 H NMR data indicated no degradation after storage in 5 M aq NaOH during 168 h at 90 °C. However, degradation started under very severe conditions (10 M, 90 °C) with ∼75% of the total ionic loss in all the AEMs assigned to Hofmann β-elimination. The overall results show that fluorene-based AEMs carrying DMP and ASU cations via methylene bridges display an attractive combination of ionic phase separation, thermal and chemical stability, and hydroxide conductivity, making them viable alternatives for use in alkaline fuel cells and water electrolyzers..

Application of Ionized Intrinsic Microporous Poly(phenyl-alkane)s as Alkaline Ionomers for Anion Exchange Membrane Water Electrolyzers

“…The chemical stability of the AEMs and ionomers under harsh alkaline conditions is an important topic of current research. Their stability is mainly determined by the structure of the polymer backbone, side chain, and cationic head groups. , The alkaline stability of the prepared ionomers was first evaluated by monitoring the changes of OH – conductivity during the treatment in a 1 M NaOH solution at 80 °C. For the CF 3 -substituted series, the OH – conductivity remained stable throughout the alkaline stability test (Figure a), indicative of excellent chemical stability.…”

“…The existence of enough water molecules in the ionomer not only is conducive to ion dissociation but also facilitates the rapid migration of ions in the percolating water-rich phase domain . Generally, the WU and SR are directly related to the IEC values, but there are outliers.…”

“…The SAXS diagrams of the ionomers were measured (Figure ), and the clear ionomer peak reveals the presence of a microphase separation structure within the ionomers. The characteristic separation length ( d ) can be calculated from the equation of d = 2π/ q max (where q max is the maximum scatting vector). , The corresponding d of QP1-CF 3 -1 , QP2-CF 3 -1 , QP2-CF 3 -2 , QP2-CF 3 -3 , QP1-CN-1 , QP2-CN-1 , QP2-CN-2 , and QP2-CN-3 is 2.96, 2.84, 2.77, 2.73, 3.12, 2.79, 2.75, and 2.71 nm, respectively (Table ). Notably, the characteristic separation length of the ionomers does not have a strong correlation with the corresponding IEC.…”

“…The existence of enough water molecules in the ionomer not only is conducive to ion dissociation but also facilitates the rapid migration of ions in the percolating water-rich phase domain. 40 Generally, the WU and SR are directly related to the IEC values, but there are outliers. As shown in Figure S14a,b, in the CF 3 -substituted series, spirobisindane-containing ionomers display higher WU and SR, particularly at high temperatures.…”

Quaternary Ammonium-Functionalized Polyethylene-Based Anion Exchange Membranes: Balancing Performance and Stability

“…Cyclic ammoniums, especially piperidinium and spirocyclic ammoniums, have recently become an increasingly frequent choice of cation for implementation in AEMs. ,,− In this study, the effect of ring size on conductivity and stability was determined by comparing ammoniums containing a 5-, 6-, or 7-membered ring ( PE14 , PE15 , PE16 ) (Scheme ). It was observed that the 6-membered piperidinium ring exhibited the highest conductivity stability followed by the 5-membered and then the 7-membered ring ( PE15 > PE14 > PE16 ) with 95, 44, and 15% conductivity retention, respectively (Figure d, Table ).…”