## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Green Hydrogen Production and Consumption
#### 2.4.1 The State of the Art
Cyclic QAs have proven to be exceptionally alkaline stable at temperatures up to 160 \({}^{\circ}\)C and NaOH concentrations up to 10 mol L\({}^{-1}\), with piperidine-based 6-azonia-spiro[5.5]undecane having the highest half-life (110 h) under the chosen conditions [141]. The high alkaline stability of the six-membered azoniaspiro ring cation is attributed to  
Figure 8: The representative cQAs, reported in the AEM literature: cN-spirocyclic QAs and their derivatives: (**I**) azoniaspiro-[m,n]-alkanes QAs; (**II**) benzo-N-spirocyclic QAs; (**III**) N-spirocyclic QAs with additional heteroatoms; (**IV**) bis-quaternized 3,6-diazaspiro-[5.5]-undecane. Reproduced with permission from [136]. Copyright 2022 Springer Nature. the constrained ring conformation, which raises the transition state energy of both the elimination and substitution degradations. The development of different anion exchange membranes (AEMs) for use in alkaline fuel cells and water electrolyzers based on the polystyrene (PS)-carrying benzyltrimethyl ammonium cations has been carried out [142]. Highly alkali-stable mono- and spirocytic piperidine-based cations have been implemented in polystyrene (PS)-carrying benzyltrimethyl ammonium cations by first performing superacid-mediated Friedel-Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by the quaternization of the piperidine rings, either using iodomethane to produce N,N-dimethyl piperidinium cations (PS-PiQdm) or by cycloquaternizations, using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary ammonium cations (PS-PiQPy and PS-PiQPi). Thus, it is possible to functionalize up to 27% of the styrene units with the piperidine rings, subsequently achieving complete quaternization (Scheme 8). The synthetic approach ensures that all of the sensitive \(\beta\)-hydrogens of the cations presented in the piperidine ring, which are attached to the polymer polystyrene (PS) backbone in the Friedel-Crafts reaction step, are protected from an attack by hydroxyl ions OH\({}^{-}\) by the two methyl groups. The AEMs based on these polymers show high alkaline stability, and less than 5% ionic loss was observed when using \({}^{1}\)H NMR spectroscopy after 30 days in 2 M aq NaOH at 90 \({}^{\circ}\)C. The AEMs functionalized with N,N-dimethyl piperidinium cations (PS-PiQdm) show higher stability than the ones carrying the N-spirocyclic quaternary ammonium (PS-PiQPy, and PS-PiQPi). A careful analysis of the latter revealed that the rings formed in cyclo-quaternization are more prone to degrade via Hofmann elimination than the rings, introduced in the Friedel-Crafts reaction. AEMs with an ion-exchange capacity of 1.5 equiv-g\({}^{-1}\) reach a hydroxide conductivity of 106 mS cm\({}^{-1}\) at 80 \({}^{\circ}\)C under fully hydrated conditions. AEMs are further tuned and improved by blending with polybenzimidazole (PBI). For example, an AEM containing 2 wt % PBI shows a reduced water uptake and much improved robustness during handling and reaches 71 mS cm\({}^{-1}\) at 80 \({}^{\circ}\)C. The study demonstrates that the critical alkaline stability of PS-containing AEMs can be significantly enhanced by replacing benchmark benzyltrimethyl ammonium cations [143] with N-alicyclic piperidine-based cations. However, the stability of these state-of-the-art cations needs to be further improved. The alkaline stability of AEMs is greatly affected by the chemical structure of the membranes, including their polymer backbones, grafted side chains, and cationic functional groups. Commercial aromatic polymers, such as polysulfones, poly(2,6-dimethyl-1,4-phenylene oxide), polyketones, etc., have been widely investigated for use as the backbones  
Figure 8: The synthetic route to PS-PiQdm, PS-PiQPy, and PS-PiQPi via the Friedel–Crafts alkylation, followed by different quaternization reactions. Reprinted with permission from [142]. Copyright 2016 American Chemical Society.