## 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
Anion exchange membranes based on spirocytic quaternary ammonium cations have been obtained and studied with the help of both experimental measurements and theoretical calculations. Spirocyclic QA cations, including 5-azoniaspiro-[4.4]-nonane [ASN]+ (**a**), 5-azoniaspiro-[4.5]-decane [ASD]+ (**b**), and 5-azoniaspiro-[4.6]-undecane [ASU]+ (**c**) have been synthesized and investigated in terms of their chemical stability in alkaline media at elevated temperatures compared to 1-butyl-1-methylpyrrolidinium [BMPy]+ (**d**), 1-methyl-4-aza-1-azonia-bicyclo-[2.2.2]-octane [MAABCO]+ (**e**), and benzyltrimethylammonium [BTMA]+ (**f**) (Figure 7). \begin{table}
\begin{tabular}{l l} \hline
**Nitrogen-Containing Groups** & **Nitrogen-Free Groups** \\ \hline Quaternary ammonium/tertiary diamines & Phosphonium \\ (Benz)Imidazolium & Sulphonium \\ Guanidinium & Metal cations (Ruthenium, Nickel, Cobalt) \\ Pyridinium \\ \hline \end{tabular}
\end{table} 
Table 2: Common anion-conducting cations in the AEM head group [131]. The degradation degree of [ASN]+ in 2M NaOH solution for 168 h has been calculated to be 4.5%, while 2.8% and 6.4%, are observed for [ASD]+ and [ASU]+, respectively. However, the degradation degree observed for nonspirocyclic QA cations [BMPy]+ (9.1%), [BTMA]+ (15.2%), and [MAABCO]+ (14.6%) is significantly higher than that for spirocyclic QA cations. Moreover, most alkali-stable cations [ASD]+ have shown the best alkaline stability in 4 M and 8 M solutions: 3.9% and 31.9%, respectively. Therefore, the alkaline stability order of the QA cations studied in this work can be ordered as follows: [ASD]+ > [ASN]+ > [ASU]+ > [BMPy]+ > [MAABCO]+ > [BTMA]+. In a theoretical study of alkaline hydrolysis, it was found that the Mulliken charge population on the spirocyclic QAs is due to the strong electron-withdrawing effect from an azonia-nitrogen atom, which shows that the neighboring \(\alpha\)-C atoms have a positive charge and the faraway \(\beta\)-C atoms show negative charge. Thus, OH\({}^{-}\) tends to attack the \(\alpha\)-C atoms instead of the \(\beta\)-C atoms. To make a parallel comparison and conform the reaction pathway from a computational view, the authors suggested that the ring-opening substitution reaction mainly happens on \(\alpha\)-C of the five-membered ring (Scheme 4). Based on the alkaline stability of the organic cations studied above, analogous AEMs based on spirocyclic QA have been obtained via the photo-crosslinking of N,N-diallyplypyrrolidinium bromide ([DAPy][Br]), N,N-diallypiperidin bromide, or N,N-diallyhexamethyleneiminium bromide([DAPi][Br] or [DAHM][Br]) with styrene and acrylonitrile, using benzoin ethyl ether as photoinitiator and divinylbenzene (DVB) as a cross-linking agent via irradiation with UV light with a 250 nm wave length. The prepared polymeric membranes ([PAPy][Br], [PAPi][Br], and [PAHM][Br], respectively) were then immersed in the alkaline solution to convert the membranes from the Br\({}^{-}\) to OH\({}^{-}\) form. AEMs based on the spirocyclic QA cation are free-standing, transparent, and could be very easily cut into appropriate sizes (Scheme 5 for [DAPy][Br]). The TGA curves of the resulting polymeric membranes based on the spirocyclic QA cation ([ASN]+, [ASD]+, [ASU]+) were recorded under nitrogen atmospheres with temperatures ranging from 30 to 800 \({}^{\circ}\)C. The weight loss for [PAHM][OH] at 200 \({}^{\circ}\)C was 3.3%, and a less than 2% weight loss was observed below 200 \({}^{\circ}\)C for [PAPy][OH] and  
Figure 7: Molecular structures of the studied QAs (**a**–**f**). Reproduced with permission from [134]. Copyright 2011 Royal Society of Chemistry. [PAPi][OH]. Weight loss attributed to polymer degradation begins at 350 \({}^{\circ}\)C. [PAPi][OH] was especially thermally stable, with a weight loss of less than 2%; it has good thermal stability and qualifies for AEM applications.