## 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
Measuring the conductivity of AEMs based on a spirocyclic QA cation in an alkaline medium was evaluated in a 2M-saturated NaOH solution at various temperatures over a period of time. The conductivities of the membranes increased with increasing temperature. Among these AEMs, [PAPi][OH] showed a loss of conductivity by 1.6% for 168 h at 80 \({}^{\circ}\)C, while [PAPy][OH] and [PAHM][OH] showed a loss of conductivity by 4.8% and 7.9%. The ion exchange capacity (IEC) values of three AEMs in the 1 M NaOH solution at 80 \({}^{\circ}\)C were almost unchanged after the test of 168 h and were equal to 0.93 \(\mathrm{mequiv}\cdot\mathrm{g}^{-1}\) for [PAPy][OH], 1.12 \(\mathrm{mequiv}\cdot\mathrm{g}^{-1}\) for [PAPi][OH], and 1.03 \(\mathrm{mequiv}\cdot\mathrm{g}^{-1}\) for [PAHM][OH]. The spirocyclic [ASD]+ cation and corresponding polymeric membranes exhibited the highest chemical stability in the alkaline solution at elevated temperatures. The results suggest that alkaline stability studies on small-molecule model cations may provide a basis for evaluating the properties of the corresponding polymer membranes and have also inspired a feasible approach for the preparation of spirocyclic QA- based AEMs with improved long-term stability and performance. There has been a period of exceptional progress in recent years concerning the development of several alkaline-stable AEMs that have remarkable AEM-FC stability. Certain AEMs based on cycloaliphatic quaternary ammonium (cQA) (mainly five- and six-membered) appear to be among those ones, having the most promising performance overall. The development of cationic polymers for anion exchange membranes (AEMs) with high alkaline stability and conductivity is a considerable challenge in materials chemistry [135]. Two spiro-ionenes (**1**,**2**) from the commercially available building blocks have been obtained. Thus, tetrakis(bromomethyl)benzene (4 BMB) has been employed in straightforward cyclopolycondensations with two commercially available dipiperidines, 4,4\({}^{\prime}\)-bipyridine (BP) and 4,4\({}^{\prime}\)-trimethylenedipiperidine (TMDP), to form spiro-ionene **1** and **2**, respectively. Spiro-centered QA cations are formed directly in the polymer backbone (Scheme 6). These polyelectrolytes show excellent thermal and alkaline stability, with no degradation detected when using NMR spectroscopy after more than 1800 h in 1 M KOD/D\({}_{2}\)O at 80 \({}^{\circ}\)C. Transparent and mechanically robust AEMs based on the ionically cross-linked blends of spiro-ionene and polybenzimidazole reach OH\({}^{-}\) conductivities of up to 0.12 S cm\({}^{-1}\) at 90 \({}^{\circ}\)C. The current findings demonstrate that spiro-ionenes constitute a new class of alkali-stable anion-exchange polymers and membranes. The stability of anion exchange membranes (AEMs) is a long-standing challenge, limiting the widespread development and adoption of AEM fuel cells (AEM-FCs). Certain AEMs based on cycloaliphatic quaternary ammonium (cQA) (mainly five- and six-membered) appear to be among those that have the most promising overall performance (Figure 8) [136]. N-Spirocyclic QAs (generalized as azoniaspiro-[m,n]-alkanes QAs), benzo-N-spirocyclic QAs, N-spirocyclic QAs with additional heteroatoms (i.e., O,N), and bis-quaternized 3,6-diazaspiro-[5.5]-undecane have been utilized as components for AEMs. Usually, the degradation mechanisms for different types of cQAs are nucleophilic substitution (path a) [137] and Hofmann elimination, leading to alkenes (path b) [Scheme 7a,b] [138; 139; 140]. However, depending on the chemical environment of the central nitrogen and structural arrangement, one degradation pathway can dominate over another; in addition, products formed along one of the paths a or b can undergo subsequent transformations [Scheme 7c-e].