Molecular Engineering of a Mixed-Spacer System Toward High-Performance Polycarbazole Anion-Exchange Membranes

This manuscript reports a mixed-spacer architecture for resolving the conductivity-swelling-stability tradeoff in ether-free anion-exchange membranes (AEMs). Unlike conventional spacer engineering, which usually relies on a single spacer length, the mixed-spacer approach provides an additional degree of freedom for regulating membrane properties. Using mixed-spacer engineering in polycarbazoles with mixed short and long spacer side chains, we show that hydroxide transport and membrane durability can be jointly optimized through control of hydration, ionic connectivity, and local cation solvation, rather than through ion exchange capacity alone. A series of membranes with varied ratios of short (C4) and long (C6) alkyl spacers were synthesized via acid-catalyzed Friedel-Crafts polymerization followed by quaternization. The optimized membrane achieves a hydroxide conductivity of 184 mS cm^-1 at 80°C, outperforming both short-spacer and long-spacer counterparts. Membranes rich in C4 spacers exhibit weaker alkaline stability, whereas those containing dominant C6 spacers show excellent stability. In a fuel cell device, the membrane delivers a peak power density of 2.51 W cm^-2, among the highest reported for polycarbazole AEMs. Mechanistic understanding was achieved through both experimental and computational investigations. These results demonstrate that mixed-spacer molecular engineering is a promising and effective strategy for designing high-performance AEMs.