Quaternised Diels–Alder Polyphenylene Anion Exchange Membranes for Efficient CO2 Electrolysis

The membrane-electrode assembly (MEA) is a highly promising configuration for CO 2 reduction reaction with anion-exchange membranes (AEMs) playing a crucial role in limiting ion crossover, increasing operational time, and maintaining low cell voltage. In this study, we investigated Diels-Alder polyphenylenes (DAPP) functionalised with five distinct combinations of quaternary ammoniums and spacers to evaluate their impact on membrane properties and performance. Comprehensive characterisation, including thermal and alkaline stability, small-angle X-ray scattering, ion exchange capacity, water uptake, and swelling ratio, was combined with MEA electrolysis testing up to 65 hours, alongside analysis of gaseous and liquid reduction products. Linear side-chain membranes (6C-TMA-DAPP and 6C-PIP-DAPP) exhibited superior alkaline stability and robust MEA performance, with 6C-PIP-DAPP achieving the best operational metrics and performing comparably to state-of-the-art AEMs. In contrast, branched side-chain membranes (5C-A-PIP-DAPP, 7C-A-PIP-DAPP and 9C-A-PIP-DAPP) displayed reduced alkaline stability and unusual voltage behaviours, likely related to restricted side-chain rotational freedom and altered polymer microstructure. These findings elucidate the importance of side-chain architecture on AEM performance, showing that linear side chains are well-suited for CO 2 RR and providing new insights into AEM behaviour.