Anion conducting polymers (ACPs) are essential materials for alkaline electrochemical energy technology such as anion-exchange membrane fuel cells (AEMFCs) and water electrolysers (AEMWEs). The aforementioned polymers are promising alternatives for proton exchange membrane-based systems due to the possibility of using platinum group metal-free electrocatalysts. However, there are still no reliable ACPs possessing the desired performance and stability, which is a major challenge for developing alkaline energy systems. Herein, we highlight an anion-exchange membrane and ionomer based on quaternised poly-carbazole (QPC-TMA) with a rigid ether-free and curved backbone structure comprised of carbazole monomers. The developed ACP exhibits excellent ionic conductivity, as well as chemical and mechanical stability. Moreover, the AEMFC using QPC-TMA shows excellent performance (1.61 W cm⁻²) compared with the other best-performing AEMFCs. In addition, the AEMWE using QPC-TMA demonstrates outstanding stability and state-of-the-art performance (3.5 A cm⁻² at 1.9 V), which is the first report of an AEMWE that outperforms the best-performing proton-exchange membrane water electrolysers.

H₂SO₄-doped dense polybenzimidazole (PBI) membranes based on poly[2,2′-(2-benzimidazole-p-phenylene)-5,5′-bibenzimidazole] (BIpPBI) containing benzimidazole side groups were developed for use in vanadium redox flow batteries (VRFBs). BIpPBI was prepared by the polycondensation of 3,3′-diaminobenzidine with 2-benzimidazole terephthalic acid (BITA) in polyphosphoric acid. Many basic properties of BIpPBI were characterized and compared with conventional PBI (mPBI). The BIpPBI membrane has an entirely amorphous structure because of the presence of additional benzimidazole side groups, and the absorption of both H₂SO₄ (65 wt%) and water (46 wt%) was significantly improved compared to the mPBI membrane in a 4 M H₂SO₄ solution. The H₂SO₄-doped BIpPBI membrane has a low area resistance of 0.17 Ω cm² and significantly lowered permeability to vanadium ions (3.45 × 10⁻⁸ cm² min⁻¹). Finally, the VRFB assembled with BIpPBI had higher coulomb efficiencies (>99%) and energy efficiencies (78–95%) than Nafion 115 under a wide range of current densities (20–100 mA cm⁻²). More importantly, the VRFB with BIpPBI exhibited stable cycling performance, running for 200 charge–discharge cycles with 4.0% energy efficiency decay and 81.5% capacity retention. Furthermore, the BIpPBI membrane was stable in a highly oxidizing VO₂ ⁺ solution, demonstrating its outstanding physicochemical stability. All experimental results indicate that the dense BIpPBI membrane is a promising material for VRFB applications.

Anion-exchange membrane water electrolysis is a promising technology for economical green hydrogen production; however, the corresponding industrial applications are limited by the lack of reliable polymer electrolytes. To address this problem, we developed chain-extender-derived high-molecular-weight hexyltrimethylammonium-tethered polycarbazoles (HQPC-TMA-x's) with compelling membrane characteristics, including high ionic conductivity, mechanical robustness, and high alkaline stability. Owing to its polycarbazole backbone, HQPC-TMA-x alleviated the problems due to ionomer adsorption on the electrode. In the single-cell configuration, the best-performing polymer (HQPC-TMA-2.4) achieved an unprecedented current density of 14.6 A cm⁻² at 2.0 V with a Ni–Fe alloy anode and low-cost cell hardware, additionally showing superior pure-water-electrolysis and direct-seawater-electrolysis performances. HQPC-TMA-2.4 exhibited in situ durability at a high current density of 1.0 A cm⁻² for 1000 h with low irreversible degradation rates of 52 and 6 μV h⁻¹ for platinum group metal (PGM) and PGM-free cells, respectively, demonstrating the reliability of this polymer in practical settings.