(Invited) Novel Polymer and Membrane Development Strategies for Water Electrolysis

Water electrolysis processes play a crucial role in transitioning to a climate-friendly society. They facilitate the integration of renewable energy, offer a clean and versatile energy carrier, decarbonize industries, improve energy storage and grid stability, and support the development of sustainable transportation solutions. As technology advances and economies of scale are realized, electrolysis is expected to play an increasingly significant role in the clean energy landscape, contributing to a more sustainable and resilient future. Various water-splitting electrolysis processes currently exist, including alkaline, solid oxide, proton exchange membrane (PEM), anion exchange (AEM), acidic-alkaline amphoteric, microbial, and photoelectrochemical methods 1 . In our research group, we are actively involved in membrane development for both PEM and AEM water electrolysis. In PEM electrolysis membrane development, our group explores several approaches, such as (a) aromatic main-chain block copolymers 2 ,(b) acid-base blend membranes using sulfonated and partially fluorinated aromatic polyether, polybenzimidazole, and a PSU-derived basic polymer 2 , (c) poly(fluorene)-based sulfonated ionomers, (d) sulfonated and phosphonated poly(pentafluorostyrene) polymers with flexible side groups, and (d) nanophase-separated block copolymers based on phosphonated 3 or sulfonated pentafluorostyrene and octylstyrene. Additionally, we investigate (e) H + -conductive fiber-mat reinforced perfluorosulfonic acid (PFSA) polymers 4 . The development of anion exchange membranes in our research group includes (a) polystyrene-based side chain anion exchange polymers and their blends with polybenzimidazole 5 , (b) polynorbornene-based optionally ionically and covalently crosslinked anion exchange polymers and membranes, (c) side chain anion exchange polymers and membranes prepared by polyhydroxyalkylation 6 , and (d) anion exchange blend membranes made from polydiallyldimethylammonium salts and polybenzimidazole. This contribution highlights the application of two polymer types in PEM and AEM membrane water electrolysis, respectively: (A) PEM Water Electrolysis (PEMWE): Membranes from PEM types (a) and (b) demonstrated good performance. PEM (a) achieved 2.2 V@6 A/cm 2 , and PEM (b) reached 2.26 V@6 A/cm 2 (compared to Nafion212: 2.26 V@6 A/cm 2 ) 2 . These performances were accomplished with non-optimized membrane-electrode assemblies using Nafion as the electrode ionomer. Further performance improvements are expected with optimized electrodes containing the same ionomers as used in the membrane. (B) AEM Water Electrolysis (AEMWE): Blend membranes from AEM type (a) exhibited excellent alkali stability (no conductivity decrease after 1000 hrs of storage in 1M KOH@85°C) and good AEMWE performance (CuCo anode catalyst, 1M KOH, 70°C, 2 V@3 A/cm 2 ) 5 . Type (c) AEMs were applied to a seawater electrolysis cell at 60°C, achieving a performance of 2 V@1 A/cm 2 using completely noble metal-free catalysts in both the anode and cathode 6 . 1 M. F. Ahmad Kamaroddin, N. Sabli, T. A. Tuan Abdullah, S. I. Siajam, L. C. Abdullah, A. Abdul Jalil, A. Ahmad, Membranes 2021 , 11 . 2 J. Bender, B. Mayerhöfer, P. Trinke, B. Bensmann, R. Hanke-Rauschenbach, K. Krajinovic, S. Thiele, J. Kerres, Polymers 2021 , 13 . 3 S. Auffarth, M. Wagner, A. Krieger, B. Fritsch, L. Hager, A. Hutzler, T. Böhm, S. Thiele, J. Kerres, ACS Materials Lett. 2023 , 5 , 2039. 4 M. S. Mu'min, M. Komma, D. Abbas, M. Wagner, A. Krieger, S. Thiele, T. Böhm, J. Kerres, Journal of Membrane Science 2023 , 685 , 121915. 5 L. Hager, M. Hegelheimer, J. Stonawski, A. T. S. Freiberg, C. Jaramillo-Hernández, G. Abellán, A. Hutzler, T. Böhm, S. Thiele, J. Kerres, J. Mater. Chem. A 2023 . 6 M. L. Frisch, T. N. Thanh, A. Arinchtein, L. Hager, J. Schmidt, S. Brückner, J. Kerres, P. Strasser, ACS Energy Lett. 2023 , 8 , 2387.