Sandwich‐Structured Covalent Organic Framework as a Proton Exchange Membrane

The development of proton exchange membranes that maintain high proton conductivity at elevated temperatures and reduced humidity remains a critical challenge. In this study, we synthesized a novel sandwich‐type layered covalent organic framework (COF) membrane (BD/SB/BD), in which a TpSB‐(SO 3 H) 2 layer is sandwiched between TpBD‐(SO 3 H) 2 layers. This composite membrane exhibited high proton conductivity (158 mS cm –1 at 90°C, 100% RH). Notably, the BD/SB/BD membrane outperformed the single‐component membranes under intermediate‐to‐low humidity conditions at 100°C. This enhancement is attributed to the combination of two COF components with different pore sizes, as well as the improved water retention and dimensional stability provided by the TpSB‐(SO 3 H) 2 layer.