Alicyclic Design of Sulfonated Polyimide Membranes with a Tricyclodecane Diamine for Improved Ion Crossover Blocking in Vanadium Redox Flow Batteries

Polyimides (PIs), known for high thermal stability, strength, and chemical resistance, are used in energy systems, such as fuel cells and redox flow batteries. Despite Nafion membranes offering high proton conductivity, their high cost, strong water dependency, and severe vanadium ion crossover limit their long-term stability and practical viability in vanadium redox flow batteries (VRFBs). Thus, designing high-performance proton exchange membranes (PEMs) based on PI with both selective proton conductivity and vanadium ion blocking capability has become a critical challenge. This study presents a design strategy that combines alicyclic and aromatic diamine monomers to achieve both structural and performance benefits. The rigid and bulky tricyclodecane diamine (TCDDA) is copolymerized with flexible aromatic diamines (ODA) and sulfonated diamines (BDSA) to synthesize segmented copolymers incorporated into the PI backbone. This design increases the free volume and enables controlled microphase separation for selective proton transport and vanadium blocking. To assess steric effects and chain stacking, a less bulky analogue, noborane diamine (NBDA), was also used for comparison. The TCDDA-based membranes exhibited outstanding comprehensive properties, including a tensile strength of up to 89 MPa and an elongation at break of 22.7%. Microstructural analysis revealed that TCDDA promoted orderly chain stacking and stable phase separation compared with the NBDA series, allowing for selective ion transport without the need for additional pore-forming treatments. In VRFB tests, the PEM with 10% TCDDA (T10) demonstrated an exceptionally low vanadium ion permeability (9.79 × 10^-8 cm²/min), significantly outperforming Nafion and NBDA-based membranes in terms of Coulombic efficiency. Energy efficiency remains above 80% across all current densities. The T10 membrane retained its integrity and conductivity after repeated cycles, confirming excellent stability. The remarkably low vanadium ion permeability of TCDDA-based alicyclic PI further underscores its high long-term durability and selectivity.