Protonated Porphyrin-Induced Ionic Aggregation Regulation for Optimizing Microphase-Separated Morphology and Performance of Sulfonated Polyimide Proton Exchange Membranes

The disordered distribution of ionic groups in sulfonated polyimide (SPI) proton exchange membranes (PEMs) hinders the construction of efficient proton transport pathways and aggravates membrane swelling and methanol crossover, thereby limiting their practical application in direct methanol fuel cells (DMFCs). Herein, a protonated porphyrin-induced ionic aggregation regulation strategy is proposed to regulate ionic aggregation behavior and optimize the ionic microphase-separated morphology, enabling simultaneous enhancement of proton conductivity, dimensional stability, and methanol resistance. A series of SPI PEMs with different porphyrin contents was initially fabricated by incorporating porphyrin branching units into the copolymer backbone. In particular, the SPI-TAPP-2.0 membrane containing 2.0% porphyrin exhibited a high proton conductivity of 156.7 mS cm–1 at 80 °C, while maintaining a low swelling ratio of 42.7%. Consequently, the SPI-TAPP-2.0 membrane achieved a peak power density of 84.3 mW cm–2 and demonstrated long-term durability over 200 h with a low voltage decay rate of 0.85 mV h–1. The characterization results from small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), atomic force microscopy (AFM), and other techniques revealed that the enhanced performance was mainly attributed to the π–π stacking interactions among porphyrin units, which established ionic assembly centers, together with acid–base interactions between pyridinic nitrogen sites and sulfonic acid groups that regulated ionic aggregation and optimized the microphase-separated structure, ultimately constructing appropriately sized, highly interconnected, and selective hydrophilic proton transport networks. This work provides a strategy for optimizing microphase-separated morphology in SPI PEMs toward high-performance DMFC applications.