Incorporating 2D porous organic polymer nanosheets into high-temperature proton-exchange membranes for low H3PO4 loss

Phosphoric acid (PA)-doped ion exchange membranes have been recognized as a classical high-temperature proton exchange membrane (HT-PEM) material. Nonetheless, the leaching of PA from membranes occurs progressively during operation, inevitably diminishing overall performance. In this study, we present a novel approach for crafting two-dimensional porous organic polymer nanosheets (NS-POPs) synthesized through nanointerfacial azo-coupling polymerization. The prepared NS-POPs possessed different pore structures with main pore sizes of 0.55 nm and 1.19 nm, which were derived from the discrepancy of monomer structure. Subsequently, the NS-POPs were successfully incorporated into polybenzimidazole to create mixed-matrix membranes due to the good interface compatibility, characterized by homogeneity and stability facilitated by the 2D nature of NS-POPs. As a comparison, POPs with non-2D properties prepared by the solution method cannot be successfully obtained in good MMMs due to poor dispersibility and interface compatibility. Furthermore, the profusion of pore structures and the presence of N/O-rich functional groups in NS-POPs significantly enhance their PA uptake capacity with up to 289 %, consequently boosting proton conductivity. As a result, the composite membrane doped with NS-POPs demonstrated a maximum conductivity of up to 143 mS cm-1 at 180 degrees C, which is approximately 2.97 times that of pure membrane. And the peak power density of H2/O2 fuel cell based on the composite membrane was 378 mW cm-2, which was higher than PBI. Most significantly, the confinement effect exerted by the small pores in NS-POPs enables composite membranes to maintain a higher PA retention of up to 74 % even after a 48 h of high-temperature and high-humidity treatment. Our study introduces an innovative and straightforward approach to synthesizing porous organic polymer nanosheets. This work not only advances the understanding of the impact of interface compatibility on composite membrane fabrication, but also establishes a versatile technology for modulating pore size and managing PA leaching in HT-PEMs.