Phosphoric acid-loaded covalent triazine framework for enhanced the proton conductivity of the proton exchange membrane

The development of proton exchange membranes (PEMs) with high loading and stable electrolytes is currently critical and challenging for applications in new energy related devices such as proton exchange membrane fuel cells (PEMFC). In this study, a novel porous organic skeleton (Covalent triazine framework, recorded as CTFp) is synthesized as a material for immobilized guest molecules via a simple nucleophilic substitution reaction. The phosphoric acid molecule (H3PO4) is extruded into the CTFp porous organic framework by vacuum assisted method (VAM). Since the molecular size of H3PO4 is smaller than the window size of the micropores in CTFp, a high loading of H3PO4 is achieved. The large amounts of basic groups distributed in CTFp can form a strong electrostatic interaction with H3PO4, which ensures the low dynamic leakage of H3PO4. PEMs with high proton conductivity are developed by embedding phosphoric acid-loaded CTFp (H3PO4@CTFp) in a SPEEK matrix. The acid-base pair formed between H3PO4@CTFp network and SPEEK optimizes the interfacial interaction and enhances the dispersion of H3PO4@CTFp in the composite membrane. H3PO4 stored in CTFp provides rich proton hopping sites for proton transport. The hydrogen bond network formed by self-dissociation of high concentration H3PO4 molecules constructs a proton transfer channel with low energy barrier for proton transfer, thereby significantly enhancing the proton conductivity of the membrane. The results show that the proton conductivity of the composite membrane at 80 °C is 0.313 S/cm when the filler content is 15%. It is worth noting that the phosphoric acid leakage rate of H3PO4@CTFp is only 15.3% after the filler is immersed in water at 60 °C for 30 days. Therefore, the SPEEK/H3PO4@CTFp composite membranes are promising to develop new PEMs with low acid loss and high proton conductivity.