Advanced composite membranes based on multifunctional fillers constructed by covalently linking phosphotungstic acid with mesoporous carbon nitride for high-performance and durable fuel cell under low humidity

Two-dimensional (2D) nanosheets possessing large specific surface area and high aspect ratio can serve as proton-conduction accelerators owing to their continuous proton transport feature. Herein, phosphotungstic acid (HPW) is chemically grafted to mesoporous carbon nitride nanosheets (CN) via a hydrothermal process to produce water-insoluble HPW/CN (PCN) nanofillers, and subsequently embedded into sulfonated poly(arylene ether sulfone) (SPAES) to develop a long-range proton transport channels for high-performance composite membranes. The loading of hydrophilic HPW enhances the dispersivity and compatibility with the SPAES polymer and improves the mechanical-dimensional-chemical stability of the composite membranes. The formed hydrogen-bonding networks and acid-base interactions between fillers and polymers could endow the SPAES/PCN membranes with favorable water-absorption ability and proton-conducting performance. Besides, the excellent interfacial interactions working as a “bridge” facilitates the formation of well-defined phase-separation structure and constructs long-range ionic/water channels at the SPAES-PCN interface. Moreover, the strong acidic HPW on surface and mesoporous structure of CN enhance effectively water-retention ability of the composite membranes, which is specially contribute to rapid proton transport at low humidity. The proton conductivity of the SPAES/PCN-7.5 membrane reaches up to 250 mS cm−1 at 90 oC, hydrous condition, and 46 mS cm−1 at 80 oC, 50 % RH, which are 42 % and 84 % higher than those of the control SPAES membrane, respectively. The SPAES/PCN-7.5 membrane achieves highest maximum power density of 404–824 mW cm−2 at 80 oC and 50%–100 % RH, exceeding that of the SPAES membrane (207–546 mW cm−2) and Nafion® 112 (359–723 mW cm−2). It still remains stable cell performance, as well as undergoes lower voltages loss and hydrogen permeation after the durability test (144 h, 80 oC/60 % RH). In addition, molecular dynamics simulation analysis is performed to offer new insight into elucidating the structure-property relationships of the composite membrane. These results indicate that the SPAES/PCN composite system is an advanced membrane for the practical applications of fuel cells.