NH2-UiO66 functionalized polyimide nanofiber to anchor HPW and fabricate high-performance sandwich-structure membrane for achieving excellent durability

Abstract

Sulfonated poly ether ether ketone (SPEEK) was used to fabricate proton exchange membrane (PEM) due to its adaptable proton conductivity. However, PEM cannot simultaneously enhance its conductivity and stability. To solve the problem mentioned above, phosphotungstic acid (HPW) with strong inherent proton conductivity was chosen to add to the SPEEK matrix, but it still faces the leaky problem. NH2-UiO-66 (NU6) with a large surface area and chemical stability was synthesized in situ on the surface of polyimide (PI) nanofiber to create NU6@PI nanofiber, which can be used to anchor HPW. Meanwhile, the stability of proton transport channel may be further enhanced by the sandwich structure, which may be advantageous for proton transport. The NU6@PI/SPEEK + HPW-20 exhibited a conductivity of 174.9 mS·cm−1 and a swelling ratio of 19 % at 60 °C and 100 % RH. After 6 weeks of testing, its conductivity decreased to 151.2 mS·cm−1 which was 24 % higher than that of SPEEK, while the swelling ratio increased to 31 % which was lower than that of SPEEK. The outcome demonstrates that the sandwich-structure NU6@PI/SPEEK + HPW membrane may really boost proton conductivity and stability at the same time. By using XPS and TEM, it is possible to confirm the acid-base interaction and acid-rich layer that exist at the interfaces of NU6@PI nanofiber and HPW/SPEEK matrix. After 6-weeks of usage, the sandwich-structure composite membrane with nanofiber at both the top and bottom of PEM can still maintain high proton conductivity and high stability, offering a novel research idea for resolving the issue that PEM cannot simultaneously achieve significant proton conductivity as well as elevated stability.