Nanocellulose-Based Proton Exchange Membranes with Excellent Dimensional Stability, Superior Mechanical Properties, and High Proton Conductivity via Composite MOF@CNT

Nanocellulose has shown significant potential in the field of proton exchange membranes (PEMs) because of its low cost, biodegradability, excellent thermal stability, and high designability. However, its development is limited by its low mechanical stability and low proton conductivity. In this study, cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) were blended as a composite matrix (CNF/CNC), and a stable metal–organic framework (MOF) with the −SO3H (S-UIO-66) was prepared on the surface of carbon nanotubes (CNTs) via an in situ growth procedure. The S-UIO-66@CNT was subsequently introduced as a filler into the CNF/CNC dispersion, and PEMs were formed via filtration. The S-UIO-66@CNT itself exhibited a certain uniform dispersion due to the presence of −SO3H groups; the incorporation of CNFs/CNCs (CCs) further enhanced the stability of the S-UIO-66 dispersion, and more unobstructed proton conduction pathways were established in the membrane. As a consequence, the resulting PEM (CC/S-UIO-66@CNT-5) composite developed superior mechanical properties (93 MPa) and high proton conductivities (0.105 S/cm at 80 °C and 100% RH and 27 mS/cm at 80 °C and 33% RH). In addition, battery performance tests showed promising potential for its application in fuel cells.