Enhanced Proton Exchange Properties of Sulfonated Nanocellulose/Poly(Ether Imide) Composite Membranes for Direct Methanol Fuel Cells

ABSTRACT This work aims to enhance the proton exchange membrane properties of cellulose nanofibrils (CNFs)‐based composites. CNFs are modified with sulfosuccinic acid (SSA) prior to immersion in sulfonated poly(ether imide) (SPEI) solution to prepare the sulfonated modified CNF/SPEI composite membranes ( x SCNF/SPEI). Various molar ratios of SSA to CNF are examined, with the ratio represented as x , ranging from 0.2 to 1.0. Results indicate that the proton conductivity of the SCNF/SPEI composite membrane could be enhanced by creating the ionic path and hydrophilicity throughout the membrane. The higher SSA/CNF molar ratio ( x ) enhances the packing of CNF via esterification, increases the number of sulfonated ionic sites, and improves proton transport of the x SCNF/SPEI composite. The 0.8SCNF/SPEI membrane achieves optimal performance, with proton conductivity reaching 16.68 mS·cm −1 and IEC of 1.01 meq·g −1 . Its proton conductivity reflects the optimal balance of water uptake, and sulfonic ionic sites, with the Grotthuss mechanism being a major mechanism for proton transport. At 80°C, the 0.8SCNF/SPEI membrane reaches a power density of 4.32 mW·cm −2 . Besides, it exhibits the lowest methanol permeability at 8.22 × 10 −8 cm 2 ·s −1 , which is two orders of magnitude lower than Nafion. This study highlights a sustainable and high‐performance membrane solution for next‐generation fuel cells.