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

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 (xSCNF/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 xSCNF/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 cm²·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.