Phosphorylated Cellulose Nanofiber Membranes with High Proton Conductivity for Polymer Electrolyte Membranes

Abstract

Proton conductive polymers are one of the basic materials for polymer electrolyte membrane (PEM) in polymer electrolyte fuel cell (PEFCs). Currently, perfluorosulfonic acid polymers (PFSAs), such as Nafion, are commonly used as PEMs due to their high proton conductivity. However, PFSA is a material with a high environmental impact. Cellulose nanofibers (CNFs) are a promising new membrane material for PEM, with a much lower environmental impact compared to current PFSAs - based PEMs. In this study, we fabricated a PEM with high proton conductivity using a phosphoric acid substituted CNF (P-CNF). P-CNFs with different amount of phosphoric groups were synthesized by phosphorylation of hydroxy groups on CNFs and their membranes were prepared by casting method. The SEM images exhibited that P-CNFs were closely packed and aligned their long axis parallel to the membrane plane. The P-CNF membrane demonstrated a proton conductivity of maximum 1.2 × 10-1 S/cm due to its high ion exchange capacity of 3.40 mmol/g and the formation of continuous proton conductive channel by the fiber surface inside the membrane. The high ion exchange capacity is achieved without sacrificing the mechanical strength of the membrane by modifying it with phosphoric acid, which is a moderate acid. The proton conductivity of the P-CNF membrane was equivalent to that of Nafion 212, measured at 20–80°C and 95% RH. Furthermore, the phosphorylation reaction performed cross-linking between CNFs which improved the mechanical property of the membranes. The results suggest that P-CNF membranes have high potential as environmentally sustainable alternative PFSA-based PEMs.