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 membranes (PEMs) in polymer electrolyte fuel cells (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 PEMs, with a much lower environmental impact compared to current PFSA-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 amounts of phosphoric groups were synthesized by phosphorylation of hydroxy groups on CNFs and their membranes were prepared by the casting method. The SEM images showed that P-CNFs were closely packed and aligned their long axis parallel to the membrane plane. The P-CNF membrane demonstrated a maximum proton conductivity of 1.2 × 10⁻¹ S cm⁻¹ due to its high ion exchange capacity of 3.40 mmol g⁻¹ and the formation of a 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. The results suggest that P-CNF membranes have high potential as environmentally sustainable alternative PFSA-based PEMs.