Cellulose Proton Conductor: Both Sulfonic Acid and Hydrophobic Group Functionalization Enable High Proton Conductivity

Abstract

Proton exchange membranes (PEMs), Nafion as a representative, are one of the key materials for energy-converting devices such as fuel cells, water electrolyzers, and redox flow batteries. Recently, environmental concerns regarding perfluoro compounds (e.g., PFAS) have been issues; thus, synthesis of PEMs with mitigated environmental impact is highly demanded. In this paper, we describe that a one-pot synthesis from cellulose provides highly proton conductive cellulose-based PEMs (SC-1) very effectively. The SC-1 with ion exchange capacity ranging from 1.07–1.49 mmol g–1was successfully prepared by controlling the feed molar ratio of the reactants.1H NMR spectra, titration, and elemental analysis supported the successful synthesis of SC-1 with a high purity and well-defined structure. Consequently, our synthetic method provided SC-1 with a high degree of substitution (1.87–2.48), which was advantageous for membrane properties. For example, the maximum H+conductivity of the SC-1 membranes exceeded 140 mS cm–1(in water at 60 °C) with a suppressed water uptake value (69%), which is one of the best performances among cellulose-based PEMs. The SC-1 membranes also showed good acid resistivity in 2 M H2SO4at 30 °C for 24 h.