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^-1 was 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 H₂SO₄ at 30 °C for 24 h.