Anhydrous proton conduction in cellulose nanofiber-heterocyclic molecule composite materials

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxylradical)-oxidized cellulose nanofibers (CNFs) form rigid rod-like structures and possess carboxyl groups on the surface of the nanofibers. We prepared CNF-heterocyclic molecule composite materials by mixing the CNF with basic heterocyclic molecules (HMs), such as imidazole (Im), benzimidazole, and pyrazole. The resulting CNF-HM composite materials formed acid-base complexes on the surface of the nanofibers. Consequently, these CNF-HM composite materials exhibited high proton conductivity under anhydrous conditions. In particular, the CNF-Im composite achieved an anhydrous proton conductivity of 1.6 × 10^-4 S cm^-1 at 140 °C. The activation energy for proton conduction in the CNF-Im composite material was 0.41 eV, and this value was one order of magnitude higher than those of conductors using water as a medium. In contrast, the gluconic acid-Im composite material, which lacked a continuous proton conducting pathway, exhibited a maximum proton conductivity of 5.8 × 10^-7 S cm^-1, which was two orders of magnitude lower than that of the CNF-Im composite material. This is due to the anhydrous proton conducting pathway formed on the CNF surface. Finally, we investigated the relationship between pK _a of the HM and anhydrous proton conduction. The anhydrous proton conductivity increased with pK _a, reaching a maximum anhydrous proton conductivity at a pK _a value of 6.5. However, at pK _a values exceeding 6.5, the anhydrous proton conductivity decreased abruptly. These results suggest that the pK _a difference (ΔpK _a) between acidic and basic molecules is a critical factor governing anhydrous proton conduction.