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 pKa of the HM and anhydrous proton conduction. The anhydrous proton conductivity increased with pKa, reaching a maximum anhydrous proton conductivity at a pKa value of 6.5. However, at pKa values exceeding 6.5, the anhydrous proton conductivity decreased abruptly. These results suggest that the pKa difference (ΔpKa) between acidic and basic molecules is a critical factor governing anhydrous proton conduction.
