Imidazole-Doped Cellulose as Membrane for Fuel Cells: Structural and Dynamic Insights from Solid-State NMR

The structure and proton tautomerism of imidazole-doped cellulose (Cell-Im), an excellent solid state proton conductor, has been studied by N-15 solid-state NMR techniques. H-1-N-15 HETCOR NMR experiments allowed us to assign the water and cellulose-OH resonances and to establish H-1-N-15 connectivities. N-15 CPMAS NMR experiments showed that imidazole is immobile and its tautomerism quenched below 263 K, whereas at higher temperatures, a broad distribution of slow and fast exchanging protons is observed, where the fraction of the latter increases with temperature. The tautomerism is found to be coupled to proton exchange with water molecules. From an analysis of the temperature-dependent fractions of both phases, a broad distribution of energies of activation of the tautomerization of Cell-Im is obtained, exhibiting a maximum at 42 kJ mol(-1) and a width of 8.2 kJ mol(-1). The tautomerization is slower than in the case of imidazole dissolved in wet organic solvents. These results indicate that imidazole is located in an aqueous fluid phase between cellulose microfibrils, where proton exchange is assisted by a fast molecular reorientation in transient hydrogen-bonded imidazole-water complexes. The implications of these findings for the mechanism of proton conductivity of Cell-Im are discussed. Finally, the potential of Dynamic Nuclear Polarization (DNP) enhanced 15N-natural abundance CP-MAS NMR of these heterocyclic systems is evaluated.