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

Abstract

The structure and proton tautomerism of imidazole-doped cellulose (Cell-Im), an excellent solid state proton conductor, has been studied by 15N solid-state NMR techniques. 1H–15N HETCOR NMR experiments allowed us to assign the water and cellulose–OH resonances and to establish 1H–15N connectivities. 15N 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.

Imidazole-doped nanocrystalline cellulose solid proton conductor: synthesis, thermal properties, and conductivity

“…The solid lines are the best fits to the Arrhenius model (Eq. 2) imidazole molecules in nanocomposites is an evidence that the protons from the imidazole molecules are the conducting species, similarly to the situation in the composites of microcrystalline Cell-Im (Zhao et al 2016). The r dc , in all NCC-Im composites, after reaching the maximum at about 150°C, starts to decrease and finally reaches the same level as for pure NCC film.…”

“…The imidazole molecules in cellulose composites differ as to the way of hydrogen bonding to the cellulose: some of them are directly attached to the OH-groups of cellulose chains and the others through water molecules. Such a bonding of Im was proved in the microcrystalline Cell-Im composites by the solidstate NMR methods (Zhao et al 2016). In favor of such interpretation are also the following results: the increase in the peak intensity at 150°C with increasing of Im content in the sample and the dc conductivity data presented in the next paragraph.…”

“…0.5-0.9 eV) reflects other than Grotthuss conduction mechanism like the vehicle (Knauth et al 2008;Kreuer et al 1982Kreuer et al , 1996 or proton hopping mechanism (Eikerling et al 2001;Luduena et al 2011). The latter mechanism was well proved and described for microcrystalline Cell-Im composites (Zhao et al 2016) in which Im molecules formed a dynamic hydrogen bond network with residual water molecules and hydroxyl groups of cellulose chains. The proton transport mechanism is supported by molecular reorientation of Im molecules and tautomerism of imidazole molecules which does not take place between imidazole molecules alone, but that it is coupled to proton exchange both with water and cellulose -OH groups.…”

“…It has to be associated with dissociation of imidazole into a ''free'' solvated imidazole cation and a ''free'' solvated imidazolate anion. This process requires a high molecular mobility of OH groups and imidazole molecules as proved by NMR methods for microcrystalline Cell-Im composites (Zhao et al 2016). We postulate the same proton conducting mechanism also in the studied NCC-Im for the following reasons: the Im molecules in the nanocomposites, like in microcrystalline composites, were introduced into the cellulose matrix through coating thus, they are bounded to cellulose chains only on the surfaces, and the activation energies for proton conduction in NCCIm (0.91-0.65 eV) and microcrystalline Cell-Im (0.82) are quite similar.…”

“…Thus, the higher temperature application requires their immobilization in polymer matrix where a high local mobility of proton solvent and the protonic charge carriers therein still have to be guaranteed. In this respect, biopolymer carrier compounds such as cellulose doped with the electro active heterocyclic molecules containing nitrogen (imidazole and its derivatives) are highly interesting (Smolarkiewicz et al 2015Zhao et al 2016). The heterocycles are attractive due to their amphoteric nature, high thermal stability, formation of a hydrogen bonding network similar to that found in water, and a high degree of self-dissociation, which is beneficial for the proton transfer (Kreuer et al 1998).…”