Imidazole-Doped Cellulose as Membrane for Fuel Cells: Structural and Dynamic Insights from Solid-State NMR Li Zhao, †,‡ Iga Smolarkiewicz, †,§,∥ Hans-Heinrich Limbach, †,# Hergen Breitzke, ‡ Katarzyna Pogorzelec-Glaser, § Radoslaw Pankiewicz, ⊥ Jadwiga Tritt-Goc,* ,§ Torsten Gutmann,* ,‡ and Gerd Buntkowsky* ,‡ ‡ Eduard-Zintl-Institute fü r Inorganic Chemistry and Physical Chemistry, Technische Universitä t Darmstadt, Alarich-Weiss-Str. 4, 64287 Darmstadt, Germany § Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland ∥ NanoBioMedical Centre, Adam Mickiewicz University in Poznań, Umultowska 85, 61-614 Poznan, Poland ⊥ Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, 61-614 Poznań, Poland # Institut fü r Chemie und Biochemie, Freie Universitä t Berlin Takustr. 3, 14195 Berlin, Germany * S Supporting Information ABSTRACT: The structure and proton tautomerism of imidazole-doped cellulose (Cell-Im), an excellent solid state proton conductor, has been studied by 15 N solid-state NMR techniques. 1 H− 15 N HETCOR NMR experiments allowed us to assign the water and cellulose−OH resonances and to establish 1 H− 15 N connectivities. 15 N 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 15 N-natural abundance CP-MAS NMR of these heterocyclic systems is evaluated. ■ INTRODUCTION In times of increasing energy requirement and global warming, new innovations such as fuel cells have been promoted to transform energy in an efficient and clean way. 1−5 In particular, the successful development of proton exchange membrane fuel cells (PEMFCs) has gained widespread interest. 6−9 The energy in such PEMFCs is generated by reaction of hydrogen, which is oxidized at the anode. During this reaction, protons are transferred to the cathode through a proton exchange membrane (PEM), while the electrons pass through the external circuit and induce electrical energy. One of the challenges is, therefore, to find an appropriate insulating film to separate anode and cathode material but, at the same time, to allow fast proton transport. In the last decades, several types of polymeric materials with high proton conductivity have been reported and used. For example, polymers containing poly(tetrafluoroethylene) back- bones with different anion-terminated side chains such as NAFION have been developed. 10−12 However, these materials exhibited a high proton conductivity only when they are hydrated, which limits the operation temperature of the device to about 353 K. Additionally, NAFION is quite expensive. 13 Consequently, intensive research has been done aimed at the development of cheaper and environmentally friendlier materials for membranes that can operate also under anhydrous conditions and at temperatures above 373 K. In particular, polymer backbones have been doped with heterocyclic nitrogen containing compounds, leading to solid state proton con- ductors, which allow one to achieve temperatures up to 473 K. 14,15 Important examples are imidazole and benzimidazole containing PEM materials, which exhibit high proton conductivities after doping with acids. 16,17 Furthermore, Received: July 14, 2016 Revised: August 19, 2016 Published: August 22, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 19574 DOI: 10.1021/acs.jpcc.6b07049 J. Phys. Chem. C 2016, 120, 19574−19585