9322
J. Phys. Chem. B 2002, 106, 9322-9334
High-Resolution Solid-State NMR Studies of Imidazole-Based Proton Conductors: Structure Motifs and Chemical Exchange from 1H NMR Gillian R. Goward, Martin F. H. Schuster, Daniel Sebastiani, Ingo Schnell, and Hans Wolfgang Spiess* Max-Planck-Institut fu¨ r Polymerforschung, Postfach 3148, D-55021 Mainz, Germany ReceiVed: April 16, 2002; In Final Form: July 8, 2002
High-resolution solid-state 1H NMR under fast magic angle spinning is used for the first time to study proton conductivity. The materials of interest, ethylene oxide tethered imidazole heterocycles (Imi-nEO), are characterized by variable temperature experiments, as well as 2D homonuclear double quantum (DQ) NMR and 2D exchange spectroscopy. Quantum chemical calculations provide a full assignment and understanding of the 1H chemical shifts, based on a single-crystal structure obtained for Imi-2EO. Three types of hydrogenbonded N-1H resonances are observed by 1H MAS NMR at 30 kHz. Double quantum NMR experiments identify those hydrogen-bonded protons that are mobile on the time scale of the experiment, and thereby, those which are able to participate in charge transport. Characterized by their spin-spin relaxation (T/2) behavior, the local mobility of these protons as a function of temperature is compared to the conductivity of the materials. Homonuclear 1H 2D DQ MAS spectra provide evidence for locally ordered domains within all the Imi-nEO materials. Disordered (mobile) and ordered components in Imi-2EO dramatically differ in their 1 H spin-lattice relaxation times. 2D NOESY spectra show no evidence of chemical exchange processes between the ordered and disordered domains. These results indicate that the highly ordered regions of the materials do not (or only poorly) contribute to proton conductivity, which is rather taking place in the disordered regions. Molecules in the disordered domains are in a state of dynamic or fluctuating hydrogen-bonding, allowing for Grotthus mechanism proton transport, while molecules in the ordered domains do not experience exchange, and do not participate in long-range proton conductivity. At the interface between these regimes a small number of molecules undergo slow exchange. With increasing temperature, this exchange becomes fast on the NMR time scale, and the final chemical shift of 12.5 ppm in Imi-5EO implies the persistence of strongly and weakly hydrogen-bonded domains, which reorganize rapidly to support the proton transport process.
1. Introduction The quest for clean portable energy sources has focused in the past decade on fuel cell technologies. In particular, the successful development of polymer electrolyte membrane fuel cells (PEM-FC) has garnered widespread interest in these types of fuel cells as a clean source of energy, potentially viable for automotive applications.1-3 Nevertheless, many hurdles remain, in particular, meeting the demands placed on the proton exchange membrane, which is traditionally a hydrated perfluorosulfonic polymer. To gain more flexibility with respect to the fuel source (ideally hydrogen gas, and alternatively other hydrogen-rich fuels such as methanol) or the operating temperature of the cell, and to avoid poisoning of the fuel cell catalyst by CO,4 various alternatives have been proposed. These include membrane modification with inorganic particles, complexation of basic polymers with oxoacids, e.g., H3PO4,5 or the replacement of water with alternative choices of proton solvents, for example imidazole, pyrazole, or benzimidazole.6 The proton conductivity remains high in polymers saturated with such heterocycles; however, in a functioning fuel cell, which is an open system, the operating temperature must still be maintained below the boiling point of the solvent. * Corresponding author: Prof. Dr. H. W. Spiess, Tel.: +49 6131 379 120, Fax: +49 6131 379 100, E-mail:
[email protected].
Motivated by this evaporation issue, a new class of materials has been designed in which proton-conducting units are tethered to a polymer backbone, yielding materials capable of operating at intermediate temperatures. In such membranes, the mechanism of proton transport must involve structural diffusion, also known as the Grotthus mechanism, in which proton transfer between heterocycles and their reorganization dominates the proton conductivity. In hydrated polymers, by contrast, rapid proton transport occurs via the hydrogen-bonded liquid water network, i.e., by vehicle transport,6 which is not possible in the tethered systems due to the immobilization of the heterocycles within the membrane. The latter concept was recently demonstrated for model compounds composed of heterocycles linked pairwise to ethylene oxide oligomers.7,8 The chemical structure of these Imi-nEO materials is shown in Figure 1. These model compounds are of much smaller molecular weight than the envisaged polymeric membrane materials, but their successful synthesis and characterization demonstrated that the concept is viable, and moreover, allowed for a thorough characterization of these well-defined materials. In this paper, we characterize the Imi-nEO materials by means of high-resolution solid-state 1H NMR spectroscopy, which allows us to directly probe the protons as the nuclei responsible for conduction - the ‘workhorse’ ions in these materials. A comparison to the parent compound, pure imidazole, provides
10.1021/jp0259521 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/16/2002
Imidazole-Based Proton Conductors
Figure 1. Chemical structure of the Imi-nEO compounds.
confirmation of the processes observed in the Imi-nEO materials. For imidazole, anisotropic conductivity has been reported and attributed to the directional H-bonding in the crystalline material.9 A recent 15N solid-state NMR study of this material concluded, however, that no reorientation of the imidazole rings occurs, and that therefore the previous ideas concerning 1H conductivity in this material were poorly founded.10 Our results provide a clearer picture of these controversial findings. 2 Methods 2.1. 1H NMR. Solid-state 1H NMR spectroscopy exhibits attractive features for studying proton conductivity. First, the nuclei of interest can be observed directly in the NMR spectra with excellent sensitivity and resolution, because the 1H chemical shift is indicative of the electronic environment of the individual protons (in particular, the presence of hydrogen bonds). Second, molecular mobility can be readily probed by measuring the relative strength of the 1H-1H dipolar interactions, which are either present at full strength in rigid molecules or reduced as a consequence of molecular motion occurring on time scales of 10 to 100 µs.13 To achieve spectral resolution in solid-state 1H NMR, magic angle spinning (MAS) is applied, with the spinning frequency requirements depending on the strength of the (residual) 1H-1H dipolar interactions encountered in the material.13 For rigid 1H systems, usually MAS frequencies of 30 kHz are used. Initial insight into the structure as well as into the nature of the dipolar interactions present in the materials is provided by variable temperature 1H MAS spectra, providing a snapshot of the number and comparative strength of the H-bonding interactions present. In systems where local motion is fast on the NMR time scale the spin-spin relaxation times (T2), as reflected in the proton line widths,11,12 are related to the correlation time for the local mobility in question, in this case, proton hopping. Motional processes that are slow with respect to the NMR time scale can be detected via a coalescence phenomenon involving the resonances of the sites in slow exchange. These processes are discussed in more detail in the results section. Saturationrecovery experiments yielding longitudinal or spin-lattice relaxation (T1) times further quantify the nature of the H-bonded domains and the degrees of molecular mobility. To investigate 1H-1H dipole-dipole couplings more directly and specifically, two alternative experimental approaches are available, depending on the strengths of the dipolar interactions in question. Strong couplings can be used for the generation of dipolar double-quantum (DQ) coherences between pairs of protons, whose signals can then be correlated with their individual resonance signals (i.e., single-quantum, SQ, coherences) in terms of a two-dimensional DQ spectrum.13 Weak couplings, in contrast, tend to give rise to incoherent processes, such as dipolar cross relaxation that forms the basis for NOE experiments.14 Thus, exploiting either coherent (having a nonzero time-averaged value) or incoherent (arising from the time-dependent part of the interactions) dipolar phenomena, it is possible to distinguish between signals arising from strongly and weakly coupled nuclei, i.e., from rigid and mobile molecules in the material. Consequently, we have applied DQ experiments
J. Phys. Chem. B, Vol. 106, No. 36, 2002 9323 to select relatively rigid components, while simple 2D NOESYtype exchange spectroscopy under MAS based on a three-pulse sequence has been used to investigate the mobile parts as well as potential interactions between mobile and rigid components. 1H-1H DQ MAS spectroscopy combines 1H chemical-shift resolution and information on 1H-1H dipolar interactions by employing dipolar recoupling pulse sequences (such as “backto-back”15) for DQ generation in conjunction with fast MAS for 1H resolution enhancement.15,13 In such DQ spectra, the observation of individual DQ signals implies the existence (or persistence) of a dipole-dipole coupling, Dij, between the pair of nuclei on the time scale of the experiment (typically 10 to 100 µs). Conversely, the absence of a DQ signal indicates a lack of the respective dipole-dipole coupling (approximately meaning that Dij/2π < 2 kHz), which can either be due to long 1H-1H distances (typically > 0.4 nm) in the structure or due to molecular motions on time scales < 100 µs. 2.2. Molecular Dynamics and DFT-Based Calculations of NMR Chemical Shifts. Quantum chemical calculations of the 1H chemical shift spectra of perfect single crystals of both imidazole and Imi-2EO are presented which allow a full assignment and interpretation of our experimental 1H NMR data. The calculations are based on a recently developed formalism for NMR chemical shifts in periodic systems in the density functional theory framework.16,17 This method is implemented in CPMD (Car-Parrinello molecular dynamics),18,19 a pseudopotential-based electronic structure program using a plane-wave basis for the electronic orbitals. All calculations were done with the BLYP gradient-corrected density functional20 and a planewave cutoff of 70 Ry. The pseudopotentials were of MartinsTrouiller type, in the Kleinman-Bylander form.21 For the ab initio calculation of NMR chemical shifts, it is important to have very good local geometries of the system. Initially, the ionic positions were taken from single crystal X-ray spectra. Since the positions of the hydrogen atoms are difficult to determine from X-ray scattering experiments, and also the position of the heavy atoms is subject to a finite uncertainty, a full geometry optimization was performed. All atoms were relaxed, in a fully periodic prescription, until the ionic forces decreased below 5 × 10-4 atomic units. As expected, the heavy atoms moved only slightly (
