Klaus-Dieter Kreuer

On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells

Abstract The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the p K a of the acidic functional groups. The less pronounced hydrophobic/hydrophilic separation of sulfonated polyetherketones compared to NAFION corresponds to narrower, less connected hydrophilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly reduce electroosmotic drag and water permeation whilst maintaining high proton conductivity. Blending of sulfonated polyetherketones with other polyaryls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to an improved swelling behaviour. Therefore, polymers based on sulfonated polyetherketones are not only interesting low-cost alternative membrane material for hydrogen fuel cell applications, they may also help to reduce the problems associated with high water drag and high methanol cross-over in direct liquid methanol fuel cells (DMFC). The relatively high conductivities observed for oligomers containing imidazole as functional groups may be exploited in fully polymeric proton conducting systems with no volatile proton solvent operating at temperatures significantly beyond 100°C, where methanol vapour may be used as a fuel in DMFCs.

On the development of proton conducting materials for technological applications

Some aspects of the simultaneous optimisation of material properties of proton conductors which are relevant for their use in electrochemical cells such as fuel cells, electrochemical reactors and sensors (high proton conductivity, chemical, electrochemical and morphological stability) are discussed. Suggestions are made for the further development of proton conducting perovskite type oxides, proton conducting polymer membranes and medium temperature proton conducting materials.

Aspects of the formation and mobility of protonic charge carriers and the stability of perovskite-type oxides

Proton conducting acceptor-doped perovskite-type alkaline earth cerates, zirconates, niobates and titanates have been investigated experimentally and by numerical simulations. For all cubic perovskites the concentration of protonic defects almost reaches the acceptor dopant concentration under appropriate conditions, and the mobility of protonic defects fall into a narrow range. Any symmetry reduction, however, leads to a reduction of the concentration and mobility of protonic defects. For all oxides, dynamical hydrogen bonding is suggested to lead to a local lattice softening, which provides an advantageous environment for high proton-mobility. This effect may explain the very high proton conductivity in covalent acceptor-doped BaZrO3, which has been found experimentally for the first time. Since this oxide also shows good thermodynamic phase stability, it is an interesting candidate as separator material in high-drain electrochemical applications such as fuel-cells.

Imidazole and pyrazole-based proton conducting polymers and liquids

The properties of imidazole (pyrazole) as a solvent for acidic protons in polymers and liquids are reported. The creation of protonic defects and the mobility of protons in these environments are found to be similar to the situation in corresponding water containing systems. The temperature stability is, however, increased and imidazole (pyrazole) is a stronger Bronstedt base compared to water, which may be useful for the application of such materials as electrochemical cells such as fuel cells and secondary batteries.

Proton conducting alkaline earth zirconates and titanates for high drain electrochemical applications

Abstract The mobility and stability of protonic defects in acceptor-doped perovskite-type oxides (ABO 3 ) in the system SrTiO 3 –SrZrO 3 –BaZrO 3 –BaTiO 3 have been examined experimentally and by computational simulations. These materials have the potential to combine high proton conductivity and thermodynamic stability. While any structural and chemical perturbation originating from the B-site occupation (poor chemical matching of the acceptor-dopant or Zr/Ti-mixing) leads to a significant reduction of the mobility of protonic defects, Sr/Ba-mixing on the A-site appears to be less critical. The stability of protonic defects is found to essentially scale with the basicity of the lattice oxygen, which is influenced by both A- and B-site occupations. The highest proton conductivities are observed for acceptor-doped BaZrO 3 . Despite its significantly higher ionic radius compared to Zr 4+ , Y 3+ is found to be optimal as an acceptor dopant for BaZrO 3 . Mulliken population analysis shows that Y does not change the oxides basicity (i.e. it chemically matches on the Zr-site of BaZrO 3 ). The highest proton conductivities have been observed for high Y-dopant concentrations (15–20 mol%). For temperatures below about 700°C, the observed proton conductivities clearly exceed the oxide ion conductivities of the best oxide ion conductors. The high conductivity and thermodynamic stability make these materials interesting alternatives for oxide ion conductors such as Y-stabilized zirconia, which are currently used as separator material for high drain electrochemical applications, such as solid oxide fuel cells.

On the complexity of proton conduction phenomena

Abstract The proton transport mechanisms in systems containing water and in cubic perovskite-type oxides are analyzed in detail in terms of chemical interactions. Some features of proton transport in hydroxides and systems containing oxo-acid anions or heterocycles as proton solvents are also described. Despite the differences between the rather complex proton conduction mechanisms in these systems, significant structural and dynamical variations of hydrogen bond interactions as a result of a good balance of chemical interactions in a wide range of configuration space have been identified as common features. These allow for high rates of proton transfer and structural reorganization, the two reactions involved in long-range proton transport. From the analyses and additional information on mesoscopic effects, suggestions are also made for the optimization of the mobility of protonic defects in hydrated, acidic polymers and perovskite-type oxides, which are interesting as separator materials for fuel cells. For the former, the importance of the microstructure is emphasized, while, for the latter, the prospects for III–III perovskites are stressed.

The diffusion mechanism of an excess proton in imidazole molecule chains: first results of an ab initio molecular dynamics study

Abstract The diffusion mechanism of an excess proton in imidazole molecule chains is studied by Car–Parrinello-type ab initio molecular dynamics simulations. The diffusion process is described by a Grotthuss mechanism (structure diffusion) involving proton transfer and local rather than long-range cooperative reorientation of the imidazole chain. At T =390 K, the proton transfer step is found to be fast with a time scale of 0.3 ps. The reorientation step is found to be rate-determining. According to our model, the time scale for the reorientation step is estimated to be approximately 30 ps in this temperature range.

Proton mobility in oligomer-bound proton solvents: imidazole immobilization via flexible spacers

Abstract A completely new approach for obtaining high proton conductivity in polymers based on proton solvating heterocycles covalently bound via flexible spacers is presented. Imidazole-terminated ethyleneoxide (EO) oligomers as model materials have been characterized with respect to their conductivity by ac impedance spectroscopy and with respect to 1 H- and 19 F-diffusion coefficients by pulsed magnetic field gradient-NMR (PFG-NMR). Comparison of conductivity and NMR diffusion coefficients of pure and acid doped materials shows ‘structure diffusion’ (intermolecular proton transfer and structural reorganization by hydrogen bond breaking and forming processes) to be the dominant conduction process, which gives rise to proton conductivities of up to 5×10 −3 S cm −1 at 120°C in completely waterfree materials.

The mechanism of proton conduction in phosphoric acid

Neat liquid phosphoric acid (H(3)PO(4)) has the highest intrinsic proton conductivity of any known substance and is a useful model for understanding proton transport in other phosphate-based systems in biology and clean energy technologies. Here, we present an ab initio molecular dynamics study that reveals, for the first time, the microscopic mechanism of this high proton conductivity. Anomalously fast proton transport in hydrogen-bonded systems involves a structural diffusion mechanism in which intramolecular proton transfer is driven by specific hydrogen bond rearrangements in the surrounding environment. Aqueous media transport excess charge defects through local hydrogen bond rearrangements that drive individual proton transfer reactions. In contrast, strong, polarizable hydrogen bonds in phosphoric acid produce coupled proton motion and a pronounced protic dielectric response of the medium, leading to the formation of extended, polarized hydrogen-bonded chains. The interplay between these chains and a frustrated hydrogen-bond network gives rise to the high proton conductivity.

Electroosmotic drag in polymer electrolyte membranes: an electrophoretic NMR study

Abstract Electrophoretic NMR has been applied for the first time to measure electroosmotic drag coefficients K drag in polymer electrolyte membranes. Theoretical and experimental details of the method are discussed and measurements of K drag as a function of water content and temperature are reported for Nafion ® 117 and sulfonated PEEKK. For a given water content n =[H 2 O]/[SO 3 H], the values for sulfonated PEEKK are lower than for Nafion, but for the water contents of the highest proton conductivities observed ( n =20 for Nafion n =40 for sulfonated PEEKK), the results are similar for both polymers ( K drag =2.6 for Nafion and K drag =3.1 for sulfonated PEEKK). A hydrodynamic model equation with the rate of proton transfer processes and the water–polymer interaction as parameters is used for the interpretation of the data.