Annette Fuchs

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.

H/D isotope effect of proton conductivity and proton conduction mechanism in oxides

Abstract The elementary steps of proton conductivity in oxides, proton transfer and hydroxyl ion reorientation, are investigated and the anticipated H D isotope effects of their rates are compared to those of proton (deuteron) conductivity. It is concluded that the rate of proton diffusivity in many oxides is limited by proton transfer between fixed oxygen sites. This is facilitated by the thermal fluctuation of the oxygen ion separation. In the case of a rigid array of oxygen ions an attempt frequency close to that of the OH (OD) oscillator and a high activation enthalpy is expected. The effect of softening the OO vibration is to decrease the attempt frequency and activation barrier. Criteria for high diffusivities of protonic defects in oxides are formulated.

Single Alkaline‐Ion (Li+, Na+) Conductors by Ion Exchange of Proton‐Conducting Ionomers and Polyelectrolytes

The vast majority of alkaline ion conductors used in electrochemical cells, especially batteries, are solutions of salts (such as LiClO4, LiCF3SO3 or LiPF6) in either aprotic polar solvents (such as ethylene carbonate) or polar polymers containing ether bridges (such as poly-ethyleneoxides, PEO). The solvating properties of such environments are poor compared to what is known for aqueous systems: there is still significant residual ionic interaction leading to correlations of the motion of cations and anions 4] and even the formation of contact ion pairs and triple ions. 6] Such effects are most pronounced for polymeric solvents. Here, the salt dissociation is driven by the coordination of alkaline ions with ether oxygens while the anions are chemically more free to move. As a consequence, such materials are more efficient anions than alkaline ion conductors. 4] Of course, such differences are smeared out in liquid electrolytes, but even for these there is strong indication for the formation of contact ion pairs and effective alkaline ion transference numbers smaller than the ones of anions (it is worth noting that alkaline ions can also be transferred by the counter diffusion of, for example, contact ion pairs and anions). The combination of relatively low total conductivity and low alkaline ion transference number is expected to lead to significant concentration polarization effects (formation of salt concentration gradients) even at moderate alkaline ion currents. Especially for high drain applications, such as high power batteries, electrolytes with a high single ion (Li, Na) conductivity may therefore help to reduce polarization effects within the electrolyte. This has been stimulating many attempts towards true single alkaline ion conductors, but in most cases either the transference number remained clearly below unity 11] or the conductivity was reduced to a non acceptable level. Most approaches aim at a retardation or complete immobilization of the anions. This may be achieved by introducing receptor functionalities for the anions (e.g. on polymers or particles) or by covalently binding the anion to a stationary phase (e.g. polymer, particle). Recently, Armand et al. have shown that by using anions with highly delocalized negative charge, even covalent immobilization may still allow for reasonable single ion conductivity. For a polystyrene functionalized with -[SO2-N-SO2-CF3] Li and solvated with PEO, they measured a conductivity of 3 10 5 S cm 1 at T = 60 8C. So far, the highest conductivities have been obtained for ionomers in their Li-form swollen with aprotic polar solvents. While for solvated polyphosphazenes with sulfonimide functional groups in their Li-form only moderate room temperature conductivities of up to 2.5 10 6 S cm 1 are reported, quite high conductivities reaching values of 7 10 4 S cm 1 have been measured for Li exchanged Nafion solvated with NMF. 21] These conductivities could be even further increased to values slightly above 1 mS cm 1 by using solvent mixtures (e.g. NMP/DMF) or by the additional use of crown-ethers as Licomplexing agent. To which extent the observed conductivities are carried by Li has not been examined yet. This Communication reports on polyelectrolytes solvated with aprotic polar solvents with conductivities reaching values so far known only for liquid aprotic electrolytes (i.e. larger 1 mS cm ), but with alkaline ion transference numbers close to unity, as evidenced by the comparison of conductivity and tracer diffusion coefficient data. Also, the relation between solvent and ion diffusion is investigated for the first time. The approach is based on our recent experience in the development of ionomers and polyelectrolytes for fuel cell applications showing high proton conductivity even at high temperature and low relative humidity. Under these conditions, also water becomes a poor solvent, that is, dissociation is no longer complete which has severe implications for the local structure and dynamics of the system. The clue for obtaining high proton conductivity turned out to be a very high local density of superacidic anions. These were sulfonic acid groups covalently bound to each phenyl ring of a poly phenylene-sulfone. Owing to the strong M effect of the sulfone links, the phenyl rings are low in electron density which is making the sulfonic acid functional group attached to these rings very acidic and the polymer backbone less sensitive towards electrophilic attack. 23] The very high density of the ionic groups allows for the formation of a continuous aqueous domain even at low hydration numbers l = [H2O]/[-SO3H] at which the very high acidity still leads to a substantial concentration of protonic charge carriers. These are actually hydronium ions and the conduction mechanism is of the vehicle type, that is, the diffusion of hydronium ions is a simple hydrodynamic process without significant proton exchange between hydronium ions and the solvent (water). In other words, the protonic charge carrier behaves like any other monovalent cation in this environment, and this suggests that also high alkaline ion conductivity may be observed in such systems. To test this hypothesis, we have ion exchanged highly sulfonated poly phenylene-sulfones in the acid form with Li and [a] Dr. K.-D. Kreuer, A. Wohlfarth, Dr. C. C. de Araujo, A. Fuchs, Prof. Dr. J. Maier Max-Planck-Institut f r Festkçrperforschung Heisenbergstraße 1, D-70569 Stuttgart (Germany) Fax: (+ 49) 711-689-1722 E-mail : kreuer@fkf.mpg.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201100506.