J. Maier

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.

Carbon-Coated Na3V2(PO4)3 Embedded in Porous Carbon Matrix: An Ultrafast Na-Storage Cathode with the Potential of Outperforming Li Cathodes

Sodium ion batteries are one of the realistic promising alternatives to the lithium analogues. However, neither theoretical energy/power density nor the practical values reach the values of Li cathodes. Poorer performance is expected owing to larger size, larger mass, and lower cell voltage. Nonetheless, sodium ion batteries are considered to be practically relevant in view of the abundance of the element Na. The arguments in favor of Li and to the disadvantage of Na would be completely obsolete if the specific performance data of the latter would match the first. Here we present a cathode consisting of carbon-coated nanosized Na3V2(PO4)3 embedded in a porous carbon matrix, which not only matches but even outshines lithium cathodes under high rate conditions. It can be (dis)charged in 6 s with a current density as high as 22 A/g (200 C), still delivering a specific capacity of 44 mAh/g, while up to 20 C, the polarization is completely negligible.

Grain Boundary Blocking Effect in Zirconia: A Schottky Barrier Analysis

The grain boundary electrical properties of high purity ZrO 2 ceramic materials doped with 2, 3, and 8 mol % Y 2 O 3 , and 8 mol % Y 2 O 3 co-doped with 0.4 mol % Al 2 O 3 were studied in the temperature range of 200 to 500°C by electrochemical techniques and were theoretically analyzed. Although the presence of a siliceous phase is shown to be a major cause for the grain boundary blocking effect, the grain boundary properties appear to he significantly influenced by space charges, particularly in materials of high purity. The oxygen vacancy distribution and the grain boundary resistivity were calculated for 8 mol % Y 2 O 3 doped ZrO 2 by assuming double Schottky barriers, and the results were compared with the experiment. It is shown that reasonable space charge potentials lead to grain boundary effects which are consistent with the experimental features. In contrary to the bulk in which defect associates prevail (at temperatures <560°C), in the boundary regions, association effects can be assumed to be much less pronounced due to the vacancy depletion.

Generalised equivalent circuits for mass and charge transport: chemical capacitance and its implications

An exact equivalent circuit including terminal parts, which takes account of electrical and chemical control parameters in a unified way, is derived for a cell with a mixed conductor (or electrolyte) without internal sources or sinks. In one-dimensional problems electrochemical kinetics can be mapped by two-dimensional circuits exhibiting the spatial and the thermodynamic displacement as two independent coordinates. One main advantage of the exact circuits with respect to the underlying differential equations is the ability to simplify the description according to specific situations. As we show in several examples in the second part of the paper, it is straightforward to select the elements relevant for the particular experimental conditions and so to make appropriate approximations. This is most helpful for the description of electrochemical systems, such as fuel cells, membranes, pumps and batteries.

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.

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.

Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes

Geometrically well-defined model electrodes have been employed to unambiguously elucidate the individual resistive and capacitive processes of various solid oxide fuel cell cathodes by means of impedance spectroscopy. The measurements were performed on dense, thin film-type microelectrodes of La 1-x Sr x Co 1-y Fe y O 3-δ and related perovskite-type materials prepared by pulsed laser deposition and photolithography. It was found that the substitution of the A-site cation La in La 1-x Sr x Co 1-y Fe y O 3-δ by Sm and especially by Ba leads to a strong enhancement of the surface exchange kinetics, whereas a variation of the Co/Fe ratio between 0 and 1 has only little effect on this quantity at temperatures around 750°C. Furthermore, it has been studied how the electrochemical activation effect, i.e., the strong reduction of the surface exchange resistance after application of a large dc bias, depends on composition.

The polarization of mixed conducting SOFC cathodes: Effects of surface reaction coefficient, ionic conductivity and geometry

Multi-dimensional finite element simulations of current distributions in mixed ionic and electronic conducting cathodes (MIEC) are presented for the case that the cathodic oxygen incorporation into an electrolyte takes place through the bulk of the electrode. The effects of the ionic conductivity and the surface reaction coefficient on the overall process are analyzed. Depending on these material parameters different parts of the cathode are involved in the oxide ion transport to the electrolyte (from a very small region close to the three phase boundary for a fast surface reaction up to the entire cathode for a very slow surface reaction). The calculations also reveal which combinations of ionic conductivity and surface reaction coefficient are appropriate to achieve acceptable polarization resistances. The influence of the particle size is discussed and interpolation formulae are given to estimate the cathodic polarization in porous MIECs.

Characterization of adsorbed water layers on Y2O3-doped ZrO2

Chemisorption and physisorption of water from a humid atmosphere, on the oxide Y2O3-doped ZrO2 (YSZ), were investigated by conductivity measurements and thermogravimetry (TG), in the temperature range 35–700°C. Ionic conduction is indicated to take place in a thin layer of water adsorbed on the oxide. That ionic conduction is most probably proton conduction. At T<600°C, it can exceed the oxygen ion conduction within the YSZ bulk when the oxide is made of fine powder. The electrical conductivity measurements of YSZ fine powder in humid atmosphere are analyzed in terms of water coverage, defect formation and proton mobility in the water layers. Further analysis of the water coverage is done using TG measurements. The enthalpies of defect formation and migration are determined. Results of preliminary conduction measurements of Al2O3 fine powder in a humid atmosphere reveal considerable similarities to those of YSZ.