W. Münch

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.

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 diffusion in perovskites : comparison between BaCeO3, BaZrO3, SrTiO3, and CaTiO3 using quantum molecular dynamics

Quantum molecular dynamics simulations have been carried out to calculate the diffusion coefficients and the activation energies of protonic defects in BaCeO3, BaZrO3, SrTiO3 and CaTiO3. The calculated activation energies are in agreement with experimental data within statistical uncertainty. The activation energy for proton transfer is found to be significantly affected by the repulsive interaction of the proton with the B-cation (B=Ce, Zr, Ti). A physical interpretation for the measured infra-red spectra can also be obtained from the numerical results.

A quantum molecular dynamics study of the cubic phase of BaTiO3 and BaZrO3

Abstract High proton mobility in perovskite-type oxides of composition ABO3 strongly depends on the dynamics of the proton environment, especially on the fluctuations of the oxide ion separations. The dynamics of the oxide host lattices of the model materials BaTiO3, and BaZrO3 have been studied using quantum molecular dynamics simulations. The simulation method has already been shown to yield numerical results in agreement with experimental findings for the cubic phase of BaCeO3. At elevated temperatures, rotational diffusion of the protons around the oxygen atoms in the plane perpendicular to the B-O-B axis is found. The free energy of the oxygen lattice vibrations is evaluated and the activation energy for proton transfer is estimated to be 0.45 eV for BaTiO3, 0.69 eV for BaZrO3, and 0.64 eV for BaCeO3.

A quantum molecular dynamics study of proton conduction phenomena in BaCeO3

Abstract Acceptor doped barium cerate dissolves significant amounts of water thus forming protonic defects. High proton mobility is generally considered to depend strongly on the dynamics of the proton environment. In this paper numerical results using quantum molecular dynamics simulations are presented to study the dynamics of the BaCeO3 host lattice. At elevated temperatures O-Ce-O bending modes mainly determine the oxygen separation coordinate Q which is considered to be responsible for modulating the proton transfer potential. It is also anticipated that this frequency coincides with the attempt frequency for proton conduction. For low proton excitations the dynamics of the proton is found to be part of a hydroxide ion rotating around the oxygen atom in the plane perpendicular to the Ce-O-Ce axis.

The relation between crystal structure and the formation and mobility of protonic charge carriers in perovskite-type oxides: A case study of Y-doped BaCeO3 and SrCeO3

Abstract Proton conductivity phenomena in 10% Y-doped barium and strontium cerate are investigated experimentally and by quantum molecular dynamics simulations. In particular the impact of deviations from the cubic perovskite structure on the formation and mobility of protonic charge carriers is investigated. For Y: SrCeO3, which shows a larger deviation from the ideal cubic perovskite structure, the concentration and mobility of protonic defects is significantly lower than for Y: BaCeO3. The first is due to the decay of the oxygen position into two sites, only one of which is involved in the formation of protonic defects. The symmetry reduction also leads to the formation of different one-dimensional proton diffusion paths, and unfavourable jumps between such paths are supposed to control the macroscopic proton diffusion coefficient in Y: SrCeO3. The analysis suggests the formation of strong but transient hydrogen bonds and inter-octa-hedra proton transfer between vertices for SrCeO3 in contrast to just ...

A molecular dynamics study of the high proton conducting phase of CsHSO4

Abstract The results of a molecular dynamics study of the high temperature phase of CsHSO 4 are presented. The estimated sulfate tetrahedra reorientation rate, and rate and activation enthalpy of the proton transfer are found to be in reasonable agreement with experimental results despite neglecting effects of hydrogen bonds on the dynamics of the host lattice.

A quantum molecular dynamics study of proton diffusion in SrTiO3 and CaTiO3

Abstract Protonic motion in cubic perovskite SrTiO3 and CaTiO3 is investigated by numerical simulations at higher temperatures. The protons are primarily found to form transient hydrogen bond complexes. The repulsive titanium/proton interaction causes a bending of the hydrogen bonds and, thus, aggravates proton transfer. However, as the proton interaction also extends to the next-next nearest oxygen sites the formation of transient, linear inter-octahedra hydrogen bonds, i.e. between the tips of neighbouring octahedra, is also possible. Whereas the time constants for proton reorientation are found to be of similar magnitude in both materials, the time constant for proton transfer is found to be larger by an order of magnitude in SrTiO3. Furthermore, the numerical simulations yield an activation energy for proton diffusion of 0.50±0.22 eV for SrTiO3 and 0.42±0.30 eV for CaTiO3.

SrTiO3 as a prototype of a mixed conductor Conductivities, oxygen diffusion and boundary effects

Abstract Partial conductivities and profiles of oxygen stoichiometry have been investigated in acceptor doped SrTiO 3 in the temperature range 450 to 1000 K and oxygen partial pressures between 10 0 and 10 5 Pa. For the first time, in-situ profiles could be observed as a function of time and evaluated according to the transport equations. Experimentally, the optical absorbance due to the corresponding valence states of internal redox-couples was recorded by a CCD-camera. If the effect of internal reaction sinks or sources being in local equilibrium is taken into account, the chemical diffusion coefficient is in remarkable agreement with the defect chemical analysis. Within the same model, partial conductivities can be understood without adjustable parameter. To achieve this, and also to apply appropriate techniques of measurement, it has to be taken into account that the surface exchange reaction becomes rate-limiting and very sluggish at low temperatures. This kinetic constraint has to be introduced in the low temperature analysis. The use of YBa 2 Cu 3 O 6 + x electrodes extends the diffusion controlled oxygen exchange to far lower temperatures ( T ≈ 450 K). Further investigations on grain boundaries by impedance, TEM, EDX and EELS measurements yield detailed information on electrical behaviour and the chemical composition of the different types of interfaces (free surfaces, crack surfaces, tilt and twist grain boundaries). Depletion layers along the grain boundaries and adjacent to the metallic electrode contact region can be identified.

Proton and apparent hydride ion conduction in Al-substituted SrTiO3

Abstract Hydrogen ion conductivity in 2% and 10% Al-substituted SrTiO 3 has been investigated by transport number measurements using the concentration cell/emf method in wet atmospheres as a function of p O 2 (10 −20 –1 atm) and temperature (350–1000 °C). Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures have been confirmed. By the present measurements, possible artefacts from the type of acceptor-dopant, gas buffer, electrode material, and porosity of the sample appear to have been ruled out. Electrochemical pumping experiments with gas chromatography were inconclusive with respect to hydride ion transport. Thermogravimetry as a function of hydrogen activity did not show evidence of hydride ion incorporation, but indicated uptake of neutral hydrogen under reducing conditions and high temperatures. Quantum molecular dynamics simulations indicate the existence of defect species or clusters that may be reminiscent of interstitial hydrogen with a tendency to associate with effective negative charge, e.g., on neighbouring titanium ions, under simulated reducing conditions.