Formation Of Protonic Defects Research Articles

Structure and simulation of a Zundel ion stabilized by 8-hydroxyquinoline-5, 7 disulphonic acid

8-hydroxyquinoline-5, 7 disulphonic was synthesized and recrystallized in methanol to strip away molecules of water. The structure of the molecule revealed that Zundel ion was stabilized in the crystal. Ab-initio molecular dynamics simulation was then carried out to understand the dynamics of proton hopping in this complex. During the course of simulation, the Zundel ion coordinates with a water molecule to form an open H7O3+ structure. This transition state structure de-solvated rapidly forming Zundel ion facilitating proton hopping in the first solvation shell. One of the sulphonic acid groups in the 5 or 7 position of the 8-hydroxyquinoline 5,7 disulphonic acid bonds with the Zundel ion favoring the proton to be transferred to the nearby water molecule through the formation of proton defects. The simulation results support the structural diffusion mechanism and that charged complex migrates through the hydrogen bond network.

Oxygen reduction reaction of PrBaCo2−xFexO5+δ compounds as H+-SOFC cathodes: correlation with physical properties

PrBaCo2−xFexO5+δ solid solution is investigated in order to understand the Oxygen Reduction Reaction (ORR) and water formation occurring at the H+-SOFC cathode. Careful attention is paid to the study of the physical properties as a function of composition by Thermogravimetry Analysis (TGA), Mossbauer spectroscopy, electrical conductivity and Seebeck coefficient measurements, with the aim to establish the correlation existing with the ORR activity for these Mixed Ionic Electronic Conductors (MIEC). The oxygen diffusion coefficients are determined by Electrical Conductivity Relaxation (ECR) and Isotopic Exchange Depth Profile (IEDP) coupled with Secondary Ion Mass Spectroscopy (SIMS) methods. An electrochemical study is then carried out and shows that the amount of oxygen vacancies is the most influential parameter. Indeed, it allows some hydration of PrBaCo2O5+δ oxide and the formation of protonic defects that can induce protonic diffusivity in these MIEC oxides.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2(x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28−xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD2O and pO2 at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10−6 K−1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

Mechanisms of proton transfer in Nafion®: elementary reactions at the sulfonic acid groups

Proton transfer reactions at the sulfonic acid groups in Nafion were theoretically studied, using complexes formed from triflic acid (CF3SO3H), H3O+ and H2O, as model systems. The investigations began with searching for potential precursors and transition states at low hydration levels, using the test-particle model (T-model), density functional theory (DFT) and ab initio calculations. They were employed as starting configurations in Born-Oppenheimer molecular dynamics (BOMD) simulations at 298 K, from which elementary reactions were analyzed and categorized. For the H3O+-H2O complexes, BOMD simulations suggested that a quasi-dynamic equilibrium could be established between the Eigen and Zundel complexes, and that was considered to be one of the most important elementary reactions in the proton transfer process. The average lifetime of H3O+ obtained from BOMD simulations is close to the lowest limit, estimated from low-frequency vibrational spectroscopy. It was demonstrated that proton transfer reactions at -SO3H are not concerted, due to the thermal energy fluctuation and the existence of various quasi-dynamic equilibria, and -SO3H could directly and indirectly mediate proton transfer reactions through the formation of proton defects, as well as the -SO3- and -SO3H2+ transition states.

Nanoionics phenomenon in proton-conducting oxide: Effect of dispersion of nanosize platinum particles on electrical conduction properties

High-temperature proton conductors are oxides in which low-valence cations are doped as electron acceptors; the incorporation of water molecules into the oxides results in the formation of protonic defects that act as charge carriers. Since the protons thus formed are in equilibrium with other electronic defects, electrons and holes, the oxides possibly have different proton-conduction properties at and near boundaries when they are in contact with another phase. In this paper, we present our recent experimental observation of a marked change in the electrical properties of a proton conductor upon the dispersal of fine platinum particles in the oxide. First, the material shows extremely low electrical conductivity in comparison with the original proton-conducting perovskite. Second, there was a threshold amount of platinum at which such a drop in conductivity occurred. A percolation model is employed to explain these experimental results; the fine platinum particles dispersed in the proton-conducting oxide wears highly resistive skin that is formed due to shifts in defect equilibriums, which prevents ionic/electronic conduction. The experiments suggest that the ion-conducting properties of oxides can be varied by introducing interfaces at a certain density; nanoionics is a key to yielding enhanced and/or controlled ionic conduction in solids.

Formation of Protonic Defects in Perovskite‐Type Oxides with Redox‐Active Acceptors: Case Study on Fe‐Doped SrTiO3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Formation of protonic defects in perovskite-type oxides with redox-active acceptors: case study on Fe-doped SrTiO3

The thermodynamics of water incorporation into Fe-doped SrTiO(3) was investigated by thermogravimetric measurements. Changes in valence states of redox-active dopant ions (Fe(3+)/Fe(4+)) with water vapor pressure were taken into account in the defect chemical analysis. The proton solubility was significantly enhanced by the presence of the redox centers. The hydration enthalpies and entropies were -60 kJ mol(-1) and -122 J mol(-1) K(-1). The defect chemical model was applied to describe the water vapor dependence of the electrical conductivity in mixed ionic and electronic conducting Fe-doped SrTiO(3) single crystals.

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

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 intra-octahedra proton transfer for BaCeO3. Whereas for BaCeO3 the proton transfer step is identified to be rate-limiting at T= 1000 K, for SrCeO3 both proton transfer and reorientation are found to be of similar magnitude.