Jonathan M. Polfus

Complete structural model for lanthanum tungstate: a chemically stable high temperature proton conductor by means of intrinsic defects

This is the first paper reporting that lanthanum tungstate, earlier believed to be La6WO12, is in fact La28−xW4+xO54+δ , where tungsten dissolves in lanthanum sites to form a stable solid-state electrolyte, exhibiting proton conduction by hydration at intermediate temperatures.

Surface defect chemistry of Y-substituted and hydrated BaZrO3 with subsurface space-charge regions

First-principles calculations were utilized to elucidate the complete defect equilibria of surfaces of proton conducting BaZrO3, encompassing charged species adsorbed to the surface, defects in the surface layer as well as in the subsurface space-charge region and bulk. Defect calculations were performed for the BaZrO3 (0 0 1) surface with focus on protons, oxygen vacancies and Y-acceptor dopants as well as adsorbed hydroxide and oxide adions. Protons were found to exhibit a particularly strong tendency to segregate to the surface with a segregation energy of −1.3 eV. While the concentration of negatively charged Y-acceptors and hydroxide species on the outer surface can be quite high, they do not fully charge compensate the protons, yielding a net positive charge of the surface. The resulting surface potential can exceed 1 V, resulting in significant depletion of charge carriers in the subsurface space-charge region. Moreover, the results are discussed in relation to surface adsorption of water, and computational approaches for treating charged point defects in periodic slab cells are evaluated with respect to symmetry and charge compensation.

Nitrogen and hydrogen defect equilibria in Ca12Al14O33: a combined experimental and computational study

The defect structure of mayenite is investigated by Density Functional Theory (DFT) defect calculations; in situ electrical conductivity measurements in NH3 atmosphere at high temperature; and X-ray photoelectron spectroscopy (XPS) and gas phase mass spectrometry (GP-MS) of NH3 treated specimens. The computational results suggest that nitrogen is primarily incorporated substitutionally on oxygen sites as NH−2 and N3−. The concentration of nitrogen was estimated to be within the same order of magnitude by XPS, GP-MS and DFT, yielding a stoichiometry close to Ca12Al14O31.5N0.5:(NH2)0.5O0.5 which corresponds well with that obtained by Boysen et al. from similarly treated samples. Out diffusion of nitrogen was found to occur around 700 °C in Ar by XPS, GP-MS and conductivity measurements, also in accordance with Boysen et al. The conductivity measurements showed that NH3 treatment had a significant effect on the defect structure of the material which became evident only after replacing the NH3 atmosphere with Ar: the conductivity increased abruptly due to a temporary non-equilibrium reduction of the material as nitrogen diffuses out while the lack of a sufficiently large source of oxygen in the surrounding atmosphere prevents the specimen from re-oxidizing. Further, based on the computational results and the pH2 dependency on conductivity after NH3 treatment, we propose dissolution of hydride ions from H2 in the reduced and highly conductive post-NH3 state.

Solubility of transition metal interstitials in proton conducting BaZrO3 and similar perovskite oxides

The defect chemistry of foreign transition metals in perovskite oxides was investigated by first-principles calculations with focus on Ni and Zn in Y-doped BaZrO3. Additional transition metals (Cu, Fe, Pd, Pt, and Ag) and perovskites (SrZrO3 and SrTiO3) were considered for comparison. The octahedral interstice coordinated with square-planar oxygen could accommodate smaller cations and Ni2+ was found to be the most stable, particularly in the presence of barium vacancies. Significant solubility of Ni was substantiated only for nominally A-site deficient materials under oxidizing conditions. The computational results were corroborated by experimental studies on BaZr0.85Y0.15O3−δ with 4 mol% NiO or ZnO sintering additives. While synchrotron radiation X-ray powder diffraction of the Ni containing sample showed the presence of a BaY2NiO5 secondary phase, it could not account for the nominal amount of Ni in the sample. STEM and EDS analyses of both the Zn and Ni containing samples showed that Zn accumulated in the grain boundaries while Ni was evenly distributed within the grains and grain boundaries indicating that Ni was dissolved in the BaZrO3 structure. Furthermore, metallic Ni particles appeared on the sample surface after treatment under reducing conditions in accordance with computational predictions. The influence of interstitially dissolved Ni on proton conductivity was evaluated based on trapping of protons. Barium vacancies were found to be strong proton traps, with a binding energy of −0.80 eV, while the binding energy of protons associated with adjacent Ni interstitials was reduced to −0.20 eV.