Lanthanum polyphosphate (LaP 3 O 9 ) is an attractive candidate for the electrolytes in fuel cells because of its relatively high proton conductivity. However, the proton conduction mechanism in LaP 3 O 9 (i.e., proton transport pathways and its relationship with the crystal structure) still remains poorly understood; thus, there has been no clear strategy for enhancing the conductivity. In this paper, we show that the fast and anisotropic proton conduction pathways exist along the b-and c-axes in the crystal lattice of LaP 3 O 9 , and the proton conductivity can be remarkably improved by controlling the microstructure of the electrolyte membranes. The first-principles calculations reveal that protons migrate only along the neighbors of specific oxide ions in the PO 4 chains, leading to the conductivity anisotropy, which is readily confirmed using Sr-doped LaP 3 O 9 single crystals. The c-axis oriented, coarse-grained polycrystalline membranes of Sr-doped LaP 3 O 9 prepared by solution synthesis techniques exhibit markedly enhanced conductivity compared to randomly oriented polycrystals prepared by solid state reaction and have direct applicability to fuel cell electrolytes. The discovery of fast proton conduction pathways in LaP 3 O 9 will motivate further development of LaP 3 O 9 -based electrolytes as well as exploration of new proton conducting crystalline polyphosphates with infinite chains of PO 4 tetrahedra.

A favorable cathode performance can be expected with La4Co3O10, due to the similarity to the representative cathode material of LaCoO3. However, its narrow stable oxygen partial pressure (pO2) range has made it difficult to synthesize and sinter La4Co3O10, and its electrical and ionic properties has not yet been studied enough. In this study, the dense body of La4Co3O10 was prepared by controlling pO2 during sintering within its stable range utilizing the equilibrium between Fe2O3 and Fe3O4. The electrical conductivity of La4Co3O10 is comparable to those of perovskite-type cathode materials such as LSCF. And its oxygen nonstoichiometry is hyperstoichiometric. Additionally, it was revealed that in air La4Co3O10 partially decomposes to LaCoO3 and La2O3 in 100 h at 800°C or higher, while no decomposition product was detected by X-ray diffraction at 700°C or lower.

Lanthanum nickelates with Ruddlesden‐Popper structure (La2NiO4, La3Ni2O7, and La4Ni3O10) and perovskite structure (LaNiO3) have attracted considerable attention due to their potential applications such as solid oxide fuel cells. Currently, the ionic and electronic conduction properties of La3Ni2O7, La4Ni3O10, and LaNiO3 are not fully understood because it is quite difficult to prepare their dense bodies required for the characterization. The difficulty arises from their narrow thermodynamic stable temperature and oxygen partial pressure ranges. In this study, we successfully obtained dense bodies of single‐phase La3Ni2O7, La4Ni3O10, and LaNiO3 via a post‐sintering oxidation process. First, dense pellets composed of fine‐grain precursors La2NiO4 and NiO (~0.5 μm) were prepared by nitrate freeze‐drying technique and low‐temperature sintering at 1150°C‐1225°C. Then they were converted into almost single‐phase La3Ni2O7, La4Ni3O10, and LaNiO3 by high‐temperature oxidation. La3Ni2O7 and La4Ni3O10 were obtained under an oxygen partial pressure pO2 of 1 bar at 1275°C and 1200°C‐1250°C, respectively, while LaNiO3 was obtained under pO2 of 392 bar at 1250°C using hot isostatic pressing. The relative densities of the pellets exceeded 90%. With regard to their phase stability, decomposition was not detected at 600°C‐1100°C in air for at least 100 hour despite their thermodynamic instability.

Doped lanthanum polyphosphate (LaP 3 O 9 ) exhibits relatively high proton conductivity. For the practical applications such as the electrolyte of fuel cells, however, its conductivity must be improved by 2 orders of magnitude. Protons are introduced into matrix by lower-valent cation doping, and proton conductivity depends on dopant species. To date, LaP 3 O 9 has been doped with only Ca, Sr and Ba. In this work, we tried to dope LaP 3 O 9 with Na + , K + , Mg 2+ and Pb 2+ , as the new dopant species, due to their close ionic radii to La 3+ . Among them, only Pb could substitute for La at a comparable concentration to those of alkaline earth metals and its highest doping level was 6.4 mol% (Doping level is defined as the concentration ratio of dopant (M) to host cation (La) site in matrix (≡M/(La + M) × 100 (mol%))). Though Pb can exist as either divalent or tetravalent state, Pb in LaP 3 O 9 was identified to be divalent state by XPS analysis. Proton conduction was demonstrated by H/D isotope effect. The electrical conductivity of Pb-doped LaP 3 O 9 increased with Pb-doping level, owing to the increase in proton concentration. The conductivity of 4.5 mol% Pb-doped LaP 3 O 9 was about one order of magnitude lower than that of 7.9 mol% Sr-doped LaP 3 O 9 .

Yttrium-doped barium zirconate (BZY) is accepted as one of the most promising electrolyte materials for protonic ceramic fuel cells (PCFCs). For the development of fuel cells with the BZY electrolytes, evaluation of electrode performance is important. In this work, electrochemical measurements of an electrolyte supported cell, Pd (wet H 2 ) | BaZr 0.8 Y 0.2 O 3-δ | La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (wet O 2 ), were performed at 600 • C in a three-electrode system. The ohmic resistances of the electrolyte under open circuit condition and DC current flow were evaluated by AC impedance and current interruption measurements, respectively. Overpotentials of cathode and anode are determined to be 0.587 and 0.053 V at 100 mAcm −2 , respectively, and exhibit a logarithmic relationship with current density. The results clearly indicate that high cathode overpotential is a significant problem restricting the fuel cell performance.

LaNi1-xFexO3 solid solutions are an interesting system exhibiting a composition-controlled metal-insulator transition and are also potential cathode materials for solid oxide fuel cells, but the composition dependence of their electrical...

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