Y‐doped BaZrO3 (BZY) is a promising candidate as an electrolyte in fuel cells, and attracts increasing attention. In this work, a systematic investigation was performed on microstructure, proton concentration, proton conductivity, and hydration induced chemical expansion in Y‐doped BaZrO3. The results revealed that the bimodal microstructure in BaZr0.85Y0.15O3−δ was composed of large grains with composition close to the nominal value, and fine grains with large compositional discrepancy. This property is considered to be one of the evidences of phase separation at lower temperature than sintering temperature (1600°C), which hinders the grain growth. Thermal expansion coefficient of BZY was measured for various dopant level, and was determined to be around 10−5 K−1 in wet and dry argon atmosphere. In addition, chemical expansion effect due to hydration was confirmed by HT‐XRD in dry and wet Ar atmospheres, and suggests an interesting relationship between the lattice change ratio and proton concentration, in the BZY system with different Y content. The change ratio of lattice constant due to hydration increased obviously with the proton concentration for the sample containing the Y content of 0.02 and 0.05, but only changed slightly when the Y content was increased to 0.1 and 0.15. However, when the Y content was further increased over 0.2, the change ratio of lattice constant due to hydration starts to increase obviously again. Such results indicate a high possibility that the stable sites of protons in BZY changed with the variation in Y content.

The proton-conducting network in lanthanum orthophosphate, LaPO 4 , has been theoretically clarified from first principles in the present study. It consists of as many as 20 kinds of migration paths with potential barriers below 1 eV, which are classified into three groups, i.e., rotations and intraand intertetrahedral hoppings. As the results of the kinetic Monte Carlo simulations using the network of the migration paths, the calculated proton diffusion coefficients have anisotropy reflecting the monoclinic crystal structure, particularly large anisotropy in the ca-plane. It results from the dominant proton migration in the directions of [011], [011̅ ], [111], and [11̅ 1], whose potential barriers are ∼0.7 eV. The calculated activation energies of the diffusion coefficients in the fastest, the slowest, and the b-axis (unique axis) directions are 0.68, 0.76, and 0.70 eV (= 66, 73, and 68 kJ/mol), respectively, in the temperature range from 500 to 1500 K. These values are lower than the experimentally reported activation energies by conductivity measurements in the range of 0.8−1.0 eV. The major origin of the discrepancy is association effects between protons and dopants. The calculated association energy between protons and strontium dopants is 0.31 eV (30 kJ/mol), which makes the slope of the conductivity curves steeper by 0.21−0.11 eV in the temperature range of 773−1073 K.

In this work, a systematic work was performed to investigate the electrochemical transport properties of acceptor-doped BaZrO3 by measuring electromotive force on various gas concentration cells. For the measurements in the wet oxidizing atmosphere, where significant hole conduction occurs, the transport numbers of the ionic conduction in the oxidizing atmosphere were corrected by taking the effect of electrode polarization into consideration. The results revealed that regardless of whether Sc, Y, In, Ho, Er, Tm or Yb was doped, proton conduction predominates in the reducing atmosphere with the transport number close to unit. However, the contribution of ionic conduction weakens, and the contribution of hole conduction enhances, when the samples are exposed to the moist oxidizing atmosphere. In addition, introducing Ba-deficiency results in degraded electrochemical conductivity, but the transport number in either the moist reducing or the moist oxidizing atmosphere does not change obviously.

The effects of carrier–carrier interaction on the proton diffusivity in a proton-conducting perovskite, Y-doped BaZrO3, have theoretically been investigated in a first-principles manner. The proton diffusivity with the proton–proton interaction was estimated by solving the master equation under the single-particle approximation. The correlation effect between successive jumps was also taken into account to estimate the proton diffusivity with more accuracy. As a result, the proton–proton interaction has two competitive effects on the proton diffusivity, i.e., the negative effect of carrier blocking and the positive effect of trap-site filling. In BaZr1–x Y x O3−δ at the typical doping level (x = 0.2), the trap-site filling effect is dominant, resulting in higher proton diffusivity than that estimated without proton–proton interaction. This positive effect suggests a possible strategy for improving the proton mobility, i.e., trap-site-killer doping, which has a function to fill, annihilate, and/or destabilize the trap sites.

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.