Analysis of transport numbers is critical for assessing the suitability of an ionconducting material for a given electrochemical application and the conditions for its employment. In this work, the proton, oxide-ion and electron transport numbers of the candidate protonic ceramic electrolyser and fuel cell material SrZr0.9Y0.1O3- (with the addition of 4 mol% ZnO as sintering aid) are measured in wet and dry oxidising atmospheres in the temperature range 700-850 °C. The determination of proton transport numbers is analysed in detail, encompassing the suitability of equivalent circuits in different conditions and the inclusion of an external parallel resistance for the correction of electrode-polarisation effects (Gorelov method). It is confirmed that transport numbers are highly inaccurate if no polarisation correction is applied. In dry oxidising conditions oxide-ion transport numbers, to, lie in the range 0.630.78. The conductivity in wet oxidising conditions is dominated by protons and an electronic component, with the proton transport number increasing from 0.79 to 0.88 with increasing pH2O in the range 1.1x10  ≤ pH2O ≤ 1.27x10  atm at 700 ºC.

The crystal structure of the technologically relevant, high-temperature proton conductor BaZr0.7Ce0.2Y0.1O3- (BZCY72) has been studied by high-resolution neutron powder diffraction performed on a deuterated sample in the temperature range 10-1173 K, complemented with synchrotron X-ray diffraction in the range RT-1173 K. A volume discontinuity on heating indicates a first-order phase transition from orthorhombic (space group Imma) to rhombohedral symmetry ( 3 Rc ) between 85 and 150 K. A further transition to cubic symmetry ( 3 Pm m ) takes place at ~ 570 K, indicated to be second order from the temperature dependence of the octahedral tilt angle. The stability field of the cubic phase was extended on cooling in the dehydrated state to 85 K. Expansion/contraction of the unit-cell volume on heating in low vacuum and air, respectively observed by neutron diffraction and synchrotron X-ray diffraction, was described with a point-defect model involving the temperature dependence of the water content and thermal expansion. Isotropic strain in the hydrated state is apparent on analysis of the broadening of the neutron-diffraction reflections during heating and cooling cycles. Rietveld refinement of the low-temperature neutron data and Fourier nuclear-density maps were employed to locate the deuterium position at a distance of ~ 0.90 Å from the bonding oxygen at 10 K.

Partial conductivities are presented for BaZr0.7Ce0.2Y0.1O3- an important proton conductor for protonic-ceramic fuel cells and membrane reactors. Atmospheric dependencies of impedance performed in humidified and dry O2, air, N2 and H2(10%)/N2(90%) in the temperature range 300-900 ºC, supported by the modified emf method, confirm significant electron-hole and protonic contributions to transport. For very reducing and wet atmospheres, the conductivity is predominantly ionic, with a higher participation of protons with decreasing temperature and increasing water-vapour partial pressure (pH2O). From moderately reducing conditions of wet N2 to wet O2, however, the conductivity is considerably influenced by electron holes as revealed by a significant dependence of total conductivity on oxygen partial pressure (pO2). With higher pH2O, proton transport increases, with a concomitant decrease of holes and oxygen vacancies. However, the effect of pH2O is also influenced by temperature, with a greater protonic contribution at both lower temperature and pO2. Values of proton transport number tH ≈ 0.63 and electronic transport number th ≈ 0.37 are obtained at 600 ºC for pH2O= 0.022 atm and pO2= 0.2 atm, whereas tH ≈ 0.95 and th ≈ 0.05 for pO2 = 10-5 atm. A hydration enthalpy of-109 kJ.mol-1 is obtained in the range 600-900 ºC.

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Pr doping in BZCY72 induces symmetry changes and enhances mixed-conductivity for electrochemical applications, with Pr concentration influencing the charge-compensation mechanism.

The mixed ionic-electronic transport properties of the protonic ceramic electrolyser material SrZr0.9Y0.1O3-, with the addition of 4mol% ZnO as sintering additive, are analysed under reducing conditions. The study is performed by means of an active-load modification of the classical electromotive-force method to account for the nonnegligible effect of the electrodes on the obtained electrical-transport numbers. The methodology is developed in detail in order to link the electrochemical criteria to simulated equivalent circuits. The observed electromotive force of the system is considerably affected by the introduction of the polarisation resistance of the electrodes in the corresponding analysis, resulting in a high deviation between the present results and those obtained by a classical analysis without attending to electrode effects. Under wet reducing conditions (pH2  0.05 atm, pH2O 3·10 3 10 2 atm), the oxide-ionic transport number is negligible in the range of 600900 ºC, whereas pure protonic conductivity is observed for temperatures ≤ 700 ºC and pH2O ≥ 5.6x10 3 atm. For higher temperatures and/or lower pH2O, mixed protonic-electronic conduction is exhibited. The electronic contribution under reducing conditions is consistent with ntype electronic behaviour.

The BaZr0.7Ce0.2Y0.1O3--BaPrO3- perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid solution with a wide interval of Ba substoichiometry in the range Ba(Ce0.2Zr0.7)1-xPrxY0.1O3-, 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, 3 ̅). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidising conditions, with total conductivity reaching a value of ~ 0.5 S.cm-1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N2) and at low Pr contents, conductivity is higher in humidified atmospheres (~ 0.023 atm pH2O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N2 due to the creation of protonic defects and the consumption of holes.