RESUMEN
SrZrO3-based perovskites are promising proton-conducting membranes for use in fuel and electrolysis cells, sensors, hydrogen separators, etc., because they combine good proton conductivity with excellent chemical stability. In the present research, the effect of Lu-doping on microstructure, phase composition, and electrical conductivity of SrZr1-xLuxO3-δ (x = 0-0.10) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy. Dense ceramic samples were obtained by the solution combustion synthesis and possessed an orthorhombic perovskite-type structure. The solubility limit of Lu was revealed to lie between x = 0.03 and 0.05. The conductivity of SrZr1-xLuxO3-δ increases strongly with the addition of Lu at x < 0.05 and just slightly changes at x > 0.05. The rise of the water vapor partial pressure results in an increase in the conductivity of SrZr1-xLuxO3-δ ceramics, which confirms their hydration ability and significant contribution of protonic defects to the charge transfer. The highest conductivity was achieved at x = 0.10 (10 mS cm-1 at 700 °C, wet air, pH2O = 0.61 kPa). The conductivity behavior was discussed in terms of the defect formation model, taking into account the improvement in ceramic sintering at high lutetium concentrations.
RESUMEN
The effect of Sr-deficiency on microstructure, phase composition and electrical conductivity of SrxZr0.95Yb0.05O3-δ (x = 0.94-1.00) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy. The samples were synthesized by a chemical solution method and sintered at 1600 °C. According to X-ray diffraction data, the samples with x = 0.96-1.00 were single-phase oxides possessing an orthorhombic perovskite-type structure; while zirconia-based minor phases arouse at x = 0.94, which was confirmed by the electron microscopy. Sr stoichiometry was shown to influence the electrical conductivity. The highest total and bulk conductivities, 6-10-4 Scm-1 and 3-10-3 Scm-1, respectively, at 600 °C in humid air (pH2O = 3.2 kPa), were observed for the x = 0.98 composition. In the temperature range of 300-600 °C, the conductivity of the samples with x = 0.96-1.00 increased with the increase in humidity, which indicates a significant contribution of protonic defects to the charge transport. Electrical conductivity of SrxZr0.95Yb0.05O3-δ was discussed in terms of the defect formation model and the secondary phases precipitation.
RESUMEN
The effect of nitric acid treatment on the electrochemical performance of porous Pt electrodes deposited on YSZ (abbreviation from yttria stabilized zirconia) electrolyte was investigated. Two identical symmetrical Pt/YSZ/Pt cells with porous Pt electrodes were fabricated, after which the electrodes of the first cell were kept as sintered, while those of the second cell were impregnated with HNO3 solution. The electrochemical behavior of the prepared electrodes was studied using impedance spectroscopy and cyclic voltammetry. Significant reduction of the polarization resistance of the HNO3-treated electrodes was revealed. The observed enhancement of the electrochemical performance of porous Pt electrodes was assumed to be caused by adsorption of NOx-species on YSZ and Pt surfaces, which promotes oxygen molecules dissociation and transport to the triple phase boundary by the "relay-race" mechanism. The obtained results allow for considering the nitric acid treatment of a porous Pt electrode as an effective way of electrode activation.
RESUMEN
The effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of SrxZrO3-δ and SrxZr0.95Y0.05O3-δ ceramics (SZx and SZYx, respectively; x = 0.94-1.02) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy followed by distribution of relaxation times analysis of impedance data. It was shown that at low Sr deficiency (x > 0.96 and 0.98 for SZx and SZYx, respectively) a solid solution of strontium vacancies in strontium zirconate crystal structure forms, whereas at higher Sr deficiency the secondary phase, zirconium oxide or yttrium zirconium oxide, is precipitated. Yttrium solubility limit in strontium zirconate was found to be close to 2 mol%. Y-doped strontium zirconates possess up to two orders of magnitude higher total conductivity than SZx samples. A-site nonstoichiometry was shown to have a significant effect on the electrical conductivity of SZx and SZYx. The highest total and bulk conductivity were observed at x = 0.98 for both systems. Increasing the conductivity with a rise in humidity indicates that proton conduction appears in the oxides in wet conditions. A defect model based on consideration of different types of point defects, such as strontium vacancies, substitutional defects and oxygen vacancies, and assumption of Y ions partitioning over Zr and Sr sites was elaborated. The proposed model consistently describes the obtained data on conductivity.