Donor- and transition metal-substituted SrTiO3: relevant issues in the development of ceramic components for SOFC anodes

Due to phase stability requirements, the choice of alternative perovskite-type materials for ceramic SOFC anodes is essentially limited to chromite-, molybdate-, vanadate- or titanate-based systems. Donor-doped SrTiO3 combines remarkable stability in both oxidizing and reducing conditions, substantial electronic conductivity, and good prospects for inhibition of carbon deposition and sulfur-tolerance. The present lecture gives a brief overview and a comparative assessment of a variety of strontium titanate-based solid solutions as fuel electrode materials with emphasis on the defect chemistry, reducibility and corresponding effects on the electrical properties, thermomechanical compatibility with solid electrolytes, and electrochemical activity. The exemplified oxide systems are (Sr,Ln)TiO3 (Ln = La-Yb), (Sr,Pr)TiO3, Sr(Ti,Ta)O3 and Sr(Ti,V)O3 series including nomically cation-stoichiometric and A-site deficient formulations [1-4].publishe

Pyrochlore-type Y2Ti2O7-based titanates as buffer layer materials for fuel-assisted solid oxide electrolysis cells

No abstract available.publishe

Structural Design of 5 mol.% Yttria Partially Stabilized Zirconia (5Y-PSZ) by Addition of Manganese Oxide and Direct Firing

In this study, 5Y-PSZ-based ceramics with 15 mol.% of manganese oxide were obtained from PSZ + MnO2 powders mixtures by pressing and direct firing. The resulting materials show a stable cubic fluorite structure with only minor traces of segregated manganese oxides and relative density from 90% to 98%. The linear thermal expansion coeffcient is in the order of 10^-5 K^-1 at 500 K and increases gradually with temperature, due to the onset of a contribution of chemical expansion, reaching about 13x10^-6 K^-1 at 1100 K. These results are suitable for prospective applicability as buffer layers to minimize degradation and delamination of electrolyte/oxygen electrode interfaces in solid electrolyte cells. The electrical conductivity remains close to 1 S/m at 973 K and close to 7 S/m at 1273 K, suggesting mixed conductivity with a prospective contribution to electrode processes occurring at electrode/electrolyte interfaces. Guidelines for further improvement were also established by a detailed analysis of the impact of heating/cooling rate, firing temperature, and time on those properties, based on Taguchi planning.publishe

Perovskite-Like Pr(A)MnO3 (A = Ca, Sr) as Anode Materials for Solid Oxide Fuel-Assisted Electrolysis Cells

Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. One of the main specific degradation mechanisms in SOECs relates to the delamination phenomena at or near the electrolyte/anode interface. The principle of so-called fuel-assisted electrolysis is to supply the carbon-containing species which can react with oxygen at the anode side thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the characterization of PrMnO3-based perovskites for potential application as anodes in solid oxide fuel-assisted electrolysis cells. Pr0.60-xA0.40MnO3±δ (A = Sr, Ca; x = 0 and 0.05) were synthesized by glycine-nitrate combustion technique. The characterization included XRD, SEM/EDS, XPS, dilatometry and thermogravimetry, measurements of electrical properties and oxygen permeability, and determination of oxygen nonstoichiometry. XRD analysis confirmed the formation of solid solutions with orthorhombic perovskite structure. The oxides exhibit negligible variations of oxygen content under oxidizing conditions while reducing p(O2) below 10-4 atm results in oxygen losses from the lattice and reduction of Mn cations. XPS results suggest that praseodymium remains in a 3+ oxidation state in the bulk of ceramics but may adopt a mixed 3+/4+ oxidation state at the surface. The lowp(O2) stability boundary of the perovskite phase at 800°C corresponds to ~10-17-10-16 atm; the stability domain is wider for Ca-substituted compositions and narrows with the introduction of A-site vacancies. Dilatometric studies confirmed good thermomechanical compatibility with common solid electrolytes under oxidizing conditions; however, reduction at operation temperatures (800°C) leads to undesirable chemical expansion. The electrical conductivity of Pr0.60-xA0.40MnO3±δ ceramics is p-type electronic and decreases with reducing p(O2) but still exceeds 40-50 S/cm under anticipated oxygen electrode operation conditions. The electrochemical activity of Pr0.6-xA0.4MnO3±δ electrodes was evaluated in contact with yttria-stabilized zirconia solid electrolyte as a function of relevant parameters (fabrication conditions, with and without buffer layers, with modifications via infiltration of praseodymia and/or doped ceria). The best performance was obtained for the cells with Pr0.55A0.40MnO3±δ electrodes (gadolinia-doped ceria buffer layers, PrOy load of ~33 wt.%) that showed anodic overpotential of around 50 mV under 500 mA/cm2 at 800°C in air.publishe

Composite LnNiO3+PrOx oxygen electrodes for solid oxide cells

Ln2NiO4+δ and its derivatives with perovskite-related K2NiF4-type structure demonstrate high mixed ionic-electronic conductivity, moderate thermal and negligible chemical expansion. As a result, these phases attracted significant attention as prospective cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFC). At the same time, perovskite-like LnNiO3 has not been considered for these applications, mostly due to the limited phase stability under ambient oxygen pressures. On heating in air, LaNiO3 decomposes at ~ 1000°C; cathodic polarization can be expected to induce the decomposition of perovskite phase at lower temperatures characteristic for IT-SOFC operation. On the contrary, redox changes imposed by anodic polarization (in solid oxide electrolysis cell mode) under oxidizing conditions should not be of risk for the phase stability of LaNiO3. The goal of the present work was the evaluation of LnNiO3-based oxygen electrodes for solid oxide fuel/electrolysis cells. The LnNiO3-δ ceramic powders with perovskite-like structure was prepared by glycine-nitrate combustion synthesis followed by calcinations in oxygen atmosphere at 800-1000°C. Porous ceramic samples for electrical and dilatometric studies were sintered in oxygen at 950-1050°C. Porous LaNiO3-δ samples were found to exhibit favorably high p-type metallic-like electrical conductivity, 400-500 S/cm at 800-600°C in air. These ceramics demonstrated also a moderate thermal expansion, with average CTE ~ 13.0 ppm/K at 25-800°C, ensuring thermomechanical compatibility with solid electrolytes. As a first step, the electrochemical performance of LaNiO3-δ electrodes was assessed in contact with three common electrolytes including (ZrO2)0.92(Y2O3)0.08 (8YSZ), Ce0.9Gd0.1O2-δ (CGO10) and (La0.8Sr0.2)0.98Ga0.8Mg0.2O3-δ (LSGM). The electrode layers were sintered at 1050°C for 2 h under oxygen flow. The studies of symmetrical cells by EIS demonstrated that the electrochemical activity of LaNiO3-δ electrodes increases in the sequence 8YSZ < CGO10 < LSGM; the corresponding values of electrode polarization resistance (Rη) at 800°C were 1.4, 0.8 and 0.25 Ohm×cm2, respectively. Significant variations of Rη with electrolyte composition correlate with the extent of chemical reactivity between LaNiO3-δ and electrolyte materials during the electrode fabrication. The Rη values of LaNiO3-δ electrodes in contact with LSGM electrolyte were further reduced to 0.03 Ohm×cm2 at 800°C and 0.11 Ohm×cm2 at 700°C by the surface modification with PrOx which is known for its electrocatalytic activity. At 750°C and current density of 0.5 A/cm3, LaNiO3+PrOx (~20 wt.%) electrodes in contact with LSGM solid electrolyte demonstrate the overpotentials of ~60 mV under cathodic polarization and ~40 mV under anodic polarization (Fig.1). The impact of substitution of lanthanum by praseodymium (in order to improve the chemical compatibility and electrochemical activity) on the relevant properties of LnNiO3 is briefly discussed.publishe

Characterization of ZrO2-Y2O3-MnO2 solid solutions as components for reversible solid oxide cells

Long-term degradation remains the main issue for the viability of reversible solid oxide fuel/electrolysis cell (SOFC/SOEC) technology as practical hydrogen production and energy storage systems. While some lifetime-limiting factors are common in both regimes, the major specific degradation mechanism in SOEC regime relates to the delamination phenomena. The experimental and modelling results suggest that high oxygen pressures can develop in electrolyte near the anode/electrolyte interface resulting in formation of voids at the grain boundaries, intergranular fractures, cracks in anode, and anode delamination; all factors contribute to irreversible degradation. The objective of this work was the characterization of ZrO2-Y2O3-MnO2 solid solution in order to design a functional material with oxygen storage ability that may be used as inclusion into electrolyte membrane or as interlayer between electrolyte and oxygen electrode with the purpose of delay or prevent degradation and irreversible changes. ((ZrO2)1-x(Y2O3)x)1-y(MnOn)y ceramics (x = 0.02-0.05, y = 0.05-0.15) were prepared by solid-state reaction route and sintered in air at 1400-1600°C. XRD results showed the formation of singlephase solid solutions with cubic fluorite-type structure for the compositions with x = 0.05, while the ceramics with lower yttria content comprised 2 or more phases based on different polymorphs of zirconia. The characterization of materials included microstructural studies (SEM/EDS), thermal analysis (thermogravimetry, dilatometry), measurements of electrical conductivity as function of temperature and oxygen partial pressure, and determination of ionic transference numbers by modified e.m.f. method. Increasing Mn content was found to results in increase of the total electrical conductivity and electronic contribution under oxidizing conditions, while ionic transport dominates under reduced oxygen partial pressures. Electrical measurements showed also a slow relaxation of electrical conductivity on redox cycling that possibly can be attributed to a variable solubility of Mn cations in fluorite lattice.publishe

Perovskite-like LaNiO3-δ as oxygen electrode material for solid oxide electrolysis cells

Perovskite-like LaNiO3-δ was evaluated as potential oxygen electrode material for solid oxide electrolysis cells. Compared to the Ruddlesden-Popper Lan+1NinO3n+1 (n = 1,2,3) counterparts, LaNiO3-δ exhibits higher p-type metallic-like conductivity under oxidizing conditions (450 S×cm-1 at 800°C for highly porous ceramics) together with a moderate thermal expansion coefficient (13.7 ppm×K-1 in air at 25-800°C) compatible with common solid electrolytes. The measured electrode polarization resistance (Rη) in contact with YSZ, CGO and LSGM solid electrolytes was 1.4, 0.77 and 0.22 Ω×cm2 at 800°C, and 208, 123 and 7.1 Ω×cm2 at 600°C, respectively, under zero-current conditions in air. Surface modification of via PrOx infiltration resulted in lower values of Rη (0.024 Ω×cm2 at 800°C and 0.76 Ω×cm2 at 600°C) and low anodic overpotentials (20 mV at 800°C and 500 mA×cm-2) in contact with LSGM.publishe

Ionic and electronic transport in calcium-substituted LaAlO3 perovskites prepared via mechanochemical route

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La1-xCaxAlO3-δ (x = 0.05-0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450°C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La1-xCaxAlO3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ~6×10-3 S/cm at 900°C. La1-xCaxAlO3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20-68% at 900°C in air and increases with Ca content, with temperature and with reducing p(O2). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La1-xCaxAlO3-δ ceramics below ~800°C are determined by prevailing grain boundary contribution to the total resistivity.This work was supported by the Slovak Research and Development Agency APVV (contracts SK-PT-18-0039 and 15-0438) and the Slovak Grand Agency (contract No. 2/0055/19). BIAS and AAY would like to acknowledge financial support by the FCT, Portugal (bilateral project Portugal-Slovakia 2019-2020, project CARBOSTEAM (POCI01-0145-FEDER-032295) and project CICECO-Aveiro Institute of Materials (FCT ref. UID/CTM/50011/2019), financed by national funds through the FCT/MCTES and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement). HK thanks to SAIA, n.o. for financial support within National Scholarship Programme of the Slovak republic (NSP).in publicatio

PrOx-infiltrated LnNiO3-based oxygen electrodes for solid oxide cells

Ln2NiO4+δ and its derivatives with perovskite-related K2NiF4-type structure demonstrate high mixed ionic-electronic conductivity, moderate thermal and negligible chemical expansion. As a result, these phases attracted significant attention as prospective cathode materials for intermediatetemperature solid oxide fuel cells (IT-SOFC). At the same time, perovskite-like LnNiO3 have not been considered for these applications, mostly due to the limited phase stability under ambient oxygen pressures. On heating in air, LaNiO3 decomposes at ~ 1000°C; cathodic polarization can be expected to induce the decomposition of perovskite phase at lower temperatures characteristic for IT-SOFC operation. On the contrary, redox changes imposed by anodic polarization (in solid oxide electrolysis cell mode) under oxidizing conditions should not be of risk for the phase stability of LaNiO3. The goal of the present work was the evaluation of LnNiO3-based oxygen electrodes for solid oxide fuel/electrolysis cells. The LnNiO3-δ ceramic powders with perovskite-like structure was prepared by glycine-nitrate combustion synthesis followed by calcinations in oxygen atmosphere at 800-1000°C. Porous ceramic samples for electrical and dilatometric studies were sintered in oxygen at 950-1050°C. Porous LaNiO3-δ samples were found to exhibit favorably high p-type metallic-like electrical conductivity, 400-500 S/cm at 800-600°C in air. These ceramics demonstrated also a moderate thermal expansion, with average CTE ~ 13.0 ppm/K at 25-800°C, ensuring thermomechanical compatibility with solid electrolytes. As a first step, the electrochemical performance of LaNiO3-δ electrodes was assessed in contact with three common electrolytes including (ZrO2)0.92(Y2O3)0.08 (8YSZ), Ce0.9Gd0.1O2-δ (CGO10) and (La0.8Sr0.2)0.98Ga0.8Mg0.2O3-δ (LSGM). The electrode layers were sintered at 1050°C for 2 h under oxygen flow. The studies of symmetrical cells by EIS demonstrated that the electrochemical activity of LaNiO3-δ electrodes increases in the sequence 8YSZ < CGO10 < LSGM; the corresponding values of electrode polarization resistance (Rη) at 800°C were 1.4, 0.8 and 0.25 Ohm×cm2 , respectively. Significant variations of Rη with electrolyte composition correlate with the extent of chemical reactivity between LaNiO3-δ and electrolyte materials during the electrode fabrication. The Rη values of LaNiO3-δ electrodes in contact with LSGM electrolyte were further reduced to 0.03 Ohm×cm2 at 800°C and 0.11 Ohm×cm2 at 700°C by the surface modification with PrOx which is known for its electrocatalytic activity. At 750°C and current density of 0.5 A/cm2 , LaNiO3+PrOx (~20 wt.%) electrodes in contact with LSGM solid electrolyte demonstrate the overpotentials of ~60 mV under cathodic polarization and ~40 mV under anodic polarization. The impact of substitution of lanthanum by praseodymium (in order to improve the chemical compatibility and electrochemical activity) on the relevant properties of LnNiO3 is briefly discussed.publishe

Mixed Ionic-Electronic Conductivity, Redox Behavior and Thermochemical Expansion of Mn-Substituted 5YSZ as an Interlayer Material for Reversible Solid Oxide Cells

Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO2)0.95(Y2O3)0.05]1−x[MnOy]x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn2+/3+→Mn2+ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x < 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion