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

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

Oxygen-deficient (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ nickelates as oxygen electrode materials for SOFC/SOEC

Perovskite-related Ln2NiO4+δ (Ln = La, Pr, Nd) nickelates with layered Ruddlesden-Popper combine redox stability with noticeable oxygen stoichiometry changes, yielding enhanced mixed transport and electrocatalytic properties. These unique features are promising for applications as oxygen electrodes with good electrochemical performance in reversible SOFC/SOEC (solid oxide fuel/electrolysis cell) systems. To date, most efforts were focused on oxygen-hyperstoichiometric Ln2NiO4+δ-based phases, whereas nickelates with oxygen-deficient lattice remain poorly explored. Recent studies demonstrated that the highest electrical conductivity in (Ln2-xSrx)2NiO4±δ series at elevated temperatures is observed for the compositions containing ~ 60 at.% of strontium in A sublattice [1,2]. The present work was focused on the characterization of (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates for the possible use as materials for reversible oxygen electrodes. The ceramic materials were prepared by Pechini method with repeated annealings at 650-1200°C and sintered at 1250-1300°C for 5 h under oxygen atmosphere. Variable-temperature XRD studies confirmed that all studied compositions retain tetragonal K2NiF4-type structure in the temperature range 25-900°C. The results of thermogravimetric analysis showed that the prepared nickelates has oxygen-deficient lattice under oxidizing conditions at temperatures above 700°C. Partial substitution of nickel by cobalt or iron results in a decrease of p-type electronic conductivity and the concentration of oxygen vacancies in the lattice, but also suppresses dimensional changes associated with microcracking effects (due to anisotropic thermal expansion of tetragonal lattice). Electrochemical performance of porous (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ electrodes in contact with Ce0.9Gd0.1O2-δ solid electrolyte was evaluated at 600-800°C employing electrochemical impedance spectroscopy and steady-state polarization (anodic and cathodic) measurements.publishe

Exploring tantalum as a potential dopant to promote the thermoelectric performance of zinc oxide

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1-xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (-464 µV/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice

Strontium titanate and zinc-oxide-based materials for high-temperature thermoelectric harvesting

Broad societal needs have focused increased attention to providing a sustainable energy supply to the population, based on technologies with minimal environmental impact and reduced fossil fuels usage. One solution is to improve energy conversion efficiency in key consuming sectors. Since most of the energy (60-70%) used worldwide is discharged as waste heat, ”green” thermoelectric (TE) conversion has received considerable attention due to its intrinsic simplicity, employing no moving parts, silent operation, excellent scalability and reliability, and self-sufficiency to enable mobile or remote applications. In some energy-conversion scenarios, the cost and thermal stability requirements may dominate over efficiency issues, making abundant, high-temperature-stable and low-toxic oxides an interesting alternative TE material. This talk will feature some oxide-specific approaches towards tuning the thermoelectric performance in strontium titanate and zincoxide-based materials, including defects engineering and in-situ induced nanostructuring.publishe

Ionic-electronic transport in zircon-type PrVO4-based ceramics

No abstract available.publishe