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Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. The principle of the so-called fuel-assisted electrolysis cell is to supply the low-grade fuel to the anode where it can react with oxygen, thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the evaluation of Sr0.7Ce0.3MnO3-d perovskite for potential application as an anode in fuel-assisted SOEC. Sr0.7Ce0.3MnO3-d was synthesized by the glycine-nitrate technique with repeated calcinations at 900-1300°C to obtain phase-pure perovskite material. Ceramic samples were sintered in air at 1450°C. The oxide exhibits negligible variations of oxygen content under oxidizing conditions while reducing p(O2) below 10-4 atm at 750-900°C results in oxygen losses and reduction of Mn cations. The low-p(O2) stability boundary of the perovskite phase at 800°C corresponds to ~3×10-17 atm. Sr0.7Ce0.3MnO3-d shows good thermomechanical compatibility with solid electrolytes under oxidizing conditions; however, reduction at operation temperatures (800°C) leads to undesirable chemical expansion. The electrical conductivity of Sr0.7Ce0.3MnO3 ceramics is p-type electronic and decreases with reducing p(O2) but still exceeds 40 S/cm under anticipated oxygen electrode operation conditions. The electrochemical activity of Sr0.7Ce0.3MnO3 electrodes was evaluated in contact with YSZ solid electrolyte as a function of relevant parameters. The best performance was obtained for the cells with a CGO buffer layer and Sr0.7Ce0.3MnO3 electrodes infiltrated with PrOy (load of ~ 30 wt.%) that can show anodic overpotentials of ~50 mV under 400 mA/cm2 at 800°C in air. published

Ln2NiO4+δ (Ln = La, Pr, Nd) and its derivatives with Ruddlesden-Popper structure demonstrate high mixed ionic-electronic conductivity, moderate thermal and negligible chemical expansion, and, therefore, attract significant attention as prospective cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFC). However, Ln2NiO4+δ are thermodynamically unstable at SOFC operation temperatures and slowly decompose to a mixture of higher order Ln1+nNinO3n+1 (n = 2, 3) phases, perovskite-like LnNiO3, and LnOy. On the contrary, LnNiO3 perovskites have limited stability at elevated temperatures under oxidizing conditions. In particular, LaNiO3 decomposes on heating in air above ~1000°C. Cathodic polarization can also be expected toinduce the decomposition of the LnNiO3 perovskite phase at lower temperatures characteristic for IT-SOFC operation. At the same time, 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 LnNiO3. Thus, the goal of the present work was the evaluation of LaNiO3-PrNiO3 solid solutions as prospective oxygen electrode materials for solid oxide electrolysis cells. La1-xPrxNiO3-δ (x = 0, 0.2, 0.5 and 1.0) solid solutions with perovskite-like structure were prepared by combustion synthesis with calcinations in oxygen at 800-1000°C. Porous ceramic samples were sintered in oxygen at 950-1050°C. The materials were synthesized and characterized by XRD, SEM/EDS, dilatometry, TGA, coulometric titration, and electrical measurements. All prepared La1-xPrxNiO3-δ perovskites were found to be oxygen-deficient with oxygen nonstoichiometry δ slightly increasing with Pr content. The cation composition was found to have a negligible impact on the low-p(O2) stability limits: all materials decompose at log p(O2) ~ -3.5 atm at 800°C. Increasing Pr content results in a decrease in p-type electronic conductivity: from ~460 S/cm for x = 0-0.2 to 220 S/cm for x = 0.5 and 115 S/cm for x = 1.0 at 800C in air, but also reduces thermal expansion coefficient from 13.7 ppm/K for x = 0 to 11.8 ppm/K for x = 1.0 at 30-950°C in air. Comparative assessment of the electrochemical performance of La1-xPrxNiO3-δ-based electrodes in contact with (ZrO2)0.92(Y2O3)0.08 (8YSZ) and (La0.8Sr0.2)0.98Ga0.8Mg0.2O3-δ (LSGM) solid electrolytes was performed in air at 600-800°C using symmetrical cell configuration. FCT/MCTES/FEDER published

Emissions of nitrogen oxides remain a significant problem associated with the use of fuel-efficient lean combustion in transportation (diesel engines) and the passivation of catalytic converters by excess oxygen in exhaust gasses. One possible way to achieve high-efficiency NOx conversion is electrochemical reduction under anodic polarization in symmetrical solid oxide cells. Recent research was focused on improving efficiency in the presence of 1-15 vol.% oxygen: seeking for NOx selective electrodes based on mixed-conducting transition metal oxides, impregnations with basic components (BaO, BaCO3, K2O) to enhance adsorption of acidic NOx or pre-oxidation of NO to NO2 which is more readily adsorbed on the electrode surface. The present work is focused on the synthesis of Ba-rich Ba(Fe,Zr,Ni)O3 perovskites and characterization of their electrical properties, phase stability, and thermomechanical behavior with the aim to assess the prospects of their use as electrocatalysts for NOx elimination. Ba(Fe1-x-yNixZry)O3-d (x = 0.05–0.5, y = 0–0.4) ceramics were synthesized by glycine-nitrate combustion technique. Single-phase perovskites with a minor NiO impurity were found to form only for prevailing Fe content and with the total Ni and Zr content x+y ≤ 0.3 at 1100°C. Sintering at T ≥ 1200°C leads to segregation of secondary phases. The electrical conductivity of single-phase ceramics is moderate and reaches 3.5 S/cm at 400-500°C; this may limit electrochemical performance. The perovskite phase is demonstrated to be stable in reducing conditions down to pO2 ~ 10-16 atm. The dilatometric studies show unusual and nonlinear behavior of thermal expansion coefficients, with the absolute values varying in the range of 15-50 ppm/K at 250-950°C. published

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. published

Rare-earth and alkaline-earth vanadates attract attention as prospective materials for electrochemical applications, in particular, as redox-reversible components for fuel electrodes of solid oxide fuel cells (SOFC). An essential advantage of (Ln,A)VOx-derived components of SOFC anodes is their anticipated resistance to carbon deposition and sulfur-containing impurities, which is critical for hydrocarbon- and biogas-fueled SOFCs. The present work was focused on the oxides of the PrVO4-Ca2V2O7 system as fuel electrode precursors, with an emphasis on phase formation, redox and thermomechanical behavior, and electrical properties. PrVO4, Ca2V2O7 and the ceramics with the nominal composition Pr1-xCaxVO4-d (x = 0.02-0.20) were prepared by the conventional solid-state route. Ceramics samples were sintered at 1000°C for Ca2V2O7 and 1300°C for other materials. XRD demonstrated the formation of phase-pure Pr1-xCaxVO4-d solid solutions with the tetragonal zircon-type structure for up to 5 at.% of calcium in Pr sublattice. At the same time, SEM/EDS suggest a lower solubility indicated by the presence of Ca-V-O phase impurities. Doping by calcium increases mixed ionic-electronic conductivity of Pr(Ca)VO4 ceramics under oxidizing conditions. The electronic contribution is p-type and decreases with reducing p(O2). The reduction of Pr1-xCaxVO4-d ceramics in a 10%H2-N2 atmosphere at 800°C leads to phase separation and formation of perovskite-like PrVO3 and CaVO3 phases. The redox behavior of PrVO4-Ca2V2O7 ceramics on isothermal cycling between air and 10%H2-N2 was studied by impedance spectroscopy, thermogravimetry, dilatometry and post-mortem XRD analysis. FCT/MCTES/FEDER published

Oxide materials with mixed ionic and electronic conductivity (MIECs) are important components of high-temperature electrochemical systems, including electrodes of solid oxide fuel and electrolysis cells (SOFC/SOEC) and dense ceramic membranes for partial oxidation of hydrocarbons and oxygen separation. Characterization of ionic transport in MIECs faces two challenges: the deconvolution of the ionic and electronic transport, and the deconvolution of bulk diffusion and kinetics of the surface electrochemical reaction. The electrical conductivity relaxation (ECR) technique [1-3] is based on assumptions that (i) the total conductivity of the MIEC has a monotonic dependence on oxygen partial pressure in a given p(O2) range with sufficient sensitivity; (ii) the nature of the ionic transport and surface exchange processes allows reasonable linearization in their numerical description; (iii) there are no unaccounted by-processes and phase transitions. In practice, the conductivity relaxation measurements are organized as a continuous recording of the changes in the sample resistivity after abrupt changes in oxygen partial pressure (or oxygen chemical potential). The collected ECR data can be fitted using numerical models that originate from the solutions described in [4]. In general, these models require well-defined and simple geometry of the sample, negligible porosity, and a known state of the surface (polished, rough, activated…). Of key importance in this technique is the precise fixation of the moment of change of p(O2) in the gas phase and the initial resistivity (conductivity) of the sample. In the ECR setup designed in the present study, oxygen partial pressure in the heated sample chamber is changed by a fast short-term connection to a gas container with different oxygen pressure. This produces a very abrupt change in the p(O2), and the time of this event is well-defined due to the high operation speed of vacuum magnet valves and also is confirmed by the response of the electrolytic oxygen sensor. All data acquisition is performed in automatic mode, enabling long-term data collection for subsequent non-linear least square analysis. The ECR technique was employed for the characterization of a series of mixed-conducting ceramics typically used as oxygen electrode materials for SOFC/SOEC, including La0.3Sr0.7CoO3-δ (LSC) and (La,Sr)(Co,Fe)O3-δ (LSCF) – based perovskite-type oxides. The ECR measurements were performed in the p(O2) range of 0.010-0.210 atm with steps of ~ 0.03-0.0.05 atm. The properties of each particular material determined the corresponding temperature range. In the case of LSC, the highest temperature was limited to 850°C by the time resolution of the data acquisition system, and the low-T limit corresponded to 450°C. For LSCF-based compositions, the studied temperature range was 650-875°C. The chemical diffusion and surface exchange coefficients obtained by the ECR technique were analyzed in combination with the data on oxygen nonstoichiometry δ and ionic transport parameters measured by the oxygen permeability technique. published

Oxide materials represent a promising alternative to traditional thermoelectrics, by providing a possibility to operate at high temperatures, resulting in a higher Carnot efficiency. Although their conversion efficiency is considerably lower, their known structural and microstructural versatility can open new horizons for thermoelectric applications. This work reviews some representative cases of engineering the composition and microstructure of oxides towards high thermoelectric performance. The selected approaches will include laser processing, in-situ formed composites, defects tailoring and aluminothermy-boosted sintering, also taking into account the unique redox-tuning capabilities of oxides. Representative examples will include ceramic materials based on SrTiO3, ZnO, CaMnO3 and Ca3Co4O9. published

Ln2NiO4 –based oxides (Ln = La, Pr, Nd) with perovskite-related K2NiF4-type structure demonstrate high oxygen diffusivity and surface exchange kinetics in combination with comparatively high electronic conductivity and are considered, therefore, as attractive mixed ionic-electronic conductors for oxygen electrodes of high-temperature SOFC/SOEC. Acceptor-type substitution of Ln3+ by Sr2+ in Ln2NiO4+d is compensated by comparatively high oxygen deficiency in Sr-rich Ln2-xSrxNiO4-d phases at elevated temperatures. The transition from oxygen excess to oxygen deficiency is expected to be accompanied by a change in the ionic transport mechanism from prevailing interstitial oxygen diffusion in rock-salt layers to oxygen vacancy diffusion in perovskite-type layers. Ceramic powders of Ln2-xSrxNiO4-d (Ln = La, Nd, Pr, x = 1.0-1.6) were prepared by the Pechini method with final calcination steps at 1150-1200°C in oxygen. Thermogravimetric studies confirmed that acceptor-type substitution by strontium is compensated by the formation of oxygen vacancies and electron-holes and progressively increases high-temperature oxygen nonstoichiometry, which reaches as high as δ = 0.36-0.40 for x = 1.6 at 900°C in air. High-temperature XRD studies demonstrated that Ln2-xSrxNiO4-δ nickelates exhibit strongly anisotropic lattice expansion interrelated with oxygen deficiency changes on heating. Highly anisotropic expansion induces microcracking phenomenon, which, in turn, results in a strong hysteresis in dimensional changes and also in variations of electrical conductivity on cycling in air for the ceramic samples sintered at 1200-1300°C. An appropriate approach to minimize or neglect the undesirable effects of anisotropic expansion and microcracking is to preserve the grain size in Ln2-xSrxNiO4-δ ceramics or porous electrodes at ~1 micron or submicron level. In particular, spark plasma sintering with subsequent careful oxidation treatment was adopted to avoid grain growth and to produce mechanically stable ceramics with smooth reversible thermochemical expansion and enhanced electrical properties. FCT/MCTES/FEDER published

Long-term degradation remains a major obstacle to the introduction of SOEC technology as a practical hydrogen production system. A specific degradation mechanism in SOECs is related to electrolyte deterioration and delamination phenomena at or near electrolyte/anode interface. The electrochemical studies of reversible solid oxide fuel/electrolysis cells (rSOCs) operation revealed that the degradation occurring at the electrolyte/oxygen electrode interface can be slowed down if a cell operates in a reversible SOEC/SOFC regime and that the degradation occurring in the SOEC regime may be partially recovered during the operation in SOFC mode. These observations raise prospects to halt degradation and to develop self-healing approaches based on reversible SOEC/SOFC cycles. One possible approach may be based on the introduction of redox-active components with oxygen storage capacity in the form of inclusions into the surface layer of solid electrolyte or as a thin buffer layer at the electrode/electrolyte interface capable of oxygen uptake under anodic polarization to prevent oxygen pressure build-up and reverting to oxygen loss in fuel cell mode. The present work was focused on the assessment of the impact of co-substitutions and processing conditions on the properties of ZrO2-Y2O3-MnOy and ZrO2-Y2O3-PrOy mixed oxides for prospective applicability as redox-active inclusions or buffer layers. Mn-substituted (ZrO2)0.95(Y2O3)0.05 (5YSZ) solid solutions were found to 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 Mn additions (5–15 mol.%). The dissolution of praseodymium oxide in 5YSZ was found to occur via the formation of pyrochlore-type Pr2Zr2O7 intermediate. Increasing PrOy additions results in a larger fraction of low-conducting pyrochlore phase which limit the total electrical conductivity. High temperature processing stabilizes the prevailing 3+ oxidation state of praseodymium cations leading to a low oxygen storage capacity. Thus, Mn-substituted 5YSZ (5–10 mol.%) is considered the most suitable for the interlayer application due to the best combination of relevant factors. published

Long-term degradation remains the main issue for the viability of solid oxide electrolysis cell (SOEC) technology as a practical hydrogen production system. The principle of the so-called fuel-assisted electrolysis cell is to supply the low-grade fuel to the anode where it can react with oxygen, thus bringing down the oxygen chemical potential at the electrolyte/anode interface and improving its stability. The present work is aimed at the evaluation of Sr0.7Ce0.3MnO3-δ perovskite for potential application as an anode in fuel-assisted SOEC. Sr0.7Ce0.3MnO3-δ was synthesized by the glycine-nitrate technique with repeated calcinations at 900-1300°C to obtain phase-pure perovskite material. Ceramic samples were sintered in air at 1450°C. The oxide exhibits negligible variations of oxygen content under oxidizing conditions while reducing p(O2) below 10-4 atm at 750-900°C results in oxygen losses and reduction of Mn cations. The low-p(O2) stability boundary of the perovskite phase at 800°C corresponds to ~3×10-17 atm. Sr0.7Ce0.3MnO3-δ shows good thermomechanical compatibility with solid electrolytes under oxidizing conditions; however, reduction at operation temperatures (800°C) leads to undesirable chemical expansion. The electrical conductivity of Sr0.7Ce0.3MnO3 ceramics is p-type electronic and decreases with reducing p(O2) but still exceeds 40 S/cm under anticipated oxygen electrode operation conditions. The electrochemical activity of Sr0.7Ce0.3MnO3 electrodes was evaluated in contact with YSZ solid electrolyte as a function of relevant parameters. The best performance was obtained for the cells with a CGO buffer layer and Sr0.7Ce0.3MnO3 electrodes infiltrated with PrOy (load of ~ 30 wt.%) that can show anodic overpotentials of ~50 mV under 400 mA/cm2 at 800°C in air. published

No abstract available. published

No abstract available. published