Solid oxide cells, i.e. solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs), are efficient energy converters for a future energy system. Poisoning of air electrodes of solid oxide cells by gaseous contaminants is a serious issue limiting the long-term stability of their electrochemical performance, and is thus a challenge for successful commercialization. This review provides an overview of poisoning of perovskite and Ruddlesden-Popper-type air electrodes by gaseous Cr- and Si-species as well as SO2, including recent results. It discusses fundamental aspects of Sr-segregation and reactions with gaseous contaminants, emphasizing the importance of size mismatch and basicity of the constituents of the ceramic oxide electrode. Studies on Cr-, Si- and SO2-poisoning of air electrodes are presented together with mitigation and recovery measures.

Substitution with Y affects phase formation and the local cation stoichiometry of the Ce-rich and Fe-rich phases of BaCe1−(x+z)FexYzO3−δself-generated composites. This is of particular importance for the water uptake in these compounds.

The rare earth nickelate La2Ni0.9Co0.1O4+delta (LNCO) was synthesized via the citrate/EDTA method. X-ray powder diffraction confirmed that the material is single-phase and crystallizes in the orthorhombic K2NiF4-type structure. In-situ dc-conductivity and conductivity relaxation measurements on a bar-shaped sample were applied to determine the electronic conductivity as well as the chemical surface exchange coefficient and chemical diffusion coefficient of oxygen from 600 degrees C to 850 degrees C and 0.01 <= pO2 / bar <= 0.1. The results indicate that substitution of nickel with cobalt in La2Ni0.9Co0.1O4+delta leads to significantly higher values of the surface exchange coefficient compared to La2NiO4+delta, while the electronic conductivity is somewhat reduced. The oxygen non-stoichiometry of LNCO was studied by thermogravimetry in the same temperature and oxygen partial pressure range. The measured thermal expansion coefficients of LNCO fit well with those of common solid electrolytes such as GDC and YSZ. Self-diffusion coefficients of oxygen and ionic conductivities were calculated from the experimentally determined diffusivities and the thermodynamic factor of oxygen by using the Nernst-Einstein relation. The results indicate that LNCO offers an attractive option for application as air electrode in solid oxide cells.

There is a strong need to store electrical energy from fluctuating renewable energy sources such as solar or wind and to decarbonize transport and industry. High-temperature electrolysis is expected to contribute significantly to reach these goals. This reference text provides a detailed guide, including the fundamental and materials aspects of solid oxide and protonic ceramic electrolysis cells at stack and system levels, as well as recent developments. Applications discussed include the production of green hydrogen as well as the combination of high-temperature electrolysis with other processes for the synthesis of ammonia, methane or e-fuels. Highly relevant to the field of renewable energy supply and conversion, the text provides a comprehensive and accessible reference for researchers, engineers, and graduate students from various disciplines. Key features • Provides comprehensive coverage of high-temperature electrolysis using solid oxide cells with oxygen ion and protonic conductors • Covers the fundamentals of solid oxide and protonic ceramic electrolysis cells and their applications, including power-to-gas and power-to-X • Includes the integration of high-temperature electrolysis into the energy system, and economic analyses of power-to-gas processes • Provides a comprehensive and accessible reference for graduate students and researchers, particularly those that are new to the field • Includes the latest developments, along with relevant open questions and methods needed to tackle them

In this work, fundamental material properties of compounds in the system (La,Pr) 2 (Ni,Co)O 4+δ as well as their performance as air electrodes in solid oxide electrolysis cells were investigated. Nickelates co‐doped with Pr and Co were characterized on a material basis by means of X‐ray diffraction and thermogravimetry. Conductivity and conductivity relaxation measurements were performed in order to obtain the electronic conductivity as well as the chemical surface exchange coefficient and the chemical diffusion coefficient of oxygen as a function of temperature and oxygen partial pressure. These parameters can be regarded as the most essential properties at the material level required to assess the suitability of mixed ionic‐electronic conducting ceramics for application as air electrode in solid oxide cells. The electrode performance of the materials was then tested on fuel electrode‐supported button cells at 800°C. The electrodes were applied by screen‐printing and the effect of varying the Pr‐content and Co‐content of the electrode powder was investigated. Cell tests were performed by means of current‐voltage measurements in electrolysis mode. While no significant impact of Pr‐doping on the investigated material properties was observed, the electrode performance of Pr‐containing materials was significantly better than for the Pr‐free compound, which has been discussed in detail.

High-resolution STEM-EELS provides information about the composition of crystalline materials at the atomic scale, though a reliable quantitative chemical analysis is often hampered by zone axis conditions, where neighbouring atomic column intensities contribute to the signal at the probe position. In this work, we present a procedure to determine the concentration of two elements within equivalent atomic columns from EELS elemental maps - in our case barium and lanthanum within the A-sites of Ba1.1La1.9Fe2O7, a second order Ruddlesden-Popper phase. We took advantage of the large changes in the elemental distribution from column to column and introduced a technique, which substitutes inelastic scattering cross sections during the quantification step by using parameters obtained from the actual experiment. We considered channelling / de-channelling effects via inelastic multislice simulations and were thereby able to count occupancies in each atomic column. The EELS quantification results were then used as prior information during the Rietveld refinement in XRD measurements in order to differentiate between barium and lanthanum.

Compounds from the series La 0.8-x Nd x Ca 0.2 FeO 3-δ (0.1 ≤ x ≤ 0.7) were synthesised by a sol–gel route. X-ray diffraction and Rietveld analysis showed that materials with 0 ≤ x ≤ 0.6 crystallize as single-phase orthorhombic perovskites. The smaller ionic radius of Nd 3+ compared to La 3+ leads to a decrease in unit cell volume with increasing x. Elemental mapping by high-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy confirms the homogeneous distribution of the A-site elements (La, Nd and Ca) in the perovskite (ABO 3 ) lattice. The thermal expansion behaviour of La 0.8-x Nd x Ca 0.2 FeO 3-δ (0 ≤ x ≤ 0.6) was characterized by dilatometry at 30 ≤ T/°C ≤ 1000 and 1 × 10 –3 ≤ pO 2 /bar ≤ 1. The thermal expansion coefficients of La 0.8-x Nd x Ca 0.2 FeO 3-δ , which were determined in regions I (40–530 °C) and II (530–990 °C), respectively, are almost independent of the Nd concentration in the range of (0 ≤ x ≤ 0.6) and increase slightly with decreasing pO 2 . The transition from orthorhombic to trigonal modification, which is observed for La 0.8 Ca 0.2 FeO 3-δ at approx. 740 °C, is suppressed for all Nd-substituted compounds with x ≥ 0.1.

Long-term stability tests are performed at 800 °C on Pr2NiO4+δ air electrodes by use of a symmetrical button cell with Ce0.9Gd0.1O1.95 as solid electrolyte. The experiments are carried out by means of electrochemical impedance spectroscopy and current-voltage measurements with and without current load under dry and humid conditions in the presence of a chromium source. Chromium poisoning of Pr2NiO4+δ air electrodes is investigated for periods of several hundred hours at 30% relative humidity. In order to separate the influence of anodic and cathodic electrode polarization on Cr-deposition, measurements are conducted using a Pt-reference electrode. The electrode performance is found to remain fairly stable under dry conditions, even when a current is drawn. However, after volatile Cr-species in a humid atmosphere are introduced, the cell performance starts to deteriorate and the polarization resistance contribution of the SOFC cathode increases significantly. After several thousand hours, the electrodes are analyzed by means of analytical electron microscopy. Detailed post-test analyses provide evidence for a correlation between the extent of Cr-deposition and electrode degradation in SOFC as well as SOEC mode. Based on these findings, enhanced resilience of Pr2NiO4+δ against Cr-poisoning in SOEC mode can be established.

Crystal structure and thermal expansion of the third-order Ruddlesden-Popper phase Pr4Ni2.7Co0.3O10-δ (PNCO) were determined by high temperature X-ray powder diffraction (HT-XRD) and Rietveld analysis in the temperature range 25–600 ​°C in air. The thermal expansion coefficient (TEC) of PNCO is smaller compared with first-order Ruddlesden-Popper phases and in excellent agreement with values of common electrolytes of solid oxide cells. HT-XRD, thermogravimetry, and differential scanning calorimetry showed a reversible phase transition at 600–650 ​°C. A single monoclinic (P21/a) phase occurs at 25–600 ​°C, while a mixture of monoclinic and tetragonal (I4/mmm) phases is found at 650–900 ​°C. The oxygen nonstoichiometry was determined by thermogravimetry as a function of temperature (300 ​≤ ​T/°C ​≤ ​900) and oxygen partial pressure (8.3 ​× ​10−4 ​≤ ​pO2/bar ​≤ ​8.3 ​× ​10−1). Data of the oxygen nonstoichiometry were further evaluated with respect to the partial molar enthalpy and entropy of oxygen, and the thermodynamic factor of oxygen.

The third-order Ruddlesden-Popper phase Pr4Ni2.7Co0.3O10-delta (PNCO43) was synthesized by a freeze drying process. Phase purity and crystal structure were determined by X-ray diffraction and Rietveld analysis. The electronic conductivity of a bulk sample obtained by a two-step sintering process was measured by the four-point dc van der Pauw method as a function of temperature (50 <= T/degrees C <= 800) and oxygen partial pressure (1 x 10(-3) <= pO(2)/bar <= 1). Dense thin-film PNCO43 microelectrodes were prepared by pulsed laser deposition and photolithography on yttria-stabilised zirconia substrates. The thin-films were characterized by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and inductively coupled plasma optical emission spectroscopy. Individual resistive and capacitive processes were investigated with electrochemical impedance spectroscopy as a function of the oxygen partial pressure (1 x 10(-3) <= pO(2)/bar <= 1) and temperature (600 <= T/degrees C <= 850). Oxygen surface exchange coefficients k(q), calculated from the resistance of the electrode, show relatively high values (e.g. k(q) = 1.5 x 10(-6) cm s(-1) at 800 degrees C and 2 x 10(-1) bar pO(2)). Chemical surface exchange coefficients k(chem )of oxygen were obtained from the peak frequency or the chemical capacitance as determined by impedance spectroscopy.

In oxides which exhibit electronic, protonic and oxide ion conductivity, water incorporation can lead to a kinetic build-up of redox gradients. We use Fe-doped SrTiO3, the defect chemistry of which is very well understood to follow the thus generated electromotive force (EMF) as a function of time. In contrast to previous works we include trapping effects in the kinetic description. The agreement between the kinetic modelling and the experimental results is remarkable, given the complexity of the process.

Ruddlesden-Popper-type oxides are promising air electrode materials for solid oxide fuel cells (SOFCs) and electrolyser cells (SOECs). Within the Ruddlesden-Popper (RP) series Lnn+1BnO3n+1, first order (n=1) RP-type rare earth nickelates with Ln=La, Nd, Pr and B=Ni show high oxygen diffusivities, high catalytic activity for the oxygen reduction reaction as well as good electronic and ionic conductivities [1]. In order to further improve the surface oxygen exchange kinetics, the effect of partial substitution of Ni by Co on the B-site in Pr2NiO4+δ was studied, similar to an earlier report for the La2NiO4+δ system [2]. Structure-composition-property–relationships were examined for the Pr2NiO4+δ system. The effect of A-site substitution of Pr with La as well as B-site substitution of Ni with Cobalt was investigated with respect to crystal structure, thermodynamic stability, oxygen non-stoichiometry, electronic conductivity as well as oxygen surface exchange and transport properties. Moreover, differences in functional properties between Ruddlesden-Popper phases of different order within the same compositional system were studied. In cases where material characterisation was problematic due to difficulties in obtaining densely sintered samples with high phase purity (including of third-order RP-Phases), electrochemical impedance spectroscopy (EIS) measurements on microelectrodes were applied in order to obtain reliable results for oxygen surface exchange rates. Oxygen surface exchange coefficients kq and kδ could be calculated from surface resistances and chemical capacitances of the thin-film electrodes between 550 and 850°C. Substitution of Ni by 10% of Co in Pr2NiO4+δ results in increased oxygen surface exchange rates expecially at lower oxygen partial pressures [3]. Performance tests with Pr2Ni0.9Co0.1O4+δ SOEC air electrodes for water electrolysis were conducted on anode supported full cells at 800°C at OCV conditions and in electrolyser mode up to current densities of 1000 mA cm-2. For anode supported cells, an electrode-electrolyte composite was used as functional anode layer in order to improve adhesion and avoid delamination issues. Moreover, the long-term stability and resistance against Cr-poisoning was examined by electrochemical impedance spectroscopy and current-voltage measurements on symmetrical cells at 800°C in dry and humid atmospheres. A three-electrode configuration allowed to analyse the time dependence of the air electrode polarization resistance during the SOFC as well as SOEC operation mode. Compared to La2NiO4+δ, a reduced susceptibility for Cr-poisoning was found for Pr2Ni0.9Co0.1O4+δ over a period of 1000 hours in SOEC operation. The development of high-performance SOEC air electrodes is part of the Austrian integrated project HydroMetha. This project combines high-temperature co-electrolysis of CO2 and H2O by solid oxide cells with catalytic methanation in order to enable storage of electrical energy from fluctuating renewable sources with high overall efficiency. References [1] C. Berger, E. Bucher, A. Egger, A.T. Strasser, N. Schrödl, C. Gspan, J. Hofer, W. Sitte, Solid State Ionics, 316, 92 (2018). [2] J.A. Kilner, C.K.M. Shaw, Solid State Ionics, 154, 523 (2002). [3] C. Berger, A. Egger, R. Merkle, E. Bucher, B. Stuhlhofer, N. Schrödl, J. Lammer, C. Gspan, G. Logvenov, J. Maier, W. Sitte, J. Electrochem. Soc., 166 (14) F1088 (2019). [4] C. Berger, E. Bucher, A. Egger, N. Schrödl, J. Lammer, C. Gspan, R. Merkle,W. Grogger, J. Maier, W. Sitte, submitted (2019).

Dense thin-film microelectrodes of the first-order Ruddlesden-Popper phases Pr2NiO4+delta (PNO) and Pr2Ni0.9Co0.1O4+delta (PNCO) were prepared by pulsed laser deposition and photolithographic patterning on yttria-stabilized zirconia (YSZ) substrates. The thinfilms were characterized by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, inductively coupled plasma optical emission spectroscopy, energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. Investigation of the phase stability of PNO and its reactivity with YSZ was performed by X-ray diffraction analyses after high-temperature treatment in air. Resistive and capacitive contributions of the individual processes occurring at the microelectrodes were determined by means of electrochemical impedance spectroscopy at various temperatures (550 <= T/degrees C <= 850) and oxygen partial pressures (1 x 10(-3) = pO(2)/bar <= 1). Oxygen surface exchange coefficients k(q) and k(chem) were calculated from the surface resistances and chemical capacitances of the thin-film electrodes. Comparing k(q)-values of PNO and PNCO shows that substitution of Ni with 10% of Co increases the oxygen surface exchange rates, especially at lower oxygen partial pressures. (C) The Author(s) 2019. Published by ECS.

The mixed ionic-electronic conducting perovskite La0.75Ca0.25FeO3-delta (LCF7525) was synthesized via a citric acid - ethylenediaminetetraacetate sol-gel route. Crystal structure, phase purity, and lattice constants were determined by powder X-ray diffraction and Rietveld refinement. The oxygen exchange kinetics (chemical surface exchange coefficients and chemical diffusion coefficients of oxygen) and the electronic conductivity were studied by in-situ dc-conductivity (relaxation) measurements at 600-800 degrees C and 1 x 10(-3) <= pO(2)/bar <= 0.1. The thermal expansion coefficient was determined by dilatometry at 30-1000 degrees C and 1 x 10(-3) <= pO(2) /bar <= 1. The oxygen nonstoichiometry was measured as a function of temperature and oxygen partial pressure by thermogravimetry and could be described by a point defect model. Experimental data of the chemical diffusion coefficient of oxygen and results from defect chemical modelling were used to estimate self-diffusion coefficients of oxygen and oxygen vacancies, as well as the ionic conductivity. Based on the results obtained for the mass and charge transport properties and the thermal expansion behaviour, it can be concluded that LCF7525 may be an attractive Sr- and Co-free material for air electrodes in intermediate temperature solid oxide fuel cells and solid oxide electrolyser cells.

La0.6Sr0.4CoO3-delta electrode layers with three different microstructures were manufactured by screen-printing, spin-coating and infiltration into a porous Ce0.9Gd0.1O1.95 backbone. Electrode performance was monitored at 700 degrees C in 20% O-2 over periods of 1,600 to 3,860 h by means of electrochemical impedance spectroscopy under open circuit conditions. Reference measurements were performed in dry atmospheres, where significant electrode activation was observed for cells with spin-coated and infiltrated electrodes. Subsequently, the relative humidity level in the surrounding atmosphere was set to 30% and further raised to 60%, thus simulating SOFC operation with ambient air without pre-drying. While no performance loss could be observed in dry atmospheres, significant degradation occurred in humid atmospheres with pronounced differences between degradation rates of half cells with different electrode microstructures. Post-test analyses by scanning electron microscopy (SEM) and transmissionscanning electron microscopy (STEM) were employed to identify the causes for the observed differences in degradation behavior. For screen-printed cells, the surface of the degraded electrodes was covered with small crystallites, probably consisting of SrO formed by Sr-segregation and surface precipitation, where humidity was found to be a crucial factor. For spin-coated and infiltrated electrodes, poisoning by impurities (Si, Cr, S) and particle coarsening were identified as potential causes.