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.

A multimaterial approach is explored for tailoring the mechanical and functional properties of zirconia‐based ceramic electrolytes to be employed in electrochemical solid oxide cells (SOCs). The combination of alumina‐toughened zirconia (ATZ) surface layers with an embedded 8 mol% yttria‐stabilized zirconia (8YSZ) core layer introduces compressive residual stress in the electrolyte surface upon cooling from sintering. Biaxial bending is employed to assess the strength distribution in the multimaterial architectures compared to the monolithic counterparts. Electrochemical impedance spectroscopy is used to measure the ionic conductivity of the samples in the temperature range between 600°C and 1000°C. The effect of alumina addition is investigated in samples containing ATZ surface layers with 10 vol% or 20 vol% alumina. Experimental results show that the strength of multimaterial electrolytes can be increased by up to ∼30% compared to the monolithic 8YSZ electrolyte, while holding similar overall ionic conductivity compared to the 8YSZ base material. The approach presented in this work offers new paths to enhance the structural properties of functional ceramics in SOCs, which may also be applied to other systems such as ceramic membranes or ceramic sensors.

It has recently been shown that the high-entropy perovskite La 0.2 Pr 0.2 Nd 0.2 Sr 0.2 Sm 0.2 CoO 3-δ (LPNSSC) exhibits outstanding performance as air electrode in both SOFC and SOEC operation [1]. For such high-performance electrode materials, it is particularly important to avoid microstructural limitations in order to achieve maximum cell performance. Optimization of the air electrode morphology and the quality of the electrode/electrolyte interface can significantly enhance the electrochemical performance and long-term stability of solid oxide cells. While many publications focus on the simulation of the electrochemical performance of electrodes with a given (artificial) microstructure, experimentalists often face challenges in correlating microstructural features with cell performance. Our approach aims to combine electrochemical cell testing with detailed 3D analysis tools of real electrodes in order to break down the cell performance into its influencing factors. This study specifically addresses the impact of the air electrode microstructure on the cell performance. In this work, cell tests were performed with anode-supported button cells using LPNSSC as air electrode material. The results were combined with 3D imaging techniques of the air electrode in order to identify microstructural features which are crucial for SOEC/SOFC applications. The microstructure of the air electrode was varied over a wide range by adjusting the grain size distribution of the LPNSSC powders used for electrode preparation. In order to assess the rate of oxygen transport towards electrochemically active regions within the electrode, the cell performance was measured at different air supply rates. For the highest air flow, the cell polarization resistance (i.e. anode plus cathode) at 800°C was found to range between 0.13 Ωcm² and 0.24 Ωcm² at 0.7 V in SOFC mode. Detailed 3D imaging was applied to provide explanations for the observed effects. Special attention is given to the combined effect of porosity, tortuosity as well as the area of the inner surface of the porous air electrodes on cell performance. [1] P. Pretschuh, A. Egger, R. Brunner, E. Bucher, Fuel Cells 23 (2023) 377–386.

The performance of solid oxide electrolyzer cell (SOEC) air electrodes depends in a complex way on various compositional and morphological features. This study provides a proof of concept for a knowledge-based AI-assisted design of SOEC air electrodes, which could circumvent the conventional time-consuming and cost-intensive workflow. Symmetrical cells with La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) - Ce0.9Gd0.1O1.95 (GDC) air electrodes with different phase ratios showed a pronounced minimum in the polarization resistance at 50:50 wt%. Using AI-assisted segmentation of SEM images, the morphological features of the electrodes were extracted with an accuracy >96 %, allowing the peak performance of the 50:50 wt% electrode to be associated with several key morphological features. To validate the derived relationships, the two best electrode designs were transferred to full cells. As predicted, the 50:50 wt% LSCF-GDC electrode delivered an exceptionally high current density of -2.37 A cm(-2) at 1.2 V and 800 degrees C, while the cell with the 70:30 wt% electrode exhibited a significantly lower current density. Finally, a 5 x 5 cm(2) cell with 50:50 wt% LSCF-GDC electrode was tested for 220 h. The excellent performance demonstrates that AI-assisted image analysis is a powerful tool to accelerate and improve the development of SOEC electrodes and cells in the future.

The functionality and structural reliability of electrolyte-supported solid oxide fuel cells depend crucially upon the load-bearing capability of the ceramic electrolyte. In this work, the effect of thickness on the mechanical strength of tape-casted 8 mol% yttria-stabilized zirconia electrolyte discs was investigated. Samples with 98 % relative density and 2-3 mu m grain size fabricated with thicknesses between 75 mu m and 360 mu m were mechanically tested using an adapted biaxial bending test. Thin electrolytes with biaxial strength up to 1 GPa could be produced. The measured characteristic strengths ranged from sigma(0) = 940 MPa to sigma(0) = 520 MPa for thin and thick samples, respectively, yielding a common Weibull modulus of m = 6. A higher strength scaled with decreasing thickness, confirming the "size effect" as in many technical ceramics. The measured ionic conductivity was above 0.1 S/cm at 1000 degrees C, which is in good agreement with commercial or conventionally prepared electrolytes.

In recent decades, numerous materials have been investigated for SOFC and SOEC air electrodes. However, many of these contain critical elements, such as cobalt and/or rare earths, in order to boost the electrode performance. In the current context of sustainability, it has become crucial to explore alternatives based on non-critical raw materials, which nevertheless provide acceptable electrochemical performance. In the present work, calcium- and iron-based oxides, which are composed of abundant and relatively inexpensive elements, are investigated for their applicability in SOC air electrodes. When examining the Ca-Fe-O system, it was found that Ca 2 Fe 2 O 5 (CFO) with a brownmillerite structure, and CaFe 2 O 4 with a spinel structure, are stable in ambient air. Although the brownmillerite exhibits higher electrical conductivity than the spinel [1], the electrical conductivity of unsubstituted CFO (~0.5 S/cm at 800°C) remains insufficient for SOC applications. Although CFO is a promising candidate in terms of sustainability, further improvements are necessary. To address this issue, the compounds Ca 2 Fe 1.3 Mn 0.7 O 5 (CFMO) and Ca 2 Fe 1.9 Ni 0.1 O 5 (CFNO) were synthesized, as previous research suggests that Mn- and Ni-substitution enhances the electronic conductivity of CFO [2]. In an alternative approach, a two-phase composite was prepared by mixing CFO with LaNi 0.6 Fe 0.4 O 3 (LNF) in a 50:50 weight ratio. At 800°C in air, the electrical conductivity of the CFO-LNF composite, CFNO, and CFMO was found to be approximately 48 S/cm, 7 S/cm, and 6 S/cm, respectively. Electrochemical impedance spectroscopy on symmetrical cells revealed polarization resistances of about 0.5 Ω cm² for both CFO and CFO-LNF electrodes in air at 800°C. [1] V. V. Kharton et al. “Mixed conductivity and stability of CaFe 2 O 4-δ ”, Journal of the Electrochemical Society, 155 (3), (2008), 13-20. [2] Q. Li et al. “Evaluation of a brownmillerite oxide as cathode for solid oxide fuel cells”, Journal of Power Sources, 238, (2013), 11-16.

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.

High-entropy perovskites (HEPs) are attracting increasing attention as air electrode materials for solid oxide cells (SOCs). In this work, three different HEPs from the series La0.2Pr0.2Nd0.2Sm0.2Sr0.2Co1-xFexO3-delta (x = 0, 0.5, 1) are synthesized using the citric acid-ethylenediaminetetraacetate (EDTA) method. X-ray diffraction analysis finds crystal structures with the orthorhombic space group 62 (Pnma) at room temperature. The lattice distortion increases with increased Fe-substitution at the B-site. The electrical conductivity (sigma(e)) is determined at temperatures from 600 to 850 degrees C and oxygen partial pressures (pO(2)) between 0.001 and 0.15 bar. For the pure cobaltate, sigma(e) is 1469 S cm(-1) at 800 degrees C and 0.15 bar pO(2). The conductivity is significantly reduced with Fe-doping, reaching 87 S cm(-1) for the pure ferrate at 800 degrees C. The chemical oxygen surface exchange coefficient (k(chem)) and the chemical oxygen diffusion coefficient (D-chem) are determined by the electrical conductivity relaxation technique. D-chem is found to be quite independent of B-site doping and pO(2), with values of approx. 5 x 10(-6) cm(2) s(-1) at 800 degrees C. In contrast, k(chem) is strongly influenced by the B-site composition, which results in an increase of more than one order of magnitude from the ferrate (3.4 x 10(-5) cm s(-1)) to the cobaltate (7.7 x 10(-4) cm s(-1)) at 800 degrees C and 0.001 bar pO(2). This clearly demonstrates the beneficial effects of Co on the electronic conductivity as well as on the catalytic activity for the oxygen surface exchange reaction.

ABSTRACTThis study investigates the novel cobalt‐free high‐entropy perovskite, La0.2Pr0.2Nd0.2Sm0.2Sr0.2FeO3–δ (LPNSSF), as an air electrode material for solid oxide cells (SOCs). When testing a button cell with a single‐phase LPNSSF electrode, a current density of 0.55 A cm−2 is obtained at 0.7 V in fuel cell mode at 800°C. In order to mitigate the moderate electronic conductivity of LPNSSF, two approaches are explored. Incorporating a Co‐free highly conductive perovskite, LaNi0.6Fe0.4O3–δ (LNF), either as an LPNSSF–LNF composite electrode or as a current collector layer (CCL), enhances the performance to 0.61 and 0.66 A cm−2, respectively, under the same conditions. Microstructural features are studied by electron microscopy and show a rather dense structure of the CCL. Optimization of the current collector increases the current density further to 0.96 A cm−2 at 0.7 V in a 5 × 5 cm2 anode‐supported cell at 800°C. This cell exhibits good long‐term stability in electrolysis mode in H2‐H2O with 80% humidification. Continuous polarization of −0.69 A cm−2 is sustained for 1000 h, with an average degradation rate of 10 mV kh−1 after an initial run‐in phase. These findings demonstrate the promising performance and durability of LPNSSF as cobalt‐free SOC air electrode.

Green hydrogen presents a promising solution for transitioning from fossil fuels to a clean energy future, particularly with the application of fuel cell electric vehicles (FCEVs). However, the hydrogen refuelling process for FCEVs requires extensive pre-cooling to achieve fast filling times. This study presents experiments and simulations of a hydrogen refuelling station equipped with an adaptable cold-fill unit, aiming to maximize fuelling efficiencies. For this purpose, we developed and experimentally validated simulation models for a hydrogen tank and an aluminium block heat exchanger. Different pre-cooling parameters affect the final tank temperatures during the parallel filling of three 350 L type IV tanks. The results indicate significant potential for optimizing the required cooling energy, with achievable savings of over 50 % depending on the pre-cooling strategy. The optimized pre-cooling strategies and energy savings aid in advancing the refuelling process for FCEVs, effectively contributing to the transition to clean energy.

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.

Strontium segregation (coupled to phase decomposition and impurity poisoning) and electrode delamination are two of the most important degradation mechanisms currently limiting the long-term stability of solid oxide fuel cell and electrolysis cell (SOFC and SOEC) air electrodes. The present study aims to demonstrate that air electrodes made of entropy-stabilized multi-component oxides can mitigate these degradation mechanisms while providing excellent cell performance. A SOEC utilizing La0.2Pr0.2Nd0.2Sm0.2Sr0.2CoO3-& delta; (LPNSSC) as an air electrode delivers -1.56 A/cm(2) at 1.2 V at 800 & DEG;C. This performance exceeds that of a commercial cell with La0.6Sr0.4CoO3-& delta; (LSC) air electrode, which reaches -1.43 A/cm(2). In a long-term electrolysis test, the LPNSSC cell shows stable performance during 700 h, while the LSC cell degrades continuously. Post-mortem analyses by scanning electron microscopy-energy dispersive X-ray spectroscopy indicate complete delamination of the LSC electrode, while LPNSSC shows excellent adhesion. The amount of secondary phases formed (esp. SrSO4) is also much lower in LPNSSC compared to LSC. In conclusion, the high-entropy perovskite LPNSSC is a promising option for air electrodes of solid oxide cells. While LPNSSC can compete with - or even outperform - LSC air electrodes in terms of electrochemical performance, it could be particularly advantageous in terms of long-term stability in SOEC mode.

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.

Reed valves are widely used in hermetic reciprocating compressors and are responsible for a major share of thermodynamic losses. The suction valve in particular, which is opened for almost the entire suction phase, has a very significant improvement potential. Suction valves in hermetic reciprocating compressors are usually opened only by the pressure difference created by the moving piston and should be closed before the compression phase starts, in order to avoid a reversed mass-flow through the valve. The valves are thus preloaded, which, on the other hand, results in a higher flow resistance. In this work, a force-assisted suction valve is investigated. An electromagnetically actuated spring generates an additional force acting on the conventional suction reed valve. This force leads to a completely different valve movement and a significant reduction of the losses caused by the suction valve. A conventional suction valve flutters during the suction phase. Thus, it hits the valve seat several times which has, beside an increased flow resistance, negative influences on the reliability of the reed valve and noise emissions. With the help of the additional force, it is able to overcome the opening delay due to the preload and the oil ‘stiction’ effect. Furthermore, the suction valve can be held open during the whole suction phase, and it closes smoothly in time near the bottom dead centre of the piston. The investigation of the force-assisted suction valve is done by numerical simulations as well as by experiments. Results show an improvement of the coefficient of performance by approximately 0.7%–1.6% depending on the operating conditions.

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.

Reversible solid oxide cells (rSOCs) present a unique possibility in comparison to other available technologies to generate electricity, heat and valuable fuels in one system, in a highly-efficient manner. The major issue hindering their commercialization are system reliability and durability. A detailed understanding of the processes and mechanisms that occur within rSOCs of industrial-size, is of critical importance for addressing this challenge. This study provides in-depth insight into behavior of large planar rSOCs based on a comprehensive experimental and numerical study. All the numerical data obtained are validated with the in-house made cells and experiments. The sensitivity analysis, which covers a wide range of operating conditions relevant for industrial-sized systems, such as varying operating temperature, H2/H2O-ratio, operating current etc., provides very good accordance of the cell performance measured and simulated. It reveals that lowering fuel volume and thus causing fuel starvation has more pronounced effect in an electrolysis mode, which is visible in both the low-frequency and the middle-frequency range. Moreover, both co- and counter-flow are appropriate for the reversible operation. However, more uniform current density distribution is achievable for the counter-flow, which is of crucial importance for the real system design. The most accurate performance prediction can be achieved when dividing the cell into 15 segments. Slightly lower accuracy is reached by logarithmic averaging the fuel compositions, thus reducing the calculation time required. A computationally- and time-efficient model with very precise performance prediction for industrial-sized cells is thus developed and validated.