Oxygen Surface Exchange Coefficient Research Articles

A-site alkali metal-doped SrTi0.3Fe0.7O3-δ: A highly stable symmetrical electrode material for solid oxide electrochemical cells

To addresses the limitations in electrode performance observed at temperatures ≤700 °C, attributed to the low oxygen surface exchange coefficient primarily caused by high Sr surface segregation, we developed an electrode performance enhancement strategy involving A-site alkali metal doping to SrTi1-xFexO3-δ (STF)-based perovskite. Here, we present the symmetrical electrode material for solid oxide electrochemical cells (SOCs), Sr0.95Mg0.05Ti0.3Fe0.7O3-δ (SMTF), demonstrating a unique combination of excellent symmetrical electrode performance and long-term stability. A-site Mg doping reduces the Sr surface segregation and improves the conductivity but also significantly enhances its electrochemical performance. At 700 °C, the performance of SMTF symmetric cell in both fuel cell and electrolysis modes is 1.5 times higher than that of STF. Moreover, under air atmosphere, SMTF demonstrates stable performance (>1000 h) at 1 A cm−2, with no degradation observed after 400 h testing under humidified hydrogen conditions. Additionally, SMTF exhibits excellent CO2 tolerance in CO2-rich atmospheres.

Comprehensive understanding of charge and mass transport in BaZr0.1Ce0.7Y0.1Yb0.1O3−δ

This work reports transport properties of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb1711), studied during oxidation/reduction and hydration/dehydration using electrical conductivity relaxation (ECR) measurements. The oxidation/reduction displayed typical monotonic conductivity relaxation behavior, attributed to the ambipolar diffusion of oxygen vacancies (Vo∙∙) and electron-holes (h∙), and oxygen surface exchange coefficient (k) were >1 order of magnitude higher than bulk diffusivities (D˜), indicating a diffusion-controlled process. Whereas, hydration/dehydration displayed a temperature-dependent gradual transition from monotonic to non-monotonic relaxation profile. At lower temperatures, transient defect concentration profile indicated that overall hydration/dehydration involved an acid-base-type reaction, comprising the diffusion of oxygen vacancy (Vo∙∙) and proton (Hi∙) couples. However, at higher temperatures and high [h∙], it involved a combination of a hydrogenation reaction (comprising component H via diffusion of h∙ and Hi∙ couple) and an oxidation reaction (involving component O via diffusion of h∙ and Vo∙∙ couple), thus giving rise to a two-fold non-monotonic relaxation profile. The k and D˜ values for oxygen vacancies (kVO∙∙; DVO∙∙) were lower than those for protons (kHi∙;DHi∙) with the respective Ea being 57.51 ± 3.14 and 60.72 ± 1.48 kJ∙mol−1 for oxygen-vacancies and 68.78 ± 12.65 and 71.05 ± 13.87 kJ∙mol−1 for protons.

Construction of high performance fuel electrode in solid oxide electrolysis cells by tuning the surface oxygen defect to promote electrochemical reduction of CO2

B-site Co–Fe-based perovskite materials have shown potential as ceramic cathodes in solid oxide electrolysis cells (SOECs) for direct CO2 electrolysis. Nevertheless, their operational utilization is constrained by insufficient catalytic activity and durability in pure CO2. In this work, F− doping and A-site deficiency strategies are proposed and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), F-doped La0.6Sr0.4Co0.2Fe0.8F0.1O2.9 (LSCFF) and A-site deficiency combined with F-doped (La0.6Sr0.4)0.95Co0.2Fe0.8F0.1O2.9 (LSCFF95) are prepared as cathode materials for CO2 electrolysis. After doping with F−, the valence states of both Co and Fe elements decrease, while generating more oxygen vacancies. With the further introduction of A-site deficiency, the valence states further decrease, resulting in more oxygen vacancies. The enhancement of oxygen vacancies leads to an improvement in the oxygen surface exchange coefficient (kchem) and the bulk diffusion coefficient (Dchem) of LSCFF and LSCFF95 materials. At 800 °C, kchem of LSCFF and LSCFF95 reach 24.21 × 10−4 and 33.54 × 10−4 cm s−1, respectively, which are 51% and 110% higher than the value of 16.03 × 10−4 cm s−1 for LSCF. Meanwhile, the Dchem values of LSCFF and LSCFF95 are measured to be 37.52 × 10−5 and 47.81 × 10−5 cm2 s−1, respectively. These values are 49% and 90% higher than the value of 25.22 × 10−5 cm2 s−1 for LSCF. Consequently, F− doping and A-site deficiency significantly increase the electrochemical performance. For example, F− doping decreases the polarization resistance (Rp) value of the SOEC with LSCF cathode from 0.19 Ω cm2 to 0.13 Ω cm2 at 800 °C. Furthermore, when the A-site deficiency is introduced, the Rp value is further reduced to 0.11 Ω cm2. Therefore, the SOEC with the LSCFF95 cathode exhibits excellent performance. Specifically, it has demonstrated an impressively high current density of 2.85 A cm−2 at 800 °C and 1.5 V, and has exhibited exceptional durability over prolonged operation.

A-site and B-site co-doped PrBaFe2O5+δ double perovskite as a symmetrical electrode of solid oxide fuel cell with potential of CO2 electrolysis

In this study, PrBa0.5Sr0.5Fe1.9Ge0.1O5+δ (PBSFG), the Sr and Ge doped PrBaFe2O5+δ (PBF), is investigated as a symmetrical electrode of solid oxide cells (SOCs). PBSFG has the potential to be a promising symmetrical electrode in SOCs owing to its high catalytic performance with redox stability. The concentration of oxygen vacancies and surface exchange coefficient of PBSFG are better than that of PBF. The results indicate that PBSFG exhibits not only adequate performance in the fuel cell, with a maximum power density of 1.05 W cm−2 at 800 °C, but also enhanced CO2 electrolysis performance, compared to the PBF. Specifically, in the CO2-electrolysis mode, a maximum current density of 825 mA cm−2 is measured at 850 °C and 1.5 V, which is substantially higher than that of 370 mA cm−2 measured for undoped PBF. These results demonstrate the effectiveness of Sr and Ge doping in PBF to improved CO2 electrolysis performance and notable fuel cell performance.

Multiplying Oxygen Permeability of a Ruddlesden-Popper Oxide by Orientation Control via Magnets.

Ruddlesden-Popper-type oxides exhibit remarkable chemical stability in comparison to perovskite oxides. However, they display lower oxygen permeability. We present an approach to overcome this trade-off by leveraging the anisotropic properties of Nd2 NiO4+δ . Its (a,b)-plane, having oxygen diffusion coefficient and surface exchange coefficient several orders of magnitude higher than its c-axis, can be aligned perpendicular to the gradient of oxygen partial pressure by a magnetic field (0.81 T). A stable and high oxygen flux of 1.40 mL min-1 cm-2 was achieved for at least 120 h at 1223 K by a textured asymmetric disk membrane with 1.0 mm thickness under the pure CO2 sweeping. Its excellent operational stability was also verified even at 1023 K in pure CO2 . These findings highlight the significant enhancement in oxygen permeation membrane performance achievable by adjusting the grain orientation. Consequently, Nd2 NiO4+δ emerges as a promising candidate for industrial applications in air separation, syngas production, and CO2 capture under harsh conditions.

Multiplying Oxygen Permeability of a Ruddlesden‐Popper Oxide by Orientation Control via Magnets

AbstractRuddlesden‐Popper‐type oxides exhibit remarkable chemical stability in comparison to perovskite oxides. However, they display lower oxygen permeability. We present an approach to overcome this trade‐off by leveraging the anisotropic properties of Nd2NiO4+δ. Its (a,b)‐plane, having oxygen diffusion coefficient and surface exchange coefficient several orders of magnitude higher than its c‐axis, can be aligned perpendicular to the gradient of oxygen partial pressure by a magnetic field (0.81 T). A stable and high oxygen flux of 1.40 mL min−1 cm−2 was achieved for at least 120 h at 1223 K by a textured asymmetric disk membrane with 1.0 mm thickness under the pure CO2 sweeping. Its excellent operational stability was also verified even at 1023 K in pure CO2. These findings highlight the significant enhancement in oxygen permeation membrane performance achievable by adjusting the grain orientation. Consequently, Nd2NiO4+δ emerges as a promising candidate for industrial applications in air separation, syngas production, and CO2 capture under harsh conditions.

Tuning the ORR catalytic activity of LaFeO3-δ-based perovskite cathode for solid oxide fuel cells by doping with alkaline-earth metal elements

Co-based perovskite cathodes have demonstrated impressive catalytic activity for oxygen reduction reaction (ORR) at intermediate temperatures. However, these cathodes face the challenges such as high cost and thermal expansion coefficient (TEC) mismatch. On the other hand, Fe-based perovskite oxides like LaFeO3 exhibit more stable properties and lower TEC, but their ORR activity is not as remarkable as that of Co-based cathodes. In this study, La3+ at the A-site of LaFeO3 was substituted by aliovalent alkaline-earth metal cations. The research findings reveal that the doping of alkaline-earth metal ions induces the formation of structural defects and alters the valence state of Fe. This leads to an increase in the conductivity and oxygen vacancy concentration, which are critical for ORR kinetics in both H–SOFC and O–SOFC. Among the various dopants, Sr doped LaFeO3 (LSrF) exhibits the most outstanding ORR catalytic activity. This can be attributed to its highest total conductivity, O2− conductivity, oxygen surface exchange coefficient, and oxygen bulk diffusion coefficient. At 700 °C, single cell with LSrF cathode achieves peak power densities of 0.781 and 0.894 W cm−2, for O2--conducting solid oxide fuel cell (SOFC) and H+-conducting SOFC, respectively. These performances are essentially twice that of single cell with LaFeO3 cathode. The findings of this study provide valuable insights into doping strategies for Fe-based perovskite cathodes and contribute to the advancement of IT-SOFC cathode research.

Estimating Degradations of La0.57Sr0.38Co0.2Fe0.8O3-δ Oxygen Diffusion and Surface Exchange Coefficients by Effective Reaction Thickness

The degradation of La0.57Sr0.38Co0.2Fe0.8O3-δ (LSCF) cathode is investigated by focusing on the change in effective reaction thickness. The durability tests showed severe degradation for the thinner electrodes, which suggests that the degradation in surface exchange coefficient (k) is the dominant degradation factor rather than the bulk diffusion coefficient (D). For the quantitative evaluation of k and D, the electrochemical impedance spectroscopy analyses demonstrated that the degradation rate of k was larger than that of D, and both degraded larger for thinner electrodes. The changes in effective reaction thickness are estimated by numerical simulation with the obtained degradation rates of D and k. The effective reaction thickness elongated in thinner electrodes due to the excessive decrease in k. This implies that the degradation is accelerated in thinner electrodes where effective reaction thickness exceeds physical electrode thickness and larger local overpotential is imposed.

Enhanced Oxygen Evolution Rate and Anti-interfacial Delamination Property of the SrCo0.9Ta0.1O3-δ@La0.6Sr0.4Co0.2Fe0.8O3-δ Oxygen-Electrode for Solid Oxide Electrolysis Cells.

Interfacial delamination between the oxygen-electrode and electrolyte is a significant factor impacting the reliability of solid oxide electrolysis cells (SOECs) when operating at high voltages. The most effective method to mitigate this delamination is to decrease the interfacial oxygen partial pressure, which can be accomplished by amplifying the oxygen exsolution rate and the O2- transport rate of the oxygen-electrode. In this study, a SrCo0.9Ta0.1O3-δ (SCT) film with an outstanding oxygen surface exchange coefficient and an outstanding O2- conductivity was introduced onto the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) surface by infiltration. This composite oxygen-electrode exhibited a notably high electrochemical catalytic activity primarily due to the significantly improved O2- transport and oxygen surface exchange rate. Single cells with a 15-LSCF oxygen-electrode achieved a peak power density of 1.33 W cm-2 at 700 °C and a current density of 1.25 A cm-2 at 1.3 V (60% H2O-H2) at 750 °C. Additionally, an electrolysis cell with a 15 wt % SCT-infiltrated LSCF oxygen-electrode demonstrated stable operation even at high current densities for over 330 h with no noticeable delamination. The remarkable durability of the 15-LSCF oxygen-electrode can be attributed to the boosted oxygen exsolution reaction (OER) activity and the suppression of Sr segregation due to SCT infiltration. The impressive OER activity and resistance to interfacial delamination make the 15-LSCF a promising candidate for a composite oxygen-electrode in SOECs.

Nd-doping-induced perovskite/Ruddlesden-Popper heterointerfaces boost the ORR activity and CO2 tolerance for SrFeO3-δ-based cathode

The development of cathode materials with high oxygen reduction reaction (ORR) activity is crucial to realize the wide application of solid oxide fuel cells. Nd-doped SrFeO3-δ (SNF) two-phase nanocomposite was fabricated by a dopant-induced in situ self-assembly process. The SNF nanocomposite comprises 63.4 wt% perovskite main phase (P–SNF) and 36.6 wt% Ruddlesden-Popper second phase (RP-SNF). The average thermal expansion coefficient (TEC) of SrFeO3-δ is reduced by 36.5 % and down to 13.9 × 10−6 K−1. At 750 °C, SNF demonstrates an impressive output power of 779 mW cm−2, which is twice that of the single-phase SrFeO3-δ. The superior performance stems from the heterointerface effect of the in situ self-assembled two phases. This effect increases the content of oxygen vacancies, thereby enhancing both chemical oxygen surface exchange coefficient and bulk diffusion coefficient. Moreover, the SNF composite exhibits excellent CO2 tolerance than the single-phase SrFeO3-δ. This work presents a promising avenue for exploring multifunctional materials through interface engineering, offering new possibilities for future energy conversion devices.

Achieving Fast Oxygen Reduction on Oxide Electrodes by Creating 3D Multiscale Micro-Nano Structures for Low-Temperature Solid Oxide Fuel Cells.

Fast oxygen reduction reaction (ORR) at the cathode is a key requirement for the realization of low-temperature solid oxide fuel cells (SOFCs). While the design of three-dimensional (3D) structures has emerged as a new and promising approach to improving the electrochemical performance of SOFC cathodes, achieving versatile structures and structural stability is still challenging. In this study, we demonstrate a novel architectural design for a superior cathode with fast ORR activity. By employing a completely new fabrication process comprising a 3D printing technique and pulsed laser deposition (PLD), we design 3D La0.8Sr0.2CoO3-δ (LSC) micro-nano structures with the desired shape. 3D-printed yttria-stabilized ZrO2 (YSZ) microstructures significantly increase the ratio of surface area to volume while maintaining suitable ionic conductivity comparable to that of single-crystalline YSZ substrates. Scanning electron microscopy and energy dispersive X-ray microanalysis reveal the formation of crack- or void-free YSZ microstructures and the uniform deposition of LSC films by PLD on the YSZ microstructures. The 3D LSC micro-nano structures show significantly enhanced oxygen surface exchange coefficients (kchem) extracted from electrical conductivity relaxation (ECR) measurements by up to 3 orders of magnitude relative to the bulk LSC. Furthermore, electrochemical impedance spectroscopy measurements verify the kchem values from ECR and no directional difference in the measured ORR activity depending on the shape of 3D microstructures. The dramatic enhancement of the ORR activity of LSC is attributed to the increased film surface areas resulting from the 3D YSZ microstructures.

Enhancing the oxygen reduction reaction activity of SOFC cathode via construct a cubic fluorite/perovskite heterostructure

Pr2NiO4+δ-based (Ruddlesden-Popper) perovskite-structured oxide materials are promising candidates for low and intermediate temperature solid oxide fuel cells (SOFCs) systems. In this study, we prepared a high-performance cubic-fluorite/perovskite heterostructure Pr6O11-Pr1.2Sr0.8NiO4+δ (PO-PSNO) cathode for intermediate temperature SOFCs by solution impregnation method. By changing the impregnation amount of Pr6O11, the micro-morphology of the heterostructure was regulated, so that the heterostructure interface had the best reactive site. The oxygen reduction reaction (ORR) performance of Pr1.2Sr0.8NiO4+δ is improved, and the content of surface Sr in the heterostructure electrode material is reduced. Our results indicate that the synergistic effect of PO particles and PSNO skeleton makes the heterostructure exhibit a high oxygen surface exchange properties and catalytic performance. When the impregnation amount of PO particles reaches about 10% of the PSNO skeleton, the heterostructure has the best ORR activity. The oxygen surface exchange coefficient (Kchem) of 2-PO-PSNO is 5.8 × 10-4 cm·s−1, being 1.45 times as that of PSNO at 750 °C. Meanwhile, the polarization impedance (Rp) of 2-PO-PSNO is reduced by 66% to only 0.114 Ω cm2. Our study provides a new approach for developing high-performance RP structured perovskite cathodes and demonstrates practical applications in the field of SOFCs.

Oxygen diffusion and surface exchange coefficients measurements under high pressure: Comparative behavior of oxygen deficient versus over‐stoichiometric air electrode materials

AbstractMixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 and 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900°C has been developed. The main goal of this study was to compare the behavior of two types of reference materials: the oxygen deficient La‐Sr‐Fe‐Co perovskites, and the overstoichiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure.

Pt Current Collectors Artificially Boosting Ceria Oxygen Surface Exchange Coefficients

The SOFC/SOEC community has an oxygen surface exchange measurement reproducibility crisis. This is exemplified by a >1,000 times variation in the literature-reported chemical oxygen surface exchange coefficient (k chem) values of Pr0.1Ce0.9O2−x (PCO) at 700 °C in air, a >10,000 times variation in the literature-reported k chem values of La0.6Sr0.4FeO3-x (LSF) at 650 °C in air, and a >100,000 times variation in the literature-reported k chem values of Ba0.6Sr0.4Co0.8Fe0.2O3-x (BSCF) at 500 °C in air. Many of these k chem values have been determined electrically (often via Electrochemical Impedance Spectroscopy or Electrical Conductivity Relaxation) using precious metal current collectors. Unfortunately, the Time-of-Flight Secondary Ion Mass Spectroscopy and Curvature Relaxation k chem measurements performed here on Pulsed Laser Deposited Pr0.1Ce0.9O2−x (PCO) thin films show that unpolarized Pt current collectors can dramatically improve PCO k chem values (by up to 2 orders of magnitude) through i) a reduction in the PCO surface Si concentration, and/or ii) Pt diffusion into the underlying PCO lattice. Further, this phenomenon can occur in PCO thin films only exposed to mild temperatures of 500 °C. Hence, it seems likely that precious metal current collectors may be responsible for at least some of the large k chem variation reported in the literature for “identical” materials tested under “identical” conditions.Reference: Ma Y, Burye TE, Nicholas JD. Pt Current Collectors Artificially Boosting Praseodymium Doped Ceria Oxygen Surface Exchange Coefficients. Journal of Materials Chemistry A. 2021; 9: 24406.

Synergistic effects of A-site deficiency and anion doping on Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode for H2O electrolysis and CO2 electrolysis

In this work, a strategy of A-site deficiency and F-doping was proposed to prepare Pr0.4Sr0.55Ni0.2Fe0.7Mo0.1F0.1O2.9 (PSNFMF) cathode. Compared with the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 (R-PSNFM), the reduced PSNFMF facilitate the exsolution reaction and generate more oxygen vacancies. Moreover, at 800 °C, the chemical oxygen surface exchange coefficient (kchem) of R-PSNFMF is 4.31 × 10−4 cm s−1, 21.8% higher than the value of 3.54 × 10−4 cm s−1 for R-PSNFM, and the oxygen chemical bulk diffusion coefficient (Dchem) of R-PSNFMF is 34.21 × 10−5 cm2 s−1, 24.3% higher than the value of 27.53 × 10−5 cm2 s−1 for R-PSNFM, suggesting faster oxygen transport kinetics are obtained for R-PSNFMF. Meanwhile, the synergistic effect of A-site deficiency and anion doping can also promote the electrolysis of H2O and CO2. For H2O electrolysis, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.75 A cm−2, 31.6% higher than the value of −0.56 A cm−2 for R-PSNFM based cell. And for CO2 electrolysis in harsh condition at 800 °C, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.43 A cm−2, 38.7% higher than the value of −0.31 A cm−2 for R-PSNFM based cell.

Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes in situ.

Wehave developed a novel in situ methodology for the direct study of mass transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, not accessible by conventional methods, enabling complementary insights for the study of ion transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) are demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary ion mass spectrometry (ToF-SIMS) characterisation and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate ourunderstanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research andbeyond. This article is protected by copyright. All rights reserved.

A novel triple-conductive cathode with high efficiency and stability for protonic ceramic fuel cells

The sluggish kinetics of cathodic oxygen reduction reaction is one of the key barriers to the development of protonic ceramic fuel cells (PCFCs), while the appearance of triple-conductive material reported in recent years has brought a new dawn for the commercialization of PCFCs. These materials need to have excellent hydration ability and water tolerance to expand the active sites and prolong the operation lifespan. Herein, a highly efficient and stable cathode material, BaCo0.4Fe0.4Zr0.1Sc0.1O3-δ (BCFZSc) with cubic perovskite structure, is proposed for PCFCs. Electrical conductivity relaxation measurements reveal that the oxygen surface exchange coefficient of BCFZSc can reach the value of 1.8 × 10−3 cm s−1 at 700 °C, almost an order of magnitude larger than those of most reported cathodes. Applying BCFZSc as cathode, single cell exhibits an excellent peak power density of 1.334 W cm−2 with a small polarization resistance of 0.044 Ω cm2 at 700 °C, and a decent durability over 150 h at 600 °C is demonstrated. This research highlights that BCFZSc is a promising candidate for PCFCs cathode.

Doping effects of alkaline earth element on oxygen reduction property of high-entropy perovskite cathode for solid oxide fuel cells

The nearly infinite arrangement and combination of multiple components in high entropy materials provide a huge platform for the modification and performance regulation of new materials. It is an attractive idea to introduce the concept of high entropy into material design and improve its electrochemical performance through component regulation. In this paper, cobalt-based high-entropy perovskite oxides with different alkaline earth metal contents were successfully synthesized. The phase structure, direct current conductivity, oxygen mobility characters and electrochemical properties of these materials are studied. It is found that with the continuous substitution of rare earth elements by alkaline earth elements in high-entropy materials, the content of oxygen vacancies increases and the electrochemical performance is enhanced. The chemical oxygen diffusion coefficient (Dchem) and surface exchange coefficient (Kchem) of the high-entropy oxide La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 are 2.8 × 10−6 cm2 s−1 and 9.1 × 10−3 cm s−1 at 700 °C, respectively. The electrochemical impedance spectroscopy measurement results show that La0.2Pr0.2Ba0.2Sr0.2Ca0.2CoO3 has the minimum polarization resistance, which is 0.03 Ω cm2 at 700 °C in air. The anode-supported fuel cell with the high entropy oxide reaches a peak power density of 792 mW cm−2 at 700 °C and exhibits attractive long-term stability and excellent CO2 resistant properties.

Oxygen exchange and transport properties of the first-order Ruddlesden-Popper phase La2Ni0.9Co0.1O4+δ

The rare earth nickelate La2Ni0.9Co0.1O4+δ (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 °C to 850 °C and 0.01 ≤ pO2 / bar ≤ 0.1. The results indicate that substitution of nickel with cobalt in La2Ni0.9Co0.1O4+δ leads to significantly higher values of the surface exchange coefficient compared to La2NiO4+δ, 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.

Considerable oxygen reduction activity and durability of BaO nanoparticles-decorated Ln0.94BaCo2O5+δ electrocatalysts

Owing to unique catalytic properties and durability, nanoparticles-containing heterostructured catalysts have attracted much interest in energy storage and conversion fields. Here we report a multi-composite electrocatalyst consisting of BaO nanoparticles and Ln0.94BaCo2O5+δ (Ln = La, Pr, Nd, Sm, and Gd) (e-Ln0.94BCO) though in situ exsolution route. As-obtained e-Ln0.94BCO catalysts exhibit desirable oxygen reduction reaction (ORR) activity and durability toward solid oxide fuel cells (SOFCs) cathode application. When varying Ln3+ from Gd3+ to La3+, the phase transformation of orthorhombic-tetragonal-cubic can be identified in Ln0.94BaCo2O5+δ, significantly demonstrating improved electrochemical performance as well as electrical conductivity, oxygen surface exchange and chemical diffusion coefficients. Resultantly, the e-L0.94BCO cathode exhibits the lowest area-specific resistance (ASR) of 0.03 Ω cm2 at 700 °C, reduced by ≈ 43% relative to pristine La0.94BaCo2O5+δ (L0.94BCO) cathode. Under realistic operating conditions, the e-L0.94BCO cathode-based button cell delivers a peak power density (PPD) 0f 1.05 W cm−2 at 700 °C, along with outstanding long-term stability. Furthermore, an exceptional CO2 durability is achieved in e-L0.94BCO with negligible degradation rate. Density-functional theory (DFT) calculations imply that interfacial BaO makes a movement of O 2p-band center to the Fermi level, which is closely related to promoted electrochemical performance.