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The understanding of surface chemistry changes on oxygen electrodes is critical for the development of reversible solid oxide fuel cell (RSOFC). Here, we report for the first time that the electrochemical potentials can drastically affect the surface composition and hence the electrochemical activity and stability of PrBaCo2O5+ δ (PBCO) electrodes. Anodic polarization degrades the activity of the PBCO electrode, whereas the cathodic bias could recover its performance. Alternating anodic/cathodic polarization for 180 h confirms this behavior. Microstructure and chemical analysis clearly show that anodic bias leads to the accumulation and segregation of insulating nanosized BaO on the electrode surface, whereas cathodic polarization depletes the surface species. Therefore, a mechanism based on the segregation and incorporation of BaO species under electrochemical potentials is considered to be responsible for the observed deactivation and recovery process, respectively. [ABSTRACT FROM AUTHOR]

Effect of Gd0.2Ce0.8O1.9 (GDC) infiltration on the performance and stability of La0.8Sr0.2MnO3 (LSM) oxygen electrodes on Y2O3-stabilized ZrO2 (YSZ) electrolyte has been studied in detail under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The incorporation of GDC nanoparticles significantly enhances the electrocatalytic activity for oxygen oxidation reaction on LSM electrodes. Electrode polarization resistance of pristine LSM electrode is 8.2 Ω cm2 at 800 °C and decreases to 0.39 and 0.09 Ω cm2 after the infiltration of 0.5 and 1.5 mg cm−2 GDC, respectively. The stability of LSM oxygen electrodes under the SOEC operating conditions is also significantly increased by the GDC infiltration. A 2.0 mg cm−2 GDC infiltrated LSM electrode shows an excellent stability under the anodic current passage at 500 mA cm−2 and 800 °C for 100 h. The infiltrated GDC nanoparticles effectively shift the reaction sites from the LSM electrode/YSZ electrolyte interface to the LSM grains/GDC nanoparticle interface in the bulk of the electrode, effectively mitigating the delamination at the LSM/YSZ interface. The results demonstrate that the GDC infiltration is an effective approach to enhance the structural integrity and thus to achieve the high activity and excellent stability of LSM-based oxygen electrode under the SOEC operating conditions. [ABSTRACT FROM AUTHOR]

Abstract: Nano-structured Pd infiltrated and Pd0.95M0.05 (M = Mn, Co, Ce, and Gd) co-infiltrated Y2O3–ZrO2 (YSZ) electrodes are studied as the oxygen electrodes of solid oxide electrolysis cells (SOECs). The infiltrated Pd-YSZ electrodes show good electrocatalytic activity for the oxygen evolution reaction. For example, the electrode polarization resistance (R E) for 2.0 mg cm−2 Pd infiltrated YSZ is 0.36 Ω cm−2 at 800 °C. R E is not significantly affected by co-infiltrating Pd with Mn and Co, but is enhanced by co-infiltration of Ce and Gd. The co-infiltration of low concentrations of metals in particular Co, Ce and Gd significantly enhances the microstructure and performance stability of the Pd-YSZ electrodes. The results demonstrate that the addition of dopants to the Pd in the form of either an alloy (Co) or a separate phase (Ce and Gd) is beneficial to enhance the performance and stability of Pd based oxygen electrodes of SOECs. [Copyright &y& Elsevier]

Abstract: A theoretical model is developed to simulate the delamination kinetics of La0.8Sr0.2MnO3 (LSM) electrode from YSZ electrolyte in solid oxide electrolysis cells (SOECs). The delamination is caused by the total stress including the internal oxygen pressure in LSM near the electrode/electrolyte interface, and the tensile stress by the oxygen migration from the YSZ electrolyte to LSM lattice. Weibull theory is used to determine the survival probability of electrode/electrolyte interface under the total stress. The relaxation time corresponding to the time for oxygen diffusion from the interface to the microcracks in La0.8Sr0.2MnO3 links the survival probability with polarization time, thus the survival interface area can be predicted with varying anodic polarization time. The model is validated with experimental data. The effects of applied anodic current and operating temperature are discussed. The present model provides a starting point to study more complex cases, such as composite oxygen electrodes. [Copyright &y& Elsevier]

Abstract: Nano-structured (La,Sr)MnO3 infiltrated Y2O3–ZrO2 (LSM–YSZ) oxygen electrodes show excellent activity and performance stability under solid oxide electrolysis cells (SOECs) operation conditions. LSM–YSZ composite electrodes are prepared by infiltrating pre-sintered YSZ scaffold with LSM nitrate solution, followed by heat-treatment at 900 or 1100°C. The electrodes heat-treated at 900°C exhibit an electrode polarization resistance as low as 0.21Ωcm2 at 800°C and are relatively stable under electrolysis operation at 500mAcm−2 for 100h, while the electrodes heat-treated at 1100°C show the increased stability with the electrolysis polarization. The results clearly indicate that LSM lattice shrinkage under anodic electrolysis conditions inhibits the agglomeration and grain growth of infiltrated LSM nanoparticles, leading to the highly stable nano-structured LSM–YSZ electrode structure. [Copyright &y& Elsevier]

Abstract: Palladium-impregnated or infiltrated La0.8Sr0.2MnO3–Gd0.2Ce0.8O1.9 (LSM-GDC) composites are studied as the oxygen electrodes (anodes) for the hydrogen production in solid oxide electrolysis cells (SOECs). The incorporation of small amount of Pd nanoparticles leads to a substantial increase in the electrocatalytic activity and stability of the LSM-GDC oxygen electrodes. The electrode polarization resistance (RE ) at 800 °C on a 0.2 mg cm−2 Pd-infiltrated LSM-GDC electrode is 0.13 Ω cm2, significantly smaller than 0.42 Ω cm2 for the reaction on the pure LSM-GDC electrodes. The overpotential loss is also substantially reduced after the Pd infiltration; at an anodic overpotential 50 mV and 800 °C, the current increases from 0.15 A cm−2 for the pure LSM-GDC anode to 0.47 A cm−2 on a 0.3 mg cm−2 Pd-infiltrated LSM-GDC. The infiltrated Pd nanoparticles enhance the stability of the LSM-GDC oxygen electrodes and are most effective in the promotion of the diffusion, exchange and combination processes of oxygen species on the surface of LSM-GDC particles, leading to the increase in the oxygen evolution reaction rate. [Copyright &y& Elsevier]

Abstract: The delamination behavior of La0.8Sr0.2MnO3 (LSM) oxygen electrode of solid oxide electrolysis cell (SOEC) is studied in detail under anodic current passage of 500 mA cm−2 and 800 °C. The delamination or failure of LSM oxygen electrode is observed after the current passage treatment for 48 h, and is accompanied by the significant increase in the electrode polarization and ohmic resistances. The delaminated electrode and electrolyte interface is characterized by the formation of nanoparticles within LSM contact rings on the electrolyte surface. SEM analysis of the interface at different stages of the polarization indicates that the formation of these nanoparticles is caused by the localized disintegration of the LSM grains at the electrode/electrolyte interface. The formation of nanoparticles is most likely due to the migration or incorporation of oxygen ions from the YSZ electrolyte into the LSM grain, leading to the shrinkage of LSM lattice. The shrinkage of the LSM lattice will create local tensile strains, resulting in the microcrack and subsequent formation of nanoparticles within LSM particles at the electrode/electrolyte interface. The formation of nanoparticle clusters weakens the anode/electrolyte interface, eventually leading to the delamination and failure of the LSM oxygen electrode under high internal partial pressure of oxygen at the interface. [Copyright &y& Elsevier]

Nanoscale perovskite oxides with enhanced electrocatalytic activities have been widely used as oxygen electrodes of reversible solid oxide cells (RSOC). Here, La0.6Sr0.4FeO3−δ (LSF) nanoscale powder is synthesized via a novel molten salt method using chlorides as the reaction medium and fired at 850 °C for 5 h after removing the additives. A direct assembly method is employed to fabricate the LSF electrode without a pre-sintering process at high temperature. The microstructure characterization ensures that the direct assembly process will not damage the porosity of LSF. When operating as a solid oxide fuel cell (SOFC), the LSF cell exhibits a peak power density of 1.36, 1.07 and 0.7 W/cm2 at 800, 750 and 700 °C, respectively, while in solid oxide electrolysis cell (SOEC) mode, the electrolysis current density reaches 1.52, 0.98 and 0.53 A/cm2 under an electrolysis voltage of 1.3 V, respectively. Thus, it indicates that the molten salt routine is a promising method for the synthesis of highly active perovskite LSF powders for directly assembled oxygen electrodes of RSOC. [ABSTRACT FROM AUTHOR]

Highlights • A novel molten salt routine is introduced to synthesize LSFN powders. • The synthesis temperature is as low as 850 °C and the LSFN particles are porous. • The average valence state of Fe in LSFN is lowered. • The LSFN electrode exhibits excellent performance for both ORR and OER. Abstract A novel molten salt method was employed to synthesize 10% Nb doped La 0.6 Sr 0.4 FeO 3−δ (LSFN). The as-synthesized powders were pure and adopted a porous structure. Using LSFN as the oxygen electrode for a reversible solid oxide cell, the button cell exhibited a peak power density of 0.79 W cm−2 in H 2 and an electrolysis current density of 0.89 A cm−2 at 1.3 V in 50% CO 2 /50% H 2 at 800 °C. A large fraction of Fe2+ was found in LSFN, resulting in the improved performance. Our results indicate that the molten salt method is a promising approach for the synthesis of highly active perovskite electrode with a dopant of Nb. [ABSTRACT FROM AUTHOR]

Symmetric cell configuration is commonly used for the study of reaction mechanism and kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) of solid oxide cells (SOCs) to avoid the issues associated with the possible inaccuracy and errors of the reference electrode in three-electrode cell configuration. The electrode impedance behavior and reaction parameters such as electrode polarization resistance are taken as half of the measured overall value of the cell, based on the assumption that the oxygen reaction measured on the symmetric electrodes is completely reversible. However, the applicability of symmetric cell configuration for such mechanistic studies of oxygen reaction has never been verified with respect to the electrode materials. In this paper, the reversibility of symmetric cell configuration is investigated on model electrodes of platinum (Pt), La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) using a three-electrode configuration. The results indicate the reversibility of ORR at working electrode and OER at counter electrode depends strongly on the reversibility of the electrode/electrolyte interface for the oxygen incorporation/exsolution process, which in turn is closely related to the oxygen ionic conductivity of the electrode and in less extent to the ionic conductivity of the electrolyte. Therefore, LSCF electrode shows a much high degree of reversibility as compared to LSM, while for the reaction on Pt electrode, the oxygen reaction is essentially not reversible. This study clearly indicates that the applicability of the symmetric cell configuration for the mechanistic study of electrode reactions in SOCs is closely related to the reversibility in the molecular level (the individual reaction steps) as well as the interface level (the regions where the individual reaction steps occur). • The applicability of symmetric cell configuration for oxygen reaction is investigated. • Pt, LSM and LSCF electrodes are used as model electrodes under various dc bias. • Applicability of symmetric cell configuration depends on the ionic conductivity of electrode and electrolyte. • O 2 reduction reaction and O 2 evolution reaction are not reversible on Pt electrode even under open circuit. [ABSTRACT FROM AUTHOR]

Abstract Reversing the direction of polarization current is essential for reversible solid oxide cells technologies, but its effect on cobaltite based perovskite oxygen electrodes is largely unknown. Herein, we report the operating stability and microstructure at the electrode/electrolyte interface of La 0.57 Sr 0.38 Co 0.18 Fe 0.72 Nb 0.1 O 3-δ (LSCFN) oxygen electrodes assembled on barrier-layer-free Y 2 O 3 ZrO 2 electrolyte under cyclic anodic/cathodic polarization mode at 0.5 A cm−2 and 750 °C. During the cyclic polarization, the electrocatalytic activity of LSCFN electrode is drastically deteriorated in cathodic mode, but the performance loss is largely recoverable in anodic mode. This is due to the fact that the surface segregation of Sr and accumulation at the electrode/electrolyte interface by cathodic polarization can be remarkably mitigated by anodic polarization. The time period in each cycle plays a key role in determining the accumulation of Sr species at the electrode/electrolyte interface. A full cell operating in a time period of 12 h fuel-cell/12 h electrolysis is reversible for a duration of 240 h, in contrast to the performance degradation in a shorter time period of 4 h fuel cell/4 h electrolysis. The present study sheds lights on applying cobaltite based perovskite oxygen electrodes on barrier-layer-free YSZ electrolyte for reliable solid oxide cells. Highlights • LSCFN oxygen electrode assembled on barrier-layer-free Y 2 O 3 ZrO 2 electrolyte. • Reversible operation of LSCFN electrode during cyclic polarization for 240 h. • Mitigated Sr surface segregation and accumulation by cyclic polarization. • Enhanced operating stability in a longer cyclic time period. [ABSTRACT FROM AUTHOR]

Bismuth oxide is as an active promoter in enhancing the ionic conductivity and electrocatalytic activity of manganite oxygen electrodes of solid oxide cells, but there are very limited reports on the formation and evolution of electrode/electrolyte interface of bismuth oxide-manganite composite electrode under the influence of electrochemical polarization. Herein, we report the effect of electrochemical polarization and direction of polarization current on the electrocatalytic performance and electrode/electrolyte interface of a (La 0·8 Sr 0.2 ) 0.95 Mn 0·95 Pt 0·05 O 3+δ -Er 0.4 Bi 1·6 O 3 (LSMPt-ESB) composite oxygen electrode assembled on zirconia electrolyte. The cell with the LSMPt-ESB electrode produces outstanding performance for power generation and steam splitting, and it is stable without noticeable degradation during operation at 600 °C for 350 h in the fuel cell mode. The cathodic polarization induces in operando formation of electrode/electrolyte interface with observation of an Er-deficient LSMPt-ESB dense layer and Er-rich (Er,Bi,Mn)O x particles on the zirconia electrolyte surface. This is different to the case of dwell under open circuit and in particular under anodic polarization conditions. The present study gains insights into the development of high performance, reliable bismuth oxide-manganite composite oxygen electrode for reduced temperature solid oxide cells. [ABSTRACT FROM AUTHOR]