Thermally and electrochemically induced electrode/electrolyte interfaces in solid oxide fuel cells: An AFM and EIS Study

In high temperature solid oxide fuel cells (SOFCs), electrode/electrolyte interfaces play a critical role in the electrocatalytic activity and durability of the cells. In this study, thermally and electrochemically induced electrode/electrolyte interfaces were investigated on pre-sintered and in situ assembled (La0.8Sr0.2)0.90MnO3 (LSM) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) electrodes on Y2O3-ZrO2 (YSZ) and Gd0.2Ce0.8O2 (GDC) electrolytes, using atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). The results indicate that thermally induced interface is characterized by convex contact rings with depth of 100–400 nm and diameter in agreement with the particle size of pre-sintered LSM and LSCF electrodes, while the electrochemically induced interfaces under cathodic polarization conditions on in situ assembled electrodes are characterized by particle-shaped contact marks or clusters (50–100 nm in diameter). The number and distribution of contact clusters depend on the cathodic current density as well as the electrode and electrolyte materials. The contact clusters on the in situ assembled LSCF/GDC interface are substantially smaller than that on the in situ assembled LSM/GDC interface likely due to the high mixed ionic and electronic conductivities of LSCF materials. The results show that the electrochemically induced interface is most likely resulting from the incorporation of oxygen species and cation interdiffusion under cathodic polarization conditions. However, the electrocatalytic activity of electrochemically induced electrode/electrolyte interfaces is comparable to the thermally induced interfaces for the O2 reduction reaction under SOFC operation conditions

A La0.8Sr0.2MnO3/La0.6Sr0.4Co0.2Fe0.8O3−δ core–shell structured cathode by a rapid sintering process for solid oxide fuel cells

A La0.8Sr0.2MnO3 (LSM)/La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) core–shell structured composite cathode of solid oxide fuel cells (SOFCs) has been fabricated by wet infiltration followed by a rapid sintering (RS) process. The RS is carried out by placing LSCF infiltrated LSM electrodes directly into a preheated furnace at 800 °C for 10 min and cooling down very quickly. The heating and cooling step takes about 20 s, substantially shorter than 10 h in the case of conventional sintering (CS) process. The results indicate the formation of a continuous and almost non-porous LSCF thin film on the LSM scaffold, forming a LSCF/LSM core–shell structure. Such RS-formed infiltrated LSCF–LSM cathodes show an electrode polarization resistance of 2.1 Ω cm2 at 700 °C, substantially smaller than 88.2 Ω cm2 of pristine LSM electrode. The core–shell structured LSCF–LSM electrodes also show good operating stability at 700 °C and 600 °C over 24–40 h

Mechanism and kinetics of Ni-Y2O3-ZrO2 hydrogen electrode for water electrolysis reactions in solid oxide electrolysis cells

© The Author(s) 2015. Published by ECS. Ni-Y2O3 stabilized ZrO2 (Ni-YSZ) cermet is the most commonly used hydrogen electrode for hydrogen oxidation reaction (HOR) under solid oxide fuel cell (SOFC) mode and water reduction reaction (WRR) under solid oxide electrolysis cell (SOEC) mode. Here we studied the electrocatalytic activity of Ni-YSZ electrodes as a function of Ni content, water concentration and dc bias for WRR and HOR under SOEC and SOFC modes, respectively. The activity of Ni-YSZ cermet increases significantly with the increase of YSZ content due to the enhanced three phase boundaries (TPB). The electrode activity for the WRR and in less degree for the HOR increases with the increase of steam concentration. The electrode polarization resistance, RE, for the WRR increases with the dc bias, while in the case of HOR, RE decreases with the dc bias, demonstrating that kinetically the WRR and HOR is not reversible on the Ni-YSZ cermet electrodes under SOFC and SOEC operation modes. The WRR can be described by two electrode processes associated with the H2O adsorption and diffusion on the oxygen-covered Ni or YSZ surface in the vicinities of TPB, followed by the charge transfer. The significant increase of high frequency electrode polarization resistance, RH and in much less extent low frequency electrode polarization resistance, RL with the dc bias indicates that the water electrolysis reaction is kinetically controlled by the reactant supply (e.g., the adsorbed H2O species) limited charge transfer process

Highly Stable Sr-Free Cobaltite-Based Perovskite Cathodes Directly Assembled on a Barrier-Layer-Free Y2O3-ZrO2 Electrolyte of Solid Oxide Fuel Cells

Direct assembly is a newly developed technique in which a cobaltite-based perovskite (CBP) cathode can be directly applied to a barrier-layer-free Y2O3-ZrO2 (YSZ) electrolyte with no high-temperature pre-sintering steps. Solid oxide fuel cells (SOFCs) based on directly assembled CBPs such as La0.6Sr0.4Co0.2Fe0.8O3-d show high performance initially but degrade rapidly under SOFC operation conditions at 750 °C owing to Sr segregation and accumulation at the electrode/electrolyte interface. Herein, the performance and interface of Sr-free CBPs such as LaCoO3-d (LC) and Sm0.95CoO3-d (SmC) and their composite cathodes directly assembled on YSZ electrolyte was studied systematically. The LC electrode underwent performance degradation, most likely owing to cation demixing and accumulation of La on the YSZ electrolyte under polarization at 500 mA cm-2 and 750 °C. However, the performance and stability of LC electrodes could be substantially enhanced by the formation of LC-gadolinium-doped ceria (GDC) composite cathodes. Replacement of La by Sm increased the cell stability, and doping of 5 % Pd to form Sm0.95Co0.95Pd0.05O3-d (SmCPd) significantly improved the electrode activity. An anode-supported YSZ-electrolyte cell with a directly assembled SmCPd-GDC composite electrode exhibited a peak power density of 1.4 W cm-2 at 750 °C, and an excellent stability at 750 °C for over 240 h. The higher stability of SmC as compared to that of LC is most likely a result of the lower reactivity of SmC with YSZ. This study demonstrates the new opportunities in the design and development of intermediate-temperature SOFCs based on the directly assembled high-performance and durable Sr-free CBP cathodes

Effect of characteristics of (Sm,Ce)O2 powder on the fabrication and performance of anode-supported solid oxide fuel cells

Effect of characteristics of Sm0.2Ce0.8O1.9 (SDC) powder as a function of calcination temperature on the fabrication of dense and flat anode-supported SDC thin electrolyte cells has been studied. The results show that the calcination temperature has a significant effect on the particle size, degree of agglomeration, and sintering profiles of the SDC powder. The characteristics of SDC powders have a significant effect on the structure integrity and flatness of the SDC electrolyte film/anode substrate bilayer cells. The SDC electrolyte layer delaminates from the anode substrate for the SDC powder calcined at 600 °C and the bilayer cell concaves towards the SDC electrolyte layer for the SDC powder calcined at 800 °C. When the calcinations temperature increased to 1000 °C, strongly bonded SDC electrolyte film/anode substrate bilayer structures were achieved. An open-circuit voltage (OCV) of 0.82–0.84 V and maximum power density of ~1 W cm−2 were obtained at 600 °C using hydrogen as fuel and stationary air as the oxidant. The results indicate that the matching of the onset sintering temperature and maximum sintering rate temperature is most critical for the development of a dense and flat Ni/SDC supported SDC thin electrolyte cells for intermediate temperature solid oxide fuel cells

Effect of Volatile Boron Species on the Electrocatalytic Activity of Cathodes of Solid Oxide Fuel Cells: III. Ba0.5Sr0.5Co0.8Fe0.2O3-δ Electrodes

The effect of volatile boron species on the electrocatalytic activity, microstructure and phase stability of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF) cathodes has been studied. The cathodes were heat-treated at 800?C for 7 days in air in the presence of boron species vaporized from borosilicate glass, and were characterized by EIS, SEM, AFM, SIMS, XRD, XPS and ICP-OES. The results have shown that after the heat-treatment in the presence of borosilicate glass, boron deposition occurs mainly on the region near electrode surface, leading to the significant Ba and in particular Sr segregation, microstructure change and phase decomposition. On the other hand, the microstructure of the inner electrode layer is almost intact. Electrode polarization resistance, RE, of an as-prepared BSCF cathode is 0.93 and 0.23 Q cm2 at 650 and 800?C, respectively, and changes to 2.08 and 0.15 Q cm2 after heat-treatment at 800?C for 7 days in the presence of borosilicate glass, respectively. The increase in RE for the O2 reduction reaction on BSCF is much lower than that observed on La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) cathodes, indicating that BSCF cathodes have a much better tolerance toward boron deposition and poisoning. The limited attack of volatile boron species on BSCF is most likely related to the much slower kinetics of the formation of strontium and barium borates as compared to the formation of lanthanum borates. This study provides a significant insight into design and development of better contaminant-tolerant cathode materials for durable solid oxide fuel cell (SOFC) technologies

catena-Poly[[[tetra­aqua­iron(II)]-μ-5,5′-diazenediylditetra­zolido] dihydrate]

In the title compound, {[Fe(C2N10)(H2O)4]·2H2O}n, the coordin­ation geometry around the Fe(II) atom, which lies on a center of inversion, is distorted octa­hedral, with bonds to four O atoms and two N atoms. The azotetra­zolate ligand displays a bridging coordination mode, forming an infinite zigzag chain. Inter­molecular O—H⋯O and O—H⋯N hydrogen bonding and offset face-to-face π–π stacking inter­actions [centroid–centroid distance = 3.4738 (13) Å] lead to a three-dimensional network

Bis[(1-methyl-1H-tetra­zol-5-yl)sulfan­yl]methane

The mol­ecule of the title compound, C5H8N8S2, lies on a twofold rotation axis that relates on 1-methyl­tetra­zolyl group to the other; the five-membered rings are twisted by 53.1 (1)°

Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes under solid oxide fuel cells operation conditions

The performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3d (LSCF)cathodes and LSCF impregnated Gd0.1Ce0.9O2d (LSCF-GDC) cathodes are investigated undersolid oxide fuel cell operation conditions. LSCF and LSCF-GDC cathodes show initiallyperformance improvement but degrade under cathodic polarization treatment at 750 C for120 h. The results confirm the grain growth and agglomeration of LSCF and in particularGDC-LSCF cathodes as well as the formation of SrCoOx particles on the surface of LSCFunder cathodic polarization conditions. The direct observation of SrCoOx formation hasbeen made possible on the surface of dense LSCF electrode plate on GDC electrolyte. Theformation of SrCoOx is most likely due to the interaction between the segregated Sr and Cofrom LSCF lattice under polarization conditions. The formation of SrCoOx would contributeto the deterioration of the electrocatalytic activity of the LSCF-based electrodes for the O2reduction in addition to the agglomeration and microstructure coarsenin

Development of Nanostructured Lanthanum Strontium Cobalt Ferrite/Gadolinian-Doped Ceria Composite Electrodes of Solid Oxide Cells Formed by In Situ Polarization

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