Electrocatalytic activity of perovskite-based cathodes for solid oxide fuel cells

A very active cathode material for intermediate temperature - solid oxide fuel cells (IT-SOFCs) is obtained by mixing La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) powders. Three different volume ratios are considered: BSCF-LSCF 50-50 v/v% (BL50), BSCF-LSCF 70-30 v/v% (BL70) and 30–70 v/v% (BL30).The electrodes are slurry coated on Ce_0.8Sm_0.2O_2-δ electrolyte and sintered at 1100 °C. After the sintering step XRD-analyses highlight relevant cation inter-diffusion within the mixed powders. As a result, an enhanced activity of BL30-BL70 electrodes towards oxygen reduction reaction is detected in comparison to LSCF or BSCF pure powders. A polarization resistance of 0.021 Ω cm2 at 650 °C for BL70 is obtained, one of the lowest value reported in literature for SOFC cathodes. Furthermore, all the electrodes show lower activation energy than the two reference materials in the considered temperature range (500–650 °C) and two different kinetic regimes are identified at the extremes of this range. Effect of the applied overpotential (0–0.3 V) on the electrode kinetic is also investigated.After a preliminary ageing, performed at 650 °C for 200 h by applying a current density of 200 mA cm-2, the electrodes preserve a remarkable performance as IT-SOFC cathodes, despite an initial degradation. A stable value of 0.048 Ω cm2 of polarization resistance for the sample richer in BSCF is recorded.     

Nanostructured MIEC Ba0.5Sr0.5Co0.6Fe0.4O3−δ (BSCF5564) cathode for IT-SOFC by nitric acid aided EDTA–citric acid complexing process (NECC)

Ba_0.5Sr_0.5Co_0.6Fe_0.4O_3−δ(BSCF5564) was synthesized by nitric acid aided EDTA–citric acid complexing sol-gel method (NECC). Both, the phase formation temperature and time of BSCF5564 synthesized NECC were found to be low i.e. single perovskite phase formation temperature is 200 °C less as compared to the conventional method of synthesis. The orthorhombic perovskite structure has been formed after calcination at 800 °C for 5 h. Scanning electron microscopy reveals the formation of porous material constituting nano-sized and irregularly shaped rod-like structure with particle size approximately ranges from 90 to 160 nm. The total weight loss of the BSCF5564 sample comes out to be 6.6%, indicating that quadrivalence state Co⁴⁺ and Fe⁴⁺ in the sample have been completely reduced to the trivalent state Co³⁺ and Fe³⁺ due to thermal analysis. The value of E_a for BSCF5564 prepared by NECC was 0.2288 eV. The electrical conductivity of BSCF5564 synthesized by NECC is observed to be steady at high temperature (above 700 °C).     

Electrochemical performance of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2–0.8) cathode on a ScSZ electrolyte for intermediate temperature SOFCs

Intermediate temperature solid oxide fuel cell cathode materials (Ba, Sr)Co_xFe_1−xO_3−δ [x = 0.2–0.8] (BSCF), were synthesized by a glycine-nitrate process (GNP) using Ba(NO₃)₂, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O as starting materials and glycine as an oxidizer and fuel. Electrolyte-supported symmetric BSCF/GDC/ScSZ/GDC/BSCF cells consisting of porous BSCF electrodes, a GDC buffer layer, and a ScSZ electrolyte were fabricated by a screen printing technique, and the electrochemical performance of the BSCF cathode was investigated at intermediate temperatures (500–700 °C) using AC impedance spectroscopy. Crystallization behavior was found to depend on the pH value of the precursor solution. A highly acidic precursor solution increased the single phase perovskite formation temperature. In the case of using a precursor solution with pH 2, a single perovskite phase was obtained at 1000 °C. The thermal expansion coefficient of BSCF was gradually increased from 24 × 10⁻⁶ K⁻¹ for BSCF (x = 0.2) to 31 × 10⁻⁶ K⁻¹ (400–1000 °C) for BSCF (x = 0.8), which resulted in peeling-off of the cathode from the GDC/ScSZ electrolyte. Only the BSCF (x = 0.2) cathode showed good adhesion to the GDC/ScSZ electrolyte and low polarization resistance. The area specific resistance (ASR) of the BSCF (x = 0.2) cathode was 0.183 Ω cm² at 600 °C. The ASR of other BSCF (x = 0.4, 0.6, and 0.8) cathodes, however, was much higher than that of BSCF (x = 0.2).     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Fabrication of porous (Ba,Sr)(Co,Fe)O3-δ (BSCF) ceramics using gelatinization and retrogradation phenomena of starch as pore-forming agent

Porous (Ba,Sr)(Co,Fe)O_3-δ (BSCF) ceramics with high open porosity and good electrical conductivity was fabricated using Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which shows a high mixed ionic-electronic conductivity. In general, during the fabrication of porous ceramics by the sacrificial template method using pore former particles, closed pores are easily formed unless sufficient pore former particles are added. In this study, we have devised a method using the gelatinization-retrogradation phenomena of starch for producing a porous body with an excellent percolated pore network structure. By dispersing BSCF and starch in an aqueous slurry (0–50% by weight) and heating, gelatinization of the starch occurred and the starch particles adhered to each other. Furthermore, in order to retain the percolated structure, the water solvent was removed by freeze-drying without heating to obtain a dried green body. The sintering behavior of the porous BSCF bodies prepared under various conditions was characterized by microstructural observations and relative density measurements. By optimizing the process conditions of the gelatinization and retrogradation, a porous body having an open porosity of 48.3%, and with 99% of the total pores open, was obtained. The matrix was also well connected and showed a sufficiently high conductivity which was similar to the porous bodies made by the traditional sacrificial template method.     

Ag decorated (Ba,Sr)(Co,Fe)O3 cathodes for solid oxide fuel cells prepared by electroless silver deposition

We reported nano-structured Ag modified Ba_0.5Sr_0.5Co_0.6Fe_0.4O_3−δ (Ag@BSCF) cathode for solid oxide fuel cells (SOFCs) that is prepared by vacuum assisted electroless deposition technique. We show that the concentration of Ag can be easily adjusted by tuning the deposition time without altering the perovskite structure of the pristine BSCF. The effect of Ag loading on the electrochemical performance of the material has been systematically studied by varying the Ag loading and the working condition (oxygen partial pressure). An optimized electrode performance is observed with an Ag loading of ∼2 wt%. We demonstrate that the presence of Ag significantly reduces the electrode ohmic resistance and enhances the catalytic O₂ reduction performance of the BSCF cathode.     

纳米NiO-YSZ复合粉体原位合成及电极微结构修饰

采用氨水沉淀原位合成法制备了NiO-YSZ(Y_2O_3稳定的ZrO_2)复合粉体,通过XRD、FESEM研究了溶液pH值对粉体性能的影响,并用干压法和丝网印刷法将氢电极分为支撑层(500 μm)、过渡层(20 μm)和功能层(10 μm)三层进行梯度化制备,采用YSZ/SDC双层电解质,通过共烧结技术将SDC(Sc掺杂CeO_2)(6 μm)作为YSZ(4 μm)电解质和Ba_0.5Sr_0.5Co_0.8Fe_0.2O_(3-δ)(BSCF,20 ìm)氧电极的隔离层.结果表明,合成NiO-YSZ复合粉体的最佳pH值为8.5,粉体呈泡沫状团聚,NiO的平均晶粒粒径为13 nm,产率为94.5%.850 ℃时制备的单体SOEC在70%、80%和90% 3种水蒸气含量的氢电极气氛下,电解池在1.5 V的产氢速率分别为266、381和558 N·mL/cm~2·h.在850 ℃、90%水蒸气含量的氢电极气氛下,以0.33 A/cm~2恒流电解1 h前、后的电解电压分别为1.09和1.16 V,电解池具备较好的稳定性.     

Oxygen reduction mechanism at Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode for solid oxide fuel cell

Perovskite structure Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) powders have been successfully synthesized by glycine–nitrate combustion process. A porous and crack-free BSCF cathode is obtained by spraying the slurry of BSCF powders and terpineol onto LSGM pellet. The oxygen reduction reaction mechanism has been investigated by AC impedance spectroscopy and cyclic voltammetry method. AC impedance spectroscopy analysis shows that there are two different processes in the cathode reaction which are related to oxygen dissociation/adsorption and bulk oxygen diffusion. And the molecular oxygen is involved in the rate-determining step. The polarization resistance decreases with an increase of temperature and the oxygen partial pressure. With an increase of the applied DC bias, the logarithm of the polarization resistance decreases linearly due to additional oxygen vacancies and the lowered chemical potential of oxygen at the BSCF/LSGM interface by the applied voltage. The exchange current density reaches to 182 mA cm⁻² at 700 °C, suggesting that the ORR kinetics at the BSCF/LSGM interface is high due to the excellent mixed ionic and electronic conductivity of BSCF.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part I: Influence on oxygen permeation,electrical properties,reductive stability,and lattice parameters

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840?°C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y³⁺ was doped in small concentrations (1–10?mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20–30%), a doping concentration of 10?mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800?°C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.     

Predicting grain size distributions in perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3–δ oxygen transport membranes

ABSTRACT

This study is conducted over a 3?×?3 time–temperature matrix on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) ceramics, and sintered bodies above 93% dense are obtained. The electron backscatter diffraction band contrast micrographs of the polished sintered samples are analysed for characterising the grain size distributions (GSDs). This study develops an algorithm for predicting the GSDs of BSCF dependence of sintering condition (time and temperature). In addition, the GSDs predicted by the algorithm agree reasonably with those experimentally observed. When individual grain size is non-dimensionalised by the median grain size, the GSDs data of all BSCF samples reduce to a single self-similar GSD curve. The median grain size is predicted by the classical kinetics equation, D^n?=?tK₀exp(?Q/RT).