Influence of electric field strength on structure,microstructure, and electrical properties of flash sintered 8% mol Yttria-stabilized zirconia as a solid oxide fuel cell electrolyte

To study the effect of electric field on the characteristics of flash sintered materials, 8% mol. Yttria-stabilized zirconia (8YSZ) was isothermally flash sintered under various electric field strengths as a solid oxide fuel cell (SOFC) electrolyte. Structural, microstructural, and electrical characteristics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Electrochemical impedance spectroscopy (EIS), respectively. Results show that the electric field did not affect the relative density of flash sintered 8YSZ. Electric fields stronger than 300 V cm^?1, however, transformed the cubic structure to tetragonal. Microstructural studies show that the average grain size of samples is independent of the applied electric field strength. Electrochemical impedance spectroscopy showed changes in the grain boundary characteristics upon using the electric field for flash sintering. Oxygen vacancy concentration in the grain boundary of flash sintered samples was more than ten times higher than conventionally sintered ones, which improved the conductivity in flash sintered samples.     

Effect of interconnect pre-oxidation on high-temperature wettability and mechanical properties of glass seals in SOFC

Metallic interconnect oxidation has a significant influence on shear strength between interconnect and sealing glass in SOFC stack operation. This is attributed to high-temperature mutual wettability. In this work, the glass having different crystallization kinetics was chosen to evaluate wettability and shear strength on Fe–16Cr alloy, which was heated treated at 750°C for 0, 50, and 100 h, forming oxide scale with a varying rough surface. Visual observation was used to quantify equilibrium contact angles between glass and substrates. The results illustrated that alloy oxidized for 50 h exhibited better wettability and shear strength, implying that thickness and roughness of the oxide scale are critical to enhancing interface joint strength. Long-term testing indicated that thermally stable glass possesses higher joint shear strength and more consistent properties. It was found that precipitated crystalline phases limited improvement of glass wettability, resulting in interfacial delamination of the glass/alloy layer over long-term operation.     

Characterization of a circular 80 mm anode supported solid oxide fuel cell (AS-SOFC) with anode support produced using high-pressure injection molding (HPIM)

The current study was oriented at analyzing the performance of an anode-supported solid oxide fuel cell produced using high-pressure injection molding. The cell with a total thickness of 550 μm was produced in the Ceramic Department (CEREL) of the Institute of Power Engineering in Poland and experimentally analyzed in the Energy Department (DENERG) of Politecnico di Torino in Italy. The high-pressure injection molding technique was applied to produce a 500 μm thick anode support NiO/8YSZ 66/34 wt% with porosity of 25 vol%. The screen printing method was used to print a 3 μm thick NiO anode contact layer, 7 μm thick NiO/8YSZ 50/50 wt% anode functional layer, 4 μm thick 8YSZ dense electrolyte, 1.5 μm thick Gd_0,1Ce_0,9O₂ barrier layer and a 30 μm thick La_0,6Sr_0,4Fe_0,8Co_0,2O_3–δ cathode with porosity 25 vol%.The experimental characterization was done at two temperature levels: 750 and 800 °C under fixed anodic and cathodic flow and compositions. The preliminary studies on the application of high-pressure injection molding are discussed together with the advantages of the technology. The performance of two generations of anode-supported cells is compared with data of reference cells with supports obtained using tape casting.     

Three-dimensional micro/macroscale simulation of planar,anode-supported,intermediate-temperature solid oxide fuel cells: I. Model development for hydrogen fueled operation

Intermediate-temperature solid oxide fuel cells (IT-SOFCs) are promising SOFC technologies that can solve many problems of high-temperature SOFCs (HT-SOFCs), such as the stringent restriction on material selection, accelerated degradation of electrode activity, limitation in thermal cycling, and requirement for long start-up times. In this study, a comprehensive three-dimensional micro/macroscale model is developed for simulating planar, anode-supported IT-SOFCs fueled with hydrogen. Many constitutive sub-models for electrode microstructure, detailed charge-transfer processes, and heat/mass transport in three-dimensional interconnect plate/gas channel geometries are combined to investigate the performance and operating characteristics of IT-SOFCs with rather standard materials (such as nickel, YSZ, LSM, and stainless steel). The current−voltage performance curves are presented along with the contribution of activation, concentration, ohmic, and contact overpotentials to total potential loss. In addition, the spatial distributions of temperature, current density, and species concentrations are also investigated for co- and counter-flow configurations. The results clearly demonstrate the capabilities of the present three-dimensional micro/macroscale model as an accurate and efficient design tool for optimizing the operating conditions, electrode microstructures, and cell geometries of planar, anode-supported IT-SOFCs.     

Electrochemical performance of novel NGCO-LSCF composite cathode for intermediate temperature solid oxide fuel cells

In this study, a co-dopant CGO was synthesized to produce more efficient cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. Neodymium (Nd) was doped into CGO in four different weight ratios in the formula Nd_xGd_0.15Ce_0.85-xO_2-δ (NGCO); the selected percentages for x were 1%, 3%, 5% and 7%. XRD patterns showed pure phase for all synthesized compositions and good compatibility at high temperature under static air with the most common ceramic cathode material in IT-SOFC (La_0·60Sr_0·40Co_0·20Fe_0·80O_2-ä, LSCF). Impedance spectroscopic characterization of symmetrical cells of the composite NGCO-LSCF at different temperatures (650–800 °C in steps of 50 °C) and a frequency range of 0.1–1 MHz in synthetic air revealed interesting results. The lowest polarization resistance (R_p) was achieved for Nd_0.05Gd_0.15Ce_0·80O_2-δ (0.06 Ω cm² at 800 °C, 0.17 Ω cm² at 750 °C, 0.31 Ω cm² at 700 °C, and 0.59 Ω cm² at 650 °C). The expected decrease in R_p was not observed for the sample with higher Nd content (7% Nd). Thus, it can be said that there is a distinction between the compositions Nd_0.05Gd_0.15Ce_0·80O_2-δ and Nd_0.07Gd_0.15Ce_0·78O_2-δ; the co-doping of Nd in NGCO incremented the oxygen ion diffusion path, thereby optimization in the triple phase boundary (TPB) sites was obtained. Furthermore, SEM and TGA measurements were conducted to clarify the reasons of such improvements. This work showed that an NGCO-LSCF composite can be considered as a potential candidate for cathode material for future IT-SOFC applications.     

Oxidation of porous stainless steel supports for metal-supported solid oxide fuel cells

Oxidation behavior of porous P434L ferritic stainless steel, used for the fabrication of metal-supported solid oxide fuel cells (MS-SOFC), is studied under anodic and cathodic atmospheres. Temperature- and atmosphere-dependence is determined for as-sintered and pre-oxidized stainless steel. Pre-oxidation reduced the long-term oxidation rate. For pre-oxidized samples, the oxidation rate in air exceeds that in humid hydrogen for temperatures above 700 °C. The influence of PrOx, LSCF-SDC, and Ni-SDC coatings is also examined. The coatings do not dramatically impact oxide scale growth. Oxidation in C-free and C-containing anodic atmospheres is similar. Addition of CO₂, CH₄, and CO to humidified hydrogen to simulate ethanol reformate does not significantly impact oxidation behavior. Cr transpiration in humid air is greatly reduced by the PrOx coating, and a PrCrO₃ reaction product is observed throughout the porous structure. A dense and protective chromia-based scale forms on steel samples during oxidation in all conditions. A thin silica enriched oxide layer also forms at the metal-scale interface. In general, the oxidation behavior at 700 °C is found to be acceptable.     

In-situ Ni exsolution from NiTiO3 as potential anode for solid oxide fuel cells

Sample NiTiO₃ (NTO) is prepared by the molten salts synthesis route as a potential anode material for solid oxide fuel cell (SOFC) applications. An additional sample impregnated with 5 mol%Ni (N-NTO) is also presented. Structural characterization reveal a pure NiTiO₃ phase upon calcination at 850 °C and 1000 °C. Redox characterization by temperature programmed reduction tests indicate the transition from NiTiO₃ to Ni/TiO₂ at ca. 700 °C. Ni nanoparticles (ca. 26 nm) are exsolved in-situ from the structure after a reducing treatment at 850 °C. Catalytic activity tests for partial oxidation of methane performed in a fixed bed reactor reveal excellent values of activity and selectivity due to the highly dispersed Ni nanoparticles in the support surface. Time-on-stream behavior during 100 h operation in reaction conditions for sample N-NTO yield a stable CH₄ conversion. Electrolyte supported symmetrical cells are prepared with both materials achieving excellent polarization resistance of 0.023 Ω cm² in 7%H₂/N₂ atmosphere at 750 °C with sample N-NTO. The maximum power density achieved is of 273 mW cm⁻² at 800 °C with a commercial Pt ink used as a reference cathode, indicating further improvement of the system can be achieved and positioning the N-NTO material as a promising SOFC anode material.     

Suitability of Pr2–xCaxNiO4+δ as cathode materials for electrochemical devices based on oxygen ion and proton conducting solid state electrolytes

In order to develop economically competitive solid oxide fuel cell (SOFC) systems it is necessary to design new functional materials with the purpose of enhancing their performance, extending operational lifetime and reducing the cost. The present work focuses on the study of Pr_2-xCa_xNiO_4+δ cathode materials prepared by a simple and cheap conventional solid state reaction method. The structure, oxygen nonstoichiometry, electrical properties and chemical compatibility of the materials with a number of well-known oxygen ion and proton conducting electrolytes were systematically investigated. Chemical composition (Ca content) and technological factors (powders pre-history, electrodes' sintering temperature), as well as external parameters (temperature, air humidity) were correlated with the electrochemical performance of the electrodes to determine the optimal compositions and conditions. Based on this study and testing results of the single anode-supported cell with BaCe_0.89Gd_0.1Cu_0.01O_3-δ electrolyte, the Pr_1.7Ca_0.3NiO_4+δ-based electrode compositions are proposed for preferred usage in SOFCs in place of Pr₂NiO_4+δ electrode.     

On the manufacturing of low temperature activated Sr0.9La0.1TiO3-δ-Ce1-xGdxO2-δ anodes for solid oxide fuel cell

Lanthanum doped strontium titanate–gadolinium doped cerium oxide (LST-GDC) anodic layers are sintered in air and further reduced in-situ at low temperature (750 °C) avoiding usually performed pre-reduction treatment at high temperature. The influence of various milling techniques and of powders with different specific surface area, on the microstructures of screen-printed anodes, is investigated. The combination of milling and sonication processes is efficient in reducing aggregation of the anode powders. The anode performance is improved when a planetary milling step is involved in the preparation of the screen printing inks. The use of gadolinium doped cerium oxide with high specific surface area decreases the polarization resistance. The rate of hydrogen oxidation is also enhanced by increasing porosity.