On the Behavior of Solid Oxide Fuel Cell Anode Materials in a Hydrogen Sulfide Containing Atmosphere

The effect of hydrogen sulfide content in a hydrogenous atmosphere on the structure, physical, and mechanical properties of solid oxide fuel cell (SOFC) anode materials has been studied. A series of specimens of porous nickel and YSZ-Ni cermet have been investigated. In order to obtain the corresponding YSZ-Ni cermet structure, specimens of the YSZ-NiO ceramics were singly reduced in a hydrogenous atmosphere (either Ar-5 vol% H2 mixture or hydrogen of 99.99 vol% H2 purity) for 4 h at 600°C under the pressure of 0.15 MPa. A part of the specimens of each series was then aged in "hydrogen sulfide in Ar-5 vol% H2 mixture" atmosphere for 4 h at 600°C. According to a test mode, the atmosphere contained 7 or 18 vol% H2S. Material microstructure and fracture surface morphology of the specimens as well as the physical and mechanical behaviors were investigated. It was revealed that the atmosphere containing up to 7 vol% H2S does not affect the strength and electrical conductivity of the YSZ-Ni cermet. Increased content of H2S (18 vol%) causes some changes in the YSZ-Ni cermet structure. A large number of completely reduced tiny nickel particles is formed. These nickel particles react with hydrogen sulfide. Sulfur is segregated on the boundaries between the zirconia and nickel phases and pores. Finally, multiple breaking of the zirconia-nickel bonds occurs that results in reduced strength of the cermet (by 39% as compared to as-received ceramics)

Effect of Water Vapor Amount in a Hydrogenous Atmosphere on Structure and Properties of Nickel-Zirconia Anode Materials for Solid Oxide Fuel Cells

Nickel-zirconia anode ceramics of YSZ-NiO system for solid oxide fuel cells (SOFCs) has been investigated. A series of specimens were singly reduced in hydrogenous atmosphere (the Ar-5 vol%H2 mixture) at 600°C under the pressure of 0.15 MPa or subjected to reduction-oxidation (redox) cyclic treatment at 600°C. Influence of water vapor concentration in hydrogenous atmosphere on structure and properties of the materials was studied. Based on structural changes in the as-received material it was revealed that a small amount of water vapor in Ar-5 vol% H2 mixture (water vapor pressure below 0.03 MPa) accelerates a reduction of the nickel phase at 600°C with formation of nanopores on tiny Ni particles. A higher concentration of water vapor (the pressure above 0.03-0.05 MPa) causes a converse change in the reduction kinetics. For as-received material, such an amount of water vapor in the mixture is an obstacle for its reduction. For the material treated by redox cycling, better physical and mechanical properties were revealed after dwelling at 600°C in a water depleted gas mixture. Based on the SEM microscopy and the data on the conductivity and strength, the dual effect of water vapor on durability of a nickel-zirconia anode is discussed

Behaviour of Solid Oxide Fuel Cell Materials in Technological Environments

The YSZ–NiO ceramics for SOFC anodes and MAX-phases of Ti-Al-C systems for interconnects have been investigated. Based on the tests of YSZ–NiO specimens preconditioned by one-time reduction or by redox cycling at 600 or 800 °C, a certain mode of the material treatment was established which provides its improved physicomechanical properties. The oxidation behaviour of MAX-phases has been investigated at 600 °C in air. It was found that the intense initial oxidation of hot-pressed Ti3AlC2-based material can be eliminated by a certain mode of pre-oxidation. The oxidation resistance of the material can be significantly improved by niobium addition

Influence of reduction conditions of NiO on its mechanical and electrical properties

Yttria stabilized zirconia with a nickel catalyst (Ni-YSZ) is the most developed, widely used cermet anode for manufacturing Solid Oxide Fuel Cells (SOFCs). Its electro-catalytic properties, mechanical durability and performance stability in hydrogen-rich environments makes it the state of the art fuel electrode for SOFCs. During the reduction stage in initial SOFC operation, the virgin anode material, a NiO-YSZ mixture, is reduced to Ni-YSZ. The volume decrease associated with the change from NiO-YSZ to Ni-YSZ creates voids and causes structural changes, which can influence the physical properties of the anode. In this work, the structural, mechanical and electrical properties of NiO samples before and after reduction in pure H2 and a mixture of 5 vol. % H2-Ar were studied. The NiO to Ni phase transformations that occur in the anode under reducing and Reduction-Oxidation (RedOx) cycling conditions and the impact on cell microstruc-ture, strength and electrical conductivity have been examined. Results show that the RedOx treatment of the NiO samples influence on their properties controversially, due to structural transformation (formation of large amount of fine pores) of the reduced Ni. It strengthened the treated samples yielding the highest mechanical strength values of 25.7 MPa, but from another side it is resulting in lowest electrical conductivity value of 1.9×105 S m-1 among all reduced samples. The results of this investigation shows that reduction conditions of NiO is a powerful tool for influence on properties of the anode substrate

Preconditioning of the YSZ-NiO Fuel Cell Anode in Hydrogenous Atmospheres Containing Water Vapor

Abstract The YSZ–NiO ceramics for solid oxide fuel cells (SOFCs) anode have been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (Ar–5 vol% H2 mixture) at 600 °C under the pressure of 0.15 MPa or subjected to ‘reduction in the mixture–oxidation in air’ (redox) cycling at 600 °C. The YSZ–Ni cermets formed in both treatment conditions were then aged in ‘water vapor in Ar–5 vol% H2 mixture’ atmosphere at 600 °C under the pressure of 0.15 MPa. Additionally, the behaviour of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar–5 vol% H2 mixture (water vapor pressure below 0.03 MPa) does not affect the reduction of the nickel phase in the YSZ–NiO ceramics, but causes some changes in the YSZ–Ni cermet structure. In particular, nanopore growth in tiny Ni particles takes place. At higher concentration of water vapor in the mixture (water vapor pressure above 0.03–0.05 MPa), converse changes in the kinetics of reduction occur. The best physical and mechanical properties were revealed for the material treated by redox cycling after holding at 600 °C in water depleted gas mixture. The dual effect of water vapor on nickel-zirconia anode behaviour is discussed basing on scanning electron microscopy analysis data, material electrical conductivity, and strength

The Effect of Treatment Temperature on Microstructure and Mechanical Behavior of a Fine-Grained YSZ–NiO(Ni) Anode Material

Reduction–oxidation (redox) cycling of a solid oxide fuel cell (SOFC) due to leakage of a fuel or standby and shutdown cycling is an issue that has attracted the attention of many research groups for a long time. The researchers mainly note the harmful effects of redox cycling on the microstructure of SOFC constituents and search for ways to mitigate or diminish them. The purpose of this study was to use reduction and oxidation stages in an appropriate mode as a positive preconditioning to improve redox cycling stability of Ni-containing SOFC anode materials. The redox treatment was applied to YSZ–NiO(Ni) anode substrate specimens at 600 °C and 800 °C. The mechanical tests (flexural strength, microhardness, and fracture toughness) were performed on these specimens and the results were compared to those for as-sintered and one-time reduced specimens. Microstructure and fracture surface morphology of material in corresponding modes were analyzed. The main findings were summarized as follows: (i) Redox treatment at 600 °C provides an increase in flexural strength and electrical conductivity of YSZ–NiO(Ni) anode cermets; (ii) the treatment at 800 °C causes formation of a gradient microstructure with lateral cracks that result in a significant decrease in flexural strength; (iii) the mode of redox treatment at 600 °C for 4 h in Ar–5% H2/air atmosphere provides an increase in flexural strength of YSZ–NiO(Ni) anode cermets (up to 127 ± 4 MPa), while electrical conductivity was provided at a comparatively high level (7 × 105 S/m)

The modal analysis of a two-link mechanical system of a robot manipulator

The method of calculation of natural frequencies and forms of oscillations of a two-link mechanical system of a robot manipulator is proposed. Links of the system are considered as straight rods with a step change of cross-sectional parameters. The equations of motion of a mechanical system are based on the technical bending theory. The analysis of oscillation processes is carried out using the matrix method of initial parameters

Effects of Sintering Temperature and Yttria Content on Microstructure, Phase Balance, Fracture Surface Morphology, and Strength of Yttria-Stabilized Zirconia

Currently, ceramics are widely used in various industry branches, especially in energy, chemistry, and aerospace, as well as in medicine. Yttria-stabilized zirconia (YSZ) having unique electrical, thermal, and mechanical properties is one of the most popular ceramics for such applications. In this study, the influence of sintering temperature and yttria percentage on the microstructure and mechanical behavior of YSZ ceramics have been investigated. Corresponding mixtures of ZrO2 powder doped with 3 and 6 mol% Y2O3 powders (hereinafter: 3YSZ and 6YSZ) were prepared, and a series of ceramic specimens were sintered in argon at 1450 °C, 1500 °C, and 1550 °C for 2 h. Changes in the morphology and size of microstructural components as well as their distribution were analyzed with respect to the sintering mode, phase composition, flexural strength, and fracture surface morphology. The 3YSZ and 6YSZ ceramics sintered for 2 h at 1550 °C and 1450 °C, respectively, exhibited the highest levels of strength due to the presence of agglomerates of fine tetragonal zirconia phase particles with high bond strength, as well as larger grains of the monoclinic zirconia phase. The dominant fracture micromechanisms in both the 3YSZ and 6YSZ ceramics related to their high strength are discussed