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

Development of oxidation-resistant and electrically conductive coating of Ti–Al–C system for the lightweight interconnects of solid oxide fuel cells

The paper studies oxidation resistance and electrical conductivity of dense coatings produced by vacuum-arc deposition technique on α-titanium thin (0.1 mm) substrate using a hot pressed Ti2AlC–TiC target. The coatings were deposited at low (7 mA/cm2) and high (15 mA/cm2) current densities on the substrate and marked LCD and HCD, respectively. This provided different local chemical and phase compositions of the coatings. It was found that phase compositions of the coatings differ from that of the target. The HCD coating has high oxidation resistance evaluated in terms of the specific weight gain (Δm/S = 0.06 mg/cm2) as well as high surface electrical conductivity (σ = 1.23·106 S/m) after long-term (1000 h) holding at 600 °C in the air due to the formation of an over thin (450 nm) Ti–Al-(C, O, N) near-surface layer. The thin titanium substrate with such Ti–Al–C coating is recommended as a lightweight interconnect of an intermediate-temperature solid oxide fuel cell

Tribological Properties at 20 and 500°C of TiN and CrN Cathodic ARC Coatings Deposited on Ti-6Al-4V Alloy

Tribological properties of TiN and CrN coatings deposited by cathodic arc method at three different bias potentials -50, -150 and -300V on Ti-6Al-4V alloy in pair with alumina have been investigated. X-ray diffraction analysis showed that single-phase textured cubic nitrides of TiN and CrN were formed in these coatings. It is shown that the friction coefficient of the coatings is practically equal to that established for the Ti6Al4V alloy, but the wear rate is more than an order of magnitude lower than for the titanium alloy substrate. Coatings deposited at a potential of -50 V show optimal tribological properties at temperatures 20 and 500°C. Friction coefficients for TiN coatings are 0.4-0.8 at 20°C and 0,75 at 500°C; for CrN coatings they are 0.5 at 20°C and 0,7 at 500°C. Wear rates for TiN coatings are 0.86·10-5 мм3/Нм at 20°C and 3.56·10-5 мм3/Нм at 500°C; for CrN coatings they are 1.43·10-5 мм3/Нм at 20°C and 7.13·10-5 мм3/Нм at 500°C