60th Annual Report on Research 2015

Within the second year of reporting, two groups of ceramic materials relevant to both the SOFC anodes and the SOFC cathodes have been synthesized and characterized.

1. Samarium and Praseodymium-based Nickelates –Cobiltites

Perovskites, as alternatives to transition and precious metal catalysts, offer significant advantages for many electrochemical energy conversion technologies, including SOFCs. A series of doped praseodymium (PrNixCo1-xO3-?) and samarium (SmNixCo1-xO3-?)-based perovskites with varied Ni and Co content (x=0.1, 0.5, 0.9) were synthesized by sol-gel approach and heat-treated at 700, 900, and 1200oC. It was demonstrated that the PrNixCo1-xO3-? perovskites were stabilized in cubic phase at x= 0.1and 0.5 in the whole temperature range, however, the cubic phase of SmNixCo1-xO3-? (x=0.1, 0.5) was observed only at lower temperatures (700-900°C).

The crystal structure and the chemical composition of the PrNixCo1-xO3-? and SmNixCo1-xO3-? perovskites was analyzed by X-ray diffraction using Rigaku Ultima Plus theta-theta X-ray diffractometer. The X-ray of Cu-k? radiation (?? = 1.54178 Å) was used to scan the materials in the range of 10°-90° (2??) with a scan rate of 0.667°C/min. The position of XRD peaks was used to calculate PrNixCo1-xO3-? and SmNixCo1-xO3-? lattice parameters. The X-ray diffraction patterns for PrNixCo1-xO3-? and SmNixCo1-xO3-? perovskites (Fig. 1) reveal that the formation and the chemical stability of the perovskite phase depends on the nature of the A-site praseodymium or samarium, transition metals Ni to Co ratio, and the heat-treatment temperature. Formation of the perovskite-phase (Fig. 1) is favorable at low Ni/Co ratio (x= 0.1and 0.5). On contrary, high Ni/Co ratio (x=0.9) do not yield the perovskites-phase. Furthermore, praseodymium based perovskites are more stable at higher heat-treatment temperatures (1200°C) than SmNixCo1-xO3-?.

Fig. 1: XRD spectra for PrNixCo1-xO3-? at (a) x=0.1; (b) x=0.5; and (c) x=0.9 sintered at 700, 900, and 1200oC where ?- cubic phase and ?-rhombohedral phase.

At the lowest nickel content (Fig. 1a), a pure single-phase perovskite is formed with a lattice parameter shifting at higher sintering temperatures. High nickel doping causes formation of mixed oxide phases.

Fig. 2: XRD spectra for SmNixCo1-xO3-? at (a) x=0.1; (b) x=0.5; and (c) x=0.9 sintered at 700, 900, and 1200oC where ?- cubic phase and ?-rhombohedral phase.

2. Doped La and Pr-based Nickelates–Cobiltites and their Nanocomposites.

The state-of-the-art ABO3 perovskites, such as strontium -doped lanthanum manganites (LSM), as well as nickelates and cobaltites with addition of iron on B-site (LSFN and LSFC) demonstrate relatively high SOFC power density although have many disadvantages including exfoliation, carbonization and interaction with electrolyte material causing accelerated cell failure. Some of these problems can be overcome by substituting lanthanum cations by Pr.

PrNi1xCoxO3? (PN1xC) are similar to lanthanum nickelates-cobaltites LaNi1xCoxO3? and Sr-doped praseodymium nickelates-cobaltites Pr1xSrxNi1xCoxO3? as materials with high mixed ionic-electronic conductivity. PN1xC and their composites with Ce0.9Y0.1O2? (PN1xC – YDC) demonstrate total electric conductivity ~ 200 S/cm, and oxygen tracer diffusion coefficient and oxygen exchange constant ~ 107 cm2/s and 106 cm/s respectively. The SOFC power density of 0.3 W/cm2 at 600 °C comparable to that of LSM, LSFC, and LSFN materials has been demonstrated. However, in terms of SOFC performance, both PN1xC and PN1xC – YDC materials have certain disadvantages while processed by conventional techniques. For example, at temperatures above 1100°C the perovskite phase becomes unstable resulting in Pr4(Ni1xCox)3O10+? Ruddlesden – Popper phase, whereas at temperatures below 1100 °C the perovskites are unable to sinter well. Thus, the microwave (MW) radiation sintering technique was used to obtain well-sintered ceramics with low porosity at temperatures below 1000 ? 1100 °C.

The goal of this study was to evaluate the effect of the microwave sintering on praseodymium nickelates-cobaltites (PrNi1xCoxO3?) perovskites and their composites with Ce0,9Y0,1O2? (YDS) fluorite in terms of phase purity, porosity, oxygen mobility, and surface reactivity. Co-doped praseodymium nickelates PrNi1-xCoxO3-? and their composites with yttrium doped ceria Ce0.9Y0.1O2-? are known to be promising materials for intermediate temperature solid oxide fuel cells and membranes for oxygen separation. Powdered samples were synthesized by sol-gel Pechini method and ultrasonic dispersion followed by mechanical activation. The pellets were sintered at 870 - 1100°C by using a microwave radiation. In comparison with conventionally sintered materials, the phase transition leading to Ruddlesden - Popper phase formation was shifted down for about 50 - 100°C. The effect of sintering by microwave radiation resulted in increased density, improved phase purity, and enhanced oxygen mobility.

Pr-based nickelates-cobaltites and their nanocomposites with yttria-doped-ceria (YDC) sintered by microwave radiation show higher phase purity and a lower residual porosity without losing oxygen mobility and surface reactivity. This makes these materials promising as IT SOFCs materials and demonstrate the advantages of MW sintering technique in SOFC manufacturing.

Fig. 3: (a) XRD patterns of the PrNi0.5Co0.5O3? sintered at 1000°C by the microwave radiation (MWS) and (b) the samples conventionally sintered (CS) at 1000°C and 1100°C.Fig. 4: XRD patterns of PrNi0,5Co0,5O3? – Ce0,9Y0,1O2? composites sintered by microwave radiation.