Improvement of Thermal Reliability and Power Density of SOFCs By Preparing Nano LSC Particles Between GDC Electrolyte and Lscf Cathode

1.Introduction

Solid oxide fuel cells (SOFCs) have received much attention as promising electrochemical power generation devices because of their high electrical efficiency and environment adaptability. Development of SOFCs is now focused on lowering the operating temperature from the conventional range of 750-900°C to an intermediate range of 500-700ºC, allowing a wider range of materials for SOFC components and improvement of thermal reliability. Preparing the Yttlia Stabilized Zirconia (YSZ) and Gadolinia-doped ceria (GDC)) bilayer electrolytes are effective for reducing operating temperature [1]. In the present work, NiO-YSZ anode supported cells with YSZ/GDC bilayer electrolytes and (La,Sr) (Co,Fe) O₃ (LSCF) thin film cathodes were fabricated by pulsed laser deposition (PLD) and screen printing. However, one thermal cycle test of a cell between 200°C and 600°C, with heating and cooling rates of 400°C /h, resulted in detachment of the LSCF thin film and thus reduced durability. To improve the adhesion between the LSCF and GDC, (La,Sr)CoO₃(LSC),  nano particles were deposited on the GDC thin films by PLD. This step was expected to increase contact area between electrolyte and cathode. We investigated the influence of the LSC nano particle on thermal reliability and I-V characteristic of SOFCs.

 2.Experimental Details

NiO-YSZ cermet (NiO:YSZ＝60:40 wt%, 500 μm thick) was used as the anode substrate, YSZ (8 mol% Y₂O₃) and GDC (10 mol% GdO₂)  as electrolyte materials, and La_0.6Sr_0.4Co_0.2Fe_0.8 (LSCF) and  La_0.6Sr_0.4Co (LSC) as cathode materials. YSZ (8 μm thick) and LSCF (30 μm thick) films were prepared by screen printing and GDC (3 μm thick) was deposited by PLD. Two sets of unit cells were prepared: one set was with LSC nano particles deposited between GDC and LSCF thin films by PLD (NiO-YSZ  / YSZ / GDC / LSC particle / LSCF) and the other set was without LSC nano particles (NiO-YSZ  / YSZ / GDC / LSCF).Thermal cycle test between 200 and 600°C with heating and cooling rates of 400° C /h were performed. Power densities and I-V characteristics were measured.

 3.Results

(a)  Deposition of LSC nano particles by PLD on the GDC thin film.

Figure 1 shows a scanning electron microscopy image of the surface of a GDC thin film. Many LSC nano particles (50-nm diameter) were deposited on the dense GDC thin film by PLD, which increased the area of contact between the GDC and LSCF thin films.

(b)  Thermal cycle test of NiO-YSZ anode supported cells with or without LSC nano particles.

 Two types of unit cells, (1) NiO-YSZ / YSZ / GDC / LSC particle / LSCF and (2) NiO-YSZ / YSZ / GDC / LSCF were prepared by screen printing and PLD. Photographs of unit cells after one thermal cycle are shown in Fig. 2. It is evident that adding the LSC nano particles succeeded in suppressing detachment of the LSCF thin film. Spike effect resulted in high adhesion between LSCF and GDC. The unit cell with LSC nano particles survived 50 thermal cycles and yielded higher thermal reliability.

(c) I-V characteristics of NiO-YSZ anode supported cells with or without LSC nano particles.

Figure 3 shows the I-V characteristics of unit cells with and without LSC nano particles. The maximum power densities of unit cells with or without LSC nano particles were 0.5 and 0.3 W·cm^-2, respectively. Cell performances were found to be better when LSC nano particles were deposited, because of increase in reactive sites.

[1] T. Mukai et al., Journal of Fuel Cell Science and Technology , 10(2013) 061006-1-6.