Densification of Gadolinia-Doped Ceria Diffusion Barriers for Solid Oxide Cells By Sol-Gel Process
Authors: Choi Hyun-Jong , Na Young-Heum , Kwak Minjun , Seo Doo-Won , Woo Sang-Kuk , Kim Sun-Dong 
Published: February 2020

A gadolinia-doped ceria (GDC)/yttria-stabilized zirconia (YSZ) bilayer has been suggested as an alternative electrolyte for the reduced-temperature operation of solid oxide fuel cells (SOFCs). The mixed ionic and electronic conduction (MIEC) of GDC with decreasing theoretical cell voltage resulting from the transition of Ce4+ to Ce3+ under a reducing atmosphere is a limitation of SOFC electrolytes. The use of YSZ on the anode side as a blocking layer could be an effective solution for inhibiting GDC reduction and leakage currents. One problem that arises from the formation of a barrier layer on YSZ electrolytes is that it becomes difficult to achieve a dense GDC layer below 1400 °C. In this study, a technology for manufacturing dense diffusion barriers below 1200 °C by the sol-gel process using metal (i.e., Gd and Ce) alkoxide precursors was developed. Thin and dense GDC layers were fabricated by GDC sol infiltration and condensation at 1000 °C. A comparative study was performed to investigate the effect of densification on the performance of SOFCs. Gadolinia-doped ceria  sols were synthesized by the controlled hydrolysis and condensation of cerium(IV) isopropoxide with each gadolinia doping agent to make a dense GDC diffusion barrier for solid oxide electrolyte cells (SOECs) and intermediate-temperature solid oxide fuel cells (IT-SOFCs). The crystallinity of a GDC sol using gadolinium(III) isopropoxide as the doping agent was five to six times stronger than that when using gadolinium(III) nitrate hydrate as the doping agent. The GDC diffusion barrier was fully densified by infiltration of a GDC sol into a porous GDC structure under heat treatment at 1000 °C. The crystallinity of the GDC sols was evaluated by powder X-ray diffractometry (Rint 2700, Rigaku Co., Japan) after drying and heat treating each sol via different synthetic routes (Case I: Ce-alkoxide/Gd-alkoxide route, Case II: Ce-alkoxide/Gd-salt route). The particle size distribution of the synthesized GDC sol was analyzed using a particle size analyzer (Zetatrac NPA152, Microtrac Inc., USA). The microstructures were examined by field-emission scanning electron microscopy (FE-SEM, S4200, Hitachi Ltd., Japan). Electrochemical measurements were performed using a Solartron 1470E and a 1455 FRA multi-channel test module. The frequency range of the Solartron 1455 system used in these experiments was 1 MHz ~ 0.1 Hz. Fifty cubic centimeters per minute of humidified hydrogen (H2 + 3vol% H2O(g)) was fed into the anode, and 150 cc/min of dry air was used as an oxidant of the cathode. Finally, comparative studies were performed to investigate the effect of densification of the diffusion barriers on the performance of SOFC cells. The performance of a cell was highly improved from 0.60 W/cm2 to 0.92 W/cm2 at 750 °C by densification of the diffusion barrier. EIS results suggest that the improved performance is mainly due to the reduced ohmic resistance through the composite electrolyte (i.e., YSZ/GDC). Finally, the degradation of a single cell with a dense GDC was maintained below 1.72%/1000 h, which was lower than that of a cell with a porous GDC (7.69%/1000 h). Based on these results, it can be concluded that the enhancement of the performance and durability of a single cell with a dense GDC was the result of reduced ohmic resistance through the composite electrolyte.

Figure 1