Yttrium and Nickel Co-Doped BaZrO3 as a Proton-Conducting Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Conventional solid oxide fuel cells (SOFCs) based on yttria-stabilized zirconia (YSZ) require high operating temperatures in the 800-1000 °C range. Such high operating temperatures hinder the commercialization of SOFCs due to degradation and high cost issues. On the other hand high temperature proton conductors (HTPCs), due to their lower activation energy for proton conduction, can achieve high conductivity at relatively lower temperatures compared to their oxygen ion conducting counterparts. BaZrO₃ related HTPC electrolyte materials possess good chemical stability and high bulk conductivity. However, due to its high refractory nature, fabrication of fully dense electrolyte films is very challenging. The poor sinterability of BaZrO₃based materials leads to a large number of resistive grain boundaries, thus lowering the total proton conductivity.

Previous reports have shown that NiO can function as an effective additive for enhancing the densification of BaZrO₃ based materials [1, 2]. These studies did not focus on controlling the stoichiometry so as to accommodate Ni at a specific site in the perovskite structure. Contrary to the previous NiO sintering aid studies, the present work focuses on substitution of Zr with Ni in the perovskite B-site in a targeted fashion. Y-doped BaZrO₃ (BZY) is chemically stable at fuel cell operating conditions and shows the best protonic conductivity when Y-content is 20 mol% [3]. In this work, the sinterability of 20 mol% Y-doped BaZrO₃ (BZY20) was significantly improved by partial substitution of Zr with Ni in the perovskite B-site. The effect of Ni-doping on the shrinkage behavior of BZY has been identified through dilatometric measurements. The SEM analysis showed a pore-free dense microstructure for 4 mol% Ni-doped BZY (BaZr_0.76Y_0.2Ni_0.04O_3-δ, BZYNi04) and therefore electrochemical characterization studies were carried out on this material. The total conductivity of BZYNi04 in wet air was observed to be 0.0037 Scm^-1 at 600 °C. The open circuit voltage (OCV) values recorded for BZYNi04 pellet fuel cell were 0.96, 0.99, 1.01, 1.03 and 1.034 V at 700, 650, 600, 550 and 500 °C respectively indicating that there is not much electronic conductivity. Ni-doped BZY electrolyte films were fabricated on an optimized NiO-BZY anode using co-pressing and co-firing process. Subsequently, a full fuel cell was fabricated with PrBaCo₂O_5+δ (PBCO) and BZY composite cathode. The ease of film electrolyte fabrication of low thickness assisted by the good sinterability of BZYNi04 coupled with employing an anode functional layer aided in attaining a superior power density output. Peak power density values of 428 and 340 mWcm^-2 were obtained at 700 and 650 °C respectively when humidified hydrogen (≈ 3% H₂O) and static air were used as the fuel and oxidant respectively. To the best of our knowledge, the power performance obtained in this work is the best reported yet for BaZrO₃based proton conducting SOFCs.

References

1. P. Babilo, S. M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362.
2. J. Tong, D. Clark, M. Hoban, R. O’Hayre, Solid State Ionics 181 (2010) 496.
3. K. D. Kreuer, S. Adams, W. Munch, A. Fuchs, U. Klock, J. Maier, Solid State Ionics 145 (2001) 295.