Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient and Film Stress Measurements Via the Curvature Relaxation Technique

Abstract

Introduction Praseodymium doped ceria (PCO) is a mixed ionic electronic conductor (MIEC) that can be used as a cathode material in solid oxide fuel cells (SOFCs). Under SOFC operating conditions, PCO undergoes the following reaction [1]: ½ O2 + 2Pr’ Ce + VӦ à 2PrCe + Oo x The kinetics of this oxygen exchange reaction are represented by the oxygen surface exchange coefficient (kchem ). In this work, simultaneous kchem and stress measurements on PCO thin films atop yttria-stabilized zirconia (YSZ) substrates were performed from 625-725 oC using the curvature relaxation (κR) technique. Experimental Methods Phase pure Pr0.1Ce0.9O1.95 powder was prepared through glycine nitrate combustion and subsequent powder calcination in air. (001) oriented 9.5% YSZ substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours. PCO thin films were sputtered onto YSZ substrates at room temperature and then sintered for 1 hour at 1100oC. The bilayer structures were tested using a multi beam optical stress sensor (MOSS) from 625oC to 725oC with 25oC increments. The κR technique determines kchem by fitting a solution of Fick’s 2nd Law to the curvature response of mechano-chemically active film | inert substrate bilayers reacting to sudden changes in oxygen partial pressure (synthetic air (21% O2 - 79% Ar) and 10% synthetic air - 90% Ar) [2,3]. Film stress values were calculated using Stoney’s equation [2,3]. In order to compare the kchem values with those in the literature, electrically-measured PCO kq values from the literature [4] were turned into kchem values by multiplying them by the corresponding PCO thermodynamic factor from Ref [5]. Results Figure 1 shows how the kchem value measured via the κR technique compares with literature [4,5] values. Conclusions The PCO kchem and activation energies measured here are significantly different than those reported in the literature. Although additional work is needed before these effects can be explained in full, these discrepancies may be caused by the different synthesis techniques (sputtering vs. pulsed laser deposition) used to produce the two films shown in Figure 1. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.