Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells Fangzhong Wang a , Ao Wang a , Jing Chen b , Bo Chi a , Jian Pu a , Li Jian a, * a Center for Fuel Cell Innovation, State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China b School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou, Henan 450001, China article info Article history: Received 9 December 2013 Received in revised form 6 February 2014 Accepted 7 February 2014 Available online xxx Keywords: Palladium Palladium oxide Oxygen reduction reaction Cathode Solid oxide fuel cells abstract Metallic Pd and PdO electrodes were prepared by using Pd and PdCl 2 slurries, respectively, and their electrochemical performance as a cathode for oxygen reduction reaction in in- termediate temperature solid oxide fuel cells was evaluated by electrochemical impedance spectroscopy (EIS) and direct current polarization (DC polarization). The electrochemical activity of metallic Pd was much higher than that of PdO for the reaction of oxygen reduction; below the decomposition temperature, a thin layer of PdO formed on the surface of metallic Pd electrode, which increased its polarization resistance. The decomposition temperature of PdO decreased from 810 to 750  C as oxygen partial pressure decreased from 20 to 5 kPa, and was further lowered under the influence of the applied current during DC polarization test. The charge transfer resistance of PdO increased by decreasing oxygen partial pressure, while that of metallic Pd was less sensitive to it. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Intermediate temperature solid oxide fuel cells (IT-SOFCs) operated at temperatures between 600 and 800  C have been investigated extensively because of their advantage of utiliz- ing metallic interconnects. However, they are facing the challenge of activity reduction of electrodes owing to the reduced operating temperature [1,2], compared to the con- ventional high temperature SOFCs that are operated near 1000  C. One effective approach to increase the activity of cathode for oxygen reduction reaction (ORR) is adding highly active precious metal Pd into the state-of-the-art cathode materials such as La x Sr 1x MnO 3 (LSM) and La x Sr 1x Co y Fe 1y O 3 (LSCF) [3e6]. It is well known that with temperature increase the metallic Pd contained in the cathode will be oxidized in air into PdO that will subsequently decompose to Pd above 800  C [7]. The reduction process of PdO is started in the bulk rather than on the surface with a hysteresis, as illustrated by Wolf et al. [8]; and metastable co-existence of Pd and PdO (or PdO x ) is commonly observed due to kinetics limitation in metal oxidation, which promotes the reaction of methane oxidation and oxygen reduction [9e12]. The adsorption of gaseous * Corresponding author. Huazhong University of Science and Technology, Materials Science and Engineering, 1037 Luo Yu Lu, Wuhan, Hubei 430074, China. Tel.: þ86 27 87557694; fax: þ86 27 87558142. E-mail address: lijian@hust.edu.cn (L. Jian). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy xxx (2014) 1 e7 Please cite this article in press as: Wang F, et al., Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.051 http://dx.doi.org/10.1016/j.ijhydene.2014.02.051 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.