A novel cobalt-free cathode material for proton-conducting solid oxide fuel cells† Cuijuan Zhang a and Hailei Zhao * ab Received 26th April 2012, Accepted 23rd July 2012 DOI: 10.1039/c2jm32627b Developing a tailored cathode is of great importance for the improvement of proton-conducting solid oxide fuel cell (SOFC-H) performance. In this work, a novel cobalt-free cathode BaCe 0.40 Sm 0.20 Fe 0.40 O 3d was designed for use in a SOFC-H. It was composed of homogeneously distributed BaCe 1x (Sm/Fe) x O 3d and BaFe 1y (Sm/Ce) y O 3d , which were synthesized by a simple in situ method, eliminating the separate synthesis and the mechanical mixing processes for the conventional composite materials. The BaCe 0.40 Sm 0.20 Fe 0.40 O 3d cathode exhibited protonic, oxygen- ionic, and electronic conduction simultaneously in wet air, expanding the triple phase boundaries to the whole cathode. The symmetrical cell tests with BaCe 0.40 Sm 0.20 Fe 0.40 O 3d as electrodes showed that the diffusion of O  ad and reduction of O  TPB were the rate limiting steps in wet air. The power density of the anode-supported single cell with Ni–BaCe 0.80 Sm 0.20 O 3d (580 mm) anode, BaCe 0.80 Sm 0.20 O 3d (70 mm) electrolyte and BaCe 0.40 Sm 0.20 Fe 0.40 O 3d (53 mm) cathode was 194.0, 169.2, and 137.1 mW cm 2 at 750, 710, and 650  C, respectively. These results are encouraging considering the cobalt-free nature and rather low electrical conductivity of the cathode material. The BaCe 0.40 Sm 0.20 Fe 0.40 O 3d material demonstrated excellent catalytic activity towards the reactions on the cathode. Accordingly, the BaCe 0.40 Sm 0.20 Fe 0.40 O 3d material can be a promising cathode for SOFC-H. Introduction Solid oxide fuel cells (SOFCs), which are characterized by their high energy conversion efficiency, environmental friendliness, excellent fuel flexibility, and entirely solid-state components, 1 are attracting increasing interest worldwide as an alternative to traditional fossil energy. The conventional SOFC based on the yttria-stabilized zirconia (YSZ) electrolyte has to work at high temperatures (800–1000  C) to obtain desirable performance. However, the high operating temperature has resulted in a series of problems such as unwanted reactions between the compo- nents, sintering of the electrodes, and gas leaking of the planar cell, thus leading to high costs. 1 Therefore, developing an inter- mediate temperature SOFC (400–750  C) is essential in order to expand the selectivity of interconnect materials and improve the stability and reliability of the system, thus becoming more economically competitive. 2 One strategy is to develop a SOFC based on a proton-conducting electrolyte (SOFC-H). Such a device can work at intermediate temperatures due to the low activation energy of proton transport. Moreover, the SOFC-H cell possesses a higher theoretical efficiency and enables the removal of the internal water-treatment system compared with its oxygen ion conductor based counterpart (SOFC-O). 3 Considering these advantages, the SOFC-H cell has received increasing attention. Great efforts have been made to develop SOFC-H cells, including developing the electrolyte 4–6 and elec- trode materials, 7–14 and studying the reaction mechanism on the electrodes. 7,15,16 Consequently, significant progress has been made. However, the highest power density of a single SOFC-H cell achieved so far is still much lower than that of SOFC-O (Table S1†). Many factors contributed to the poor power density, but one of the greatest challenges for high-performance SOFC-H cells lies in the development of purposely tailored cathode materials. The research in this field is still in its infancy. The present cathode materials for SOFC-H are generally trans- ferred from SOFC-O cells. Although both kinds of SOFCs have similar requirements for the cathode material, such as high electronic conductivity, appropriate porosity, desirable catalytic ability towards oxygen reduction, and good thermal expansion compatibility with the electrolyte, 1,2 an ideal cathode material for SOFC-H should also conduct protons as it is the site for the reaction of protons with oxygen. As shown in Fig. S1†, when a pure electronic conductor such as Pt is employed as a cathode, the reaction is limited to the gas– cathode–electrolyte triple phase boundaries (TPBs). Therefore, a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail: hlzhao@ustb.edu.cn; Fax: +86-10-82376837; Tel: +86-10-82376837 b Beijing Key Lab of New Energy Materials and Technology, Beijing 100083, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm32627b This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 18387–18394 | 18387 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 18387 www.rsc.org/materials PAPER