Solid oxide fuel cells (SOFCs) with a high energy conversion efficiency and low emissions are considered as promising substitutes for traditional thermal power devices^[1,2]. However, the conventional Ni based anode suffers from agglomeration, sulfur poisoning and carbon coking with hydrocarbon fuels, which limit its application^[3]. Various alternative materials have been studied as promising SOFC anodes. Meanwhile, in situ exsolution has been developed as a fabrication strategy to prepare perovskite oxides with uniformly dispersed nanometallic particles^[4]. Recently, A-site ordered PrBaMn₂O_5+d has been reported as a promising anode with high electrical conductivity and good catalytic activity for the electrochemical oxidation of both hydrogen and hydrocarbons^[5]. In this work, La_0.5Ba_0.5Mn_1-2xCo_xFe_xO_3-δ (x=0, 0.05, 0.1) has been synthesized with the Pechini method and investigated as an anode material of SOFCs with H₂ and methane as fuels. The structure of the anode converts from a mixture of cubic and hexagonal phases to a perovskite structure with core-shell nanoparticles on the surface after reduction. The in situ exsolution process of the metals on the B sites is studied with an X-ray photoelectron spectrometer, a thermogravimetric analyzer and a transmission electron microscope. The results of the electrochemical tests demonstrate that the doping of Co and Fe into B sites effectively improve the performance of the single cell with H₂ as the fuel. A single cell with a 2CF-LBM anode layer and a 300-μm La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ electrolyte layer exhibits a maximum power density (P_max) of 98, 210, 383, 653 and 1479 mW cm^-2 with wet H₂ as the fuel at 650, 700, 750, 800 and 850 ^oC, respectively, and achieves a peak power density of 503 mW cm^-2 at 850 ^oC when fueled with wet CH₄. Moreover, the 2CF-LBM anode exhibits a high coking resistance, and no remarkable degradation of the performance is observed when the cell is operated with methane as the fuel for more than 200 hours.

Keywords: Solid oxide fuel cell; Perovskite; Anode; In situ exsolution

Table 1. Abbreviations of various anode materials and the maximum output power densities of the cells fed with H₂ and CH₄ at 850 ^oC

Anode composition	Abbreviation	Pmax,H2 (mW cm-2)	Pmax,CH4 (mW cm-2)
La0.5Ba0.5MnO3-δ	LBM	962	336
La0.5Ba0.5Mn0.9Co0.05Fe0.05O3-δ	1CF-LBM	1241	389
La0.5Ba0.5Mn0.8Co0.1Fe0.1O3-δ	2CF-LBM	1479	503

Figure 1. (a) Bright-field TEM image, (b) HAADF imagine with the EDS linear scanning and (c) EDS elemental map of the reduced 2CF-LBM; (d) I-V and I-P curves of the single cell in 650-850 ^oC with H₂ as fuel.

References

1. S. Tao, J.T. Irvine, Nature materials 2003, 2, 320-323.
2. Z. Shao, S.M. Haile, Nature 2004, 431, 170-173.
3. X.M. Ge, S.H. Chan, Q.L Liu, et al., Advanced energy materials 2012, 2, 1156-1181.
4. D. Neagu, G. Tsekouras, D. N. Miller, et al., Nature Chemistry 2013, 5, 916-923.
5. S. Sengodan, S. Choi, A. Jun, et al., Nature materials 2015, 14, 205-209.