Anodes Derived from Fluorite-Type and Perovskite-Type Metal Oxides for SOFCs

Solid oxide fuel cells (SOFCs) are alternative energy conversion devices, which deliver high quality power with a high efficiency using variety of fuels, including H₂ and hydrocarbons. At present, composite Ni with Y-stabilised zirconia (YSZ) is widely used as an anode for SOFCs [1]. However, technical challenges due to coking and sulfur poisoning are yet to overcome when direct hydrocarbon-containing fuels are used. Ni not only possess good catalytic activity towards H₂ oxidation and hydrocarbon reforming, it also catalyses carbon formation and poorly resists sulfur poisoning [2]. To overcome these problems, several alternatives such as Cu/CeO₂/YSZ [3] and LSCM (La_1-xSr_xCr_1-xMn_xO₃) [4] have been investigated. Though, these materials were able to resist coke and sulfur poisoning, they were not able to satisfy all the requirements of ideal SOFC anodes.

In the present study, mixed ionic electronic conducting (MIECs) Ce_0.8Y_0.1Mn_0.1O_2-d and BaZr_0.1Ce_0.7Y_0.1M_0.1O_3−δ (M = Ni and Co), and composite Ni and Ba_0.5Sr_0.5Ce_0.6Zr_0.2Gd_0.1Y_0.1O_3-δ (BSCZGY) have been investigated as SOFC anodes. The cell performance of a button cell: Ce_0.8Y_0.1Mn_0.1O_2-d/Zr_0.84Y_0.16O_2-d/La_0.6Sr_0.4MnO₃+Zr_0.84Y_0.16O_2-d was tested at 800 ^oC in 3 % H₂O-H₂ as a fuel and air as an oxidant. There was an obvious enhancement of the cell performance upon exposure to 10 ppm H₂S-H₂ for 48 h. Another area of the MIEC anodes development involves investigation of perovskite-type BaZr_0.1Ce_0.7Y_0.1M_0.1O_3−δ (M = Ni and Co). The BaZr_0.1Ce_0.7Y_0.1Ni_0.1O_3−δ compound shows a promising low polarization resistance of 0.4 Ω.cm² at 800^oC in H₂. Low polarization resistance of the BaZr_0.1Ce_0.7Y_0.1Ni_0.1O_3−δ perovskite, as a non-composite anode without any additional electron conductive phase, makes it a promising material for SOFC anodes.

Proton conducting doped BaCeO₃ materials is the frontrunners as electrolyte in the field of low temperature (400-700 ^oC) proton-SOFCs. A and B site co-doped Ba_0.5Sr_0.5Ce_0.6Zr_0.2Gd_0.1Y_0.1O_3-δ has shown good chemical stability in CO₂ and humid atmospheres at elevated temperature [5]. In the present work, anode performance of Ni-BSCZGY composites (with Ni: BSCZGY ratios as 30:70, 40:60 and 50:50) has been studied. Screen-printing and co-firing processes were employed to make symmetrical cell: Ni-BSCZGY/BSCZGY/Ni-BSCZGY. Area specific resistance (ASR), capacitance and activation energy values were studied to evaluate the electrochemical performance of the anode composites. 50:50 composite shows the lowest ASR value among the current investigated anode compositions (30:70 and 50:50).

References

1)     M. Ihara, T. Kusano and C. Yokoyama, J. Electrochem. Soc., 148, A209 (2001).

2)     Zhe Cheng and Meilin Liu, Solid State Ionics, 178, 925 (2007).

3)     H. Kim, S. Park, J.M. Vohs and R.J. Gorte, J. Electrochem. Soc., 148, A693 (2001).

4)     S. Tao and J.T.S. Irvine, Nature Letters, 2, 320 (2003).

5)    R. Kannan, S. Gill, K. Singh, T. Fürstenhaupt and V. Thangadurai, Sci. Report, 3, 2138 (2013).