Chemical Stability of La 1-X Sr x Mn0.8Ni0.2O3-δ x={0.1 to 0.5} Cathode Compositions with 8YSZ at High Temperature

Abstract

INTRODUCTION : A one-step co-sintered solid oxide fuel cell (SOFC) stack consists of several cells sintered at high temperature at once (>1250oC). Typically, each SOFC cell is composed of a cathode (strontium-doped lanthanum manganite, LSM), an electrolyte (8YSZ), an anode (NiO/8YSZ), a ceramic interconnector and a dedicated gas distribution system [1]. It is believed that reducing the step processing should lead to cost reduction of SOFC cells fabrication and finally promote a wider use of SOFC devices. Unfortunately, several technological issues still have not had an adequate solution, mainly: thermal expansion mismatch between stack components [2] and lack of efficiency caused by formation of a high electrical resistance secondary phase at the LSM/8YSZ interface, La2Zr2O7 (LZO) or SrZrO3 (SZO) [3]. Several methods have been evaluated for reducing secondary phase formation, for instance: protector buffer layer [4], Ce addition into LSM structure [5] and A-site deficiency LSM compositions [6], etc. However, they show a limited performance or are complicated to use at the temperatures required for fabricating a one-step co-sintered SOFC stack. This work evaluates the impact of a Ni-addition, into B-site of LSM composition, on the formation of secondary phase at the LSM/8YSZ interface. Equal Ni addition (20 at%) was used in several LSM compositions, with different strontium contents, and their chemical stability with 8YSZ at 1300oC was evaluated. Moreover, a chemical stability comparison between a commercial LSM composition (La0.8Sr0.2MnO3-δ) and a similar Ni-added one (La0.8Sr0.2Mn0.8Ni0.2O3-δ) is presented. EXPERIMENTAL: La1-xSrxMn0.8Ni0.2O3-δ with x={0.1 to 0.5} compositions, hereinafter called Ni-added LSM, were fabricated by a glycine route using nitrates as reactive and glycine as fuel. Both, reactive and glycine, were dissolved in deionized water at room temperature until obtain a clear solution. Water and organic components were removed by different heat treatment at 180oC and 450oC, respectively. Afterwards, all powders were reacted and calcinated at 850oC and 1000oC for 5h, respectively. Chemical reaction with 8YSZ was evaluated by mixing Ni-added LSM powders with 8YSZ in 50/50 wt%. All samples, whatever 8YSZ mixed or not, were sintered at 1300oC for 5h. Electrical conductivity measurements were performed by using four-probe method from room temperature to 800oC in air, while chemical stability with 8YSZ was evaluated by x-ray diffraction (XRD). RESULTS AND DISCUSSION: XRD patterns from Ni-added LSM compositions showed no secondary phase formation after sintered at 1300oC for 5h (not shown). Ni-added LSM compositions with low strontium content (x≤0.3) were indexed as rhombohedral crystal structure, while at higher strontium contents (x>0.3) an orthorhombic crystal structure tends to gradually form instead of the rhombohedral one, thus at x=0.5 only orthorhombic crystal structure was indexed. Electric conductivity of La1-xSrxMn0.8Ni0.2O3-δ with x={0.1 to 0.5} compositions have a maximum (115 S cm-1) at x=0.3, well correlated with the crystal structure evolution as a function of strontium content. (not shown) Fig.1 shows XRD patterns from Ni-added LSM compositions after mixing with 8YSZ (50/50 wt%) sintered at 1300oC for 5h. No crystal structure change was observed respect to non 8YSZ mixed condition. However, at strontium contents over x≥ 0.3 a secondary phase (SZO) was observed, while below x<0.3 no secondary phase was observed. In addition, XRD patterns from a commercial La0.8Sr0.2MnO3-δ and La0.8Sr0.2Mn0.8Ni0.2O3-δ compositions mixed with 8YSZ and sintered at 1300oC for 5h were recorded. LZO secondary phase was observed only in commercial LSM composition, while it was not observed in La0.8Sr0.2Mn0.8Ni0.2O3-δ composition, even after sintering at 1300oC for 24h (not shown) Results suggest that Ni addition into the B-site of LSM (with low strontium content), could be a suitable and simple method for reducing, and eventually suppressing, the secondary phase formation at the LSM/8YSZ interface at high temperature. [1] S. Suda, J. P. Wiff and S. Shimada, ECS Trans. 57(1), 543-548 (2013) [2] B. Ahmed, S. B. Lee, R. H. Song, J. W. Lee, T. H. Lim and S. J. Park, ECS Trans. 57, 2075-2082 (2013) [3] C. Levy, Y. Zhong, C. Morel and S. Marlin, ECS Trans. 25, 2815-2823 (2009) [4] H. S. Noh, J. W. Son, H. Lee, H. R. Kim, J. H. Lee and H. W. Lee, J. Korean Ceram. Soc. 47, 75-81 (2010) [5] J. P. Wiff, K. Jono, M. Suzuki and S. Suda, J. Power Sources 196(15) 6196-6200 (2011) [6] A. Chen, J. R. Smith, K. L. Duncan, R. T. DeHoff, K. S. Jones and E. D. Wachsman, J. Electrochem. Soc. 157, B1624-B1628 (2010) Figure 1