INTRODUCTION: A one-step co-sintered solid oxide fuel cell (SOFC) stack consists of several cells sintered at high temperature at once (>1250^oC). 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 1300^oC was evaluated. Moreover, a chemical stability comparison between a commercial LSM composition (La_0.8Sr_0.2MnO_3-δ) and a similar Ni-added one (La_0.8Sr_0.2Mn_0.8Ni_0.2O_3-δ) is presented.

EXPERIMENTAL: La_1-xSr_xMn_0.8Ni_0.2O_3-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 180^oC and 450^oC, respectively. Afterwards, all powders were reacted and calcinated at 850^oC and 1000^oC 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 1300^oC for 5h. Electrical conductivity measurements were performed by using four-probe method from room temperature to 800^oC 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 1300^oC 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 La_1-xSr_xMn_0.8Ni_0.2O_3-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 1300^oC 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 La_0.8Sr_0.2MnO_3-δ and La_0.8Sr_0.2Mn_0.8Ni_0.2O_3-δ compositions mixed with 8YSZ and sintered at 1300^oC for 5h were recorded. LZO secondary phase was observed only in commercial LSM composition, while it was not observed in La_0.8Sr_0.2Mn_0.8Ni_0.2O_3-δ composition, even after sintering at 1300^oC 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.

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