UNA INTRUSIÓN ACIAGA

The Sr-doped LaMnO3 (LSM), a p-type perovskite semiconductor, is one of the most intensively investigated and commercially usable materials for HT-SOFCs. The selection of this material compared to others such as LaCoO3- and LaFeO3- based perovskites with superior electronic conductivity, is based on consideration of compatibility with the YSZ electrolyte used in HT- SOFCs, in terms of both reactivity and thermal expansion behavior[3, 70]. Pure LaMnO3 is neither a good electrical conductor nor a good catalyst for oxygen reduction. Doping on the La site by Sr increases the overall electronic conductivity of the material by enhancing the electron-hole concentration due to oxidation of the manganese ion[71]. The electronic conductivity of LSM increases approximately linearly with increasing Sr concentration, as the Mn4+ fraction increases, up to a maximum around 50 mol%[13]; with conductivity of 320 S.cm-1 at 800oC for the La0.6Sr0.4MnO3-δ composition[42, 72]. However, Sr doping levels greater than 30% mol result in the

Page | 33 significant formation of insulatingzirconate phases due to reaction of LSM with YSZ at the high temperatures needed for operation[42, 67]. For this reason the most common cathode composition for use with YSZ electrolyte cells is considered to be La0.8Sr0.2MnO3-δ[3]. In LSM, the reduction of oxygen is restricted to the triple phase boundary (TPB), at the interface of the electrolyte-cathode where O2 gas can be reduced and transferred, due to the absence of oxygen vacancies. This is due to the very low ionic conductivity in the range of 10-7 S.cm-1 at 900 oC [17, 72]and it is considered to be the main reason that the performance of LSM is not sufficient at lower temperatures and hence not a good candidate for IT-SOFCs. The incorporation of an ionic conductive component is reported to enlarge the electrochemical reaction zone; composites with the ionically conductive electrolytes YSZ and GDC have been reported to enhance the performance of LSM[68].

Page | 34 1.4.3.1.2. LSC and LSCF cathodes

The Sr-doped LaCoO3 (LSC) based perovskites family is a mixed electronic-ionic conductor (MIEC) as it displays both high p-type semiconductive electronic and fast ionic conductivity, due to the ability of cobalt to alter its oxidation state and subsequent formation of oxygen vacancies[73, 74]. Hence, the reactive area is extended to the bulk of the cathode and not limited to the TPB and makes it applicable for IT-SOFCs. The total electrical conductivity is reported to be very high, in the order of 1500 S.cm-1in air at 600oC[75]. Moreover, cobalt positioned at the right of the first transition series, results in strong covalency with oxygen and it is considered as a good electrocatalyst for the oxidative dissociation of the dioxygen O2 molecule to oxygen ions (O2−)[29, 68]. However, a large amount of Co and oxygen vacancies result in high thermal expansion coefficients (TEC), typically ca. 20x10-6 K-1 for the LSC system[44, 76], due to the relative weak Co-O bond, resulting in significant mismatch with the common electrolytes in IT- SOFCs, such as GDC, SDC and LSGM [20, 77] (Section1.4.1). This can be critical for long term operation as thermal cycling results in cracking and delamination of the cathode[42]. Introducing Fe in the place of Co in LSC lowers the TEC but results in lower MIEC properties [76, 78, 79]. The most commonly used composition is La0.6Sr0.4Co0.2Fe0.8O3-δ, which has a thermal expansion

coefficient of 15.3x10-6 K-1, giving a good match with that of ceria-based electrolytes[80]. The total electrical conductivity is reported to be 350-400 S.cm-1 at 750oC [80, 81], with the electronic conductivity of 102S.cm-1 and ionic conductivity of 10-3 S.cm-1 at 750oC [82, 83]. The development of LSCF composite cathodes with GDC has been reported to further enhance the ionic conductivity and reduce the TEC values closer to that of CGO[84, 85]. The main limitation for long-term operation of the LSCF cathodes is the degradation with time, which is due to strontium diffusion out of the cathode leading to reduced performance[86, 87].

1.4.3.1.3. BSCF

The Co-rich cubic BSCF (Ba0.5Sr0.5Co0.8Fe0.2O3-δ) perovskite is a recently developed IT-SOFCs cathode material, in the BaCoO3-δ- SrCoO3-δ system, exhibiting very low electrode polarization resistance (of less than 0.1 Ω cm2 at 600oC and high power densities of about 1 W cm-2 at 600oC [88]

. It originates[69] from SrCo0.8Fe0.2O3-δ, which was developed for use in oxygen separation membranes and exhibiting the highest oxygen permeation flux, a property that is closely related to the oxygen ionic and electronic conductivities, in the Ln1−xAxCo1−yByO3−δ (Ln= La, Pr, Nd,

Page | 35 Sm, Gd; A= Sr, Ca, Ba; B =Mn, Cr, Fe, Co, Ni, Cu) system. Substitution with Ba was introduced in order to suppress the limited mechanical and phase stability encountered in the cubic SrCo0.8Fe0.2O3-δ while sustaining the high oxygen permeation flux.

The main limitations of BSCF are the incompatibility with the commonly used electrolyte materials due to reactivity and its high thermal expansion coefficient of 19.0(5)–20.8(6) ×10−6 K−1 between 600-900oC[89]. The thermal mismatching with electrolytes (common IT-SOFCs electrolytes having TEC= 10-13x10-6 K-1) is a common feature of cobalt containing cathodes exhibiting TEC often larger than 14x10-6K-1) [90]. Moreover, BSCF undergoes decomposition into a mixture of hexagonal barium-rich, iron-free cobalt perovskite phase and a cubic strontium- rich, iron-cobalt perovskite phase at temperatures below 900oC for long time periods in air[91, 92]. It is believed that the cubic perovskite tends to decompose to two of its end-members, BaCoO3-δ, which is known to be a hexagonal perovskite, and SrFeO3-δ, which adopts a cubic polymorph of the perovskite structure[93]. According to the literature, the driving force for the decomposition of the cubic BSCF, is the cobalt preference to low-spin configuration in the Co3+ (d6) oxidation state, upon heating at temperatures below 900oC, which prefers to form smaller bonds in the hexagonal structure (chains of face-sharing BO6 octahedra, having B-O distances of 1.84 Å) instead of longer bonds in the cubic structure (corner-sharing octahedra with B-O distances of 1.95Å)[94]

. The competing hexagonal phases are favored at high (>+3) cobalt oxidation states in BSCF (cobalt oxidation state: 3-4) resulting in reducing of the concentration of oxygen vacancies as well as the number of charge carriers leading to poorer electrochemical performance compared to the highly efficient BSCF cubic polymorph[69]. It is worth noticing though that lower Co containing Ba0.5Sr0.5Co0.2Fe0.8O3-δ shows improved stability compared to Ba0.5Sr0.5Co0.8Fe0.2O3-δ, but lower electrochemical performance[95].