3.1 Phase analysis

According to the HT-XRD results (Fig. 1), an amorphous material was observed from room temperature to 350°C with crystallization starting above 350°C. Whereas peaks corresponding to the perovskite were evidenced from the beginning of crystallization for LNC37 and LNC55, an intermediate domain was observed for LNC73 with crystallization of the perovskite only above 525°C. The HT-XRD shows small peaks at 350°C corresponding to the residue, after the combustion of organics and nitrates, plus the formation of an intermediate phase of the perovskite. Thermogravimetric analysis was reported by Ivanova et al. ¹⁷, in the case of LaNi_0.5Co_0.5O₃, where an intermediate product formed at 350-400°C, a spinel and a carbonate (La₂O₂CO₃) were identified, which is in agreement with our HT-XRD results.

Figure 1. High-temperature XRD showing the crystallization profiles of the perovskite (*) in (a) LNC37, (b) LNC55 & (c) LNC73. 

Unit cell parameters were refined in an R 3¯c space group (Table I), in agreement with LaNiO₃ and LaCoO₃ perovskite cell without distortion¹⁸. The a-lattice parameter shows a systematic increase with the decrease of cobalt content in the LaNi_xCo _1-x O₃ structure. The increase of the lattice parameter is likely due to a higher ionic radius, which is dependant of the mixed valences of nickel and cobalt.

Table 1. Perovskite unit cell parameters refined in R 3¯c on LaNi_xCo _1-x O₃ powders. 

In all of the cases, the perovskite structure is obtained after 600°C, where a well-defined XRD peak can be identified around that temperature, after which the structure becomes crystalline, gradually forming the final structure at 1000°C. One advantage of this method is the time taken to reach crystallization, which is only 5 h or less to convert the amorphous powder into a full crystalline material. In comparison, solid state procedures require a longer amount of time at the same temperature to obtain the phase in a pure state or at least with enough order to be fully detected.

It can be inferred that the method may have influenced the structure and properties, as reported before for other materials¹⁵. In brief, it is well known that the cathode performance depends on its microstructural and morphological properties, for which reason these features must be well controlled and understood.

Preliminary stability tests were made by mixing the materials (LCN) with YSZ, then firing the mixture at 1000°C for 5 h, well above the typical SOFC working temperatures, which are around 700°C. For the three cathode materials, well-defined peaks corresponding to YSZ and the perovskite material can be observed in Fig. 2, where no impurity peaks are discerned in the spectrum, which suggests that no reaction appears at the interface and proves that these materials produced by the polymeric route have a good potential for SOFC cathodes. Of course, further studies are needed to judge on the feasibility of these new cathodes interacting with different electrolytes such as gadolinium doped ceria (GDC).

Figure 2. X-ray profiles showing the mixture of LaNi_xCo _1-x O₃ and YSZ fired for 5 h at 1000°C. 

3.2 Morphology

Figure 3 show SEM micrographs of the calcined samples at 1000°C. In Fig. 3 (a-c), a mixture of YSZ and the cathode materials (50%/50% weight) are shown, while the pure perovskites are shown in Figs. 3 (d-f). Figure 3 (a-c) show fine particles ≈ 150 nm of YSZ and the larger grains of cathode material which can be distinguished by their different morphology. In Fig. 3(d), corresponding to LNC37, particles of ≈ 170-600 nm with a semispherical shape can be observed. In Fig. 3(e), round elongated shapes of LNC55 can been seen, the particles becoming larger and the interaction between them producing a denser structure. In Fig. 3(f), LNC73 a structure with a higher number of planar faces is observed; as the composition approaches the structure of the LaNiO₃, the structure become more compact, exhibiting well connected sub-micrometric grains. With all these micrographs, it is possible to conclude that increasing the amount of cobalt promotes the increase of the porosity and the formation of rounded shape structures, which could be beneficial for free air flow passing through the material. Considering the three samples, it can be seen in Fig. 3(a-c) that the amount of YSZ interacting with the cathode is higher when the amount of cobalt increases (LNC37), because the YSZ particles can pass through the channels easily, increasing the TPBs, which is recommended in order to produce a good cathode material. No degradation can be observed, neither the change in composition nor the change in structure have been detected in the samples by either XRD or SEM.

Figure 3. SEM micrographs showing the mixture of YSZ/LaNi _x Co _1-x O₃, where x = 3, 5, 7 in (a), (b) and (c) respectively and the crystallized LaNi _x Co _1-x O₃ material, where x = 3, 5, 7 in (d), (e) and (f) respectively 

Given that the electrochemical reaction can only occur at the triple-phase boundaries (TPBs), area is an important factor for reaction rate, therefore, it is more probable that LNC37 and LNC55 will be able to extend the TPB length, due to its open, less dense structure, thus enabling the contact between the perovskite, electrolyte and oxygen anions (O^2-), improving the material properties and its feasibility as a new cathode for SOFC.

The particles shown in Fig. 3(d), with the highest proportion of Co, are more likely to be spherical in shape, having the smallest particle size (307 nm), while the smaller amount of Co produces significant faceting of the surface (grains) with a polygonal shape, creating a denser, more sintered structure with coarser grains (432 nm). Therefore, finer powders give rise to larger specific areas at the cathode/electrolyte interface and, consequently, a larger amount of triple phase boundary points.

LNC37 has the smallest grain size of 307 nm, in comparison with LNC55 showing a particle size of 416 nm, has a higher number of planar faces and appears to be in an intermediate stage of sintering. Even when the material is totally crystallized, the volume of the structure can be reduced by increasing the heating time at 1000°C, allowing the grains to grow through grain boundary diffusion. Further studies will be made using a full dense single phase material, which may have poor oxygen flow due to the lack of porosity, but with the probability of higher mechanical resistance. When conductive paths become clearer (LNC55 and LNC73), as the conductive particles are closer to each other, as shown in Fig. 3, then conductivity improves.

3.3 Area-specific resistance analysis

Area-specific resistance (ASR) as a function of temperature was obtained for each composition to determine the materials performance as electrode. All samples exhibit the same behavior, with resistivity decreasing with temperature, as shown in Fig. 4.

Figure 4. Area-Specific-Resistance (ASR) for each compound, indicating the Activation Energy (E_a). 

Intermediate work temperatures in the range of 800 to 500°C are considered, the ASR results at 762°C for the LNC73, LCN55 and LNC37 were 2.35 Ω⋅ cm², 2.61 Ω⋅ cm² and 4.53 Ω⋅ cm², respectively. At this point, LNC73 had the best performance, and LNC37 showed the highest resistance. When the temperature is 625°C the resistance increases, this time the LNC73 resistance is 63.71 Ω⋅ cm², while LNC55 keeps the lower value, 50.13 Ω⋅ cm², showing an improvement as the temperature decrease. This is mainly because the activation energy (E_a) of the LNC55 is E_a = 0.68 eV, while LNC73 is E_a = 0.85 eV, then it is expected that the values will be lower, since the starting point for both samples was very similar. Electrical resistance activation energy is a barrier for conduction, the lower electrical resistance activation results in a higher conductivity. Thus, the intention is to decrease the work temperature of the SOFC, then at low temperature, less than 600°C, significant improvement was observed for the sample LNC55, which shows the lowest resistance at that temperature (102.07 Ω⋅ cm²) and the lowest E_a=0.16 eV, which means that even if the temperature continues decreasing, the rate of the resistance values will not increase as drastically as in LNC73 and LNC37. Note that for the three cases the slope changes at around 600°C, which implies changes in the activation energy and thus the rate limiting process. A similar behavior has been reported by Sayers et al¹⁹ for lanthanum nickelate and has been attributed to changes in electronic conductivity, which according to Daroukh et al²⁰ has a maximum at around this temperature. It is clear that 600°C is the breaking point for all the samples, where the E_a is reduced but the resistance is too elevated after this point. For the LNC55 cathode, the improvement of its performance lies in the fact that there are more active sites for oxygen reduction reaction, because the electrochemical reactions can only occur at the TPBs. Figure 5 shows representative impedance diagrams, corresponding to LNC37 at 300°C, 400°C and 500°C confirming that as the temperature increases the impedance decreases. Note that, while we are reporting significant effects of cobalt doping based on concentrations, microstructural features and ASR measurements, along with impedance measurements, our results can also be associated to the fact that cobalt doping increases both mixed conductivity and catalytic activity of lanthanum nickelate.

Figure 5. (a) Impedance diagrams for LNC37 at different temperatures. (b) The curve for the 500±C test, amplified for more clarity. 

4. Conclusions

A cathode material with different compositions of LaNi_xCo _1-x O₃ (x =0.3, 0.5, 0.7) was synthesized without phase contamination. The material crystallization began at 350°C according to high-temperature XRD which also confirmed the presence of pure compounds after sintering at 1000°C. The influence of composition on the structure was investigated; finding out that LNC55 has an open structure with rounded grains and is likely to have a better performance when used as a SOFC cathode, as shown by the ASR results. LNC55 has an intermediate structure showing the possibility of controlling the microstructure through the composition of the material, opening the possibility to design a material with the desired properties. According to our results, the importance and flexibility of the modified Pechini method is evidenced, and it can be used to design and produce cathode materials. The ASR performance of the materials in the range from 500 to 800°C for each compound showed that LNC55 got the lowest resistance (102.07 Ω⋅ cm²) at intermediate temperatures (578°C) and the lowest activation energy (E_a = 0.16 eV). When conductive paths become clearer (LNC55), as the conductive particles are closer to each other, as shown in Fig. 3, then conductivity improves. The electrical properties are also consistent with the morphologies shown in the SEM images.