The samples were characterized and identified as shown in Table 1.

Analysis by UV spectroscopy confirmed the obtention of metal oxide species, associated with electronic transitions located at 332 and 475nm, where the small variation in absorbance is associated with the level of cation modification. In Fig. 1, the highest absorbances are associated with the presence of chromium species, which indicates the formation of corresponding Cr2O3 oxide attributed to its more stable state of oxidation. The binding energy associated with each wavelength, using the Planck equation, display transition energies around 1.85 and 2.15eV for corresponding wavelengths, in which it is possible to identify similar energy values based on electronic transitions related with typical perovskite oxides [28]. A detailed analysis between 200 and 700nm confirms the presence of electronic transitions π→π* of moderate energy corresponding to the region of the far and near UV, while transitions n→π* significantly diminished, belonging to the visible region of the spectrum. Such transitions are characteristic for elements of transition series with d configuration [29–31].

Fig. 1.

UV spectra of LSNO, LSNCO1, LSNCO2, LSNCO3, LSNCO4 and LSCO oxide materials obtained between 300 and 700nm.

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The band-gap values obtained for each system are shown in Table 1, where it is clear that a semiconductor behavior marked for the sample with the highest concentration of nickel cation, with a value of 1.85eV in comparison with the LSCO system, demonstrating that obtained band-gap values are directly proportional to the concentration of chromium cation. These values obtained are in agreement with the studies of Strehlow et al., in which these values are characteristic for semiconducting materials related with perovskite structures [32].

The infrared spectroscopy analysis (FTIR) allowed to evaluate and identify the effectiveness of synthesis process. The performed analysis using the SDBS databases and is shown in Fig. 2, with the six samples exhibiting a high degree of correspondence between characteristic vibrational bands related with metaloxygen bonds, validating the proposed synthesis method.

Fig. 2.

Infrared spectra of LSNO, LSNCO1, LSNCO2, LSNCO3, LSNCO4 and LSCO oxides, in KBr pellets at 25°C, with its main transmittance signals.

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A detailed analysis of the spectra shows the presence of characteristic bands, located at 3750cm−1 and related with vibrational modes of OH link associated with hydroxyl-bonded species. The signal located at 2935cm−1 is characteristic of vibrational mode OH group linked by hydrogen bonds. The signal located at 2460cm−1 represents the vibrational modes associated with the presence of occluded CO2, by the effect of thermal treatments applied on the obtention of ceramics. The signal located at 1855cm−1 can be identified with vibrations associated with MO bond.

Moreover, the signal located around 1591cm−1 may be associated with asymmetric vibrational modes of metal oxides in configuration MOM, which can be moved toward lower wavenumbers associated with the same vibrational modes but in configuration MMO. The signals located at 1261cm−1 and 1078cm−1 correspond with the vibrational mode of CO bond, derived from environmental CO2. Finally, signals located at low wavenumber around 1000cm−1 corresponding with vibrational signals related with metal cations of La3+, Sr2+, Ni3+ and Cr3+ with hydroxyl groups, demonstrating a clear evidence of the interaction between metal cations and oxygen as shown in Table 2[30].

The analysis of the calcined oxides was performed using X’Pert High Score software. The results confirm that progressive insertion of chromium cation in the structure causes a change in the structural parameters of ceramic, allowing evolution from a rhombohedral arrangement related with the LSNO sample until an orthorhombic structure associated with the LSCO sample. This could explain the change in conformation of X-ray diffraction patterns, as shown in Fig. 3, and it is consistent with previous works [33,34], where it is possible to evaluate the cation chromium effect in the crystal structure. According to Oliveira et al., it is well known that substituting a trivalent metal ion at site A for a bivalent metallic cation as Sr promotes a modification on structure of perovskite, with corresponding presence of SrCrO4 secondary phases assisted by the simultaneous change in the concentration of chromium cation. However, in the case of pure compositions of La0.8Sr0.2NiO3 and La0.8Sr0.2CrO3, the effect disappears and a pure structure is possible only under null concentration of a second cation in the B position [35].

Fig. 3.

X-ray diffraction patterns for La0.8Sr0.2Ni(1−x)CrxO3 systems.

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In Fig. 3, for increasing values of chromium cation, the signals show the formation of secondary phases of SrCrO4, which is related with low kinetic reaction rates and the simultaneous effect between strontium and chromium cations, which finally disappears when the pure phase of La0.8Sr0.2CrO3 is consolidated [36]. The signals of LSNCO 1–4 solids, around 57° 2θ, confirm the presence of nickel oxide and is associated with the signal present at 44° 2θ, as has been shown in previous works by Gallego et al. [37]. According to Montenegro and Rodríguez [38], the signals located at 34 and 39° 2θ in the intermediate compounds of nickel and chromium indicate the presence of strontium oxide (II) (SrO), which does not show evidence of integration to the main crystalline phase, even after the thermal treatment developed at 1000°C under oxygen flow (50mLmin−1) for 120min [39,40].

According to Sakai and Waernhus et al. [41,42], the strontium chromate is characterized by its high resistivity and low catalytic activity. Reason why, the formation of this compund is related directly with the interaction and mobility between nickel and chromium cations in the stabilization process of crystalline phases, where strontium cation shows an important role in the stabilization of the perovskite structure. Therefore, vacancies and mobility of ions during thermal treatments cause deficiencies in the A site of the structure, which provoke that B site elements to form binary oxides; this is the reason why the crystal phases of pure LSNO and LSCO solids remain stable along the synthesized series.

Table 3 shows the cell parameters and the crystal characteristics obtained for each sample, which displays the concentration of chromium cation and the systematic distortion of rhombohedral crystalline structure related with LSNO sample until stabilization of an orthorhombic arrangement of LSCO oxide, in coherence with the perovskite structure and the type and size of atomic bonding of atoms [43].

The crystal sizes of all samples were calculated using the Debye–Scherrer equation, using the intensity of highest signals in experimental X-ray patterns, whose data are shown in Fig. 4, in which the effect of the cation concentration is clear, as well as the consolidation of crystallites in the nanometric scale.

Fig. 4.

Comparison of crystal size and chromium cation content on La0.8Sr0.2Ni(1−x)CrxO3 system.

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According to X-ray diffraction results of Fig. 5a and b, the LSCO system offered the best characteristics related with crystallinity in comparison with obtained systems. A more detailed analysis is shown in Fig. 6 using the Rietveld refinement and PCW software, confirming an excellent correlation with La0.8Sr0.2CrO3 reference structure of ICSD databases. The red line in Fig. 6 corresponds with the theoretical model provided by the GSAS software. The crosses represent the experimental pattern, and the blue line, the difference between theoretical and experimental patterns, confirming the effectiveness of the proposed synthesis method for the obtention of a chromite crystalline phase, the oxide displayed a perovskite structure with a Pnma (62) space group.

Fig. 5.

(a) X-ray spectrum and (b) unit cell of LSCO sample obtained using the PCW software.

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Fig. 6.

Rietveld refinement profile for the LSCO system.

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Tables 4 and 5 summarize the statistical parameters, atomic positions, and cell volume obtained from Rietveld refinement.

Results derived from scanning electron microscopy are shown in Fig. 7, and the results corroborate the obtention of homogeneous materials, with textural and morphological characteristics derived from synthesis method, with some degree of heterogeneity and porosity consistent with properties for anodic materials in SOFC technologies [44].

Fig. 7.

Images of scanning electron microscopy and spectra of energy dispersion by X-rays of (a) LSNO, (b) LSNCO1, (c) LSNCO2, (d) LSNCO3, (e) LSNCO4 and (f) LSCO samples.

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From the point of view of microstructure, obtained oxides are sufficiently heterogeneous to facilitate the migration and the transport of gas to active phase sites. This is the reason why the proposed methodology may achieve the maximum efficiency of materials. Thus, the materials with a high degree of densification and crystallization may limit the mass transport, whereas materials with a high concentration of surface defects can generate low mechanical strength for electrocatalytic applications related with the migration of gas species [45].

The Fig. 7 shows the scanning electron microscopy images and its corresponding EDX spectrums, showing that the cation contents correspond to the proposed content as shown in Table 6. The obtained values do not exceed a difference of 0.107% w/w); which means that the loss of cations during the synthesis process and the posterior thermal treatments to consolidate the crystalline phases were succesfully avoided.

Figs. 8 and 9 correspond with the impedance analysis of the six oxides, with obtained results at room temperature in a modified solid-state cell. Two curves are shown for each sample: the impedance vs frequency and the phase vs frequency change.

Fig. 8.

Bode impedance plots of (a) LSNO, (b) LSNCO1, (c) LSNCO2, (d) LSNCO3, (e) LSNCO4 and (f) LSCO samples.

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Fig. 9.

Bode impedance plots compiled for the La0.8Sr0.2Ni(1−x)CrxO3 system.

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A meticulous analysis of the impedance curves with respect to frequency identified as LSNO, LSNCO1, LSNCO2, LSNCO3, LSNCO4 and LSNO, respectively, shows low values of resistance for LSNO, LSNCO1, LSNCO2 and LSNCO3 samples, with a high concentration of nickel, associated with an electrical semiconducting behavior due to the nickel incidence at B sites of the perovskite. According with Alvarado et al. [46], the preferential formation of ions in oxides with a high content of nickel is strongly linked with the change of ionic radii of the elements that are replaced in the perovskite structure. Hashimoto et al. [47] have shown that the electrical conductivity increases with the nickel content and this behavior is related with the formation of load carriers during synthesis process; additionally, it has been found that elements with d-orbital partially filled allowed the transfer of charge under a semiconducting regime, in accordance with the reports of Gómez-Garcia et al. [48].

On the other hand, for LSNCO4 and LSCO oxides, the obtained impedance signals are related with an opposite behavior associated with an ionic conductivity effect, in which the value of electrical conductivity decreases notably and the charge transferring are fully identified with high electrical resistivity values [49]. Zheng et al. [50] have observed that a decrease in the electrical conductivity with respect to increase in concentration of chromium is related with development of charge carriers polaron type, which permit to understand the interactions between electrons and atoms in a solid material. In this form, the obtained resistivity values show dependence with respect to development of grain boundaries, usually characterized by promoting a high resistivity to electrical conduction as has been established by Gómez-Cuaspud et al. [33].

The Fig. 10 shows the resistance values obtained for each system with an exponential behavior, where the increase in resistance is proportional to chromium concentration, causing variations in resistance between 5.13 and 613Ω range. These results are explained according to several authors, by the nature of modification cation that favors major structural changes, influencing significantly the conductive properties of oxides [46–50]. According to previous studies and the present work [51,52], the lanthanum chromites, modified with nickel, are characterized by a mixed ionic and electronic behavior that improves the oxidation of hydrogen fuel in the surface of the catalyst as an important alternative in design of materials for solid oxide fuel cells.

Fig. 10.

Resistance values related to the content of the chromium cation in the La0.8Sr0.2Ni(1−x)CrxO3 system.

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