Novel Protonic Conductor SrLa2Sc2O7 with Layered Structure for Electrochemical Devices

Novel materials with target properties for different electrochemical energy conversion and storage devices are currently being actively created and investigated. Materials with high level of protonic conductivity are attracting attention as electrolytes for solid oxide fuel cells and electrolyzers. Though many materials are being investigated as potential electrolytic components for these devices, many problems exist, including comparability between electrodes and electrolytes. In this paper, layered perovskite SrLa2Sc2O7 was investigated as a protonic conductor for the first time. The possibility for water uptake and protonic transport was revealed. It was shown that the SrLa2Sc2O7 composition can be considered a prospective ionic conductor. The layered perovskites can be considered as very promising materials for electrochemical devices for energy applications.


Introduction
Novel materials with target properties for different electrochemical energy conversion and storage devices are currently being actively created and investigated [1][2][3][4]. These devices must meet certain requirements, such as high effectiveness, low cost, eco-friendliness and safety. Hydrogen energy satisfies those criteria well, and can be considered one of the most promising energy sources for the future [5][6][7][8][9]. Accordingly, the development of systems for the production, transportation and conversion of hydrogen is necessary. Protonic ceramic fuel cells are electrochemical devices that convert the chemical energy of hydrogen oxidation into electrical energy. The main components of such devices are electrolytes [10][11][12][13][14] and electrodes [15,16]. Though many materials have been investigated as potential electrolytic and electrode components for these devices, many problems exist, including comparability between electrodes and electrolytes [17][18][19][20]. The most studied proton-conducting materials for use as electrolytes in protonic ceramic fuel cells are barium cerate-zirconates BaCeO 3 -BaZrO 3 , which are characterized by a perovskite structure [21,22]. However, promising electrode materials such as nickelites [23][24][25][26] and cobaltates [27][28][29] have layered perovskite structure. Consequently, the creation of protonconductive materials with layered perovskite structure is very important from the point of view of comparability between electrolyte and electrode materials. Layered perovskites can be described by the general formula AA' n B n O 3n+1 , where A is the alkali-earth metal, such as barium or strontium, A' is the rare-earth metal, such as lanthanum or neodymium, and B is the trivalent metal, such as indium or scandium. Monolayer perovskites AA'BO 4 (n = 1) were described as protonic conductors several years ago for the first time [30]. Such matrix compositions as BaNdInO 4 [31][32][33][34][35], BaNdScO 4 [36], SrLaInO 4 [37][38][39][40][41], BaLaInO 4 [42][43][44][45][46][47] and compounds based on them were investigated, and general regularities of proton transport in doped monolayer perovskites were revealed [48]. Two-layer perovskites with the general formula AA' 2 B 2 O 7 (n = 2), such as BaLa 2 In 2 O 7 [49][50][51][52] and BaNd 2 In 2 O 7 [53], Figure 1a represents the results of the XRD-analysis for the obtained SrLa 2 Sc 2 O 7 composition. All peaks correspond to the Fmmm space group, and their calculated lattice parameters (Table 1) are well correlated with previously reported data [54,55] (ICSD 67625).    Local structure of the SrLa 2 Sc 2 O 7 composition was investigated using the Raman spectroscopy method. Figure 2 represents the deconvolution of the Raman spectrum for the SrLa 2 Sc 2 O 7 composition. Local structure of the SrLa2Sc2O7 composition was investigated u spectroscopy method. Figure 2 represents the deconvolution of the Ram the SrLa2Sc2O7 composition. The low-wavenumbers region (120−200 cm −1 ) contains several signa to the stretching and bending vibrations of alkali-earth-and rare-earth-c polyhedra [51,[56][57][58][59] ( Table 2). The tilting/bending and stretching vibra The low-wavenumbers region (120−200 cm −1 ) contains several signals corresponded to the stretching and bending vibrations of alkali-earth-and rare-earth-containing metal polyhedra [51,[56][57][58][59] ( Table 2). The tilting/bending and stretching vibrations of trivalent metal with small ionic radii polyhedra (scandium, in our case [60]) should be located in the mid-and high-wavenumbers region (higher 200 cm −1 ). This region contains more signals as compared to BaLa 2 In 2 O 7 [53], which can indicate an increase in the deformation of polyhedra [ScO 6 ] in the structure of SrLa 2 Sc 2 O 7 compared with polyhedra [InO 6 ] in the structure of BaLa 2 In 2 O 7 . The signals in the 500−900 cm −1 wavenumbers region correspond to the repulsion between the Sr 2+ /La 3+ ions and oxygen ions in compressed Sc-contained polyhedra [61], i.e., it proves the deformation of Sc-contained polyhedra. The additional confirmation of this is the decrease in the lattice parameters and unit cell volume in the row BaLa 2 In 2 O 7 −SrLa 2 Sc 2 O 7 (Table 1). The possibility of interaction of the investigated composition with water vapors was checked using thermogravimetry (TG) measurements ( Figure 3). As can be seen, the SrLa 2 Sc 2 O 7 composition can dissociatively intercalate some amount of water molecules, however, the water uptake is not much; about 0.05 mol H 2 O per mol complex oxide. The mass spectroscopy (MS) results confirm the release of water during heating. At the same time, water uptake for the BaLa 2 In 2 O 7 composition was about 0.17 mol H 2 O per formula unit [53]. For the layered perovskites, the possibility of water uptake is due to the presence of enough space between the perovskite blocks and the rock-salt layers [48]:

Results
where ( OH) • o is the hydroxyl group in the regular oxygen position; (OH) i is the hydroxyl group located in the interlayer space. The increase of the size of this space leads to the increase of the water uptake. Accordingly, the decrease of the unit cell volume and the decrease of this space should lead to a decrease of the water uptake. In other words, the concentration of protons decreases in the BaLa 2 In 2 O 7 −SrLa 2 Sc 2 O 7 row in accordance with the decrease of the unit cell volumes (Table 1).  The electrical conductivity values were collected using the impedance spectroscopy method. The EIS plots for the SrLa2Sc2O7 composition obtained at different temperatures are presented in Figure 4. All EIS plots consist of two semicircles. The fitting of the spectra was made using ZView software (Scribner, Southern Pines, NC, USA), and the obtained results are presented in Table 3. The first (high-frequency) corresponds to volume resistance and has a capacitance of ~10 -12 F/cm. The second semicircle (very small) corresponds to grain boundaries resistance and has a capacitance of ~10 -10 F/cm. To calculate was made using ZView software (Scribner, Southern Pines, NC, USA), and the obtained results are presented in Table 3. The first (high-frequency) corresponds to volume resistance and has a capacitance of~10 −12 F/cm. The second semicircle (very small) corresponds to grain boundaries resistance and has a capacitance of~10 −10 F/cm. To calculate conductivity, we used the resistance value of the sample obtained by extrapolating the high-frequency semicircle to the abscissa axis (approximation with using the Zview software). The electrical conductivity values were collected using the impedance spectroscopy method. The EIS plots for the SrLa2Sc2O7 composition obtained at different temperatures are presented in Figure 4. All EIS plots consist of two semicircles. The fitting of the spectra was made using ZView software (Scribner, Southern Pines, NC, USA), and the obtained results are presented in Table 3. The first (high-frequency) corresponds to volume resistance and has a capacitance of ~10 -12 F/cm. The second semicircle (very small) corresponds to grain boundaries resistance and has a capacitance of ~10 -10 F/cm. To calculate conductivity, we used the resistance value of the sample obtained by extrapolating the high-frequency semicircle to the abscissa axis (approximation with using the Zview software).  3.1 × 10 −10 3.5 × 10 −10 2.0 × 10 −10 R2 9.2 6.5 4.5

MS (H 2 O) TG
The temperature dependencies of conductivity are presented in Figure 5. The conductivity values obtained under high temperatures and dry air conditions (pO2 = 0.21 atm) are higher than those obtained under dry Ar conditions (pO2~10 −5 atm, conditions of dominance of oxygen-ionic conductivity), which confirms the mixed oxygen-hole nature of conductivity: where V •• is the oxygen vacancy; h • is the hole. However, the temperature decreasing leads to the increase in the oxygen transport share from 30% at 900 °C to 70% at 300 °C. It  The temperature dependencies of conductivity are presented in Figure 5. The conductivity values obtained under high temperatures and dry air conditions (pO 2 = 0.21 atm) are higher than those obtained under dry Ar conditions (pO 2~1 0 −5 atm, conditions of dominance of oxygen-ionic conductivity), which confirms the mixed oxygen-hole nature of conductivity: where V •• O is the oxygen vacancy; h • is the hole. However, the temperature decreasing leads to the increase in the oxygen transport share from 30% at 900 • C to 70% at 300 • C. It should be noted that the BaLa 2 In 2 O 7 composition is characterized by mixed oxygen-hole conductivity, with a 20% share of oxygen transport in the entire temperature 900−300 region [53]. should be noted that the BaLa2In2O7 composition is characterized by mixed oxygen-hole conductivity, with a 20% share of oxygen transport in the entire temperature 900−300 region [53].  Figure 6 represents the comparison of the temperature dependencies for the SrLa2Sc2O7 and BaLa2In2O7 compositions obtained under dry conditions. As can be seen, the conductivity values obtained under dry air are close, and the conductivity values obtained under dry Ar (oxygen-ion conductivity) are higher for the SrLa2Sc2O7 composi-  Figure 6 represents the comparison of the temperature dependencies for the SrLa 2 Sc 2 O 7 and BaLa 2 In 2 O 7 compositions obtained under dry conditions. As can be seen, the conductivity values obtained under dry air are close, and the conductivity values obtained under dry Ar (oxygen-ion conductivity) are higher for the SrLa 2 Sc 2 O 7 composition. In other words, the SrLa 2 Sc 2 O 7 composition is more preferable from the point of view of oxygen-ionic conductivity compared with the BaLa 2 In 2 O 7 composition.  Figure 6 represents the comparison of the temperature dependencies for the SrLa2Sc2O7 and BaLa2In2O7 compositions obtained under dry conditions. As can be seen, the conductivity values obtained under dry air are close, and the conductivity values obtained under dry Ar (oxygen-ion conductivity) are higher for the SrLa2Sc2O7 composition. In other words, the SrLa2Sc2O7 composition is more preferable from the point of view of oxygen-ionic conductivity compared with the BaLa2In2O7 composition. The conductivity values obtained for the SrLa2Sc2O7 composition under wet conditions are presented in Figure 5 (open symbols). The effect of humidity on the conductivity values started lower, at ~400 °C, which correlates well with TG-data. At the same time, the conductivity values obtained under wet air and wet Ar below 400 °C are very close, which indicates the ionic (protonic) nature of conductivity under wet air conditions and low temperatures. The proton conductivity was calculated as the difference between the conductivity values obtained under wet Ar and dry Ar, i.e., as: The conductivity values obtained for the SrLa 2 Sc 2 O 7 composition under wet conditions are presented in Figure 5 (open symbols). The effect of humidity on the conductivity values started lower, at~400 • C, which correlates well with TG-data. At the same time, the conductivity values obtained under wet air and wet Ar below 400 • C are very close, which indicates the ionic (protonic) nature of conductivity under wet air conditions and low temperatures. The proton conductivity was calculated as the difference between the conductivity values obtained under wet Ar and dry Ar, i.e., as: and its temperature dependences are shown in Figure 7. The calculation was made for the temperatures 300, 350, 400, 450 and 500 • C.
Materials 2022, 14, x FOR PEER REVIEW 7 of 10 and its temperature dependences are shown in Figure 7. The calculation was made for the temperatures 300, 350, 400, 450 and 500 °C. As can be seen, the protonic conductivity values for the SrLa2Sc2O7 composition are lower than for the BaLa2In2O7 composition. It is clear that this decrease is due to a decrease of the proton concentration for SrLa2Sc2O7 compared with BaLa2In2O7. Meanwhile, the SrLa2Sc2O7 composition is very promising prospective ionic conductor. The increase As can be seen, the protonic conductivity values for the SrLa 2 Sc 2 O 7 composition are lower than for the BaLa 2 In 2 O 7 composition. It is clear that this decrease is due to a decrease of the proton concentration for SrLa 2 Sc 2 O 7 compared with BaLa 2 In 2 O 7 . Meanwhile, the SrLa 2 Sc 2 O 7 composition is very promising prospective ionic conductor. The increase of the unit cell volume by the doping, for example, can lead to an increase of the proton concentration in the structure and an increase of the proton conductivity.

Conclusions
The layered perovskite SrLa 2 Sc 2 O 7 was investigated as a protonic conductor for the first time. The local structure, possibility for water uptake and protonic transport was revealed. The doping of the layered perovskite structure potentially can increase the proton conductivity. Based on this, the layered perovskite SrLa 2 Sc 2 O 7 can be considered as a very promising material for energy applications in electrochemical devices.

Conflicts of Interest:
The authors declare no conflict of interest.