Novel co-doped protonic conductors BaLa1.9Sr0.1In1.95M0.05O6.925 with layered perovskite structure published by Ural Federal University eISSN 2411-1414 chimicatechnoacta.ru ARTICLE 2023, vol. 10(2), No. 202310206 DOI: 10.15826/chimtech.2023.10.2.06 1 of 7 Novel co-doped protonic conductors BaLa1.9Sr0.1In1.95M0.05O6.925 with layered perovskite structure Anzhelika Bedarkova * , Nataliia Tarasova , Irina Animitsa , Ekaterina Abakumova, Irina Fedorova, Polina Cheremisina, Evgenia Verinkina Institute of Hydrogen Energy, Ural Federal University, Ekaterinburg 620009, Russia * Corresponding author: a.o.galisheva@urfu.ru This paper belongs to a Regular Issue. Abstract Active development of electrochemical devices such as proton-conducting fuel cells and electrolyzers should ensure sustainable environmental de- velopment. An electrolyte material of a hydrogen-powered electrochemi- cal device must satisfy a number of requirements, including high proton conductivity. Layered perovskites are a promising class of proton-conduct- ing electrolytes. The cationic co-doping method has been successfully ap- plied to well-known proton conductors with the classical perovskite struc- ture ABO3. However, the data on the application of this method to layered perovskites are limited. In this work, the bilayer perovskites BaLa1.9Sr0.1In1.95M0.05O6.925 (M = Mg2+, Ca2+) were obtained and investigated for the first time. Cationic co-doping increases oxygen-ion and proton con- ductivity values. Keywords layered perovskite oxygen-ion conductivity proton conductivity hydrogen energy Ruddlesden-Popper structure Received: 20.03.23 Revised: 06.04.23 Accepted: 06.04.23 Available online: 11.04.23 Key findings ● Cationic co-doping leads to an increase in proton conductivity of BaLa2In2O7 values of up to ~0.8 orders of magnitude. ● The cationic co-doping strategy is a promising way to improve the transport properties of bilayer perovskites. © 2023, the Authors. This article is published in open access under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction Electrochemical devices such as proton-conducting fuel cells [1–3] and electrolyzers [4, 5] are in dire need of highly efficient materials with targeted properties includ- ing proton conductivity [6–8]. Active development and im- plementation of these devices as a part of the “hydrogen energy in everyday life” strategy should ensure sustaina- ble environmental development [9–16]. The electrolyte material of a hydrogen-powered electrochemical device must satisfy a number of requirements, including high proton conductivity. One of the known ways to improve the conductivity is by co-doping of cationic sublattices of complex oxides. This method has been successfully ap- plied to well-known proton conductors such as barium ce- rate-zirconates [17–22] and lanthanum scandates [23–26]. At the same time, the possibility of applying this method to a new class of proton-conducting materials, such as lay- ered perovskites AA′nBnO3n+1 [27], is currently under in- vestigation. The possibility of oxygen-ion transport in the monolayer perovskites AA′BO4 was opened about ten years ago by Fujii et al. for compositions based on BaNdInO4 [28–32] and by Troncoso et al. for compositions based on SrLaInO4 [33–35]. The realisation of proton transport in the layered structures was demonstrated several years later for compositions based on BaLaInO4 [36]. Currently, a large class of materials with the monolayer perovskite structure Ba(Sr)La(Nd)In(Sc)O4 [37–41] is described in terms of proton transport. The possi- bility of proton conductivity in bilayer AA′2B2O7 perovskites such as on BaLa2In2O7 [42–44], BaNd2In2O7 [45] and SrLa2Sc2O7 [46] and compositions based on them was de- scribed last year. It was shown that doping cationic sublat- tices can improve oxygen-ion and proton conductivity by up to 1.5 orders of magnitude [12]. It can be predicted that co- doping can also promote an increase in conductivity. In this work, we performed acceptor Sr2+→La3+ and M2+→In3+ (M = Mg2+, Ca2+) co-doping in the cationic sublattices of the bilayer perovskite BaLa2In2O7. The doping effect on the pro- ton conductivity was revealed. http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2023.10.2.06 http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0003-4346-5644 https://orcid.org/0000-0001-7800-0172 https://orcid.org/0000-0002-0757-9241 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2023.10.2.06&domain=pdf&date_stamp=2023-04-11 Chimica Techno Acta 2023, vol. 10(2), No. 202310206 ARTICLE 2 of 7 DOI: 10.15826/chimtech.2023.10.2.06 2. Experimental The compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 were prepared by the solid state method. The powders of the starting reagents BaCO3, SrCO3, CaCO3, MgO, La2O3, In2O3 were dried and used in stoichio- metric amounts. The agate mortar was used for grinding. The compositions were heated after each grinding. The an- nealing was carried out in the temperature range of 800– 1300 °C with 100 °C steps and 24 h dwell time at each step. The phase identification of the obtained compositions was carried out using the Bruker Advance D8 Cu Kα diffrac- tometer. The scanning electron microscope VEGA3 TESCAN was used to define the morphology of the samples. The thermogravimetric and mass spectrometric investigations were carried out using the NETZSCH STA 409 PC analyser equipped with the NETZSCH QMS 403C Aëolos mass spec- trometer. Initially hydrated samples were used. The hy- drated samples were obtained by slow cooling from 1100 to 150 °C (1 °C /min) under a flow of wet Ar. The ceramic samples were prepared for the electrical properties studies. The powders were pressed into pellets and then sintered at 1300 °C for 24 h in dry air. The pellets had a relative density of ~92% (the density of the sintered samples was determined by the Archimedes method). The electrical conductivity was measured with an impedance spectrometer Z-1000P, Elins, RF. The investigations were carried out from 1000 to 200 °C at a cooling rate of 1o/min under dry air or dry Ar. The dry gas (air or Ar) was prepared by circulating the gas through P2O5 (pH2O = 3.5·10−5 atm). The wet gas (air or Ar) was obtained by bubbling the gas at room temperature first through distilled water and then through a saturated solution of KBr (pH2O = 2·10−2 atm). 3. Results and discussions The phase attestation of the obtained compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 was made using the X-ray diffraction method. The results of the Le Bail analysis of the XRD-data are presented in Fig- ure 1. Both samples are single phase and have orthorhombic symmetry with P42/mnm space group. The introduction of bigger ions (Sr2+) into La3+-sublattice (𝑟La3+ = 1.216 Å, rSr2+ = 1.31 Å, 𝑟In3+ = 0.8 Å, 𝑟Ca2+ = 1.0 Å, 𝑟Mg2+ = 0.72 Å [47]) leads to an increase in the lattice parameter (Table 1). However, the additional introduction of Mg2+ and Ca2+ ions into the In3+ sublattice (co-doping) leads to a small decrease in the c lattice parameter and unit cell volume. The most probable reason for this is the local distortion of the crystal lattice due to the presence of several different cations in the cationic sublattices. In addition, the appear- ance of oxygen vacancies in the crystal lattice during accep- tor doping also contributes to the distortion: 2SrO La2O3 → 2SrLa ′ +2Oo × +Vo •• (1) 2Ca(Mg)O In2O3 → 2Ca(Mg)In ′ +2Oo × +Vo •• (2) Morphological analysis of the powder samples was car- ried out using scanning electron microscopy (SEM). The di- ameter of the grains was about ~2–5 μm (Figure 2). The typical EIS-plots are presented in Figure 3. The con- ductivity values were calculated using resistance values taken at the intersection of the high-frequency semicircle with the abscissa axis. The capacitance values for these semicircles was ~ 10–12 F/cm, which corresponds to the re- sistance of the grain volume of the polycrystalline sample. Figure 1 The results of XRD investigations for the compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 (a) and BaLa1.9Sr0.1In1.95Ca0.05O6.925 (b). 10 20 30 40 50 60 70 80 90 (a) 5 000 BaLa 1.9 Sr 0.1 In 1.95 Mg 0.05 O 6.925 15 000 10 000 2  In te n si ty 0 10 20 30 40 50 60 70 80 90 (b) 5 000 BaLa 1.9 Sr 0.1 In 1.95 Ca 0.05 O 6.925 15 000 10 000 2  In te n si ty 0 https://doi.org/10.15826/chimtech.2023.10.2.06 https://doi.org/10.15826/chimtech.2023.10.2.06 Chimica Techno Acta 2023, vol. 10(2), No. 202310206 ARTICLE 3 of 7 DOI: 10.15826/chimtech.2023.10.2.06 Table 1 Lattice parameters, unit cell volume and water uptake for the investigated compositions. Composition a, b (Å) c (Å) Vcell (Å 3) Water uptake (mol) BaLa2In2O7 5.914(9) 20.846(5) 729.33(6) 0.17 BaLa1.9Sr0.1In2O6.95 [42] 5.916(3) 20.870(4) 730.51(8) 0.18 BaLa1.9Sr0.1In1.95Mg0.05O6.925 5.916(3) 20.849(5) 729.78(6) 0.17 BaLa1.9Sr0.1In1.95Ca0.05O6.925 5.916(4) 20.852(0) 729.89(9) 0.17 Figure 2 SEM result for the composition BaLa1.9Sr0.1In1.95Ca0.05O6.925 Figure 3 The EIS plots for the composition BaLa1.9Sr0.1In1.95Ca0.05O6.925 obtained at 400 °C, 420 °C and 440 °C in dry air (a), and at 400 °C in dry and wet air (b). The temperature dependences of the conductivity ob- tained under dry/wet air/Ar are presented in Figure 4a and Figure 4b for the compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925, respectively. As can be seen, the conductivity values under dry Ar (pO2 ~ 10−5 atm) are lower than under dry air (pO2 = 0.21 atm), indicating the hole contribution to the electrical conductivity: Vo •• +1 2⁄ O2 ⇔ Oo × +2h• (3) The increase in water partial pressure leads to an in- crease in the conductivity values due to the formation of proton charge carriers: h• +1 2⁄ H2O+Oo × ⇔ 1 4⁄ O2 +(OH)o • . (4) Vo •• +H2O+Oo × ⇔ 2(OH)o • . (5) It should be noted that the conductivity values under wet air and wet Ar are very close at low temperatures, in- dicating the dominance of ion (proton) conductivity. Figure 5 represents the comparison of the conductivity val- ues for the co-doped compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 compositions with undoped BaLa2In2O7 and only Sr-doped BaLa1.9Sr0.1In2O6.95 composi- tions. As can be seen, co-doping leads to an increase in con- ductivity of up to one order of magnitude. The conductivity values increase in the BaLa2In2O7 − BaLa1.9Sr0.1In1.95Mg0.05O6.925 − BaLa1.9Sr0.1In1.95Ca0.05O6.925 − BaLa1.9Sr0.1In2O6.95 series, which correlates with the in- crease in the lattice parameter and unit cell volume. The activation energy values for the oxygen-ion and pro- ton conductivities were calculated and are presented in Ta- ble 2. As can be seen, the activation energy of the oxygen- ion conductivity of the co-doped compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 is lower than for undoped BaLa2In2O7 and only Sr-doped BaLa1.9Sr0.1In2O6.95 compositions. A more detailed study of these samples is required to explain this fact. Table 2 Activation energy values for the oxygen-ion and proton conductivities for the investigated compositions. Composition Ea for O 2–(eV) Ea for H +(eV) BaLa2In2O7 0.80 0.63 BaLa1.9Sr0.1In2O6.95 [42] 0.80 0.57 BaLa1.9Sr0.1In1.95Mg0.05O6.925 0.72 0.75 BaLa1.9Sr0.1In1.95Ca0.05O6.925 0.77 0.75 0 5 10 15 20 25 30 0 5 10 15 20 25 30 -Z '' , k  Z', k (a) BaLa 1.9 Sr 0.1 In 1.95 Ca 0.05 O 6.925 1 kHz 100 kHz1000 kHz 440 o C 420 o C 400 o C dry air 0 5 10 15 20 25 30 0 5 10 15 20 25 30 (b) -Z '' , k  Z', k BaLa 1.9 Sr 0.1 In 1.95 Ca 0.05 O 6.925 wet 400 o C dry https://doi.org/10.15826/chimtech.2023.10.2.06 https://doi.org/10.15826/chimtech.2023.10.2.06 Chimica Techno Acta 2023, vol. 10(2), No. 202310206 ARTICLE 4 of 7 DOI: 10.15826/chimtech.2023.10.2.06 Figure 4 The temperature dependences for the compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 (a) and BaLa1.9Sr0.1In1.95Ca0.05O6.925 (b). Figure 5 The temperature dependences of electrical conductivity for the compositions BaLa2In2O7, BaLa1.9Sr0.1In1.95Mg0.05O6.925, BaLa1.9Sr0.1In1.95Ca0.05O6.925, BaLa1.9Sr0.1In2O6.95 [42] in dry air (a), dry Ar (b), wet air (c), wet Ar (d). https://doi.org/10.15826/chimtech.2023.10.2.06 https://doi.org/10.15826/chimtech.2023.10.2.06 Chimica Techno Acta 2023, vol. 10(2), No. 202310206 ARTICLE 5 of 7 DOI: 10.15826/chimtech.2023.10.2.06 It should be noted that the co-doped BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 compositions contain 50% more oxygen vacancies than the BaLa1.9Sr0.1In2O6.95 composition. We can conclude that the change in the geometric characteristic of the unit cell volume has the most significant effect on the conductivity value than a change in the concentration of oxygen vacancies. The temperature dependences of the proton conductivi- ties, calculated as the difference between the ionic conduc- tivity under wet conditions (wet Ar) and dry conditions (dry Ar), are presented in Figure 6. The values of the activation energy of the proton conductivity calculated from Figure 6 are given in Table 2. The activation energy decreases from 0.63 eV for the undoped BaLa2In2O7 composition to 0.57 eV for the BaLa1.9Sr0.1In2O6.95 composition by introducing only strontium as a dopant. Interestingly, co-doping leads to a decrease in the values of the activation energy of proton conduction. The same regularity of increase in conductivity values for the doped compositions is observed. The values of the proton concentrations are required for the correct analysis of these dependences. Figure 7 shows the results of thermogravimetry (TG), mass spectrometry (MS) and differential scanning calorime- try (DSC) analysis for the BaLa1.9Sr0.1In1.95Ca0.05O6.925 compo- sition. Mass loss occurs at the temperatures below 700 °C (TG curve) and is solely due to the release of water (MS(H2O) curve). The values of water uptake are close for all doped BaLa1.9Sr0.1In1.95M0.05O6.925 (M = Mg, Ca), BaLa1.9Sr0.1In2O6.95 and undoped BaLa2In2O7 compositions and are about ~ 0.17−0.18 mol of water per formula unit (Table 1). In other words, the proton concentration (𝑐H+) is close for all compo- sitions. Consequently, the increase in proton conductivity (𝜎H+) x is due to the increase in proton mobility (𝜇H+): 𝜎H+ = 𝑧 ∙𝑒 ∙ 𝜇H+ ∙ 𝑐H+. (6) We conclude that co-doping positively affects the ionic conductivity of BaLa2In2O7. An increase in lattice parame- ters leads to facilitation of oxygen-ion transport, which, in turn, leads to an increase in oxygen-ion conductivity. At the same time, co-doping also leads to an increase in proton conductivity, probably due to an increase in proton mobil- ity. The proton conductivity values for co-doped composi- tions are 2.9∙10−6 S/cm and 5.4∙10−6 S/cm at 400 °C for the compositions BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925, respectively. The comparison of the electrical conductivity values ob- tained in wet air for the undoped BaLa2In2O7, monodoped BaLa1.9Sr0.1In2O6.95 and co-doped BaLa1.9Sr0.1In1.95M0.05O6.925 (M = Mg, Ca) compositions with known proton conductors such as doped barium and strontium ceramics is shown in Figure 8. The conductivity of the studied compositions under wet conditions is lower than that of doped barium and stron- tium cerates. Nevertheless, acceptor doping can increase conductivity values by up to ⁓1.5 orders of magnitude. Figure 6 The temperature dependences of protonic conductivity for the compositions BaLa2In2O7, BaLa1.9Sr0.1In1.95Mg0.05O6.925, BaLa1.9Sr0.1In1.95Ca0.05O6.925, BaLa1.9Sr0.1In2O6.95 [42] Figure 7 The results of TG, DSC and MS(H2O) investigations for the composition BaLa1.9Sr0.1In1.95Ca0.05O6.925. Figure 8 The temperature dependences of conductivity obtained in wet air for compositions BaLa1.9Sr0.1In2O6.95 [42], BaLa1.9Sr0.1In1.95Mg0.05O6.925, BaLa1.9Sr0.1In1.95Ca0.05O6.925, BaLa2In2O7, BaCeO3 (10 mol.% Y2O3) [48], SrCeO3(10 mol.% Y2O3) [48] https://doi.org/10.15826/chimtech.2023.10.2.06 https://doi.org/10.15826/chimtech.2023.10.2.06 Chimica Techno Acta 2023, vol. 10(2), No. 202310206 ARTICLE 6 of 7 DOI: 10.15826/chimtech.2023.10.2.06 4. Limitations Firstly, we have some limitations in the measurement of electrical conductivity. The maximum frequency measured with the Elins Z-1000P impedance spectrometer is 1000 kHz. The measurement of the conductivity at higher frequencies will give a more accurate representation of the EIS-plots at T > 500 oC. Secondly, a full explanation of the activation energy val- ues obtained for co-doped compositions is difficult at this stage of the study. For the BaLa2In2O7 composition, which is co-doped in the A and B sublattices, a more detailed study of the ion transport mechanisms is required. Thirdly, doping the BaLa2In2O7 composition leads to an increase in electrical conductivity of up to 1.5 orders of magnitude. However, the conductivity of the compositions studied in this article is lower than that of doped barium and strontium ceramics. 5. Conclusions In this paper, we performed acceptor Sr2+→La3+ and M2+→ In3+ (M = Mg2+, Ca2+) co-doping in the cationic sub- lattices of the bilayer perovskite BaLa2In2O7. The bilayer perovskites BaLa1.9Sr0.1In1.95Mg0.05O6.925 and BaLa1.9Sr0.1In1.95Ca0.05O6.925 were obtained and investigated for the first time. The phase attestation, morphology, pos- sibility of water uptake and electrical conductivity were in- vestigated and discussed. The doping effect on the oxygen- ion and proton conductivity was revealed. It was shown that cationic co-doping leads to an increase in proton con- ductivity values of up to ~0.8 orders of magnitude. ● Supplementary materials None. ● Funding The research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority- 2030 Program) is gratefully acknowledged. ● Acknowledgments None. ● Author contributions Conceptualization: N.T., I.A. Data curation: A.B., N.T. Methodology: N.T., I.A. Investigation: A.B., E.A., I.F., P.C., E.V. Validation: A.B., N.T. Visualization: A.B., E.A., N.T. Writing – original draft: N.T. Writing – review & editing: N.T., A.B., ● Conflict of interest The authors declare no conflict of interest. ● Additional information Author IDs: Natalia Tarasova, Scopus ID 37047923700; Anzhelika Bedarkova, Scopus ID 57195274932; Irina Animitsa, Scopus ID 6603520951. Website: Ural Federal University, https://urfu.ru/en. References 1. Zhang W, Hu YH. Progress in proton-conducting oxides as electrolytes for low-temperature solid oxide fuel cells: From materials to devices. Energy Sci Eng. 2021;9:984–1011. doi:10.1002/ese3.886 2. Nayak AP, Sasmal A. Recent advance on fundamental proper- ties and synthesis of barium zirconate for proton conducting ceramic fuel cell. J Cleaner Product. 2023;386:135827. doi:10.1016/j.jclepro.2022.135827 3. Guo R, He T. High-entropy perovskite electrolyte for protonic ceramic fuel cells operating below 600 °C. ACS Mater Lett. 2022;4:1646–1652. doi:10.1021/acsmaterialslett.2c00542 4. Wang C, Li Z, Zhao S, Xia L, Zhu M, Han M, Ni M. 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