One of the key positions of modern materials science is the research and development of solid oxide fuel cells and their components, including electrolytic membrane materials. It is necessary to find inexpensive and technological solid electrolyte with high conductivity and stability at high temperature, in oxidizing and reducing atmosphere. Medium temperatures are the most optimal region in terms of energy costs. Prospective ionic conductors for this temperature range are proton electrolytes based on the complex oxides.
The most studied method for modification of their structure and optimization of physicochemical properties is a homogeneous cationic doping [1–5]. However, the homogeneous anionic doping is a new promising way for the obtaining of new materials with improved properties. Earlier, we have reported a new route for increasing oxygen-ion and proton conductivities by F -doping of brownmillerite Ba In O . It has been proved that small F -concentrations can improve the oxide-ion (mixed anion effect) and the proton conductivities. The other prospective method of improving transport properties is a heterogeneous doping. The composites based on Ba In O with chemically inert Ba InNbO oxide phase as a heterogeneous dopant demonstrate significant increase of conductivity level. The maximum conductivity corresponds to the ratio of the components 0.7:0.3 . In this work the possibility of application of simultaneous homogeneous and heterogeneous doping has been described for the first time. The composite 0.7Ba In O F 0.3Ba InNbO has been chosen for the investigation.
We used in situ solid-state method for preparing the composite. This method consisted in simultaneous synthesis of the components from starting materials in the same reaction mixture:
(1) using temperature treatments 800–1300 C, six stage 24h each.
The X-ray powder diffraction (XRD) measurements were made on a Bruker Advance D8 diffractometer with Cu K radiation. The crystal structure of the sample was determined through Rietveld refinement using FULLPROF software.
The surface morphology and local chemical composition were studied using a workstation AURIGA CrossBeam (Carl Zeiss NTS) and JEOL JSM 6390 LA scanning electron microscope with console JEOL JED-2300. The detection limit at ordinary energies (5–20 kV) was 0.5 at.%; the concentration measurement error was 2%.
Thermogravimetric analysis was carried out on STA (Simultaneous Thermal Analyzer) 409 PC analyzer (Netzsch) coupled with a quadrupole mass spectrometer QMS 403 C Aëolos (Netzsch). For the preparation of hydrated forms of the specimens, the powder samples were hydrated at slow cooling from 900 to 200 C (1 C/min) under a flow of wet air (pH O = 2·10 atm). The cooling was performed to a temperature not lower than 200 C to avoid the appearance of adsorbed water. The hydrated forms of the samples were heated at the rate of 10 C/min in a corundum crucible under a flow of argon.
The ceramics used for the electrical measurements were prepared by pressing disk-shaped samples at 250–300 MPa and sintering them at 1300 C for 24h in dry air. After polishing, the platinum paste electrodes were applied from both sides of the samples by painting and fired at 900 C for 3h. The ac conductivity of the samples (2-probe method) was measured using a Z-1000P (Elins) impedance spectrometer within the frequency range of 1-10 Hz. The conductivity measurements were carried out under dry and wet air varying the temperature and partial oxygen pressure pO . The bulk resistance was calculated from a complex impedance plot using the Zview software fitting. The `wet' air was obtained by bubbling the gas at room temperature first through distilled water and then through the saturated solution of KBr (pH O = 2·10 atm). The `dry' air was produced by flowing the gas through P O (pH O = 3.5·10 atm). The humidity of gases was measured by H O-sensor (`Honeywell' HIH-3610).
According to XRD analysis, the sample contained two phases – cubic perovskite type Ba InNbO phase (Pm3m space group) and brownmillerite type Ba In O F phase (I4cm space group) with partial disordering of oxygen vacancies. The lattice parameters were a = 4.142 Å for Ba InNbO and a = 5.950(2) Å, c = 16.813(9) Å for Ba In O F . They were in a good agreement with previously reported data . XRD-pattern for 0.7Ba In O F 0.3Ba InNbO refined by Rietveld analysis is presented in Figure 1.
The morphology of the samples was studied by scanning electron microscopy (SEM) (Figure 2). It can be seen for individual phases Ba In O F (Figure 2(a)) and Ba InNbO (Figure 2(b)) and for composite system (Figure 2(c)) that the grain size was approximately 5–10 μm and the grain boundaries were clean. The microelement analysis showed the presence of all main elements in the samples.
Thermal analysis of the composite system showed that the composite changed mass at temperatures 300–500 C in wet atmosphere (pH O = 2 10 atm), which corresponded to the processes of removing water molecules (Figure 3). The maximal water uptake for composite system is proportional to the content of the phase Ba In O F with incompletion in the oxygen sublattice and is 0.60 mole H O per formula 0.7Ba In O F 0.3 Ba InNbO . The Ba InNbO phase is nominally complete in the oxygen sublattice and is capable of absorbing only small amounts of water due to an insignificant change in stoichiometry during the synthesis. Thus, the main amounts of proton defects are concentrated in the grains of the phase Ba In O F .
The conductivity measurements were carried out under dry (pH O = 3.5·10 atm) and wet (pH O = 2·10 atm) air by varying the temperature (250–1000 C) (Figure 4). The conductivity values of composite system 0.7Ba In O F 0.3 Ba InNbO are significantly higher than those for both undoped Ba In O composition and F-doped Ba In O F one in the whole temperature range. The increasing in conductivity under wet air for the composite proves the ability of the sample to the proton transfer.
In this article, the possibility of application of simultaneous homogeneous and heterogeneous doping has been described for the first time. The composite 0.7Ba In O F 0.3Ba InNbO has been obtained by in situ solid-state method. It has been proved that composite sample is capable for water uptake and for proton transfer. The increasing in the conductivity values for the composite sample comparing with conductivity for the individual phases allows concluding that simultaneous homogeneous and heterogeneous doping is prospective method for obtaining high-conductive proton electrolytes.
This work was supported in parts by the Ministry of Education and Science of the Russian Federation (State Task 4.2288.2017) and by Act 211 Government of the Russian Federation, agreement 02.A03.21.0006. The equipment of the Ural Center for Shared Use `Modern nanotechnology' SNSM UrFU was used.