Synthesis and Characterization of BaCe0.4Zr0.4Pr0.2O3−δ Mixed Conductor Perovskite


 Compounds based on barium cerates and zirconates Ba(Ce,Zr)O3−δ are oxides able to transport protons through their crystal lattice by proton hopping between oxygen sites. This feature makes them potential candidates as hydrogen sensors, membranes for hydrogen purification and isotopic exchange, and electrolyte for Proton Conducting Solid Oxide Fuel Cell (PC-SOFC) and Solid Oxide Electrolyzer Cells (PC-SOEC). Pr-doping on these family compounds decreases sintering temperature and introduces electronic conductivity. This work presents a systematic study of the high temperature properties of the BaCe0.4Zr0.4Pr0.2O3−δ (BCZP) perovskite in view of its potential use for these applications. At room temperature, BCZP presents rhombohedral structure, which transforms reversibly to cubic at 550 °C in dry air. Electrochemical measurements under dry air, wet air with water vapor (~ 2% H2O) and heavy water vapor (~ 2% D2O) were employed to describe bulk ionic and electronic transport above 200 °C. The total conductivity data exhibit behavior changes between 400 and 600 °C which could be associated to a crystal structure transition. Below 600 ºC BCZP presents low total conductivity, which limits its applications as single phase cathode material for electrochemical applications. However, the material has good thermomechanical compatibility with electrolytes (~ 11 × 10− 6 K− 1) and high CO2 tolerance (T > 900 ºC). These properties suggest that this compound could be used as oxide support for composite or impregnated electrodes, buffer layer to improve the performance of another cathode material or gas separation co-ionic membranes.


Introduction
Proton conducting oxides are interesting materials for economic and sustainable energy conversion and storage devices such as protonic ceramic fuel cells, protonic ceramic electrolysis cells, gas puri cation and separation membranes, syn-gas membrane, ammonia synthesis, etc. [1][2][3][4].
Solid Oxide Fuel Cells (SOFC) are electrochemical devices based on oxides that transform the chemical energy from hydrogen or other fuels into electricity and heat. Proton conductor SOFC (PC-SOFC) have the particularity that electrolytes exhibit protonic conduction. These devices presents high e ciency, low environmental impact, operating temperatures between 400 and 800 ºC, good stability and durability [5][6][7][8]. The search for new materials for PC-SOFC is a major topic in materials research. In the case of electrodes, different synthesis routes and compositions have been studied to obtain porous materials with small particle sizes and high purity, to ensure a high active surface area, i.e. low polarization resistance and power density increase [9,10]. Several compositions were proposed as PC-SOFC cathodes: BaCe 0.4 Sm 0.2 Fe 0.4 O 3−δ [11], NdBaCo 2 O 5+δ [12], Ba 0.5 Sr 0.5 Fe 0.8 Cu 0.2 O 3−δ [13,14], Sm 0.5 Sr 0.5 Fe 0.8 Cu 0.2 O 3−δ [15,16], etc. They were tested as single cells, reaching power density values between 150 and 500 mWcm − 2 in the temperature range of 600 to 700 ºC. In addition, mixed proton-ionic and electronic conductors as well as co-H + /O − 2 ionic conductors are proposed for technological applications, such as gas separation membrane reactors in non-oxidative dehydrogenation processes [17][18][19] or high temperature humidity sensors [20].
On the other hand, it was reported that the incorporation of Pr in barium cerates and zirconates decreases sintering temperatures, improves the mechanical properties, increases CO 2 tolerance and introduces Mixed Ionic Electronic Conductivity (MIEC) [21][22][23][24][25][26][27] both thermal treatments, powders were uniaxially pressed into pellets at 50 kgcm − 2 using a 10 mm matrix. In cases where powders were required, the dense pellets were grinded in mortar. The phase purity and crystallographic structure were evaluated by X-Ray diffraction (XRD), by using PANalytical Empyrean diffractometer with Bragg Brentano geometry, Cu K α radiation, a graphite monochromator and a PIXcel 3D detector. Structural and microstructural parameters were obtained from the diffraction pattern by using the Rietveld method and the Fullprof Suite software [28]. The instrumental line broadening was obtained from XRD standard.
Microstructure of dense pellets was observed by Scanning Electron Microscopy (SEM) with a ZEISS Crossbeam 340 microscope. In addition, a powder sample was studied by Transmission Electron Microscopy (TEM) by using a TECNAI F20 G2 FEI microscope. Both TEM and SEM microscopes are equipped with Energy Dispersive Spectroscopy (EDS) detectors for elementary analysis.
Crystal structure as a function of temperature was studied by XRD, between room temperature and 800 °C under dry 20% O 2 /He atmosphere. The heating and cooling rates were 10 °Cmin − 1 with dwell time of 10 min before measurements. Diffraction patterns at high temperatures were collected by using an Anton Paar HTK1200N furnace coupled to the PANalytical Empyrean diffractometer. Structural and experimental parameters at high temperatures were obtained by using sequential mode of the Fullprof Suite software [28].
The mass evolution of BCZP hydrated powder was studied by Thermogravimetry (TG), using a Cahn 1000 electrobalance [29]. Previously, the powder was annealed at 200 ºC under wet air during 4 hours. Reversibility of water incorporation into the structure was studied by thermal cycling from room temperature to 900 °C with a heating/cooling rate of 5 °Cmin − 1 under atmospheric air. The thermal behavior for the same sample was analyzed by Differential Scanning Calorimetry (DSC) with a Modulated DSC 2910 TA Instruments at a heating/cooling rate of 5 °Cmin − 1 between room temperature and 500 °C under air.
The density of fresh dense pellet of BCZP was measured by Archimedes method through hydrostatic weighing using the same electrobalance and diethyl phthalate as the immersion uid [30,31]. CO 2 tolerance must be evaluated through accelerated ageing tests due to Ba basicity and some gas impurities could affect the performance of the material under operating conditions. The accelerated ageing test of BCZP powders was performed by using TG under 10% CO 2 /Ar, between room temperature and 1000 °C, with a heating/cooling rate of 5 °Cmin − 1 .
Linear expansion coe cient of dense BCZP pellets was determined by dilatometry, between room temperature and 900 °C, under atmospheric air by using a heating/cooling rate of 1 °Cmin − 1 . The measurements were performed by using LINSEIS L75VS1000C vertical dilatometer.
The total electrical conductivity measurements were performed by the Two Probe Method (TPM) using an Agilent 3497A multimeter. The evolution of conductivity with temperature was studied using a square bar geometry and a heating/cooling rate of 1 °Cmin − 1 under air. The total electrical conductivity of the BCZP bar was measured between room temperature and 900 °C under (i) dry synthetic air (20% O 2 /N 2 ), (ii) synthetic air with water vapor (~ 2% H 2 O), and (iii) synthetic air with heavy water vapor (~ 2% D 2 O). In addition, transport mechanisms were evaluated by Electrochemical Impedance Spectroscopy (EIS) on disc samples. Measurements were performed by using an Autolab PGSTAT30 potentiostat/galvanostat from 1 MHz to 0.1 Hz with 50 mV amplitude between 100 and 600 ºC under same conditions (i.e., dry air, wet air with water vapor, and wet air with heavy water vapor). The spectra were tted with Equivalent Electrical Circuits (EEC), by using Zview2 software considering electrical resistance in parallel with a constant phase element (R//CPE). The dense bars and discs were painted on both sides with Pt ink.
Results And Discussion Figure 1 shows the diffraction pattern of BCZP powder under atmospheric air at room temperature. The X-Ray data shows a single phase and it was indexed in the rhombohedral symmetry, R-3c space group, in agreement with previous works on barium cerates and zirconates with Pr content [32][33][34]. Wyckoff positions for Ba, (Ce,Zr,Pr) and O were assigned as 6a, 6b and 18e, respectively. From Rietveld method, the calculated lattice parameters (a, b, c) were: a = b = 6.087(1) Å and c = 14.958(1) Å. The angles (α, β, γ) were considered as α = β = 90 º and γ = 120 °. Also, the microstructural parameters such as average crystallite size (CS) and isotropic strains (IS) were calculated from the XRD pattern re nement, being 317(1) nm and 40.0(1) %%, respectively. Heras-Juaristi et al. and Antunes et al. indicate that low Prcontent on barium cerates and zirconates increases the phase transition temperature, and high Pr content induces several phase transitions to lower symmetry structures (Pnma, Imma, etc.) [32][33][34]. The TEM characterization of BCZP powder is complementary to XRD. Figure 2a displays a low-resolution image with an overall particle size of ~ 1 μm. Figures 2b and 2c show a high resolution TEM image and the Selected Area Electron Diffraction (SAED) pattern, respectively. The TEM interplanar distances are consistent with the assumption of a rhombohedral symmetry corresponding to the R-3c space group. Figure 2d presents the microstructure of a sintered pellet of BCZP where it can be observed a micrometric grain size ranging from ~ 2-5 µm and some degree of surface porosity, with pores of ~ 2 µm. A density of 6.02 gcm -3 was determined by Archimedes method, which corresponds to ~ 95 % of the theoretical density (6.36 gcm -3 by XRD). Similar compositions such as Ba(Ce,Zr,Y,Pr)O 3-δ [35], Ba(Ce,Zr,Gd,Pr)O 3-δ [36] and Ba(Ce,Y,Pr)O 3-δ [37] were also synthetized by SSR and sintered between 1400 and 1650 ºC with relative densities higher than 90 % (i.e., porosities < 10 %). The good sintering capability of these compounds is related with Pr-content in the structure.
MIEC properties (oxygen non-stoichiometry, hydration degree of oxygen vacancies, electronic charge carriers, etc.) and the operating temperature determine the different energy application perspectives for this material. It is desirable that oxygen vacancies are saturated by water vapor to improve protonic conductivity allowing to decrease operating temperatures of the electrochemical devices. Therefore, the hydration process of oxygen vacancies and its reversibility for BCZP was studied by TG. Figure 3a compares mass change of BCZP wet powder, under atmospheric air (with moist), between room temperature and 900 °C. During the heating rate, a slight increase of mass between 100 and 200 °C could be associated to water uptake from atmospheric air. A similar compound, BaCe 0.8 Pr 0.2 O 3-δ perovskite, did not evidence a mass variation under oxidizing atmosphere [21,24]. In this sense, the TG plot shows a structural water loss above 300 ºC. Also, there is not a signi cant mass variation between 600 and 950 ºC which indicates that the oxygen content remains almost constant and no cation reduction occurs due to oxygen loss [38]. It was assumed a negligible CO 2 adsorption/desorption and/or BaCO 3 formation during thermal cycling, due to the high carbonation resistance of this material (see CO 2 tolerance test, Figure 7). In addition, a hysteresis process was observed between the heating and cooling cycle.
Lyagaeva et al. observed the same trend studying the hydration/dehydration process of Ba(Ce,Zr,Y)O 3-δ proton conductors by TG under air. These authors associated differences on heating and cooling cycles to kinetic processes [39]. A DSC study was performed to detect at which temperature the water is lost. During the heating run, a broad endothermic peak appears centered at ~ 320 ºC (see Figure 3b). Although, Ohzeki et al. reported a reversible 2 nd order phase transition of R-3c to Pm-3m at 895 ºC for BaCeO 3-δ [40], this is discarded due to the fact that the phase transition of BCZP occurs close to 550 °C (see phase transition, Figure 4). Moreover, we observed the same response at 280 ºC for the perovskite Ba 0.5 Sr 0.5 Fe 0.8 Cu 0.2 O 3-δ [38] due to water and CO 2 loss. According to these results, the endothermic peak at 320 ºC corresponds to structural water loss instead of a 2 nd order phase transition. Moreover, the temperature region of weight loss in the TG measurement agrees well with the DSC result.
Previous results demonstrated that the loss or incorporation of interstitial protons into the crystal structure cause distortions of lattice parameters and ionic conductivity [31]. BCZP was studied by HT-XRD under dry synthetic air to determine if such distortions are present in this compound. A phase transition R-3c to Pm-3m occurs between 550 and 575 °C (see Figure 4a), in agreement with phase transitions detected between 400 and 600 ºC in similar compounds [32][33][34]. Figure 4b shows the variation of the pseudocubic lattice parameter with temperature during the heating run. A subtle slope change indicates a 2 nd order phase transition between 400 and 600 °C. There is no difference between the cooling or heating behavior for the lattice parameters as a function of temperature. The lattice distortions by water incorporation/loss could not be resolved due to the low oxygen non-stoichiometry and degree of hydration. However, TG and DCS con rmed the proton incorporation on BCZP. Electric charge transport of MIEC materials for PC-SOFC is given by hopping of interstitial protons between oxygen sites [7] and the electronic transport associated to small polaron conductivity [43]. High electronic and protonic conductivities are necessary in order to improve ORR and proton diffusion. The total electrical conductivity measured by TPM should be a representative of the MIEC conductivity. To clarify this point, we used TPM instead of Four Probe Method due to the high resistance values of BCZP bar and negligible errors during the measurements. Otherwise, EIS technique should discriminate the transport mechanisms of the material. Therefore, TPM and EIS techniques give us complementary information. Figure 6a shows the Arrhenius plots of total conductivity by TPM under air ow. For example, the total conductivity values are 250, 64, 5 and 0.09 µS/cm at 800, 600, 400 and 200 ºC, respectively. Also, this plot shows three different zones independently of the water content and hydrogen isotope (H/D). The change in the activation energies from one zone to other suggests a change in the transport mechanism. Below 300 ºC when H 2 O is incorporated into the structure, the conductivities have a high ionic transport component (possibly mixed protonic-oxygen vacancy conductivity), whereas, once sample is dehydrated above 600 ºC, an electron-hole transport mechanism would be dominant. However, between 400 and 600 ºC the conductivity is apparently affected by the 2 nd order phase transition. The ionic dominating nature of conductivity at low temperature is evidenced by EIS, showing a capacitive component typical of ionic diffusive process (see Figure 6b). To assure that conductivity measurements using low heating rates were near-equilibrium condition, different tests were performed at xed temperatures between room temperature and 800 °C and in all cases conductivity values remain similar to the ones acquired during heating/cooling. Although previous experiments (see Figure 3) indicate water incorporation into the structure, the electrical measurements are not sensible enough to distinguish changes by change in water content and H/D nature, mainly since the high electronic conductivity with respect to ionic conductivity prevents the observation of these changes on the whole temperature range. Also, no signi cant differences were observed between impedance spectra under dry, H 2 O and D 2 O vapor atmospheres. Nevertheless, high frequency arcs present capacitance values ~ 2 pF/cm, which are associated with bulk ionic transport (b) [44]. Medium and low frequency arcs related to grain boundaries (gb) and ORR, respectively, were not detected during measurements in the whole temperature range. The bulk frequency values increase with temperature, and the arcs are no longer observed by EIS at T > 300°C . Conductivity values between 400 and 600 °C can be extrapolated from lower temperature values (T < 200 °C), where impedance arcs are clearly observed. Similar behaviors were observed for BaCe 0.9 Tb 0.1 O 3−δ [44] and Ba(Ce,Pr)O 3-δ [21,24] perovskites under air. Magrasó et al. proposed that Prdoping produces mixed conductor materials, where electrons and electron-holes are the minority charge carriers and defects concentration is dominated at low temperature by protons and at high temperature by oxygen vacancies [26]. These authors suggest that electron-hole conductivity prevails at high pO 2 , proton conductivity becomes more signi cant at low temperature, low pO 2 and wet conditions, whereas the n-type conductivity is signi cant at low pO 2 and high temperatures [26]. Fabbri et al. calculated the transport numbers for Ba(Zr,Pr,Y) compounds from electrical conductivity measurements as a function of temperature and wet pO 2 . [45]. These authors indicate that these perovskite presents mixed conductivity between 500 and 700 ºC. However, Heras-Juaristi et al. studied Ba(Ce,Zr,Pr,Y)O 3−δ perovskites by EIS and observed no difference in the electrical conductivity between wet and dry air for low Pr content sample (x = 0.2), due to the dominating character of the electron−hole component at such high oxygen activities [32]. These authors suggested that at high pO 2 , electron−hole (p-type) conductivity is higher with respect to protonic (see Equation 1). On the other hand, water incorporation would be signi cant with respect to electron-hole formation at low pO 2 (see Equation 2). Although oxygen vacancies are consumed in both cases, Equation 1 indicates oxygen sites production, meanwhile Equation 2 indicates sites consumption.
Activation energy (E a ) calculated from EIS data is 0.51(1) eV, the same value obtained by TPM between 100 and 300 ºC (see Figure 6a and c). This E a is comparable to the values for proton transport mechanism, but also to activation energies reported for other MIEC compositions with Pr as Ba(Zr 0.7 Ce 0.2 ) 0.8 Pr 0.2 Y 0.1 O 3−δ [32] and Ba(Ce,Pr)O 3-δ [21,24]. Besides, the electrical conductivity values are too low as compared to these materials (~ 2 µS/cm at 200 ºC) [21,24,32], but they are comparable to the values obtained for BaZr 0.8 Pr 0.2 O 3−δ [43]. Then, all evidences con rm that BCZP presents mixed ionic and electronic conductivity. The low electrical conductivity does not ful ll the requirements for its application as PC-SOFC single phase cathode, although it would be enough as support in a composite electrode or gas separation membranes. CO 2 tolerance must be guaranteed under speci ed operating conditions. Therefore, ageing tests were carried out under harsh temperature and CO 2 concentrations in order to estimate the material stability under SOFC operating condition. Figure 7a shows the weight change with temperature for BCZP powder between room temperature and 950 °C under 10 % CO 2 /Ar, where no signi cant weight change is observed. The high stability was con rmed by XRD post CO 2 thermal treatment, where no secondary phases were detected (see Figure 7b). In the same way, Antunes  At room temperature, BCZP presents rhombohedral structure, which transforms reversibly to cubic symmetry at 550 °C in dry air. Also, there is a good agreement between HT-XRD and dilatometric data, i.e., TEC ≈ 11 × 10 − 6 K − 1 and the 2nd order phase transition at 550 °C. TG and DSC data indicated the incorporation of a small amount of water into the structure below 300 °C. Chemical stability under CO 2rich atmosphere was con rmed by TG.
The electrical conductivity was studied by TPM and EIS under dry and wet air, with water vapor (~ 2% H 2 O) and heavy water vapor (~ 2% D 2 O). Impedance spectra above 200 °C suggest mixed ionic bulk and electronic transport conduction. A transition in total conductivity was observed between 400 and 600 °C. According to this set of measurements, BCZP would present ionic conductivity below 300 ºC. BCZP low conductivity limits its applications as single phase PC-SOFC cathode. However, its high stability at PC-SOFC operating temperatures, good thermomechanical compatibility with electrolytes and excellent CO 2 tolerance indicate that this compound could be used as oxide backbone for composite or impregnated electrodes, buffer layer, gas separation co-ionic membranes or another electrochemical application.