Structural and electrochemical characterization of BaCe0.7Zr0.2Y0.05Zn0.05O3 as an electrolyte for SOFC-H

As a potential electrolyte for proton-conducting solid oxide fuel cells (SOFC-Hs) and to get better protonic conductivity and stability, zinc doped BCZY material has been found to be promising. In this study, we report a new composition of proton conductors BaCe0.7Zr0.2Y0.05Zn0.05O3 (BCZYZn5) which was investigated using XRD, SEM and conductivity measurements. Rietveld refinement of the XRD data revel a cubic perovskite structure with Pm-3m space group. BaCe0.7Zr0.2Y0.05Zn0.05O3 shows cell parameter a = 4.3452(9) Å. Scanning electron microscopy images shows that the grain sizes are large and compact which gives the sample high density and good protonic conductivity. The total conductivity in wet atmosphere is significantly higher than that of dry condition and the conductivity was found to be 0.276 × 10-3 Scm-1 and 0.204 × 10-3 Scm-1 at 600°C in wet and dry Ar, respectively. This study indicated that perovskite electrolyte BCZYZn5 is a promising material for the next generation intermediate temperature solid oxide fuel cells (IT-SOFCs).


Introduction
These invention of Solid Oxide Fuel cells (SOFC) brings a great advantage in renewable and sustainable energy system for future generation. This is the most efficient device among the energy technology invented so far [1]. In recent years proton conducting electrolytes brings a great advantage in SOFCs technologies to operate in intermediate temperatures. They have a growing interests in protonconducting oxide electrolytes for SOFCs and a wide range of technological applications in fuel cells, batteries, gas sensors, hydrogenation or dehydrogenation of hydrocarbon electrolysers [2][3][4]. A lots of perovskite type oxides show high proton conductivity in H2 and H2O containing atmospheres at relatively low operating temperatures (400-700°C), low activation energy and high efficiency [5,6]. Finding the best proton conducting and proper compromise perovskite material between conductivity and chemical stability is a great challenge. To get highly densified materials at low sintering temperature for proton conducting electrolyte is also a major challenges. BaCeO3-based materials may exhibit mixed ionic (oxide ions and protons) conduction [7,8]. Doped barium cerates (BaCeO3) possess high proton conductivity and good sintering behavior [9,10], though they are unstable in CO2 and steam, destroying the perovskite structure [11], which is essential to the maintenance of high proton conductivity. On the other hand doped zirconates which offer low proton conductivity and poor sinter ability but high chemical stability in CO2 and H2O atmospheres [12][13][14][15][16]. In combining these two doped, an apparent optimized material could be obtained. In recent reports the solid solutions of BaCeO3 and BaZrO3 combined the high proton conductivity of barium cerate with the good chemical stability of barium zirconate [17,18], despite the fact that the sintering temperature was still very high [19]. Recently it was reported that the better chemical stability obtained by partial substitution of Zr +4 cations into Ce +4 cations as an electrolyte material [20,21]. The introduction of small amount of Zn at B-site into the structure allows a reduction in high sintering temperatures and a remarkable improvement in the stability, relative density and conductivity [22]. As a potential high performance proton conductor BCZYZ has been offering the benefits of both stability and increased proton conductivity [23]. These oxides also exhibit mixed proton and oxide-ion conductivity [24]upon exposure to humid atmosphere. This new material for proton conducting electrolyte offers to minimize those challenges. In this study we report a new proton conducting perovskite series BaCe0.7Zr0.2Y0.05Zn0.05O3 to overcome those major challenges. We have studied the thermal and electrochemical properties of the series using XRD, SEM and conductivity measurements.

Experimental
The zinc doped BCZY powder materials were prepared by using solid state reaction method. Stoichiometric amount of BaCO3, CeO2, ZrO2, Y2O3 and ZnO were mixed by ball milling with zirconia balls and acetone to ensure thorough mixing for 24 h. The finely ground dried materials were fired at 1000°C for 8 h with a fixed heating rate of 5°C min-¹ and subsequently ground and palletized using 13mm diameter die and Kennedy Hydraulic Bench Press under pressure of 202 MPa (3 ton) and sintered at 1200°C in air for 18 h with a fixed heating rate of 5°C min-¹. The final sintering temperature was 1400°C in air for 22 h with a fixed heating rate of 5°C min-¹ using Nabertherm box furnace.
The phase structure, purity, identity and homogeneity of the mixed oxide was examined by at ambient temperature using Shimadzu-7000 diffractometer (CuK1=1.5406 Å) in the 2 range from 10° to 80°. The data were collected with a 0.01 o step size and a count time of 60 s/step. The obtained data were refined by the Rietveld method using the FullProf software [25]. The microscopic features of the prepared electrolytes were characterized using sub-nanometer resolution Scanning Electron Microscope (JSM-7610F). The SEM data were collected in atmospherically-isolated chamber.
To measure the deformation of the sample materials under non-oscillating stress against time or temperature, with programmed temperature using SETARAM Instrumentation-SETSYS Evolution TMA S60/28682-LCT10414-3. For the BCZYZn5 material, the thermal expansion coefficients properties are measured by thermomechanical analysis. To determine both the size of particles and their state of distribution of BCZYZn5 material, Horiba Particle Size Distribution Analyzer LA-920 and Horiba Reservoir Unit LY-201 were used in wet mode.
The conductivity of the sample was analyzed by using Impedance Spectrometry. A Solartron 1260 frequency response analyzer connected to a ProboStat (NorECs, Norway) conductivity cell was used to measure impedance over the frequency range 6 MHz to 1Hz and the applied sine wave amplitude was 1 V rms. a sintered pellet of the as-prepared material, 13 mm in diameter, and with approximately 0.5 cm 2 platinum paste electrodes were used. Impedance data were initially collected on cooling from 1000 to 150°C (cooling cycle) in 50ºC steps under flowing dry and wet Ar. The gas was passed through two beds of P2O5 desiccant before entering the conductivity cell and is called the 'Dry Ar'. Whereas, when Ar gas was flowed through water at 22ºC (p(H2O) = 0.026 atm) and the measurement is called 'Wet Ar'. Each time, sufficient time was allocated at each temperature to ensure equilibrium before spectra was recorded. The least squares refinement program Z-View (Scribner Associates Inc.) was used to fit the obtained impedance data.

X-ray Diffraction Analysis
Rietveld refinement of XRD data shows the BaCe0.7Zr0.2Y0.05Zn0.05O3 material as cubic symmetry in the space group Pm-3m. Table 1 shows the unit cell parameter is a = 4.3452(9) Å.  1. Rietveld analysis of X-ray diffraction data for BCZYZn5 as Cubic. Figure 1 shows the Rietveld refinement of XRD of BCZYZn5 in cubic symmetry. The refinement converged quickly with significant improvements to the fit achieved when oxygen occupancies were allowed to vary from full occupancy. The occupancies of A and B site components were also checked and found to be fully occupied. The refinement of oxygen sites indicate the overall oxygen contents of O2.96 which is very close to the value required to conserve charge neutrality. The B-site occupancies were found as the expected stoichiometry within the limit of standard deviation. Decreasing Zr concentration leads to an increase of the lattice parameters. This increase of parameters reflects the substitution of small Zr4+ for Y3+-site. These results suggest that these compositions could be good approach as electrolyte materials in SOFC-H.

Scanning Electron Microscope Analysis
To observe the microstructures of the sample series, scanning electron microscopy was applied. Figure  2 a) shows the SEM top-view of surface morphology of the as-prepared BCZYZn5 materials. The surface was smooth and free of cracks. Thus, this compositions indicated non porosity, no trace of liquid or secondary phases were found at the grain boundary region in any of the samples investigated. It also shows that the grain sizes are larger and compact full which gives high density. Figure 2 b) shows the  [26]. Comparing this two figures that for Figure 2 a), the grain size is much bigger than 2 b). For BCZYZn5 the small spherical phase is approximately 1µm. The large grain size gives less grain boundary resistance also. The SEM data were collected with a voltage of 5 kV and with magnification of 3000 times. These results suggest that the introducing of Zn leads to a compactness of the grain size.

Thermo-Mechanical Analysis
The result of thermal expansion curves for BaCe0.7Zr0.2Y0.05Zn0.05O3 oxide materials, shows the lowest of all temperature ranges. In Figure 3, the thermal expansion coefficient (TEC) increases with the increase of Yttriam % and also with the temperature. The TEC is also increased with the decrease of Zirconium %. To analyze the thermomechanical properties of the composition, the TEC is 10.919 × 10 -6 /ºC at 898.92ºC.

Particle Size Measurement
The The particle size distribution for the metal oxides used in this investigation was calculated in powder after sintering at 1400ºC. The metal-oxide powders were previously sonicated during 4 h at 60ºC to obtain a homogeneous particles dope. Table 2 shows that, as the increase of increasing of Y concentration with Zr leads to a decrease of the particle size. The addition of metal oxide particles not only increases water diffusion into the growing membrane due to its higher hydrophilic nature but also affects the interaction between polymer and solvent molecules by the hindrance effect of the particles. From figure 4 and, it was indicated that the quantity (%) of the sample varies with the diameter of particle size and the diameter range of the BCZYZn5 material remain within 0-2.5 µm.

Impedance Spectroscopy
The total and bulk conductivity increases along with the concentration of Y 3+ dopant. This trivalent, dopant creates more vacant oxygen sites and in turn increases the proton concentration. Figure 5 shows the Arrhenius plot of the sample in hydrate (under dry Ar) and as-prepared (under wet Ar) form. The bulk conductivity in wet and dry Ar atmosphere was 0.00136 Scm -1 and 0.000946 Scm -1 at 600°C and, 0.004242 Scm -1 and 0.00331 Scm -1 at 900°C, respectively. Similarly the total conductivity in wet and dry Ar atmosphere was 0.000276 Scm -1 and 0.000204 Scm -1 at 600°C, and 0.003014 Scm -1 and 0.00218 Scm -1 at 900°C respectively. Figure 6 shows that the conductivity increases with temperature as well as wet atmosphere. The increase of total conductivity is smoother than the bulk conductivity. Slodczyk et al. reported that the BCZYZ ceramic -offering an high conductivity and stability as a potential candidate for use as an electrolytic membrane which was suited to the surface and bulk analysis [23]. In the present work, we have got higher conductivity in BCZYZn5 in comparison to BCZYZ reported by Slodczyk et al.   Figure 6. Conductivity vs Temperature of the sample BCZYZn5 in dry and wet Ar. Bulk and total conductivity is also shown.

Conclusions
The In this study, BaCe0.7Zr0.2Y0.05Zn0.05O3 proton conducting electrolyte was successfully fabricated. Rietveld analysis of the XRD data indicates that the samples crystallize in cubic symmetry. The SEM analysis shows the electrolyte materials are non-porous and highly dense. The electrical measurements performed in different atmospheres have shown that adding Zn was beneficial in terms of increased conductivity values. For BCZYZn5, Bulk conductivity reaches 1.36×10 -02 at 600ºC in wet condition with an activation energy of 0.58 eV. To get high protonic conductivity and high stability zinc doped BCZYZn5 material has been found to be promising as a potential electrolyte for proton-conducting SOFCs.