Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell

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Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell Aarthi Uthayakumar, Arunkumar Pandiyan, Suresh Babu Krishna Moorthy* Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University), Puducherry 605014, India

article info

abstract

Article history:

Development of high proton conducting, chemically stable electrolyte for solid oxide fuel

Received 21 July 2018

cell application still remains as a major challenge. In this work, yttrium (0, 5, 10, 15 and

Received in revised form

20 mol%) doped barium zirconate synthesised by hydrothermal assisted coprecipitation

10 October 2018

exhibited highly crystalline cubic perovskite. The results demonstrate that the proton

Accepted 24 October 2018

conductivity is higher than oxygen ion conductivity measured in the temperature range of

Available online xxx

200e600
C. The 20 mol% Y doped BaZrO3 exhibited higher protonic conductivity (6.1 mScm1) with an activation energy 0.64 eV under the reducing atmosphere. The Mott

Keywords:

eSchottky analysis carried out in hydrogen atmosphere at 200
C revealed that the barrier

Conductivity

height of doped BaZrO3 reduced from 0.6 to 0.2 V. The Schottky depletion layer width also

Proton

decreased from 4 to 2 nm with the increase in yttrium concentration and the boiling water

Space charge

test showed good phase stability. Our study highlights the critical role of space charge in

Barrier height

the grain boundary and its suppression with the increase in dopant concentration. The

Fuel cell

results demonstrate that Y doped BaZrO3 sintered at low temperature is a promising candidate as the electrolyte material for the intermediate temperature proton conducting solid oxide fuel cells. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Solid oxide fuel cells (SOFC) are electrochemical devices which convert chemical (fuel) to energy with high efficiency. High operational temperature of SOFC is associated with the thermally activated mobility of ions in an electrolyte which is still a foremost challenge for the commercialization. Efforts are being made with respect to material property to enhance the nature of transport (i.e., oxide/proton) across the electrolyte in

order to operate at the intermediate temperature range of 300e600
C. The proton conducting ceramic electrolytes have received much attention due to its reduced activation energy in hydrogen fuel which is an efficient and clean fuel for combustion due to the carbon-free emission [1e3]. In particular, the perovskite (AIIBIVO3) based oxides exhibit higher protonic conductivity and lower activation energy (~0.3e0.6 eV) in the hydrogen or water-based atmosphere [4,5]. Unlike oxide ion conductors, proton conducting

* Corresponding author. E-mail address: [email protected] (S.B. Krishna Moorthy). https://doi.org/10.1016/j.ijhydene.2018.10.185 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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electrolytes avoid fuel dilution at the anode by producing water at the cathode, thereby improving the performance durability [6]. BaZrO3 is one of the promising perovskite electrolytes which shows proton conductivity better than many oxygen ion conductors at temperature below 700
C [7]. The substitution of the tetravalent B site with a trivalent element leads to the formation of oxygen vacancy. In perovskites, the proton is transported through the Grotthuss mechanism in which protons migrate from one ion to another by hopping in the crystal lattice without strongly binding to any oxygen atom [8]. BaZrO3 owns good chemical stability due to the higher covalency of ZreO bond in the fuel cell operating environment. The phase formation of cubic barium zirconate requires the higher calcination temperature (1350
C) and, the refractory nature of BaZrO3 needs higher sintering temperature (1700e1800
C) and prolonged soaking time (12e24 h) that results in BaO evaporation. Various attempts have been made to improve the protonic conductivity as well as to lower the sintering temperature by the addition of different dopants (Al, Sc, In, Lu, Tm, Y, Gd, Sm, Nd, La etc.) [9e14]. However, the protonic conductivity of the electrolyte was found to be modulated with respect to dopant nature. In spite of the larger ionic radii difference, doping with yttrium in BaZrO3 (ionic A and Zr4þ ¼ 0.72
A) [15] resulted in higher radii Y3þ ¼ 0.91
proton conductivity of 7.9  103 S cm1 at 600
C [16]. Also, Y has good chemical matching such as basicity and electronegativity with respect to the host material which plays the crucial role in proton mobility. In addition, sintered barium zirconate offers higher grain boundary resistance which results in undesirable conductivity for fuel cell applications [17]. The high grain boundary resistivity or the blocking grain boundary resistance [18] arise from intrinsic effect, either due to the segregation of impurities or the formation of a space charge layer along the grain boundary [19,20]. In the space charge layer model, the excess positively charged grain boundary core is compensated by the negative charge present in the adjacent layers of the grains. The positively charged protonic defects are repelled by grain boundary core creating a depletion layer [21e23]. Thus, the depletion layer in the grain boundary reduces the proton transport. Iguchi et al., proposed that the increase in dopant concentration would consequently reduce the width of the space charge layer [24]. Unfortunately, the study did not provide a correlation between the dopant concentration and the accumulation of dopants in the grain boundary. Shirpour et al., studied the space charge effect in the Y doped BaZrO3 and found that the barrier height was reduced from 0.7 to 0.3 V with an increase in the dopant concentration but the samples were sintered at 1700
C for 20 h with the evaporation of BaO [25]. Similarly, Ricote et al., investigated the resistivity and the space charge layer of the grain boundaries in four different sample prepared by solid state synthesis with and without the sintering aid [26]. Various attempts by chemical methods, solid state and solegel synthesis have been carried out to solve the high sintering temperature issues. Also, the existence of the space charge layer and the role of dopant concentration on space charge by these reported methods remains to be explored. Reddy et al., achieved shapecontrolled cubic BaZrO3 nanospheres at relatively lower

calcination temperature without any desirable BaCO3 phase yet to explore the transport properties [27]. Furthur to improve the crystallinity and to decrease the particle size and agglomeration, the hydrothermal assisted co-precipitation method was adopted. To the best of authors knowledge, the hydrothermal assisted coprecipitation method was adopted for the first time to reduce the phase formation and sintering temperature in order to analyse the electrical properties as an electrolyte. In this work, we report the formation of BaZr1-xYxO3(x ¼ 0,0.05,0.10,0.15 and 0.20) phase around 600
C for 2 h by d hydrothermal assisted coprecipitation and sintered at relatively 1200
C without any sintering aid. The role of yttrium concentration on the structural, morphological and electrical properties were studied in detail. The grain boundary resistivity and the application of space charge concept to the acceptor doped BaZrO3 and its effect on yttrium concentration is quantified through the experiments.

Experimental details The BaZr1-xYxO3–x/2 samples (x ¼ 0,0.05,0.10,0.15 and 0.20 coded as B0YZ, B5YZ, B10YZ, B15YZ, and B20YZ, respectively) were prepared by hydrothermal assisted coprecipitation method. The required stoichiometric amounts of barium chloride, zirconyl oxychloride, yttrium nitrate (Analytical grade, Himedia, India) dissolved in 200 ml of deionised water was used as the precursor solution. Aqueous sodium hydroxide (15 M) solution was added dropwise to the above mixture and stirred for 90 min while maintaining the pH around 11. Then, the solution was transferred to a teflon coated stainless steel container and autoclaved at 120
C for 12 h. The resultant precipitate was washed five times with deionised water, filtered and dried at 100
C. After calcining at 600
C for 2 h, the powders were uniaxially pressed with 40 MPa using a hydraulic press into a cylindrical pellet of 13 mm diameter and 1 mm thick and then sintered in air at 1200
C for 4 h. Phase analysis was carried out using Rigaku Ultima IV Xray diffractometer with Cu Ka source (l ¼ 1.5406
A) in the 2q range of 20e80
at a scan rate of 0.02
/sec. The mean crystallite size, lattice parameter, and tolerance factor were calculated using Scherrer's formula, Bragg's equation, and Goldschmidt tolerance factor equation, respectively. An RM2000 Renishaw laser confocal microscope was used for recording the Raman spectra with 785 nm laser excitation source in the range of 100e800 cm1. Thermo Nicolet 6700 spectrometer used to record the Fourier transform infrared (FTIR) spectra from 4000 to 400 cm1. The microstructural analysis was carried out using scanning electron microscopy (ULTRA 55, GEMINI) and high-resolution transmission electron microscopy (FEI-Technai -G2) at 200 kV. X-ray photoelectron spectra (XPS) were recorded by AXIS 165 ULTRA to analyse the elemental state and its chemical composition. The conductivity of the sample was measured using Novacontrol impedance analyser (Alpha-A) in the frequency range of 10 MHz to 1 Hz with an applied bias voltage of 10 mV from 200 to 600
C with the temperature interval of 50
C. The opposite sides of sintered pellets were painted with silver paste

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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(current collector) and placed between two platinum electrodes to measure the conductivity.

Results and discussion X-ray diffraction The X-ray diffraction patterns of the barium zirconate with various yttrium content after sintering are shown in Fig. 1. The patterns confirm the presence of perovskite-structured barium zirconate with Pm3m space group (ICDD No 98-0107880). The absence of secondary phases such as yttria stabilized zirconia (YSZ) in B5YZ denotes the presence of yttrium as the dopant in barium zirconate lattice. However, as shown in the zoomed part in Fig. 1, two additional peaks emerged at 29.6
and 34
on increasing the Y content from 10 mol% onwards which can be attributed to the presence of cubic yttria stabilized zirconia (YSZ) as a secondary phase. The calculated lattice parameter and tolerance factor for the pellets are shown in Fig. 2. The linear increase in lattice parameter with yttrium denotes the lattice expansion due to A) the substitution of Y3þ which has the larger ionic radius (0.9
A). Generally, the ionic radii mismatch bethan Zr4þ (0.72
tween the host (Rh) and dopant (Rd) results in either lattice expansion (Rh < Rd) or contraction (Rd < Rh). The calculated lattice parameter was found to be in the range of 4.1920
A which matches with that of undoped barium zirconate [28,29]. Thus, Y3þ ions were completely incorporated into the BaZrO3 lattice up to 5 mol% whereas the 10, 15 and 20 mol% Y doped samples show the presence of secondary YSZ phase. The crystalline stability of the perovskite structured oxides (ABO3) can be calculated from the Goldschmidt tolerant factor (t) given as: RA þ RO t ¼ pffiffiffi 2 ðRB þ RO Þ

(1)

Fig. 2 e Calculated lattice parameter (left) and tolerance factor (right) with respect to the yttrium content and the inset represents the crystal structure of barium zirconate.

where RA and RB are the ionic radius of the cations occupying the A and B sites, respectively, and RO is the oxygen ionic radius. The tolerance range for the stability of the cubic perovskite structure is given as 0.95  t  1.04. The undoped barium zirconate (B0YZ) has the tolerance factor near to 1 which represents the ideal cubic symmetry. A marginal variation in tolerance factor with yttrium content but within the tolerance range confirms the retention of cubic structure. Sammells et al., reported that the further increase in the dopant percentage reduce the tolerance factor leading to orthorhombic symmetry because of the mismatch between A-O and BeO bond lengths [30]. The calculated mean crystallite size from Scherrer equation varied between 32 and 41 nm and the size was found to be increasing with yttrium content. Due to the increase in the atomic radius, the doping of Y increases the possibility of agglomeration and results in the grain growth.

Fig. 1 e XRD spectra of the pellets sintered at 1200
C for 4 h with the zoomed part showing the projection of (110) plane. Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Vibrational spectra To understand the local structural changes, Raman spectra were recorded for the sintered samples (Fig. 3). Though an ideal cubic perovskite oxide does not exhibit any Raman active modes, sintered samples in the present case showed several vibrational modes. The observed peaks of Raman forbidden modes in perovskite samples can be ascribed to lattice disorder due to the perturbation in the cubic symmetry and formation of oxygen vacancy or second-order Raman scattering [31]. The ideal cubic structure exhibits four types of optically active modes as given in Eq. (2). G ¼ 3f1u ðIRÞ þ f2u ðSÞ

(2)

which represents 3f1u ðIRÞ - three IR active, f2u ðSÞ  one silent and no Raman active modes. The intense Raman scattering observed around 100 and 200 cm1 arise from the deformational motion and stretching vibrations of Ba-[Zr/Y]-O6 unit. Ba2þ ions being heaviest ions, the vibrations involving this compound are expected to vibrate at the low-frequency bands [32]. The bands from 224 to 690 cm1 represent the different oxygen motion in the cubic crystal lattice, due to the bending and symmetric stretching of oxygen bonds, respectively [33,34]. The presence of bands at higher frequency provides evidence that the displacement of cations and oxygen ions in the crystallographic sites of cubic symmetry. The vibrational modes 405 cm1 corresponds to the asymmetric stretching of OeZreO in the opposite directions. Specifically, the intensity of the stretching mode at 405 cm1 was found to be stronger in 15 and 20 mol% Y doped samples due to the presence of secondary phase stretching vibration from [Zr/Y]-O2. The emergence of 405 cm1 increases with dopant concentration due to the formation of oxygen vacancy and substitution of Y3þ in the perovskite lattice. The substitution of trivalent dopant in the Zr4þ results in oxygen vacancy for€ gereVink notation. mation denoted in Kro 2ZrXZr þ OXo þ Y2 O3 4 2Y 'Zr þ V,, O þ 2ZrO2

(3)

The observed bands slightly red shifted for the doped samples in comparison to undoped barium zirconate due to

Fig. 3 e Raman spectra of the pellets sintered at 1200
C for 4 h.

the incorporation of larger ionic radii Y3þ and the creation of the oxygen vacancies and tilt in the octahedra. To get further insight into the structure, FTIR spectra were recorded in the range of 400e3000 cm1 as shown in Fig. 4. The strong absorption peak at 532 cm1 can be attributed to the BaeO & ZreO (MeO type) stretching and bending vibration, respectively, which is the characteristic feature of the octahedral BO6 group in perovskite compounds. With an increase in dopant concentration, the 532 cm1 peak became stronger and intensity increases, especially B20YZ, due to the presence of YSZ as the secondary phase by creating new MeO with the neighbouring oxygen atoms as observed in the XRD. The wideband from 1440 to 1135 cm1 represents the presence of BaeO bending vibration. The presence of all these peaks together confirms the formation of yttrium doped barium zirconate.

Microstructural analysis Fig. 5 shows the microstructural features of the pellets sintered at 1200
C for 4 h. All the samples exhibited a faceted structure consisting of agglomerated particles. With the increase in dopant concentration, a well-connected and irregular shaped grains were observed in comparison to undoped BaZrO3. In comparison to other materials, B15YZ and B20YZ showed a dense microstructure, and well crystallized polyhedral grains. The increase in the dopant concentration assist the grain growth which results in well-defined grains in the B20YZ. The increased doping concentration of Y3þ effectively increases the density of the sintered pellets [35]. The density calculated by Archimedes method, by immersing the sintered pellet in water showed an increase in relative density from 90.2 to 97.3% with yttrium concentration. The SEM images also show the grain size less than the 100 nm which are in agreement with the mean crystallite size calculated from XRD. The concentration of yttrium obtained from EDAX spectra shown in Fig. 5 indicates the presence of yttrium near to its theoretical stoichiometry. Fig. 6e shows the HRTEM image for the undoped and 20 mol % yttrium doped BaZrO3 pellets sintered at 1200 C. The HRTEM

Fig. 4 e FTIR spectra of the pellets sintered at 1200
C for 4 h.

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Fig. 5 e FESEM images of the pellets sintered at 1200
C for 4 h and the Y concentration of the samples obtained from EDAX spectra.

image shows spherical grains in the three-dimensional network. The image shows that there is an absence of secondary phase and amorphous phase indicating the single phase of BaZrO3. As observed in the SEM images, the spherical grains are grown to a polyhedral structure by addition of yttrium dopant indicates that the yttrium aids nucleation and grain growth [36]. The representative high magnification image shows the lattice fringes with the interplanar distance of 0.289 and 0.27 nm indexed to the (110) planes of cubic BaZrO3 for B0YZ and B20YZ respectively. The selected area electron diffraction (SAED) pattern of the TEM image images are shown in Fig. 6 (c and g). The series of rings were observed in the SAED pattern can be indexed to (111), (110), (100), (210), (310), and (311) plane of cubic perovskite structure of BaZrO3 which confirm the crystalline nature of B0ZY and B20YZ. The absence of the additional rings represents the single-phase formation in both samples. The presence of YSZ phase in B20YZ does not show any inevitable change in the structure. The energy dispersive X-ray diffraction (EDAX) spectra of the TEM image images are shown in Fig. 6(d and h). The EDAX spectra denote the composition as expected in theoretical

stoichiometry. The elemental composition of B20YZ reveals the presence of Y dopant in barium zirconate in the expected level.

X-ray photoelectron spectra To understand the oxidation state modification, B0YZ and B20YZ pellets were analysed by X-ray photoelectron spectra. The obtained spectra were charge corrected with reference to C 1s (284.8 eV) binding energy. The high-resolution spectra were fitted using fityk software after subtracting the background with Shirley function [37]. Fig. 7a shows XPS survey scan for both B0YZ and B20YZ which confirms the presence of Ba, Zr, and O. In spite of the observed weak Y 3d peak in survey scan while high-resolution XPS spectra (Fig. 7b) clearly shows the presence/absence of Y in B20YZ/B0YZ, respectively. The high-resolution scan for Ba 3d, Zr 3d, Y 3d and O 1s are shown in Fig. 7bee, respectively. A doublet peak observed for Ba 3d corresponds to Ba 3d5/2 (778.7 eV) and Ba 3d3/2 (793.9 eV) with the spin-orbital splitting of 15.29 eV (Fig. 7b). A similar binding energy observed for B0YZ and B20YZ without any shift in binding energy confirms the absence of yttrium

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Fig. 6 e (a-b), (e-f) HRTEM images of B0YZ and B20YZ, (c,g) and (d,h) SAED pattern and EDAX spectra of B0YZ and B20YZ respectively. The inset in (b) and (f) corresponds to interplanar distance for 110 plane of B0YZ and B20YZ.

substitution in A site (Ba) in ABO3. As shown in Fig. 7c, the Zr 3d peak also observed with doublet peaks at 181.4 and 184.6 eV with spin-orbital splitting of 2.8 eV. However, Zr 3d5/2 and Zr 3d3/2 peaks for B20YZ shifted to the lower binding energy which indicates the doping of Y3þin Zr site in B20YZ. The attained results are in reliable with the tetravalent and divalent oxidation state of Zr and Ba, respectively. The lower binding energy shift attributed to the doping of yttrium at the Zr site creating the localized modification. Since Y3þ has a higher ionic radius than Zr4þ which results in a strong lattice distortion leading to the shift towards lower binding energy. Notably, the intensity of Zr 3d5/2 was reduced in the B20YZ compared to the B0YZ confirming the substantial substitution of Y3þ at Zr4þsites. The XPS spectrum of Y 3d also exists in doublet peak, with splitting between Y 3d5/2 and Y 3d3/2 with corresponding binding energies of 159.0 and 156.8 eV for B20YZ samples. The spin-orbital splitting between Y 3d5/2 and Y 3d3/2 is 2.2 eV as shown in Fig. 7d. The O 1s owns two principle peak split under the influence local environment between 531.2 and 533.8 eV, respectively with the splitting of 2.6 eV. The higher binding energies of O 1s can be attributed to the hydroxyl and surface adsorbed oxygen, whereas the lower binding energies signify the lattice oxygen [38]. A shift towards lower binding energy in O 1s upon doping with Y3þ indicates the presence of O bonding in the different bond such as BO6, YO6, and ZrO6 octahedra. The O 1s binding energy is in good basicity scale and reduces with increase in basicity and this basicity of oxides favours the proton formation [39,40]. In order to determine the BaO evaporation, the intensity ratio of the Ba 3d and Zr 3d levels were calculated from the survey scan. The calculated intensity ratio of Ba: Zr (0.98:0.78) was found to be in agreement with the theoretical stoichiometry of B20YZ thereby by implying a minimal evaporation of BaO.

Impedance spectra The ionic conductivity was measured in both reducing (4% H2/ 96% N2) and ambient air (O2) atmosphere from 200 to 600
C with the temperature interval of 50
C. Fig. 8 shows the Nyquist plot for the sintered pellets measured in H2 atmosphere at 600
C and the inset gives a comparison of electrical property for BYZ20 in H2 and O2 atmosphere. The spectra exhibited two depressed arcs which are the typical characteristic of the proton conductors. The depression in the semicircle is due to the inductance of the instrument setup [41]. The highest frequency arc corresponds to the delay in the time constant during the transport of ions via grain and the lowest frequency arc represents the grain boundary transport. The ohmic resistance of the electrolyte given by the intercept of the high-frequency arc at the real axis and the diameter of the semicircle represents the polarisation resistance of the electrolyte. The obtained impedance spectra were analysed using brick layer model and fitted by the equivalent circuit consisting of ohmic resistance due to the cell components (RS) and two pairs of parallel resistor (R) - constant phase element (CPE) connected in series (as shown in Fig. 10 inset). The usage of CPE instead of capacitor suppress the inductance effect from the instrument, the relaxation time or time constant (t ¼ RC) extricates the grain and grain boundary contribution of the material. Both R and C values were obtained from the best fit of the impedance spectra. With the increase in temperature, the arc shifted to the high frequency due to lowering of relaxation time and the conductivity increases with the dopant concentration. The calculated conductivity of the sintered pellets are tabulated in Table 1. All the sintered samples unveiled superior protonic conductivity than the oxygen ionic conductivity (Fig. 8, inset) proving that the BYZ materials are more favourable for the transport of protons. B20YZ exhibited the superior conductivity (6.1 mScm1) in H2 atmosphere due to

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Fig. 7 e Wide scan XPS spectra of the B0YZ and B20YZ (a), high-resolution spectra with the Gaussian curve fitting for Ba 3d (b), Zr 3d (c), Y 3d (d) and O 1s (e); scatter plot and solid line indicates the experimental and fitted value respectively.

the highest protonic defect formation by doping via the following reaction [42,43].
(4) H2 þ 2OXO 4 2ðOHÞO þ 2e' For further understanding, the specific grain boundary conductivity which is the average grain boundary conductivity was calculated using Eq. (3) assuming that the dielectric constant is the same for the bulk and grain boundary [26]. sspecific grain boundary ¼

1 L Cbulk Rgb A Cgb

(5)

where Rgb denotes the grain boundary resistivity, L is the length of the sample, A is an area of the sample and Cbulk and

Cbg is capacitance of the bulk and grain boundary, respectively. The specific grain boundary resistivity was found to be four-fold higher than the grain interior and the specific grain boundary resistivity decreases with the increase in the dopant concentration. The higher grain boundary resistivity effectively reduces the overall protonic conductivity of the electrolyte. The intercepts of the semicircle in the complex impedance plane plots with the real axis were derived and reported in the Arrhenius plots and the activation energy and the preexponential factor of the bulk, specific grain boundary and

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Fig. 8 e Nyquist plot of the sintered pellets measured at 600
C in H2 atmosphere; inset showing comparision graph of B20YZ measured under H2 and air atmosphere.

the total conductivity were calculated using Arrhenius equation as follows Ea

s ¼ AO eKB T

(6)

where so is a pre-exponential factor and Ea is activation energy. Fig. 9 shows the Arrhenius plots of the measured conductivity for grain, specific grain boundary and total for both H2 and O2 atmosphere. The calculated total Ea range shifts from 0.69 to 0.64 eV to 0.74 to 0.70 eV in reducing and air atmosphere, respectively, which agrees with the values reported in the literature [44e48]. The lower activation energy in reducing condition in comparison to air atmosphere indicates the variation in transport mechanism with respect to the atmosphere. The transport of protons requires minimal activation energy because of its lighter weight than the oxygen ions, due to the fact it poses lesser activation energy in reducing atmosphere compared to air. Interestingly, activation energy did not show major variation with respect to the dopant concentration. However, under the reducing conditions, grain boundary activation energy was found to be higher than that of grain interior. The migration of protons is harder in the grain

boundary compared to the grain interior because of the discontinuity in periodicity. This fact also confirms the presence of barrier for the transport of ions across the grain boundary and requires higher activation energy. The pre-exponential factor (Ao) is proportional to the number of free charge carriers present in the lattice [49,50] and reveals transport factors such as hydration entropy, hoping distance, and migration entropy. B20YZ shows around 1.5 times higher proton content in reducing atmosphere than other samples in grain interior. Also, the grain boundary proton concentration was found to be lower due to the presence barrier along the grain boundary which prohibits the transport of protons along the grain boundary region. Eventhough barium zirconate is highly refractory in nature and requires high sintering temperature and soaking time, the used precipitation assisted hydrothermal method affords the dense single phase barium zirconate sintered at the lower temperature (1200
C for 4 h) as supported by XRD. The substitution of yttrium dopant in the host lattice increased the oxygen vacancy concentration and increases the transport of ions via the defect formation. The low-temperature sintering prevented the BaO evaporation from the A site and maintains the stoichiometry as observed from XPS analysis. Eventhough the protonic conductivity is higher than that of the oxygen ionic conductivity, the specific grain boundary conductivity is four-fold lesser than the grain conductivity. The lesser specific grain boundary conductivity remains a challenge for enhancing the conductivity of BaZrO3 based electrolyte materials. The increase in the grain boundary resistivity can be investigated in following phenomenon, (1) due to the presence of the physical barrier in the grain boundary such as segregation of impurities along the grain boundary whereas the EDAX and XPS analysis showed the absence of remarkable impurities in the sintered pellets. (2) The presence of the space charge along the grain boundary region. Kim et al., proposed that the decrease in grain boundary conductivity is due to the space charge effect [19] Similar phenomenon was observed by Nyman [48]. In order to understand the higher specific grain boundary resistivity, the impedance measurement was carried out for the sample at different bias voltages. The impedance spectra with respect to variation in bias voltages, shown in Fig. 10 were recorded at the lower temperature (200
C) as it is difficult to distinguish the grain boundary contribution at higher temperatures. The MotteSchottky approximation was

Table 1 e The activation energy and pre-exponential factor of sintered pellets in both reducing and air atmosphere. BXYZ

Conditions

Grain interior Ea (eV)

B0YZ B5YZ B10YZ B15YZ B20YZ

Reducing Air Reducing Air Reducing Air Reducing Air Reducing Air

0.62 0.75 0.62 0.75 0.63 0.70 0.63 0.69 0.60 0.68

Grain boundary 1

Ao,GI (SKcm ) 3.1 2.8 8.0 3.0 1.0 4.5 4.1 6.9 9.6 4.0

         

3

10 103 103 103 104 103 104 103 104 104

Ea (eV) 0.75 0.73 0.74 0.72 0.73 0.71 0.74 0.73 0.70 0.71

Total 1

Ao,GB (SKcm ) 2

0.9  10 4.2  102 1.1  102 6.7  102 1.35  102 9.6  102 5.71  102 1.2  103 1.1  103 5.6  103

Ea (eV) 0.69 0.74 0.68 0.73 0.66 0.70 0.68 0.71 0.64 0.70

Ao,tot (SKcm1) 2.2 7.0 6.8 1.0 8.8 2.1 1.2 5.4 5.6 1.4

         

103 102 103 103 103 103 104 103 104 104

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Fig. 9 e Arrhenius plot of the (a) grain interior and specific grain boundary and (b) total conductivity in reducing atmosphere, (c) grain interior and specific grain boundary and (d) total conductivity in an air atmosphere.

used to analyse the high grain boundary resistivity of the materials. The distorted semicircles were fitted using the equivalent circuit shown in Fig. 10 and the best-fitted values considered for analysis.

The capacitance and the relaxation time were calculated using CPE1, CPE2, Rg, and Rgb of grain interior and grain boundary respectively. According to the Motteschottky approximation, the barrier height is equal to the potential energy when the proton concentration is minimum (x ¼ 0), thus the barrier height DF(0) was determined by the following equation eD4ð0Þ

rGI tGI e kT ¼ ¼ eD4ð0Þ rGB tGB

(7)

kT

where, k is the Boltzmann constant and T is the temperature. The dielectric constant(εr), Debye length(l), and Mott-Schottky depletion layer width (l*) were calculated using Eqs. (8)e(10), respectively c ¼ εr εo

Fig. 10 e Nyquist plot of the sintered pellets measured at 200
C at H2 atmosphere in different bias voltage with inset showing equivalent circuit used to fit the nyquist plot.

l¼

A L

sffiffiffiffiffiffiffiffiffiffiffiffiffiffi kTεr εo 2e2 CH

(8)

(9)

Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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Table 2 e The dielectric constant and barrier height of the sintered pellets at 200
C. Samples

Barrier height (V)

Dielectric constant at 200
C

0.60 0.54 0.42 0.38 0.21

23 39 48 61 53

B0YZ B5YZ B10YZ B15YZ B20YZ

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi eD4ð0Þ kT

l* ¼ 2l

(10)

where A, the area of the sample, εo, vacuum permittivity, L, the thickness of the sample and CH, proton concentration in the bulk [29]. According to the space charge model, the oxygen vacancy created by the trivalent doping segregated at the grain boundary results in the electrostatic potential barrier. Many researchers observed the oxygen vacancy segregation at the grain boundary using Density Functional Theory (DFT) calculations [51,52]. The depletion layer created along both sides of the positive grain boundary core results in the proton transport blockage. The barrier height and the depletion layer width calculated using Eqs. (6)&(8) for the B0YZ and B20YZ were found to be 0.6 V/4 nm and 0.2 V/2 nm, respectively shown in Table 2.

Thus, the increase in dopant concentration reduces the barrier height and depletion layer width. The aggregation of the dopants along the grain boundary would suppress the positive charge accumulation due to oxygen vacancy that results in the reduction of barrier height. The acceptor yttrium dopant could segregate to the grain boundary due to the elastic strain created by the size mismatch of Y3þ and Zr4þ (ionic radii A and Zr4þ ¼ 0.72
A). Fig. 11 shows the schematic Y3þ ¼ 0.91
representation of space charge model for B0YZ and B20YZ. Increasing the yttrium concentration more than 20 mol% can increase the proton conductivity and also reduce the barrier height. However, increasing the dopant concentration would probably trap the proton on the edges of YO6 octahedra to form the hydrogen bond. Thus, the larger local distortions created due to proton trapping at dopant site reduces the proton mobility [3].

Chemical stability analysis The chemical stability of the sintered pellets was analysed by boiling water stability test. The sintered pellets were immersed in the boiling water at 90
C for 24 h. After the powders were collected and dried to analysed by XRD. The XRD diffraction spectra recorded after the water stability test is shown in Fig. 12. The spectra predominately match with the XRD spectra of the sintered pellets shown in Fig. 1. The spectra

Fig. 11 e Schematic representation of the space charge layer for B0YZ and B20YZ. Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185

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11

Acknowledgements Authors acknowledge the funding from DST-SERB (EMR/ 2016/007577) fast-track scheme for a young scientist, India. One of the authors, U. Aarthi, acknowledges Senior Research Fellowship (09/559/0115/2016-EMR-I) received from Council of Scientific and Industrial Research (CSIR), Government of India and Indian Nanoelectronics user Programme (INUP), IISC Bangalore for characterisation facility. P. Arunkumar is thankful for Indo-Korea Research Internship funded by Department of Science and Technology, Government of India.

references Fig. 12 e XRD spectra of the sintered pellets after the boiling water stability test for 24 h.

showed no evidence of significant phase changes before and after the boiling water chemical stability test except the emergence of two minor peaks around ~24 and 35 indicating the formation of barium hydroxide [53,54].

Conclusion In this study, the effect of dopant concentration (Y ¼ 0,0.05,0.1,0.15 & 0.20) on BaZrO3 and its structural, microstructural, stoichiometric and electrical properties were investigated. The samples were prepared by precipitation assisted hydrothermal synthesis method and sintered at 1200
C for 4 h. The cubic phase of barium zirconate was observed in all the pellets and higher dopant concentration showed the very minor peak of the secondary phase. The chemical stability of the pellets were analysed using boiling water treatment and materials showed good chemical stability. The pellets were densified with the increase in the dopant concentration analysed by microstructural analysis. The stoichiometric was maintained in all the pellets confirmed by EDAX of FESEM and HRTEM. The low-temperature sintering reduced the BaO evaporation to maintain the composition revealed by XPS analysis. The proton conductivity in reducing atmosphere is greater than the oxygen ionic conductivity whereas the B20YZ possess 6.1 mScm1 with the activation energy of 0.64 eV. The protonic conductivity requires minimal activation energy and higher pre-exponential factor than the oxygen-ionic conductivity. The accumulation of Y dopant on the grain boundary core with increasing dopant concentration reduces the positive grain boundary core therewith minimizing the depletion layer width. The barrier height falls from 0.7 to 0.2 V with increasing dopant concentration. Furthur, increase in dopant concentration would trap the proton prohibiting the mobility. Thus the obtained results, Y doped BaZrO3 prepared by low-temperature sintering is the potential candidate as an electrolyte for the solid oxide fuel cell.

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Please cite this article as: Uthayakumar A et al., Yttrium dependent space charge effect on modulating the conductivity of barium zirconate electrolyte for solid oxide fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.185