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Improved Conductivity of Spark Plasma Sintered HoSubstituted BaZrO Electrolyte Ceramics for IT-SOFCs 3

Deepash Shekhar Saini, Avijit Ghosh, Shuvendu Tripathy, Sanjeev Kumar Sharma, Aparabal Kumar, and Debasis Bhattacharya ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00655 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Energy Materials

Improved Conductivity of Spark Plasma Sintered Ho-Substituted BaZrO3 Electrolyte Ceramics for IT-SOFCs Deepash Shekhar Saini1, 3*, Avijit Ghosh2, Shuvendu Tripathy1 Sanjeev Kumar Sharma1, Aparabal Kumar1 and Debasis Bhattacharya1 1

Materials Science Centre, Indian Institute of Technology, Kharagpur - 721302, India

2

Centre for Applied Physics, Central University of Jharkhand, Ranchi - 835205, India 3

Department of Physics, National Institute of Technology, Raipur - 492010, India

ABSTRACT The effect of spark plasma sintering (SPS) on Ho-substituted BaZrO3 electrolyte ceramics was investigated to enhance the electrical conductivity for intermediate temperature solid oxide fuel cells (IT-SOFCs). The cubic crystal system with Pm3 m space group symmetry, unit cell parameters and bond length were revealed out through Rietveld refined X-ray diffraction (XRD) pattern. The microstructure of BaZr0.9Ho0.1O3-δ (BZH10) and BaZr0.8Ho0.2O3-δ (BZH20) SPS samples was found highly dense and exhibits the mixed inter/trans granular fracture mode through FESEM analysis. The average grain size and relative density of SPS at 1600 °C for 20 min of Ho-substituted BaZrO3 ceramics were found to decrease with the increase of Ho-substitution in BaZrO3 ceramics. The Electrical Impedance Spectroscopic (EIS) analysis was performed under air and 3% humidified O2 atmosphere as a function of probing temperature from 250 to 800 °C. Thus, the total electrical conductivity of SPS BZH20 sample was estimated to be 4.91 × 10-2 S-cm-1 at 700 °C under 3% humidified O2 atmosphere, which is the highest-class conductivity among all reported trivalent cation substituted BaZrO3 ceramics follow by spark plasma sintering. Keywords: Perovskite materials, Spark plasma sintering, Rietveld analysis, Microstructure, Electrical impedance, Conductivity

*

Corresponding author: [email protected]

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1. INTRODUCTION For several years, the demand of global energy is growing up due to high consumption of energy sources (Hydrocarbons) leads to an increase in greenhouse gas emissions in the atmosphere. Therefore, the alternative energy sources are being explored to reduce the greenhouse gases [1]. Recently, the field of fuel cells have been gaining growing interest because of an effective alternative way to produce clean energy. Out of several kinds of fuel cells, intermediate temperature solid oxide fuel cells (IT-SOFCs) have many advantages such as low temperature, high efficiency and high-power density, etc. Furthermore, the efficiency of IT-SOFCs essentially depends upon its main components such as electrolytes and electrodes [2]. Among the various factors, high density, large proton conductivity and chemical stability of electrolytes strongly affect the performance of ITSOFCs [3, 4]. Therefore, the fabrication of a highly stable dense electrolyte membrane with acceptable proton conductivity is the main requirement for the fabrication of IT-SOFCs. In view of the above, trivalent cation substituted barium zirconate (BaZrO3) has been received considerable attention as a proton conductor due to its acceptable ionic conductivity and excellent chemical stability compared to trivalent cation substituted barium cerate (BaCeO3). However, the preparation of low-cost, highly dense electrolyte membrane is the major challenge due to refractory nature of BaZrO3 [5-9]. In the past recent years, yttrium substituted BaZrO3 ceramic has been widely considered as a proton conductor at low temperature (300-700 °C) regime due to its high ionic conductivity compared to other trivalent cation substituted BaZrO3 ceramic [7-9]. Unfortunately, a very high sintering temperature (above 1700 °C) is required to achieve more than 90% dense due to its refractory nature. In this context, many researchers have been extensively focused on to achieve fully dense Y-substituted BaZrO3 ceramic: for example, (a) Duval et al. [10] used solid state reaction method to synthesize crystalline BZY10 powder and achieved 91% dense after sintering at 1720 °C for 24 h, (b) Park et al. [11] synthesized crystalline BZY15 powders using four different approaches: solid state, combustion, hydrothermal and polymer-gelation methods to achieve densities of 84.1, 80.1, 86.5, and 95.5% after sintering at 1670 °C for 24 h, respectively, and (c) Schober et al. [12] reported 96% dense BZY10 ceramics after sintering at 1715 °C for 30 min. However, barium (Ba) evaporation is taking placed during sintering at high temperature and affects the conductivity of trivalent substituted BaZrO3 ceramic. To overcome this problem, the pellets were covered


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with powder of the material that to be sintered with extra amount of barium oxide during sintering [14]. Another possible way to get dense ceramics is to utilize of small amounts of sintering additives such as Cu, Ni, Zn, Co and Fe. During the sintering, this small amount of additives is in the form of liquid with the solid one, which promotes compactness of ceramics due to enhancement in the diffusion processes. However, these sintering additives segregate at grain boundaries and reduced conductivity of the ceramics [11, 15, 16]. Other than the above, spark plasma sintering (SPS) is another possible way to improve the densification of such ceramics by using electrical discharge between the ceramic particles under pressure. The processing conditions of SPS promote to suppress evaporation of components from ceramics. In SPS, the ceramic powders easily transform into dense form under uniform heating at comparatively low temperature and much shorter time periods, typically few minutes, compared to conventional sintering (CS) process [18]. Kjølseth et al. [19] reported 95% density of BZY10 ceramics after using SPS with two sintering conditions: at 1650 °C under 50 MPa for 1 h or at 1600 °C under 100 MPa for 5 min. Furthermore, Anselmi-Tamburini et al. [20] obtained densities between 94.4 and 97.9% of BZY8 and BZY16 ceramics using SPS process at 1600 °C, respectively, while the 98% density of BZY8 ceramic was also achieved by Park et al. [21] with the help of SPS process. Furthermore, the conductivity of electrolyte ceramic materials mainly depends on various factors such as choice of substitution, synthesis route, sintering conditions and environment during conductivity measurement, etc. In this context, the Ho-substituted BaZrO3 nano-crystalline ceramic powders are synthesized by flash pyrolysis (FP) route followed by SPS process. Hence, the impact of SPS on Ho-substituted BaZrO3 ceramics in terms of phase relationship, microstructure, and electrical conductivity to improve the conductivity has been investigated thoroughly. 2. EXPERIMENTAL DETAILS In this study, the Ho-substituted BaZrO3 nano-crystalline ceramics powder were synthesized by flash pyrolysis route using Ba(NO3)2, ZrOCl2⋅8H2O and Ho(NO3)3⋅5H2O (Alfa Aesar) as precursors. The details of the synthesis of Ho-substituted BaZrO3 nanocrystalline ceramics powder have been reported elsewhere [22, 24]. The X-ray diffractometer of Bruker D8 Advance system was used to identify the phase of spark plasma sintered samples within 20 to 120° with a step size of 0.019°. The field emission scanning electron



microscope (FESEM) of Zeiss (Merlin-Gemini II) was employed for microstructural analysis of sintered samples such as grain morphology and grain size. The electrical parameters of studied materials were measured using complex plane AC impedance spectroscopy through a Frequency Response Analyser from 100 Hz to 1 MHz range of ModuLab, Solartron, UK. For these measurements, pure platinum paste (Metalor) were painted on both sides of perfectly flat surface of bar specimens (length: 6 mm, breath: 4 mm and thickness: 1-2 mm) and followed by heating at 1000 °C for 2 h in air to prepare Pt-BaZr1-xHoxO3-δ-Pt (x = 0.10 and 0.20) symmetric cell configuration. Finally, the electrical measurements were performed in dry air as well as in 3% humidified O2 atmosphere (3% H2O + 97% O2 atmosphere). The 3% humidified O2 atmosphere was produced by passing oxygen gas continuously through water bubbler at room temperature (RT). The experimental setup for electrical measurement is also shown in Fig.1. 3. RESULTS AND DISCUSSION 3.1 Sintering BaZr(1-x)HoxO3-δ (x = 0.10 and 0.20) powders were calcined at 1100 °C after obtained by flash pyrolysis route. The calcined powders were sintered in graphite die-punch (20 mm inner diameter) in a SPS system (FUJI SPS 625, Fuji Electronic Industrial Co. Ltd., Japan). The SPS process was performed at 75 MPa in four steps (i) from RT to 600 °C for 6 min (heating rate ∼ 100 °C/min), (ii) 600 to 1400 °C for 8 min (heating rate ∼ 100 °C/min), (iii) 1400 to 1600 °C for 4 min (heating rate ∼ 50 °C/min), and finally (iv) holding at 1600 °C for 20 min. An optical pyrometer was used to detect the temperature of sample during SPS process and focused on surface of the graphite die, externally. A dc current pulse of width = 3 ms with ON/OFF = 12:2 was used for heating, which was entered through the conductive graphite die that directly performs as a heating element. After completion of process, samples could cool down at RT with cooling rate of 400 °C/min. The as-sintered materials were found to be covered with a carbon layer from graphite die, which provides that black colour. The samples were then heated at 900 °C for 2 h in air and polished up to 1-micron finish. This process was utilized to release residual stresses (arising either from SPS process or from polishing) and to overcome the surface contamination by carbon. The relative density of BaZr0.90Ho0.10O3-δ (BZH10) and BaZr0.80Ho0.20O3-δ (BZH20), measured by the Archimedes method was established to be 99.5 and 99.1, respectively.


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3.2 Phase analysis The XRD patterns of SPS samples consisted of single phase BaZr(1-x)HoxO3-δ (x = 0.10 and 0.20) are displayed in Fig. 2(a). The Rietveld refined XRD patterns exhibit cubic crystal system with Pm3 m space group symmetry as shown in Fig. 2(a). The crystallographic information file (CIF) of XRD pattern for x = 0.20 has been created through Fullprof program. Using Diamond software, the unit cell of cubic structure for x = 0.20 was drawn as shown in Fig. 2(b) [23, 24]. The Rietveld refined unit cell parameters corresponding to x = 0.10 and 0.20 samples were found to be 4.1983 and 4.2092 Å, which are excellent agreement with literature [12]. Thus, it reveals that Ho was fully substituted at the Zr-site in BaZrO3 ceramics and its extra negative charge is naturalized by oxygen vacancies. It was observed that progressive shift of XRD peaks towards lower diffraction angle (2θ) with the increase of Ho-content as presented in Fig. 3, revealed that enlargement of the unit cell. The refined structural parameters for BaZr1-xHoxO3-δ (x = 0.10 and 0.20) samples obtained by SPS process at 1600 °C with a holding time of 20 min are tabulated in Table 1. Since, the ionic radius of Ho3+ (∼0.901 Å) is more than that of Zr4+ (∼0.72 Å), the lattice parameter as well as Zr/Ho-O and Ba-O bond are increased as Ho-substitution increases. Furthermore, it has been observed that crystallite size is decreased with increase of Ho-substitution, i.e. the degree of crystallization is decreased with increase of Ho-substitution in BaZrO3 ceramics. 3.3 Effect of spark plasma sintering on microstructure Figs. 4 and 5 exhibit the FESEM micrographs for fracture surfaces of BZH10 and BZH20 ceramics prepared by SPS process at 1600 °C for 20 min under pressure of 75 MPa, respectively. A mixed inter/trans granular fracture mode was observed in both the samples. The grains from 600 to 1000 nm range can be visualized in BZH10, meanwhile, the grain size in BZH20 ceramics is from 200 to 600 nm range. It is clear that grain growth seems to occur in both the samples. However, the relative density and average grain size are larger for BZH10 sample than that for BZH20 sample. This suggests that lower rate of cation diffusion with the increase of Ho-substitution in BaZrO3 ceramics [25, 26]. Fig. 6 {(a)-(d)} shows the EDS (energy dispersive X-ray spectra) and elemental mapping of fracture surface of BZH10 and BZH20 ceramics prepared by SPS at 1600 °C for 20 min under 75 MPa, respectively. The EDS of both samples exhibit presence of Ba, Zr, Ho and O elements. The atomic percentage of elements in both samples is also displayed in



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Table 2. The EDS elemental mapping of fracture surface for BZH10 and BZH20 ceramics indicates the homogeneous distribution of all elements. 3.4 Impedance analysis The impedance spectra (Nyquist plots) of BaZr(1-x)HoxO3-δ (x = 0.10 and 0.20) ceramics in air as well as in 3% humidified O2 atmosphere at different temperatures are presented in Figs. 7-10. The fitting of impedance spectra has been performed through ZSimpWin 3.21 program to evaluate the contribution from grain and grain boundary [24]. The equivalent electric circuit shown in impedance spectra plots were employed for a detailed fitting. In these electric circuits, Rg, Rgb and Re represent the resistance from grain (bulk) interior, grain boundary and electrode, respectively, while CPE provides the constant phase element. The fitting equivalent circuit parameters are shown in Tables 3-6. After the impedance spectra fitting, the conductivity of all samples was evaluated by using the following equations:

σ=

 L  1   A  Rg + Rgb 

(1)

where σ is the conductivity, L is the thickness of electrolyte, A is the tested electrolyte area, Rg is grain resistance and Rgb is grain boundary resistance, respectively. The impedance spectra provided that the coexistence of three depressed semicircular arcs. According to “brick layer” model, the first (at high frequency) depressed semicircular arc in impedance spectra (Nyquist plot) corresponds to the bulk (grain interior) contribution and the second depressed semicircular arc is mainly attributed to the grain boundary response. The third semicircle (at low frequency) is responsible to electrode polarization effect [27, 28]. In this work, at the high temperature regime, the first semicircle cannot be completely resolved due to frequency limitation of the frequency response analyser. Fig. 7 (a-d) exhibits the impedance spectra for BZH10 sample in air. The coexistence of two depressed semicircular arcs from 250 to 500 °C temperature range {shown in Fig. 7(a, b)} leads to the presence of two types of relaxation phenomena with sufficient and different relaxation times. The shape and size of semicircular arcs modify with temperature and decreases as the temperature increases. This indicates the variation in resistive and capacitive component of the sample. The depressed semicircular arcs at low and high frequency regions are mainly attributed to contribution from the grain boundary and grain (bulk), respectively.


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Fig. 7(c) provides that two depressed semi-circular arcs transforms into three depressed semicircular arcs for further increase of temperature from 550 to 600 °C. This additional semicircular arc at much lower frequency is due to electrode polarization effect. Moreover, Fig. 7(d) exhibits that transformation of three semicircular arcs into two semicircular arcs for further increase of temperature from 650 to 700 °C. This reveals that grain interior response is not clearly separated out from 650 to 700 °C temperature range. It may be attributed to overlapping of the grain interior response with response from grain boundary. [28, 29]. Fig. 8(a-c) provides the complex impedance spectra for BZH20 sample in air. The features of these complex impedance spectra are different from impedance spectra for BZH10 sample. The impedance spectra {shown in Fig. 8(a)} display only one semicircular arc from 250 to 350 °C temperature range which is attributed to the contribution from the grain boundary. Fig. 8(b) and 8(c) exhibit that transformation of single semicircular arc into two semicircular arcs with further increase of temperature from 400 to 700 °C. This additional semi-circular arc at low frequency is because of electrode polarization [28, 29]. Fig. 9(a-c) displays the impedance spectra for BZH10 sample at some selected temperatures in 3% humidified O2 atmosphere. This sample reveals two semicircular arcs from 300 to 800 °C range under complex impedance spectra. However, the characteristics of semicircular arcs are different in different temperature regimes. The sample shows two semicircular arcs from 300 to 450 °C range leading to presence of two types of relaxation mechanism with sufficient different relaxation times {Fig. 9(a)}. The depressed semicircular arcs at low and high frequency regions are mainly attributed to contribution from the grain boundary and grain (bulk) from 300 to 450 °C range, respectively. Furthermore, two semicircular arcs from 500 to 800 °C range in the impedance spectra are possibly due to contribution from grain boundary and electrode {Fig. 9(b) and 9(c)}, respectively [28, 29]. Fig. 10(a-c) exhibits the impedance spectra for BZH20 sample at some selected temperatures in 3% humidified O2 atmosphere. The nature of impedance spectra is same as that of the BZH20 sample in air, while the shape and size of semicircular arcs are different only. The sample exhibits two semicircular arcs {Fig. 10(a)} from 300 to 350 °C range possibly due to contribution from grain interior and grain boundary. Furthermore, the signature of two semicircular arcs from 500 to 800 °C range are possibly due to contribution from grain boundary and electrode {Fig. 10(b) and (c)}, respectively [28, 29].

3.5 Total electrical conductivity



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The electrical response of these samples has been evaluated using impedance analysis from 100 Hz to 1 MHz range. The electrical characterization of these samples has been performed in air as well as 3% humidified O2 atmosphere from 250 to 800 °C ranges, respectively. Generally, the electrical conductivity of perovskite materials is responsible for various factors such as, ionic radius of substituent, concentration of defect sites in the lattice, types of charge species, surrounding gas atmosphere, and operating temperature, etc. The factors such as operating temperature and surrounding atmosphere can control the dominant effect of charged species and defect sites in perovskite materials [30]. Furthermore, it has been observed that the electrical conductivity increasing as temperature increases for both the samples in air atmosphere (Fig. 11). In air atmosphere, oxygen vacancies are main defect sites and majority charge carriers in the materials [31]. These oxygen vacancies (defect sites) are introduced in lattice by substitution of lower valance of Ho (3+) at the Zr (4+) sites. This mechanism may be expressed as per Eq. (2) [31, 32].

2 ZrZrX + OOX + Ho2O3 → 2 Ho′Zr + VO•• + 2 ZrO2

(2)

The concentration of oxygen ion vacancy raises as per the increment of Ho-content in barium zirconate ceramics. This leads to increase total electrical conductivity throughout the temperature range (Fig. 11). Furthermore, the grain boundary conductivity is dominating (Nyquist plots) due to core-space charge layer behaviour in the grain boundary. This behaviour promotes to rise the depletion layer of positive charge carriers in a layer near to the boundary core [19]. Moreover, the maximum total conductivity in air atmosphere is found to be 9.64 × 10-3 S-cm-1 at 700 °C for BZH20 sample, which is approximately one order higher than that for BZH10 sample due to additional oxygen vacancy in BZH20 sample. In 3% humidified O2 atmosphere, a water molecule from gas phase dissociates into OH − and H + ions in the oxide. The OH − ion fills up an oxygen ion vacancy; meanwhile H + ion forms an OH − ion with oxygen lattice [33, 34] as per the Eq. (3).

H 2O( gas) + VO•• + OOX → 2OH O−

(3)

As a result, the protons are introduced by filling of oxygen vacancies generated by substitution of Ho into the Zr site in BaZrO3 ceramics. Thus, the major conducting species are proton ion (H+) in wet conditions. Furthermore, beside proton (H+) ions, oxygen vacancies and holes may also play a vital role during the conduction progression depending upon gas atmosphere, and temperature, etc [32]. The conduction of proton (H+) in this


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perovskite material may be explained by Grotthus mechanism. In this mechanism, the proton diffuses by combined process of molecular reorientation around the oxygen and hop of the proton from oxygen to nearest neighbour ion as shown in Fig.12 [35-37]. The total conductivity of the samples rises significantly in 3% humidified O2 atmosphere [38, 39]. The increased total electrical conductivity in 3% humidified O2 may be due to (1) proton ion, and (2) larger ionic radius of Ho3+ (90.1 pm) than that of Zr4+ (72 pm). The dominant proton (H+) charge species are increased as Ho-concentration increases because of additional oxygen vacancies. Because of larger ionic radius of Ho3+, volume of unit cell for BaZrO3 increases as Ho-content increases leading to enhance the width of migration channel for proton conduction progression. Thus, total conductivity in 3% humidified O2 atmosphere is higher than that for all the samples measured in air atmosphere indicating the additional charge species (i.e. H+) conduction in 3% humidified O2 atmosphere. In 3% humidified O2 atmosphere, the conductivity associated to grain boundary is again dominating (Nyquist plots) due to core-space charge layer behaviour in the grain boundary. This leads to raise the depletion of the positive charge carriers in a layer near to boundary core [19]. The maximum total conductivity in 3% humidified O2 atmosphere is found to be 4.91 × 10-2 S-cm-1 at 700 °C for BZH20 sample. This result provides the highest-class conductivity among all reported trivalent cation substituted BaZrO3 ceramics followed spark plasma sintering [40-44]. For comparison, this result is presented in Table 7 and therefore, this material is suitable as an electrolyte for IT-SOFCs. The conductivity plots reveal that slope as per Arrhenius relation changing at the high temperature region. This indicates that the conductivity is due to hole or oxygen vacancy, and not for the proton conduction. Similar results have also been reported by Bohn and Schober for Y-doped BaZrO3 ceramics [12]. However, an increase in total conductivity at higher temperatures, under O2 environment confirms that hole conduction is dominating, according to the following reaction: 1/2O2 (g) + Vo•• ↔ Oox + 2h•

(4)

4. CONCLUSIONS In summary, all the Rietveld refined SPS samples granted pure perovskite cubic crystal system with Pm 3 m space group symmetry. The FESEM micrograph indicating relative high density and average grain size of Ho-substituted BaZrO3 samples are found to



decrease with the increase of Ho-substitution. Furthermore, the electrical impedance spectroscopic study provides the contributions from grain interior, grain boundary, and electrode on electrical properties of the samples in air and 3% humidified O2 atmosphere. A general feature of the impedance spectrograph suggests a decrease in bulk resistance with prevailing grain boundary contribution as temperature increases. Furthermore, the obtained total electrical conductivity from Nyquist plots indicated that highest conductivity of 4.91 × 10-2 S-cm-1 at 700 °C in 3% humidified O2 atmosphere for BZH20 sample, which is appropriate as an electrolyte for IT-SOFCs.

ACKNOWLEDGMENTS The authors are thankful to Dr. Rajendra Nath Basu, Fuel Cell & Battery Division, Central Glass and Ceramic Research Institute (CGCRI) Kolkata, India for providing impedance measurement facility. Dr. Deepash Shekhar Saini specially thanks to the MHRD, Government of India, in part of financial support.

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2006, 26, 2313-2318. [21] Park, H. J.; Munir, Z. A.; Kim, S. T. Grain Boundary Protonic Conductivity in Highly Dense Nano-crystalline Y-doped BaZrO3. J. Kore. Ceram. Soc., 2010, 47, 71-74. [22] Saini, D. S. Study of Proton Conduction and Dielectric Properties of Ho-Substituted BaZrO3 Electrolyte Ceramic for Proton Conducting SOFCs, Ph. D. Thesis, Indian Institute of Technology Kharagpur, Kharagpur, India, 2017. [23] Saini, D. S.; Bhattacharya, D. (2016). Electrical Properties of BaZrO3 Ceramic Synthesized by Flash Pyrolysis Process. AIP Conference Proceedings, 2016, 1724, 020104-1 – 020104-8. [24] Saini, D. S.; Tripathy, S.; Kumar, A.; Sharma, S. K.; Ghosh, A.; Bhattacharya, D. Impedance and Modulus Spectroscopic Analysis of Single Phase BaZrO3 Ceramics for SOFC Application. Ionics, 2018, 24, 1161-117. [25] Kang, S. J. L. Sintering: Densification, Grain Growth and Microstructure. 2004, Butterworth-Heinemann. [26] Yamazaki, Y.; Hernandez-Sanchez, R.; Haile, S. M. Cation Non-Stoichiometry in Yttrium-Doped Barium Zirconate: Phase Behavior, Microstructure, and Proton Conductivity. J. Mater. Chem. 2010, 20, 8158-8166. [27] Tuller, H. L. Ionic Conduction in Nanocrystalline Materials. Solid State Ionics, 2000, 131, 143-157.

[28] Barsoukov, E.;

Macdonald, J. R. (Eds.). Impedance Spectroscopy: Theory,

Experiment, and Applications. 2005, John Wiley & Sons. [29] West, A. R.; Sinclair, D. C.; Hirose, N. Characterization of Electrical Materials, Especially Ferroelectrics, by Impedance Spectroscopy. J. Electroceram. 1997, 1, 6571. [30] Gdula‐Kasica, K.; Mielewczyk‐Gryn, A.; Lendze, T.; Molin, S.; Kusz, B.; Gazda, M. Synthesis of Acceptor‐Doped Ba‐Ce‐Zr‐O Perovskites. Cryst. Res. and Tech. 2010, 45, 1251-1257.


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[31] Gorbova, E.; Maragou, V.; Medvedev, D.; Demin, A.; Tsiakaras, P. Investigation of the Protonic Conduction in Sm Doped BaCeO3. J. Power Sources, 2008, 181, 207213. [32] Haile, S. M.; Staneff, G.; Ryu, K. H. Non-Stoichiometry, Grain Boundary Transport and Chemical Stability of Proton Conducting Perovskites. J. Mater. Sci., 2001, 36, 1149-1160. [33] Bonanos, N. Oxide-Based Protonic Conductors: Point Defects and Transport Properties. Solid State Ionics 2001, 145, 265-274. [34] Medvedev, D.; Brouzgou, A.; Demin, A.; Tsiakaras, P. Proton-Conducting Electrolytes for Solid Oxide Fuel Cell Applications. In Advances in Medium and High Temperature Solid Oxide Fuel Cell Technology, Springer International Publishing,

2017, 77-118. [35] Cherry, M.; Islam, M. S.; Gale, J. D.; Catlow, C. R. A. Computational Studies of Protons in Perovskite-Structured Oxides. J. Phys. Chem. 1995, 99, 14614-14618. [36] Münch, W.; Seifert, G.; Kreuer, K. D.; Maier, J. A Quantum Molecular Dynamics Study of the Cubic Phase of BaTiO3 and BaZrO3. Solid State Ionics. 1997, 97, 39-44. [37] Cherry, M.; Islam, M. S.; Gale, J. D.; Catlow, C. R. A. Computational Studies of Proton Migration in Perovskite Oxides. Solid State Ionics. 1995, 77, 207-209. [38] Jena, H.; Kutty, K. G.; Kutty, T. R. N. Proton Transport and Structural Relations in Hydroxyl-Bearing BaTiO3 and its Doped Compositions Synthesised by Wet-Chemical Methods. Mater. Res. Bull. 2004, 39, 489-511. [39] Islam, Q. A.; Nag, S.; Basu, R. N. Electrical Properties of Tb-Doped Barium Cerate. Ceram. Int., 2013, 39, 6433-6440.

[40] Ricote, S.; Bonanos, N.; Wang, H. J.; Boukamp, B. A. Conductivity Study of Dense BaZr0.9Y0.1O(3− δ) Obtained by Spark Plasma Sintering. Solid State Ionics, 2012, 213, 36-41. [41] Wang, S.; Liu, Y.; He, J.; Chen, F.; Brinkman, K. S. Spark-Plasma-Sintered Barium Zirconate Based Proton Conductors for Solid Oxide Fuel Cell and Hydrogen Separation Applications. Int. J. Hydrogen Energy, 2015, 40, 5707-5714. [42] Gorelov, V. P.; Balakireva, V. B. Synthesis and Properties of High-Density Protonic Solid Electrolyte BaZr0.9Y0.1O3−α. Russ. J. Electrochem. 2009, 45, 476-482. [43] Gorelov, V. P.; Balakireva, V. B.; Kuz’min, A. V. Ionic, Proton, and Oxygen Conductivities in the BaZr1−xYxO3−α System (x = 0.02−0.15) in Humid air. Russ. J. Electrochem. 2010, 46, 890-895.



[44] Kuz’min, A. V.; Balakireva, V. B.; Plaksin, S. V.; Gorelov, V. P. Total and Hole Conductivity in the BaZr1−xYxO3−α system (x = 0.02−0.20) in oxidizing atmosphere. Russ. J. Electrochem. 2009, 45, 1351-1357.


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Figure Captions:

Fig. 1: The experimental setup for electrical measurement.



(a)

(b)

Fig. 2: (a) The Rietveld refined XRD patterns of BaZr(1-x)HoxO3-δ (x = 0.10 and 0.20) ceramics and (b) Refined structure of BaZr0.80Ho0.20O3-δ samples obtained by spark plasma sintering at 1600 °C for the holding time of 20 min.


Page 16 of 34

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Fig. 3: Shifting of peaks in XRD patterns of BaZr(1-x)HoxO3-δ (x = 0.10 and 0.20) samples obtained by spark plasma sintering at 1600 °C for the holding time of 20 min.



(a)

(b)

2 µm

400 nm

Fig. 4: FESEM image with a magnification of (a) 10 KX and (b) 50 KX for facture surface of BaZr0.90Ho0.10O3-δ ceramics sintered at 1600 °C for 20 min through spark plasma.


Page 18 of 34

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(a)

(b)

2 µm

400 nm

Fig. 5: FESEM image with a magnification of (a) 10 KX and (b) 50 KX for facture surface of BaZr0.80Ho0.20O3-δ ceramics sintered at 1600 °C for 20 min through spark plasma.



Fig. 6: (a) Energy Dispersive X-ray Spectra (EDS), and (b) elemental mapping of facture surface of BaZr0.90Ho0.10O3-δ ceramics, (c) EDS and (d) elemental mapping of facture surface of BaZr0.80Ho0.20O3-δ ceramics sintered at 1600 °C for 20 min by spark plasma.


Page 20 of 34

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Fig. 7: Impedance spectra of spark plasma sintered BaZr0.90Ho0.10O3-δ sample at different temperature ranges (a) 250 to 350 °C, (b) 400 to 500 °C, (c) 550 to 600 °C, and (d) 650 to 700 °C in air.



Fig. 8: Impedance spectra of spark plasma sintered BaZr0.80Ho0.20O3-δ sample at different temperature ranges (a) 250 to 350 °C, (b) 400 to 550 °C, and (c) 600 to 700 °C in air.


Page 22 of 34

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Fig. 9: Impedance spectra of spark plasma sintered BaZr0.90Ho0.10O3-δ sample at different temperature ranges (a) 300 to 450 °C, (b) 500 to 650 °C, and (c) 700 to 800 °C in 3% humidified O2 atmosphere.



Fig. 10: Impedance spectra of spark plasma sintered BaZr0.80Ho0.20O3-δ sample at different temperature ranges (a) 300 to 350 °C, (b) 400 to 600 °C, and (c) 650 to 800 °C in 3% humidified O2 atmosphere.


Page 24 of 34

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Fig. 11: Total conductivity of spark plasma sintered BaZr(1-x) HoxO3-δ ceramics for x = 0.10 and 0.20 in dry air and 3% humidified O2 atmospheres, respectively.



Fig. 12: Schematic representation of the Grotthus mechanism for proton conduction.


Page 26 of 34

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Table 1: Rietveld refined structural parameters for BaZr1-xHoxO3-δ (x = 0.10 and 0.20) ceramic samples obtained by spark plasma sintering at temperature of 1600 °C for the holding time of 20 min.

Bond length (Å) x

ɑ (Å)

Vl 3

(Å )

0.10

4.1983

73.998

0.20

4.2092

74.576

Refined Rietveld Parameters Rp = 16.2, Rwp = 15.2, Rexp = 14.72, χ2 = 1.06 Rp = 14.6, Rwp = 13, Rexp = 13.42, χ2 = 1.09

D [ZrO6]

[BaO12]

Zr/Ho-O

Ba-O

2.0992

2.9686

107.98

2.1046

2.9763

81.83


(nm)


Table 2: Atomic percentages of elements calculated from EDX analysis in BaZr1-xHoO3-δ. x 0.10 0.20

Ba (%) 18.27 21.31

Zr (%) 17.24 16.83

Ho (%) 1.75 3.65

O (%) 63.01 58.21


Page 28 of 34

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Table 3: Electrical equivalent circuit fitting parameters from impedance spectra for BZH10 sample in dry air. T (°°C)

Rg

Qg

ng

n 2 (Ω Ω/cm2) (S-sec /cm )

Rgb

Qgb

(Ω Ω- cm2)

(S-secn/cm2)

ngb

Re

Qe

ne


300

6088

1.10 × 10 -9

1

8.04 × 106

5.64 × 10-9

0.899

-

-

-

400

1174

1.01 × 10 -9

1

7.55 × 105

1.26 × 10-8

0.860

-

-

-

500

292.3

1.08 × 10 -9

1

1.60 × 104

2.51 × 10-8

0.822

-

-

-

600

84.65

1.59 × 10 -9

0.999

2042

2.61 × 10-8

0.852

1151

5.16 × 10-6

0.548

700

-

-

-

356

4.99 × 10-8

0.802

212.6

1.77 × 10-4

0.323



Page 30 of 34

Table 4: Electrical equivalent circuit fitting parameters from impedance spectra for BZH20 sample in dry air. T

Rg

Qg

ng

n 2 (°°C) (Ω Ω/cm2) (S-sec /cm )

Rgb

Qgb

(Ω Ω- cm2)

(S-secn/cm2)

ngb

Re

Qe

(Ω Ω/cm2)

(S-secn/cm2)

ne

300

-

-

-

7.47 × 105

2.51 × 10-9

0.877

-

-

-

400

-

-

-

4.55 × 104

4.86 × 10-9

0.855

2.72 × 104

6.78 × 10-7

0.818

500

-

-

-

9288

1.92 × 10-9

0.820

7815

4.32 × 10-6

0.512

600

-

-

-

1066

6.98 × 10-9

0.840

1132

9.99 × 10-6

0.479

700

-

-

-

302.7

6.11 × 10-9

0.839

314.8

3.44 × 10-5

0.438


Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5: Electrical equivalent circuit fitting parameters of impedance spectra for BZH10 sample in 3% humidified O2. T (°°C)

Rg

Qg

ng

Rgb

Qgb

(Ω Ω- cm2)

(S-secn/cm2)

0.986

1.20 × 106

5.73 × 10-9


300 2.70 × 104 1.03 × 10-10

ngb

Re

Qe

(Ω Ω/cm2)

(Ssecn/cm2)

0.821

-

-

-

ne

400

1645

2.75 × 10-10

1

7.04 × 104

1.26 × 10-8

0.798

-

-

-

500

-

-

-

4845

1.45 × 10-8

0.860

3719

2.05 × 10-6

0.497

600

-

-

-

1026

2.46 × 10-8

0.814

392.1 6.77 × 10-6

0.625

700

-

-

-

107.8

4.71 × 10-9

1

63.01 1.97 × 10-5

0.578

800

-

-

-

13.63

3.92 × 10-8

1

8.479 5.65 × 10-4

0.534



Page 32 of 34

Table 6: Electrical equivalent circuit fitting parameters of impedance spectra for BZH20 sample in 3% humidified O2. T (°°C)

Rg

Qg

ng


Rgb

Qgb

ngb

n 2 (Ω Ω- cm2) (S-sec /cm )

Re

Qe

ne


300

-

-

-

1.89×105

2.24×10-8

0.689

-

-

-

400

-

-

-

7926

2.72×10-8

0.735

3132

9.80×10-7

0.648

500

-

-

-

638

9.02×10-8

0.872

522.8

4.53×10-6

0.534

600

-

-

-

70.75

5.30×10-8

1

56.48

8.60×10-6

0.636

700

-

-

-

10.74

3.53×10-8

1

3.615

3.73×10-5

0.786

800

-

-

-

1.863

9.38×10-7

1

1.513

3.78×10-5

0.892


Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 7: The total conductivity of electrolyte ceramic materials as a function of measurement conditions (atmosphere and probing temperature), the quality of ceramics (relative density), and technological regimes for preparation of ceramics (synthesis method and sintering temperature). Sample

Synthesis method

Sintering conditions

Relative density (%)

Total Conductivity (S-cm-1)

Measurement conditions

References

BZH10

FPR

1600 °C for 20 min. (SPS)

99.5

1.23 × 10-3 (700 °C)

Air

Present work

BZH20

FPR


99.1

9.64 × 10-3 (700 °C)

Air

Present work

BZH10

FPR


99.5

4.84 × 10-3 (700 °C)

3% humidified O2

Present work

BZH20

FPR


99.1

4.91 × 10-2 (700 °C)

3% humidified O2

Present work

BZY10

SS


99.8

2.4 × 10-3 (600 °C)

N2/H2 (9%), H2O (0.015 atm)

40

BZY10

SS


99.8

3.8 × 10-3 (600 °C)

N2/H2 (9%), H2O (0.030 atm)

40

BZY10

Pechini

1400 °C for 5 min, (SPS)

92.4

9.4 × 10-7 (500°C)

Wet H2

41

BZY10

CC +SSRM

1850 °C (CS)

∼100

1.41× 10-2 (650 °C)

Wet air

42

BZY15

SSRM

1800 °C (CS)

∼95

6.0 × 10-4 (700 °C)

Wet air

43

BZY20

SSRM

1600 °C (CS)

95

5.30 × 10-3 (700 °C)

Wet air

44

∗

FPR - Flash Pyrolysis Route, SS - Solid State, CC - Co-precipitation, SSRM – Solid State Reaction Method



Table of Contents (TOC)


Page 34 of 34

Improved Conductivity of Spark Plasma Sintered ... - ACS Publications

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