Effect of thermal treatment on the phase structure and electrical properties of Ce x Gd 1-x O 2-δ electrolytes

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1 Indian Journal of Chemistry Vol. 53A, December 2014, pp Effect of thermal treatment on the phase structure and electrical properties of Ce x Gd 1-x O 2-δ electrolytes Ping Fang a, *, Shiping Li b, Jiqing Lu b, Yueqing Lu a & Mengfei Luo b a School of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing , China fangping@usx.edu.cn b Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua , China Received 16 May 2014; revised and accepted 3 November 2014 A series of Ce x Gd 1-x O 2-δ electrolytes has been prepared by typical and improved sol-gel methods with different thermal treatment conditions. The Ce x Gd 1-x O 2-δ phase structures have been characterized by XRD and Raman spectroscopy, and the electrolytes properties studied by the AC impedance spectroscopy. It is found that the improved sol-gel method is beneficial for the formation of CeO 2 -based or Gd 2 O 3 -based solid solutions and consequently oxygen vacancies, through which the electrolytes can absorb and desorb oxygen continuously, resulting in improved electrical conductivity. Amongst the studied electrolytes, the Ce 0.6 Gd 0.4 O 2-δ -A electrolyte prepared by typical method and the Ce 0.6 Gd 0.4 O 2-δ -N electrolyte prepared by improved method show the highest electrical conductivity. Keywords: Solid electrolytes, Sol-gel method, Oxygen vacancy, Phase structure, Electrical conductivity Solid oxide fuel cells (SOFCs) are an important type of fuel cells because of their clean and high efficient production of electricity 1. Solid electrolytes are the most important and indispensable components of SOFC, and their structures and properties directly affect the performance of SOFC. Rare earth doped-ceria solid electrolytes are of growing interest for their applications in intermediate temperature solid oxide fuel cells (IT-SOFCs), due to their higher electrical conductivities as compared to the traditional yttria stabilized zirconia (YSZ). Since the ionic radius of Gd 3+ is close to that of Ce 4+ 2, recently, Gd-doped ceria (GDC), was used as electrolyte for intermediate temperature SOFCs 3-6. In our previous research 7, the Ce x Gd 1-x O 2-δ electrolytes prepared by a traditional citrate method showed good electrical conductivity. However, during the sol gel process, organic compounds were burnt up violently accompanied with great heat when heated directly at high temperature in air atmosphere, resulting in the aggregation of particles 8. The sintering particles with large grain boundary of electrolyte block the flow of current and consequently, decrease the electrical conductivity 9. To avoid the heat emissions, observed in our previous work 10, in the present study the CeO 2 -based citrate precursor was placed in a tube furnace and heated in the nitrogen atmosphere which yielded a black mixture of CeO 2 -based oxides and carbon powders. Subsequently, the intermediate mixture was calcined in air to remove the carbon species. This improved sol-gel method avoids the intense combustion of citric acid, but produces amorphous carbon powders, which can encapsulate or isolate CeO 2 -based oxides and prevent them from sintering. Ultrafine particles could thus be synthesized, which may reduce the grain boundary resistivity, and improve the electrical conductivities of electrolytes. In this article, Ce x Gd 1-x O 2-δ electrolytes were prepared by sol gel methods with typical and improved thermal treatments. The complex impedance spectroscopy of the electrolytes was measured at various temperatures. The influence of microstructures of these electrolytes on their electrical properties was investigated. Materials and Methods Ce(NO 3 ) 3 6H 2 O (> 99.5%), Gd 2 O 3 (> 99.99%), and citric acid (> 99.5%) were used as starting materials. Ce(NO 3 ) 3 6H 2 O dissolved in deionized water was added to a solution of Gd 2 O 3 dissolved in the least amount of conc. nitric acid to form a Ce-Gd nitrate solution. Citric acid with double mole ratio of total metal cations was added to obtain a mixed nitrate and citrate solution. After being stirred for several

2 1514 INDIAN J CHEM, SEC A, DECEMBER 2014 minutes, the pellucid solution was heated at 90 ºC on a magnetic stirrer for several hours until a viscous gel was obtained. The gel was dried at 110 ºC overnight, followed by two thermal treatment methods. In the typical sol-gel method, the precursor was calcined at 600 ºC for 4 h in air to obtain the powder, which was denoted as Ce x Gd 1-x O 2-δ -A6. In the improved sol-gel method, the precursor was first placed in a tube furnace and heated in N 2 at 600 ºC for 2 h, and then calcined at 500 for 4 h to burn off the carbon. The resulting sample was denoted as Ce x Gd 1-x O 2-δ -N6. The mole ratio of Ce/Gd was x/(1-x) (x = 0, 0.3, 0.6, 0.8, 1.0). The calcined powder was ground and pressed into pellets at 400 MPa using a die of 10 mm diameter. These pellets were sintered at 1450 o C for 4 h in air to obtain the solid electrolytes with a thickness of 0.5 mm and denoted as Ce x Gd 1-x O 2-δ -A and Ce x Gd 1-x O 2-δ -N electrolytes. X-ray diffraction (XRD) patterns were collected on a PANalytical X pert PRO diffractometer using Cu Kα radiation. The working voltage of the instrument was 40 kv and the current was 40 ma. The intensity data were collected in a 2θ range from 20 o 90 o with a scan rate of 1.0 o min -1 at 25 o C. The phase compositions of the samples were calculated by a full curve fitting method using JADE 6.5 software. Raman spectroscopy was performed using a Renishaw RM 1000 confocal microscope, with an excitation laser line of nm (Ar + laser) and a resolution of ±1 cm -1. Data acquisition was carried out at 25 C at the scanning range from 100 to 1000 cm -1. Electrical conductivities of the sintered pellets of the samples were measured. Two Pt sheets were used as the electrodes and platinum paint was used to fix the Pt sheets onto both sides of the sintered pellets. The AC impedance spectra of the sintered electrolytes were obtained in air with a PAR potentiostat (model 283) and EG&G (model 1025) frequency response detector. The samples were heated during the measurement in a homemade furnace. The test frequencies were in the range from 0.1 Hz to 4 MHz and the measurements were carried out in the temperature range of C with an interval of 50 o C. Ziew 2.0 software was used to analyze the impedance data and calculate the electrical conductivity of the samples. Results and Discussion XRD patterns The XRD patterns of (a) Ce x Gd 1-x O 2-δ -A and (b) Ce x Gd 1-x O 2-δ -N samples are shown in Fig. 1. For the Ce 0.8 Gd 0.2 O 2-δ -A samples, Fig. 1(a) reveals that the diffraction patterns are in good agreement with those of pure ceria typical fluorite-like cubic structure (CeO 2, JCPDS: ). When x 0.6, some weak diffraction peaks (e.g o, 39.0 o, 42.5 o ) are observed, which are attributed to the cubic Gd 2 O 3. In order to identify the phase compositions, JADE 6.5 software was used to analyze these samples. Table 1 lists the phase compositions and crystallite sizes of Ce x Gd 1-x O 2-δ -A and Ce x Gd 1-x O 2-δ -N samples. It can be seen that the crystallite sizes of all Ce x Gd 1-x O 2-δ -N samples are smaller than those of Ce x Gd 1-x O 2-δ -A, which indicates that the improved sol-gel methods prevents the intense combustion of citric acid, but produce amorphous carbon powders, which can encapsulate or isolate CeO 2 -based oxides and prevent them from sintering. Smaller particles could thus be synthesized. From Table 1, it is found that Ce 0.8 Gd 0.2 O 2-δ -A is in form of CeO 2 -based solid solution, while the Ce 0.6 Gd 0.4 O 2-δ -A is combined in a mixture of CeO 2 - Fig. 1 XRD patterns of (a) Ce x Gd 1-x O 2-δ -A and (b) Ce x Gd 1-x O 2-δ -N.

3 FANG et al.: PHASE STRUCTURE & ELECTRICAL PROPERTIES OF Ce x Gd 1-x O 2-δ ELECTROLYTES 1515 Table 1 Phase compositions and crystallite sizes of Ce x Gd 1-x O 2-δ -A and Ce x Gd 1-x O 2-δ -N samples Sample Phase comp. (%) Crystallite size (nm) CeO 2 (FCC) CeO 2 -based solid solutions (FCC) Gd 2 O 3 -based solid solution (BCC) Gd 2 O 3 (BCC) Gd 2 O 3 (base-centered monoclinc) CeO 2 -A Ce 0.8 Gd 0.2 O 2-δ -A Ce 0.6 Gd 0.4 O 2-δ -A Ce 0.3 Gd 0.7 O 2-δ -A Gd 2 O 3 -A CeO 2 -N Ce 0.8 Gd 0.2 O 2-δ -N Ce 0.6 Gd 0.4 O 2-δ -N Ce 0.3 Gd 0.7 O 2-δ -N Gd 2 O 3 -N Fig. 2 Laser Raman spectra of (a) Ce x Gd 1-x O 2-δ -A and (b) Ce x Gd 1-x O 2-δ -N. based and Gd 2 O 3 -based solid solution, in the ratio of 6.8:93.2(%). However, the Ce 0.3 Gd 0.7 O 2-δ -A sample, consists of Gd 2 O 3 -based solid solution and Gd 2 O 3, in the ratio of 65.0:35.0(%). The single Gd 2 O 3 -A sample, consists of body-centered cubic Gd 2 O 3 and base-centered monoclinc Gd 2 O 3 (80.6:19.4(%)). The XRD patterns of the Ce x Gd 1-x O 2-δ -N samples (Fig. 1(b)) are similar to those of the corresponding Ce x Gd 1-x O 2-δ -A samples. However, the single Gd 2 O 3 -N sample consists of only the base-centered monoclinc Gd 2 O 3 phase. The XRD results indicate that both Ce 0.8 Gd 0.2 O 2-δ -A and Ce 0.8 Gd 0.2 O 2-δ -N exist as CeO 2 -based solid solution, while the electrolytes are as two phases at high Gd 3+ contents (x = 0.6, 0.3). The single Gd 2 O 3 sample prepared by typical method exists in the cubic and monoclinc phases, while in that prepared by the improved method, only the monoclinc phase was observed. Raman spectra Raman spectra of the Ce x Gd 1-x O 2-δ samples are shown in Fig. 2. From Fig. 2, the x = 0 (Gd 2 O 3 ) sample shows an entirely different Raman spectra. The single Gd 2 O 3 -A shows Raman bands at 315, 361, 445 and 568 cm -1, attributed to the body-centered cubic Gd 2 O 3 11, and the bands at 110, 175, 218, 252, 267, 300, 385, 416, 428, 445, 482 and 580 cm -1 of the base-centered monoclinc phase of Gd 2 O 3. The band at 445 cm -1 is overlapped for the two phases 11,12. For the Gd 2 O 3 -N sample, only the bands belonging to monoclinc phase can be observed. For the Ce 0.8 Gd 0.2 O 2-δ -A electrolyte, there is a feature band at

4 1516 INDIAN J CHEM, SEC A, DECEMBER cm -1, which is attributed to the F 2g typical vibrational mode of the cubic CeO 2 structure 13,14, while the broad peaks around 260 and 580 cm -1 are attributed to oxygen vacancies generated by the incorporation of other metal cations into the ceria lattice Gd O 2Gd +3O +V 2CeO Ce O O These oxygen vacancies can increase the diffusion rate of oxygen, by which the material can absorb and desorb oxygen continuously 17. When x = , except for the bands at 260 and 580 cm -1, new bands at 375 and 480 cm -1 are observed, which in tune with the XRD results, are attribute to the vibrational mode of the cubic Gd 2 O 3 -based solid solutions. Figure 2(b) shows the Raman spectra of the Ce x Gd 1-x O 2-δ -N samples. The bands are essentially the same as those of the corresponding Ce x Gd 1-x O 2-δ -A samples. For the single Gd 2 O 3 -N, only bands for monoclinc phase are observed. The Raman results indicate that the formation of either CeO 2 -based or Gd 2 O 3 -based solid solution can promote the generation of oxygen vacancies (260 and 580 cm -1 ). AC impedance spectroscopy Table 2 lists the equivalent parameters of AC impedance of the Ce x Gd 1-x O 2-δ -A and Ce x Gd 1-x O 2-δ -N electrolytes measured at 800 o C. On comparing the samples prepared by the two thermal treatment methods, it is found that the Ce x Gd 1-x O 2-δ -N samples show better conductivities than the Ce x Gd 1-x O 2-δ -A samples. This is because the improved method is beneficial for the formation of smaller CeO 2 -based or Gd 2 O 3 -based solid solutions, which can enhance the diffusion rate of oxygen. Thus, the electrolytes can absorb and desorb oxygen continuously, resulting in the improvement of its electrical conductivity. Although the crystallite size of Ce 0.3 Gd 0.7 O 2-δ -N Table 2 Equivalent parameters of AC impedance of Ce x Gd 1-x O 2-δ -A and Ce x Gd 1-x O 2-δ -N electrolytes measured at 800 o C σ ( 10-5 S cm -1 ) Electrolyte Ce x Gd 1-x O 2-δ -A Ce x Gd 1-x O 2-δ -N CeO Ce 0.8 Gd 0.2 O 2-δ Ce 0.6 Gd 0.4 O 2-δ Ce 0.3 Gd 0.7 O 2-δ Gd 2 O electrolyte is smaller than that of Ce 0.6 Gd 0.4 O 2-δ -N, since Ce 0.3 Gd 0.7 O 2-δ -N contains only 65.1% Gd 2 O 3 - based solid solution, the Ce 0.6 Gd 0.4 O 2-δ -N electrolyte still has the better conductivity. From Table 2 it can be seen that for either the Ce x Gd 1-x O 2-δ -A or the Ce x Gd 1-x O 2-δ -N samples, the conductivity of the electrolyte first increases with increasing Gd 3+ dopant concentration and reaches the maximal value at x = 6, however, further increasing Gd 3+ content in the sample results in a decline of conductivity. Among these samples, the maximum electrical conductivity is seen for a dopant concentration of x = 0.6 (Ce 0.6 Gd 0.4 O 2-δ -A or Ce 0.6 Gd 0.4 O 2-δ -N). This trend is probably related to the presence of oxygen vacancies in the sample. At low Gd 3+ doping concentrations, substituting Gd 3+ for Ce 4+ increases the number of free oxygen vacancies. The effective concentration and the transition probability of oxygen vacancy increases, which leads to the gradual increase of the conductivity. However, with the further increasing of the Gd 3+ doping concentration (i.e. x = 0.3), oxygen vacancies will associate with the Gd lacuna to form { Gd V Ö } Ce lacuna cluster, which decreases the truly free volume of crystal lattice and the effectual oxygen vacancy concentration, leading to blocking the passage of oxygen, thus decreasing the conductive ability of the electrolytes 3,18,19. The AC impedance spectra of the Ce 0.6 Gd 0.4 O 2-δ -A measured at 700, 750, 800 and 850 o C are presented in Fig. 3. The resistance of grain interior, grain boundary and semicircle of electrode polarization cannot be observed clearly in this figure. The electrical resistance was obtained from the point of intersection of semicircle and the abscissa, and then the electrical conductivity was calculated. With increasing testing temperature, the transfer rate of oxygen increases and therefore, the resistance decreases, i.e., the electrical conductivity increases synchronously. The impedance spectra can be modeled with the equivalent circuit shown in Fig. 4. R0 represents the unavoidable resistance associated with testing equipment design. Rgi is the grain interior resistance, meanwhile, Rgb and Rct are the grain boundary and electrode polarization resistance respectively. CPEgi, CPEgb and CPEct are the grain interior, grain boundary, electrode polarization constant phase element, respectively. Ce

5 FANG et al.: PHASE STRUCTURE & ELECTRICAL PROPERTIES OF Ce x Gd 1-x O 2-δ ELECTROLYTES 1517 Fig. 3 AC impedance spectra of Ce 0.6 Gd 0.4 O 2-δ -A measured at varying temperatures. [(a) 700 o C; (b) 750 o C; (c) 800 o C; (d) 850 o C]. Fig. 4 Equivalent circuit used in analyzing impedance spectra. Figure 5 shows the AC impedance spectra of Ce 0.6 Gd 0.4 O 2-δ -N measured at 700, 750, 800 and 850 C. Compared with Fig. 3, it can be seen that while the resistance of grain interior and grain boundary is divided, the semicircle of electrode cannot be observed obviously. With increasing test temperature, the resistance decrease, i.e., the electrical conductivity increases synchronously. For analysis of the total resistance, the grain interior and grain boundary resistance are combined. The electric mechanism of oxygen in the solid electrolytes depends on the transfer of oxygen vacancy. Combined with the Arrhenius equation, the equation of electrical conductivity and temperature is obtained as σ = (A/T)exp(-E a /kt) lnσt = lna - E a /kt The Arrhenius plots of the total conductivity of Ce 0.6 Gd 0.4 O 2-δ electrolytes are shown in Fig. 6. The electrical conductivity of Ce 0.6 Gd 0.4 O 2-δ -N (1.57 ± 0.02 ev) is lower than the sample prepared by the typical method (2.83 ± 0.18 ev). Lower activation energies can help the oxygen vacancy transit from out-of-order to ordinal at lower temperature and hence the electrolyte prepared by the improved method can accelerate the oxygen vacancy transition and show higher electrical conductivity. The Ce 0.6 Gd 0.4 O 2-δ -N electrical conductivity value (1.57 ± 0.02 ev) was close to the value of Ce 0.8 Gd 0.2 O 1.9 solid electrolyte (1.04 ev) prepared by Modified pechini method 20, and Ce 0.8 Gd 0.2 O 1.9 (0.93 ev) synthesized by the EDTA-citrate method 21. The Ce 0.6 Gd 0.4 O 2-δ -N electrical conductivity value is lower than the electronic activation energies of (CeO 2 ) 0.9 (CaO) 0.1 (1.77 ± 0.03 ev at o C, 2.18 ± 0.03 ev at o C and 2.24 ± 0.07 ev at o C) 22.

6 1518 INDIAN J CHEM, SEC A, DECEMBER 2014 Fig. 5 The AC impedance spectra of Ce 0.6 Gd 0.4 O 2-δ -N measured at varying temperatures. [(a) 700 o C; (b) 750 o C; (c) 800 o C; (d) 850 o C]. Conclusions With the improved thermal treatment method, the formation of the CeO 2 -based or Gd 2 O 3 -based solid solutions in the Ce x Gd 1-x O 2-δ -N can be enhanced. The presence of such solid solutions helps in the generation of oxygen vacancies, and hence fastens the diffusion rate of oxygen. Thus the electrolytes can absorb and desorb oxygen continuously, resulting in improvement of its electrical conductivity. Amongst the studied samples, Ce 0.6 Gd 0.4 O 2-δ -N presents with the best electrical conductivity. As the oxygen vacancy number increases with increasing Gd 3+ doping, but, when dopant concentration is higher than 40%, the formation of { GdCe V Ö } lacuna cluster decreases the conductivity. Fig. 6 Arrhenius plots of the total conductivity of Ce 0.6 Gd 0.4 O 2-δ electrolytes. This shows that Ce x Gd 1-x O 2-δ prepared by the improved sol-gel method had better electrical properties than similar samples prepared by other methods. Acknowledgement The support by the Natural Science Foundation of Zhejiang Province, PR China (grant Z404383) and the Foundation of Shaoxing University, PR China (2012LG09) are acknowledged.

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