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Superionic Conductivity of Sm3+, Pr3+, and Nd3+ Triple-Doped Ceria through Bulk and Surface Two-Step Doping Approach Yanyan Liu,†,# Liangdong Fan,‡,# Yixiao Cai,*,§,∥ Wei Zhang,⊥ Baoyuan Wang,⊥ and Bin Zhu*,†,⊥ †

Department of Energy Technology, Royal Institute of Technology, Stockholm SE-10044, Sweden Shenzhen Key Laboratory of New Lithium-Ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, Guangdong 518060, P. R. China § Department of Engineering Sciences, Ångström Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden ∥ NUS Environmental Research Institute, National University of Singapore, 1 Create Way, Singapore 138602, Singapore ⊥ Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Faculty of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062, P. R. China ‡

S Supporting Information *

ABSTRACT: Sufficiently high oxygen ion conductivity of electrolyte is critical for good performance of low-temperature solid oxide fuel cells (LT-SOFCs). Notably, material conductivity, reliability, and manufacturing cost are the major barriers hindering LT-SOFC commercialization. Generally, surface properties control the physical and chemical functionalities of materials. Hereby, we report a Sm3+, Pr3+, and Nd3+ triple-doped ceria, exhibiting the highest ionic conductivity among reported doped-ceria oxides, 0.125 S cm−1 at 600 °C. It was designed using a two-step wet-chemical coprecipitation method to realize a desired doping for Sm3+ at the bulk and Pr3+/Nd3+ at surface domains (abbreviated as PNSDC). The redox couple Pr3+/ Pr4+ contributes to the extraordinary ionic conductivity. Moreover, the mechanism for ionic conductivity enhancement is demonstrated. The above findings reveal that a joint bulk and surface doping methodology for ceria is a feasible approach to develop new oxide-ion conductors with high impacts on advanced LT-SOFCs. KEYWORDS: LT-SOFCs, doped ceria, bulk and surface doping, oxygen ion conductivity, redox

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INTRODUCTION Solid oxide fuel cells (SOFCs) are advanced electrochemical energy-conversion devices to realize the direct transformation from chemical into electrical energy. They are promising candidates for the ever-increasing energy demand with the advantages of high conversion efficiency, low emission, zero noise, and a variety of applications.1 Solid electrolyte, as the major component in SOFC devices, determines the operating temperature and related systems.2 High ionic conductivity of electrolyte is indispensable for fuel cells to achieve good performances. However, high operating temperature, e.g., 800− 1000 °C, is strictly required. The main drawbacks for such elevated temperature may give rise to high cost and system unreliability for commercialization. Goodenough3 proposed that oxide-ion conductors as alternative materials were necessary to be developed into low enough temperatures and to be technically useful. Until now, extensive attempts have been devoted to explore highly conductive ionic conductors, but very few were available to reach 0.1 S cm−1.4 Ceria-based oxide electrolytes have attracted intensive attention for their higher ionic conductivity and lower activation energy comparing with yttria-stabilized zirconia © 2017 American Chemical Society

(YSZ), with additional virtues, for instance, high catalytic activity, allowing intermediate- or low-temperature operation.5−7 Notably, doping with lower valence metal cations plays a central role in introducing oxygen vacancies for developing high ionic conductive ceria.8,9 To date, some typical doped ceria materials, such as Ce0.8Sm0.2O1.9 (SDC) and Ce0.8Gd0.2O1.9 (GDC), have been widely investigated for LTSOFCs as single-ion substituted oxides.10−12 However, one of the crucial issues for these doped-ceria oxides is the redox instability induced by the reduction from Ce4+ to Ce3+, resulting in electronic conduction and undesirable structural change.13 To address those issues, many efforts have been devoted in the SOFC field.14−17 Virkar14 utilized YSZ as the electronblocking layer to prevent electronic leakage current. Wachsman and Lee15 reported codoped bismuth-oxide-based materials with the known highest conductivity as the solid-state oxide electrolyte for SOFCs. Sanna et al.16 further developed δ-Bi2O3 Received: February 15, 2017 Accepted: June 26, 2017 Published: June 26, 2017 23614

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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• (Cell I) Ni-foam/SDC+NCAL (anode)|SDC (electrolyte)|SDC +NCAL (cathode)/Ni-foam; • (Cell II) Ni-foam/PNSDC+NCAL (anode)|PNSDC (electrolyte)| PNSDC+NCAL (cathode)/Ni-foam. The fuel cells were copressed at a pressure of 150 MPa to form a disc-type green cell of 13 mm in diameter, 1 mm in thickness including the SDC or PNSDC of about 0.5 mm in thickness, with an active area of 0.64 cm2. The electrochemical performances of the fuel cells were measured using a programmable electronic load (ITECH8511, ITECH Electrical Co., Ltd.) at an operation temperature of 550 °C. Hydrogen was used as the fuel with a flow of 120 mL min−1 at a pressure of 1 bar. Microstructural Characterization and Electrochemical Test. The surface and particle morphology of PNSDC sample were detected by a field-emission scanning electron microscope (MERLIN, Germany) and FP5021/20 transmission electron microscope (TEM, Czech Republic). Raman spectra analysis was performed by using a Renishaw Ramascope equipped with a Lieca LM optical microscope and a charge-coupled device (CCD) camera. The excitation laser lines were at the wavelengths λex = 325 and 532 nm. Before measurements, calibration was performed using a piece of Si wafer. X-ray photoelectron spectroscopy (XPS, Physical Electronics Quantum 2000, Al Kα X-ray source) was used to investigate the surface chemical properties of the SDC and PNSDC samples in terms of oxidation states and species of surface atoms. The IGOR software (Wavemetrics, Lake Oswego, OR) was further used for curve fitting and quantitative chemical analysis. The binding energies were calibrated using the C 1s peak at 284.6 eV as a reference and were quoted with a precision of ±0.2 eV. The electrical conductivities of as-prepared SDC and PNSDC were measured in air atmosphere by electrochemical electrochemical workstation (CHI660B, Chen Hua Corp.), in a temperature range of 400−600 °C upon cooling at an interval of 50 °C under opencircuit condition. Each pellet has a thickness of 2 mm prepared by drypressing. The silver pastes served as current collectors. The electronic conductivity was tested using a Hebb-Wagner ion-blocking cell by dc polarization method. The measurement was carried out by a digital micro-ohm meter (KD2531, Kangda Electrical Co., Ltd.). A Gamry Reference 3000 instrument was employed to analyze the electrochemical performance of fuel cell pellets under H2/air condition at 550 °C. The measured frequency ranges from 0.01 Hz to 1 MHz under a bias voltage of 10 mV, and the electrical conductivity σ can be calculated depending on the following equation:27

using highly coherent interfaces of alternating layers of Er2O3stabilized δ-Bi2O3 and Gd2O3-doped CeO2 displaying exceptionally high chemical stability in reducing conditions and redox cycles at elevated temperature. Double or codoping has been demonstrated to be a successful strategy to enhance ceria conductivity both from theoretical and experimental aspects.18,19 For example, Banerjee et al.18 developed a Ca−Sm codoped ceria exhibiting an admirable ionic conductivity of 0.122 S cm−1 at 700 °C. Besides, the nanotechnology approach has also attracted significant attention on improving the ionic conductivity of ceria-based oxides. Recently, nanocomposites have been proposed and developed by introducing functional ceria-based composites into LT-SOFCs.20−24 These studies revealed that the surface/interfacial conduction played a significant role in enhancing the total ionic conductivities. It gives hints to the authors to further improve the performance by surface/grain boundary modification or functionalization. On the basis of the impact of surface properties on the physical or chemical functionalities of materials, we design a novel approach to modify the surface property of samariumdoped ceria (SDC) with Pr3+ and Nd3+ dopants. Pr3+ (1.126 Å) and Nd3+ (1.109 Å) are selected because of their potentials in extending the electrolytic domain boundary to lower oxygen partial pressure as well as the potential catalytic activity for oxygen reduction reaction (ORR).25,26 Remarkably, the doped ceria reached the highest ionic conductivity so far, 0.125 S cm−1 at 600 °C in air, and displayed excellent fuel cell performance as electrolyte. We further discuss the mechanisms on the enhancement of ionic conduction and redox capability. This work reveals a promising approach to design functional materials for next-generation LT-SOFCs.

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EXPERIMENTAL SECTION

Materials Preparation. All the chemicals in this study were analytical grade purchased from Sinopharm Chemical Reagent Co., Ltd., China, and used as received without further purification. Samarium-doped ceria (SDC) was prepared via a facile carbonate coprecipitation approach. Designed amounts of cerium nitrate and samarium nitrate with the stoichiometric ratio of 4:1 were dissolved in deionized water. Sodium carbonate solution was prepared in parallel. Then, the metal salt precursors (cerium nitrate and samarium nitrate) and sodium carbonate solution were mixed with a molar ratio of 1:2 and continuously vigorously stirred for 6 h followed by aging for another 6 h. After filtration, the excess carbonate was removed thoroughly by repeated washing. The obtained precipitate was dried in the oven overnight and then calcined at 800 °C for 4 h to obtain the product, denoted as SDC. Pr3+ and Nd3+ ions as secondary dopants (with molar ratio of 9:1) were introduced into the SDC skeleton to prepare the Sm3+, Pr3+, and Nd3+ triple-doped ceria (PNSDC) sample through a modified solidstate reaction route. Initially, the quantities of commercial praseodymium neodymium nitrate (1.8 mol % in the total metal ions of PNSDC sample) were added into deionized water to form a transparent solution. Then, the as-prepared SDC was added into the above solution. Subsequently, the homogeneous suspension was dried and then calcined at 750 °C for 2 h. The final product was vigorously ground, denoted as PNSDC. Commercial nickel cobalt aluminum lithium oxides (Ni0.8Co0.15Al0.05Li-oxide, NCAL) were purchased from Tianjin Bamo Sci. & Tech. Joint Stock Ltd., China, and used as the symmetrical electrode catalyst and current collectors. Fuel Cell Fabrication and Performance Measurement. Two kinds of fuel cells were fabricated for using the SDC (cell I) and PNSDC (cell II) as the electrolytes, respectively, and the related composites with NCAL as symmetrical electrodes are shown in the following configuration:

σ=

L RA

(1)

Here, L is the thickness of the samples, R is the total resistance, and A represents the cross-sectional area.

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RESULTS AND DISCUSSION Powder Phase and Microstructure Analysis. Figure 1 displays the X-ray diffraction (XRD) patterns of monocation doping (SDC) and trication doping (PNSDC) ceria oxides. All the diffraction peaks are assigned to the cubic fluorite structure CeO2 according to JCPDS 34-0394. The diffraction peaks of PNSDC are slightly shifted to lower Bragg due to the bigger ionic radius of Pr3+ (1.126 Å) and Nd3+ (1.109 Å) substitutions at Ce4+ (0.97 Å) sites leading to the slight lattice expansion.25 The calculated lattice constants of SDC and PNSDC are 5.414 and 5.415 Å, respectively, higher than that of CeO2 (5.411 Å).28 The crystallite sizes of as-prepared materials are calculated to be approximately 55 and 61 nm for SDC and PNSDC, respectively, according to Scherrer’s equation.29 The average particle size of PNSDC is mainly around 70 nm in the scanning electron microscope (SEM, Figure 2a) and transmission electron microscope (TEM, Figure 2b) images, which agrees with the results calculated from XRD analysis, indicating that 23615

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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concentration of Pr3+/Nd3+ ions is associated with the relative proportion of the intensity of the ultraviolet absorption peak and the introduction of Pr3+/Nd3+ ions is related to the oxygen vacancies according to the charge-compensation mechanism.30 With the wavelength increasing, the gradually decreasing intensity of the absorption peak for the resulting samples demonstrates that the information is collected from variable sampling depths by means of changing the excitation laser lines on the Raman spectroscopy measurements. This provides a solution that the accurate and complete oxygen vacancy information including the surface and bulk can be analyzed by using different excitation laser lines, such as 325 and 532 nm. The surface vibrational mode and lattice structural characteristics are investigated using the ultraviolet and visible Raman spectroscopy.26,30 Figure 3b and c display the Raman spectra with UV (325 nm) and visible (532 nm) excitation laser wavelengths (λ ex ) of the SDC and PNSDC samples, respectively. The dominant Raman-active mode of the symmetrical oxygen lattice Ce−O stretching vibration (F2g) in a fluorite-type CeO2 is indexed to around 460 cm−1 (λex = 532 nm).26,31−33 In this case, the peak is located at 461 cm−1 (denoted as peak α), showing a lower energy shift in both assynthesized SDC and PNSDC samples compared with the CeO2.34 Also a new feature mode appears at around 554 cm−1 (denoted as peak β), as shown in Figure 3b, due to the substitutions of Sm3+, Pr3+, or Nd3+ ions in ceria inducing the formation of oxygen vacancies.30 The Raman lines for F2g mode and the spectrum of oxygen vacancy introduced modes of PNSDC that exhibit more broadly and asymmetrically compared with SDC, implying a higher dopant concentration in the ceria lattice.34 The bulk and surface oxygen vacancy defects can be reflected by the ultraviolet (λex = 325 nm) and visible (λex = 532 nm) excitation laser lines, as shown in Figure 3d. The majority of excitation laser and scattering lines are absorbed at a strong excitation laser (λex = 325 nm, Figure 3a), while a small amount of light carrying the surface information is captured. Therefore, the information related to the oxygen vacancies on the surface of the doped ceria samples can be obtained using the UV excitation laser lines. That is, it is hard to collect Raman signals from the bulk of the tested samples at a strongly absorbed excitation laser. With increasing the wavelength of the excitation laser, the obtained information is closer to the sample’s bulk.30,35−37 More information in bulk can be obtained by using the increasing excitation laser, such as the visible region, 532 nm.26,30 Figure 3c shows that an obvious increasing peak intensity located at around 554 cm−1 (peak β) is observed for PNSDC compared to the SDC when the UV excitation laser in 325 nm was used. Further evidence was performed to support the substitutions of Ce4+ ions with Pr3+ and Nd3+ ions on the ceria surface regions. The intensity ratio of peaks β reflecting the oxygen vacancies and peak α representing the F2g mode of the fluorite-type structure, calculated by their peak areas and noted as Aβ/Aα, indicates the relative oxygen vacancy concentration and the defect-site types for doped ceria.30 The Aβ/Aα value for PNSDC (∼0.298) is similar to that of SDC (∼0.301) in Figure 3b, indicating that the secondary-doped Pr3+ and Nd3+ did not induce the apparent changes in bulk of ceria lattice. Obviously, a significant enhancement of the Aβ/Aα value for PNSDC (∼1.512) in comparison to that of SDC (∼1.294), representing a higher oxygen vacancy concentration, is obtained from the Raman spectra using the UV excitation laser line (Figure 3c). In

Figure 1. X-ray diffraction patterns of (a) SDC and (b) PNSDC.

Figure 2. (a) SEM and (b) TEM micrographs for PNSDC powder.

the nanoscale particles are highly dispersed by coprecipitation procedure. The energy-dispersive X-ray spectroscopy (EDS) element mapping analysis (see Figure S1 in Supporting Information) indicates the successful distribution of Pr3+ and Nd3+ onto the SDC scaffold. Combining with the XRD results, we confirm that the Pr3+/Nd3+ ions are successfully doped into the samarium-doped ceria, instead of forming a second phase. UV−Visible Raman Scattering Analysis. Ultraviolet− visible (UV−vis) diffuse reflectance spectroscopy is a typical technique to analyze the surface coordination and oxidation states of various metal ions. The UV−vis diffuse reflectance spectra of SDC and PNSDC samples are presented in Figure 3a. From the absorption spectra, strong absorption is observed in the UV region (400 nm, e.g., 532 nm) for the PNSDC, while no increase in this region is obtained for the SDC. The increase of the UV absorption for the PNSDC indicates that the substitutions of Ce4+ ions with Pr3+ and Nd3+ ions benefit the increase of the oxygen vacancy concentration because the 23616

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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Figure 3. (a) UV−vis diffuse reflectance spectroscopy of SDC and PNSDC; UV−visible Raman spectra of SDC and PNSDC with an excitation laser wavelength (b) λex = 532 nm and (c) λex = 325 nm; (d) schematic description for Raman scattering by varying the ultraviolet (UV, λex = 325 nm) and visible (λex = 532 nm) excitation laser lines to collect the surface and bulk information.

addition, the increasing β peak intensity reflecting the inhomogeneous distributions of the doped elements, e.g., Pr3+ and Nd3+, for the PNSDC also implies a significant enhancement of oxygen vacancies at the surface of PNSDC because the 325 nm laser line induces excitation on the shallow part of the doped ceria samples; see the schematic description in Figure 3d. Combining with XRD pattern for PNSDC in Figure 1, we can see slight shifting from SDC peaks, indicating the Pr3+/ Nd3+ indeed doped in SDC. Thus, the surface doping of Pr3+ and Nd3+ can be concluded. XPS Analysis. XPS is used to study surface properties of the SDC and PNSDC samples. Figure 4a shows the survey spectra, and it can be clearly found that the characteristics of Ce, Sm, Nd, Pr, Co, O, and C are present in the PNSDC sample. To identify the chemical state, the O 1s, Pr 3d, and Ce 3d corelevel spectra are discussed in detail below, and the relevant XPS peaks with multiple components were resolved by Gaussian functions. The O 1s core-level spectra shown in Figure 4b includes the chemisorbed oxygen species (Oα) and lattice oxygen (Oβ). The binding energy at 531.0−532.6 eV can be assigned to the oxide defects or the surface oxygen species (Oα) adsorbed on the oxygen vacancies (i.e., O−, OH−, and CO32−), while the binding energy at 528.8−529.4 eV is ascribed to lattice oxygen (Oβ).38 Usually, once the dopants are incorporated into ceria sample, the relevant Ce−O-dopant bonds can be formed, and then the oxygen becomes more labile, which results in the facilitation of the migration process from internal lattice oxygen to surface lattice oxygen.39 In our case, it is interesting to observe that the

Oα/Oβ value in the PNSDC sample is 1.06, which is close to that of SDC (1.14). Figure 4c indicates the coexistence of Ce4+ and Ce3+ ions on the surface of each sample. To better understand the effect of dopant on the surface chemistry and determine the relative fraction of Ce4+ and Ce3+ oxidation states, the Ce 3d spectra were deconvolved into five spin-split doublets in accordance with common use in the literature.40,41 Note that the peaks v, v″, v‴, u, u″, and u‴ are due to cerium ions in the 4+ state, while v0, v′, u0, and u′ reflect 3+ ions. The relative abundance of the Ce3+ species in each sample is estimated by calculating the deconvolution peaks. The analysis shows up to 2% increase in the Ce3+ ion concentration in the PNSDC sample as compared to the SDC. It has been reported that the existence of Ce3+ is associated with the formation of oxygen vacancies.42,43 Thus, the larger Ce3+ content in the PNSDC sample can result in more oxygen vacancies on the surface. Wolfframm et al. claimed that unambiguous fitting of the Pr 3d core levels is difficult and remains controversial.44 According to some previous SOFC studies,44,45 the Pr 3d spectra that envelope the Pr3+ and Pr4+ oxidation states were fitted into three distinguishable peaks, as shown in Figure 4d. The two characteristic peaks of Pr4+ 3d5/2 and 3d3/2 are observed at 931.6 and 950.2 eV, respectively. The peak pairs of 933.3/928.4 eV at 3d5/2 and 953.1/947.2 eV at 3d3/2 can be assigned to Pr3+. This observation indicates that the oxidation states of Pr in the compound are 3+ and 4+. More importantly, it clearly shows that the areas of Pr3+ peaks are significantly larger than that of Pr4+. This authenticates that the oxidation state of Pr3+ is dominant in the PNSDC sample. As reported, the reduction 23617

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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Figure 4. (a) XPS survey spectra of SDC and PNSDC samples; (b) XPS O 1s spectra of SDC and PNSDC samples; (c) XPS Ce 3d spectra of SDC and PNSDC samples; (d) XPS Pr 3d spectra of PNSDC sample.

Figure 5. Electrochemical impedance spectra of SDC and PNSDC measured in air at (a) 400, (b) 450, (c) 500, (d) 550, and (e) 600 °C, and (f) the empirical equivalent circuit to analysis the ionic conductivity of SDC and PNSDC materials in air.

fact, the electrical conductivity measurement greatly depends on the sample in situ environments or applied gas atmospheres. Generally, for the EIS measured in air, the intrinsic charge carrier makes main contributions to the conductivity. In a typical complex impedance spectroscopy for ionic conductivity materials case, the EIS mainly contains three arcs: the highfrequency one, the middle-frequency one, and the lowfrequency tail, which are contributed from grain interior,

from higher valence state cations to a lower one can lead to the formation of oxygen vacancies with the temperature increases.45 Hence, the presence of a mixed valence in the triple-doped lattice plays a vital role in the ionic conduction and electrochemical performance in a SOFC system. Electrical Property Analysis. Electrochemical impedance spectroscopy (EIS) is undertaken to study the electrical conductivities of the as-synthesized samples. As a matter of 23618

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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under ion-blocking conditions. On the basis of this measurement, the electronic conductivity of PNSDC sample is determined to be ∼6.4 × 10−4 S cm−1 at 600 °C. The current (I) versus time (t) curves obtained at a temperature range of 450 and 600 °C, at an interval of 50 °C, are plotted in Figure S2 to determine the electronic conductivity of the PNSDC sample. It indicates that the electronic conductivity of PNSDC is quite negligible compared with its overall conductivity. Also as seen from EIS curves, the enhanced total ionic conductivity is mainly ascribed to the grain boundary conductivity enhancement. In a word, the secondary doping Pr3+ and Nd3+ ions promote the surface ionic incorporation and enhance the grain boundary conductivity, resulting in a lower activation energy and higher conductivity. Triple-doping method using rare earth elements has been implemented in various fields to design desirable materials.50 Praseodymium, as a surface dopant to enhance the ionic conductivity by decrease of the material grain boundary resistance, has been reported.49 The Pr3+ and Nd3+ ions were discovered to promote the surface oxygen transfer and conduction. The secondary Pr3+ and Nd3+ doping of ceria causes negligible change on the surface crystal lattice of SDC and trivially influences the structural distortion, leading to a potential catalytic role.51 The formation and migration of oxygen vacancy in ceria are closely related to the ionic conductivity.52 It is a well-known that a high concentration of surface oxygen vacancies within a certain limit, for the doped ceria, promote the adsorption and dissociation of oxygen from the gas phase, surface regions, and even subsurface regions.53 Here, the extensive results give credible evidence that such tridopant, combining Sm3+ in bulk and Pr3+/Nd3+ at surface, is an efficient approach to enhance oxygen ions conduction through surfaces, so that O2− can be transported through both surface and bulk paths to make a great synergetic conductivity enhancement. In addition, the surface-doping of Pr3+/Nd3+ significantly improved the redox property through promoting the O2− transports from the oxygen reduction interface, traversing the electrolyte with assistance of the redox couple of Pr3+/Pr4+ ions, to the hydrogen oxidation region. This process is an accelerating ion-delivery process to enhance the redox property of the whole device and make the electrode polarizations significantly reduced. Fuel Cell Performance Analysis. Figure 7a shows I−V and I−P characteristics for cell I and II using SDC and PNSDC as the electrolyte. Their open circuit voltages (OCV) reached 0.922 and 1.021 V at 550 °C, respectively. The higher OCV for PNSDC electrolyte-based device indicates that PNSDC, as a better electrolyte, can prevent the ceria electronic current leakage. Correspondingly, a higher peak power density (710 mW cm−2) was achieved for cell II than that of cell I (490 mW cm−2). Such a big enhancement on the power output reflects higher ionic conductivity of the PNSDC electrolyte and reduced electrode polarization resistances. Figure 7b presents the cell voltage recorded over time at a discharge current density of 80 mA cm−2 operated at 530 °C. As seen, the device exhibited relatively good durability during 13 h operations. To investigate the kinetic process in cell reaction processes, we further carry out the EIS analyses for cell I, Ni-foam/SDC +NCAL|SDC|SDC+NCAL/Ni-foam, and cell II, Ni-foam/ PNSDC+NCAL|PNSDC|PNSDC+NCAL/Ni-foam, under air and H2/air condition at 550 °C. The results are presented in Figure 8. The empirical equivalent circuit Ro(R1-CPE1)(R2CPE2) is employed to analyze the intrinsic oxygen ions

grain boundaries, and electrode processes, respectively. However, not all of the arcs can be observed owing to the limited frequency range of the instrument and applied temperatures.46 Figure 5 displays typical Nyquist plots. In this case, the arcs representing grain interior cannot be observed for all samples due to the high-frequency limitation. Instead, two evident arcs, corresponding to grain boundaries and electrode/ electrolyte interface polarization, are presented in the Nyquist plots. EIS arcs’ sizes decrease with the increasing temperatures. It is worth noting that the grain boundary arc of the PNSDC decreases sharply compared with those of SDC. According to the fitting results, the capacitors of grain, grain boundary, and electrode process for both SDC and PNSDC samples are 10−12 F, 10−10−10−8 F, and 10−4 F, respectively, in this case, which is in agreement with the reported frequency ranges in the literature.47,48 Temperature dependences of conductivities for SDC and PNSDC are presented in Figure 6 in the form of

Figure 6. Temperature dependence of total conductivity and Arrhenius plots for SDC and PNSDC. The inset represents area specific resistance (ASR) versus temperature of SDC and PNSDC.

Arrhenius plots. Conductivities calculated for PNSDC and SDC materials are 0.125 and 0.0114 S cm−1 at 600 °C, respectively. Obviously, the conductivity of PNSDC is 1 order of magnitude higher than that of SDC at 600 °C. The former is the highest value reported for doped ceria materials. Moreover, the activation energy of PNSDC for ionic conduction is around 0.41 eV, which is significantly lower than 0.65 eV of SDC. Both the conductivity and the activation energy of SDC are comparable with literature values.49 The significantly enhanced ionic conductivity and much reduced activation energy of PNSDC imply different ionic conduction mechanisms. To further confirm the improved conductivity resulting from the enhancement of the ionic conduction, the electronic conductivity of PNSDC is measured by Hebb-Wagner ion-blocking technique arrangement with a gold blocking electrode and silver paste is painted on the opposing electrode. The electronic conductivity measurement is carried out using a digital microohm meter (KD2531, Kangda Electrical Co., Ltd.) with the varied voltage range from 0.01 to 1.0 V. When the HebbWagner type cells are polarized, the early current is contributed from the mixed motion of electrons and oxide ions. With the time increasing, the oxygen chemical potential at the inner Au/ PNSDC interface rapidly decreases and the constant electronic current is finally reached when the steady state is established 23619

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enhanced.54 The electrode polarization resistance, related to the basic electrode reaction process, including the O2− ions migration toward the triple-phase boundary (TPB) through the electrode to enter the electrolyte, is simultaneously improved. The enhancement of oxygen ionic conductivity is accompanied with the decrement of grain boundary and improvement of surface conduction property of PNSDC. Figure 9 schematically illustrates the oxygen transfer/transport

Figure 9. Schematic diagrams representing (a) the overall O2− transfer with the assistance of reduction of Pr3+/Pr4+ ions in the fuel cell; (b) the cathode reaction process and (c) brief routes for the O2− surface and bulk transfer and conduction process in PNSDC electrolyte.

Figure 7. (a) Typical I−V and I−P curves of fuel cells based on SDC and PNSDC electrolyte, respectively; the mixed NCAL and electrolyte materials were used as the electrode material. The measurements were performed at 550 °C with H2 and air as the fuel and the oxidant, respectively. (b) Durability test based on Cell II: Ni-foam/PNSDC +NCAL (anode)|PNSDC (electrolyte)| PNSDC+NCAL (cathode)/ Ni-foam at a constant discharge current density of 80 mA cm−2 operated at 530 °C.

routes in PNSDC electrolyte. As reported, the oxygen transfer relies on the ion chains, where charge is transferred during the redox process.55,56 In the redox process, the oxygen molecules were activated first under dissociation and reduction with the assistance of electrons from external circuit. At the surface of the PNSDC particles, oxygen ion transfer is promoted through

transport processes.7 Obviously, the ohmic resistance, Ro indexing to the intercept in high frequency, significantly decreases for PNSDC versus that of the SDC. It demonstrates that the O2− transport process in the electrolyte is markedly

Figure 8. Electrochemical impedance spectra of symmetric cell using SDC and PNSDC samples as electrolyte measured under (a) air and (b) H2/air conditions at 550 °C. 23620

DOI: 10.1021/acsami.7b02224 ACS Appl. Mater. Interfaces 2017, 9, 23614−23623

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the electron exchange arising from the redox between Pr3+/Pr4+ ions as described below:55,56 Pri 3 + + Prj4 + ↔ Pri 4 + + Prj3 +

CONCLUSIONS We have successfully developed an effective approach to significantly enhance oxygen ionic conductivity by >1 order of magnitude and overcome the redox instability issue of doped ceria oxide via a novel two-step wet-chemical coprecipitation method to realize with Sm3+ doped in bulk and Pr3+/Nd3+ doped in surface domains of ceria. The modified surface property greatly reduced the grain-boundary resistance, leading to an exceptional electrical conductivity of 0.125 S cm−1 at 600 °C, which is the highest known recording so far. The presence of Pr3+/Nd3+ also enhances the surface oxygen exchange rate and helps to improve the redox and electrode reaction kinetics, largely enhancing the voltage and power efficiency. SOFC based on PNSDC electrolyte with a thickness of 0.5 mm obtained a maximum power density of 710 mW cm−2 at 550 °C. Thus, the improved electrical and redox properties suggest a new promising approach for practical application of ceriabased materials for high-performance, low-temperature SOFC applications, which will lead to a great impact on SOFCs development and commercialization.

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Vice versa, the oxygen ion transfer can be reinforced at the presence of Pr4+ ions, and the reaction process is Pr 4 + + O2 − ↔ Pr 3 + + O−

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On the H2-contact side, the tetravalent Pr4+ is reduced to trivalent Pr3+. Simultaneously, the electrons diffuse away from the initial sites on the surface of PNSDC at a specific rate, and certain defects largely limit the mobility of donated electrons. On air side, the Pr3+ is reoxidized to Pr4+ to complete the entire Pr4+/Pr3+ redox cycle, in which the O2− is preferably involved and transferred. The assistance role of Pr3+/Pr4+ in the oxygenmobility path has been verified in the well-understood mechanism of the kinetic and structural studies of oxygen mobility in Pr−Zr oxide.55,56 The characteristic of the redox property for PNSDC is to prevent the reduction of Ce4+ ions and electronic conduction, resulting in a high open-circuit voltage, e.g., 1.021 V at 550 °C. To further clarify the mechanism of enhanced redox process owing to variable valence of the Pr element, the superficial Pr-doping with only ceria as electrolyte was compared via fuel cell performance. The current density−voltage and power density characteristics for this device using the Pr-doped ceria electrolyte measured at 550 °C were presented in Figure S3. As shown, a comparable output power density of 548 mW cm−2 was observed for this fuel cell under the open-circuit voltage of 1.058 V. This result provides indirect evidence to indicate enhanced redox process to improve the fuel cell performance. Also, it gives a hint to further delve into the potential of single Pr-doped ceria applied for LT-SOFCs. It has been reported that the oxygen reduction reaction (ORR) process dominates the electrode polarization loss for LT-SOFCs.57 The high ionic conductivity electrolyte, PNSDC, facilitates the good power output; meanwhile, the cathode ORR process is impacted. The ORR is realized by the participation of O2− in SOFC cathode side. O2− ions are generated through the role of the catalytic cathode and transferred to PNSDC electrolyte faster in comparison with the SDC electrolyte, thus realizing the cathodic reaction through both the surface and internal of these particles.58 The enhanced ionic transfer capability improves the ORR efficiency, which may explain why the performance of cell II is better than cell I. It has been recently reported that the lithiated transition metal oxide as electrode material exhibits excellent catalytic activity both in hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) processes.7 The existence of NCAL in both anode and cathode sides can promote the anodic HOR process while enhancing the cathode ORR process in the air side.59 The electronic conductor NCAL mixed with electrolyte material PNSDC may extend the electrolyte/electrocatalyst/gaseous reactants triple-phase boundary (TPB), leading to the enhancement of the fuel cell performances.60−62 Moreover, the PNSDC can simultaneously play a catalytic role in ORR process, resulting in a higher power output. The electrochemical catalytic property of Praseodymium alleviates the electrode/ electrolyte polarization. Therefore, the existence of the Pr3+ ions can significantly promote the O2− conduction in electrolyte and the ORR process in cathode, thus providing superior performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02224. SEM image and EDS element mapping analysis, current vs time curves, and current density−voltage and power density characteristics (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yixiao Cai: 0000-0001-7073-4964 Bin Zhu: 0000-0003-1479-0464 Author Contributions #

Y.L. and L.F. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (NSFC, Grant no. 51402093), the Natural Science Foundation of Hubei Province, major project (Grant no. 2015CFA120), the Swedish Research Council (Grant no. 6212011-4983), and the European Commission FP7 TriSOFCproject (Grant no. 303454). B.Z. acknowledges support from Hubei University through Hubei Provincial 100-Talent Distinguished Professor grants, and Y.Y.L. acknowledges support from China Scholarship Council for the Ph.D. fellowship.

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REFERENCES

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