Effective Concentration of Mobile Oxygen-Vacancies in Heavily Doped

Article pubs.acs.org/cm

Effective Concentration of Mobile Oxygen-Vacancies in Heavily Doped Cubic Zirconia: Results from Combined Electrochemical Impedance and NMR Spectroscopies Chien-Ting Chen, Sabyasachi Sen,* and Sangtae Kim* Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616, United States ABSTRACT: The effective concentration of mobile oxygen vacancies in heavily doped solid electrolytes (SE) used in electrochemical applications such as oxide fuel cells, electrolyzers, and gas sensors has long been of great interest. It is well-known that not all the oxygen vacancies in such heavily doped SEs are sufficiently mobile due to their association with the dopant cations, and thus, the effective number of mobile oxygen vacancies is smaller than the total number created by acceptor doping. However, the effective concentration of oxygen vacancies in a heavily doped SE cannot be directly obtained from electrical conductivity measurements, unless their mobility is known a priori. Here, we report the combined application of dopant−cation magic angle spinning nuclear magnetic resonance (MAS NMR) and electrochemical impedance (EI) spectroscopic techniques to determine the effective concentration and the mobility of the oxygen vacancies in a heavily doped (59 cat % Y) yttria stabilized zirconia (YSZ). The results clearly demonstrate that the effective concentration of oxygen vacancies in this SE is lower than their nominal concentration by more than a factor of 2, even at 700 °C. Furthermore, their mobility in this heavily doped YSZ is lower than that in 17 cat % Y-doped zirconia, one of most widely used SE, by orders of magnitude and is characterized by an activation energy (1.38 eV) that is significantly higher in the former than (1.02 eV) in the latter. These results provide a unique and direct mechanistic understanding of the respective roles of concentration and mobility in controlling ionic transport in SEs. KEYWORDS: effective concentration, effective mobility, YSZ, 89Y MAS NMR, electrochemical impedance spectroscopy

1. INTRODUCTION

cannot be determined from the electrical conductivity alone, unless the information about their mobility is available. Recently, we have demonstrated using a dopant−cation MAS NMR line shape simulation technique that the characteristic length scale of the oxygen vacancy hopping in relatively lightly doped (5 cat %) SEs is directly related to the average dopant− dopant separation in the lattice.7 Specifically, high-temperature 45 Sc MAS NMR line shape in 5 cat % Sc-doped CeO2 was shown to be dynamically controlled by the oxide-ions moving in and out of the nearest neighbor coordination shell of Sc3+ ions, resulting in virtual exchange between seven- and eightcoordinated Sc sites with one and zero oxygen vacancy, respectively. The hopping frequency, ωNMR, and activation energy, Ea, of this virtual cation site exchange were found to be in excellent agreement with those characteristic of the electrical conduction process determined by electrochemical impedance spectroscopy (EIS). According to the Nernst−Einstein relation for a random walk diffusion model (eq 1), the electrical conductivity of the oxygen vacancy σV is directly related to the hopping distance d

Oxide-ion migration in a solid electrolyte (SE) results from thermally activated hopping of these ions to oxygen vacancies formed in the crystal lattice. In such SEs, a relatively large number of these vacancies are created to charge compensate the acceptor dopant, which is typically an aliovalent cation deliberately introduced to replace the host cation in the lattice.1−3 Therefore, the total number of oxygen vacancies in the lattice can be readily determined from the acceptor concentration. However, it is not as straightforward to estimate the number of the mobile oxygen vacancies in such a heavily doped material at a given temperature. This is because not all the vacancies present in the material are sufficiently mobile, especially at relatively low temperatures, due to their substantial association with the dopant.4−6 It has long been of particular interest to the solid-state ionics community to estimate the “effective” concentration of mobile oxygen vacancies in a heavily doped material since most oxide-ion conducting SEs used for practical applications such as oxide fuel cells, chemical sensors, and oxygen pumps possess rather high concentrations (typically >20 cat %) of an acceptor dopant cation to present sufficiently high conductivity.1−3 It should be noted that electrical conductivity is proportional to the product of the concentration of the mobile charge carriers and their mobility. Therefore, the effective concentration of the oxygen vacancies © 2012 American Chemical Society

Received: July 1, 2012 Revised: August 20, 2012 Published: August 30, 2012 3604

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Chemistry of Materials ⎛z e⎞ σ V = z Vec Vμ V = z Vec VD V ⎜ V ⎟ ⎝ kBT ⎠

Article

ray spectroscopy (EDX) in an SEM (microscope FEI XL30-SFEG operated at 10 kV of accelerating voltage) and was found to be within ±1 cation % of the nominal composition. The average particle size of the sample was examined with SEM and was found to be on the order of ∼0.3 μm. 2.2. High-T 89Y NMR. For NMR measurements, a sintered pellet of YSZ59 was ground in an agate mortar. 89Y MAS NMR spectra were collected at a Larmor frequency of 24.5 MHz using a Bruker wide-bore magnet and a Bruker Avance-500 solid state NMR spectrometer. An ambient-temperature 89Y MAS NMR spectrum was collected using a low-gamma Bruker 4 mm MAS probe, crushed samples were spun at 10 kHz and free induction decays (FID) were collected using a π/2 rf pulse (6 μs) and a recycle delay of 5 s. The high-temperature 89Y MAS NMR spectra of Y-doped ZrO2 were collected using a hightemperature MAS probe (Doty, Inc.) that was suitably modified for operation with low-gamma nuclides. The powdered sample was loaded into a boron nitride capsule that was inserted into a 7 mm Si3N4 rotor and was spun at spinning rates of between 4 and 5 kHz. N2 gas boil off from a high pressure liquid nitrogen dewar was used for spin and temperature control of the sample. The temperature of the probe was calibrated externally using the well-known temperature dependence of the 207Pb chemical shift of Pb(NO3)2, before and after the 89Y NMR measurements. The sample temperature was raised from 25 to 500 °C stepwise, and the sample was allowed to reach equilibrium at each temperature for 15 min before data collection. All high-temperature 89 Y MAS NMR spectra were collected using a π/2 rf pulse (1 μs) and a recycle delay of 0.2 s. Approximately 2000−6000 FIDs were averaged and Fourier-transformed to obtain each 89Y MAS NMR spectrum. The 89 Y NMR chemical shifts were externally referenced to that of crystalline Y2Sn2O7 (δiso = 150 ppm). 2.3. EIS. For impedance measurements, both faces of a sintered pellet were polished and painted with Pt paste (5349 Heraeus, U.S.A.). The pellet was then annealed at 1000 °C for 2 h in air with heating and cooling rates of 5 °C/min to ensure good contact of the electrodes (Pt) with the sample. The EIS measurements of the YSZ59 pellet were carried out under air in the temperature range 350−700 °C, using a Novocontrol Alpha-AN modulus analyzer in the frequency range from 10−1 to 107 Hz. The fittings of the measured impedance spectra with an appropriate equivalent circuit model were performed using the software Z-View (Scribner).

(1)

where

DV =

γ 2 d ωh 6

with z, e, kB, and T being charge number, elementary charge, Boltzmann constant, and absolute temperature, respectively, and γ is a correlation factor, often referred to in the literature as the Haven ratio. c, μ, and D denote the concentration, the mobility, and the diffusivity, respectively, and the subscript V corresponds to oxygen vacancy. Since ωh is an experimentally measurable parameter, eq 1 suggests that both cV and μV in an oxide-ion conducting SE can be estimated if d in the lattice is known. To estimate cV and μV in a heavily doped material, it is thus, essential to determine the hopping distance d in a lattice where cooperative processes and vacancy clustering may become significant. However, the above-mentioned 45Sc MAS NMR experiments in a heavily Sc-doped CeO2 or ZrO2 cannot be carried out, primarily because of the limited solubility of Sc2O3 in the host lattice.8 The solubility of Y2O3 in CeO2 or ZrO2 lattices, on the other hand, is significantly higher but the 89Y MAS NMR line shape measurements, in contrast to 45Sc MAS NMR, pose significant practical problems especially at high temperatures, mainly because 89Y is a low-gamma nuclide with low sensitivity and long spin−lattice relaxation time.9−12 Even after doping with paramagnetic ions, the data collection time for a one-pulse 89Y MAS NMR spectrum in Y-doped CeO2 or ZrO2 may take in excess of 20 h even for samples with relatively high Y content.9,10 Therefore, the low sensitivity combined with long data acquisition time make 89Y MAS NMR at high temperature particularly challenging. To the best of our knowledge, high-temperature MAS NMR spectroscopy has never been attempted before on low-gamma nuclides in solids. Here, we report the results of a combined 89Y MAS NMR and EIS study of the oxygen-vacancy hopping dynamics in a 59 cat % (i.e., 42 mol %) Y-doped ZrO2 (YSZ59) sample over a wide range of temperatures (from ambient to 500 °C for NMR and from 350 °C to 700 °C for EIS measurements). The results of this study provide important and unique site-specific information regarding the long-range oxide-ion/vacancy dynamics in the heavily doped ZrO2, enabling us to determine the effective cV as well as μV from electrical conductivity measurements.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of the sintered YSZ59 pellet that corresponds well to the cubic fluorite structure. The shifts in the Bragg peak positions in the XRD pattern with respect to those in 17 cat % (i.e., 8 mol %) Y-doped ZrO2 (YSZ17),

2. EXPERIMENTAL DETAILS 2.1. Sample Synthesis and Preliminary Characterization. The YSZ59 power was synthesized via a coprecipitation method where an aqueous solution of NH4OH was added dropwise to an aqueous solution containing the stoichiometric amounts of constituent metal nitrates. Gd3+ was incorporated in the solution phase at a level of ∼1000 ppm to shorten the spin−lattice relaxation time of 89Y nuclides and hence reduce the collection time of 89Y NMR spectra. The precipitated oxide powders were collected by centrifugation and were washed with water, a 50 vol % ethanol−water solution, and subsequently with pure ethanol. The resulting powders were then dried at 120 °C for 12 h, ground, and finally annealed at 800 °C in air for 2 h. The synthesized YSZ59 powder was pelletized via cold isostatic pressing at 276 MPa and subsequently sintered at 1600 °C for 10 h. Powder X-ray diffraction (XRD) measurement (Scintag XDS2000) indicated that the sample has the cubic fluorite crystal structure. The Y content of the sample was analyzed using energy dispersive X-

Figure 1. X-ray diffraction pattern (black) of YSZ59 sample. Blue stick pattern corresponds to main Bragg peaks for 8 cat % Y-doped YSZ from the JCPDS database. 3605

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average coordination number of 7.12 for Y atoms, which is slightly higher than the ideal coordination number of 6.82 expected from a random distribution of vacancies in the YSZ59 structure. This result therefore indicates a slight preference of the vacancies for the Zr atoms over the Y atoms in YSZ59 that is consistent with the results reported in previous studies in the literature.9,11 As the temperature increases, the peaks in the 89Y MAS NMR spectrum become broader and start to overlap but still remain distinguishable, at least up to 330 °C (Figure 2). The 89 Y MAS NMR spectrum at 400 °C is dominated by a broad peak centered near the weighted average of the positions of the three original peaks corresponding to the Y[8], Y[7], and Y[6] sites. This dynamically averaged 89Y MAS NMR line shape implies rapid cross-exchange between these sites at a rate that is on the order of the peak separation of ∼2.5 kHz, which results in coalescence of the peaks. Further increase in temperature to up to 500 °C results in narrowing of the width of this dynamically averaged peak that is indicative of increasing frequency of cross-exchange between the Y sites. The diffusion coefficient of Y3+ ions in this temperature range is expected to be negligibly small and, hence, cannot be responsible for the observed dynamical cross-exchange between the Y[8], Y[7], and Y[6] sites. Instead, the cross-exchange must result from the dynamical transformation between these sites when an oxygen vacancy hops in and out of the nearest neighbor oxygen coordination shells of these Y3+ ions. These processes can be represented in the form of a transformation reaction: Y[n] + [n−x] xV•• where V•• O ↔ Y O represents an oxygen vacancy and n = 8 with x = 1 or 2 or n = 7 with x = 1. The high-temperature 89Y MAS NMR line shapes can then be simulated using a standard three-site random cross-exchange model in order to obtain the average hopping frequency of oxygen vacancies between Y[6], Y[7], and Y[8] sites in the lattice as a function of temperature. The analytic expression for the line shape g(ω) resulting from cross-exchange between N distinct sites is given by the real part of g(ω), given as15 g(ω) = (1/N)[L/(1 − L/τNMR)] where L = ∑ j=1,N [i(ω − ωj) + 1/T2j + N/τNMR]−1, ωj is the resonance frequency, T2j is the reciprocal of the intrinsic line width corresponding to the site j, and 1/τNMR is the frequency of exchange or dynamical transformation between the Y sites. The simulated 89Y NMR line shapes based on a three-site random exchange model (i.e., N = 3) are compared with the experimental spectra in Figure 2. The value of T2j has been kept constant at 0.6 ms for all Y sites in all of the simulations. A single average temperature-dependent hopping frequency τNMR−1 was used for each simulation. A qualitative inspection of Figure 2 clearly indicates that the experimental 89Y NMR line shapes can be simulated well with such a random exchange model at all temperatures up to 500 °C. The frequency dependent ac conductivity σ(ω) of YSZ59 is computed over a wide range of frequency (10−1 to 107 Hz), f = ω/2π, with ω being angular frequency, using the values of the real and imaginary parts of the impedance Z′(ω) and Z″(ω), respectively, as obtained from EIS measurements and using the relation:

indicate unit cell expansion that can be attributed to the higher content of Y3+, which has a larger ionic radius than Zr4+. According to the phase diagram,13 the cubic phase at this Y2O3 composition is stabilized at temperatures above ∼1300 °C and the expected equilibrium phases below this temperature are a mixture of the ordered Y4Zr3O12 and hexagonal yttria-rich phases. On the other hand, it was also reported that the order− disorder transition is extremely slow and takes up to months at around 1200 °C to take place.13,14 In light of the fact that the pellet was sintered at 1600 °C, and that we do not see any peaks that correspond to the Y4Zr3O12 ordered phase in Figure 1, it was deduced that there is not enough time for the ordered−disordered transition to occur during the furnace cooling of the sample. Therefore, we can safely assume that the vast majority of the sample, if not all, is of the cubic fluorite structure. This observation is supported by a recent report where the cubic fluorite structure of ZrO2 is stabilized even up to a doping level of 64 cat % Y.11 Figure 2 shows the 89Y MAS NMR spectra of the YSZ59 sample collected at temperatures ranging from ambient to up to

Figure 2. Experimental (solid line) and simulated (dashed line) 89Y MAS NMR spectra of YSZ59. Temperatures and corresponding exchange frequencies are indicated alongside each spectrum. RT denotes room temperature. The three resonances centered at ∼295, 190, and 86 ppm in the room temperature spectrum correspond to Y[6], Y[7], and Y[8] sites, respectively.

500 °C. At ambient temperature, the 89Y MAS spectrum exhibits three resonances centered at isotropic chemical shifts of ∼295, 190, and 86 ppm, which can be readily assigned on the basis of previous studies to six, seven, and eight coordinated Y cations, Y[6],Y[7], and Y[8], respectively.9,10 It may be noted here that Y[8], Y[7], and Y[6] sites correspond to Y atoms with zero, one, and two oxygen vacancies, respectively, in their nearestneighbor coordination environment. Integration of the corresponding peak areas yield the relative ratios of Y[6], Y[7], and Y[8] sites to be 13:62:25. These relative ratios imply an

σ(ω) =

⎛L⎞ ⎜ ⎟ Z′(ω) + Z″(ω) ⎝ A ⎠ 1

2

2

where L and A denote the sample thickness and the electrode area, respectively. The σ(ω) of YSZ59 measured at different temperatures presents two distinctive regions over the 3606

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frequency range of interest (Figure 3, only the data acquired at selected temperatures are shown in the plot for the sake of

Figure 3. Bode plot displaying frequency dependence of the YSZ59 sample at different temperatures. Symbols correspond to experimental data measured by EIS and dashed lines are the best fits obtained using the Almond−West expression (see text for details). Crosses are data points from Nyquist plots (see Figure 4).

Figure 5. Arrhenius plots of the bulk dc conductivity (inverted triangles), conductivity relaxation frequencies (circles and squares) determined from EIS results (see text for details), and oxygen vacancy hopping frequencies determined from simulation of 89Y MAS NMR line shapes (open triangles).

clarity) that can be described by the Almond−West expression:16 σ(ω) = σdc{1 + (ω/ωh)n}, where σdc and ωh denote the bulk dc conductivity and the average hopping frequency of oxygen vacancies, respectively. n is a fitting parameter. This expression implies that σ(ω) = σdc, where ω is sufficiently lower than ωh, while σ(ω) = σdc(ω/ωh)n if ω ≫ ωh. The σ(ω) data at various temperatures have been fitted to this expression to obtain the temperature dependence of σdc and ωh in YSZ59. The ωh is expected to be consistent with the conductivity relaxation frequency ωbulk = (RbulkCbulk)−1 with Rbulk and Cbulk being the resistance and the capacitance of the bulk, respectively, which can be found at the apex of the bulk semicircular arc in a Nyqusit plot such as that shown in Figure 4. Figure 5 presents an Arrhenius plot of σdcT. Also included are those of the “virtual” exchange frequency ωNMR (= 2πτ−1 NMR) between the Y sites, as obtained from the three-site exchange modeling of the 89Y NMR line shapes, and of ωh and ωbulk, as obtained from the best fits of the σ(ω) data and the Nyquist

plot, respectively. The Ea of σdcT estimated from the slope of the plot is 1.52 eV consistent with the value previously reported for heavily doped ZrO2.17,18 It should be noted that the Ea of σdcT (1.52 eV) is higher than that of ωh (1.46 eV) because the former includes the energy associated with the temperaturedependent variation in the concentration of mobile charge carriers (see below for details). The Ea for the YSZ59 sample remains practically constant over the entire temperature range studied here. This is in contrast with previous reports on the lightly doped YSZ17 sample that indicated a clear reduction in the Ea of σdcT from ∼1.1 eV at temperatures lower than ∼500 °C to ∼0.9 eV at temperatures higher than ∼800 °C.18 The higher value of Ea at lower temperatures is attributed to the association between the oxygen vacancies and the dopant cations (hereafter, the defect interactions). At high temperatures, the thermal energy available to the system is sufficient to result in complete dissociation of the oxygen vacancies from the dopant cations, and the Ea of σdcT decreases to the value characteristic of the activation energy of migration. On the other hand, in a heavily doped system, the defect interactions remain substantial even at relatively high temperatures due to the large numbers of oxygen vacancies and the dopants, leading to Ea invariant even in the high temperature region of interest. In addition, the absolute value of the Ea in the heavily doped system is greater than that of the lightly doped system because the oxygen vacancy hopping may require even higher energy as a result of the more complex nature of the defect interactions in the former. For instance, previous studies based on computer simulation as well as X-ray absorption spectroscopy have indicated the possibility of significant vacancy clustering in doped zirconia and ceria.19−24 Figure 5 also demonstrates that the values of ωh are in excellent agreement with those of ωNMR over frequencies spanning nearly 3 orders of magnitude. Therefore, these results confirm that the characteristic length scale associated with oxygen vacancy hopping and resulting ionic conduction in heavily doped SEs is on the order of the average separation distance between dopant cations. As mentioned above, such observation was shown in a previous

Figure 4. Representative impedance spectrum (Nyquist plot) of the YSZ59 sample measured at 450 °C under air. The solid line indicates the best fit using an equivalent circuit model (see the text for details). The numbers indicate the logarithmic frequencies. 3607

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oxygen vacancy hopping distance d to be similar to the Y[7]− Y[8] distance in the lattice as the concentration of the Y[6] sites in these materials is negligibly small.9,11 It is remarkable that the difference in the effective μV between these materials with different dopant concentrations is several orders of magnitude (Figure 6a). The Ea of the effective μV (0.71 eV) measured for YDC2 in the relatively low temperature region (700 °C). Therefore, previous literature reports of the observation that YSZ displays its conductivity maximum at around 17 cat % can primarily be attributed to the reduction in the effective μV in heavily doped YSZ.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. was supported by the National Science Foundation grant DMR1104869. S.K is grateful for partial support from UCMEXUS grant for this work.

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REFERENCES

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