Correlation among Oxygen Vacancies, Protonic Defects, and the

Sep 18, 2015 - Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopant in Sc-Doped BaZrO3 Studied by 45Sc Nuclear Magnetic Reson...
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Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopant in Sc-Doped BaZrO3 Studied by 45Sc Nuclear Magnetic Resonance Itaru Oikawa and Hitoshi Takamura* Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan ABSTRACT: The distribution of protons and oxygen vacancies at room temperature at different proton concentrations in 10 mol % Sc-doped BaZrO3 was investigated to clarify the influence of proton concentration and oxygen vacancies in the trapping of protons caused by the acceptor dopant. To enhance proton conductivity for practical use, it is essential to understand this phenomenon known to limit the long-range transport of protons. In this study, 45Sc nuclear magnetic resonance (NMR) spectroscopy combined with thermogravimetric analysis (TGA) and 1H NMR is used to elucidate the protonic defects and oxygen vacancies formed around Sc and Zr. The results reveal that the high protonic defect concentration around Sc with a 9−10 mol % proton concentration is a clear indication for proton trapping and that the high protonic defect concentration around Zr at an intermediate proton concentration of 4 mol % suggests that the protons are residing in the nontrapping sites. The oxygen vacancies that tend to be located around Sc apparently prevent the formation of protonic defects due to the repulsive interaction between the protonic defect and the association of Sc and an oxygen vacancy, both of which have a positive net charge. This study suggests that the formation of oxygen vacancies around the acceptor may inhibit proton trapping and therefore have a positive effect on the long-range transport of protons.

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× M 2O3 = 2M′Zr + 3OO + V •• O

INTRODUCTION Acceptor-doped perovskite-type proton conductors are a candidate material for use as an electrolyte in intermediate temperature solid oxide fuel cells (SOFCs) due to their relatively high proton conductivity at temperatures below 700 °C. These doped conductors compare favorably to yttriumstabilized zirconia, an oxide-ion conductor widely used as an electrolyte in present SOFCs.1−3 Rare-earth-doped cerates and zirconates have been reported to show proton conduction in a water-containing atmosphere.4,5 The cerates-based proton conductors exhibit high proton conductivity and well sinterability;4,6,7 however, these materials easily decompose under water vapor and CO2.8−10 Meanwhile, because BaZrO3based materials are chemically stable, they do not react with water vapor or CO2.11 Among them, the Y-doped BaZrO3 is regarded as the most promising electrolyte for use in intermediate-temperature SOFCs, largely because its proton conductivity is comparable to that of BaCeO3-based materials.5,11,12 Mobile protons in these materials are derived from the formation of protonic defects produced by the reaction between oxygen vacancies and water vapor. In BaZrO3-based perovskite-type protonic conductors BaZr1−xMxO3−δ where M is a rare-earth element, oxygen vacancies are introduced by substituting the tetra-valent cation with the trivalent one. This is expressed by the following equation using Kröger-Vink notation: © XXXX American Chemical Society

(1)

Here, MZr ′ represents the acceptor dopant at Zr sites, OO× is the oxide-ion at O sites, and V•• O indicates the oxygen vacancy. When this material is exposed to an H 2O-containing atmosphere, oxygen vacancies react with water vapor to create protonic defects as follows: × • H 2O + OO + V •• O = 2OH O

(2)

OH•O is a proton bonded to an oxide-ion (protonic defects). Once the protonic defects are introduced in the material, they are able to migrate through the lattice by a hopping mechanism: they literally hop from one oxide-ion to another. The drawbacks of these materials have been well-reported: their proton conductivity is insufficient for practical use, and they have poor sinterability and also low grain boundary conductivity.13−15 To improve their sinterability, a variety of sintering additives have been investigated including ZnO and NiO.16−18 In another study, total conductivity was improved by decreasing the influence of the grain boundary. Yamazaki et al. reported high total conductivity in Y-doped BaZrO3 with large grains.19 However, for these materials to be suited to a wider array of practical applications, the further enhancement of Received: June 25, 2015 Revised: September 17, 2015

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DOI: 10.1021/acs.chemmater.5b02441 Chem. Mater. XXXX, XXX, XXX−XXX

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proton conductivity is necessary at intermediate temperatures below 500 °C. To achieve high proton conductivity, ideally the long-range transport of protons would occur without hindrance. However, transport is known to be limited by the mode of transport itself: protons hop from one oxide-ion to a neighboring oxide-ion.20 This limitation is expected to be stronger in the vicinity of the acceptor dopant because the acceptor has a negative net charge, which electrostatically attracts protons, in effect trapping them. This trapping of protons was observed in Yb-doped SrCeO3 by the quasielastic neutron scattering technique, which revealed the presence of protons free from the trapping effect as well as protons trapped around Yb.21 This attractive interaction between protons and dopants has also been reported for BaZrO3-based proton conductors by means of computational methods, that is, by density functional theory calculations and atomic simulations.22−24 In addition, the influence of the trapping on long-range proton migration has been revealed for Y-doped BaZrO3 by thermogravimetric analysis and A.C. impedance methods combined with proton nuclear magnetic resonance (NMR) techniques.25 As mentioned earlier, a detailed understanding of proton trapping is crucial in the effort to enhance proton conductivity; nevertheless, too few investigations from the perspective of local structure, including the concentration of protonic defects formed around ions, have been carried out to add to the macroscopic data on electrical conductivity and proton concentration reported by various techniques for comparison purposes. Previously, the local structure of perovskite-type proton conductors has been explored by extended X-ray absorption fine structure (EXAFS) and infrared and Raman spectroscopy, which revealed the symmetry of local coordination octahedra of the acceptor, proton sites, and the difference in the strength of hydrogen bonding.26−31 In addition to these methods, NMR spectroscopy has been used to elucidate the local structure of specific ions. 1H NMR studies on Y-doped SrCeO3, BaCeO3 and BaZrO3 regarding dynamics of protons have been reported.25,32,33 Meanwhile, an NMR study on the acceptor in Sc- and Y-doped BaZrO3 has clarified the formation of protonic defects and oxygen vacancies around the acceptor as a difference in the chemical environment.34−36 While the distribution of protons and oxygen vacancies has been studied at given proton concentrations, for example, fully hydrated samples, the effect of proton concentration on the distribution of defects has yet to be clarified. Positively charged oxygen vacancies and protonic defects are correlated with each other. The distribution of the defects as a function of proton concentration appears to contribute to the proton trapping phenomena in acceptor-doped BaZrO3 due to the correlation between the negatively charged acceptors, positively charged oxygen vacancies, and protonic defects. The aim of this study, therefore, is to elucidate the effect of oxygen vacancies on the formation of protons around Sc in Sc-doped BaZrO3 with respect to proton trapping. To achieve this purpose, the distribution of protonic defects and oxygen vacancies at room temperature was clarified by 45Sc NMR combined with thermogravimetric analysis (TGA) since 45Sc has been proved to be an effective probe to elucidate defects by the NMR technique in fluorite-type oxide-ion conductors and perovskitetype proton conductors as well as picometer-scale changes in the local structure around the probe nucleus.34,35,37−41

Article

EXPERIMENTAL SECTION

Sample Preparation. Samples of 10 mol % (moles of Sc per mole of Ba(Zr, Sc)O3−δ) Sc-doped BaZrO3 were prepared by solid-state reaction from BaCO3 (99.99%, Wako Pure Chemical Industries), ZrO2 (99.9%, Kanto Chemical), and Sc2O3 (99.9%, Mitsuwa Chemicals). After drying at 150 °C overnight in air, stoichiometric amounts of raw materials were mixed by ballmilling for 6 h using zirconia balls and a zirconia pot with ethanol. The mixture was dried in air, uniaxially pressed into pellets, and calcined at 1300 °C for 12 h in a laboratory atmosphere. Calcined samples were ground and ballmilled for 12 h. After ballmilling, the samples were isostatically pressed into pellets at 200 MPa and sintered at 1600 °C for 24 h in the laboratory atmosphere. All the sintering pellets were embedded in calcined powder to prevent the evaporation of Ba due to the high vapor pressure at the sintering temperature. Control of Proton Concentration. To control the proton concentration, samples were heat-treated in a humidified atmosphere at temperatures between 350 and 900 °C. In the series of perovskitetype protonic conductors including rare-earth-doped BaZrO3, protonic defects are stable up to approximately 400 °C, and the concentration of protonic defects starts to decrease as the temperature rises.12 By making stepwise changes in temperature in the humidified atmosphere from 350 to 900 °C, it is possible to control the proton concentration. Samples were first heated to 900 °C in N2 without humidification for 3 h to remove the protonic defects that formed during sample preparation. Then the temperature was decreased to 350−800 °C, and the atmosphere was changed to N2 humidified with a saturated vapor pressure of H2O at 10 °C: this corresponds to a water vapor partial pressure of 1.2 kPa. Heat treatment in this humidified atmosphere was carried out for 15 h. To conserve the proton concentration and prevent water adsorption to the sample, the heat treatment atmosphere was changed to unhumidified N2 and cooled to 350 °C. Thereafter, the sample was taken out of the electric furnace and immediately moved to an Ar-filled glovebox. The same procedure was carried out in an unhumidified N2 environment at 900 °C for a period of 15 h to obtained dehydrated samples. A thermogravimetric analysis (TGA) was carried out using Pyris 1 TGA (PerkinElmer) with synthetic air (79% N2, 21% O2) without humidification as a carrier gas to determine the concentration of protons in the sample heat-treated at 350 °C in a humidified atmosphere since this sample was expected to have the highest proton concentration. Structural Analysis. The crystalline phases and lattice parameters were determined from powder X-ray diffraction (XRD) measurements using D8 ADVANCE (Bruker). XRD patterns of samples were collected by CuKα radiation with 2θ from 10−120° at increments of 0.02°. The lattice constants were determined using TOPAS 4 software. XRD measurements were carried out in Ar atmosphere using an airtight cell and in air. XRD patterns observed in Ar and air did not show any difference. Before XRD measurements, all samples were kept in the glovebox after hydration at different temperatures to avoid adsorption of water. 1 H magic-angle spinning(MAS) NMR measurements were carried out at room temperature using JNM-ECA600 (JEOL) at a magnetic field strength of 14.1 T, which corresponds to an 1H Lamor frequency of 600.17 MHz. 1H NMR spectra were collected with 400 scans at a repetition time of 10 s using a 1.3 mm probe with a zirconia sample rotor spinning rate of 33 kHz. The 1H chemical shift was referenced to tetramethylsilane at 0 ppm. 45Sc MAS NMR experiments were carried out at room temperature using JNM-ECA800 and a 3.2 mm CPMAS probe operating at a magnetic field strength of 18.8 T, which corresponds to a 45Sc Lamor frequency of 194.34 MHz. The spectra were obtained from a single-pulse experiment with 6400 scans at a repetition time of 1 s. The spinning rate of the zirconia sample rotor was 20 kHz. The excitation pulse width was set to 1.2 μs, which corresponds to a π/24 pulse of the reference solution. The 45Sc chemical shift was referenced to 1 M Sc(NO3)3 aqueous solution at 0 ppm. Samples of 10 mol % Sc-doped BaZrO3 subject to various heat treatment conditions were packed into the sample rotor in an Ar-filled B

DOI: 10.1021/acs.chemmater.5b02441 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials glovebox, and dry air was used to power the rotor during 1H and 45Sc NMR experiments to prevent the adsorption of water since this may affects the 1H and 45Sc NMR spectra. All spectra were deconvoluted using Dmfit program.42

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RESULTS AND DISCUSSION Proton Concentration in 10 mol % Sc-Doped BaZrO3. To determine the proton concentration in 10 mol % Sc-doped BaZrO3 annealed at various temperatures, TGA and 1H NMR were performed. Figure 1 shows the TGA result of 10 mol %

Figure 2. 1H NMR spectra of the samples heat-treated at different temperatures.

intensity of NMR signals, the absolute amounts of protons are calculated using the TGA results of the sample hydrated at 350 °C. The dependence of the proton concentration on hydration temperature is shown in Figure 3. The proton concentration

Figure 1. Weight change of the sample heat-treated at 350 °C during heating to 900 °C.

Sc-doped BaZrO3 heated at 350 °C in the humidified atmosphere. The sample weight decreased to 0.329 ± 0.004% of its initial weight as the temperature rose to 900 °C. The proton concentration is determined from the weight change of the sample assuming this change is due to the release of water vapor originating from the reverse reaction of protonic defects formation in eq 2. The proton concentration in the sample heat-treated at 350 °C in humidified N2 atmosphere was 9.9 ± 0.2 mol %, which indicates that a fully hydrated sample can be achieved by the heat treatment at this temperature. The proton concentration in the sample heated from 500− 800 °C in wet N2 and 900 °C in unhumidified N2 was determined from the 1H magic-angle spinning(MAS) NMR spectra at room temperature. Figure 2 shows the 1H MAS NMR spectra of 10 mol % Sc-doped BaZrO3 hydrated at different temperatures, ranging from 350−900 °C. Intensities of all the spectra are displayed in the absolute value so as to show the change in the intensity clearly with respect to the heat treatment temperature since the integral intensity of the 1H NMR spectra is proportional to the proton concentration. The maximum peak intensity of the 1H signal was found for the sample heat-treated at 350 °C, and it was shown that the intensity of this peak decreases as the hydration temperature increases. The proton concentration of the samples was calculated from the integral intensity of each signal including the integral intensity from spinning sidebands. Errors in the proton concentration were estimated from the uncertainty of the integral intensity of 1H signals derived from the baseline correction and expressed in ± two standard deviations. The baseline correction of the spectra was carried out by selecting the slightly different points in the baseline, and integral intensity was calculated for each spectrum. Since only the relative amounts of protons are derived from the integral

Figure 3. Proton concentration as a function of heat treatment temperature.

gradually decreased with increasing hydration temperature from 350 to 800 °C in wet N2, and the lowest concentration was obtained from the sample dried at 900 °C in unhumidified N2. The small amounts of protons observed even in the dried sample at high temperature, (1.1 ± 0.2 mol %) may be due to the residual protons inside the sample that failed to be removed from the pelletized sample because of insufficient dehydration time. The proton concentration determined from both TGA and 1H NMR is summarized in Table 1. As expected, the proton concentration is well controlled by changing the hydration temperature. Protonic Defects and Oxygen Vacancies around Sc. That proton concentration can be controlled was also confirmed from a structural perspective. The XRD patterns of hydrated and dried samples, shown in Figure 4, were identified as a single phase of cubic perovskite-type structure (space group: Pm3¯m), and no other phase was observed. Lattice constants obtained from the XRD patterns are shown in Figure 5. The lattice constants linerly increased with increasing the proton concentration, which indicates the lattice expansion accompanied by the formation of protonic defects and decrease of oxygen vacancies. This liner trend of lattice constants to the C

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Chemistry of Materials Table 1. Proton Concentration of 10 mol % Sc-Doped BaZrO3 Anealed at Different Temperature. Atmosphere from 350−800 °C Is Wet N2 at Water Partial Pressure of 1.2 kPa, while That of 900 °C Is Unhumidified N2 heat treatment temperature (°C) 350 500 600 700 800 900

proton concentration (mol %) 9.9 9.5 5.9 4.2 3.9 1.1

± ± ± ± ± ±

0.2 0.3 1.0 0.3 0.1 0.2

Figure 6. 45Sc NMR spectra at different proton concentration in the sample. Asterisks indicate spinning sidebands.

Sc, which results in the Sc signal without the influence of the quadrupole broadening. The peak from ScO5(OH) is assigned because this peak changes its intensity with respect to the proton concentration. The peaks from ScO5 show a characteristic line shape of the second-order quadrupolar interaction, which originates from a large electric-field gradient at the Sc site due to the formation of an oxygen vacancy. The NMR spectrum of the fully hydrated sample (9.9 ± 0.2 mol %) was deconvoluted into ScO6 and ScO5(OH), and a small negligible peak for ScO5 was detected. The peaks for ScO6 and ScO5 decreased as the concentration of protons increased in the sample, while those for ScO5(OH) increased. The protonic defect concentration and oxygen vacancy concentration around Sc was calculated from the integral intensity of peaks from the distinguished Sc sites observed in the 45Sc NMR spectra. Figure 7 shows the peak deconvolution of the 45Sc NMR spectra for samples with 1.1 ± 0.2, 5.9 ± 1.0, and 9.9 ± 0.2 mol % protons. The spectra of the sample with 1.1 ± 0.2 mol % protons consisted of two components: ScO5 and ScO6. In the case of the sample with 5.9 ± 1.0 mol % proton concentration, the spectra were deconvoluted into three peaks: ScO5, ScO5(OH) and ScO6, as shown in the middle of Figure 7. At the proton concentration of 9.9 ± 0.2 mol %, the peak from ScO5 almost disappears leaving the peaks for ScO5(OH) and ScO6 only. Integral intensities of each Sc site obtained from the deconvolution of the spectra were corrected by taking quadrupolar parameters of the Sc sites and the contribution from the spinning sidebands into account.43,44 Quadrupole coupling constant, CQ, and asymmetry parameter, η, used in the intensity correction were CQ = 29 MHz and η = 0, respectively, for ScO5 determined from 45Sc NMR spectra of 10 mol % Sc-doped BaZrO3.35 The parameters of the other two sites were reported to be CQ = 7 MHz and η = 0 for ScO5(OH), and CQ = 0.8 MHz and η = 0.5 for ScO6, which are determined from the static and the MQMAS NMR spectra of 15 mol % Sc-doped BaZrO3.34 The integral intensities of the peaks from the three Sc sites were converted to the defect concentration around Sc. The integral intensity of NMR spectra for each peak is proportional to the concentration of 45Sc in each specific chemical environment, that is, the coordination number, the vacancies, and the nearest neighbor ions. The relative amounts of Sc sites

Figure 4. XRD patterns of the samples at different proton concentrations.

Figure 5. Lattice constant of the samples as a function of proton concentration.

proton concentration confirms that the concentration obtained in this study is well controlled. To clarify the distribution of protonic defects and oxygen vacancies around Sc with respect to the concentration of protons in the sample, 45Sc MAS NMR measurements were carried out. Figure 6 shows 45Sc MAS NMR spectra of the samples with different proton concentrations. The three peaks at approximately 144, 163, and 230 ppm were observed in the spectra. These peaks can be assigned to 6-coordinated Sc (ScO6) for the peaks at 144 ppm, 6-coordinated Sc with protonic defects residing in its vicinity (ScO5(OH)) at 163 ppm, and 5-coordinated Sc (ScO5) at 230 ppm, respectively.34,35 The second-order quadrupolar interaction determined the line shape of the Sc signals. The sharp peak from ScO6 is derived from negligibly small electric-field gradient at D

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concentration around Zr was estimated by subtracting the concentration around Sc from the total. Since an oxygen vacancy is formed by substituting a Zr ion with Sc and one oxygen vacancy reacts with water vapor to create two protonic defects (as shown in eqs 1 and 2), the total protonic defect concentration and oxygen vacancy concentration, defined by the contents of Sc, can be expressed by the following equation: δtotal =

x 1 •• = [V •• ([OH•O]Sc + [OH•O]Zr ) O ]Sc + [V O ]Zr + 2 2 (5)

[V•• O ]Sc

Here, and are the oxygen vacancy concentrations around Sc and Zr. [OH•O]Sc and [OH•O]Zr are the protonic defect concentrations around Sc and Zr. In eq 5, the protonic defect concentration and the oxygen vacancy concentration around Sc were determined from the integral intensity of the 45Sc NMR spectra as mentioned before. The protonic defect concentration around Zr can thus be estimated by subtracting the protonic defect around Sc from the proton concentration in the sample. Consequently, the oxygen vacancy concentration around Zr is obtained by assuming the Sc content of x is 0.1 (10 mol %). Errors in the defect concentration around Sc were estimated by the same procedure used in the estimation of errors in the proton concentration from 1H NMR results, and errors in the defect concentration around Zr were calculated from the errors in the proton concentration and the defect concentration around Sc. The distribution of protonic defects and oxygen vacancies to Sc and Zr is shown in Figure 8 with respect to the proton

Figure 7. Peak deconvolution of 45Sc NMR spectra of 1.1, 5.9, and 9.9 mol % proton concentration. Asterisks indicate spinning sidebands.

with protonic defects (ScO5(OH)) or oxygen vacancies (ScO5) observed in the 45Sc NMR spectra were converted to the protonic defect concentration and oxygen vacancy concentration by taking the concentration of the acceptor dopant into consideration. Assuming the concentration of Sc as 10 mol %, the protonic defect concentration and oxygen vacancy concentration can be expressed by the following equations: ⎤ ⎡ ••⎤ x ⎡ V = × ⎢ScO5⎥ ⎦NMR ⎣ ⎣⎢ O ⎦⎥Sc 2

(3)

[OH•O]Sc = x × [ScO5(OH)]NMR

(4)

[V•• O ]Zr

Here, [ScO5]NMR and [ScO5(OH)]NMR are the relative integral intensities of ScO5 and ScO5(OH), respectively, in the 45Sc NMR spectra, and x is the concentration of Sc. Since the oxygen vacancy is coshared by Sc and Zr in the second nearest neighbor, 1/2 is multiplied to the right-hand side of eq 3. Multiplying a factor of 1/2 is required to convert the relative amounts of 5-coordinated Sc into that of oxygen vacancy formed around Sc. The 5-coordinated Sc includes one oxygen vacancy per one Sc. At the same time, the oxygen vacancy is coshared by not only Sc, but also Zr. Taking this into account, a half of oxygen vacancy per one Sc is regarded as locating around Sc, and the other half is located to the cosharing cation. The 45Sc NMR results were combined with the TGA and 1H NMR results to get information on the distribution of protonic defects and oxygen vacancies around Zr. 45Sc NMR spectroscopy is capable of revealing the protonic defect concentration and oxygen vacancy concentration around Sc; however, it is not able to reveal the defect concentration around Zr. To elucidate the distribution of all the defects in the sample, the proton concentration as determined by the TGA and 1H NMR results is required. By taking the total proton concentration in the sample and the protonic defect concentration around Sc obtained from 45Sc NMR into account, the protonic defects

Figure 8. Distribution of protonic defects and oxygen vacancies as a function of proton concentration.

concentration in the sample. An increase in protonic defects around Sc occurred as the total proton concentration increased, and approximately 80% of all defects formed around Sc in the fully hydrated samples were protonic defects. Meanwhile, the maximum protonic defect concentration around Zr occurs at a proton concentration of around 4 mol %. When the proton concentration is increased beyond this, the protonic defect concentration around Zr starts to decrease. The trend in the oxygen vacancy concentration around Sc and Zr is the same, that is, both decrease linearly with increasing proton concentration. E

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Chemistry of Materials Distribution of Protonic Defects and Oxygen Vacancies. On the basis of the distribution analysis of protonic defects and oxygen vacancies (Figure 8), it is possible to discern the energetically preferred sites of protonic defects and oxygen vacancies with respect to the proton concentration. At low proton concentrations, most of the defects are distributed around Sc and Zr as oxygen vacancies. As the proton concentration increases, the number of oxygen vacancies decreased, and more protonic defects formed. It should be noted that in the fully hydrated samples, almost all the oxygen vacancies disappeared, and the vast majority of the defects formed around Sc and Zr were protonic defects. The distribution of protonic defects was as follows: the proton concentration around Sc increased rapidly along with the total proton concentration, and approximately 80% of all the defects that formed around Sc were protonic defects. This implies that the protons are trapped around Sc in the fully hydrated samples. Since the distribution of the defects was based on an analysis of the local structure at room temperature observed from NMR spectroscopy in this study, it is not surprising to observe the proton trapping around Sc. In a similar study, Yamazaki and co-workers reported that almost all protons are trapped around Y at temperatures below 200 °C in Y-doped BaZrO3.25 Our results for Sc in the fully hydrated samples are in good agreement with the behavior reported in their study. Computational methods have also indicated that proton trapping occurs around Sc, and clarify a strong association between Sc and the protonic defect.23,24 It is interesting to note that proton trapping does not always occur. The protonic defect concentration around Zr is higher than that around Sc when the total proton concentration is approximately 4 mol %, or about half that of a fully hydrated sample. The relatively high protonic defect concentration around Zr at this proton concentration range indicates more than 50% of the protons are located in nontrapping sites. These results suggest it is possible to avoid or weaken the influence of the proton trapping that hinders the long-range migration of protons. The comparative weakness of the electrostatic net charge of the interaction between a proton with a positively charged (OH•O) and Zr ion with a zero net charge (ZrZr× ) compared to the interaction between a proton with a positively charged (OH•O) and the acceptor of Sc with a negatively charged (Sc′Zr) can be explained to avoid, or at least weaken, proton trapping. It is important to stress, however, that there are possible flaws in these results due to the method employed to determine the proton concentration around Zr. As explained earlier in this text, it is determined by subtracting the proton concentration around Sc from the total proton concentration. By using this method, it is possible to consider the influence of other sorts of protons such as adsorbed water molecules. However, since all the samples used in this study were heat-treated at temperatures higher than 350 °C and treated in an argon-filled glovebox after heat treatment, the influence of adsorbed water was minimized. As such, the protons that remain once the proton concentration around Sc is subtracted can be assumed to be those not in the vicinity of Sc, that is, the protons residing around Zr. Oxygen Vacancy as an Inhibitor of Proton Trapping. To clarify the influence of oxygen vacancies on the formation of protonic defects, the energetically favorable sites of oxygen vacancies to Sc and Zr are discussed based on the distribution of oxygen vacancies in the sample with a 1.1 ± 0.2 mol % total

proton concentration (Figure 8). Assuming the random distribution of oxygen vacancies and the formation of negligibly small amounts of protonic defects in 10 mol % Sc-doped BaZrO3, the ratio of 5-coordinated Sc to 6-coordinated Sc is 1:10 (those with fewer than 5-coordination sites account for less than 1% of the Sc). The ratio of 5-coordinated Sc to 6coordinated Sc in the 1.1 ± 0.2 mol % proton sample determined from the integral intensity of the 45Sc NMR signals was approximately 1:3 when the integral intensity of ScO5(OH) (less than 1%) was neglected. It is important to note that the 5-coordinated/6-coordinated ratio in random distribution should be compared to that of the completely dehydrated sample; however, a comparison between experimental and random distribution reveals that the oxygen vacancy concentration around Sc is approximately three-times higher than that of the random distribution of oxygen vacancies. This result is in good agreement with previous works on BaZrO3 system with the same acceptor dopant.34,35 All the possible configurations of oxygen vacancies, Sc−V•• O− 45 •• Sc, Sc−V•• Sc O −Zr, and Zr−VO −Zr, are indicated by the NMR spectra of the lowest proton concentration (1.1 ± 0.2 mol %). There are three possible configurations of oxygen •• •• vacancies, Sc−V•• O −Sc, Sc−VO −Zr, and Zr−VO −Zr. When all oxygen vacancies are formed between two Sc (Sc−V•• O −Sc), only the peaks from 5-coordinated Sc are observed in 45Sc NMR spectra. Meanwhile, in the case of all oxygen vacancies formed between Sc and Zr, the peaks from 5- and 6coordination are observed in the same ratio. The ratio of 5coordinated Sc to 6-coordinated Sc from the 45Sc NMR result of the 1.1 ± 0.2 mol % proton concentration was 1:1.7, which indicates the possible configurations of oxygen vacancies •• between two Zr (Zr−V•• O −Zr) as well as Sc−VO −Sc and Sc−V•• −Zr. O To explain the relatively high concentration of protonic defects around Zr at approximately 4 mol % proton concentration in Figure 8, the influence of oxygen vacancies around Sc on the formation of protonic defects is considered. At a proton concentration of approximately 4 mol %, both protonic defects and oxygen vacancies coexist in the material, and these oxygen vacancies are assumed to be preferentially located around Sc. As illustrated in Figure 9, when an oxygen

Figure 9. Schematic view of the interaction between oxygen vacancies and protonic defects around the acceptor.

vacancy is formed around Sc, the associated defects still possesses a single positive charge, and this positive charge around Sc with one oxygen vacancy prevents the formation of a protonic defect in its vicinity due to the repulsive electrostatic interaction between positively charged protonic defects and the association of Sc and an oxygen vacancy. F

DOI: 10.1021/acs.chemmater.5b02441 Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials In conclusion, this study suggests that the preferential formation of oxygen vacancies around the acceptor dopant hinder proton trapping from the perspective of local structure, that is, the distribution of protonic defects and oxygen vacancies at room temperature. At full hydration (9−10 mol % protons), the majority of the formed defects are protonic defects with a negligible number of oxygen vacancies also formed around Sc. In this case, the protons become trapped by the negatively charged acceptors. Meanwhile, when the total proton concentration is approximately 4 mol % and a sufficient amount of oxygen vacancies is located around Sc, the positively charged association of Sc and an oxygen vacancy seems to prevent the formation of protonic defects around the acceptor; consequently, the protonic defects are distributed in the vicinity of Zr. This indicates that the protons reside in the nontrapping sites. While it was revealed that protons exist around Zr when protonic defects and oxygen vacancies coexist in the sample, it should be noted that the influence of oxygen vacancies on the mobility of protons and the distribution of protonic defects and oxygen vacancies in the range of operating temperatures was not clarified. Further investigation on the influence of oxygen vacancies on mobility and trapping effect of protons will contribute to the development of our understanding of proton trapping and the migration mechanism.

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CONCLUSION

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

I.O. thanks a financial support by Grant-in-Aid for JSPS Fellows. NMR experiments in this work were supported by the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, at the Center for Integrated Nanotechnology Support, Tohoku University. H.T. would like to gratefully acknowledge financial support from JSPS KAKENHI Grant Number 26249103.

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The distribution of protonic defects and oxygen vacancies with respect to proton concentration in 10 mol % Sc-doped BaZrO3 at room temperature was investigated by combining TGA and NMR techniques. Proton concentration in the sample was controlled by heat treatment and determined by TGA and 1H NMR. The protonic defect concentration and oxygen vacancy concentration around Sc were clarified by 45Sc NMR. By combining TGA and 1H and 45Sc NMR results, the defect concentration around Zr was estimated to reveal the distribution of the defects in the samples. At a high proton concentration of 9−10 mol %, the results strongly supported the proton trapping effects. When the proton concentration is approximately 4 mol % and protonic defects and oxygen vacancies coexist, while the protonic defect concentration around Zr was shown to be relatively high, they seemed to be located in the nontrapping sites. Protonic defects around Zr in this concentration range can be explained by the existence of oxygen vacancies around Sc. These oxygen vacancies seem to prevent the formation of protonic defects around Sc due to the repulsive interaction between protons and the association of Sc and an oxygen vacancy, both of which have a positive net charge. Further investigation on the influence of oxygen vacancies on the mobility of protons will contribute to a more detailed understanding of proton trapping and the effect of oxygen vacancies on long-range migration.

Corresponding Author

*E-mail: [email protected]. Phone: +81 22 795 3938. Fax: +81 22 795 3938. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.chemmater.5b02441 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

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DOI: 10.1021/acs.chemmater.5b02441 Chem. Mater. XXXX, XXX, XXX−XXX