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Influence of Yttrium Concentration on Local Structure in BaZr YO Based Proton Conductors 1-x

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Caroline W. Mburu, Samuel M. Gaita, Christopher S Knee, Michael J. Gatari, and Maths Karlsson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05023 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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The Journal of Physical Chemistry

Influence of Yttrium Concentration on Local Structure in BaZr1−x Yx O3−δ Based Proton Conductors Caroline W. Mburu,† Samuel M. Gaita,†,‡ Christopher S. Knee,§ Michael J. Gatari,† and Maths Karlsson∗,k † Institute

of Nuclear Science & Technology, University of Nairobi, P. O. Box 30197 00100, Nairobi, Kenya

‡ Department

of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Göteborg, Sweden

§ Department

of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

k Department

of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

ABSTRACT: The evolution of local structure, coordination of protons and proton conductivity in yttrium doped barium zirconate, BaZr1−x Yx O3−δ (x = 0 - 0.5), has been investigated using thermal-gravimetric analysis, impedance spectroscopy, and infrared spectroscopy. Low frequency (50–1000 cm−1 ) infrared absorbance spectra provide evidence of increasing local structural distortions as a function of yttrium concentration as well as subtle differences, mainly linked to the oxygen sub-lattice, between the dry and hydrated samples. High frequency (1700–4500 cm−1 ) spectra of the hydrated samples, distinguished by a broad O-H stretch continuum, manifest a varying degree of hydrogen bond interactions between the protons and nearest neighbour oxygens due to the disordered crystal structure, with a general weakening in particular of the strongest hydrogen bonding interactions with increasing dopant levels. It is argued that compositions within the range 0.15 6 x 6 0.3 possess a favourable level of local structural distortions to facilitate high proton conductivity, and this, coupled with a significant proton concentration, may be a factor in explaining the high proton conductivity these phases display.

1

INTRODUCTION

Yttrium doped barium zirconate (Y-doped BaZrO3 , Y:BZO) – a perovskite type material built up of corner-sharing (Y/Zr)O6 octahedra and central Ba2+ ions – has emerged as an attractive electrolyte for future solid oxide fuel cells (SOFCs) operating in the so called intermediate temperature region, ca. 200–600 ◦ C, because of its combination of excellent chemical stability and high proton conductivity. 1–3 The substitution of Y3+ for Zr4+ creates an oxygen-deficient perovskite structure, BaZr1−x Yx O3−δ , which can be filled with protons during a hydration procedure. Hydration of these materials is usually performed by heat treatment in a humid atmosphere, at which water molecules from the gaseous phase dissociate into hydroxyl (OH− ) groups, which fill the oxygen vacancies, and protons (H+ ) which bond to oxygens of the perovskite lattice, leading, ideally, to a material of the form BaZr1−x Yx O3 Hx . The proton transport is governed by proton hopping from one oxygen to another one, with rotational diffusion of the -OH group between such hops, resulting in a high proton conductivity, as high as 0.01 Scm−1 at 450 ◦ C depending on sample composition. 4 However, fundamental questions encompassing the relationship between structure and proton conduction in these materials remain, especially regarding the effect of the dopant (Y) atom on the materials’ proton transport properties. In this work we investigate the influence of the Y-concentration on the local structure and proton conductivity of Y:BZO over a

large range of dopant concentrations, i.e. x = 0, 0.1, 0.15, 0.3, 0.4, and 0.5. Previous work by Fabbri et al., 3 has highlighted a novel wet chemistry route that allows a complete Y solubility for all these compositions, which far exceeds that achievable when using normal solid state routes, i.e. x ≈ 0.2. Specifically, it was found, through a combined thermal-gravimetric analysis (TGA) and impedance spectroscopy (IS) study, that the proton concentration increases and the proton conductivity decreases with increasing Y-concentration for x > 0.2. It was suggested that the observed lowering of the proton conductivity with increasing Y-dopant level might be related to larger structural distortions due to the difference in ionic radii between Zr4+ (0.72 Å) and Y3+ (0.90 Å), 5 and to proton trapping, meaning that the dopant atoms could trap protons and impede their long-range transport, especially at larger Yconcentrations. 3 Recent results on the 20% substituted material (20Y:BZO), obtained from TGA, IS, and nuclear magnetic resonance (NMR) measurements, show that such proton traps are indeed present. 4 Moreover, a study on 10Y:BZO, substituted with 1% of Eu3+ on the Y3+ site acting as a luminescent probe for the local structure around the Y3+ /Eu3+ ions, suggests an attractive interaction between protons and dopant atoms. 6 However, the dependency on Y-concentration on the local structural properties of Y:BZO, in particular concerning the local coordination of protons, and its relationship to proton conduction, is not fully understood. The aim of the present study is, therefore, to obtain new insight into the nature of local structure and proton sites in 1–11 | 1

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Y:BZO (x = 0–0.5). Specifically, we use infrared (IR) spectroscopy to study the local structural properties of Y:BZO over a wide substitution level. IR spectroscopy is a very sensitive probe of the local structure of proton conducting oxides, for which the protons are manifested as a, typically, broad band in the IR absorbance spectrum and whose position, intensity and shape provide fingerprint information about the local coordination(s) of protons. In previous IR spectroscopy studies on the In-doped equivalent BaZr1−x Inx O3−δ (x = 0.25–0.75), we showed that the O-H stretch vibrations are manifested as a very broad region of bands between around 2000 and 3600 cm−1 , as a result of several different types of local proton sites present due to dopant-induced local structural distortions in the material. 7,8 Here we compare the spectroscopic results from the yttrium and indium substituted systems as well as discuss the possible link with proton conductivity as measured on the same samples.

2 2.1

EXPERIMENTAL DETAILS Sample Preparation

The BaZr1−x Yx O3−δ samples with x = 0, 0.1, 0.15, 0.3, 0.4 and 0.5 were prepared via a Pechini process as outlined in ref. 9. The chemicals used were barium (II) nitrate (Aldrich, 99.98% purity), zirconium (IV) acetylacetonate (Aldrich, 98% purity) and yttrium (III) nitrate hexahydrate (Aldrich, 99.9% purity). In addition, citric acid (Sigma Aldrich, 99.99% purity) was used as a chelating agent while ethylene glycol (Sigma Aldrich, 99.8% purity) was used as the only solvent. After prolonged heating and stirring the gel dried to a brownish mass, which was ground using an agate pestle and mortar. The obtained powder was calcined consecutively at 300 and 500 ◦ C for 1 h intervals, prior to manual grinding and further calcinations at 800 and 1100 ◦ C for 5 h durations. Finally, the synthesis was completed by heating the off-white powders at 1400 ◦ C for 8 h. Portions of the as-prepared samples were put in an alumina crucible and heated to 500 ◦ C under a flow of Ar gas, saturated with water vapour at 70 ◦ C (pH2 O = 0.3 atm). The hydration process took 3 days and the samples were removed after cooling in steps of 100 ◦ C (2 h dwells) to 150 ◦ C. Pellets for impedance measurements were prepared by uniaxially pressing the as-prepared powder into pellets 10 mm in diameter at 600 MPa. The pellets were covered in remaining powder to minimise barium loss and sintered at 1550 ◦ C for 8 h. After sintering, densities of ca. 70% of the theoretical density were obtained. The surfaces of the pellets were covered in Pt paste and two point ac-conductivity measurements were then performed using a Solartron 1260 frequency response analyser from 1Hz to 4.5 MHz with the sine wave amplitude of 1 V rms. The samples were heated to 1000 ◦ C in a flow of dried argon, held for one hour at this temperature and

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data collected on slowly cooling to 150 ◦ C in steps of 50 ◦ C after adequate equilibration dwells. The procedure was then repeated but with the argon gas now passing through a water bottle at room temperature to provide a wet gas condition. Further details of the impedance set-up can be found in ref. 10. 2.2

Powder X-ray Diffraction

Powder X-ray diffraction (PXRD) data were collected for the as-prepared samples on a Bruker D8 Advance diffractometer operating with Cu-Kα1 radiation in the 2θ -range 19 to 100◦ with a step size of 0.009◦ and count time of 4 s per step. The data were analysed using the CELREF 11 program to refine the cell parameters. 2.3

Thermal-Gravimetric Analysis

TGA was performed using a NETZCH STA 409 PC TGA/DSC instrument on the hydrated samples. About 100 mg of each sample was placed in an alumina crucible and heated in a flow of nitrogen gas (50 ml/min) from ambient temperature to 1050 ◦ C at a rate of 15 ◦ C/min. 2.4

Scanning Electron Microscopy and Energy Dispersive X-ray Analysis

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis of selected impedance pellets (x = 0.1, 0.3 and 0.5) after sintering at 1550 ◦ C were performed using a Leo Ultra 55 FEG SEM equipped with an Oxford Inca EDX system. EDX analysis was done randomly, on a minimum of 5 spots on each pellet surface with the instrument operating at 30 kV to obtain an average of the chemical concentration of Ba, Zr and Y. 2.5

Infrared Spectroscopy

The IR spectroscopy measurements were performed using three different setups. The lowest-frequency part, 50–450 cm−1 , was measured in transmittance on a Bruker IFS 66v/s FT-IR spectrometer equipped with a Mylar 6 beam splitter and a MCT detector. The middle-frequency part, 400–1000 cm−1 , was measured on the same instrument, but with the Mylar 6 beam splitter exchanged for a KBr beam splitter. Sample pellets for the measurements were prepared by uniaxially pressing mixtures of approximately 5 wt% of the sample dispersed in polyethylene, for measurements in the range 50–450 cm−1 , and KBr, for measurements in the range 400– 1000 cm−1 . The highest-frequency part, 1700–4500 cm−1 , was measured in diffuse reflectance mode directly on the samples, using a Bruker Alpha spectrometer. All measurements were performed in vacuum (50–1000 cm−1 part) or inside a glove box (1700–4500 cm−1 part).

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Average Structural and Thermal-Gravimetric Analysis

The PXRD patterns confirmed the formation of an averagecubic perovskite structure for all Y:BZO samples (x = 0–0.5 ) after the treatment at 1400 ◦ C. A trend of increasing cell parameter with increasing Y-concentration is shown in Figure 1. The cell dependence is of similar magnitude to that previously observed 3 and primarily reflects substitution of the Zr4+ ion (0.72 Å) in 6-fold coordination by the larger Y3+ ion (0.90 Å). 5 The increasing level of Y substitution was further confirmed from the EDX analysis on the sintered pellets [Figure S1 in the Supporting Information (SI)]. In Figure 2 are shown the TGA curves, recalculated to the number of moles of water per formula unit perovskite, for the Y:BZO samples. A general trend is the increasing water molar concentrations with increasing Y concentration. It should be noted that in all cases the H2 O molar concentration exceeds the maximum value expected based solely on hydration of oxygen vacancies created via acceptor doping, c f . Eq. (1). As discussed in previous contributions 3,12 the mass loss recorded from acceptor doped perovskites can reflect both protons dissolved into the bulk of the material following Eq. (1) below, and physisorbed water molecules. · H2 O +VO·· + O× O ⇋ 2OHO

(1)

This makes a quantitative estimate of the protons dissolved via filling of oxygen vacancies difficult. However, assuming the contribution of bulk protons is dominant at T > 300 ◦ C, then the level of dissolved protons clearly increases with increasing Y concentration (Fig. 2). This trend was found previously for Y:BZO. 3,13 Furthermore, the high partial pressure of H2 O utilised for hydration of the current samples will push the equilibrium of the hydration reaction to the right hand side and favour close to complete filling of the available vacancies. 3.2

Cell parameter (Å)

3.1

RESULTS

Y concentration, x Figure 1 Y-concentration dependence of the cell parameter of Y:BZO, as extracted from room temperature PXRD data.

2

3

n H O /noxide

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Temperature (°C) Figure 2 TGA curves for Y:BZO, with x = 0.10 (bottom), 0.15, 0.30, 0.40, and 0.50 (top).

Proton Conductivity

Figure 3 summarises the total conductivity of the samples recorded under wet argon at 300, 400 and 500 ◦ C. Nyquist plots from the IS measurements showed typically the presence of two or more heavily depressed arcs. It was not possible to reliably de-convolute the data in order to determine the bulk and grain boundary responses and hence the total conductivity is reported. Typically the conductivity recorded under wet gas was one order greater than the equivalent dry gas measurements at T 6 500 ◦ C, indicating that protons are the dominant charge carrier in elevated pH2 O. Figure 3 shows that the proton conductivity does not display a strong correlation to the Y-concentration, with the perfor-

mance of the x = 0.15, 0.3 and 0.4 samples being very similar at the three temperatures shown. The values recorded for these three samples are higher than those for the x = 0.1 sample, and significantly higher than the level of the x = 0.5 sample. Based purely on a capacity to dissolve protons the x = 0.5 phase may have been expected to support the highest levels of proton conductivity. Similar findings were reported by Fabbri et al. 3 for Y:BZO where a gradual decrease in both bulk and total proton conductivity was observed from x = 0.2 to 0.5. The total conductivity values reported here lie approximately half an order lower than those reported by Fabbri et al. 3 The synthesis of the samples was similar with the exception of the final sinter1–11 | 3


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Total conductivity (Scm-1)


500 °C 400 °C 300 °C

Y concentration, x

Figure 3 Total conductivity of the Y:BZO samples, as measured under wet argon at 300, 400, and 500 ◦ C.

ing temperature employed to produce the conductivity pellets being 50 ◦ C higher in the study of Fabbri et al. 3 The variation in overall magnitude of conductivity between the two sets of samples may therefore reflect microstructural factors such as the grain size /density of the pellets used for the IS experiments. 3.3

Local Structural Analysis with Infrared Spectroscopy

Figure 4 shows the IR spectra of dry and hydrated samples of Y:BZO with x = 0, 0.10, 0.15, 0.30, 0.40, and 0.50, in the lowfrequency range 50–1000 cm−1 . This part of the IR spectra relates to vibrations of the perovskite host lattice. Considering first the undoped compound, BZO, the spectrum is dominated by three main bands, located at approximately 135, 270, and 530 cm−1 , which are assigned as Ba-(ZrO6 ) stretch (ν1 ), O-Zr-O bend (ν2 ), and Zr-O stretch (ν3 ) modes, respectively, and which are the only IR-active modes for a perfectly cubic perovskite material. 8 The broad and asymmetric nature of the bands, and in particular the presence of a weaker feature in the range 150–200 cm−1 , which is most likely related to a torsional motion of the unit cell, are clear signatures of local structural distortions. 8 Compared to our previous IR studies on the same composition prepared by a solid state sintering route instead of the wet chemical synthesis route used here, 8,13 the spectrum in Figure 4 exhibits less distinct bands. Given the differences in synthesis method, small differences in the structure and morphology of the samples can be expected. These are not detected by PXRD, but may give rise to significant features in the vibrational spectra, which is more sensitive to the local structural arrangement of atoms in materials.

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For the spectra of the yttrium substituted, dried, perovskite samples (dashed lines), several significant, although weak, features are observed, suggesting a slight change of the structure of the material. More specifically, we observe a systematic down-shift of the ν1 band from 135 cm−1 for x = 0 to 125 cm−1 for x = 0.5. This behaviour is most likely an effect of the lattice expansion of the material (c f . Figure 1) which increases the distance between Ba and ZrO6 and hence can be expected to lower the frequency of the Ba-(ZrO6 ) stretch mode. The slight broadening of the ν1 band with increasing Y-concentration results from a more disordered local structure. Furthermore, we observe the generally more asymmetric shape of the ν2 and ν3 bands for all the dried Y:BZO samples, compared to BZO. In particular, we observe a quite different spectral intensity in the range 200–350 cm−1 amongst all samples, with the presence of additional, small but distinct, bands, visible as shoulders of the ν2 and ν3 bands at around 300 and 570 cm−1 , for the three highest dopant concentrations, x = 0.30, 0.40, and 0.50. The 570 cm−1 band grows gradually in intensity with increasing dopant concentration, and for x> 0.30 it is even more intense than the 530 cm−1 peak. The observation of asymmetric bandshapes and the appearance of new peaks for the higher Y-dopant concentrations is similar to what was observed for the corresponding In-doped system, BaZr1−x Inx O3−δ (x = 0–0.75), and reflects the pronunciation of dopant-induced local structural distortions of the average cubic perovskite structure as determined by PXRD. 8 Given the large size and charge difference between Zr4+ (0.72 Å) and Y3+ (0.90 Å), 5 as well as the creation of charge-compensating oxygen vacancies accompanying the Y-doping, some local structural rearrangements will occur as discussed further below.

For the spectra of the doped, hydrated, perovskite samples (solid lines), the spectra are overall similar to the spectra of the dried samples, however, some important differences can be discerned. In particular, we observe a higher spectral intensity in the range of the ν2 band (200–400 cm−1 ), with a notably stronger 570 cm−1 peak for x = 0.30 and x = 0.40 compared to the spectra of the dried samples. Also, the x = 0.10 material shows a significantly higher intensity in this range after hydration, although with no such a marked shoulder that can be distinguished. As for the ν3 band (450–650 cm−1 ), this is less distinct for the hydrated samples, whereas the ν1 band is essentially the same in the two series of spectra. Given the filling of oxygen vacancies with -OH groups upon hydration, it is obvious that some changes in the local structure will occur resulting in subtle vibrational differences, especially in relation to the vibrations of the oxygen sub-lattice, as manifested by the ν2 and ν3 bands.

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x = 0.50

Absorbance (arb. units)

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Hydrated Dried

Hydrated Dried x = 0.50

x = 0.40

x = 0.40

x = 0.30

x = 0.30

x = 0.15

x = 0.15

x = 0.10

x = 0.10

x=0

x=0

ν1

ν2

ν3

Wavenumber (cm-1)

Wavenumber (cm-1)

Figure 4 IR spectra of the Y:BZO (x = 0–0.5) samples over the host-lattice region. The three major bands are related to Ba-(ZrO6 ) stretches (ν1 ), O-Zr-O bends (ν2 ), and Zr-O stretches (ν3 ), respectively. The spectra have been baseline corrected with a straight line between 50 and 450 cm−1 (left) and 415 and 1050 cm−1 (right), as well as vertically offset.

3.4

Local Coordination of Protons Investigated with Infrared Spectroscopy

Figure 5 shows the high-frequency part (1700–4500 cm−1 ) of the IR spectra for all hydrated Y:BZO samples, as well as for the undoped one, BZO, and one vacuum dried (deprotonated) sample of the composition x = 0.50. The spectra of all Y-doped materials (x = 0.10–0.50) exhibit a broad, asymmetric, O-H stretch continuum over the range 2500 to 3700 cm−1 , as well as three weaker bands at around 1900, 2400, and 4500 cm−1 , respectively. The absence of a water bend band at around 1650 cm−1 (see Figure S2), suggests that the O-H stretch band in Figure 5 relates to dissolved protons, whereas the observation of a very broad O-H stretch region reflects a broad range of local proton sites in the material. This is consistent with the low-frequency spectra reported in Figure 4. A comparison with the literature on proton conducting oxides generally suggests that the bands between 2400 and 3600 cm−1 are due to fundamental O-H stretch modes, 7,14 whilst the higher-frequency band at 4500 cm−1 band is assigned as a combination of O-H wag (IR-inactive) and O-H stretch modes. 7,14 Previous works, e.g., by computer simulations, have not confirmed fundamental O-H stretch modes as low as the 1900 cm−1 band as observed here. Nevertheless, we note that the 1900 cm−1 band is absent in the undoped material, BZO, grows in intensity upon hydration, c f . the spectra of dry and hydrated 50Y:BZO, suggesting it is indeed related

to O-H stretch vibrations. It could be related, e.g., to quite “extreme” proton sites, such as between two dopant atoms and/or in the vicinity of other protons, that, to our knowledge, have not been thoroughly investigated in previous works. Upon increasing the Y-dopant concentration, from x = 0.10 to x = 0.50, the broad band between 2500 and 3700 cm−1 changes slightly in shape, and the two low-frequency components at 1900 and 2400 cm−1 increase slightly in intensity. For comparison, the spectra of the undoped sample (BZO) and the vacuum-dried one show considerably lower spectral intensity over the 1700–4500 cm−1 region, in agreement with the expectedly much lower proton concentrations in these samples. The yet significant O-H stretch band intensity for the 50% yttrium substituted, vacuum-dried sample reflects the difficulty to remove all protons from the perovskite matrix, even during heat treatment in high vacuum, which is consistent with other studies on the same and similar systems. 7 Importantly, the spectral shape of the dried sample is similar to the hydrated one, suggesting a similar distribution of proton sites in the structure, despite the expectedly higher concentration of unfilled oxygen vacancies in the dried one. In a more detailed, quantitative, analysis, we find that the O-H stretch band can be reproduced/fitted with four Gaussian components (marked as A, B, C, and D), centered around 2700, 3100, 3400, and 3600 cm−1 , respectively, and a linear, sloping, background between 1700 and 4500 cm−1 . Fig1–11 | 5




Dried x = 0.50


x = 0.40 x = 0.30 x = 0.15 x = 0.10

(a)

x = 0.40 (40Y:BZO) Exp. spectrum Fit

B C A

x=0

Wavenumber (cm-1) Absorbance (arb. units)

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Figure 5 IR spectra of the Y:BZO (x = 0–0.50) samples over the O-H stretch region. Included in the figure is the spectrum for one vacuum dried (de-protonated) sample of composition x = 0.5 (dashed line). The two lowest-frequency bands at around 1900 and 2400 cm−1 are marked with vertical dashed lines. The spectra have been baseline corrected with a straight line between 1700 and ∼3900 cm−1 and vertically separated for increased clarity.

D

(b)

x = 0.375 (37.5In:BZO) Exp. spectrum Fit

A B

C D

ure 6(a) shows, as an example, the peak fit to 10Y:BZO, after the linear background has been subtracted from the spectrum. Figure 7(a) shows the Y-concentration dependence of the relative integrated intensity of each Gaussian component and in Figure 7(b-c) are shown their peak frequency and full width at half maximum (FWHM), respectively. An important result is the large redistribution of intensity from the two middlefrequency bands (B and C) to the lowest-frequency band (A) [Figure 7(a)], as well as the large upshift of band A, with increasing Y-concentration [Figure 7(b)]. The larger FWHM of band A for x = 0.3 [Figure 7(c)] does not follow the generally featureless Y concentration dependence of the FWHMs of the four Gaussians and should hence be considered with caution. A fit with the peak position of the four Gaussians fixed to the same value for all Y concentrations equally well reproduce the spectra with the general trends in Figure 7(a,b) being essentially the same. Taken together, it implies that the main effect upon increasing Y-concentration is a systematic redistribution of protons from proton sites characterized by relatively weak hydrogen-bonding interactions to proton sites characterized by stronger hydrogen bonds in the material, as well as a general weakening of the overall hydrogen-bond interactions as judged from the upshift in frequency of all modes [Figure 7(b)]. The decrease in the average strength of the hydrogen bonding is especially strong for protons related to band A.

Wavenumber (cm-1) Figure 6 (a) IR spectrum of 40Y:BZO together with the fit to four Gaussians (marked as A, B, C, and D). (b) IR spectra of 37.5In:BZO together with the fit to seven Gaussians, where the four Gaussians fitted to the range 2500–3700 cm−1 are marked as A, B, C, D. The sum of the four Gaussians (blue) fits well to the experimental spectrum (black) for both materials.

4

DISCUSSION

The wet chemical synthesis route 9 utilised here extends the yttrium substitution and allows the detailed exploration of the structural properties of a heavily substituted region of BaZr1−x Yx O3−δ with x > 0.2 for the first time. The incorporation of the yttrium dopant into the perovskite structure is supported by the lack of secondary phases present in the XRD scans and the increasing cell parameters (Figure 1). The cell parameter trend with increasing yttrium levels recorded in this work is not perfectly linear. This probably reflects that the as-prepared samples hydrated to differing levels during the cooling period of the final synthesis heat treatment, and hence display different levels of hydration induced chemical expansion. A significant chemical expansion usually accompanies

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(a)

(b)

Relative integrated intensity (arb. units)

(c) D

A

C

C A B

A

FWHM (cm-1)

B

Peak frequency (cm-1)

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B C D

D

Y concentration, x

Y concentration, x

Y concentration, x

Figure 7 Y concentration dependence of the (a) relative integrated intensity, (b) peak frequency, and (c) FWHM, of the peak fitted Gaussians (A, B, C, and D), according to Figure 6. Lines are linear fits to the data and serve as guidance for the general trends.

the hydration reaction [Eq. (1)] of proton conducting oxides, reflecting the greater size of the OH- defects in comparison to the oxygen vacancies. 12 The deviation from linearity may also be partly linked to the increasing levels of structural distortions associated with the incorporation of yttrium ions (and accompanying vacancy generation) at the B site of the ABO3 type perovskite. The dopant-induced local structural distortions are most likely related to tilts/rotations of the octahedral units, which reduces bond strain from the competing Ba-O and Y-O interactions, in a manner analogous to that observed for perovskite systems accommodating large B site ions, for example BaCeO3 . 8 In particular, the activation of additional bands upon increasing Y-concentration as shown in Figure 4 provides strong evidence for displacements of the cations and the oxygens away from their high-symmetry crystallographic sites expected in a cubic structure. In general, a larger (Y/Zr) site ion separation is expected with increasing Y-concentration, which is reflected by the expansion of the cubic lattice parameter (Figure 1). The magnitude of lattice expansion is, however, smaller than would be expected from a linear dependence reflecting a simple addition of the average ionic radii of the (Y/Zr) and O ions. This further suggests the presence of localized tilts/rotations of the (Y/Zr)O6−δ moieties, which, in effect, relax the (Y/Zr)-O(Y/Zr) bond angle from linear, thus allowing shorter (Y/Zr) site separations and which leads to the activation of otherwise IR-inactive modes. Given a random distribution of the Y dopants, it is clear that the effect of local structural distortions will not be limited to the immediate neighbourhood of the dopant atom, rather the effect of tilts/rotations of the (Y/Zr)O6−δ moieties are expected to distribute throughout the lattice already for quite low dopant concentrations. In order to better understand the effect of Y-concentration on the local structure and proton environments in Y:BZO, we

compare our results with our previous IR spectroscopy and density functional theory (DFT) investigations of the In-doped system, BaZr1−x Inx O3−δ (x = 0.25–0.75), from now on referred to as In:BZO. Similarly to Y:BZO, the In:BZO system exhibits an average cubic structure, with increasing cell parameters 15 and more pronounced local structural distortions upon increasing x. 8 Figure 6(b) shows, as an example from our previous study, the O-H stretch region for 37.5In:BZO, which similarly to 40Y:BZO [Figure 6(a)] can be fitted to four Gaussian components, now at approximately 3000, 3300 (B), 3400 (C), and 3500 (D) cm−1 . DFT based simulations of the O-H stretch frequencies for the proton placed in different local configurations in the material suggest that band A is assigned to protons in non-symmetrical environments, such as Zr-OH-In, whereas bands B, C, and D relate to more symmetrical proton sites, such as Zr-OH-Zr and In-OH-In. It was argued that protons in non-symmetrical sites are generally displaced towards a neighbouring oxygen, because of the difference in oxidation state between In3+ and Zr4+ and/or the local structural distortions as attributed to octahedral tilting. This increases the tendency for hydrogen bond formation between the proton and nearest-neighbour oxygen and, in effect, lowers the frequency of the O-H stretch mode. Protons in more symmetrical sites are less displaced and the tendency for hydrogen-bond formation is therefore generally lower and the O-H stretch frequency is higher. The evolution of the O-H stretch region of the IR spectra for In:BZO (0.25–0.75) is different from that of Y:BZO in a comparable dopant regime (0.15–0.5). For In:BZO a downshift of all bands is observed as the In concentration increases, from x = 0.25 to x = 0.75, while the relative integrated intensities of the bands remained essentially unchanged. 7 The lowest frequency band, band A, in particular showed a downshift from 3100 cm−1 (x = 0.25) to 2900 cm−1 (x = 0.75) that was in1–11 | 7



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terpreted as evidence for increasing amounts of local structural disorder in the In-doped system leading to more strongly hydrogen bonded protons. In comparison, the data for Y:BZO presented here, show that these samples have, relatively, more strongly hydrogen bonded sites as judged by the lowest frequency band (band A) position being located at ∼2700 cm−1 . This we interpret as evidence for enhanced tendency for hydrogen bonding in Y:BZO, probably originating from comparatively larger local distortions due to the larger size of Y3+ (0.9 Å) vs. In3+ (0.8 Å). 5 Overall, the evolution of the IR spectra with increasing dopant concentration differs in that for Y:BZO the O-H stretch band region is broader and its width narrows, if anything, reflecting the upshift of band A [Figure 7(b)] whilst for In:BZO a distinct broadening of the O-H stretch region occurs due mainly to the downshift of band A. In summary, the spectroscopic information therefore indicates that the two systems accommodate increasing proton concentrations in quite different ways. In this context, it is relevant that the trends in proton conductivity with increasing dopant concentration also differs between the two systems. In contrast to Y:BZO, In:BZO shows a significant increase in proton conductivity with increasing dopant, and proton, concentration from x = 0.25 to 0.75. 14,15 The behaviour of Y:BZO also contrasts with recent findings on the heavily substituted BaTi1−x Scx O3−δ perovskite series that shows increasing proton conductivity with increasing Sc levels in the region where cubic phases are stabilised (0.5 6 x 6 0.7). 10 Initially it is worth emphasising that any efforts to establish a link between the IR spectra and proton mobility must remain speculative in nature as the influence of hydrogen bonding strength on proton mobility, and particular the elementary proton migration steps, is not yet established. In addition it is worth noting that the changes in the O-H stretch regions of Y:BZO are not dramatic, and this is, in some respects, consistent with the plateau-like behaviour of the proton conductivity as observed here. Nevertheless, consideration of the widely accepted Grotthuss-type proton migration mechanism that consists of a proton transfer step between two neighbouring oxygen ions and a re-orientational motion of the proton around a given oxygen between such steps, is merited. In such a scenario, hydrogen-bonded configurations are likely to favour proton transfer between two oxygens, although a too strong hydrogen bond may hinder the long-range migration by decreasing the likelihood of proton re-orientation. The IR data for Y:BZO show that, in comparison to In:BZI, BZY samples have relatively more, strongly hydrogen bonded sites. Since Y:BZO with x ≈ 0.20 is generally accepted to display the highest proton conductivity of all proton conducting perovskite materials, it can then be argued that a large number of protons in sites characterized by strong hydrogen bonds may be favourable for high proton conductivity. The general trend of an upshift of band A with increas-

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ing Y-concentration, signifying a systematic weakening of the hydrogen bonding interactions, would not be expected, however, as the structure becomes successively more and more locally distorted as manifested by the low-frequency spectra (Figure 4). Instead, the upshift may result from other features, such as proton-proton interactions or perhaps more complicated Y-Zr environments, both of which should gain in importance as the Y- and proton concentration increase and which may dominate over the structural distortions associated to octahedral tilting. We therefore hypothesise that a Y-dopant level of x = 0.15–0.3 may be the best compromise as it has a higher proton concentration than x = 0.1 and a favourable amount of structural distortions creating a relatively large amount of protons sites in which the proton mobility is high. For larger Y-dopant concentrations, the increase in proton concentration may be balanced by the general weakening of hydrogen bonding interactions, i.e. by the occupation of less favourable proton sites, resulting in the plateau/decrease in conductivity. This is consistent with the plateau-like behaviour of the total conductivity in our samples (assuming resistive grain boundary contributions are more or less equal) and the gradual decrease in bulk conductivity for x > 0.2 reported by Fabbri et al. 3 Finally, it is noteworthy, that for both the Y:BZO and In:BZO systems a marked growth of bands at < 2400 cm−1 is seen. Further investigations of what types of local structural configurations this O-H stretch intensity represents are needed but as mentioned in the Results section, these may be indications of proton sites experiencing extremely strong hydrogen bonding and thus potentially strongly “trapped” protons. Such an interpretation is in line with the decrease in charge carriers indicated by Fabbri et al. 3 An alternative explanation of the presence of these bands below 2400 cm−1 , could be that they relate to higher-order transitions of lower-frequency O-H wag and/or lattice modes.

5

CONCLUSIONS

Using a combination of PXRD, TGA, IS, and IR spectroscopy, we have investigated the evolution of local structure, coordination of protons and proton conductivity in yttrium doped barium zirconate, BaZr1−x Yx O3−δ , over a large dopant range (x = 0–0.5) for the first time. The results provide evidence of increasing local structural distortions of the perovskite lattice with increasing yttrium concentration as well as subtle differences between dry (essentially proton-free) and hydrated samples. In particular, we find that the pronunciation of local structural distortions with increasing yttrium concentration is responsible for the decreasing proton conductivity despite increasing proton concentration for x > 0.2 samples, as speculated on by Fabbri et al. 3 It is hypothesized that a Y-dopant level of x = 0.15–0.3 may be the best compromise for high

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proton conductivity, as it exhibits higher proton concentration than x = 0.1 and a favourable amount of structural distortions leading to high proton mobility in the perovskite lattice. For larger Y-dopant concentrations, the increase in proton concentration may be balanced by the creation/occupation of, from a proton mobility point of view, less favourable proton sites, resulting ultimately in a decrease of proton conductivity.

ASSOCIATED CONTENT Supporting Information Chemical composition determined by SEM-EDX analysis, IR absorbance spectra over the range 700 to 4500 cm−1 , and a full list of author names in ref. 15 (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present addresses M. K.: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden; C. S. K.: ESAB AB, Lindholmsallèn 9, SE-402 77, Göteborg, Sweden. Funding Sources The authors declare no competing financial interest.

ACKNOWLEDGMENTS M. K. thanks the Swedish Research Council (grant No. 20103519 and 2011-4887) and the Swedish Foundation for Strategic Research (grant No. ICAIO-0001) for research funding. C. M. and S. G. thank the International Science Program, Uppsala, Sweden, and the Institute of Nuclear Science and Technology, for a fellowship, enabling a one-year long research visit to Chalmers University of Technology.

References 1 Kreuer, K. D.; Adams, St.; Münch, W.; Fuchs, A.; Klock, U.; Maier, J. Proton Conducting Alkaline Earth Zirconates and Titanates for High Drain Electrochemical Applications. Solid State Ionics 2001, 145, 295–306. 2 Malavasi, L.; Fisher, C. A. J.; Islam, M. S. Oxide-Ion and Proton Conducting Electrolyte Materials for Clean Energy Applications: Structural and Mechanistic Features. Chem. Soc. Rev. 2010, 39, 4370–4387.

3 Fabbri, E.; Pergolesi, D.; Licoccia, S.; Traversa, E. Does the Increase in Y-Dopant Concentration Improve the Proton Conductivity of BaZr1−x Yx O3−δ Fuel Cell Electrolytes? Solid State Ionics 2010, 181, 1043–1051. 4 Yamazaki, Y.; Blanc, F.; Okuyama, Y.; Buannic, L.; LucioVega, J. C.; Grey, C. P.; Haile, S. M. Proton Trapping in Yttrium-Doped Barium Zirconate. Nat. Mater. 2013, 12, 647–51. 5 Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides Acta Cryst., 1976, A32, 751–767. 6 Haro-Gonzáles, P.; Karlsson, M.; Gaita, S. M.; Knee, C. S.; Bettinelli, M. Eu3+ as a Luminescent Probe for the Local Structure of Trivalent Dopant Ions in Barium Zirconate-Based Proton Conductors Solid State Ionics, 2013, 247-248, 94–97. 7 Karlsson, M.; Björketun, M. E.; Sundell, P.; Matic, A.; Wahnström, G.; Engberg, D.; Börjesson, L.; Ahmed, I.; Eriksson, S.-G.; Berastegui, P. Vibrational Properties of Protons in Hydrated BaInx Zr1−x O3−x/2 . Phys. Rev. B 2005, 72, 094303. 8 Karlsson, M.; Matic, A.; Knee, C. S.; Ahmed, I.; Eriksson, S. G.; Börjesson, L. Short-Range Structure of ProtonConducting Perovskite BaInx Zr1−x O3−x/2 (x = 0–0.75). Chem. Mater. 2008, 20, 3480–3486. 9 D’Epifanio, A.; Fabbri, E.; Bartolomeo, E. D.; Licoccia, S.; Traversa, E. Design of BaZr0.8 Y0.2 O3−δ Protonic Conductor to Improve the Electrochemical Performance in Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs) Fuel Cells, 2008, 8, 69–76. 10 Rahman, S. M. H.; Norberg, S. T.; Knee, C. S.; Biendicho, J. J.; Hull, S.; Eriksson, S. G. Proton Conductivity of Hexagonal and Cubic BaTi1−x Scx O3−δ (0.1 6 x 6 0.8) Dalton Trans., 2014, 43, 15055–15064. 11 Laugier, J.; Bochu, B. CELREF C3, Developed at the Laboratoire des Materiaux et du Genie Phyique, Ecole Nationale Superieure de Physique de Grenoble, 2003. 12 Eriksson, A. K. E.; Selbach, S. M.; Knee, C. S.; Grande, T. Chemical Expansion Due to Hydration of ProtonConducting Perovskite Oxide Ceramics J. Amer. Cer. Soc., 2014, 97, 2654–2661. 13 Yamasaki, Y.; Babilo, P.; Haile, S. M. Defect Chemistry of Yttrium-Doped Barium Zirconate: A Thermodynamic Analysis of Water Uptake Chem. Mater., 2008, 20, 6352– 6357. 14 Ahmed, I.; Eriksson, S. G.; Ahlberg, E.; Knee, C. S.; Karlsson, M.; Matic, A,; Engberg, D.; Börjesson, L. Proton Conductivity and Low Temperature Structure of Indoped BaZrO3 Solid State Ionics, 2006, 177, 2357-2362. 15 Ahmed, I.; Eriksson, S. G.; Ahlberg, E.; Knee, C S.; Berastegui, P.; Johansson, L. G.; Rundlöf, H.; Karlsson, M.;

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Matic, A.; Börjesson, L.; et al. Synthesis and Structural Characterization of Perovskite Type Proton Conducting BaZr1−x Inx O3−δ (0.0 6x 60.75) Solid State Ionics, 2006, 177, 1395–1403.

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