Atomistic Insight into the Correlation among Oxygen Vacancies

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Atomistic Insight into the Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopants in Sc-Doped BaZrO using First-Principles Calculations 3

Hiroki Takahashi, Itaru Oikawa, and Hitoshi Takamura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11742 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

1

Atomistic Insight into the Correlation among Oxygen Vacancies, Protonic

2

Defects, and the Acceptor Dopants in Sc-Doped BaZrO3 using First-Principles

3

Calculations

4

Hiroki Takahashi,1,2,* Itaru Oikawa,1 Hitoshi Takamura,1,*

5

1

6

6-6-02 Aramaki Aoba, Sendai 980-8579, Japan

7

2

8

*Corresponding Authors

9

E-mail: [email protected]

10

Department of Materials Science, Graduate School of Engineering, Tohoku University,

Mitsui Mining & Smelting Co., Ltd., 1333-2 Haraichi, Ageo, Saitama 362-0021, Japan

E-mail: [email protected]

11 12

Abstract

13

It is necessary to elucidate the correlation between hydration properties and proton

14

distributions in electrolytes as proton conductors to allow for further improvements in

15

solid oxide fuel cells (SOFCs). In this study, the hydration properties of Sc-doped BaZrO3

16

(BZO) were investigated by means of density functional theory calculations capable of

17

taking both the local structural configurations and the hydration levels into account. At a

18

low hydration level, Sc-doped BZOs gained a negatively larger hydration energy, i.e. more

19

exothermic reaction, by incorporating an H2O molecule with unstable oxygen vacancies

20

adjacent to Zr. At a high hydration level, the configuration of ScO4(OH)2, which has a

21

positive net charge as a local structure, was formed with a smaller but negative hydration

22

energy by the reaction of H2O with oxygen vacancies adjacent to Sc. This indicates that the

23

stability of the whole system, and not only the local electrostatic interactions of point

24

defects, needs to be taken into account when considering the hydration energy. The

25

characteristic local structure of ScO4(OH)2 was identified using 1 ACS Paragon Plus Environment

45

Sc nuclear magnetic

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

1

resonance (NMR) chemical shift calculations. It is proposed that the resolution of current

2

45

3

Sc-doped BZOs, and that a higher resolution

4

existence of ScO4(OH)2.

Sc NMR spectroscopy techniques does not allow for the detection of ScO4(OH)2 in 45

Sc NMR technique will likely reveal the

5 6

1. Introduction

7

Proton conductors for intermediate temperature solid oxide fuel cell (IT-SOFC) have

8

been attracting much attention as potential electrolytes. An acceptor-doped

9

perovskite-type oxide, ABO3, is one of the candidate materials for proton conductors.1–3

10

Among a number of perovskite-type oxides, BaZrO3 (BZO) exhibits high proton

11

conductivity and is chemically stable in water vapor and CO2.4–6 For BZO, the substitution

12

of the tetravalent Zr4+ cation by a trivalent cation leads to the formation of oxygen

13

vacancies. The subsequent creation of protons proceeds by the reaction of oxygen

14

vacancies with the H2O molecule according to the following equation using the

15

Kröger-Vink notation:

16

•• • Hଶ Oሺgሻ + O× ୓ + V୓ ↔ 2OH୓

(1)

17

In this reaction, the H2O molecule reacts with an oxygen vacancy and a host oxide ion,

18

resulting in the generation of two OH groups, generally referred to as protonic defects.

19

These protonic defects migrate from one oxide ion to the adjacent oxide ion in a process

20

known as protonic conduction. Proton conductivity depends on both dopants and host

21

cations. There have been various studies reported focused on the relationship between

22

the local structural property of dopants and protonic defects, including proton trapping.7–9

23

In these reports, nuclear magnetic resonance (NMR) is used as one of the most powerful

24

techniques for determining the local structures around specific ions.8–17 Meanwhile,

25

computational simulations represented by density functional theory (DFT) calculations are 2 ACS Paragon Plus Environment

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

also effective in the investigation the state of protonic defects under ideal conditions. In

2

ref.s 9 and 11, the proton configurations of Y-doped BZO and Y-doped BaSnO3 have been

3

successfully studied by combining NMR measurements with DFT calculations.

4

Oikawa et al. revealed the distribution of oxygen vacancies and protons in Sc-doped 45

Sc NMR.17 Their key finding was that, since

5

BZO for various hydration levels using

6

protonic defects and oxygen vacancies coexist under partially hydrated situations, protonic

7

defects tend to avoid the occupation around the Sc dopant adjacent to the oxygen vacancy

8

due to electrostatic repulsion. It is, however, as yet unclear whether such electrostatic

9

interactions exist in partially hydrated BZOs. In this study, the combination of DFT

10

calculations with the experimental results provides deeper insight into the proton

11

configuration in BZOs, in effect opening a pathway for the design of a better protonic

12

conductor not limited to BZOs. Though many studies on the DFT calculations of hydration

13

in BZOs have been reported,18–33 few studies have taken both the hydration level and

14

configurations of local structures into account because of use of restricted small cell size.

15

In this paper, we focus on the hydration properties of Sc-doped BZO in the change of the

16

hydration level, and investigate the stability of various defect configurations using a large

17

supercell in DFT calculations. Moreover, we consider appropriate defect structures

18

consistent with the previous experimental results of

19

projector augmented wave (GIPAW) methods.

45

Sc NMR by the gauge-including

20 21

2. Methods

22

2.1

23 24 25

DFT Calculations According to the above equation eq 1, the hydration energy (Ehydr) of Sc-doped BZO

can be evaluated by the following equation: ′

•

′

×

••

Ehydr = Et(2ScZr, 2OHO) − Et(2ScZr, OO, V O ) − Et(H2O) 3 ACS Paragon Plus Environment

(2)


Page 4 of 26

1

where Et denotes the total energy, and the defects and the species in parentheses are

2

taken into account. These values were calculated using the CASTEP code, which is based

3

on the density functional theory (DFT).34 DFT calculations were performed with

4

plane-wave

5

Perdew-Burke-Emzerhof form (GGA-PBE). The ionic cores were represented by on-the-fly

6

generated (OTFG) ultrasoft pseudopotentials.35 The plane-wave cutoff energy was set at

7

630 eV, and a 4×4×4 supercell of a BaZrO3 unit cell was used with a single k-point sampling

8

at Γ-point. Geometry optimizations were performed by relaxing all atom positions and

9

lattice volumes under the following conditions: the total energy convergence for the

10

geometry optimization was 2×10-5 eV/atom and the stress was smaller than 0.1 GPa with

11

an energy convergence of 5×10-7 eV/atom for self-consistent calculations.

12

shielding was calculated using the GIPAW method.36,37 A k-point spacing of 0.03 Å-1 was

13

used, which corresponded to 2×2×2 k-point sampling. To allow for a direct comparison

14

with experimental data, isotropic NMR shielding (σiso) was converted to the isotropic

15

chemical shift (δiso) according to the following equation:38–40

expansions

using

the

generalized

δiso = σref − σiso

16

gradient

approximation

in

45

the

Sc NMR

(3)

17

As can be seen from eq 3, the correlation between experimental NMR chemical shifts and

18

calculated NMR shieldings provides minus unity of the slope. We used a value of σref = 783

19

ppm as the reference of

20

shielding of Ba3Sc4O9, ScPO4 and NaScO2.41,42 Details are provided in Figure S1 in the

21

Supporting Information.

45

Sc NMR shielding which was decided using the isotropic

45

Sc

22 23

2. 2

Modeling of Structures and Procedure of Calculations

24

The initial structure before hydration of Sc-doped BZO was constructed by referring to

25

the local structure experimentally observed by 45Sc NMR in ref 17. The concentration of Sc 4 ACS Paragon Plus Environment

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


′

••

1

was set at 12.5 mol%; that is, the simulated model was expressed as (8ScZr, 4V O ), with four

2

oxygen vacancies introduced to maintain overall electroneutrality. Figures 1 a-c show the

3

schematic representation of the local structures of the initial state containing four oxygen

4

vacancies before hydration. Two Zr−VO−Zr pairs (in Figures 1 a and b), one Sc−VO−Zr pair (in

5

Figure 1 a) and one Sc−VO−Sc pair (in Figure 1 c) were considered. In addition to the

6

5-coordinated Sc (ScO5), 6-coordinated Sc (ScO6) (not shown in Figure 1) was also arranged

7

in the calculation model to have a ratio of 1:1.67, which is close to the experimental value

8

(1:1.7) reported in ref 17. Note that even though two Sc−VO−Zr pairs can be adopted

9

instead of one Sc−VO−Sc pair, we adopted the Sc−VO−Sc pair, which is the associated

10

complex defect and is more energetically favored than the isolated configuration (two

11

Sc−VO−Zr pairs).43 These configurations were allocated in the 4×4×4 BZO supercell so that

12

they were the second nearest neighbor (2NN) when close or were even further apart in

13

order to avoid artificial interactions among them due to the limited cell size.

14

Since the calculated model includes four oxygen vacancies, as mentioned above, the

15

hydration level can be increased by 25% with every reaction with an H2O molecule

16

according to eq 1. There appear to be several possible configurations during the hydration

17

reaction, which involves the production of two protons by incorporating an H2O molecule

18

into an oxygen vacancy site. In this study, one of the protons expressed as OH in eq 1 was

19

fixed at the filled oxygen vacancy site; the other proton searches for an appropriate site

20

only within 1NN from the oxygen vacancy site, as shown in Figures 1 d-j. By using the most

21

stable configuration for the 25% hydration level, another H2O molecule was then

22

incorporated into the other oxygen vacancy site to construct a candidate model for a

23

hydration level of 50%. The same procedures of the hydration process were repeated by

24

incorporating an H2O molecule step by step to reproduce the 75% and 100% hydration

25

levels. 5 ACS Paragon Plus Environment


1 2

3. Results and discussion

3

3.1

Stable Configurations of Protons in Sc-Doped BZO

4

The computationally derived lattice constant of a non-doped BZO unit cell was

5

calculated as a = 4.230 Å, while the experimentally obtained lattice constant was reported

6

as a = 4.194 Å.44 The calculated value is in good agreement with the experimental value

7

within 1% error, indicating that the quality of this calculation method is such that the

8

structure of BZO can be reproduced.

9

First of all, we deal with the model Figure 1 b as a trial case to reveal the effect of

10

proton configurations, and especially the bonding direction of two protons, on stability in

11

the hydration reaction. Figure 2 shows the schematic representation which expresses

12

three configurations (b) on plane, (c) parallel and (d) perpendicular at the 25% hydration

13

level based on the Figure 2 a model before hydration. Among the three configurations (b)

14

to (d), the configuration of (d) perpendicular model was found to be the most energetically

15

stable. The total energies of (b) on plane and (c) parallel are 0.1 eV and 0.2 eV larger than

16

that of (d) perpendicular type, respectively. According to this result, the perpendicular

17

bonding direction of two protons was used as a standard stable hydrated model in the

18

series of calculations.

19 20

3.2

Hydration Energy as a Function of Hydration Level

21

The hydration energies for the seven possible configurations at the 25% hydration

22

level (case 1 to 7) are shown in the 4th column in Table 1. Case 1 is the most stable

23

configuration. No significant difference was found between the hydration energies of cases

24

3 (-1.11 eV), 6 (-1.07 eV) and that of case 1 (-1.13 eV). These three cases 1, 3 and 6 have

25

similar defect configurations, with an oxygen vacancy of the Zr−VO−Zr pair hydrated by an 6 ACS Paragon Plus Environment

Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

H2O molecule to form OH, and then the remaining H locates the O adjacent to Sc. The

2

reason for the negatively larger hydration energy, i.e. stable configuration, of these three

3

cases is that VO in Zr−VO−Zr pair is less stable than the VO in Sc−VO−Zr and Sc−VO−Sc,

4

allowing energetically unstable oxygen vacancies to be readily hydrated.21,22,43,45,46 The

5

remaining positively charged H (OH O ) then favors the O site adjacent to the negatively

6

charged ScZr site due to the Coulombic attractive interaction. The opposite is true in cases

7

4 and 7. That is, the hydration reaction of the oxygen vacancy adjacent to Sc shows

8

moderate hydration energy because the oxygen vacancy adjacent to Sc is stable as it

9

is.21,22,43 Meanwhile, based on the negatively smaller hydration energy for case 2 (-0.79 eV),

10

it can be assumed that the incorporation of two protons on the O site adjacent to the

11

same Zr without the nearest-neighboring Sc is unstable even when the hydration reaction

12

takes place at the oxygen vacancy in Zr−VO−Zr. A similar tendency can be seen in cases 4

13

and 5.

•

′

14

According to ref. 43, protons are the most stable at 1NN of Sc. This means that case 1

15

at 25% hydration level with two protons located at 1NN (Sc−OH−Zr) and 2NN (Zr−OH−Zr)

16

of Sc is not the global minimum state. More stable proton configurations were then

17

explored based on case 1. In the cases of 8 and 9 shown in Figure S2, two protons diffuse

18

to 1NN of Sc and are located apart from each other; their hydration energies are

19

negatively larger than that of case 1, as shown in Table S1 (a). This implies that protons

20

essentially favor the 1NN of Sc: this is interpreted as proton trapping behavior.7–9

21

Meanwhile, we assume in this study that H2O instantaneously reacts with oxygen

22

vacancies successively without proton diffusion in the process of the hydration reaction.

23

This means that the systems under consideration are not energetically global minimum

24

states. To estimate the effect of neglecting proton diffusion on the hydration energy, the

25

hydration energy was calculated based on the isolated models in ref. 43: in these models 7 ACS Paragon Plus Environment


1

the dopants are located apart from each other and two protons exist at 1NN and 2NN of

2

the dopants. The results are shown in Table S1 (b). As can be seen, even though the

3

hydration energies of the model which takes proton diffusion into account (H diffusion) is

4

negatively larger, as expected, than those of the original models without proton diffusion

5

(After hydration), the order of hydration energy with respect to the kind of dopants does

6

not change (Al > Ga > Sc). This trend indicates that, even with the assumption that H2O

7

instantaneously reacts with oxygen vacancies successively without proton diffusion, it is

8

possible to discuss their hydration properties, including site preferences. Meanwhile, it

9

should also be noted that because of the assumption, the absolute values of hydration

10

reactions may be further increased by approximately 0.25 eV due to the effect of proton

11

diffusion searching for its global minimum sites. We then constructed candidate models

12

with a hydration level of 50% based on case 1.

13

The 50% hydrated level was prepared by incorporating one more H2O molecule for

14

case 1. The results are summarized in Table 1 (see the 5th column). In case 6, the 50%

15

hydrated level shows the largest negative hydration energy among the three cases, 4, 6,

16

and 7, in Table 1. The structural feature of case 6 for the 50% hydration level is that, like

17

case 1 at the 25% hydration level, an oxygen vacancy of Zr−VO−Zr pair is hydrated by an

18

H2O molecule to form OH and then the remaining H locates at the O site adjacent to Sc.

19

The 75% hydrated models were also prepared by incorporating one more H2O molecule

20

into case 6 for the 50% hydration level. In case 4 for the 75% hydration level by containing

21

the oxygen vacancy adjacent to Sc, the negative hydration energy becomes larger. At 100%

22

hydration, an H2O molecule finally reacts with an Sc−VO−Sc associated complex

23

configuration. In other words, the VO in the Sc−VO−Sc associated configuration is extremely

24

stable; this VO can be hydrated only at the full hydration level.43 Figure 3 includes a

25

summary of the hydration energy with respect to the hydration level. As the hydration 8 ACS Paragon Plus Environment

Page 8 of 26

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

reaction progresses, the hydration energy becomes negatively less, that is, unstable

2

oxygen vacancies around Zr readily react with the H2O molecule and gain larger negative

3

hydration energy. On the other hand, more stable oxygen vacancies remain intact at low

4

hydration levels and finally accept H2O molecules at the high hydration level, leading to

5

less negative hydration energy. It should be also noted that the full hydration has been

6

experimentally achieved in 10 mol% Sc-doped BZO even with negatively smaller hydration

7

energy at a high hydration level.17

8 9

3.3

′

••

•

Repulsive Interaction between (ScZr−V O ) and OH O ′

10

Oikawa et. al. concluded there was a Coulombic repulsive interaction between (ScZr

11

−V O ) and OH O adjacent to Sc for partially hydrated Sc-doped BZO.17 To elucidate the

12

Coulombic repulsive interaction between them, a comparison can be made between case

13

1 with Zr−OH−Sc−O and case 3 with Zr−OH−Sc−VO at the 25% hydration level from the data

14

in Table 1. Even though the case 3 with Sc−VO is unstable by 0.02 eV, the difference is

15

significantly smaller than expected. Meanwhile, the proton locating O adjacent to Sc in

16

case 3 shows a peculiar perpendicular bonding with respect to the O−Sc bonding direcƟon:

17

this repulsive interaction affects the geometry of proton site around O−Sc−VO.

••

•

18

Figure 4 b shows the relaxed structure around O−Sc−VO (VO(2)) for case 3 represented

19

in Figure 4 a. To clarify the stable geometry for the proton around O−Sc−VO, the change in

20

the total energy was calculated by moving the initial proton H(2) site in Figure 4 b within

21

the region of the cross sections near the oxygen vacancy shown in Figure 4 c. This is the

22

so-called potential energy surface (PES) of the proton.47 As can be seen from the PES in

23

Figure 4 d, the proton located on the plane of the ScO4 unit which faces the oxygen

24

vacancy is stable with a smaller relative total energy (Erel), in the range of 0 ~ 0.35 eV,

25

which corresponds to the area indicated from cyan to blue in the PES. On the other hand, 9 ACS Paragon Plus Environment


1

the proton on the plane including Sc−VO−Zr is less stable by Erel = 0.35 ~ 0.45 eV (the blue

2

area in Figure 4 e). The protons bonding with the O located on the opposite side of the

3

oxygen vacancy are the least stable (Erel = 0.75 ~ 0.90 eV, the red area in Figure 4 e). This is

4

because the atomic distance between Sc and the O located on the opposite side of the

5

oxygen vacancy was as short as 1.93 Å (the other Sc−O distances were 2.0 to 2.2 Å as

6

shown in Figure 4 b) due to the repulsive interaction of the facing cations (Sc and Zr) which

7

resulted from the absence of oxide ions at the VO(2) site. It was thus determined that a

8

repulsive interaction between (ScZr−V O ) and OH O does indeed exist and may affect the

9

anisotropy required for proton migration. Even though steric interactions between the two

10

hydrogen atoms may be also involved, given that Figure 4 e indicates that (1) the region of

11

the least stable area around H1 is very limited, and (2) stable areas facing the VO site are

12

basically symmetrical, we can conclude that it is the repulsive interaction which appears to

13

be the main contributor to the defect configurations.

′

••

•

14 15

3.4

Occupation Site of Protons as a Function of Hydration Level

16

According to the data reported in an experimental NMR study on Sc-doped BZO in ref

17

17, protons are distributed not only in the vicinity of Sc but also Zr. The local structure of

18

ZrO5(OH) with a positive net charge is likely to be unstable on the basis of the local

19

electroneutrality. Based on a series of calculated cases, the energetically favored protonic

20

sites are plotted as a function of their hydration levels. Figure 5 shows the ratio of protons

21

located on O adjacent to Sc (blue line) or Zr (red dashed line) as a function of the hydration

22

level. Protons exist not only at 1NN of Sc but also 1NN of Zr even when the hydration

23

levels are a low 25% or at 50%, which has at least 1NN of Sc still available. This is because

24

when the hydration reaction consumes the oxygen vacancy adjacent to Zr, the stability of

25

the whole system is improved, i.e. negatively large hydration energy of ≈ -1 eV (exothermic 10 ACS Paragon Plus Environment

Page 10 of 26

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

reaction), even though the local instability, which seems to be a minor issue, is the result

2

of the formation of the positive net charge of ZrO5(OH). At high hydration levels, a

3

hydration reaction takes place which utilizes the oxygen vacancies near Sc (case 4:75% and

4

7:100% in Table 1). In these cases, two protons originating from H2O exist at the oxygen

5

adjacent to the identical Sc atom. While the positive net charge of ScO4(OH)2 suggests this

6

configuration is less stable, even such a configuration with a positive net charge shows

7

sufficient hydration energy (-0.78 eV of case 4(75%), -0.71 eV of case 7(100%)). This

8

indicates that besides classical treatments based on the local net charge, quantum

9

treatment based on first-principles calculations which takes the covalency of chemical

10

bonding as well as the total energy of the whole system into account is required to

11

evaluate the stability of protons.48

12 13

3.5

The Relationship between Local Structures and the 45Sc NMR Spectra

14

The atomistic calculations for Sc-doped BZO provide useful information for NMR

15

studies on this material. The chemical shift in 45Sc NMR for various defect configurations

16

can be derived, as shown in Table 2. These chemical shifts in

17

structure around Sc. The 45Sc NMR chemical shifts of ScO6, ScO5(OH) and ScO5 in Sc-doped

18

BZO were calculated at 150 ppm, 158 ppm and 230 ppm, respectively. Figure 6 shows a

19

45

20

chemical shifts calculated in this study. As can be seen, these calculated chemical shifts are

21

in good agreement with the experimental data (dashed lines). This implies that our

22

simulated models are well-suited for the reproduction of experimental results. In the case

23

of ScO5(OH), the chemical shift ranges from 153 ppm to 164 ppm, which is within the

24

range of the experimental value. This also indicates that the

25

sensitive to a slight difference in proton position. As for ScO4(OH)2, though no such

45

Sc NMR reflect the local

Sc NMR spectrum taken for a partially hydrated 10 mol% Sc-doped BZO17 with the

11 ACS Paragon Plus Environment

45

Sc NMR chemical shift is


1

configuration has yet to be identified experimentally, the chemical shift is estimated to be

2

approximately 160 ppm (red line), which is within the range of the chemical shift of

3

ScO5(OH) mentioned above. While experimentally distinguishing these two local structures

4

when taking the broadness of

5

improved resolution of

6

configurations of ScO5(OH) and ScO4(OH)2 and the distribution of protons around Sc can be

7

interpreted with higher accuracy.

45

45

Sc NMR spectrum into account is clearly difficult, with

Sc NMR spectra, it will be possible to distinguish the two

8 9

4. Conclusion

10

We investigated the hydration properties of Sc-doped BZOs by taking both the

11

hydration level and the local structural configurations into account. At the low hydration

12

level, Sc-doped BZOs gain larger negative hydration energy by incorporating H2O with the

13

oxygen vacancies adjacent to Zr. Protons near ScZr−V O do not show any significant

14

repulsive interaction in terms of the hydration energy; rather, the interaction affects the

15

spatial anisotropic distribution of the protons. At the high hydration level, the protons

16

become finally incorporated with stable oxygen vacancies near Sc resulting in a relatively

17

small hydration energy. Even when the configuration includes two protons at identical Sc

18

(ScO4(OH)2), which will not be favored considering the positive net charge of the local

19

structure, negatively hydration energy is sufficiently high. This implies that the correlation

20

among defects (dopants, oxygen vacancies, and protons) needs to be considered by taking

21

the total energy of the whole system and not just the local electrostatic interactions of

22

those point defects into account. Furthermore, it was found that the NMR peak which has

23

been experimentally assigned to ScO5(OH) may overlap with ScO4(OH)2 at the high

24

hydration level.

′

••

25 12 ACS Paragon Plus Environment

Page 12 of 26

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


Supporting Information

2

Relationship between the calculated isotropic 45Sc NMR shielding and the experimental

3

chemical shift of Ba3Sc4O9, ScPO4 and NaScO2 as reference materials (Figure S1), schematic

4

representation of the proton configuration at 25% hydration level after the diffusion of

5

two protons towards the most stable site at the first nearest neighbor of Sc (Figure S2),

6

hydration energy of (a) cases 1, 8 and 9 at the 25% hydration level, and (b) isolated models

7

of M-doped BaZrO3 (M=Al, Sc, Ga) in ref. 43, and H diffusion models calculated in this

8

study (Table S1), and convergence test of cutoff energy for calculations of PES map (Figure

9

S3).

10 11

Acknowledgement

12

This work has been financially supported in part by JSPS KAKENHI Grant Number

13

26249103.

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

References (1) (2) (3) (4) (5) (6) (7) (8) (9)

Kreuer, K.-D. Proton Conductivity: Materials and Applications. Chem. Mater. 1996, 8 (3), 610–641. Norby, T. Solid-State Protonic Conductors: Principles, Properties, Progress and Prospects. Solid State Ionics 1999, 125 (1–4), 1–11. Iwahara, H.; Yajima, T.; Hibino, T.; Ozaki, K.; Suzuki, H. Protonic Conduction in Calcium, Strontium and Barium Zirconates. Solid State Ionics 1993, 61 (1–3), 65–69. Imashuku, S.; Uda, T.; Nose, Y.; Taniguchi, G.; Ito, Y.; Awakura, Y. Dependence of Dopant Cations on Microstructure and Proton Conductivity of Barium Zirconate. J. Electrochem. Soc. 2009, 156 (1), B1. Kreuer, K. D. Aspects of the Formation and Mobility of Protonic Charge Carriers and the Stability of Perovskite-Type Oxides. Solid State Ionics 1999, 125 (1), 285–302. Ryu, K. H.; Haile, S. M. Chemical Stability and Proton Conductivity of Doped BaCeO3-BaZrO3 Solid Solutions. Solid State Ionics 1999, 125 (1), 355–367. Björketun, M. E.; Sundell, P. G.; Wahnström, G.; Engberg, D. A Kinetic Monte Carlo Study of Proton Diffusion in Disordered Perovskite Structured Lattices Based on First-Principles Calculations. Solid State Ionics 2005, 176 (39–40), 3035–3040. Yamazaki, Y.; Blanc, F.; Okuyama, Y.; Buannic, L.; Lucio-Vega, J. C.; Grey, C. P.; Haile, S. M. Proton Trapping in Yttrium-Doped Barium Zirconate. Nat. Mater. 2013, 12 (7), 647–651. Blanc, F.; Sperrin, L.; Lee, D.; Dervişoğlu, R.; Yamazaki, Y.; Haile, S. M.; De Paëpe, G.; 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

(10) (11)

(12) (13) (14) (15) (16)

(17) (18) (19) (20) (21) (22) (23) (24) (25) (26)

Grey, C. P. Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3. J. Phys. Chem. Lett. 2014, 5 (14), 2431–2436. Buannic, L.; Blanc, F.; Hung, I.; Gan, Z.; Grey, C. P. Probing the Local Structures and Protonic Conduction Pathways in Scandium Substituted BaZrO3 by Multinuclear Solid-State NMR Spectroscopy. J. Mater. Chem. 2010, 20 (30), 6322. Buannic, L.; Blanc, F.; Middlemiss, D. S.; Grey, C. P. Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO3 Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility. J. Am. Chem. Soc. 2012, 134 (35), 14483–14498. Oikawa, I.; Ando, M.; Kiyono, H.; Tansho, M.; Shimizu, T.; Maekawa, H. On the Symmetry of Defects in Perovskite-Type Protonic conductors:A Sc-45 NMR Study. Solid State Ionics 2012, 213, 14–17. Kreuer, K. D.; Dippel, T.; Baikov, Y. M.; Maier, J. Water Solubility, Proton and Oxygen Diffusion in Acceptor Doped BaCeO3: A Single Crystal Analysis. Solid State Ionics 1996, 86–88 (PART 1), 613–620. Maekawa, H.; Kashii, N.; Kawamura, J.; Hinatsu, Y.; Yamamura, T. High Temperature 1 H NMR Study of Proton Conducting Oxide SrCe0.95Y0.05H0.004O3–δ. Solid State Ionics 1999, 122 (1–4), 231–236. Maekawa, H.; Ukei, Y.; Morota, K.; Kashii, N.; Kawamura, J.; Yamamura, T. High Temperature Proton NMR Study of Yttrium Doped Barium Cerates. Solid State Commun. 2004, 130 (1–2), 73–77. Cervera, R. B.; Oyama, Y.; Miyoshi, S.; Oikawa, I.; Takamura, H.; Yamaguchi, S. Nanograined Sc-Doped BaZrO3 as a Proton Conducting Solid Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Solid State Ionics 2014, 264, 1–6. Oikawa, I.; Takamura, H. Correlation among Oxygen Vacancies, Protonic Defects, and the Acceptor Dopant in Sc-Doped BaZrO3 Studied by 45Sc Nuclear Magnetic Resonance. Chem. Mater. 2015, 27 (19), 6660–6667. Dawson, J. A.; Miller, J. A.; Tanaka, I. First-Principles Insight into the Hydration Ability and Proton Conduction of the Solid State Proton Conductor, Y and Sn Co-Doped BaZrO3. Chem. Mater. 2015, 27 (3), 901–908. Dawson, J. A.; Tanaka, I. Proton Trapping in Y and Sn Co-Doped BaZrO3. J. Mater. Chem. A 2015, 3 (18), 10045–10051. Stokes, S. J.; Islam, M. S. Defect Chemistry and Proton-Dopant Association in BaZrO3 and BaPrO3. J. Mater. Chem. 2010, 20 (30), 6258. Sundell, P. G.; Björketun, M. E.; Wahnström, G. Thermodynamics of Doping and Vacancy Formation in BaZrO3 Perovskite Oxide from Density Functional Calculations. Phys. Rev. B - Condens. Matter Mater. Phys. 2006, 73 (10), 104112. Björketun, M. E.; Sundell, P. G.; Wahnström, G. Structure and Thermodynamic Stability of Hydrogen Interstitials in BaZrO3 Perovskite Oxide from Density Functional Calculations. Faraday Discuss. 2007, 134, 247–265. Bjørheim, T. S.; Kotomin, E. A.; Maier, J. Hydration Entropy of BaZrO3 from First Principles Phonon Calculations. J. Mater. Chem. A 2015, 3 (14), 7639–7648. Polfus, J. M.; Bjørheim, T. S.; Norby, T.; Bredesen, R. Surface Defect Chemistry of Y-Substituted and Hydrated BaZrO3 with Subsurface Space-Charge Regions. J. Mater. Chem. A 2016, 4 (19), 7437–7444. Bévillon, E.; Dezanneau, G.; Geneste, G. Oxygen Incorporation in Acceptor-Doped Perovskites. Phys. Rev. B 2011, 83 (17), 174101. Gomez, M. A.; Fry, D. L.; Sweet, M. E. Effects on the Proton Conduction Limiting Barriers and Trajectories in BaZr0.875Y0.125O3 Due to the Presence of Other Protons. J. 14 ACS Paragon Plus Environment

Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50


(27) (28) (29) (30) (31) (32) (33) (34) (35)

(36) (37) (38) (39) (40) (41) (42) (43) (44)

Korean Ceram. Soc. 2016, 53 (5), 521–528. 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 (11), 4370. Kang, S. G.; Sholl, D. S. First Principles Assessment of Perovskite Dopants for Proton Conductors with Chemical Stability and High Conductivity. RSC Adv. 2013, 3, 3333. Bjørheim, T. S.; Kuwabara, A.; Ahmed, I.; Haugsrud, R.; Stølen, S.; Norby, T. A Combined Conductivity and DFT Study of Protons in PbZrO3 and Alkaline Earth Zirconate Perovskites. Solid State Ionics 2010, 181 (3–4), 130–137. Zhang, Q.; Wahnström, G.; Björketun, M. E.; Gao, S.; Wang, E. Path Integral Treatment of Proton Transport Processes in BaZrO3. Phys. Rev. Lett. 2008, 101 (21), 215902. Karlsson, M.; Björketun, M. E.; Sundell, P. G.; Matic, A.; Wahnström, G.; Engberg, D.; Börjesson, L.; Ahmed, I.; Eriksson, S.; Berastegui, P. Vibrational Properties of Protons in Hydrated BaInxZr1−xO3−x/2. Phys. Rev. B 2005, 72 (9), 94303. Björketun, M. E.; Sundell, P. G.; Wahnström, G. Effect of Acceptor Dopants on the Proton Mobility in BaZrO3 : A Density Functional Investig. Phys. Rev. B 2007, 76 (5), 54307. Sundell, P. G.; Björketun, M. E.; Wahnström, G. Density-Functional Calculations of Prefactors and Activation Energies for H Diffusion in BaZrO3. Phys. Rev. B - Condens. Matter Mater. Phys. 2007, 76 (9), 94301. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Zeitschrift für Krist. - Cryst. Mater. 2005, 220 (5/6), 567–570. Lejaeghere, K.; Van Speybroeck, V.; Van Oost, G.; Cottenier, S. Error Estimates for Solid-State Density-Functional Theory Predictions: An Overview by Means of the Ground-State Elemental Crystals. Crit. Rev. Solid State Mater. Sci. 2014, 39 (1), 1– 24. Pickard, C. J.; Mauri, F. All-Electron Magnetic Response with Pseudopotentials: NMR Chemical Shifts. Phys. Rev. B 2001, 63 (24), 245101. Yates, J. R.; Pickard, C. J.; Mauri, F. Calculation of NMR Chemical Shifts for Extended Systems Using Ultrasoft Pseudopotentials. Phys. Rev. B 2007, 76 (2), 24401. Yates, J. R.; Dobbins, S. E.; Pickard, C. J.; Mauri, F.; Ghi, P. Y.; Harris, R. K. A Combined First Principles Computational and Solid-State NMR Study of a Molecular Crystal: Flurbiprofen. Phys. Chem. Chem. Phys. 2005, 7 (7), 1402. Harris, R. K.; Cadars, S.; Emsley, L.; Yates, J. R.; Pickard, C. J.; Jetti, R. K. R.; Griesser, U. J. NMRcrystallography of Oxybuprocaine Hydrochloride, Modification II°. Phys. Chem. Chem. Phys. 2007, 9 (3), 360–368. Dumez, J.-N.; Pickard, C. J. Calculation of NMR Chemical Shifts in Organic Solids: Accounting for Motional Effects. J. Chem. Phys. 2009, 130 (10), 104701. Kim, N.; Hsieh, C. H.; Stebbins, J. F. Scandium Coordination in Solid Oxides and Stabilized Zirconia: 45Sc NMR. Chem. Mater. 2006, 18 (16), 3855–3859. Oikawa, I.; Takamura, H. 45Sc NMR Spectroscopy and First-Principles Calculation on the Symmetry of ScO6 Polyhedra in BaO–Sc2O3-Based Oxides. Dalt. Trans. 2014, 43 (25), 9714. Takahashi, H.; Yashima, I.; Amezawa, K.; Eguchi, K.; Matsumoto, H.; Takamura, H.; Yamaguchi, S. First-Principles Calculations for the Energetics of the Hydration Reaction of Acceptor-Doped BaZrO3. Chem. Mater. 2017, 29 (4), 1518–1526. Levin, I.; Amos, T. G.; Bell, S. M.; Farber, L.; Vanderah, T. A.; Roth, R. S.; Toby, B. H. Phase Equilibria, Crystal Structures, and Dielectric Anomaly in the BaZrO3-CaZrO3 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14

(45) (46) (47) (48) (49)

System. J. Solid State Chem. 2003, 175 (2), 170–181. Bévillon, É.; Geneste, G. Hydration Properties of BaSn 0.875M0.125O3−δ Substituted by Large Dopants (M=In, Y, Gd, and Sm) from First Principles. Phys. Rev. B 2008, 77 (18), 184113. Bévillon, É.; Geneste, G.; Chesnaud, A.; Wang, Y.; Dezanneau, G. Ab Initio Study of La-Doped BaSnO3 Proton Conductor. Ionics (Kiel). 2008, 14 (4), 293–301. Toyoura, K.; Hatada, N.; Nose, Y.; Tanaka, I.; Matsunaga, K.; Uda, T. Proton-Conducting Network in Lanthanum Orthophosphate. J. Phys. Chem. C 2012, 116 (36), 19117–19124. Liu, Q. J.; Liu, Z. T.; Feng, L. P. Elasticity, Electronic Structure, Chemical Bonding and Optical Properties of Monoclinic ZrO2 from First-Principles. Phys. B Condens. Matter 2011, 406 (3), 345–350. Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44 (6), 1272–1276.

15 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2 3 4

Figure 1 (a)-(c) Schematic representation of the local structures of the initial state

5

containing four oxygen vacancies before hydration. (d)-(j) Representation of hydrated

6

cases by incorporating an H2O molecule with four oxygen vacancies.



1 2 3

Figure 2 Schematic representation of three configurations of two protons, (b) on plane and

4

face to face, (c) parallel and (d) perpendicular at 25% hydration level. H bonding direction

5

is expressed by the Natta projection way. Depicted triangular in stripe pattern means

6

bonding direction retreating into the paper. Model (a) corresponds to Figure 1 b.


Page 18 of 26

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


Table 1 Hydration energy of each reaction cases at 25% to 100% hydration level (unit: eV) Case

OH site(VO)

H site(OO)

Ehydr at 25%

Ehydr at 50%

1

Zr−VO(1)−Zr

Sc−OO−Zr

-1.13

−

−

−

2

Zr−VO(1)−Zr

Zr−OO−Zr

-0.79

−

−

−

3

Zr−VO(1)−Zr

Zr−OO−Sc−VO

-1.11

−

−

−

4

Sc−VO(2)−Zr

Sc−OO−Zr

-0.80

5

Sc−VO(2)−Zr

Zr−OO−Zr

-0.51

6

Zr−VO(3)−Zr

Sc−OO−Zr

-1.07

-0.96

7

Sc−VO(4)−Sc

Sc−OO−Zr

-0.82

-0.79

-0.80 −

2 3


Ehydr at 75%

-0.78

Ehydr at 100%

−

−

−

−

−

-0.73

-0.71


1

2 3

Figure 3 Summary of the hydration energy with respect to the hydration level.

4 5


Page 20 of 26

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 4 (a) Schematic representation of the local structures of 25% hydrated case 3 in

3

Figure 1. The area surrounded by the green dashed line corresponds to (b). (b) Relaxed

4

structure and Sc−O atomic distance around the oxygen vacancy VO(2). (c) Schematic

5

illustration of the cross sections calculated the potential energy surface (PES) of the proton

6

H(2). (d) and (e) PES cross sections corresponding to the orange and blue regions shown in

7

(c), respectively. Cross section (d) includes the ScO4 unit and lies in the yz-plane, and (e)

8

includes Sc−VO−Zr on the xy-plane. Under the condition that the proton H(1) is fixed at the

9

initial stable site, the proton H(2) moves within the region of the cross sections with 0.1 Å

10

grid spacing for the calculation of PES. The cutoff energy was decreased to 380 eV to

11

reduce calculation time. A convergence test of cutoff energy is shown in Figure S3. Ba

12

atoms are omitted for clarity. Structures and PES cross sections are visualized using

13

VESTA.49 21 ACS Paragon Plus Environment


1


Page 22 of 26

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2 3

Figure 5 Distribution of protons distinguished for each the nearest cations with respect to

4

the hydration level.

5



45

Page 24 of 26

1

Table 2

Sc NMR chemical shift classified by the local structure around Sc (unit: ppm).

2

When there were plural identical structures, the chemical shifts were averaged. Local structure Hydration ScO5

ScO6

ScO5(OH)

0 (initial)

232.9

150.2

−

−

25 (case 1)

230.3

151.7

164.2

−

50 (case 6)

228.8

151.5

157.8

−

75 (case 4)

231.0

152.1

152.9

159.5

100 (case 7)

−

152.6

155.6

163.7

ScO4(OH)2

level (%)

3 4


Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2 45

3

Figure 6 Experimental (10 mol% Sc-doped BZO) and calculated

4 5

with various defect configurations. The 45Sc NMR spectrum is taken from ref. 17.


Sc NMR chemical shifts


1

Table of Contents Image

2


Page 26 of 26

Atomistic Insight into the Correlation among Oxygen Vacancies

Recommend Documents