Mechanistic Insights into Hydration of Solid Oxides

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Cite This: Chem. Mater. 2018, 30, 138−144

Mechanistic Insights into Hydration of Solid Oxides Yuhang Jing,†,‡,§ Hiroshige Matsumoto,∥ and Narayana R. Aluru*,§ †

Department of Astronautical Science and Mechanics, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P.R. China National Center for Supercomputing Applications (NCSA) and §Department of Mechanical Science and Engineering, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ∥ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

S Supporting Information *

ABSTRACT: Some of the solid oxide materials, used in solid oxide fuel and electrolysis cells, are known to conduct protons once they are hydrated. However, the mechanisms by which solid oxide materials get hydrated is not clear. By performing detailed density functional theory calculations, we investigate hydration of two typical solid oxides with a single-crystal structurea proton-conducting yttrium-doped strontium zirconate (SZY) and an oxide ion-conducting yttria-stabilized zirconia (YSZ). We suggest a four-step process to understand the hydration of solid oxideswater adsorption on the surface, proton migration from the surface to bulk, proton migration in the bulk, and oxide ion vacancy migration in the bulk. The hydroxide ion migration with a lower energy barrier, compared to the proton hopping mechanism, is proposed for the conduction of proton between the surface and subsurface of the perovskite oxide. Our analysis provides mechanistic insights into the hydration of single-crystal SZY and nonhydration of single-crystal YSZ. The study presented here not only explains the hydration of materials but also provides the importance of structural rearrangement when a proton is incorporated into the bulk of the solid oxide material.

1. INTRODUCTION Hydrogen production has attracted great interest as it is a clean and sustainable fuel and is a promising choice for the storage of intermittent renewable energies (e.g., solar and wind power).1 Water electrolysis using renewable energy is considered to be one of the cleanest methods to produce hydrogen2 and electrolysis cells are widely used for this purpose. Compared to a commercially available polymer electrolyte membrane (PEM)based low-temperature electrolysis cell, a solid oxide electrolysis cell (SOEC) operating at a high temperature can take advantage of thermal energy to reduce electrical energy demand, resulting in a reduced cost and enhanced efficiency for hydrogen production.3 However, the design and development of solid oxide electrolytes with sufficient stability and enhanced conductivity for SOEC is still a challenge. Oxide ion conductors are commonly used as electrolytes in SOECs. For example, ZrO2 doped with Y2O3 (YSZ) exhibits sufficient oxide ion conductivity as well as thermal and chemical stability at high temperature.2 Certain oxide ion-conducting acceptor-doped perovskites (of the type ABO3, where A = Sr, Ba; B = Zr, Ce) have been experimentally shown to be proton conductors in a wet environment.4 For instance, Y-doped SrZrO3 (SZY) can function as a proton conductor in the presence of humidity. As for YSZ, proton diffusion through grain boundaries in a nanocrystalline YSZ has been observed,5−9 while other findings state that the proton transport is along the internal surfaces provided by microcracks and pores and not through their grain boundaries.10−12 In addition to this ongoing debate © 2017 American Chemical Society

on whether protons migrate using grain boundaries, to our knowledge, there is no significant literature on proton transport in a single-crystalline YSZ.13 Comparing SOECs with oxide ionconducting electrolytes, SOECs with proton-conducting electrolytes can produce pure and dry hydrogen. To design highconductivity proton conductors, a fundamental understanding of what determines whether an oxide ion conductor can become a proton conductor is needed, which is currently missing. The origin of proton conduction, attributed to the hydration process, is of significant importance for the development of proton conductors. The hydration reaction can be written in × • Kröger-Vink notation14 as H2O (g) + V•• O + OO ⇄ 2OHO, where × • V•• , O , and OH denote the oxygen vacancy, lattice oxygen, O O O and protonic defect in the hydrated structure, respectively. The schematic illustration of the hydration reaction is shown in Figure 1a. The hydroxide ion from the water molecule fills the oxygen vacancy, and the remaining proton bonds to the neighboring lattice oxygen, resulting in the formation of two hydroxide ions. The corresponding equilibrium constant is ΔS ° R [OH•O],

e x p r e s se d a s K = exp

−ΔH ° RT [O×O], [V•• O ],

( )exp( ) =

[OH•O]2 × [OO][V •• O ]pH2O

,15

where ΔS°, ΔH°, R, T, and pH2O are the hydration entropy, enthalpy, ideal gas constant, temperature, hydroxide ion concentration (equal to proton concentration in Received: August 16, 2017 Revised: December 4, 2017 Published: December 5, 2017 138

DOI: 10.1021/acs.chemmater.7b03476 Chem. Mater. 2018, 30, 138−144

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Chemistry of Materials

population analysis is performed to calculate the atomic charges.34 The migration energy barriers are calculated using the climbing image nudged elastic band (CI-NEB) method.35 The orthorhombic bulk SrZrO3 and cubic bulk ZrO2 unit cells are optimized using a 6 × 6 × 4 and 6 × 6 × 6 Monkhorst−Pack k-points mesh, respectively. The computed lattice parameters are consistent with previous experimental results36,37 and DFT calculations,38−40 as shown in Table 1. On the basis of the optimized unit cell, SrZrO3 supercell with 2 × 2 × 2 orthorhombic unit cells and ZrO2 supercell with 3 × 3 × 2 unit cells containing (111) plane are constructed and optimized. Next, one oxide ion vacancy is introduced in the supercells with two Zr ions substituted with two Y ions. The preferred positions of these two Y ions and the oxide ion vacancy are obtained through comparing the total energies of the system while locating them at different atomic sites. Finally, bulk SZY and YSZ are obtained and the (001) SZY surfaces and (111) YSZ surfaces are cleaved from the optimized supercells. For the (001) SZY surfaces, SrO-terminated surfaces are used for both sides since this surface is more stable than ZrO2terminated surface of SrZrO3.41 The slabs contain nine layers. Two different surfaces are considered. In one structure, the vacancy is located in the bulk, and the vacancy is still in the bulk after optimization, which is shown in Figure S1a of the Supporting Information. In the other structure, the vacancy is initially located at the subsurface, and the vacancy ends up on the surface after optimization, which is shown in Figure 1b. For the YSZ surface structure, the (111) YSZ surface has been found to be the most stable surface from both experiment42 and computation43 and has been widely investigated in previous YSZ adsorption calculations.44−46 Here the (111) YSZ surface structure is also used in our study with the slabs containing six trilayers, and two surface structures are constructed. In one surface, the dopants are far away from the surface and the oxide ion vacancy is located on the surface, whose final optimized structure is shown in Figure S1b of the Supporting Information. In the other structure, the dopants are near the surface and the oxide ion vacancy is located in the third layer from the surface, and the vacancy is still in the third layer from the surface after optimization, which is shown in Figure 1c. All the surfaces are in the XY-plane and perpendicular to the Z axis. The Z coordinate of ions in the bottom three layers is fixed. A vacuum slab larger than 15 Å is used to minimize the interactions between the surface structure and its images. The 3 × 3 × 1 (or 2 × 2 × 1) and 3 × 3 × 2 (or 2 × 2 × 2) k-points mesh is used for the surface and bulk calculations of SZY (or YSZ), respectively.

Figure 1. Schematic illustration of (a) hydration reaction, (b) SZY surface, and (c) YSZ surface. Sr, Zr, Y, O, and H ions are green, light blue, dark blue, red, and white, respectively, and black hollow circles represent oxygen vacancies.

this system), lattice oxygen concentration, oxygen vacancy concentration, and water partial pressure, respectively. Significant effort has gone into obtaining correlations between hydration thermodynamics (hydration enthalpy and entropy) and material properties such as the type and concentration of acceptor dopant on the B-site of perovskites,16,17 difference in electronegativity between B- and A-site constituents in perovskites,15 and chemical expansion of perovskites.18 Both experiments19−21 and simulations22−26 have focused on the hydration thermodynamics of proton-conducting perovskites, and hydration enthalpy and entropy are widely used to describe the hydration properties. However, the concentration of protonic defects is affected by many factors,27−29 and existing approaches have not been able to describe the detailed dynamic process of hydration. In this paper, density functional theory (DFT) calculations are performed to investigate the hydration behavior of solid oxides. We suggest four key steps for hydration: these are (1) water adsorption on the oxide surface, (2) proton migration from the surface to the bulk of the oxide, (3) proton migration in the bulk, and (4) oxide ion vacancy migration in the bulk. We investigate two solid oxides with a single-crystal structurea protonconducting SZY and an oxide ion-conducting YSZand demonstrate the significance of the four steps in the hydration process. Our analysis provides detailed insights into the hydration of a single-crystal SZY and the lack of hydration of single-crystal YSZ. To our knowledge, this is the first systematic study on the dynamics of hydration, providing a deeper understanding of the underlying mechanisms governing hydration of solid oxides.

2. COMPUTATIONAL DETAILS All DFT calculations are performed using the Vienna Ab initio simulation package (VASP).30−32 The Perdew−Burke−Ernzerhof (PBE)33 exchange-correlation functional is employed based on the projector augmented wave (PAW) method.32 The cutoff energy for the plane wave basis set was 500 eV for all calculations. For improved accuracy, the pseudopotentials with valence states of Sr (4s, 4p, 5s, 4d), Zr (4s, 4p, 5s, 4d), Y (4s, 4p, 5s, 4d), and O (2s, 2p) are used for all calculations. All ionic positions are optimized by a conjugate gradient method until the forces on each ion are less than 0.04 eV/Å (0.01 eV/Å is used for enthalpy calculation). All the calculations are spin-polarized. Bader

3. RESULTS AND DISCUSSION 3.1. Water Adsorption on the Oxide Surfaces. The optimized water adsorption geometries were obtained with several initial configurations with the water molecule in different orientations and positions on the SZY (001) surface as shown in Figure S2a of the Supporting Information. The minimum energy configuration is the one where the water molecule is split with the

Table 1. Calculated and Experimental Lattice Parameters for Orthorhombic SrZrO3 and Cubic ZrO2 solid oxide SrZrO3 (orthorhombic)

ZrO2 (cubic)

lattice parameters a (Å) b (Å) c (Å) a (Å)

this work, GGA-PBE 5.81 5.87 8.24 5.11 139

GGA-PBE 38

5.84 5.9138 8.2938 5.1339

LDA 38

5.73 5.8038 8.1338 5.03140

experiment 5.78636 5.81536 8.19636 5.0937

DOI: 10.1021/acs.chemmater.7b03476 Chem. Mater. 2018, 30, 138−144

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Chemistry of Materials hydroxide ion filling the oxide ion vacancy on the surface and the remaining proton bonding to the adjacent oxide ion on the surface. The water adsorption process is shown in Figure S3, and the final optimized water adsorption geometry is shown in Figure 2a. The length of Zr−O bonds on the surface is 2.08 Å; however,

3.2. Proton Migration from the Surface to the Bulk. It is well-known that the protons diffuse by hopping from an oxide ion to a neighboring oxide ion in a solid oxide material.47 In perovskite structures, protons move via rotation, intraoctahedral hopping, and interoctahedral hopping. For proton migration in SZY, the energy barrier associated with proton intraoctahedral hopping from the surface to the subsurface is shown in Figure S5 of the Supporting Information. It can be seen that the most preferred hopping position is near the dopant because of the lower dopant electronegativity and the NEB calculated energy barrier for this proton hopping is 0.81 eV. Further migration of the proton from the second layer to the third layer is studied and the results are shown in Figure 3a. The proton rotation,

Figure 2. Optimized structure for water adsorption on the (a) SrO (001) SZY surface and (b) (111) YSZ surface. Sr, Zr, Y, O, and H are shown in green, light blue, dark blue, red, and white, respectively, and a slightly bigger red O is from the water molecule.

the distance between the exposed Zr at the vacancy site and the O of the filled hydroxide ion is 2.66 Å and the Y−O bond elongates from 2.23 to 2.55 Å due to the adsorbed proton. In addition, the length of the two O−H bonds is 0.97 Å which is identical to the length of the O−H bonds in the water molecule. On the basis of Bader population analysis, charges of the surface O and hydroxide ion are −1.4 electrons and −0.8 electrons, respectively. Therefore, the oxygen bonded to hydrogen has weaker interaction with neighboring Zr and Y ions compared to the oxygen not bonded to hydrogen, which indicates that the hydroxide ion can move easily compared to the surface oxygen. The geometry and charge analyses suggest that the water molecule is adsorbed as two hydroxide ions on the surface. The adsorption energy is calculated using the expression, ΔEads = E[surface + H2O] − E[surface] − E[H2O],39 where E[surface + H2O], E[surface], and E[H2O] are the total energies of the adsorbed system, a bare surface, and an isolated water molecule, respectively. The adsorption energy is −2.49 eV which is smaller than that of other optimized water adsorption configurations on the SrO (001) SZY surface as shown in Figure S2a and Figure S4a (the vacancy is far away from the surface in this case) of the Supporting Information. Therefore, we choose this structure as the most stable water adsorption configuration on the surface, and it is used as the initial structure for proton migration from the surface to bulk in the rest of this study. For the YSZ (111) surface, different adsorption configurations and energies of dissociative and molecular adsorptions were also studied as shown in Figure S2b (the dopants are near the surface) and Figure S4b (the dopants are far away from the surface) of the Supporting Information. The optimized adsorption configuration that is further considered for hydration is shown in Figure 2b. Similar to the stable water adsorption configuration on the SrO (001) SZY surface, the length of the two O−H bonds is 0.97 Å. The calculated Bader charges of the surface O and hydroxide ion are −1.3 electrons and −0.7 electrons, respectively. Therefore, the water molecule is adsorbed as two hydroxide ions at the surface. The calculated adsorption energy of −0.86 eV is slightly larger than the other configurations shown in Figure S2b and Figure S4b of the Supporting Information. However, the two protons from the water molecule are bonded to the oxide ions at the surface, which is a better initial structure for subsequent steps of proton migration from the surface to the bulk compared to the other adsorption structures where the protons are farther away from the surface.

Figure 3. NEB results for (a) traditional proton hopping and (b) hydroxide ion migration.

intraoctahedral hopping, and interoctahedral hopping are considered and the energy barriers calculated using NEB range from 0.18 to 0.51 eV, which are consistent with the previous results (less than 0.6 eV47). The proton migration from the third layer is assumed to be the same as the proton migration in the bulk perovskite structure. On the basis of the above discussion, proton migration from the surface to the subsurface is a difficult and critical step involving an energy barrier of at least 0.81 eV. This suggests that one possible mechanism by which SZY hydrates is from proton hopping from the surface to the subsurface, even though the energy barrier associated with it is somewhat unfavorable. We now investigate another possible mechanism for proton migration from the surface to the subsurface. Since a water molecule is adsorbed as a hydroxide ion on the surface, hydration of the material can also take place due to the migration of the hydroxide ion from the surface to the subsurface. To investigate this, we propose a hydroxide ion migration mechanism and study the hydroxide ion migration process (see Figure S6). To compare the proton hopping (from the surface to the subsurface) and hydroxide ion migration (from the surface to the subsurface), NEB results for the hydroxide ion migration are shown in Figure 3b. We note that the barrier for hydroxide ion migration from the surface to the subsurface is 0.27 eV, which is smaller than that of proton hopping (0.81 eV, from surface to subsurface). Furthermore, the hydroxide ion migration from the 140

DOI: 10.1021/acs.chemmater.7b03476 Chem. Mater. 2018, 30, 138−144

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Chemistry of Materials surface (containing the hydrogen bonds) to the subsurface is also investigated (see Figure S6c). It can be found that the hydrogen bond facilitates the hydroxide ion migration from the surface to the subsurface and results in a much smaller barrier (about 0.01 eV). However, the barrier for hydroxide ion migration from the second layer to the third layer away from the dopant is 0.53 eV, while the migration barrier near the dopant is 0.73 eV, which are slightly larger than that for proton hopping. This indicates that the hydroxide ion migration from the surface to subsurface is more favorable for this step. To understand this, first, there is a large energy difference between proton adsorption on the surface and proton in the subsurface (shown in Figure S5), which indicates that the proton is trapped by the surface oxide ion and does not easily move to the subsurface. Second, the subsurface zirconium (or yttrium) ion’s attraction to the surface oxide ion not only results in oxide ion vacancy ending up on the surface from the subsurface (shown in Figure 1b) but may also facilitate hydroxide ion migration. In bulk, there is not a big difference between oxide ions (i.e., oxide ion vacancies) and the suitable O−O distance can result in proton hopping rather than the hydroxide ion migration. Thus, proton rotation, intraoctahedral hopping, and interoctahedral hopping are preferred. For the (111) YSZ surface, both the proton hopping and hydroxide ion migration are studied. The details are shown in Figure S7. Similar to the SrO (001) SZY surface, the hopping position near the dopant is preferred. The energy barrier from NEB calculations for proton hopping is 1.1 eV and the barrier for hydroxide ion migration from the surface to the subsurface is 1.22 eV (see Figure 4a). We also investigated the YSZ (111) surface

Figure 5. Schematic diagrams and energy barriers for proton migration obtained from NEB in the bulk SZY (a) and YSZ (b). Sr, Zr, Y, O, and H ions are green, light blue, dark blue, red, and white, respectively. “T” denotes proton intraoctahedral hopping and “J” denotes proton hopping between oxide ions bonded to the same Zr or Y.

Since YSZ is a typical oxide ion conductor and data in the literature on the proton migration in single-crystal YSZ is limited,13 several proton migration paths are studied to compute the energy barriers using NEB calculations. The hopping structures and energy barriers are shown in Figure 5b. The barrier for proton hopping varies from 0.22 to 0.56 eV and the value is dependent on the O−O distance, which is consistent with the recent DFT calculations.13 Details on the energy barriers are shown in Figure S9a of the Supporting Information. These results, comparable to the energy barriers for proton hopping in proton conductors, indicate that the energy barrier for proton migration in bulk YSZ is smaller than that from the surface to the bulk. Thus, the energy barriers for proton migration in both bulk SZY and YSZ structures are small and comparable. 3.4. Oxide Ion Vacancy Migration in the Bulk. The migration of oxide ion vacancy from the bulk to the surface of the oxide is important as the oxide ion from water needs to fill the vacancy on the surface for the hydration process. For the bulk SZY, as shown in Figure 6a, we first focus on the oxide ion vacancy migration near the dopants. Four oxide ions, which can move to the oxide ion vacancy from different directions, are considered. The energy barriers with NEB calculations are in the range of 0.24−0.65 eV. In addition, we also studied the energy barrier for oxide ion vacancy migration far away from the dopants and the NEB result is 0.59 eV. The details on the structures and

Figure 4. NEB results for proton hopping and hydroxide ion migration (from surface to subsurface in both cases) for two YSZ surfaces: (a) dopants near the surface and (b) dopants far away from the surface.

without dopants and the results for both proton hopping and hydroxide ion migration are shown in Figure 4b and additional details are shown in Figure S8. From these calculations, we note that both proton hopping and hydroxide ion migration have larger barriers, implying a more difficult process for protons to migrate from the surface to the bulk of YSZ. 3.3. Proton Migration in the Bulk. For the bulk SZY, two different structures are considered for proton migration: in the first case, the proton is far away from the dopant and in the second case, the proton is near the dopant. The schematic diagrams are shown in Figure 5a. Here, we focus only on the intraoctahedral hopping since this is a common step for proton diffusion in bulk perovskite.48 The energy barriers from NEB calculations for these two structures are 0.27 and 0.25 eV, respectively, which are consistent with the previous DFT calculations.38,49

Figure 6. Schematic diagrams and energy barriers, calculated using NEB, for oxide ion vacancy migration in bulk SZY (a) and YSZ (b). Sr, Zr, Y, O, and H ions are green, light blue, dark blue, red, and white, respectively. “T” denotes proton intraoctahedral hopping and “J” denotes proton hopping between oxide ions bonded to the same Zr or Y. 141

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Table 2. Comparison of Water Adsorption Energy and Energy Barriers Associated with Different Steps of the Hydration Process in SZY and YSZ

NEB results are shown in Figure S10a of the Supporting Information. The results confirm that SZY shows oxide ion conductivity.27 Thus, the oxide ion vacancy migration from the bulk to the surface ensures the hydration process for SZY during which water is not only adsorbed on the surface but also migrates to the subsurface using the oxygen vacancy appearing near the surface. For the bulk YSZ, we investigated the oxide ion vacancy migration near the dopants and far away from the dopants. The structures and NEB results are shown in Figure 6b and additional details on the structures and NEB results are shown in Figure S10b of the Supporting Information. Far away from the dopants, the energy barriers are 0.20 and 0.30 eV for the oxide ion migration within the same trilayer and between two trilayers, respectively. Near the dopants, the energy barriers are 0.21 and 0.32 eV for the oxide ion migration within the same trilayer and between two trilayers, respectively. The largest energy barrier in both cases is about 0.32 eV, which is consistent with previous DFT calculations.50,51 Since YSZ is a typical oxide ion conductor, the low energy barrier for oxide ion vacancy migration in bulk is expected. 3.5. Discussion. On the basis of the above results, in Table 2, we summarize the hydration process in SZY and YSZ by comparing the energy barriers (and the adsorption energy in the case of Step I) associated with each step. From the table, we can conclude that the primary difference in the hydration of SZY and YSZ arises from the energy barriers associated with proton migration from the surface to the bulk. The more favorable energy barrier associated with the migration of OH in SZY gives rise to the hydration of SZY. The larger energy barriers associated with the proton migration (from the surface to the bulk) in YSZ can be attributed to the structural differences between YSZ and SZY. For SZY, as shown in Figures S5b and S6b, no significant change in the structure is observed for the proton hopping as well as hydroxide ion migration from the surface to the subsurface. However, for YSZ, as shown in Figures S7 and S8, the proton hopping as well as hydroxide ion migration from the surface to subsurface causes a large structural relaxation. This is because there are only two bonds for each oxide ion with B site ions in SZY and the Zr−O−Zr angle is about 155°, while there are four bonds for each oxide ion with Zr ions in YSZ and they form a three-dimensional structure. Figure 7 shows the schematic illustration. When a proton bonds to the oxide ion, the proton slightly changes the Zr−O length and the Zr−O−Zr angle in SZY. However, in YSZ, the formation of the hydroxide ion breaks one of the O−Zr bonds and the other three O−Zr bonds become planar, which needs to overcome a larger energy barrier. We also calculated the hydration enthalpy at zero pressure and zero temperature based on the total energy of each system in the Kröger-Vink hydration notation: ΔEhyd = Etot(hydrated) − 24,52 where Etot(hydrated) is the total Etot(V•• O ) − Etot(H2O),

Figure 7. Schematic illustration of proton incorporation into SZY and YSZ. Zr, O, and H ions are shown in light blue, red, and white, respectively.

energy of the system containing two dopants and two protons, Etot(V•• O ) is the total energy of the system containing two dopants and one oxide ion vacancy, and Etot(H2O) is the total energy of an isolated water molecule. The calculated hydration enthalpies for SZY and YSZ are shown in Table 3. For SZY, the hydration Table 3. Hydration Enthalpies for SZY and YSZa hydration enthalpy (eV) SZY GGA-PBE (this work) GGA-PBE52 experiment (SZYb)53 classical computation13

YSZ −1.26 −1.25 −1.1

0.71

5.50−6.35

a

Previous theoretical and experimental results are also shown for comparison.

enthalpy is consistent with previous DFT calculation for SZY52 and experimental result for Yb-doped SrZrO3 (SZYb).53 The hydration process is thus exothermic and is favored at low temperatures. However, the hydration enthalpy for YSZ is positive which is consistent with the previous simulation results,13 implying that YSZ cannot be hydrated easily. The calculated hydration enthalpies confirm the structural analysis presented above. These results suggest that the formation of a three-dimensional structure primarily between oxide ion and metal atoms is a significant barrier for the hydration of the material. It is worth pointing out that the grain boundary effect on the dynamic process of hydration is not considered in the current work. However, some previous calculations have showed that the grain boundaries significantly reduce the hydration energy, and some experiments have also observed the proton conduction in nanocrystalline YSZ at low temperatures. Therefore, it is important to clarify the role of grain boundaries on hydration, especially in nanocrystalline YSZ. This will be investigated in 142

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future work to obtain a complete understanding of the hydration process in polycrystalline materials. It is important to note that the adsorption energies and energy barriers should be calculated using the Gibbs free energy. The total energy obtained from DFT does not account for the entropic contribution, which can be quite involved as discussed in ref 54. However, the entropic contribution to the adsorption energy and energy barrier is small.54,55 We also note that while the hydration thermodynamics such as the adsorption energy and hydration energy can determine whether the solid oxide is hydrated or not, the hydration kinetics such as the energy barriers associated with the four steps of the hydration process can provide detailed insights into the reasons for the hydration (or the lack of it) in the solid oxide. Thus, on the basis of the above discussion, bond breaking results in a larger energy barrier for proton migration from the surface to the bulk in YSZ.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuhang Jing: 0000-0001-5520-3866 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (USA) under Grant No. 1545907, the NSF of China under Grant No. 11304059, the NSF of Heilongjiang Province of China under Grant No. QC2015001, and the International Postdoctoral Exchange Fellowship Program of China under Grant No. 20140016. This research is part of the Blue Waters sustainedpetascale computing project, which is supported by the National Science Foundation (awards OCI-0725070 and ACI-1238993) and the state of Illinois. Blue Waters is a joint effort of the University of Illinois at Urbana−Champaign and its National Center for Supercomputing Applications (NCSA). We acknowledge the financial support of NCSA for Dr. Yuhang Jing’s Postdoctoral Research Associate. We thank Tao Sun and Linjian Ma for helpful discussions.

4. CONCLUSIONS In this study, we have performed DFT calculations to investigate the hydration processes in two different solid oxides with a singlecrystal structurea proton-conducting SZY and an oxide ionconducting YSZ. We present a hypothesis where the hydration of the solid oxide involves four key steps: water adsorption on the oxide surface, proton migration from the surface to the bulk of the oxide, proton migration in the bulk, and oxide ion vacancy migration in the bulk. The water adsorption calculations show that both SrO (001) surface of SZY and (111) surface of YSZ are favorable for the dissociative adsorption of water. The structural minimization and charge analyses indicate that the water molecule is adsorbed as two hydroxide ions on the surface. The calculations for proton migration from the surface to bulk show that the energy barrier for YSZ is larger than that of SZY for both hydroxide ion migration and proton hopping. In addition, the energy barriers for proton migration in both bulk SZY and YSZ structures are relatively small and comparable. The energy barrier for the oxide ion vacancy migration in bulk SZY is slightly larger than that in bulk YSZ, but the results do confirm that bulk SZY shows oxide ion conductivity. The migration of the oxide ion vacancy from the bulk to the surface of the oxide ensures that water is not only adsorbed on the surface but also hydrates the interior in the case of SZY. Our results indicate that the primary difference between the hydration of SZY and YSZ comes from the energy barriers for proton migration from the surface to the bulk of the oxide material. Our results not only provide mechanistic insights into the hydration of single-crystal SZY and nonhydration of the singlecrystal YSZ but also show the importance of structural rearrangement when a proton is incorporated into the bulk of the solid oxide material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03476. Surface models, water adsorption geometries, and adsorption process, structures and energy data for proton hopping and hydroxide ion migration from the surface to the subsurface, proton migration in the bulk, and oxide ion vacancy migration in the bulk (PDF) 143

DOI: 10.1021/acs.chemmater.7b03476 Chem. Mater. 2018, 30, 138−144

Article

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