Strain Effects on Oxygen Reduction Activity of Pr2NiO4 Caused by

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

www.acsaem.org

Strain Effects on Oxygen Reduction Activity of Pr2NiO4 Caused by Gold Bulk Dispersion for Low Temperature Solid Oxide Fuel Cells Sun Jae Kim,† Taner Akbay,† Junko Matsuda,‡ Atsushi Takagaki,†,‡ and Tatsumi Ishihara*,†,‡ †

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

‡

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 5.62.155.65 on 02/09/19. For personal use only.

S Supporting Information *

ABSTRACT: Effects of tensile strain induced by the dispersion of Au nanoparticles in Pr2NiO4-based oxide on the oxygen reduction reaction were investigated. Au-dispersed Pr1.9Ni0.71Cu0.41Ga0.05O4+δ (PNCG) showed a much decreased cathodic overpotential, and a cell using Au-dispersed PNCG showed a significantly higher power density, approximately 2.5 times higher than that of a cell using PNCG without dispersed Au. The smallest overpotential was achieved at 3−5 mol % dispersion of Au nanoparticles, at which the largest tensile strain was observed. Impedance measurements suggested that the impedance arc in the lower frequency range was mainly decreased; therefore, the increased activity to oxygen reduction was attributed to the increased bulk and surface diffusivity of oxide ions. Electron energy loss spectroscopy (EELS) shows that oxygen in the strained region was in a more reduced state and this oxygen could be assigned to interstitial oxygen which is highly mobile. In addition, density functional theory (DFT) analysis suggested that bond destabilization was attributed to the increase in energy of occupied π* orbitals of surface peroxo species on tensile strained surfaces. Therefore, increased cathodic performance of PNCG by Au nanoparticle dispersion could be assigned to the increased diffusivity by tensile strain. KEYWORDS: solid oxide fuel cells, cathode, strain effect, oxygen reduction reaction, computational simulation degrades cathodic performance.14 A high TEC mismatch between a perovskite cathode material (ca. 20 × 10−6 K−1) and the electrolyte produces interface instability (or delamination) during heating or cooling cycles.15 Therefore, the application of Ln2NiO4 compounds (Ln = Pr, La, and Nd, etc.) as cathode materials has been attempted because they are Sr-free and have similar TECs as the electrolyte. Pr2NiO4 (PNO), La2NiO4 (LNO), and Nd2NiO4 (NNO) lanthanide nickelates have been tested as cathode materials.16−18 Among these, PNO exhibited the lowest cathodic polarization resistance and highest oxygen permeation rate.17−21 In addition, the substitution of Cu and Ga in PNO, Pr1.9Ni0.71Cu0.24Ga0.05O4 (PNCG), increased the ionic conductivity to a value comparable with that of La1−xSrxGa1−yMgyO3 (LSGM).22−24 The reason for the conductivity enhancement was assigned to GaO7 formation by Ga doping and an increased amount of interstitial oxygen. On the other hand, strain effects in the lattice have been extensively studied recently to increase electrical and electrochemical properties. Some strategies have been suggested for strain inducement, such as TEC mismatch between a metal and metal oxide or lattice mismatch between a substrate and

1. INTRODUCTION Solid oxide fuel cells (SOFCs) are considered one of the promising power generation systems because of their low environmental impact, high energy-conversion efficiency and their capability of using a variety of fuels (hydrogen, hydrocarbons, and so on).1−3 However, to achieve high power density at low temperature, the electrochemical activity of the cathode should be improved because potential loss occurs mostly at the cathode rather than at the anode as the operating temperature decreases.4−9 Many materials have been investigated to increase the electrochemical activity for the oxygen reduction reaction (ORR) and decrease potential loss at the cathode. Lanthanide nickelates, Ln2NiO4 (Ln = La, Pr, and Nd, etc.), have been suggested as promising cathode materials when compared with widely used perovskite materials such as La1−xSrxCoO3 (LSC), La1−xSrxCoyFe1−yO3 (LSCF), and Ba1−xSrxCo1−yFeyO3 (BSCF) because lanthanide nickelates have advantages such as non-cation segregation because they are Sr-free and have thermal expansion coefficients (TECs) similar to that of the electrolyte (ca. 11 × 10−6 K−1).10,11 For the perovskite cathode materials, cation (Sr) segregation and diffusion out from the lattice to the surface or the interface between the cathode and electrolyte decrease electrical conductivity and degrade performance.3,12,13 SrO surface segregation can also play a role as a reaction seed with Cr vapor from the interconnector material, which also © XXXX American Chemical Society

Received: October 17, 2018 Accepted: January 18, 2019

A

DOI: 10.1021/acsaem.8b01776 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials epitaxial oxide film.25−31 For example, a metal oxide was sintered at high temperature after metal particles were dispersed in the oxide, and large strain was introduced at the interface between the metal particles and the metal oxide during cooling due to differences in their TECs. We have previously reported on strain induced by TEC mismatch between Au (as a metal) and PNCG (as metal oxide), so-called three-dimensional (3D) tensile strain.25,26 Although tensile strain in PNCG was successfully induced by a dispersion of Au particles and the fundamental electrical properties were investigated, the performance of such a material as a cathode for SOFCs has not been reported until now. The effects of strain on surface activity to oxygen dissociation have been reported recently and have attracted much interest.28−31 However, these studies were mainly performed using thin films and far from the structure of a realistic electrode. In this study, to investigate the electrochemical performance of the PNCG cathode in the strained state, tensile strain was generated in PNCG by the TEC mismatch between Au and PNCG and the cathode performance investigated. Lattice strain and valence numbers were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS). Au is poor as a catalytically active metal; therefore, Au was used as a dispersed metal for PNCG to clarify the strain effects on the surface activity separated from metal loading effects. This study demonstrates the strong relationship between the ORR activity and tensile strain that results in the formation of oxygen vacancies.

500 rpm for 4 h in ethanol (Pulverisette 6, Netzsch, Germany). Preparation of the NiO−Fe2O3 anode and LSGM electrolyte has been reported elsewhere.32,33 Au-dispersed PNCG and NiO−Fe2O3 paste were prepared for the screen printing method by mixing the powders with a solution of isobutyrate (Tokyo Chemical Industry, Japan) and ethyl cellulose (Kinda Chemical, Japan). Each electrode was screen printed on a side of the LSGM (300 μm thick) electrolyte. Pt reference electrode was deposited beside the cathode using Pt paste. All electrodes were then co-fired at 1373 K for 1 h. After co-firing, the active area of the cathode was ca. 0.196 cm2. 2.2. Cathodic Performance Analysis. The LSGM electrolytesupported cell (Au-PNCG/LSGM/NiO−Fe2O3) was set between alumina tubes and sealed with molten Pyrex glass for testing at 873− 1073 K with humidified hydrogen (97% H2 + 3% H2O) as a fuel and O2 as an oxidant. The O2 and fuel flow rates were both 100 mL/min. An impedance analyzer (Solartron 1260 and 1287) was used to obtain impedance spectra at 873−1073 K. Current−voltage (I−V) curves were measured with the four-probe method using a galvanostat (Hokuto HA301) as an electric load. The internal resistance of the cell was measured by the current interruption method. A current pulse was generated with a current pulse generator (Hokuto HC111), and the residual potential response was analyzed with a memory recorder (Hioki 8835). Impedance and current interruption of each electrode were measured against the reference electrode to separate the anode and cathode overpotentials. 2.3. Microstructure and Phase Analysis. Microstructural and compositional analysis was performed using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy, respectively. XPS (Shimadzu Ultra 165) and scanning transmission electron microscopy (STEM; JEOL JEM-ARM 200F) measurements were performed to determine electronic states and morphology, respectively. XRD (Rigaku, Japan) measurements were conducted for phase analyses. 2.4. Computational Method. In order to elucidate the underlying principles of the effect of strain on the oxygen reduction reaction, spin polarized plane wave DFT analyses were carried on a nine-layer symmetric slab of Ln2NiO4, where Ln is a lanthanide series element. Since the DFT analyses for Ln = Pr require elaborate consideration of complex valence states as well as the 4f valence electrons, in this work, Ln = La was chosen as a representative model case for understanding the strain effects on oxygen dissociation. The generalized gradient approximation (GGA) parametrized by Perdew− Burke−Ernzerhof (PBE) as implemented in the Vienna ab initio simulation package (VASP) was used for approximating the nonlocal exchange and correlation functionals.34−37 Cutoff kinetic energies for the projector augmented wave (PAW) pseudopotentials were chosen as 520 eV.38,39 Dudarev’s rotationally invariant GGA+U approach has been used with an effective Hubbard parameter of 6 eV on Ni 3d states.40,41 A 5 × 5 × 5 Monkhorst−Pack k-point mesh was used for sampling the Brillouin zone when optimizing the bulk unit cell.42 The electronic self-consistency and ionic relaxation cycles were considered to be converged when the total energy difference and the force acting on atoms were less than 10−5 eV and 0.01 eV/Å, respectively. The nine-layer symmetric nonpolar slab with LaO-terminated (001) basal planes was created by expanding the optimized bulk unit cell 2 × 2 × 4/3 times along its principle axes. A vacuum layer of 30 Å was placed above the slab to prevent interactions between the periodic images. During the ionic relaxations, the ions in the middle NiO2 layer of the slab were constrained to their optimized bulk unit cell positions. A 3 × 3 × 1 Monkhorst−Pack k-point grid was used for sampling the anisotropic reciprocal space of the slab model. In-plane compressive and tensile strain were applied by altering the basal-plane lattice parameters (a and b) of the slab accordingly. Following the constrained ionic relaxations, the projected crystal orbital Hamilton population (pCOHP) analyses were carried out to partition the electron probabilities into bonding, antibonding, and nonbonding domains between the atomic orbitals of the dissociated oxygen and the lattice oxygen on the surface.43−47 Optimized structures and the

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Au-dispersed PNCG powder was prepared by the combination of solid state reaction (SSR) and a liquid process, as schematically presented in Figure 1. After a stoichiometric

Figure 1. Schematic image for the preparation of Au-dispersed PNCG to induce lattice strain in PNCG by the thermal expansion mismatch between PNCG and Au particles. amount (1, 3, 5, and 7 mol %) of HAuCl4·6H2O (99.0%, Kinda Chemical, Japan) was dissolved in deionized water, Pr6O11 (99.9%, Soekawa Chemical, Japan), NiO (Wako, Japan), CuO (99.0%, High Purity Chemicals, Japan), and Ga2O3 (99.9%, High Purity Chemicals) were mixed in the Au solved aqueous solution. The solution was then heated on a hot plate to evaporate water. To decompose the nitrate, the resultant powder was then fired at 673 K for 2 h. The powder obtained was uniaxially pressed into a pellet and then cold isostatically pressed (CIP) at 300 MPa for 30 min. The pellet was calcined at 1073 K for 2 h and then at 1573 K for 6 h. The sintered pellet was crushed and the particle size was decreased by milling with zirconia balls at B


Article

ACS Applied Energy Materials

Figure 2. (a) XRD patterns of x mol % Au-dispersed PNCG powders (x = 0, 1, 3, 5, and 7) after calcination at 1573 K for 6 h. Main peaks of (b) PNCG and (c) Au magnified for comparison. (d) Lattice parameters calculated as a function of Au content (top) and strain estimated from the lattice volume (bottom).

Figure 3. TEM-EDX results for x mol % Au-PNCG powders (Au = 1, 3, and 5). The particle size of Au is estimated by the line profiles for (a) 1, (b) 3, and (c) 5 mol % Au.

the difference in the TECs of Au and PNCG.25,26 However, with an excess amount of Au, phase separation and/or large diameter Au particles were annealed and the tensile strain formed. From this result, 5 mol % Au was the upper limit to generate tensile strain while maintaining the tetragonal structure of PNCG. For analysis of reaction between dispersed Au and PNCG, the chemical state of Au was analyzed with XPS (Figure.S1 in the Support Information). It is obvious that Au peaks from 4f 7/2 and 4f 5/2 orbital were observed at 84 and 87 eV and no peak from the oxidation state of Au was observed. In addition, a stable oxidation state of Au is +3 and the ionic size of Au3+ is 85 pm, which is smaller than that of Pr3+, so if the substitution of Au into the Pr site of PNCG may occur, the lattice should shrink. Considering the stable metallic state of Au, reaction between Au with PNCG maybe negligibly occurred. Therefore, shift in diffraction peaks of PNCG was assigned to the residual tensile strain but not reaction. The lattice parameters and strain were also calculated from the XRD results (Figure 2d). Rietveld refinement was applied to calculate the lattice parameter (Figure 2d, top). The lattice parameter of a (=b) had a maximum value at 3 mol % Au, and that of the c-axis became a maximum at 5 mol % Au. However, both values were decreased at 7 mol % Au, and the change in

electron density differences were visualized by using the VESTA software package.48

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of Au-Dispersed PNCG. XRD phase analysis was performed for Au-dispersed PNCG (1, 2, 3, 5, and 7 mol %) after calcination at 1573 K, and the results are shown in Figure 2. The wide-angle XRD patterns indicated that PNCG was successfully synthesized as a tetragonal structure, regardless of the Au content, and the formation of the PNCG phase was not influenced by Au dispersion (Figure 2a). However, secondary phase formation was observed at ca. 28° in the case of 7 mol % Au-PNCG, which was indexed as PrO2 in the magnified pattern (2θ = 27− 35°) (Figure 2b). Unlike the phase separation of PNCG, the Au peak was detected at ca. 38.3° as a single phase and gradually increased with the amount of dispersed Au (Figure 2c). Clear Au peaks without a shift in diffraction angle indicated that no reaction between Au and PNCG occurred. In contrast, the main peak of PNCG was shifted to lower angle for up to 5 mol % Au, as shown in Figure 2b, which implies that tensile stress was induced in PNCG. However, the peak position returned to high angle at 7 mol % Au. Therefore, tensile strain was successfully introduced and is attributed to C


Article


Figure 4. (a) Representative I−V and I−P curves for 3 mol % Au-dispersion PNCG cathode and the PNCG cathode without Au dispersion. (b) Maximum power density of cells as a function of the amount of dispersed Au (x = 0, 1, 3, 5, and 7 mol %) in the PNCG cathode.

Figure 5. Current interruption data for x mol % Au-PNCG (x = 0, 1, 3, 5, and 7) at various temperatures. (a) Ohmic and (b) polarization resistances of the cathode part measured using the reference electrode.

and no reaction occurred with PNCG, as confirmed by STEM line analysis. 3.2. Cathodic Performance of Au-Dispersed PNCG. The cathodic performance of Au-dispersed PNCG was measured. Figure 4 shows the I−V (current density−voltage) and I−P (current density−power density) performance of the cell with the Au-dispersed PNCG cathode. The open circuit voltage (OCV) was higher than 1.10 V after anode reduction at 1073 K, which indicated the gas tightness of the cell was reasonably high for this measurement. When 3 mol % Audispersed PNCG was applied for the cathode of the cell, the maximum power density was increased to ca. 2.5 times (450 mW/cm2) that without an Au-dispersed PNCG cathode (155 mW/cm2), as shown in Figure 4a. Although the power performance was also enhanced using simply Au mixed with PNCG (Au-mixed-PNCG), the power performance was still lower than that with the Au-dispersed PNCG cathode. Because Au-mixed-PNCG was in a strain-free state, this result indicates that the tensile strain by Au dispersion contributes to further improve electrochemical properties of PNCG than the increased conductivity by mixing Au. All components of the

the lattice parameters were similar to the positional change of the main peak in the XRD patterns. The change in the strain value was also calculated based on the results of lattice volume (Figure 2d, bottom). The strain was gradually increased to ca. 0.55% at 3−5 mol % Au, which indicated tensile strain formed in the PNCG powder. On the other hand, the strain was relaxed to ca. 0.32% at 7 mol % Au. Au particles dispersed in PNCG were characterized using TEM-EDX as shown in Figure 3. Au particles were detected as bright spots in the TEM images and were uniformly dispersed in the PNCG phase (see Figure S2 in the Supporting Information). To estimate the particle size, STEM-EDX line profiles were measured for Au particles. At 1 mol % Au, the particle size was ca. 2 nm, which increased in proportion to the amount of Au and was distributed from ca. 5 to 10 nm (Figure 3b,c). Particle growth proceeded by sintering during powder calcination. The tensile stress increased with the particle size of Au in PNCG. Therefore, in Au-dispersed PNCG, since clear interface between Au and PNCG was observed, Au particles with diameters around 2−10 nm were distributed uniformly D


Article


Figure 6. (a) Impedance spectra for Au-PNCG (0, 1, 3, 5, and 7 mol %) at various temperatures (873−1073 K) with the cathode part divided from the reference electrode. (b) Comparison of impedance results for 3 mol % Au dispersed, mixed, and no added PNCG cathode. For ease of comparison, impedances for Au-PNCG (0 and 3 mol % mixture and dispersion) are labeled, and the ohmic resistance values are subtracted from the data. Numbers show the frequency values in the logarithmic scale.

On the other hand, overpotential was measured at 873, 973, and 1073 K, whereby it decreased gradually from 1 to 5 mol % (Figure 5b). The results indicate that the electrochemical performance of the cathode can be increased by Au dispersion, and this may be related to the tensile strain caused by Au dispersion. As shown in Figure 5b, the polarization resistance changed with the amount of Au with a tendency similar to that for the strain curves shown in Figure 2d, which suggests that the tensile strain in PNCG is strongly related to the increased electrochemical performance of the PNCG cathode. On the other hand, polarization resistance was increased again at 7 mol % Au, due to strain relaxation and the formation of an impurity phase (PrO2). Arrhenius plots of the current density at an overpotential of 50 mV were calculated for the temperature range of 873−1073 K (Figure S3a in the Supporting Information). The current density was enhanced as tensile stress was induced in PNCG, which suggests that the activity for oxygen adsorption or dissociation on the PNCG surface, i.e., ORR activity, is increased. The activation energy and pre-exponential terms were estimated from the Arrhenius plots of current density at 50 mV with potential drop, and the results are shown in Figure S3b. The pre-exponential term was increased by the dispersion of Au, and the largest was obtained at 3 mol % Au, which reveals a tendency similar to the strain. However, the apparent activation energy was almost unaffected by Au dispersion within the operation temperature range of the SOFC (873− 1073 K). The pre-exponential term, which is related to reaction area, was increased when 2 mol % Au was added. Therefore, it can be said that increase in the reaction site might be the main reason for decreasing cathodic overpotential by Au. On the other hand, with increasing tensile strain, overpotential of the cathode was decreased (see Figure S4 in the Supporting Information); therefore, tensile strain seems to be the main reason for increased performance of PNCG cathode. Electrochemical performance at OCV was also analyzed by impedance measurements. Impedance plots measured at the cathode side are shown in Figure 6 for various operating

cell were made by the same processes, except for the cathode; therefore, the difference in power density can be explained by improved cathodic performance as a result of Au dispersion. The improved performance was maintained at all temperatures, which indicates that the tensile strain remained even during high temperature operation. The maximum power density of each cell was compared at Au amount and each temperature (Figure 4b). The cell performance was increased by the addition of 1 and 3 mol % Au, and the largest power density was achieved at 3 mol % Au, as shown in Figure 4b. On the other hand, the power density was decreased with decreasing operating temperature; moreover, the dependence of the power density on the amount of Au became less as the operating temperature decreased. This is because, at 873 K, overpotential of the anode was also increased and influence of cathode overpotential was not large, as discussed later. To identify the reason for the increased power density, the internal resistance was analyzed with the current interruption method and the potential drop estimated from the cathodic overpotential and ohmic resistance is shown in Figure 5 as a function of current density. At 1 mol % Au, the ohmic resistance and the cathode overpotential were significantly decreased compared with that for the non-Au-dispersed 3PNCG. The internal resistance was proportionally decreased with the increase in the amount of Au (Figure 5a), and a minimum was achieved at around 3 mol % Au. These results showed a similar tendency over the entire temperature range examined (873−1073 K). The ohmic resistance of PNCG was not affected by the cathodic activity, but the resistance was related with the interface condition between the PNCG cathode and the LSGM electrolyte.49−51 At 1 mol % Au, Au, which has a low melting point, may improve the interface quality through liquid sintering behavior during electrode firing. However, when 1 mol % is exceeded, Au has an adverse effect on the interface contact quality. When there was excess Au at the interface between PNCG and LSGM, partial delamination of the cathode occurred during heating and cooling treatments due to the different TECs. E


Article


Figure 7. XPS spectra for O1s (a−d) and Ni2p (e−h) in x mol % Au-dispersed PNCG (x = 1, 3, and 5). The open symbols show the measured values, and the dotted lines show the fitting results (four Gaussian curves). (a, e) 0 wt %, (b, f) 1 wt %, (c, g) 3 wt %, and (d, h) 5 wt %.

(i.e., Au-mixed-PNCG); therefore, the strain in the PNCG lattice can improve the cathodic reaction. After power generation measurements, the microstructure of the cathode was analyzed using SEM (Figure S5). The PNCG cathode was still porous and good adhesion was observed after the power generation measurement. The porous electrode thickness was almost similar (ca. 10 μm), which guarantees that the electrochemical performance was not dependent on the thickness. Moreover, there was no cation segregation or reaction between PNCG and LSGM (Supporting Information. Figure S6). Therefore, Au-PNCG has chemical compatibility with the LSGM electrolyte. The morphology, thin thickness, and porous structure, of the PNCG cathode was almost the same as the PNCG cathode with 0−7 mol % Au; therefore, the diffusion overpotential of PNCG could be negligible in electrochemical parameters determining the performance. 3.3. Improved Mechanism for Oxygen Reduction Activity by Tensile Strain Effects. Figure 7 shows XPS spectra for Ni and Au in PNCG with and without Au dispersion, where only oxygen (O1s) and Ni (Ni2p) peaks are shown for comparison because the binding energies of Pr (Pr3d) and Cu (Cu2p) overlapped with each other and made it difficult to define the oxidation states of Pr and Cu by XPS measurement. The oxygen peaks were mainly fitted with three Gaussian peaks (ca. 533.5, 532, and 529 eV), as shown in Figure 7a−d. The peak located at ca. 533.5 eV is assigned to oxygen in adsorption species such as CO2 or H2O, and that at ca. 532 eV is due to lattice oxygen.56,57 The peak at low binding energies is attributed to a more reduced state (ca. 529 eV), which could be assigned to oxygen at interstitial sites.56,58 A comparison of the XPS results for 0 and 1 mol % Au revealed the peak areas show that the reduced state (ca. 529 eV) of oxygen was increased by the addition of 1 mol % Au, which was correlated with an increase in interstitial oxygen (δ) of the PNCG.26 As shown in Figure 7, the peak area at ca. 529 eV was increased relative to that at ca. 532 eV with an increase in the amount of dispersed Au. In our previous study, hole conduction in PNCG was significantly increased by the dispersion of Au.22,26 Electronic holes are formed by charge compensation of interstitial oxygen; therefore, an increase in

temperatures (873−1073 K), and the cathodic resistances are compared (Figure 6a). The ohmic resistance, which is the xaxis intercept at high frequency, was decreased at 1 mol % Au and then increased at 7 mol % Au, which is behavior similar to that for the current interruption measurements.52 On the other hand, polarization resistance, which is the x-axis intercept value subtracted from low frequency to high frequency, was also decreased at ca. 5 times at 1 mol % Au and gradually increased with Au content (1−7 mol %).35 There was a slightly different dependence on the amount of Au when compared with the current interruption results, and this could have originated from the current activating the electrode. Because Au is known to have low or almost no catalytic activity toward oxygen dissociation, the reaction sites for oxygen reduction were decreased by Au covered on the surface at OCV. On the other hand, under operation conditions, the oxygen dissociation reaction by current may be more enhanced by activating oxygen diffusivity and electrical conductivity than that without current. For further analysis, impedances of different cathode morphologies (e.g., non-Au-dispersed, 3 mol % Au-dispersed, and Au-mixed-PNCG) were compared at 1073 K and frequency values were marked on Figure 6b. The impedance of the cathode typically consisted of two resistance−capacitor (RC) components; a semicircle at high frequency (103−10 Hz) and a semicircle at low frequency (10−10−1 Hz). Electrochemical reaction at high frequency is related to a charge transfer reaction; the transport of ionic species across from the electrode to the electrolyte. On the other hand, semicircle at low frequency was related to oxygen dissociation on the electrode surface.53−55 The dispersion or mixture of Au nanoparticles in PNCG resulted in a decrease of the high frequency semicircle, which suggests an increase in the interface contact between the electrode and electrolyte. As with the result in Figure 6a, reduced ohmic resistance indicates an improvement in the interface and the transport of ionic species. Moreover, a decrease of the low frequency semicircle was also observed due to the improved oxygen dissociation reaction by tensile strain as compared to the strain-free case F


Article


Figure 8. (a) 18O diffusion profiles at 873 K and (b) calculated self-diffusion (D*) and surface exchange reaction (k) coefficients as a function of the Au content.

Figure 9. STEM image of Au particle dispersed in PNCG (a) and O-EELS spectra at points 1 and 2 shown in the STEM image (b, c).

conductivity, the oxidation state of Ni was increased from Ni2+ to Ni3+ and interstitial oxygen was introduced by charge compensation when Au was dispersed in PNCG, according to the following equation expressed by Kröger−Vink notation.

the peak areas of the 529 eV peak corresponds well with the conductivity results reported previously.22,23,26 XPS peaks for Ni are also shown in Figure 7e−h. From peak fitting, the Ni peak was divided into four components, as shown in Figure 7e−h, which can be assigned to the satellite peaks (ca. 863.9 and 861.0 eV), Ni3+ (ca. 858.3 eV), and Ni2+ (ca. 855.6 eV).59−61 The Ni2+ peak is dominant in Pr2NiO4, and a small amount of Ni3+ is observed. However, the XPS peak assigned to Ni2+ was weakened with an increase in the amount of dispersed Au. The peak ratio of Ni2+/Ni3+ was estimated and is shown in Figure 7. The value of Ni2+/Ni3+ decreased with an increase of Au, which indicates that Ni was in a more oxidized state under tensile stress. This is reasonable because the ionic size of Ni3+ is smaller than that of Ni2+ and tensile strain is relaxed by the formation of smaller cations. Ni was in a higher oxidation state with Au dispersed in PNCG. Considering the O 1s XPS results and the electrical

1 O2 (g) → O″i + 2h• 2

(1)

The introduction of interstitial oxygen is also reasonable to explain the increased lattice parameter. In the K 2NiF4 structure, interstitial positions in a rock salt block are considered to be fast oxide ion mobile routes because of a large free volume.62 In our previous work, we reported the oxygen permeation properties of PNCG with a dispersion of Au and suggested that the oxide ion conductivity increased.26 On the other hand, an increase in bulk oxygen dissociation and electronic density is also related with increased surface ORR because the charge transfer step is reported as the rateG


Article


Considering that the activation energy was not changed, which means activity of one site was not changed, and the decreased overpotential was mainly explained by an increase in the pre-exponential term for the Arrhenius plots, then the large positive effects of Au dispersed in PNCG could be assigned to the increased diffusivity of oxygen. The introduction of tensile strain has increased the diffusivity of surface and bulk oxygen on PNCG resulting in the increase in the ORR activity of the cathode. It is also noted that PNCG with Au loading was almost the same diffraction angle before and after power generation measurement (see Figure S6 in the Supporting Information), so the tensile strain formed by Au dispersion is stably sustained during power generation measurements at 873−1073 K. 3.4. Effect of In-Plane Strain on Os-Ol Bond Strength. Since the defect-free LaO-terminated (001) basal planes of La2NiO4 have recently been shown as active to oxygen reduction reaction,64 the effect of in-plane strain on the chemical bonding between the adsorbed oxygen and the lattice oxygen was investigated using the pCOHP analysis techniques based on plane wave DFT calculations. Figure 11 shows the

determining step for oxygen dissociation on the cathode oxide.63 Therefore, the effects of Au dispersion on oxygen dissociation were further studied using tracer diffusion reactions. Figure 8 shows 18O tracer diffusion in Au-dispersed PNCG at 873 K and its detail measurement procedure was reported elsewhere.25 Not only the diffusion length but also the surface concentration of 18O increases by the dispersion of Au in PNCG. From depth profiles of 18O shown in Figure 8a, oxygen tracer diffusion (D*) and surface exchange (k) coefficients were estimated by fitting the diffusion coefficient as a function of the dispersed Au content, as shown in Figure 8b. D* and k values were increased as with the Au content and achieved a maximum at 2 mol % Au; enhancement was closely related to an increased number of carriers (Oi″ and h•). Therefore, the increasing number of carriers induced by strain enhances cathodic performance for ORR activity and is thus anticipated to increase cathodic activity. Changes in the oxygen electronic state were studied using EELS with STEM (Figure 9). Figure 9a shows a STEM image of Au dispersed in PNCG, and panels b and c of Figure 9 show EELS spectra for oxygen at points 1 and 2 marked in Figure 9a. Au particle with a size of 50 nm was dispersed in bulk PNCG without reaction; therefore, it is expected that PNCG around Au (point 1) would exhibit large strain compared with that far from Au particle (point 2). On both points, strong EELS peaks were observed at 536 eV, which was assigned to lattice oxygen, and a weak peak around 526 eV, which was assigned to oxygen in the more reduced state. The O-EELS spectra suggest that oxygen close to an Au particle was in a more reduced state; however, oxygen far from the Au-PNCG interface was in a more oxidized state and more predominant (Supporting Information Figure S7). Therefore, the electronic state of oxygen in the strained area is in a more negatively charged state, which could be assigned to interstitial oxygen. The reduced state of oxygen in the strained area was conducted by high angle annular dark-field STEM (HAADF-STEM) imaging and EELS mapping of O, and the results are shown in Figure 10. The localization of interstitial oxygen around the Au

Figure 11. Optimized structure for the LaO-terminated nine-layer symmetric surface slab with 30 Å vacuum separation. (a) Threedimensional projection, and (b) top view. La is depicted in green, Ni is depicted in gray, lattice Ol is depicted in red, and surface Os is depicted in purple.

minimum energy structure of the nine-layer symmetric slab with an adsorbed surface oxygen (Os) on the LaO-terminated surface of La2NiO4. Due to the involvement of La in the surface charge transfer process for dioxygen activation and dissociation, the adsorbed Os was found to be preferentially attached to the La−La bridge position and covalently bonded to the nearest lattice oxygen (Ol). As depicted in Figure 12a for the relaxed surface, the Os−Ol bond lengths were calculated as 1.50 ± 0.01 Å for not only the relaxed but also all in-plane strained surfaces. The invariant bond length implies that, on both strained and unstrained LaO-terminated surfaces, the diatomic species (made up of Os and Ol) was found in the activated peroxo state. In support of this, the electron density difference plot, shown in Figure 12b, depicts the electron gain (shown in blue color) becoming slightly larger on the Os compared to the Ol due to the electron transfer mediated by the surface La in addition to the actual charge transfer through the oxygen sublattice of La2NiO4. In order to further analyze the chemical bonding states, the pCOHP of the activated peroxo species under different in-plane strain conditions were

Figure 10. (a) HAADF-STEM image and (b) reduced state of oxygen EELS mapping corresponding to the HAADF-STEM image.

particle dispersed in PNCG is more visibly shown by the EELS mapping, and the length of the strained region around Au seems to be a few tens of nanometers. Therefore, the formation of Ni3+ may induce the formation of interstitial oxygen, and this occurs more significantly close to the Au particle dispersed in PNCG because of the large strain effects. H


Article


EELS analysis. Considering the stable metallic state of Au, tensile strain was successfully introduced by the dispersion of Au nanoparticles in PNCG by difference in thermal expansion coefficient. Although PNCG has low activity toward oxygen reduction, the cathodic overpotential can be significantly decreased by the dispersion of Au nanoparticles, which induces tensile strain. From TEM-EELS and XPS analysis, a reduced state of oxygen which could be assigned to interstitial oxygen, was increased around Au nanoparticles which is a highly tensile region (few tens of nanometers), and Ni3+ was also increased by charge compensation. The increased surface exchange coefficient (k0 value) for PNCG by Au dispersion was mainly assigned to increased diffusivity of oxide ions in the surface and also bulk due to an increased amount of interstitial oxygen. As a result of the increased k value, the cathodic overpotential of PNCG was decreased by Au dispersion and the power density of the cell was increased by ca. 2.5 times (450 mW/cm2) compared with that using a non-Au-dispersed PNCG cathode (160 mW/cm2). The DFT calculations together with pCOHP analyses on a model system of La2NiO4 provided the fundamental support for the observed activity increase in the tensile strained surfaces of PNCG for oxygen reduction reaction. Thus, tensile strain is highly effective to increase ORR activity in the PNCG cathode, and this is related to an increase in diffusivity of oxygen on the surface by weakening the adsorption state and also in bulk by interstitial oxygen.

Figure 12. Close up view of the surface perovskite layer of the slab showing the interaction between the surface Os (purple) and the lattice Ol (red) as well as the surface La (green) species. (a) Ion separations for Os−Ol and Os−La are calculated as 1.50 and 2.47 Å, respectively. (b) Electron density difference plot of the surface oxygen species showing the covalent bonding between Os and Ol with a partial charge mediated by the surface La. Positive and negative electron densities are denoted blue and yellow, respectively.

calculated. It was found that the occupied π* antibonding states energetically shift up to the Fermi level as the in-plane strain state changes from compression to tension (see Figure 13). Figure 13 summarizes the variation of average energies of the occupied π* orbitals of the surface peroxo species with respect to the applied in-plane strain. The pCOHP results imply that the in-plane tensile strain on La2NiO4 surfaces can readily destabilize the activated peroxo species by raising the energies of the occupied π* states. This destabilization could be the main reason for increased dissociation of oxygen molecule into oxide ion and also increased surface diffusivity. Therefore, from these DFT calculations, it is reasonably considered that the increased activity of PNCG under tensile strain applied could be assigned to the increased surface diffusivity of oxygen. Effects of mixed valence state of Pr on the surface electronic state of oxygen is now under investigation and will be reported in the future.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01776. Experimental details and additional results, e.g., Au-XPS, TEM, Arrhenius plots of cathodic overpotential, overpotential and strain relationship, SEM, and XRD (PDF)

■

4. CONCLUSION The effects of tensile strain on cathodic activity were studied using PNCG by dispersion of Au nanoparticles. XRD peak from PNCG was shifted to lower angle, and reaction between Au and PNCG was negligibly observed by XPS and TEM-

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Taner Akbay: 0000-0002-7115-9970

Figure 13. Chemical bonding analyses for the Os−Ol bond on the LaO-terminated (001) surfaces of La2NiO4. Hamilton populations calculated for (a) 2% compressive strained surface, (b) relaxed surface, and (c) 2% tensile strained surface. Solid and dashed lines are for the majority and the minority spin levels of O 2p electrons, respectively. (d) Variation of the occupied π* antibonding Os−Ol molecular orbital energy against strain. Os−Ol bond gets destabilized (i.e., average energy of the occupied π* molecular orbital is shifted up toward the Fermi energy level) due to the inplane tensile strain. I


Article


assembled La0.6Sr0.4Co0.2Fe0.8O3‑δ oxygen electrode on Y2O3-ZrO2 electrolyte of solid oxide electrolysis cells. J. Power Sources 2018, 384, 125−135. (14) Chen, Y.; Yoo, S.; Li, X.; Ding, D.; Pei, K.; Chen, D.; Ding, Y.; Zhao, B.; Murphy, R.; deGlee, B.; Liu, J.; Liu, M. An effective strategy to enhancing tolerance to contaminants poisoning of solid oxide fuel cell cathodes. Nano Energy 2018, 47, 474−480. (15) Khan, M. Z.; Mehran, M. T.; Song, R. H.; Lee, S. B.; Lim, T. H. Effects of applied current density and thermal cycling on the degradation of a solid oxide fuel cell cathode. Int. J. Hydrogen Energy 2018, 43, 12346−12357. (16) Montenegro-Hernández, A.; Vega-Castillo, J.; Mogni, L.; Caneiro, A. Thermal stability of Ln2NiO4+δ (Ln: La, Pr, Nd) and their chemical compatibility with YSZ and CGO solid electrolytes. Int. J. Hydrogen Energy 2011, 36, 15704−15714. (17) Philippeau, B.; Mauvy, F.; Mazataud, C.; Fourcade, S.; Grenier, J. C. Comparative study of electrochemical properties of mixed c o n d u ct i ng L n 2 NiO 4 + δ ( Ln = L a , P r a n d N d ) a n d La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3−δ as SOFC cathodes associated to Ce0.9Gd0.1O2‑δ, La0.8Sr0.2Ga0.8Mg0.2O3−δ and La9Sr1Si6O26.5 electrolytes. Solid State Ionics 2013, 249−250, 17−25. (18) Boehm, E.; Bassat, J. M.; Dordor, P.; Mauvy, F.; Grenier, J. C.; Stevens, P. Oxygen diffusion and transport properties in nonstoichiometric Ln2−xNiO4+δ oxides. Solid State Ionics 2005, 176, 2717−2725. (19) Ishihara, T.; Nakashima, K.; Okada, S.; Enoki, M.; Matsumoto, H. Defect chemistry and oxygen permeation property of Pr2Ni0.75Cu0.25O4 oxide doped with Ga. Solid State Ionics 2008, 179, 1367−1371. (20) Ishihara, T.; Sirikanda, N.; Nakashima, K.; Miyoshi, S.; Matsumoto, H. Mixed Oxide Ion and Hole Conductivity in Pr2−αNi0.76−xCu0.24GaxO4+δ Membrane. J. Electrochem. Soc. 2010, 157 (1), B141−B146. (21) Miyoshi, S.; Furuno, T.; Sangoanruang, O.; Matsumoto, H.; Ishihara, T. Mixed Conductivity and Oxygen Permeability of Doped Pr2NiO4-Based Oxides. J. Electrochem. Soc. 2007, 154 (1), B57−B62. (22) Hyodo, J.; Tominaga, K.; Ju, Y. W.; Ida, S.; Ishihara, T. Electrical conductivity and oxygen diffusivity in Cu- and Ga-doped Pr2NiO4. Solid State Ionics 2014, 256, 5−10. (23) Yashima, M.; Yamada, H.; Nuansaeng, S.; Ishihara, T. Role of Ga3+ and Cu2+ in the High Interstitial Oxide-Ion Diffusivity of Pr2NiO4-Based Oxides: Design Concept of Interstitial Ion Conductors through the Higher-Valence d10 Dopant and Jahn−Teller Effect. Chem. Mater. 2012, 24, 4100−4113. (24) Gao, Z.; Miller, E. C.; Barnett, S. A. A High Power Density Intermediate-Temperature Solid Oxide Fuel Cell with Thin (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O3‑δ Electrolyte and Nano-Scale Anode. Adv. Funct. Mater. 2014, 24, 5703−5709. (25) Hyodo, J.; Tominaga, K.; Ida, S.; Ishihara, T. Effects of threedimensional mechano-chemical tensile strain on fast oxygen diffusion in Au-dispersed Pr1.90Ni0.71Cu0.24Ga0.05O4+δ. J. Mater. Chem. A 2016, 4, 3844−3849. (26) Hyodo, J.; Tominaga, K.; Hong, J. E.; Ida, S.; Ishihara, T. Effects of Three-Dimensional Strain on Electric Conductivity in AuDispersed Pr1.90Ni0.71Cu0.24Ga0.05O4+δ. J. Phys. Chem. C 2015, 119, 5− 13. (27) Ishihara, T.; Hyodo, J.; Schraknepper, H.; Tominaga, K.; Ida, S. Effects of Pt dispersion on electronic and oxide ionic conductivity in Pr1.90Ni0.71Cu0.24Ga0.05O4. Phys. Chem. Chem. Phys. 2016, 18, 11125− 11131. (28) Kubicek, M.; Cai, Z.; Ma, W.; Yildiz, B.; Hutter, H.; Fleig, J. Tensile Lattice Strain Accelerates Oxygen Surface Exchange and Diffusion in La1−xSrxCoO3−δ Thin Films. ACS Nano 2013, 7, 3276− 3286. (29) Jalili, H.; Han, J. W.; Kuru, Y.; Cai, Z.; Yildiz, B. New Insights into the Strain Coupling to Surface Chemistry, Electronic Structure, and Reactivity of La0.7Sr0.3MnO3. J. Phys. Chem. Lett. 2011, 2, 801. (30) la O’, G. J.; Ahn, S. J.; Crumlin, E.; Orikasa, Y.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. Catalytic Activity Enhancement for

Atsushi Takagaki: 0000-0002-7829-3451 Tatsumi Ishihara: 0000-0002-7434-3773 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was financially supported by a Grant-in-Aid for Specially Promoted Research (No. 16H06293) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan through the Japan Society for the Promotion of Science (JSPS). We also acknowledged the useful discussion of Dr. Hajime Kusaba in Kyushu University of XPS measurement.

■

REFERENCES

(1) Singhal, S. C. Advances in solid oxide fuel cell technology. Solid State Ionics 2000, 135, 305−313. (2) Irvine, J. T. S.; Neagu, D.; Verbraeken, M. C.; Chatzichristodoulou, C.; Graves, C.; Mogensen, M. B. Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers. Nat. Energy 2016, 1, 15014. (3) Kim, S. J.; Choi, M. B.; Park, M.; Kim, H.; Son, J. W.; Lee, J. H.; Kim, B. K.; Lee, H. W.; Kim, S. G.; Yoon, K. J. Acceleration tests: Degradation of anode-supported planar solid oxide fuel cells at elevated operating temperatures. J. Power Sources 2017, 360, 284− 293. (4) Connor, P. A.; Yue, X.; Savaniu, C. D.; Price, R.; Triantafyllou, G.; Cassidy, M.; Kerherve, G.; Payne, D. J.; Maher, R. C.; Cohen, L. F.; Tomov, R. I.; Glowacki, B. A.; Kumar, R. V.; Irvine, J. T. S. Tailoring SOFC Electrode Microstructures for Improved Performance. Adv. Energy Mater. 2018, 8, 1800120. (5) Chang, I.; Ji, S.; Park, J.; Lee, M. H.; Cha, S. W. Ultrathin YSZ Coating on Pt Cathode for High Thermal Stability and Enhanced Oxygen Reduction Reaction Activity. Adv. Energy Mater. 2015, 5, 1402251. (6) Li, M.; Zhao, M.; Li, F.; Zhou, W.; Peterson, V. K.; Xu, X.; Shao, Z.; Gentle, I.; Zhu, Z. A niobium and tantalum co-doped perovskite cathode for solid oxide fuel cells operating below 500 °C. Nat. Commun. 2017, 8, 13990. (7) Chen, Y.; Bu, Y.; Zhang, Y.; Yan, R.; Ding, D.; Zhao, B.; Yoo, S.; Dang, D.; Hu, R.; Yang, C.; Liu, M. A Highly Efficient and Robust Nanofiber Cathode for Solid Oxide Fuel Cells. Adv. Energy Mater. 2017, 7, 1601890. (8) Laguna-Bercero, M. A.; Kinadjan, N.; Sayers, R.; El Shinawi, H.; Greaves, C.; Skinner, S. J. Performance of La2−xSrxCo0.5Ni0.5O4±δ as an Oxygen Electrode for Solid Oxide Reversible Cells. Fuel Cells 2011, 11, 102−107. (9) Choi, H. J.; Kim, M.; Neoh, K. C.; Jang, D. Y.; Kim, H. J.; Shin, J. M.; Kim, G. T.; Shim, J. H. High-Performance Silver Cathode Surface Treated with Scandia-Stabilized Zirconia Nanoparticles for Intermediate Temperature Solid Oxide Fuel Cells. Adv. Energy Mater. 2017, 7, 1601956. (10) Kim, S. J.; Kim, K. J.; Dayaghi, A. M.; Choi, G. M. Polarization and stability of La2NiO4+δ in comparison with La0.6Sr0.4Co0.2Fe0.8O3−δ as air electrode of solid oxide electrolysis cell. Int. J. Hydrogen Energy 2016, 41, 14498−14506. (11) Flura, A.; Dru, S.; Nicollet, C.; Vibhu, V.; Fourcade, S.; Lebraud, E.; Rougier, A.; Bassat, J. M.; Grenier, J. C. Chemical and structural changes in Ln2NiO4+δ (Ln = La, Pr or Nd) lanthanide nickelates as a function of oxygen partial pressure at high temperature. J. Solid State Chem. 2015, 228, 189−198. (12) Hardy, J. S.; Templeton, J. W.; Edwards, D. J.; Lu, Z.; Stevenson, J. W. Lattice expansion of LSCF-6428 cathodes measured by in situ XRD during SOFC operation. J. Power Sources 2012, 198, 76−82. (13) Ai, N.; He, S.; Li, N.; Zhang, Q.; Rickard, W.; Chen, K.; Zhang, T.; Jiang, S. P. Suppressed Sr segregation and performance of directly J


Article

ACS Applied Energy Materials Oxygen Reduction on Epitaxial Perovskite Thin Films for Solid-Oxide Fuel Cells. Angew. Chem., Int. Ed. 2010, 49, 5344. (31) Tsvetkov, N.; Lu, Q.; Chen, Y.; Yildiz, B. Accelerated Oxygen Exchange Kinetics on Nd2NiO4+δ Thin Films with Tensile Strain along c-Axis. ACS Nano 2015, 9, 1613. (32) Ishihara, T.; Jirathiwathanakul, N.; Zhong, H. Intermediate temperature solid oxide electrolysis cell using LaGaO3 based perovskite electrolyte. Energy Environ. Sci. 2010, 3, 665. (33) Ishihara, T.; Matsuda, H.; Takita, Y. Doped LaGaO3 Perovskite Type Oxide as a New Oxide Ionic Conductor. J. Am. Chem. Soc. 1994, 116, 3801. (34) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558. (35) Kresse, G.; Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal−amorphous-semiconductor transition in germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251. (36) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15. (37) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396. (40) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 1505. (41) Hautier, G.; Ong, S. P.; Jain, A.; Moore, C. J.; Ceder, G. Accuracy of density functional theory in predicting formation energies of ternary oxides from binary oxides and its implication on phase stability. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 155208. (42) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (43) Dronskowski, R.; Blöchl, P. E. Crystal orbital Hamilton populations (COHP): energy-resolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 1993, 97, 8617−8624. (44) Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. Crystal Orbital Hamilton Population (COHP) Analysis As Projected from Plane-Wave Basis Sets. J. Phys. Chem. A 2011, 115, 5461−5466. (45) Maintz, S.; Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids. J. Comput. Chem. 2013, 34, 2557−2267. (46) Maintz, S.; Deringer, V. L.; Tchougreeff, A. L.; Dronskowski, R. LOBSTER: A tool to extract chemical bonding from plane-wave based DFT. J. Comput. Chem. 2016, 37, 1030−1035. (47) Maintz, S.; Esser, M.; Dronskowski, R. Efficient Rotation of Local Basis Functions Using Real Spherical Harmonics. Acta Phys. Pol., B 2016, 47, 1165. (48) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (49) Keane, M.; Mahapatra, M. K.; Verma, A.; Singh, P. LSM−YSZ interactions and anode delamination in solid oxide electrolysis cells. Int. J. Hydrogen Energy 2012, 37, 16776−16785. (50) Kim, S. J.; Choi, G. M. Stability of LSCF electrode with GDC interlayer in YSZ-based solid oxide electrolysis cell. Solid State Ionics 2014, 262, 303−306. (51) Li, N.; Keane, M.; Mahapatra, M. K.; Singh, P. Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ. Int. J. Hydrogen Energy 2013, 38, 6298− 6303. (52) Wu, W.; Guan, W. B.; Wang, G. L.; Wang, F.; Wang, W. G. InSitu Investigation of Quantitative Contributions of the Anode,

Cathode, and Electrolyte to the Cell Performance in AnodeSupported Planar SOFCs. Adv. Energy Mater. 2014, 4, 1400120. (53) Giuliano, A.; Carpanese, M. P.; Panizza, M.; Cerisola, G.; Clematis, D.; Barbucci, A. Characterisation of La0.6Sr0.4Co0.2Fe0.8O3‑δ − Ba0.5Sr0.5Co0.8Fe0.2O3‑δ composite as cathode for solid oxide fuel cells. Electrochim. Acta 2017, 240, 258−266. (54) Amin, R.; Karan, K. Characterization of La0.5Ba0.5CoO3 − δ as a SOFC Cathode. J. Electrochem. Soc. 2010, 157 (2), B285−B291. (55) Esquirol, A.; Brandon, N. P.; Kilner, J. A.; Mogensen, M. J. Electrochem. Soc. 2004, 151 (11), A1847. (56) Yan, L.; Yu, R.; Liu, G.; Xing, X. A facile template-free synthesis of large-scale single crystalline Pr(OH)3 and Pr6O11 nanorods. Scr. Mater. 2008, 58 (8), 707−710. (57) Wei, L.; Sun, L. P.; Li, Q.; Huo, L. H.; Zhao, H. Doping Effect of Alkaline Earth Metal on Oxygen Reduction Reaction in Praseodymium Nickelate With Layered Perovskite Structure. J. Electrochem. En. Conv. Stor. 2016, 13 (4), 041003. (58) Barr, T. L. An ESCA study of the termination of the passivation of elemental metals. J. Phys. Chem. 1978, 82 (16), 1801−1810. (59) Peck, M. A.; Langell, M. A. Comparison of Nanoscaled and Bulk NiO Structural and Environmental Characteristics by XRD, XAFS, and XPS. Chem. Mater. 2012, 24 (23), 4483−4490. (60) Prabu, M.; Ketpang, K.; Shanmugam, S. Hierarchical nanostructured NiCo2O4 as an efficient bifunctional non-precious metal catalyst for rechargeable zinc−air batteries. Nanoscale 2014, 6 (6), 3173. (61) Li, C.; Shen, Y.; Zhu, S.; Shen, S. Supported Ni−La−Ox for catalytic decomposition of N2O I: component optimization and synergy. RSC Adv. 2014, 4 (55), 29107−29119. (62) Yashima, M.; Enoki, M.; Wakita, T.; Ali, R.; Matsushita, Y.; Izumi, F.; Ishihara, T. Structural Disorder and Diffusional Pathway of Oxide Ions in a Doped Pr2NiO4-Based Mixed Conductor. J. Am. Chem. Soc. 2008, 130, 2762−2763. (63) Ju, Y. W.; Hyodo, J.; Inoishi, A.; Ida, S.; Tohei, T.; So, Y. G.; Ikuhara, Y.; Ishihara, T. Double Columnar Structure with a Nanogradient Composite for Increased Oxygen Diffusivity and Reduction Activity. Adv. Energy Mater. 2014, 4, 1400783. (64) Akbay, T.; Staykov, A.; Druce, J.; Téllez, H.; Ishihara, T.; Kilner, J. A. The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4. J. Mater. Chem. A 2016, 4, 13113−13124.

K


Strain Effects on Oxygen Reduction Activity of Pr2NiO4 Caused by

Recommend Documents