Improving the Electro-catalytic Activity and Durability of La0.6Sr0.4Co0

Subscriber access provided by University of Sunderland

Energy, Environmental, and Catalysis Applications

Improving the Electro-catalytic Activity and Durability of La0.6Sr0.4Co0.2Fe0.8O3-# Cathode by Surface Modification Huijun Chen, Zheng Guo, Lei A Zhang, Yifeng Li, Fei Li, Yapeng Zhang, Yu Chen, Xinwei Wang, Bo Yu, Jian-min Shi, Jiang Liu, Chenghao Yang, Shuang Cheng, Yan Chen, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14693 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 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.

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 28 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

ACS Applied Materials & Interfaces

Improving the Electro-catalytic Activity and Durability of La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode by Surface Modification Huijun Chen,1 Zheng Guo,2 Lei A Zhang,3 Yifeng Li,4 Fei Li,1 Yapeng Zhang,1 Yu Chen,3 Xinwei Wang,2 Bo Yu,4 Jian-min Shi,5 Jiang Liu,1 Chenghao Yang, 1 Shuang Cheng, 1 Yan Chen 1 ,6 *and Meilin Liu 3* 1

Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Institute, School

of Environment and Energy, South China University of Technology, Guangzhou510006, China 2

School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055,

China 3 Materials

Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA

4

Institute of Nuclear and New Energy Technology (INET), Tsinghua University, Beijing 100084, China

5

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang

621000, China 6

Guangdong Engineering and Technology Research Center for Surface Chemistry of Energy Materials,

School of Environment and Energy, South China University of Technology, Guangzhou, 510006, PR China KEYWORDS: surface engineering, oxygen reduction reaction, oxygen defects, electrodes, oxide hetero-structures

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 28

ABSTRACT: Electrode materials with high activity and good stability are essential for commercialization of energy conversion systems such as solid oxide fuel cells or electrolysis cells at the intermediate temperature. Modifying existing perovskite-based electrodes surface to form a heterostructure has been widely applied for rational design of novel electrodes with high performance. Despite many successful developments in enhancing electrode performance by surface modification, some controversial results are also reported in the literature and the mechanisms are still not well understood. In this work, the mechanism of how surface modification impacts the oxygen reduction reaction (ORR) activity and stability of perovskite-based oxides were investigated. We took La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the thin-film model systems and modified its surface with additive PrxCe1-xO2 layers of different thicknesses. We found a strong correlation between surface oxygen defects and the ORR activity of hetero-structure. Inducing higher oxygen vacancy concentration compared to bare LSCF, PrO2 coating is proved to greatly facilitate the rate of oxygen dissociation, thus significantly enhancing ORR activity. Due to low oxygen vacancy density introduced by Pr0.2Ce0.8O2 and CeO2 coating, on the one hand, it does not boost the rate of ORR, but successfully suppresses surface Sr segregation, leading to an enhanced durability. Our findings demonstrate the vital role of surface oxygen defects and provide with important insights for rational design of high-performance electrode materials through surface defect engineering.

1.INTRODUCTION Due to the high efficiency for chemical-to-electrical energy conversion, excellent fuel flexibility, and the capability to produce fuels from electricity or solar energy, high-temperature electrochemical systems have attracted great attentions, including solid oxide fuel cells, solid oxide electrolysis cells, oxygen separation membrane, and solar-thermal H2O/CO2 splitting systems.1,

2

However, broader

commercialization of these systems is hindered by the lack of electrode materials with high performance and long lifespan.3 Modifying existing perovskite based electrode surface with a secondary phase to form a hetero-structure has been widely used to improve the electrodes activity and stability.4, 5-6 Such 2 ACS Paragon Plus Environment

Page 3 of 28 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


secondary phases include perovskite-based oxides,7-11 alkaline earth metal oxides,12 transition metal oxides,13-17-18 lanthanides oxides19 and their composites phases.20 Despite many successful developments in enhancing electrode performance by surface modification, some controversial results are also reported in the literature. For instance, the decoration of CoOx was reported to improve the ORR activity of (La,Sr)CoO3 (LSC)14 and La0.8Sr0.2FeO3 electrode,15 but show little impact on that of LSCF16 and (La,Sr)MnO3 (LSM)/ yttria stabilized zirconia (YSZ) electrode.17

Mutoro et al.12 showed that the

decoration of SrOx nano-particles on LSC surface greatly enhanced the oxygen surface exchange kinetics, while Rupp et al.14 reported a severe deactivation effect of sub-monolayer of SrOx on the LSC. The proposed mechanisms for enhancement in catalytic activity arisen from surface modification are still controversial. 21-24 It is well known that oxygen defects play an essential role in determining the activity and stability of oxide electrodes.13, 25-30 For many perovskite with high electronic and ionic conductivity such as LSC, LSCF, and BSCF, more oxygen vacancies on the surface are reported to lead to higher surface oxygen exchange coefficient. 31, 32 Consistently, perovskite oxides with a higher O 2p band center energy level, which are associated with lower oxygen vacancy formation energy, are reported to have better ORR activity.27

On the other hand, high concentration of oxygen vacancies on the surface is claimed to

destabilize perovskite oxide surface,13,

29

attributed to the electrostatic interaction between positively

charged oxygen vacancies on the surface and negatively charged cation dopant such as Sr, Ca, and Ba.29 Such interaction can lead to cation segregation and separation on the surfaces, which is one major cause for the degradation of perovskite oxide materials.4 For oxide hetero-structure electrodes, the oxygen vacancies in the surface coating layer are found to influence the activity and stability of electrodes.13, 20, 33

The mechanism for such influence, however, is still unclear. In this work, praseodymium doped ceria (PrxCe1−xO2) modified La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) thin

films synthesized by Pulsed Laser Deposition (PLD) were used as the model hetero-structure electrodes. 3 ACS Paragon Plus Environment


Page 4 of 28

The controversial reports about the impact of surface modification in literature, as mentioned above, in many cases originated from the inability to control the coating's thickness, microstructure, density, composition and uniformity. The use of PLD technique in this study ensures that the surface coating has a well-defined thickness, composition, and morphology, enabling a systematic investigation into the impact of surface modification. PrxCe1−xO2−δ was chosen as the modification layer due to its tunable oxygen defect chemistry.34-37 The objective is to provide critical insight into the mechanism of how surface modification impacts the ORR activity and stability of perovskite-based oxides. Particularly, we focus on understanding the impact of oxygen defect in the modification layer on the overall ORR performance. We found that PrO2 modified LSCF showed much higher ORR activity compared with bare LSCF, which were attributed to the high oxygen vacancy concentration in the top layer that facilitated the oxygen dissociation process. In contrast, Pr0.2Ce0.8O2 (PCO) and CeO2 modified LSCF only showed moderate enhancement in ORR activity, but presented much better long-term stability. We believe this can be related to the high oxygen formation energy and low Sr solubility in the top layer, which suppress Sr segregation to LSCF surface during high temperature operation. Our results provide a guide line for the rational design of oxide hetero-structure through surface oxygen defects engineering. 2. EXPERIMENTAL SECTION Highly textured LSCF (001) films with ~25 nm thickness were prepared by PLD on YSZ (001) single crystals substrates (CTMwafer, China) with GDC as a buffer layer. A surface modification layer of PrO2, Pr0.2Ce0.8O2 (PCO) and CeO2 with two different thicknesses (~2nm and ~6 nm) was deposited subsequently on top of LSCF/GDC/YSZ(001) thin film hetero-structure. The PLD growth was carried out at 600 °C under oxygen pressure of 10 mTorr. A KrF excimer laser with wavelength of 248 nm, energy of 300 mJ per pulse and a pulse frequency of 10 Hz were used for the deposition. The target substrate distance was set to be 10 cm. The films were cooled down to room temperature in 2 torr oxygen pressure with a cooling rate of 5 oC/min. PrO2 , PCO and CeO2 reference samples were 4 ACS Paragon Plus Environment

Page 5 of 28 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


deposited on to YSZ substrate with GDC buffer layer using the same PLD condition for comparison. The procedure to make PLD target can be found in Supporting Information Section 1. X-ray diffraction (Bruker D8 Advance, Germany) was used to confirm that all the powders and targets are the right phase. HRXRD 2θ−ω scans were performed to determine the orientation and the crystallinity of the thin films synthesized by PLD. The HRXRD measurements employed a highresolution four-circle diffractometer with a two-bounce Ge(022) channel-cut monochromator and a scintillation counter, using Cu Kα1 radiation(Rigaku SmartLab, Japan). XPS measurement was performed for surface chemistry analysis, using Thermo Scientificinstrument (ESCALAB 250Xi) with monochromated Al Kα (15kV) X-ray radiation equipped with charge neutralization. The C 1s peak at 284.6 eV was used as the internal reference peak for calibrating binding energies. The local cation composition on the surface was quantified by AES (PHI-700 , ULVAC-PHI) with energy resolution of 1‰. Surface morphologies were investigated using a SEM (ZEISS SUPPRA 55VP, German) with ET secondary electron detector, and AFM using Asylum Research instrument (MFP-3D-S). High resolution transmission electron microscope(HRTEM) , high angle annular dark field scanning TEM (HAADFSTEM) and energy dispersive spectroscopy (EDS) elements linear scanning was performed by

a JEOL

JEM -3200FS machine to determine the interface structure. Focus ion Beam (FIB, FEI, Scios) milling technique was used for HRTEM sample preparation. The ORR activity and stability of modified LSCF and reference samples were evaluated by EIS using Zahner Im6 System. The EIS test was carried in frequency from 10 kHz to 5 mHz with an excitation voltage of 10 mV. First-principles calculations were carried out in Vienna Ab-initio Simulation Package (VASP) 38. Generalized gradient approximation parameterized by Perdew, Burke and Ernzerholf 39 with spinpolarization was adopted to describe the unpaired f electrons in Pr and Ce ions. Plane-wave basis set with energy cutoff 400 eV and a 4 × 4 × 4 and 2 × 2 × 2 Monkhorst-Pack 40 k-points were used for 5 ACS Paragon Plus Environment


the fluorite conventional unit cell and the 40-atom supercell. To overcome the delocalization tendency of DFT method in describing small polarons, local lattice distortions surrounding them were created by using Ce_3 and Pr_3 pseudo-potentials prior of spin-polarized calculations. 3. RESULTS AND DISCUSSION Using PLD, bare LSCF thin films and LSCF with a ~2nm (thin) and ~6nm (thick) coherent coating of PrO2, Pr0.2Ce0.8O2−δ (PCO) and CeO2 were grown on Yttrium stabilized Zirconia (YSZ) (001) single crystal substrates to serve as model systems. A Gd-doped CeO2 (GDC) layer buffer layer was grown between LSCF and YSZ substrate to avoid undesired reaction between electrode and electrolyte. High resolution X-ray diffraction (HRXRD) results showed that the LSCF, the PrxCe1−xO2−δ coating, and the GDC buffer layers were all highly textured in (001) orientation. Representative HRXRD results for the LSCF, the thick CeO2 (~6nm) modified LSCF are shown in Figure 1a. More detailed HRXRD results are shown in Figure S2. All the as prepared samples presented very smooth surface with roughness of

20

30

LSCF

40

GDC (004) YSZ (004)

(b)

RMS 0.21nm

LSCF as-prep.

CeO2(004) GDC (004) YSZ (004)

LSCF (002)

CeO2 thick

LSCF (002)

CeO2 (002)GDC (002) YSZ (002)

LSCF (001)

LSCF (001)

(a)

GDC (002) YSZ (002)

less than 1 nm (Figure 1b, Figure S4).

Intensity (a.u.)

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 6 of 28

(c)

RMS 0.45nm

CeO 2 thick as-prep.

50 60 2 (degree)

70

80


Page 7 of 28 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


Figure 1.(a) HRXRD pattern of as prepared LSCF, thick CeO2 (~6 nm)/LSCF (CeO2 thick). The peaks of top CeO2 layer overlap with the GDC buffer layer . All the layers are highly textured in (001) orientation. AFM images of (b)as prepared LSCF, (c) thick CeO2/LSCF. The surface roughness of LSCF and thick CeO2/LSCF were 0.21 nm and 0.45 nm, respectively. The ORR activity of the LSCF and the modified LSCF electrodes at different temperatures and oxygen partial pressures were evaluated using electrochemical impedance spectroscopy (EIS) performed on model cells with the thin-film electrode as the working electrode, YSZ substrate as the electrolyte, and porous Ag-YSZ as the counter electrode (Figure 2a). Representative EIS spectra are shown in Figure 2b for the bare LSCF and the LSCF with a thick modification layer (~6 nm) tested at 600 oC in air. Since the contribution of the counter electrode (porous Ag-YSZ) to the total impedance of the cell is much smaller than that of the dense thin-film working electrode (Figure S3), the Nyquist plots are dominated by a semicircle attributed to the surface oxygen exchange process of the thin-film electrode. Using the equivalent circuit shown in Figure 2b, the ORR area specific resistance (ASR) of the LSCF and the modified LSCF at different temperatures in air are obtained (Figure 2c-d). Oxygen exchange coeffient kq can be calculated from the ORR ASR using equations (1-2). kq=kBT/4e2ASR co

(1)

co =(3-δ)/Vm

(2)

where kB is Boltzmann constant, T is the absolute temperature, e is the electron charge. The lattice oxygen concentration co is calculated based the bulk molar volume Vm and oxide ion vacancy content δ of LSCF at 873K reported by Niwa et al41. Among the samples tested, PrO2/LSCF shows the most pronounced enhancement in ORR kinetics. The ORR kinetic of the thick PrO2/LSCF is nearly 6 times faster than that of the bare LSCF at 600 °C in air. The CeO2/LSCF samples showed only a slight enhancement in oxygen reduction kinetics. The 7 ACS Paragon Plus Environment


Page 8 of 28

performance of the PCO/LSCF was between that of the PrO2/LSCF and that of the CeO2/LSCF electrode. The effect of the modification layer thickness was also studied. For both PrO2 and PCO modification, the ORR kinetics increased with the thickness of the modification layer, leading to higher kq value for a thicker layer (Figure 2 c and d). In contrast, for the CeO2 modification, the performance became worse with the increasing thickness of the top CeO2 layer.The dependence of kq on oxygen partial pressure of the modified LSCF and corresponding single phase were fitted using the following equation kq ∝ p𝑂𝑚 2

(3)

where pO2 is the oxygen partial pressure. The exponent m can provide information about the species involved in the rate limiting step (RLS) of the ORR. Considering a simple mode for the ORR, the reaction steps and the corresponding exponent m are as follows: 42-45 O2,gas ⇔ O2,ads; m=1

(4)

O2,ads ⇔ 2Oads; m=0.5

(5)

O2, ads  4e -  2 V'O'  2 OOx ; m=0.25 (6)


Page 9 of 28 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


Figure 2. (a) Schematics of the EIS measurement setup. (b) Nyquist plot for the bare LSCF and the thick (~6nm) PrO2, PCO and CeO2 modified LSCF measured in air at 600 °C. The inset figure shows the equivalent circuit for the fitting of EIS data. (c-d) Temperature dependence of oxygen surface exchange coefficient, kq for the bare LSCF and the LSCF coated with a thin (c) and a thick (d) modification layer



Page 10 of 28

Figure 3. The oxygen partial pressure dependence of the kq for (a) PrO2/LSCF, (b) PCO/LSCF, (c) CeO2/LSCF and corresponding reference samples at 600 oC. The test pressure ranges from 1 bar to 10-3 bar. (a) PrO2/LSCF, (b) PCO/LSCF (c) CeO2/LSCF. Thin and thick are referred to the thickness of the decoration layer, which are 2 nm and 6 nm, respectively. Pure is referred to bare PrxCe1−xO2−δ films deposited on YSZ with GDC as the buffer layer. After surface modification, the dependence of kq on oxygen pressure showed noticeable changes. For bare LSCF, m was close to 0.5 (Figure 3), indicating that the dissociation of molecular oxygen to atomic oxygen is likely to be RLS (O2,ads ⇔ 2Oads).42-45 This result is consistent with previous report for mixed ionic and electronic conductors (MIEC) such as LSCF and LSC, which have sufficiently high electronic and ion conductivity.31, 32 The m value for the PrO2 modified LSCF was between 0.5 and 0.25, implying that the rate of ORR may be limited by both oxygen dissociation and charge transfer process. The m value for the PCO and the CeO2 modified LSCF electrode were close to 0.25, indicating that the charge transfer process ( O2, ads  4e -  2 V'O'  2 OOx ) may be the rate-limiting step of ORR. It needs to note that the ORR reaction steps proposed above may contain several elemental steps. Determining the exact elemental step as the RLS is beyond the scope of this work. Our EIS results, however, provided evidence that surface modifications critically influence the mechanism and kinetics of ORR, due to the different oxygen defects chemistry of the top layer, as to be discussed in a later section. 10 ACS Paragon Plus Environment

Page 11 of 28

Besides the initial ORR activity, long-term stability of bare LSCF and modified LSCF were also compared. The kq of all samples at 600 oC in air as a function of operation time are shown in Figure 4. The kq of the bare LSCF reduced continuously with operation time, becoming more than 6 times lower than the initial value after 60 hours operation. In contrast, all surface modified electrodes showed enhanced stability. In particular, the LSCF electrode coated with a 6 nm thick CeO2 layer showed much higher kq after long time operation, although the initial kq was slightly lower than that of the bare LSCF.

-7

10

PrO2 thin

PrO2 thick

PCO thin CeO2 thin

PCO thick CeO2 thick

LSCF

q

-1

k (cms )

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


-8

10

0

10

20

30

Time (h)

40

50

60

Figure 4. The kq of bare LSCF and modified LSCF at 600 oC in air for about 60 hours. The thin and thick are referred modification layer with the thickness of ~2nm and ~6nm. To understand the mechanism for the enhancement in stability of the surface modified LSCF, we have characterized the detailed surface morphology and composition of the bare LSCF and the surface modified LSCF electrodes before and after annealing at 600 oC in air. As shown in Figure 1b and Figure 5a, as prepared LSCF films presented very smooth surface. Auger Electron Spectroscopy (AES) results from different locations of the as-prepared LSCF surface did not show any noticeable difference (Figure 5b), indicating a chemically homogeneous surface. After annealing in air at 600 oC for 3 hours, 11 ACS Paragon Plus Environment

X


the LSCF surface became very rough, with particles presented on the surface (Figure 5c). HRXRD results showed no secondary phase form in bulk (Figure S6). As shown in AES element mapping of Sr on annealed LSCF, the particles that form on surface were Sr enriched phases with low concentration of Fe and Co. (Figure 5d, Figure S7) Further increasing the annealing temperature to 750 oC resulted in even larger Sr-enriched particles on the LSCF surface (Figure S8). Such preferential segregation of Sr to the surface after high temperature annealing have been widely observed in perovskite-based materials,4 such as (La,Sr)MnO3, 29, 46-48 Sr(Ti,Fe)O3,49 (Ba,Sr)(Co,Fe)O350 and (La,Sr)2CoO4.51 Due to the insulating nature of the Sr enriched particles, they physically block the active sites for oxygen reduction reaction and impede the paths for ionic and electronic transport. Therefore, we believed the segregation and separation of Sr cation on the surface is the reason for the degradation of LSCF at 600 oC

(Figure 4). 0302.8.map:

Company Name

2017 Mar 2 10.0 kV 0 nA FRR Sr1

1

LSCF as prep.

2

Intensity

(a)

500 nm

70.3

Sr1

La

(c)

LSCF 600 oC

500 nm

10.0 keV

400

Fe

Co

600

800

Sr 1600

Kinetic Ener gy (eV)

1800

(d)

1

2

O

C 200

50000 X

10.92 min RSF

(b)

Sr mapping

0.200 ?m

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 12 of 28

0.5 µm 3/2/2017 0.200 ?m



3 Figure 5. (a) SEM image of as prepared LSCF (b) Representative AES spectra from different location, 0.0

as marked in (a), on as prepared LSCF surface (c) SEM image of LSCF after annealing at 600oC in air, (d) AES mapping of Sr for annealed LSCF from the same location as for (c)). The brightness in (d) represents the intensity of Sr concentration.

10.0 keV

12

2.0 µm

ACS Paragon Plus Environment

Page 13 of 28 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 PrxCe1-xO2 modified LSCF films presented very smooth surface in their as prepared states (Figure 1 and Figure S4). After annealing at 600oC, although there were still particles showed up on the surface of PrO2/LSCF and PCO/LSCF surface (Figure 6 c-f), the coverage of these particles were much less than that on bare LSCF. For annealed CeO2/LSCF, only a few scattered particles were observed on the surface (Figure 6 g-h). These results indicate that the modification layer on top of LSCF can suppress Sr segregation and phase separation on the LSCF surface, which explains the enhanced stability of modified LSCF.

Figure 6. SEM images of the bare LSCF (a-b) and the LSCF modified by (c) thin PrO2, (d) thick PrO2, (e) thin PCO (f) thick PCO, (g) thin CeO2, and (h) thick CeO2 after annealing at 600°C in air for 3 hours; 13 ACS Paragon Plus Environment


Page 14 of 28

(i) Comparison of the Sr 3d XPS spectra for the bare LSCF and the modified LSCF in their as prepared state (left) and after annealing (right). The yellow and purple doublets are attributed to bulk coordinated Sr in LSCF lattice (Srlattice) and surface Sr species (Srnon-lattic). (j) Comparison of Srnon-lattic/Srlattice ratio for bare LSCF and modified LSCF. To further understand the influence of surface modification on the Sr segregation behavior in LSCF, we compared the chemical environment of Sr in the near surface for bare LSCF and modified LSCF. Sr 3d spectra of the bare LSCF and the LSCF with a thin surface modification layer in their as prepared state and after 600 oC annealing are presented in Figure 6i. The Sr 3d spectra were deconvoluted with two sets of spin–orbit split doublets. The doublets located at lower binding energy (Srlattice) are attributed to Sr within LSCF lattice, while the ones located at higher binding energy are attributed to Sr-OH, SrCO3 and under coordinated Sr (Srnon-lattice).4, 49, 51-54 The Srnon-lattice / Sr lattice ratio reflects the extent of Sr segregation and separation in bare LSCF and modified LSCF. The Sr non-lattice/Sr lattice ratio of bare LSCF showed large increase after annealing (Figure 6i-j), indicating a strong Sr segregation towards the surface. This is consistent with the AES results for bare LSCF as shown in Figure 5. After covering LSCF with a thin modification layer, we can see non-lattice Sr becomes much less pronounced both for as prepared and annealed samples. The Srnon-lattice/Srlattice ratios are in the order of LSCF> PrO2/LSCF> PCO/LSCF> CeO2/LSCF (Figure 6j), suggesting that CeO2 modification is most effective for suppressing Sr segregation in LSCF (Figure 6j), which is consistent with SEM results (Figure 6g-h). As shown in Figure 6 c-f, the particle density on PCO/LSCF surface is larger than that on PrO2/LSCF, but the particles size is considerably smaller. Although SEM is a powerful tool to reveal surface morphology of thin films, it cannot provide a quantitative comparison of the coverage of these surface particles among different samples. The XPS results provide direct evidence that the PCO can more successfully suppress Sr segregation than PrO2, which is consistent with the better stability of PCO/LSCF than PrO2/LSCF shown in Figure 4. For the 14 ACS Paragon Plus Environment

Page 15 of 28

LSCF with a thick modification layer (~6nm), due to the detection depth limit of XPS, we were not able to probe the Sr environment. Therefore, instead of XPS measurement, we did High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive Spectroscopy (EDS) characterization for the annealed thick CeO2/LSCF film. As shown in Figure 7, there was no clear Sr segregation and secondary phase formation near the interface of CeO2 and LSCF. Due to the surface damage introduced by FIB milling during sample preparation, we were not able to obtain the local structure information of Sr segregation on the top surface of bare LSCF with TEM measurement. Using Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS, Figure S10), the presence of Sr segregation on LSCF surface and the suppression effect of PCO coating were observed and were consistent with our XPS and SEM results.

(a)

(b)

YSZ 0.5 mm

(e)

Fe Co Sr La

CeO 2 5.6 nm

GDC

40 LSCF

2 nm

Ce

60

CeO 2

LSCF 25.0 nm

Substr ate

20

CeO 2 GDC 37.8 nm

0

LSCF

Distance (nm)

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


80 Intensity (a.u.)

Sur face

Figure 7. (a) HRTEM images of thick (~6 nm )CeO2/LSCF thin film after annealing at 600 oC for 3 hours in air; (b) EDS analysis from the substrate to the surface. Considering the essential role of oxygen vacancy for ORR activity and stability for LSCF, we 15 ACS Paragon Plus Environment


Page 16 of 28

hypothesize that the mechanism for the different improvement of ORR performance for PrO2, PCO and CeO2 modified LSCF are related to their very different oxygen defect chemistry. To provide more insight into the underlying mechanism, we did theoretical computation of the oxygen vacancy formation energy 𝐸𝑣𝑎𝑐 for PrO2, PCO and CeO2. The calculation details can be found in the Supporting Information Section 2. As shown in Figure 8a, we found that 𝐸𝑣𝑎𝑐 decreased with the increase of Pr dopant concentration in the CeO2 host materials, which are consistent with previous experiment and theoretical work.20, 34, 35 The reason for the different improvement in ORR activity for PrO2, PCO and CeO2 modified LSCF can be related their different oxygen defect concentration (Figure 8b). The oxygen vacancy formation energy (Evac) in PrO2 (0.91eV) is found to be much lower than that in LSCF (1.3-2.4 eV).20, 55 Depositing a PrO2 layer onto LSCF can lead to higher concentration of oxygen vacancies on surface compared with the bare LSCF, which strongly facilitate the dissociation and incorporation of oxygen molecular oxygen, leading to a significant enhancement of ORR activity for PrO2 coated LSCF.32, 56, 57 This interpretation is consistent with our previous near ambient XPS results. 20CeO2, on the other hand, shows very high oxygen vacancy formation energy (3.7eV) (Figure 8 a). As a result, CeO2 single phase exhibited very low kq, which was also independent of oxygen partial pressure (Figure 3c). Covering LSCF with a CeO2 lead to a blocking effect for the absorption of oxygen gas molecular and charge transfer process. Therefore, CeO2 modification showed the smallest improvement in ORR activity. Furthermore, due to the different oxygen vacancy concentration in the top layer, the ORR activity got improve when PrO2 thickness increased, but decreased with CeO2 top layer thickness. The oxygen formation energy of PCO was between PrO2 and CeO2, leading to less improvement in ORR activity compared with PrO2/LSCF but better performance than CeO2/LSCF. Oxygen defect concentration on the top layer of modified LSCF can also impact the Sr segregation behavior of LSCF. This is why kq of the modified LSCF does not completely follow the order of oxygen 16 ACS Paragon Plus Environment

Page 17 of 28 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


vacancy formation energy of the modification layer after long time operation (Figure 4).Our recent results showed that the oxygen defect chemistry of perovskite oxides were strongly impacted by the presence of hetero-interfaces.58 A less reducible CeO2 on top of LSCF leads to a decrease in oxygen vacancies on the LSCF surface. As a results, the electrostatic attraction between the positively charged oxygen vacancy and negatively charge Sr dopant decreased, resulting in a decrease in the driving force for Sr to segregate to the surface.29 Therefore, CeO2/LSCF presents much less Srnon-lattice than the bare LSCF does. This effect is similar to the enhanced stability of LSC by surface modification of less reducible oxides reported by Tsvetkov et al.13 By doping Pr into CeO2, the oxygen formation energy decreases and surface oxygen concentration increases. As a result, PrO2 and PCO showed less apparent effect in suppressing Sr segregation. The reason why a more reducible PrO2 on top of LSCF can still improve the stability of LSCF is that the PrO2 layer physically blocks the direct contact of the LSCF with CO2 and H2O. As is well known, direct exposure of LSCF to CO2 and H2O in the air may facilitate the formation of insulating phases such as Sr(OH)2 and SrCO34, 49, 51 It needs to note that, besides oxygen vacancy formation energy, the different substitution energy of Sr in PrO2, PCO and CeO2 (Figure 8) can also contribute to the variation of Sr segregation in LSCF. CeO2 with highest Sr substitution energy can better suppress Sr segregated from the bulk of LSCF to the interface and surface of hetero-structure. To compensate the charge difference, substituting Pr or Ce with Sr is normally accompanied with the formation of oxygen vacancies. Therefore, the large Sr substitution energy in CeO2 is correlated with its high oxygen vacancy formation energy. Our results demonstrate that surface oxygen vacancies have critical impact on the ORR kinetics and stability of perovskite electrodes. While high oxygen vacancies promote the ORR reaction, it may destabilize the surface by introducing more surface dopant segregation. Therefore, an ideal material design can be the use of composite coating which can both block the Sr segregation and provide oxygen defects on the surface to facilitate the reaction, as demonstrated in some of recent works.20, 33 It is also worth to emphasis that besides materials characteristics, the microstructure of the top layer also strongly 17 ACS Paragon Plus Environment


Page 18 of 28

impact on the ORR activity of the hetero-structure. For instance, we found the ORR activity of PrO2/LSCF and PCO/LSCF increased with the thickness of top layer. In contrast, a thicker the top CeO2 lead to a worse ORR activity. Therefore, one reason for the large variation in performance of surface modified electrode in literature can be the different microstructure such as thickness and density of the surface coating. Techniques such as atomic layer deposition 59 that can realize surface modification with well controlled thickness and composition are believed to be powerful tools for the synthesis of high performance electrode material.

Figure 8. (a) Vacancy formation energy (Black square) and Sr substitution energy (Blue sphere) in CeO2, PCO (Pr0.25Ce0.75O2) and PrO2. PCO have two scenarios in Sr substituting Pr and Ce site. Dashed line is for the guiding of eyes. The inset figure shows the spin charge density of PCO with an oxygen vacancy in the cell. (b) Illustration shows the critical impact of oxygen vacancies in the modification layer on the performance of hetero-structure electrodes. High densities of oxygen vacancy in the top layer can facilitate oxygen dissociation and incorporation on the surface, leading to high ORR activity, as for the case of PrO2/LSCF. Low concentration oxygen in the top layer can help suppress Sr segregation to the surface, leading to better stability, as exampled by CeO2/LSCF. 4. Conclusion 18 ACS Paragon Plus Environment

Page 19 of 28 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


In summary, we have systematically investigated the effect of surface modification on LSCF electrode performance, providing critical insight into the role of oxygen defect in the surface modification layer on the ORR activity and stability of oxide hetero-structure electrodes. In particular, LSCF thin-film electrodes with PrxCe1-xO2-δ (x=0, 0.2, 1) coating are synthesized by PLD to serve as the model system with well-defined hetero-structures for fundamental studies. Among the coating materials studied, PrO2 modification displays the most pronounced enhancement in ORR activity, attributed largely to the high oxygen vacancy concentration of PrO2. Degradation in performance of a bare LSCF electrode was due mainly to surface Sr segregation at high temperature during operation. Such Sr segregation is successfully suppressed by the application of a surface coating, leading to greatly improved stability. The effects of suppressing Sr segregation follow the order of CeO2/LSCF > PCO/LSCF > PrO2/LSCF, because oxygen vacancy formation energy and Sr substitution energy increases as the Pr content is reduced. Not only Sr solubility is the lowest in CeO2, its lowest oxygen vacancy concentration in CeO2 helps to suppress the Sr segregation to the surface. These results demonstrate the vital role of surface oxygen defects and point to a potential direction for achieving highperformance electrodes through surface defect engineering. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional experimental data including pulsed Laser Deposition Target synthesis ;XRD patterns of PLD target after sintering (S1); HRXRD patterns of modified LSCF and corresponding single phase reference samples (S2);Resistance comparison between porous Ag-YSZ and LSCF(S3); SEM image of the modified LSCF in their as prepared states (S4); Nyquist plot for modified LSCF(S5); HRXRD patterns of thin film before and after annealing(S6);SEM and AES maps for LSCF(S7);XPS and EDS analysis on annealed LSCF surface(S8);La 3d 5/2 and Ce 3d XPS spectra of PCO single phase and PCO modified LSCF (S9); TOF-SIMS pattern of modified LSCF and bare LSCF(S10) and DFT calculation 19 Environment ACS Paragon Plus


Page 20 of 28

of oxygen vacancy formation energy and Sr substitution energy; Summary of fitting parameters for Sr 3d peak (Table S1). AUTHOR INFORMATION Corresponding Author *(Y. C.)E-mail address: [email protected]. *(M. L.)E-mail address: [email protected]. ORCID *Yan Chen : 0000-0001-6193-7508 *Meilin Liu: 0000-0002-6188-2372 Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by Guangzhou Science and Technology Program General Projects (No. 201707010146), the Fundamental Research Funds for the Central Universities (2018MS40), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200), the National Natural Science Foundation of China (11605063, 91745203), the Science and Technology Planning Project of Guangdong Province, China (No. 2017B090916002), the Recruitment Program of Global Youth Experts of China. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

20 Environment ACS Paragon Plus

Page 21 of 28 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


REFERENCES 1. Zheng, Y.; Wang, J. C.; Yu, B.; Zhang, W. Q.; Chen, J.; Qiao, J. L.; Zhang, J. J., A Review of High Temperature Co-electrolysis of H2O and CO2 to Produce Sustainable Fuels Using Solid Oxide Electrolysis Cells (SOECs): Advanced Materials and Technology. Chemical Society Reviews 2017, 46, 1427-1463. 2. Wachsman, E. D.; Marlowe, C. A.; Lee, K. T., Role of Solid Oxide Fuel Cells in a Balanced Energy Strategy. Energy & Environmental Science 2012, 5, 5498-5509. 3. Zhang, Y.;

Knibbe, R.;

Sunarso, J.;

Zhong, Y. J.;

Zhou, W.;

Shao, Z. P.; Zhu, Z. H., Recent

Progress on Advanced Materials for Solid-Oxide Fuel Cells Operating Below 500 Degrees C. Adv. Mater. 2017, 29, 1700132-1700165. 4. Li, Y. F.; Zhang, W. Q.; Zheng, Y.; Chen, J.; Yu, B.; Chen, Y.; Liu, M. L., Controlling Cation Segregation in Perovskite-based Electrodes for High Electro-catalytic Activity and Durability. Chemical Society Reviews 2017, 46, 6345-6378. 5. Ding, D.; Li, X. X.; Lai, S. Y.; Gerdes, K.; Liu, M. L., Enhancing SOFC Cathode Performance by Surface Modification through Infiltration. Energy & Environmental Science 2014, 7, 552-575. 6. Zhu, Y.; Zhou, W.; Ran, R.; Chen, Y.; Shao, Z.; Liu, M., Promotion of Oxygen Reduction by Exsolved Silver Nanoparticles on a Perovskite Scaffold for Low-temperature Solid Oxide Fuel Cells. Nano letters 2015, 16, 512-518. 7. Ding, D.; Liu, M. F.; Liu, Z. B.; Li, X. X.; Blinn, K.; Zhu, X. B.; Liu, M. L., Efficient ElectroCatalysts for Enhancing Surface Activity and Stability of SOFC Cathodes. Adv. Energy Mater. 2013, 3, 1149-1154. 8. Crumlin, E. J.; Mutoro, E.; Ahn, S. J.; la O, G. J.; Leonard, D. N.; Borisevich, A.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y., Oxygen Reduction Kinetics Enhancement on a Heterostructured Oxide Surface for Solid Oxide Fuel Cells. J. Phys. Chem. Lett. 2010, 1, 3149-3155. 9. Sase, M.; Yashiro, K.; Sato, K.; Mizusaki, J.; Kawada, T.; Sakai, N.; Yamaji, K.; Horita, T.; Yokokawa, H., Enhancement of Oxygen Exchange at the Hetero Interface of (La,Sr)CoO3/(La,Sr) 21 Environment ACS Paragon Plus


2CoO4

Page 22 of 28

in Composite Ceramics. Solid State Ionics 2008, 178, 1843-1852.

10. Lee, D.; Lee, Y. L.; Hong, W. T.; Biegalski, M. D.; Morgan, D.; Shao-Horn, Y., Oxygen Surface Exchange Kinetics and Stability of (La,Sr)2CoO4 ±δ/La1-xSrxMO3-δ(M = Co and Fe) Hetero-interfaces at Intermediate Temperatures. Journal of Materials Chemistry A 2015, 3, 2144-2157. 11. Hayd, J.; Yokokawa, H.; Ivers-Tiffée, E., Hetero-Interfaces at Nanoscaled (La, Sr) CoO3-δ Thin-Film Cathodes Enhancing Oxygen Surface-exchange Properties. Journal of The Electrochemical Society 2013, 160, F351-F359. 12. Mutoro, E.; Crumlin, E. J.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y., Enhanced Oxygen Reduction Activity on Surface-decorated Perovskite Thin Films for Solid Oxide Fuel Cells. Energy & Environmental Science 2011, 4, 3689-3696. 13. Tsvetkov, N.; Lu, Q.; Sun, L.; Crumlin, E. J.; Yildiz, B., Improved Chemical and Electrochemical Stability of Perovskite Oxides with Less Reducible Cations at the Surface. Nat Mater 2016, 15, 10101016. 14. Rupp, G. M.; Opitz, A. K.; Nenning, A.; Limbeck, A.; Fleig, J., Real-time Impedance Monitoring of Oxygen Reduction during Surface Modification of Thin Film Cathodes. Nature Materials 2017, 16, 640-645. 15. Li, M.; Zheng, M. H.; Hu, B. B.; Zhang, Y. X.; Xia, C. R., Improving Electrochemical Performance of Lanthanum Strontium Ferrite by Decorating Instead of Doping Cobaltite. Electrochim. Acta 2017, 230, 196-203. 16. Cox-Galhotra, R. A.; McIntosh, S., Unreliability of Simultaneously Determining kchem and Dchem via Conductivity Relaxation for Surface-modified La0.6Sr0.4Co0.2Fe0.8O3-δ. Solid State Ionics 2010, 181, 1429-1436. 17. Huang, Y. Y.; Vohs, J. M.; Gorte, R. J., An Examination of LSM-LSCo Mixtures for Use in SOFC Cathodes. Journal of the Electrochemical Society 2006, 153, A951-A955. 18. Kong, Q.; Kim, D.; Liu, C.; Yu, Y.; Su, Y.; Li, Y.; Yang, P., Directed Assembly of Nanoparticle Catalysts on Nanowire Photoelectrodes for Photoelectrochemical CO2 Reduction. Nano Lett 2016, 16, 22 Environment ACS Paragon Plus

Page 23 of 28 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


5675-5680. 19. Saher, S.; Naqash, S.; Boukamp, B. A.; Hu, B. B.; Xia, C. R.; Bouwmeester, H. J. M., Influence of Ionic Conductivity of the Nano-particulate Coating Phase on Oxygen Surface Exchange of La0.58Sr0.4Co0.2Fe0.8O3-δ .Journal of Materials Chemistry A 2017, 5, 4991-4999. 20. Chen, Y.; Chen, Y.; Ding, D.; Ding, Y.; Choi, Y.; Zhang, L.; Yoo, S.; Chen, D.; deGlee, B.; Xu, H.; Lu, Q.; Zhao, B.; Vardar, G.; Wang, J.; Bluhm, H.; Crumlin, E. J.; Yang, C.; Liu, J.; Yildiz, B.; Liu, M., A Robust and Active Hybrid Catalyst for Facile Oxygen Reduction in Solid Oxide Fuel Cells. Energy Environ. Sci. 2017, 10, 964-971. 21. Chen, Y.; Cai, Z. H.; Kuru, Y.; Ma, W.; Tuller, H. L.; Yildiz, B., Electronic Activation of Cathode Superlattices at Elevated Temperatures - Source of Markedly Accelerated Oxygen Reduction Kinetics. Adv. Energy Mater. 2013, 3, 1221-1229. 22. Feng, Z. X.; Yacoby, Y.; Gadre, M. J.; Lee, Y. L.; Hong, W. T.; Zhou, H.; Biegalski, M. D.; Christen, H. M.; Adler, S. B.; Morgan, D.; Shao-Horn, Y., Anomalous Interface and Surface Strontium Segregation in

(La1-ySry)2CoO4 ±δ/La1-xSrxCoO3-δ Heterostructured Thin Films. J. Phys. Chem. Lett.

2014, 5, 1027-1034. 23. Han, J. W.; Yildiz, B., Mechanism for Enhanced Oxygen Reduction Kinetics at the (La,Sr)CoO3-δ /(La,Sr)2CoO4±δ Hetero-interface. Energy & Environmental Science 2012, 5, 8598-8607. 24. Gadre, M. J.; Lee, Y. L.; Morgan, D., Cation Interdiffusion Model for Enhanced Oxygen Kinetics at Oxide Heterostructure Interfaces. Physical Chemistry Chemical Physics 2012, 14, 2606-2616. 25. Feng, Z. L. A.; El Gabaly, F.; Ye, X. F.; Shen, Z. X.; Chueh, W. C., Fast Vacancy-mediated Oxygen Ion Incorporation across the Ceria-gas Electrochemical Interface. Nature Communications 2014, 5,4374-4384. 26. Grimaud, A.; Hong, W. T.; Shao-Horn, Y.; Tarascon, J. M., Anionic Redox Processes for Electrochemical Devices. Nature Materials 2016, 15, 121-126. 27. Lee, Y. L.; Kleis, J.; Rossmeisl, J.; Shao-Horn, Y.; Morgan, D., Prediction of Solid Oxide Fuel Cell Cathode Activity with First-principles Descriptors. Energy & Environmental Science 2011, 4, 396623 Environment ACS Paragon Plus


Page 24 of 28

3970. 28. Lee, Y. L.; Lee, D.; Wang, X. R.; Lee, H. N.; Morgan, D.; Shao-Horn, Y., Kinetics of Oxygen Surface Exchange on Epitaxial Ruddlesden-Popper Phases and Correlations to First-Principles Descriptors. J. Phys. Chem. Lett. 2016, 7, 244-249. 29. Lee, W.; Han, J. W.; Chen, Y.; Cai, Z.; Yildiz, B., Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites. Journal of the American Chemical Society 2013, 135, 7909-7925. 30. Li, F.; Li, Y.; Chen, H.; Li, H.; Zheng, Y.; Zhang, Y.; Yu, B.; Wang, X.; Liu, J.; Yang, C.;

Chen,

Y.; Liu, M., Impact of Strain-Induced Changes in Defect Chemistry on Catalytic Activity of Nd2NiO4 +δ Electrodes. ACS applied materials & interfaces 2018, DOI: 10.1021/acsami.8b11877. 31. Merkle, R.; Maier, J.; Bouwmeester, H. J. M., A Linear Free Energy Relationship for Gas-solid Interactions: Correlation between Surface Rate Constant and Diffusion Coefficient of Oxygen Tracer Exchange for Electron-rich Perovskites. Angew. Chem.-Int. Edit. 2004, 43, 5069-5073. 32. Kuklja, M. M.; Kotomin, E. A.; Merkle, R.; Mastrikov, Y. A.; Maier, J., Combined Theoretical and Experimental Analysis of Processes Determining Cathode Performance in Solid Oxide Fuel Cells. Physical Chemistry Chemical Physics 2013, 15, 5443-5471. 33. Chen, Y.; Yoo, S.; Pei, K.; Chen, D.; Zhang, L.; deGlee, B.; Murphy, R.; Zhao, B.; Zhang, Y.; Chen, Y.; Liu, M., An In Situ Formed, Dual-Phase Cathode with a Highly Active Catalyst Coating for Protonic Ceramic Fuel Cells. Advanced Functional Materials 2017, 1704907-1704914. 34. Ahn, K.; Yoo, D. S.; Prasad, D. H.; Lee, H.-W.; Chung, Y.-C.; Lee, J.-H., Role of Multivalent Pr in the Formation and Migration of Oxygen Vacancy in Pr-Doped Ceria: Experimental and First-Principles Investigations. Chemistry of Materials 2012, 24, 4261-4267. 35. Bishop, S. R.; Stefanik, T. S.; Tuller, H. L., Defects and Transport in PrxCe1−xO2−δ: Composition trends. Journal of Materials Research 2012, 27, 2009-2016. 36. Chen, D.; Bishop, S. R.; Tuller, H. L., Nonstoichiometry in Oxide Thin Films Operating under Anodic Conditions: A Chemical Capacitance Study of the Praseodymium-Cerium Oxide System. 24 Environment ACS Paragon Plus

Page 25 of 28 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


Chemistry of Materials 2014, 26, 6622-6627. 37. Chen, D.; Bishop, S. R.; Tuller, H. L., Praseodymium-cerium Oxide Thin Film Cathodes: Study of Oxygen Reduction Reaction Kinetics. Journal of Electroceramics 2012, 28, 62-69. 38. Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for ab initio Total-energy Calculations using a Plane-wave Basis Set. Phys Rev B 1996, 54, 11169-11186. 39. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys Rev Lett 1996, 77, 3865-3868. 40. Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-zone Integrations. Phys Rev B 1976, 13, 5188-5192. 41. Niwa, E.; Hashimoto, T., Dependence of Thermal Expansion of LaNi0.6Fe0.4O3−δ and La0.6Sr0.4Co0. 2Fe0. 8O3− δ on

Oxygen Partial Pressure. Solid State Ionics 2016, 285, 187-194.

42. Takeda, Y.; Kanno, R.; Noda, M.; Tomida, Y.; Yamamoto, O., Cathodic Polarization Phenomena of Perovskite Oxide Electrodes with Stabilized Zirconia. Journal of the Electrochemical Society 1987, 134, 2656-2661. 43. Chen, D.; Ran, R.; Zhang, K.; Wang, J.; Shao, Z., Intermediate-temperature Electrochemical Performance of a Polycrystalline PrBaCo2O5+δ Cathode on Samarium-doped Ceria Electrolyte. Journal of Power Sources 2009, 188, 96-105. 44. Chueh, W. C.; Haile, S. M., Electrochemistry of Mixed Oxygen Ion and Electron Conducting Electrodes in Solid Electrolyte Cells. Annual Review of Chemical and Biomolecular Engineering, 2012, 3, 313-341. 45. Siebert, E.; Hammouche, A.; Kleitz, M., Impedance Spectroscopy Analysis of La1-x SritxMnO3yttria-stabilized Zirconia Electrode Kinetics. Electrochim. Acta 1995, 40, 1741-1753. 46. Huber, A.-K.; Falk, M.; Rohnke, M.; Luerssen, B.; Amati, M.;

Gregoratti, L.;

Hesse, D.; Janek,

J., In Situ Study of Activation and De-activation of LSM Fuel Cell Cathodes – Electrochemistry and Surface Analysis of Thin-film Electrodes. Journal of Catalysis 2012, 294, 79-88. 47. Wang, W.; Jiang, S., A Mechanistic Study on the Activation Process of (La, Sr)MnO3 Electrodes of 25 Environment ACS Paragon Plus


Page 26 of 28

Solid Oxide Fuel Cells. Solid State Ionics 2006, 177, 1361-1369. 48. Hu, B.; Mahapatra, M. K.; Singh, P., Performance Regeneration in Lanthanum Strontium Manganite Cathode during Exposure to H2O and CO2 Containing Ambient Air Atmospheres. Journal of the Ceramic Society of Japan 2015, 123, 199-204. 49. Chen, Y.; Jung, W.; Cai, Z.; Kim, J. J.; Tuller, H. L.; Yildiz, B., Impact of Sr Segregation on the Electronic Structure and Oxygen Reduction Activity of SrTi1−xFexO3 Surfaces. Energy & Environmental Science 2012, 5, 7979. 50. Kim, Y.-M.; Chen, X.; Jiang, S. P.; Bae, J., Effect of Strontium Content on Chromium Deposition and Poisoning in Ba1− xSrxCo0. 8Fe0. 2O3− δ (0.3≤ x≤ 0.7) Cathodes of Solid Oxide Fuel Cells. Journal of The Electrochemical Society 2011, 159, B185-B194. 51. Chen, Y.; Téllez, H.; Burriel, M.; Yang, F.; Tsvetkov, N.; Cai, Z.; McComb, D. W.; Kilner, J. A.; Yildiz, B., Segregated Chemistry and Structure on (001) and (100) Surfaces of (La1–xSrx)2CoO4 Override the Crystal Anisotropy in Oxygen Exchange Kinetics. Chemistry of Materials 2015, 27, 54365450. 52. Van Der Heide, P., Systematic x‐Ray Photoelectron Spectroscopic Study of La1− xSrx‐based Perovskite‐type Oxides. Surface and interface analysis 2002, 33, 414-425. 53. Jiang, S. P.; Zhang, S.; Zhen, Y., Deposition of Cr species at (La, Sr)(Co, Fe) O3 Cathodes of Solid Oxide Fuel Cells. Journal of The Electrochemical Society 2006, 153, A127-A134. 54. Dupin, J.-C.; Gonbeau, D.; Vinatier, P.; Levasseur, A., Systematic XPS Studies of Metal Oxides, Hydroxides and Peroxides. Physical Chemistry Chemical Physics 2000, 2, 1319-1324. 55. Jun, A.; Yoo, S.; Gwon, O. H.; Shin, J.; Kim, G., Thermodynamic and Electrical Properties of Ba0.5Sr0.5Co0.8Fe0.2O3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ for Intermediate-temperature Solid Oxide Fuel Cells. Electrochim. Acta 2013, 89, 372-376. 56. Gupta, A.; Waghmare, U. V.; Hegde, M. S., Correlation of Oxygen Storage Capacity and Structural Distortion in Transition-Metal-, Noble-Metal-, and Rare-Earth-Ion-Substituted CeO2 from First 26 Environment ACS Paragon Plus

Page 27 of 28 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


Principles Calculation. Chemistry of Materials 2010, 22, 5184-5198. 57. Dholabhai, P. P.; Adams, J. B.; Crozier, P.; Sharma, R., Oxygen Vacancy Migration in Ceria and Prdoped Ceria: a DFT+U Study. J Chem Phys 2010, 132, 094104-094112. 58. Chen, Y.; Fong, D. D.; Herbert, F. W.; Rault, J.; Rueff, J.-P.; Tsvetkov, N.; Yildiz, B., Modified Oxygen Defect Chemistry at Transition Metal Oxide Hetero-structures Probed by Hard X-Ray Photoelectron Spectroscopy and X-ray Diffraction. Chemistry of Materials 2018, DOI: 10.1021/acs.chemmater.8b00808. 59. Meng, X. B.; Wang, X. W.; Geng, D. S.; Ozgit-Akgun, C.; Schneider, N.; Elam, J. W., Atomic Layer Deposition for Nanomaterial Synthesis and Functionalization in Energy Technology. Mater. Horizons 2017, 4, 133-154.



Table of Contents Graphic


Page 28 of 28

Improving the Electro-catalytic Activity and Durability of La0.6Sr0.4Co0

Recommend Documents