1

Replacement of Ca by Ni in a Perovskite Titanate to Yield a Novel Perovskite Exsolution

Architecture for Oxygen-Evolution Reactions

Jin Goo Lee,* Jae-ha Myung, Aaron B. Naden, Ok Sung Jeon, Yong Gun Shul, John T. S.

Irvine*

Dr. J. G. Lee, Dr. A. B. Naden, Prof. J. T. S. Irvine

School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK

E-mail: jtsi@st-and.ac.uk, jgl3@st-andrews.ac.uk

Prof. J. H. Myung

Department of Materials Science and Engineering, Incheon National University, Songdo-dong,

119 Academy-ro, Yeonsu-gu, Incheon, Republic of Korea

Dr. O. S. Jeon, Prof. Y. G. Shul

Department of Chemical and Bio-molecular Engineering, Yonsei University, 134 Shinchon-

dong, Seodaemun-gu, Seoul, 120-749, Republic of Korea

Keywords: perovskite, exsolution, hydration, oxygen-evolution reaction, electrochemical

catalysis

For efficient catalysis and electrocatalysis well-designed, high-surface-area support

architectures covered with highly dispersed metal nano-particles with good catalyst-support

interactions are required. In-situ grown Ni nano-particles on perovskites have been recently

reported to enhance catalytic activities in high-temperature systems such as solid oxide cells

(SOCs). However, the micron-scale primary particles prepared by conventional solid-state

reactions have limited surface area and tend to retain much of the active catalytic element

within the bulk limiting efficacy of such exsolution processes in low-temperature systems.

Here, we demonstrate a new, highly efficient, solvothermal route to exsolution from smaller

scale primary particles. Furthermore, unlike previous reports of B-site exsolution, it seems

that the metal nanoparticles are exsolved from the A-site of these perovskites. The catalysts

show large active site areas and strong metal-support interaction (SMSI), leading to ~ 26 %

higher geometric activity (25 times higher mass activity with 1.4 V of Eon-set) and stability for

oxygen-evolution reaction (OER) with only 0.72 μg base metal contents compared to typical

20 wt. % Ni/C and even commercial 20 wt.% Ir/C. The findings obtained here demonstrate

2

the potential design and development of heterogeneous catalysts in various low-temperature

electrochemical systems including alkaline fuel cells and metal-air batteries.

1. Introduction

Oxygen-evolution reaction (OER) is a key process for various energy storage technologies

such as rechargeable metal–air batteries (MxO2 → Mx + O2) and H2 production through water

splitting reactions (H2O → H2 + 1/2O2).[1-4] Since the reaction rate of the water electrolysis

largely depends on the OER (4OH- → 2H2O + O2 + 4e-) in alkaline media, RuO2 or IrO2 has

been typically considered as the catalyst to produce high OER activity. However, their high

costs limit scalable applications, and therefore a target for development of the OER catalysts

has been moved from precious metals into first-row transition metals and other oxides with

perovskite or spinel structures. This research trend suggests that controlling microstructure or

stoichiometry becomes highly important to generate maximized surface reactivity and strong

metal-support interactions (SMSI) using the latter. [5-10]

Morphology and microstructure plays a key role in determining the surface reactivity in

many fields including catalysis, photo-catalysis, and electrochemical energy storage and

conversion.[11-17] A variety of the preparation methods have been developed to build optimal

microstructures, such as impregnation and physical and chemical deposition.[18,19] However,

there is still limited control over distribution and anchorage of the deposited particles during

preparation or ageing, which means that metal particles on oxide supports can easily become

agglomerated because of the mobility of the particles. In-situ metal exsolution from A-site

deficient perovskite frameworks (A1-yMxB1-xO3-δ e.g. y=0.2, x=0.06) has been recently

reported for use in high-temperature catalysis applications such as in solid oxide cells

(SOCs).[20-25] Exciting nanoarchitectures with even and highly dispersed metal nano-particles

anchored on a perovskite framework have been observed and these are found to inhibit metal

agglomeration and carbon formation in reactions such as reforming. However, since the metal

exsolution is limited, a number of metal ions deep inside the micron scale perovskite bodies

3

have little opportunity for surface exsolution.[21] Furthermore, it is also important to optimise

the base perovskite active sites for low-temperature applications as it is known that perovskite

surfaces can act as catalysts for the oxygen-evolution reaction (OER).

To date the main approach towards designing metal exsolution from perovskites has been to

place the catalytically active transition metal ions on the B-site of perovskites (ABO3) in

oxidizing conditions and then to drive particles to form on the surfaces via a reduction process.

Previous studies reveal that A-site deficiency can serve as an effective driving force to trigger

stable B-site metal exsolution.[21-25] Such studies have typically involved larger A-site ions

such as Sr2+ or La3+ with larger A-site to B-site size ratios. There is also a possibility to

substitute transition metal ions onto the A-site of perovskites with Ca on the A-site as the

difference between Ca and transition metal, e.g. Ni, ionic radii is small compared to other

possible A-site to B-site pairings (Supplementary Figure S1). For example, Zhang et al.

recently reported that Pt ions can be moved onto the A-site of doped calcium titanate

perovskite films after reduction and oxidation.[26]

In this study, we investigate well-designed heterogeneous catalysts, Ni-exsolved CaTiO3

sub-micron particles (spherical and rectangular shapes), for OER as a near-ambient

temperature application. This improves the exsolution efficiency, the availability of

perovskite contributions to the reaction, and SMSI. Furthermore, the experimental data seem

to suggest that Ni nanoparticles can be initially exsolved from the A-site of perovskites, which

is quite different from the usually observed B-site exsolution process. The results shown here

demonstrate the potential of the metal exsolution for the design and development of low-

temperature electrochemical catalysts.

2. Results and discussion

2.1. Effects of perovskite scaffold on metal exsolution

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In order to overcome limitations of low support surface area as well as incomplete metal

exsolution at depth, reducing the size of the initial perovskite grains has been attempted for

Ni-doped CaTiO3. The approach was to solvothermally react at 180 °C for 10 h, a

stoichiometrically balanced mixture of Ca(NO3) and small (~300 nm) amorphous TiO2

nanospheres, that had been prepared via hydrolysis, with a 4% addition of NiO, with targeted

Ca:Ti: Ni ratio of 1: 1: 0.04, see Supplementary text and Figure S2 for details.[27] This is

perhaps a surprising ratio to drive exsolution as it might be expected to have B-site excess;

however the hydrolysis and subsequent solvothermal chemistry provides a different pathway.

Exsolution behavior from these samples was then compared with that of CaTi0.94Ni0.04O3-δ

prepared by conventional solid-state reaction. Crystal structures of both samples were defined

as orthorhombic structures and slightly smaller unit-cell volume appeared in the

solvothermally prepared Ni-doped CaTiO3 compared to CaTiO3 and Ni-doped CaTiO3

prepared by solid-state method (Supplementary Figure S3 and Table S1). Estimated

stoichiometry showed (Ca0.92Ni0.04)TiO2.96 and Ca(Ti0.96Ni0.04)O2.96 in the solvothermal and

solid-state samples, respectively (Table S2). These imply the potential of the A-site metal

exsolution from the solvothermally prepared (Ca0.92Ni0.04)TiO2.96, and it will be discussed in

the final section.

Exsolution from the submicron-spherical morphologies yields increased Ni particle

population on the perovskite scaffold compared to when Ni nanoparticles were exsolved from

the micron-scale Ca(Ti0.96Ni0.04)O2.96 (Figure 1). On exsolution, the spherical morphology

was preserved, although the spheres became less homogeneous in size with the diameter

slightly decreasing to 300-450 nm and also appear to being more porous following the

chemical changes due to exsolution and dehydration processes duringon this CaTiO3 thermal

treatment (Supplementary Figure S4). After reduction at 900 °C for 30 min at 900oC in 5%

H2/Ar, the peaks associated with Ni0 (~851.3 eV) and Ti3+ (456.2 eV) were observedfound in

the Ni 2p3/2 and Ti 2p3/2 spectra of XPS spectra-ray photoelectron spectroscopy (XPS)

5

(Supplementary Figure S5), which indicates that a large proportion of the Ni ions dissolved

within the perovskite lattices were exsolved on the CaTiO3 spheres, with a small,er but

significant proportion of Ti reduced to 3+. The scanning electron microscopy (SEM) image

clearly shows that Ni nanoparticles of ~ 20-30 nm size were exsolved on the approximately

450 nm CaTiO3 spheres with apparent porosity existing in the spheres (Figure 1b). These are

different from those produced at the micron scale Ca(Ti0.96Ni0.04)O2.96 grains prepared by

solid-state reactions where the 30-50 nm-sized Ni nanoparticles were exsolved on the μm size

CaTiO3 (Figure 1a). Comparing the physical characteristics of the two samples, one can

observe a large variation in specific surface area (SSA) (Figure 1c and Supplementary

Figure S6). The SSA of the Ni-exsolved CaTiO3 spheres was greatly increased to 17.310 m2

g-1 compared with 1.191 m2 g-1 of the micron scaled prepared by solid-state reactions and

2.981 m2 g-1 of the mixtures: wet-chemical (>10 m2 g-1), mechanochemical (<6 m2 g-1).[28-30]

The higher SSA value results from the submicron perovskite scaffold with nano-porous

structures and spherical morphology. Meanwhile, it is noticeable that Ni particle population

was remarkably increased from ~50 μm-2 on the micron scaled particles to ~ 150 μm-2 on the

submicron spheres notwithstanding shorter duration time for reduction (Figure 1c), which

corresponds to a 30 % increase in amount of exsolved Ni allowing for particle size differences.

This confirms that the perovskite size and morphologies are an important factor in the

exsolution process for better utilization of Ni ions and higher surface area of the perovskite

bodies.

2.2. Electrochemical characteristics

To prove the potential of the A-site metal exsolution from the perovskite scaffold

where size and morphologies are modified for low-temperature catalytic applications,

we compared electrochemical activities for oxygen-evolution reactions (OER). Firstly,

the electrochemically catalytic active surface areas (ECSAs) were compared among the

three samples: Ni/CaTiO3 mixtures, micron scale Ni-doped CaTiO3 by solid-state

6

reaction, and Ni-doped CaTiO3 submicron spheres by solvothermal synthesis (Figure

2a). The ECSA data were obtained from Supplementary Figures S7-S9. The as

prepared micron scaled particles showed an ECSA of only ~4.57 cm2, lower than 7.41

cm2 for a simple mixture, whereas the as-prepared submicron spheres were higher

(8.30 cm2) than the mixtures in the ECSA. It means that the perovskite substrate

contribution to the electrochemically catalytic reactions can be enhanced in the

submicron spheres owing to their high SSA value. The OER activity was slightly

higher in the as-prepared submicron spheres (1.34 mA cm-2geo) than the as-prepared

micron scaled particles (1.07 mA cm-2geo) at 1.7 V (Supplementary Figure S10).

Meanwhile, the ECSAs were significantly increased in both the Ni-exsolved CaTiO3

prepared by solid-state reactions and solvothermal synthesis, but the latter (16.82 cm2)

was higher than the former (14.87 cm2) (Figure 2a) (previous metal: >30 cm2,

transition metal oxides: ≥1 cm2, and perovskites: ~3 cm2 depending on synthesis

methods).[31-33] The geometric OER activities were 3.28 mA cm-2geo, 4.80 mA cm-2

geo,

and 10.88 mA cm-2geo at 1.7 V in the mixture, solid state prepared micron scale Ni-

exsolved, and Ni-exsolved CaTiO3 submicron spheres by solvothermal synthesis,

respectively (Figure 2b), which is comparable to state of the art results of both base

metals and perovskites (Supplementary Table S3).[34,35] The results clearly show that

the increase in Ni utilization from the porous submicron spheres can enhance the

electrochemical activities.

Eliciting high catalytic activity concurrently with low metal loading has been a key

issue in the microstructure design of catalysts because it is directly connected with

catalyst cost and system commercialization.[36] It is noteworthy that mass activity of

the solvothermally produced catalysts for the amount of Ni loading reached to ~2900

mA mg-1 at 1.7 V, which not only was 25 times higher mass activity than 20 wt. %

Ni/C (~110 mA mg-1) but also surpass the mass activities of 20 wt. % Ir/C (~180 mA

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mg-1) and other catalysts previously reported except one data of 1-nm thick

Ba0.5Sr0.5Co0.8Fe0.2O3−δ nanofilm coated on Ni foam (Figure 2c and Supplementary

Figure S11).[37,38] Despite a very small amount of Ni loading (0.72 μg), geometric

current density was also ~ 26 % higher in the Ni-exsolved CaTiO3 spheres than both 20

wt. % Ni/C and 20 wt. % NiO/C (Supplementary Figure S12). Here, interesting point

is that when we consider mass activities, on-set potential of the Ni-exsolved CaTiO3

submicron spheres was moved from 1.6 V into 1.4 V (~1.4 V of 20 wt.% Ir/C

catalysts) due to very small amount of base metals, which attests to the effectiveness of

the metal exsolution from the size- and morphology-modified perovskite scaffold for

the design of the low-temperature electrochemical catalysts. The selective exsolution

of transition metals and wide tenability of the host perovskite materials enable this

strategy to be applied to a wide range of fields as shown in Supplementary Figure

S13 which demonstrates good OER activities for various metal exsolved CaTiO3

catalysts that are competitive with state of the art systems.[22,39]

2.3. Strong metal-support interaction (SMSI)

Inhibiting degradations of metal nanoparticles is one of the key issues to retain stable

activities during reaction cycles.[37] In the electrochemical activity for OER, the degradation

rates after 500 cycles were approximately 19.4 % in the Ni/CaTiO3 mixtures (Figure 2d).

However, the Ni-exsolved CaTiO3 spheres significantly increased in the OER activity from

initial to 2000 cycles (Figure 2e). It may be due to in-situ progression of Ni exsolution via the

electrochemical activation.[21] The submicron spheres suffered only ~1.1 % degradation rate

from 2000 cycles to 4000 cycles (Figure 2e) and was stable in chronopotentiometry responses

at 10 mA cm-2 (Figure 2f). To identify metal amorphization and aggregation, we applied high

resolution transmission electron microscopy (HR-TEM) to the sample after the cycles

(Supplementary Figure S14). There is no evidence of either metal amorphization or

aggregation. The spherical morphologies were distorted, but nonetheless the size of Ni

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nanoparticles remained unchanged as 10~20 nm. It would seem to strongly indicate strong

metal-support interaction effects (SMSI) which have positive impacts on catalyst stability.[5-7]

In order to prove that, we measured Ni K-edge X-ray absorption near edge structure

(XANES) spectra on the samples (Figure 3a). All samples had the same resonance, thereby

indicating that there was metallic Ni at all the catalysts studied, there was also evidence some

higher oxidation state Ni in all the samples except metallic Ni foil, arising from unexolved Ni

within the CaTiO3 perovskites. The dominant XANES peak at the Ni K-edge, which is an

absorption threshold resonance, can be assigned to the allowed orbital transition from 1s to

4p.[40] The high intensity around 8350 eV shows that the orbital electron density of Ni atoms

is modified by strong interaction between the Ni nanoparticles and the CaTiO3 perovskite

supports.[41] This further indicates that the metal exsolution from the perovskites leads to

SMSI between the Ni nanoparticles and CaTiO3 perovskites due to the higher intensity at this

energy, than observed in the mixture (Figure 3b). The metal-support interactions on the

mixtures and the Ni-exsolved CaTiO3 can be further illustrated as shown in Figure 3c and 3d.

The TEM images show that the exsolved Ni nanoparticles were well embedded on the CaTiO3

perovskites compared to the mixtures mixed physically, which correspond with their

embedded nature (Figure 3c and 3d).

2.4. The Unusual Solid Chemistry of Ni-incorporation into these CaTiO3 Hosts

Undoubtedly the described materials are a very promising new variant of exsolution solids;

however, it transpires that the underlying stoichiometry relations may be quite different to that

of the previously described B-site substitutions (eg Neagu et al[22]). Obviously the crystal

stoichiometry from solution methods can vary from starting stoichiometry due to solubility,

so composition of the crystals needs checking. Initial EDX data on the solvothermal prepared

Ni-doped CaTiO3 clearly indicated a stoichiometry of Ca0.90(Ni0.04Ti0.96)Ox (Table S4). Such

low Ca stoichiometry is surprising in terms of anticipated charge balance for ABO3

perovskites and the overall values suggest possible A-site substitution by Ni, e.g.

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(Ca0.90Ni0.04)(Ti1)Ox so further studies were performed investigating unit cell parameters and

densities. First of all the densities of the solvothermally produced materials were significantly

lower than expected for normal CaTiO3 and the unit cell volumes larger than expected (Table

S5. S6 and Supplementary Figure S15). On heating the solvothermally prepared calcium

titanates in TGA there was a mass loss of 1.58 % and 1.78 %, at around 400 °C

(Supplementary Figure S16), consistent with dehydration of (Ca0.90Ni0.04)Ti1O2.88(OH)0.12

and Ca0.92Ti1O2.85(OH)0.14 to form 0.04Ni/Ca0.90Ti1O2.90 and Ca0.92Ti1O2.92, respectively

(Figure 4), which explains the low A-site stoichiometry.

3. Conclusion

we developed a well-designed heterogeneous electrocatalyst via Ni exsolution from the

CaTiO3 submicron spheres to increase active surface area, Ni utilization in surface exsolution

and the number of perovskite active sites for OER. Here, we found that the Ni ions can be

substituted on the A-site of CaTiO3, and exsolved as Ni nanoparticles in reducing conditions,

which is beneficial to enhance the reversibility between exsolution and re-incorporation and

to prevent alkaline earth metal segregations due to the formation of A-site deficiency after

exsolution. The solvothermal method of small particle synthesis yielded a number of

additional features, the incorporated hydroxyl groups altered stoichiometry and afforded Ca

deficiency. In turn this allowed A-site substitution by Ni and added well-organized nano-scale

porosity which in turn favoured electrochemical activity. The Ni-exsolved CaTiO3 spheres

yielded larger surface area and greater Ni population than the micron scale Ni-exsolved

CaTiO3 prepared by conventional solid-state reactions. It resulted in the higher OER activity

than its counterpart owing to the improvement in the electrochemically active sites of both Ni

nanoparticles and perovskite itself. The excellent OER stability of the Ni-exsolved CaTiO3

spheres was obtained because the SMSI was established between the exsolved Ni

nanoparticles and CaTiO3 spheres. We consider that the concept in this study can be applied

to a wide variety of catalytic fields owing to selective metal exsolution and flexibility of

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perovskite materials. Thus, the metal exsolution on perovskite frameworks can be a suitable

method to prepare well-designed heterogeneous catalysts in low-temperature electrochemical

systems, not only in high-temperature systems.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author. The

research data underpinning this publication can be accessed at

https://doi.org/10.17630/736ee412-dd08-4148-8357-707643dbeb0d

Acknowledgements

This research was supported by EPSRC (grant code: EP/R023522/1 and EP/P007821/1),

Basic Science Research Program through the National Research Foundation of Korea (NRF)

funded by the Ministry of Education (NRF-2017R1A6A3A03004416). Experiments at PLS-II

6D beamline were supported in part by MEST, POSTECH, and UNIST.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

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Figure 1. Comparison of physical characteristics in Ni-doped CaTiO3. a. Schematic

diagram of the micron-scale Ni-doped CaTiO3 prepared by conventional solid-state reaction,

and corresponding SEM image. Scale bar refers to 100 nm. b. Schematic diagram of the Ni-

doped CaTiO3 submicron spheres prepared by solvothermal synthesis, and corresponding

SEM image. Scale bar refers to 100 nm. c. Physical characteristics of the Ni-doped CaTiO3

prepared by solid-state reactions and solvothermal synthesis. SSA refers to specific surface

area.

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Figure 2. Electrochemical characteristics for oxygen-evolution reactions (OER). a.

Electrochemically catalytic active areas (ECSA) of the Ni/CaTiO3 mixture prepared by

physical mixing and the Ni-doped CaTiO3 prepared by conventional solid-state reactions and

solvothermal synthesis. b. OER activities of the Ni-exsolved CaTiO3 catalysts. Inset shows

mass activities of the solvothermally produced sample and 20 wt.% Ir/C. c. Mass activities

based on the amount of Ni loading (20 wt.% Ni/C and 20 wt.% Ir/C: 0.012mg and

solvothermal: ~0.72 μg). Inset represents mass activities based on the amount of catalysts

loading with perovskites and Ni (0.012 mg). M, SS, and ST indicate mixtures, solid-state, and

solvothermal, respectively. Ni/C and Ir/C contained 20 wt.% metals. d. OER stability of the

Ni/CaTiO3 mixture. e. OER stability of the Ni-exsolved CaTiO3 catalyst prepared by

solvothermal method. f. Chronopotentiometric responses of the Ni-exsolved CaTiO3 catalyst

prepared by solvothermal method. A constant current density was 10 mA cm-2disk. All

electrochemical measurements were carried out at 1600 rpm of a rotating speed and 10 mV s-1

of a scan rate in O2-saturated 0.1 M KOH.

Figure 3. Strong metal-support interaction (SMSI). a. Ni K-edge XANES spectra of the

Ni/CaTiO3 mixture and the Ni-exsolved CaTiO3 prepared by conventional solid-state reaction

and solvothermal synthesis. b. The Ni K-edge XANES spectra of the mixture and the Ni-

exsolved CaTiO3 prepared by solvothermal synthesis in the specific range represented as

dotted rectangular. The higher intensity of the peak around 8350 eV can imply the formation

of strong metal-support interaction (SMSI) effects. c. The schematic diagram of the

interaction between Ni and perovskites in the mixture and corresponding HR-TEM image.

Scale bar refers to 20 nm. d. The schematic diagram of the interaction between Ni and

15

perovskites in the Ni-exsolved CaTiO3, and corresponding HR-TEM image. Scale bar refers

to 10 nm.

Figure 4. A-site deficiency and structural OH contents of each samples. (CaNi)TiO3 (ST),

CaTiO3 (ST), and CaTiO3 (SS) refer to (Ca0.90Ni0.04)TiO2.88(OH)0.12, Ca0.92TiO2.85(OH)0.14, and

CaTiO3 synthesized by solid-state method, respectively: (ST) is solvothermal and (SS) is

solid-state. Each data was derived from EDX and TGA measurements.

16

Ni exsolution from A-site deficient hydrated CaTiO3 submicron spheres shows high activity

and stability for oxygen-evolution reactions (OER) due to increase in their active sites and

strong metal-support interaction (SMSI) between Ni and CaTiO3. Furthermore, structural OH

ions are involved in such A-site deficiency and Ni incorporation into the A-site of CaTiO3.

Keyword perovskite, exsolution, hydration, oxygen-evolution reaction, electrochemical

catalysis

Jin Goo Lee,* Jae-ha Myung, Aaron B. Naden, Ok Sung Jeon, Yong Gun Shul, John T. S.

Irvine*

Replacement of Ca by Ni in a Perovskite Titanate to Yield a Novel Perovskite Exsolution

Architecture for Oxygen-Evolution Reactions

17

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

Replacement of Ca by Ni in a Perovskite Titanate to Yield a Novel Perovskite Exsolution

Architecture for Oxygen-Evolution Reactions

Jin Goo Lee,* Jae-ha Myung, Aaron B. Naden, Ok Sung Jeon, Yong Gun Shul, John T. S.

Irvine*

18

Methods

TiO2 nano-spheres: Dodecylamine (1.2 g, Sigma Aldrich, 98 %) was dissolved in the mixed

solution of distilled water (1.5 g), acetonitrile (50 mL, Duksan), and ethyl alcohol (150 mL,

Duksan) with stirring for 10 min. Titanium(IV) isopropoxide (4 mL, Sigma Aldrich, 97 %)

was rapidly dropped in the solution, and then the solution was aged for 6 h. The precipitates

were centrifuged and washed for 1 time using ethyl alcohol and for 2 times using distilled

water. The amorphous TiO2 nano-spheres were dried at 80 °C overnight.

Ni-doped CaTiO3 submicron spheres: The Ni-doped CaTiO3 submicron spheres (4 mol% Ni)

were prepared by solvothermal synthesis. Poly ethylene glycol (30 mL, PEG-400, Sigma

Aldrich) was used as a solvent. The amorphous TiO2 nano-spheres were dispersed in the

solvent, and the desired stoichiometric amount of Ca(NO3)2·4H2O (Junsei Chemical) and

Ni(NO3)2·6H2O (Junsei Chemical) and 2 mmol NaOH (Sigma Aldrich) were added to the

solution. After stirring for 30 min, the solution was transferred into a Teflon-lined autoclave,

and remained at 180 °C for 10 h: the porosity of the CaTiO3 spheres may vary depending on

the reaction time. The precipitates were centrifuged, and then washed using acetone, 0.1 N

acetic acid and distilled water, respectively. The materials were dried at 80 °C for 1 h, and

calcined at 650 °C for 2 h in air. The Ni exsolution process was carried out at 900 °C for 30

min in 5 % H2/Ar. For comparison between A-site and B-site, Ti[OCH(CH3)2]4 was directly

used as a precursor to ensure exact compositions, which induces rectangular shapes of the

samples. Synthesis conditions were slightly modified as follows: temperature was 190 °C,

holding time was 20 h, and NaOH was 0.02 mol. As-prepared cubes were without heat

treatments. Reduction temperature was 750 °C for 20 h in 5% H2/Ar.

Micron-scale Ni-doped CaTiO3 particles: The CaTiO3 and micron-scale Ni-doped CaTiO3

were prepared by conventional solid-state reaction. Initially intended stoichiometry of Ni-

doped CaTiO3 was Ca(Ti0.94Ni0.04)O3-δ. Stoichiometric amount of CaCO3 (Sigma Aldrich),

TiO2 (Junsei Chemical), and Ni(NO3)2·6H2O (Junsei Chemical) was dispersed in acetone

19

(Sigma Aldrich), and stirred until the acetone was completely evaporated. The mixed powders

were transferred into an alumina crucible, and then calcined at 1000 °C for 20 h in air. The

calcined powders were ball-milled by using planetary milling machine, and then pelletized.

The pellets of CaTiO3 and Ni-doped CaTiO3 were sintered at 1400 °C for 10 h in air. These

were X-rayed to confirm sample purity, Figure S3. The Ni exsolution process was then

carried out by heating at 900 °C for 5 h in 5 % H2/Ar.

Ni/CaTiO3 mixture: The Ni/CaTiO3 mixture was prepared by physical mixing. The NiO (0.04

Ni as the molar ratio for CaTiO3, Junsei Chemical) and the CaTiO3 was ball-milled in ethyl

alcohol for 24 h, and dried at 80 °C for 1 day. The powders were calcined at 650 °C for 2 h in

air, and then reduced at 900 °C for 30 min in 5 % H2/Ar.

Materials characterizations: X-ray diffractometer (PANalytical, Empyrean) was used to

detect the crystal structures of the materials. The basic rietveld refinements were conducted

by using WinXPow program. Specific surface area was measured by using BELSORP-mini II

(BEL. Japan Inc.), flowing N2 gas. To confirm the morphologies of the materials, field-

emission scanning electron microscopy (FE-SEM, JEOL, JEOL-7800F) and spherical

aberration correction scanning transmission electron microscope (Cs-corrected STEM, JEOL,

JEM-ARM 200F) were used. Individual particles have been numbered on the basis of pixel

contrast. The changes in chemical status and elemental compositions were measured by X-ray

photoelectron spectroscopy (XPS, Thermo U. K., K-alpha). The specific gravities were

measured by using a density bottle and dichloromethane as a solvent. The elemental

compositions were detected by FIB equipped with EDX (Scios Duelbeam, Thermo Fisher

Scientific). In order to identify strong metal-support interaction (SMSI) effects, XAS

measurements at the Ni K-edge were collected in total electron yield (TEY) mode at Pohang

Light Source at the BL-6D beamline of the Pohang Accelerator Laboratory (PAL),

Gyeongsangbuk-do, Korea.

20

Electrochemically catalytic surface area (ECSA): Electrochemical surface areas (ECSAs)

were estimated by measuring the electrochemical capacitance of the electrode-electrolyte

interface in the double-layer regime of the voltammograms.

iDL = CDL∙ν

ECSA = CDL/Cs

where iDL is the capacitive current, CDL is the specific capacitance of the electrode double

layer, ν is the scan rate, and Cs is the specific capacitance. The cyclic voltammograms were

obtained in N2-saturated 0.1 M KOH from 0.05 V to 0.15 V versus Hg/HgO at different scan

rates between 5 and 100 mV s-1.

Electrochemical measurements: Electrode was prepared by drop-casting a catalyst ink. The

catalyst ink consisted of the catalysts (2 mg), carbon black (8 mg, VULCAN® XC72R,

surface area: ~220 m2 g-1), and Nafion (60 μL, 5 wt. %, Sigma Aldrich) in ethyl alcohol (1

mL). The catalyst ink was sonicated for 3 min, and then stirred for 30 min. The mixing

process was repeated more than 3 times. The catalysts ink (6 μL) was drop-cast onto a glassy

carbon electrode (0.196 cm2, Pine Instruments), yielding a perovskite loading of 0.06 mg cm-2.

OER measurements were performed with a rotating disk electrode setup. Pt and Hg/HgO in

saturated 1 M NaOH (~0.14 V versus reversible hydrogen electrode (RHE) for Pt) were used

as a counter and reference electrodes, respectively. A 0.1 M KOH (99.99 %, Sigma Aldrich)

electrolyte was prepared with distilled water. With respect to the measurements of the OER

activity on the catalysts, oxygen was saturated during OER cycling due to O2/HO- equilibrium

at 1.23 V versus RHE. The potential was measured by using a potentiostat (Biologic VSP

model) at a scan rate of 10 mV s-1 and a rotation speed of 1600 rpm. The

chronopotentiometric responses were obtained at a rotation speed of 1600 rpm and a constant

current density of 10 mA cm-2disk in O2-saturated 0.1 M KOH for 10 h. The potential,

measured against an Hg/HgO electrode, was converted to potential versus RHE according

to Evs RHE = Evs Hg/HgO + Eø Hg/HgO + 0.059 pH. All potentials represent iR-corrected potentials.

21

Figure S1. Relation between ionic radii and coordination numbers for Ti4+, Ni2+, Ca2+,

Sr2+ and Ba2+. The difference of ionic radii between Ni2+ ions and Ca2+ ions is relatively

small, especially in the crossover region between A-site and B-site domains, shaded area. The

dotted lines linking the radii values for each ion are simply a guide to the eye

22

Figure S2. Amorphous TiO2 nano-spheres. a. SEM image of the amorphous TiO2 nano-

spheres. b. Magnified SEM image of the amorphous TiO2 nano-spheres. Scale bar refers to 1

μm (a) and 100 nm (b), respectively.

23

24

Figure S3. X-ray diffraction patterns. a. (Ca0.92Ni0.04)TiO2.96 sphere, b. CaTiO3 grain and c.

Ca(Ti0.96Ni0.04)O2.96 grain. SS refers to solid-state synthesis and Bragg position of phase

CaTiO3 shows orthorhombic structures (Pbnm) of CaTiO3.

25

Figure S4. As-prepared Ni-doped CaTiO3 submicron spheres. a. SEM image of the as-

prepared Ni-doped CaTiO3 spheres. b. Magnified SEM image of the as-prepared Ni-doped

CaTiO3 spheres. Scale bar refers to 1 μm (a) and 100 nm (b), respectively.

26

Figure S5. X-ray photoelectron spectroscopy (XPS) results of Ni-doped and -exsolved

CaTiO3 spheres. a. and b. Ni 2p in the Ni-doped and -exsolved CaTiO3, respectively. c. and

d. Ti 2p in the Ni-doped and -exsolved CaTiO3, respectively. Calcination and reduction were

carried out at 650 °C in air and at 900 °C in 5% H2/Ar, respectively. The as-prepared samples

showed Ni2+ and Ti4+ oxidation states of Ni and Ti, respectively. Ni0 and a small amount of

the peak assigned as Ti3+ appeared upon reduction.

서식 지정함: 위 첨자

서식 지정함: 위 첨자

27

Figure S6. Specific surface area (SSA). The mixture refers to Ni/CaTiO3 prepared by

physical mixing. Solid-state reaction and solvothermal indicate the preparation methods for

the micro-scale Ni-doped CaTiO3 and Ni-doped CaTiO3 submicron spheres, respectively.

28

Figure S7. Electrochemically active surface area (ECSA). a. Cyclic voltammograms

measured for the Ni/CaTiO3 mixtures prepared by physical mixing in N2-saturated 0.1 M

KOH at different scan rates from 5 to 100 mV s-1. b. A plot of anodic and cathodic currents

measured at 0.1 V vs. Hg/HgO as a function of scan rate. The ECSA of the mixtures was 7.41

cm2.

29

Figure S8. Electrochemically active surface area (ECSA). a. Cyclic voltammograms

measured for the as-prepared micro-scale Ni-doped CaTiO3 prepared by solid-state reactions

in N2-saturated 0.1 M KOH at different scan rates from 5 to 100 mV s-1. b. A plot of anodic

and cathodic currents measured at 0.1 V vs. Hg/HgO as a function of scan rate. The ECSA of

the as-prepared micro-scales was 4.57 cm2. c. Cyclic voltammograms measured for the Ni-

exsolved micro-scale CaTiO3 in N2-saturated 0.1 M KOH at different scan rates from 5 to 100

mV s-1. d. A plot of anodic and cathodic currents measured at 0.1 V vs. Hg/HgO as a function

of scan rate. The ECSA of the Ni-exsolved micro-scale CaTiO3 was 14.87 cm2.

30

Figure S9. Electrochemically active surface area (ECSA). a. Cyclic voltammograms

measured for the as-prepared Ni-doped CaTiO3 submicron spheres prepared by solvothermal

synthesis in N2-saturated 0.1 M KOH at different scan rates from 5 to 100 mV s-1. b. A plot of

anodic and cathodic currents measured at 0.1 V vs. Hg/HgO as a function of scan rate. The

ECSA of the as-prepared submicron spheres was 8.30 cm2. c. Cyclic voltammograms

measured for the Ni-exsolved CaTiO3 submicron spheres in N2-saturated 0.1 M KOH at

different scan rates from 5 to 100 mV s-1. d. A plot of anodic and cathodic currents measured

at 0.1 V vs. Hg/HgO as a function of scan rate. The ECSA of the Ni-exsolved CaTiO3

submicron spheres was 16.82 cm2.

31

Figure S10. Electrochemical characteristics for oxygen-evolution reactions (OER). OER

activities of the as-prepared Ni-doped CaTiO3 catalysts. All electrochemical measurements

were carried out at 1600 rpm of a rotating speed and 10 mV s-1 of a scan rate in O2-saturated

0.1 M KOH.

32

Figure S11. Mass activities of each sample. a. Mass activities based on the amount of

catalysts loading with perovskites and Ni, b. Mass activities based on the amount of Ni

loading.

33

Figure S12. Catalytic activities for oxygen-evolution reactions (OER). Linear sweep

voltammograms of the 20 wt. % Ni/C and 20 wt. % NiO/C. Electrochemical measurements

were carried out at 1600 rpm of a rotating speed and 10 mV s-1 of a scan rate in O2-saturated

0.1 M KOH.

34

Figure S13. OER activities for different metal exsolution. Initially 4 mol % metal ions

were incorporated into CaTiO3 structures, and then exsolved on the surfaces. It proves

flexibility for metal selections in our proposed concept, and OER activities of each metal-

exsolved catalyst clearly show compatible activity to the catalysts with high contents of metal

loading previously reported by other researchers.[1,2]

Log Current density (mA cm-2

)

0.001 0.01 0.1 1 10

Po

ten

tia

l (V

vs.

RH

E)

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Ru

Ni

Co

35

Figure S14. HR-TEM images of the Ni-exsolved CaTiO3 spheres. a. HR-TEM image of

the Ni-exsolved CaTiO3 spheres after 4000 cycles of OER. Scale bar refers to 200 nm. The Ni

nano-particles with 10-20 nm size were remained on the CaTiO3 supports, but nano-spherical

morphologies were distorted. b. Magnified HR-TEM image of the Ni-exsolved CaTiO3

spheres after 4000 cycles of OER in the yellow dot circle. Scale bar refers to 10 nm. There

was no trace in surface amorphization.

36

Figure S15. X-ray diffraction patterns. a. Ca0.92TiO2.85(OH)0.14 and b.

(Ca0.90Ni0.04)TiO2.88(OH)0.12 showing that all peaks could indexed to orthorhombic CaTiO3

structure, see table

37

Temperature (℃)

100 200 300 400 500 600 700 800 900 1000

We

igh

t lo

ss (

%)

96

97

98

99

100

101

102

(Ca0.90Ni0.04)TiO2.88(OH)0.12

Ca0.92TiO2.85(OH)0.14

CaTiO3 by solid-state synthesis

Figure S16. TGA results. Weight losses of (Ca0.90Ni0.04)TiO2.88(OH)0.12,

Ca0.92TiO2.85(OH)0.14, and CaTiO3 prepared by solid-state method.

38

Table S1. Unit Cell parameters. Rietveld refinement was carried out for each sample. All

samples showed orthorhombic structure (pbnm) of CaTiO3. (Ca0.92Ni0.04)TiO2.96 was

synthesized by solvothermal method and then calcined at 650 °C in air. SS refers to solid-state

synthesis

(Ca0.92Ni0.04)TiO2.96 CaTiO3 (SS) Ca(Ti0.96Ni0.04)O2.96 (SS)

a (Ǻ) 7.6411 (10) 7.6475 (6) 7.6510 (8)

b (Ǻ) 5.4401 (3) 5.4452 (9) 5.4491 (6)

c (Ǻ) 5.3742 (3) 5.3838 (5) 5.3863 (6)

V (Ǻ3) 223.32 224.21 224.56

39

Table S2. Elemental compositions. (Ca0.92Ni0.04)TiO2.96 seems to have Ni ions incorporation

into Ca sites of CaTiO3 with the small amount of A-site deficiency. SS refers to solid-state

synthesis.

Element

(Atomic %) (Ca0.92Ni0.04)TiO2.96 CaTiO3 (SS) Ca(Ti0.96Ni0.04)O2.96 (SS)

Ca 45.88 50.33 50.09

Ti 50.01 49.67 47.85

Ni 2.11 - 2.06

Total 100 100 100

40

Table S3. Various state-of-the-art catalysts for OER and their catalytic activities.

Compounds Electrolyte Loading (mg cm-2disk) ɳ (mV) at

10 mA cm-2

Ref No.

Ir 0.1 M KOH 0.05 >~450 [3]

Ru 0.1 M KOH 0.05 >~350 [3]

Ni2P 1 M KOH 0.14 ~290 [4]

Ni-Fe LDH 0.1 M KOH 0.20 ~310 [5]

Co2O3 1 M KOH 0.24 ~350 [6]

Co2O3/N-rmGO 1 M KOH 0.24 ~320 [6]

Ni-Cu/Au 0.1 M KOH 0.075 >~570 [7]

Ni-Zn/Au 0.1 M KOH 0.075 >~570 [7]

CoMn2O4/Graphene 0.1 M KOH N.A. ~460 [8]

LaCoO3 0.1 M KOH 0.25 >~570 [9]

La0.5Ca0.5CoO3 0.1 M KOH 0.25 >~470 [9]

Ba0.5Sr0.5Co0.8Fe0.2O3-δ 0.1 M KOH 0.25 ~370 [9]

Pr0.5Ba0.5CoO3-δ 0.1 M KOH 0.25 ~320 [10]

LaNiO3 1 M NaOH N.A. ~350 [11]

NdBa0.25Sr0.75Co2O5.9 0.1 M KOH 0.8 ~450 [12]

41

NdBa0.25Sr0.75Co2O5.9/pP

y/C

0.1 M KOH 0.8 ~390 [12]

BaNiO3 0.1 M KOH 0.29 ~390 [13]

a-Fe36Co64Ox 0.1 M KOH N.A. >~590 [14,15]

a-Fe40Ni60Ox 0.1 M KOH N.A. >~570 [14,15]

a-Fe40Co39Ni22Ox 0.1 M KOH N.A. >~560 [14,15]

a-Co41Ni59Ox 0.1 M KOH N.A. >~590 [14,15]

Ir 0.01 KOH 0.012 490 In this

study

Ni 0.1 M KOH 0.012 517 In this

study

NiO 0.1 M KOH 0.012 594 In this

study

Ni/CaTiO3 mixtures 0.1 M KOH 4 mol % Ni in 0.012

perovskites

N/A In this

study

Ni-exsolved CaTiO3

(Bulk)

0.1 M KOH 4 mol % Ni in 0.012

perovskites

568 In this

study

Ni-exsolved CaTiO3

(Sphere)

0.1 M KOH 4 mol % Ni in 0.012

perovskites

489 In this

study

42

Table S4. Elemental compositions. (Ca0.90Ni0.04)TiO2.88(OH)0.12 and Ca0.92TiO2.85(OH)0.14

were prepared by solvothermal method. All elemental compositions were estimated via EDX

measurements.

Element (Atomic %) (Ca0.90Ni0.04)TiO2.88(OH)0.12 Ca0.92TiO2.85(OH)0.14

Ca 46.64 47.74

Ti 51.54 52.26

Ni 1.82 -

Total 100 100

43

Table S5. True densities. (Ca0.90Ni0.04)TiO2.88(OH)0.12 and Ca0.92TiO2.85(OH)0.14 were

prepared by solvothermal method. SS refers to solid-state synthesis. Numbers inside

parenthesis indicates relative density (%). Heat treatment was conducted at 600 °C for 5 h.

(Ca0.90Ni0.04)TiO2.88(OH)0.12 Ca0.92TiO2.85(OH)0.14 CaTiO3 (SS)

Density (g cm-3) 3.8622

(97.03)

3.9140

(98.34)

4.0054

(100.63)

Density (g cm-3)

after heat treatment

3.9687

(99.71)

3.9769

(99.92) -

44

Table S6. Unit Cell parameters. (Ca0.90Ni0.04)TiO2.88(OH)0.12 and Ca0.92TiO2.85(OH)0.14 were

prepared by solvothermal method. Rietveld refinement was carried out for each sample.

(Ca0.90Ni0.04)TiO2.88(OH)0.12 Ca0.92TiO2.85(OH)0.14

a (Ǻ) 7.6690 (7) 7.6731 (5)

b (Ǻ) 5.4574 (4) 5.4421 (5)

c (Ǻ) 5.4013 (5) 5.4200 (6)

V (Ǻ3) 226.06 226.31

45

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