Strontium-doped Lanthanum Iron Nickelate Oxide as Highly Efficient Electrocatalysts

for Oxygen Evolution Reaction

Mengran Li,a Abi Rafdi Insani,a Linzhou Zhuang,a Zhanke Wang,a Ateeq ur Rehman,a Lian X.

Liu,b Zhonghua Zhua,*

a. School of Chemical Engineering, the University of Queensland, St. Lucia, Queensland,

4067

b. School of Chemical and Process Engineering, Senate House, University of Surrey,

Guildford, Surrey, GU2 7XH, UK

* Corresponding Author: Zhonghua Zhu: z.zhu@uq.edu.au, (T) +61 7 336 53528, (F) +61 7

336 54199

Abstract: Pursuing efficient and low-cost catalysts for the sluggish oxygen evolution

reaction (OER) is imperative for the large-scale deployment of promising electrochemical

technologies such as water splitting and CO2 electrochemical reduction. The earth-abundant

perovskite catalysts based on LaNiO3-δ show promise in OER catalysis because of their

relatively low cost and their optimal electronic structure but suffer from low electrode-area

normalized activity. In this work, we partially substituted La with Sr and Ni with Fe to enable

a remarkably high OER activity with an ultra-low overpotential of 374 ± 3 mV vs RHE at a

current density of 10mA cm-2 normalized by electrode geometric area. This performance even

surpasses the performance of benchmark RuO2. Our results show that Sr could promote OER-

active sites including Ni(III), O2-2/O-, and optimal Ni/Fe ratios, which significantly improve

the surface intrinsic activity at the perovskite surface. Therefore, this work not only

developed a highly efficient earth-abundant catalyst towards OER, but also demonstrated the

effective modulation of catalyst surface interactions through A-site doping for perovskite

Abbreviations: OER, oxygen evolution reaction; RHE, reverse hydrogen electrode; EDTA, ethylenediaminetetracetic acid; LNF, LaNi0.8Fe0.2O3-δ; LSNF91, La0.9Sr0.1Ni0.8Fe0.2O3-δ; LSNF73, La0.7Sr0.3Ni0.8Fe0.2O3-δ; LSNF55, La0.5Sr0.5Ni0.8Fe0.2O3-δ; EIS, electrochemical impedance spectroscopy; ECSA, electrochemically active surface area; TEM, transmission electron microscopy; SEM, scanning electron microscopy; TOF, turnover frequency; LSV, linear sweep voltammetry.

oxides for key applications such as water splitting, CO2 electrochemical reduction and N2

electrochemical fixations.

Keywords: oxygen evolution reaction, perovskite, electrocatalysis, heterogeneous catalyst

1. Introduction

Technologies that electrochemically facilitate electrical and chemical energy conversions,

such as water splitting, CO2/N2 electrochemical fixation, or rechargeable metal-air batteries,

have great promise in renewables and carbon neutralization.[1-6] As a key half reaction,

however, the oxygen evolution reaction (OER) is very sluggish and normally requires a

considerably high overpotential to achieve a desired current density, thus severely lowering

the efficiency and the rate of energy conversion.[7, 8] Therefore, developing highly efficient

electrocatalysts for OER is imperative for the deployment of these promising energy

conversion technologies. Material based on precious metals such as IrO2 and RuO2 are widely

known as the state-of-art OER catalysts that can lower the overpotential to initiate OER and

thereby significantly improve the voltage efficiency.[9, 10] However, their high material cost

and unacceptable durability limit their practical applications in large scale.

Perovskite oxides with a general formula of ABO3, containing alkaline-earth or rare-earth

elements as A-site elements and transition metals (e.g. Co, Ni) as the B-site elements, are

emerging as promising candidates as OER catalysts because of their relatively low-cost, high

intrinsic electroactivity[11-13] and high flexibility in physical-chemical properties. LaNiO3-δ-

based oxides have particularly received extensive research interest due to their high intrinsic

activity and metallic conductivity of LaNiO3-δ.[11, 14-17] LaNiO3-δ outperforms most of the

conventional perovskite catalysts as it lies near the top of the volcano-type intrinsic OER

activity due to the balanced binding strength with oxygen species, or in other word, the e g

orbital fillings of B-site atoms.[11, 18]

2

The crystal structure, specific surface area and chemical compositions all play a significant

role in affecting the electroactivity of the LaNiO3-δ-based perovskites. Through manipulating

the crystal structures of LaNiO3-δ, Zhou et al. found that LaNiO3-δ has a strong structure-

performance correlation, and a cubic crystal structure is more favourable for its OER

electrocatalysis as compared to rhombohedral structure.[15] Johnston and Stevenson et al.

successfully synthesized nano-structured LaNiO3-δ that shows a high surface area (~ 11 m2 g-

1) and high mass activity comparable to IrO2 through reverse phase arrested growth

precipitation followed by calcination.[16] Additionally, partial substitutions at La-site or Ni-

site have also been demonstrated beneficial for OER catalysis.[13, 19, 20] Inspired from the

synergistic effects of Ni/Fe catalysts, Goodenough et al. and Lin et al. partially substituted Ni

with Fe, and observed a significant enhancement of the OER electroactivity, which likely

results from the increase of Ni(III) concentration on the surface as imparted by the Fe

incorporation.[19, 20] However, limited work has been done on doping Sr2+ into A-site of the

La(Ni,Fe)O3-δ,[21] though Sr2+ substitution has been effective in tuning the crystal structures,

optimising electronic structures, and promoting OER active sites on the surface such as

oxygen vacancy defects, hydroxylation, or transition metal ions with higher valences for Co

or Fe-based perovskite OER catalysts.[12, 22-25]

In this work, we partially replaced La at A-site of LaNi0.8Fe0.2O3-δ perovskite oxides with

10%, 30%, and 50% Sr and studied the role of Sr in OER catalysis through investigating the

OER-related properties of La1-xSrxNi0.8Fe0.2O3-δ (x = 0, 0.3, 0.5) oxides, including crystal

structures, specific surface area, surface chemistry and electrochemical properties. We chose

LaNi0.8Fe0.2O3-δ perovskite oxide as the parent oxide because LaNi0.8Fe0.2O3-δ perovskite oxide

has been reported to show an optimal efficiency in OER catalysis among La(Ni,Fe)O3-δ

perovskite oxides.[19] Although our results showed that the Sr may have negative impact on

the specific surface area and promote unwanted NiO secondary phase, the promoted OER-

3

active Ni(III) and O2-2/O- species and optimal Ni/Fe ratio by Sr incorporation lead to a

significant improvement of the catalytic stability and OER electroactivity normalized by the

electrode geometric area in the alkaline electrolyte.

2. Materials and methods

2.1 Materials

The La1-xSrxNi0.8Fe0.2O3-δ (x = 0, 0.1, 0.3, and 0.5) and Ni0.8Fe0.2Oy powders were synthesized

through a facile one pot combustion method.[13] The nitrate precursors (Sigma Aldrich) were

weighed according to their corresponding stoichiometry and dissolved in deionized water.

The complexing agents, ethylenediaminetetracetic acid (EDTA) and citric acid, were

subsequently dissolved in ammonia solution (~ 32 %) and added into the prepared aqueous

nitrate solution subsequently (molar ratio of the metal ions, EDTA and citric acid = 1:1:1.5).

The solution with a pH at ~8 was thereafter evaporated at 80˚C on a hot plate until the

formation of a viscous gel. The formed gel was fired at 260 ˚C and then calcinated at 900 ˚C

in static air for 5 h.

2.2 OER activity evaluation

A rotating disk ring electrode (RRDE, from ALS Co Ltd.) was used to characterize OER

activity of the targeted samples in a KOH aqueous solution (0.1M) in a typical three-

electrode configuration where a Pt wire and Ag/AgCl (sat. KCl) serve as the counter

electrode and reference electrode respectively. An ink of the sample was drop casted and

dried to form a thin film on a glassy carbon (GC) disk electrode (0.126 cm2 in area), which

was polished beforehand and cleansed using sonication. The ink was prepared by mixing the

synthesized powder (10 mg), carbon black (Super P® Conductive, from Alfa Aesar, 10mg),

ethanol (1 mL) and Nafion solution (5 wt%, 0.1 mL) and subsequently agitating the

4

suspension for ~30 min by sonication. The addition of the carbon black is to improve the

electronic conductivity of the deposited thin film electrode.

Voltammetry and chronopotentiometry were carried out using a Biologic VMP2/Z

multichannel potentiostat to evaluate the OER catalytic activity and stability. The

electrochemical impedance spectroscopy (EIS) was conducted using an Autolab 302N

electrochemical work station. The KOH aqueous electrolyte was continuously bubbled with

O2 during the voltammetry analysis. We estimated the electrochemically active surface areas

(ECSAs) of the thin film electrode through dividing their double layer capacitance by a

specific capacitance of 0.04 mF cm-2, a typical value for a metal electrode in alkaline media.

The double layer capacitance of the targeted thin film electrode is equal to the slope of the

charging current curve as a function of scan rates, which can be obtained through performing

cyclic voltammetry at a non-Faradaic potential range (0.1V) with different scan rates from

500mV s-1 to 8mV s-1. All the potentials reported in this work were converted into potentials

versus reversible hydrogen electrode according to the following equation:

EiR−corrected=E Ag∨AgCl+0.9717−i Ru (1)

Where i is the current and Ru is the uncompensated ohmic resistance (52Ω).

The turnover frequency (TOF) was calculated using the equation shown below:

TOF= i4 × F × n (2)

Where F is the Faraday constant (96485 C mol-1), and n represents the moles of the total

actives in the thin film electrode. In this work, all the transition metal atoms are assumed to

be the active sites.

The Tafel slope was calculated from the Tafel equation in a logarithm form:

5

η=log( jj0

) (3)

Where η, b, j, j0 are overpotential, Tafel slope and current density and overall current density

at equilibrium.

2.3 Other characterisations

A Bruker D8 Advanced X-ray diffractometer with a nickel-filtered Cu-Kα radiation was

performed to study the crystal structures of the targeted powders. High-resolution electron

transmission microscopy (HRTEM) and selected area electron diffraction (SAED) was

applied to further confirm the crystal structure of the particle samples. The SAED diffraction

pattern was analyzed and indexed using CrysTBox software.[26] Scanning electron

microscopy and transmission electron microscopy were used to characterize the

microstructures and morphologies of the targeted powder samples. The surface chemistry and

compositions were investigated using an X-ray photoelectron spectrometer (XPS). All the

XPS spectra raw data were corrected by setting the adventitious C1s binding energy to be

284.8 eV. The O2 temperature programmed desorption (O2-TPD) was performed through

monitoring the desorbed oxygen from the powder samples using BELMASS mass

spectroscopy (from MicrotracBEL Corp.) equipped with a selected ion monitor. The samples

for O2-TPD measurement were first pre-treated at 150 °C for 30 min under a flowing Ar and

heated from room temperature to 750 °C with a ramping rate of 10 °C with Ar also as the

carrier gas.

3. Results and discussion

3.1 Structure and microstructure

The analysis of X-ray powder diffraction in Figure 1 showed that incorporating Sr2+ has a

significant impact on the crystal structures of the (La, Sr) Ni0.8Fe0.2O3-δ perovskites. With an

6

increase of Sr content from 0 to 50mol%, the major crystal structure transforms from a

rhombohedral structure in R-3c space group symmetry for LaNi0.8Fe0.2O3-δ (LNF) and

La0.9Sr0.1Ni0.8Fe0.2O3-δ (LSNF91) into a major tetragonal phase in a I4/mmm symmetry for

La0.5Sr0.5Ni0.8Fe0.2O3-δ (LSNF55). The La0.7Sr0.3Ni0.8Fe0.2O3-δ (LSNF73) is in a structure

transition state, where it has a combined rhombohedral and tetragonal structures as the major

phases. However, NiO phase (monoclinic symmetry, C2/m space group) is promoted when of

Sr is incorporated. Ni0.8Fe0.2Oy (NF) oxide, prepared following the same synthesis route for

comparison, is a composite of a NiO phase and a cubic phase (Fd-3m space group) that is

likely to be the NiFe2O4 spinel.

Further high-resolution transmission electron microscopy (HRTEM) images confirm the

major phases of the samples. As revealed by the Figure 2(a)-(d), the d-spacing measured from

the HRTEM is 0.385 nm for LNF, 0.273 nm for LSNF91, 0.365 nm for LSNF73, and 0.206

nm for LSNF55, corresponding to (10-2) planes (2θ = ~ 23.1 °) of LNF, (104) planes (2θ = ~

32.7 °) of LSNF91, (101) planes (2θ=~24.3°) of the tetragonal phase of LSNF73, and (114)

planes (2θ = ~ 44.0 °) of the tetragonal LSNF55, respectively. The selected area diffraction

(SAED) patterns of the TEM, as shown in Figure 2(e)-(h), also confirms the transition of the

major phases from the rhombohedral phase to tetragonal phase when Sr increases.

Such phase transition can be explained by the larger ionic size of Sr2+ (1.44 Å in ionic radii)

than La3+ (1.36 Å in ionic radii),[27] which increases the tolerance factor of the perovskite

samples through increasing A-O bond average length, and therefore reduces the

rhombohedral distortions in the lattice.

The microstructures of the powder samples were characterized by the scanning electron

micrographs. As presented by Figure S1, the samples become less porous when the Sr content

increases. The particle size of the samples also increases with the Sr content. For example,

7

the average particle size of LNF is in a range of 200-300 nm, while the size of LSNF55

particles are over 1 micron. This suggests that the incorporation of Sr element increases

sinterability of the samples, and therefore may have negative impact on the OER catalysis

due the relatively low availability of active sites for catalysis.

3.2 OER activity

We evaluated the OER activity of the targeted samples through using linear sweep

voltammetry (LSV) on a rotating disk electrode in an O2-saturated 0.1 M KOH aqueous

solution. To rule out the potential background contribution from the carbon black in the

catalyst, we also studied the catalytic activity of carbon black. As shown in Figure 3(a), the

current density of carbon black is the lowest among the tested samples, indicating that our

results can truly reflect the OER activity of the studied metal oxides. By comparing with the

NF sample synthesized through the same route, we noticed that all the samples with

perovskite-related structures show much better performance, highlighting the essential role of

perovskite-related structure in OER electrocatalysis. At the same potential where OER takes

place, as shown in Figure 3(a), the current density, which is normalized by electrode

geometric area, increases following the trend: LSNF55 > LSNF73 > LSNF91 > LNF,

indicating that incorporating Sr significantly boosts the OER electroactivity of the

La(Ni,Fe)O3-δ-based catalysts. The poor OER activity of the commercial NiO, as observed

from Figure 3a, suggests that the promoted NiO phase by Sr incorporation is not the major

reason for the improvement of OER activity.

Figure 3(b) further compares the overpotentials of the samples at 10 mA cm-2, which is a

current density that is expected for an efficient device in converting solar to fuels.[28] Under

the same OER current density, a lower overpotential indicates a higher catalysis activity. We

noticed that at 10 mA cm-2 LSNF55 shows remarkably high OER activity with an

8

overpotential as low as 374 ± 3 mV, outperforming the benchmark RuO2 with an

overpotential of 403 ± 24 mV (Error: Reference source not found(b)). In addition, the

LSNF55 catalysts, even in micron size, has a high OER activity that surpasses many of the

state-of-art perovskite-based OER electrocatalysts developed recently, such as the nano-

structured Ba0.5Sr0.5Co0.8Fe0.2O3-δ,[29] SrNb0.1Co0.7Fe0.2O3-δ,[30] and NiO-(La0.613Ca0.387)2NiO3.562

nano hybrid,[13] especially with respect to the current density normalized by electrode

geometric area (Table S1). Tafel analysis over the LSV voltammograms, as shown in Figure

3(b), also suggested that the Tafel slope is lower for LSNF73 (64 mV dec -1) and LSNF55 (70

mV dec-1) than for LNF (92 mV dec-1) and LSNF91 (79 mV dec-1). In addition, results of the

electrochemical impedance spectroscopy (EIS) revealed that both LSNF73 and LSNF55 have

significantly lower area specific resistance (ASR) as compared with LSNF91 and LNF,

indicating a more efficient charge-transfer process as imparted by Sr incorporation during the

OER catalysis.

By assuming that the B-site transition metals (i.e. Ni and Fe) are the dominant active sites and

that all the Ni and Fe sites are involved in OER catalysis, we compared the estimated

turnover frequencies (TOF) for the LNF, LSNF91, LSNF73 and LSNF55 perovskite samples

at an overpotential of 350 mV. A higher TOF value suggests a more efficient intrinsic

catalysis over the active sites. As shown in Error: Reference source not found, the TOFs of

these samples follow the trend: LSNF55 > LSNF73 > LSNF91 > LNF, meaning a higher

intrinsic OER activity for samples with higher content of Sr element. Particularly, the

LSNF55 has the highest TOF value as high as ~0.0119 s-1, over 8 times higher than that of

LNF (~0.0014 s-1). Note that the OER catalysis only takes place at the surface of these

perovskite oxides, so the actual TOF value for these samples could be much higher due to

their limited specific surface, particularly for the Sr-containing samples with larger particle

sizes (Error: Reference source not found and Figure S1)

9

The specific surface area plays a role in OER catalysis, and thus the specific surface area and

electrochemically active surface area (ECSA) of the samples were studied. The specific

surface area of LNF, LSNF91, LSNF73 and LSNF55 were calculated using Brunauer-

Emmett-Teller (BET) method from N2 adsorption/desorption isotherms at 77K. As shown in

Figure 4(a), the BET surface area slightly increases with Sr content. We also found from

Figure 4(a) that the ECSA estimated from the double layer capacitance (shown in Figure S2)

reaches the maximum value for LSNF91 and decreases with Sr thereafter. Such ECSA trend

could be a result of the concentration of the electrochemically active (Ni, Fe) species at the

surface and the specific surface areas of the samples. Surface chemistry analysis over LNF,

LSNF73, and LSNF55 catalysts from general scan of X-ray photoelectron spectra (XPS)

revealed that introducing Sr decreases the content of (Ni, Fe) species likely due to the Sr

surface enrichment, as widely reported for Sr-containing perovskites.[31] With an optimal

combination of (Ni, Fe) content and BET specific surface area, for example, the LSNF91

possesses the highest ECSA of ~17.6 m2 g-1 among all the tested samples. The low

concentration of (Ni, Fe) species at the surface also explains the observed small ECSA for

LSNF73 and LSNF55. Although there are low ECSA and availability of OER active sites

due to Sr doping, the superior OER activity for LSNF55 highlights the essential role of Sr in

enhancing the intrinsic activity (i.e. aforementioned higher TOF values) of the catalysts.

Interestingly, Sr is also found effective in modulating the Ni/Fe atomic ratio at the surface,

which could be significantly different from the Ni/Fe ratio in the bulk. Figure 4(b) shows that

Ni/Fe ratio at the surface decreases from ~ 6.74 for LNF to ~ 4.39 for LSNF55. As reported

in literature, the (Ni, Fe)-based catalysts such as (Ni, Fe) oxides or phosphides with a Ni/Fe

atomic ratio close to 1.04 -1.5 normally show an optimal performance in catalysing OER.[29,

32, 33] Given the crucial role of Ni/Fe ratio in determining the OER catalysis, we believe that

10

the decreased Ni/Fe ratio for samples with higher content of Sr partially contribute to the

observed superior OER activity of LSNF55.

3.3 Surface chemistry of the catalysts

As the catalyst surface predominates the OER catalysis, we studied the surface chemistry of

the nickel and oxygen constituents of the LNF, LSNF73 and LSNF55 through high resolution

XPS. Figure 5(a)-(c) show the O 1s spectra and the corresponding deconvoluted peaks. The

deconvoluted peak of the O 1s denoted by O1 at a binding energy of ~ 528 eV was assigned

to lattice oxygen for La-O lattice.[19] O2 peak at ~ 529 eV is related to the lattice oxygen

species for Ni-O, while O3 peak at ~530.6eV and O4 peak at ~531eV correspond to the

surface adsorbed O2-2/O- oxygen species and Ni-OH, respectively.[19, 34] O5 peak at ~ 532-

533 eV can be assigned to the less-electron-rich oxygen species and adsorbed water.[34] As

shown in Figure 5(d)-(f) previously, Ni 2p3/2 peaks for La-containing samples are interfered

with La 3d3/2 peaks at lower binding energy. Nevertheless, the deconvoluted peaks denoted by

1 at ~ 854 eV and by 2 at ~ 855.5 eV are related to Ni(II) and Ni(III) respectively.[35]

A detailed quantification analysis based on the XPS spectra deconvolution is shown in Table

1. The deconvolution results of Ni 2p3/2 XPS spectra revealed that doping Sr also promotes

Ni(III) species at the surface in the sample, increasing the Ni(III)/Ni(II) atomic ratio from

33.4 % for LNF, to 48.1 % for LSNF55 (Error: Reference source not found). The Ni(III) was

also reported to be more active in OER catalysis as compared with Ni(II)[19, 36] due to the

unity of eg filling of Ni(III), which allows an optimal binding strength with the OER

intermediates.[37] Such Ni(III) promotion by Sr2+ can be explained by the fact that Ni species

are forced to increase their overall valence to compensate the charge imbalance due to the

replacement of trivalent La3+ with divalent Sr2+, and that the exsolution of NiO secondary

phase which further increases the average valence of Ni in the major lattice. This result well

11

explains the improvement of the turnover frequencies with increment of the Sr content, as

shown in Figure 4(a).

Additionally, we also observed a higher population of O2-2/O- species at the surface of

samples containing more Sr: the O2-2/O- content at the surface increases from 12.6 % for LNF

to 34.0 % for LSNF55. (Table 1) As reported elsewhere, the OER rate of LaNiO3 perovskite

is likely limited to the formation of O2-2 groups, i.e. S-OOH+OH- S-O2

2- + H2O +e-, where

S stands for the active nickel atom at the surface that binds with the reaction intermediates.

[11] Previous studies have widely reported the essential role of O2-2/O- species in effecting an

efficient OER catalysis of perovskite oxide in alkaline media. [13, 38-43] Therefore, a higher

density of O2-2/O- species at surface as a result of increasing content of Sr indicates an

accelerated rate determining step for LaNiO3-δ-based perovskite catalysts.

Table 1 Quantification of surface Ni 2p3/2 and O1s convoluted XPS spectra for LNF, LSNF73,

and SNF

Sample nameNi(III)/[Ni(III)

+Ni(II)]La-O Ni-O O2

2-/O- Ni-OH H2Oadsorbed

LNF 33.4 25.4 20.5 12.6 33.0 8.7

LSNF73 34.7 23.0 16.1 25.7 19.7 15.5

LSNF55 48.1 15.9 15.6 34.0 27.2 7.2

3.4 OER short-term stability

These perovskite samples show different stabilities when catalysing OER in alkaline media.

As shown in Figure S4, for example, the current density of the samples containing lower Sr

content (i.e. LNF and LSNF91) degraded significantly with the number of cyclic

voltammetry sweeps. Due to the observable degradation of OER catalysis for LNF and

LSNF91, we only evaluated the stability of LSNF73 and LSNF55 samples for ~ 3 h under a

12

constant current density of 10 mA cm-2 in 0.1 KOH electrolyte. The results are shown in

Figure 6. At a constant current density, an increment of potential means a higher

overpotential required to drive the same rate of reaction, which is a sign of catalyst

performance degradation. As shown in Figure 7, LSNF73 exhibited a degradation of ~ 2 %

after 3h treatment under 10 mA cm-2. In contrast, LSNF55 with a higher content of Sr still

preserve its OER activity after the same treatment, demonstrating a better short-term stability

in alkaline media. We can conclude that doping Sr not only improves the electroactivity of

La(Ni,Fe)O3-δ towards OER, but also boosts the short-term stability of water oxidation.

4. Conclusions

In summary, we have synthesized a series of La1-xSrxNi0.8Fe0.2O3-δ (x = 0, 0.1, 0.3 and 0.5)

perovskite oxides and evaluated these perovskites as catalysts for OER catalysis in alkaline

media. With a higher concentration of Sr2+ incorporated, the (La, Sr) Ni0.8Fe0.2O3-δ -based

catalysts achieves a greatly enhanced OER electrocatalytic activity, with an overpotential as

low as 374 ± 3 mV vs RHE for La0.5Sr0.5Ni0.8Fe0.2O3-δ under 10 mA cm-2, normalized by

electrode geometric area, even surpassing the benchmark precious-metal-based RuO2

catalysts and some of the state-of-the-art perovskite oxides.[13, 29, 30, 44] Through

examining the OER-related properties such as crystal structures, surface area, and surface

chemistry, it is found that the substitution of trivalent La3+ with divalent Sr2+ can promote

OER-active Ni(III) species, increases O2-2/O- oxygen species, and optimise Ni/Fe surface

ratio, all of which contribute to the excellent intrinsic OER activity of Sr-doped samples.

Additionally, the short-term stability OER catalysis are also significantly enhanced by

incorporating Sr2+ for ~ 3 h operation under 10 mA cm-2. This work not only showed a new

earth-abundant perovskite catalyst for efficient OER catalysis for water splitting, CO2 and

nitrogen electrochemical fixation, but also demonstrated an effective strategy in modulating

13

the catalysis-related surface properties of perovskite oxides such as transition metal valences,

transition metal ratios and consequently the binding strength with reaction intermediates.

5. Acknowledgements

This work was supported by Australian Research Council (ARC) [DP170104660] and ARC

Future Fellowship Project [FT120100720]. The authors also acknowledge the technical

support from the Australian Microscopy and Microanalysis Research Centre at the University

of Queensland.

6. Appendix A. Supplementary data

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Figure 1 X-ray diffraction patterns of La1-xSrxNi0.8Fe0.2O3-δ (x=0, 0.1, 0.3, 0.5, denoted as

LNF, LSNF91, LSNF73 and LSNF55) and Ni0.8Fe0.2Oy (NF)oxides. The peaks denoted by *

belong to NiO phase. (single column image)

Figure 2 High resolution transmissions electron micrographs of (a) LNF, (b)LSNF91, (c)

LSNF73, and (d) LSNF55. Selected area electron diffraction of the major phases for (e) LNF,

(f) LSNF91, (c)LSNF73, and (d)LSNF55. (double column image)

18

Figure 3 (a) Linear sweep voltammograms (LSVs) of LNF, LSNF91, LSNF73 and LSNF55,

NF and RuO2 catalysts. (b) A comparison of Tafel plots for the NF, LNF, LSNF91, LSNF73

and LSNF55 catalysts. (c) Electrochemical impedance spectra (EIS) of perovskite-based

electrodes at an overpotential of 400mV, with ohmic resistance subtracted from the Nyquist

plot. (d) A comparison of the corresponding averaged overpotentials for the samples, and all

the samples are tested at least three times for accuracy. All the samples were composited

with carbon black and deposited as a thin film on a glassy carbon substrate of a rotating disk

electrode with rotating speed of 1600 rpm, and tested in 0.1M O2-saturated KOH aqueous

electrolyte. (double column image)

19

Figure 4 (a) A comparison of Brunauer-Emmett-Teller (BET) specific surface area ,

electrochemically active surface area (ECSA), and estimated Turnover Frequency (TOF) for

LNF, LSNF91, LSNF73 and LSNF55 at an overpotential of 350mV vs RHE in O2-saturated

0.1M KOH. (b) Atomic ratios of surface Ni/Fe and (Ni, Fe)/(La, Sr) estimated from general

scan of X-ray photoelectron spectra (XPS) for LNF, LSNF73, and LSNF55. (single column

image)

20

Figure 5 Deconvoluted high resolution X-ray photoelectron spectra (XPS) of O1s for (a)

LNF (b) LSNF73 (c) LSNF55, and of Ni2p3/2 for (d) LNF (e) LSNF73 (f) LSNF55. With

respect to O1s, 1-5 are used to denote La-O lattice oxygen, lattice O, adsorbed O2-2/O-

species, Ni-OH, and less-electron rich oxygen and adsorbed water. For Ni2p3/2 spectra, 1

and 2 represent Ni(II) and Ni(III) species. In LNF and LSNF55 samples, the Ni2p 3/2 peaks

are interfered with La d3/2 peaks at lower binding energies. (double column image)

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Figure 6 Stability test of LSNF73 and LSNF55 under 10mA cm-2 for ~3h. (single column

image)

22