Impact of strontium-substitution on oxygen evolution reaction of lanthanum nickelates in alkaline solution

Article abstract of DOI:10.1149/2.0301815jes

In the present study, activity for anodic oxygen evolution reaction (OER) has been evaluated by varying the degree of Sr²⁺-substitution in La_1-x Sr_x NiO₃ from x = 0.0 to 1.0. EDS was utilized to measure the elemental composition. XRD and Rietveld refinement were employed for the phase and crystal structure analysis. It was observed that the crystal structure of LaNiO₃ was distorted after Sr²⁺-substitution and formed tetragonal lanthanum-strontium nickelates (LSN) for x ≤ 0.8, and rhombohedral strontium nickelate for x = 1.0. For all the samples, secondary phases (NiO for 0.2 ≤ × ≤ 1.0 and SrCO₃ for 0.8 ≤ × ≤ 1.0) were also observed. Rietveld analysis suggests that Sr²⁺-substitution caused the cell volume to contract. The oxidation state of Ni in the samples were investigated by XPS for the elusive Ni⁴⁺. An increase in the mass specific activity for OER was observed as the degree of Sr²⁺-substitution increase until x = 0.6, however, the activity decreased for higher values of x. The LSN samples were significantly more active than that of LaNiO₃, and the state-of-the-art electrocatalyst Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ.

Full text of DOI:10.1149/2.0301815jes

J3236
Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
JES FOCUS ISSUE ON ELECTROCATALYSIS — IN HONOR OF RADOSLAV ADZIC
Impact of Strontium-Substitution on Oxygen Evolution Reaction of
Lanthanum Nickelates in Alkaline Solution
1
,∗
2,3
2
1
Ravi Sankannavar,
K. C. Sandeep, Sachin Kamath, Akkihebbal K. Suresh,
1
,z
and A. Sarkar
1
Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai - 400 076, India
Heavy Water Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India
Homi Bhabha National Institute, Mumbai - 400 085, India
2
3
In the present study, activity for anodic oxygen evolution reaction (OER) has been evaluated by varying the degree of Sr2 -substitution
in La1−x Srx NiO3 from x = 0.0 to 1.0. EDS was utilized to measure the elemental composition. XRD and Rietveld refinement were
employed for the phase and crystal structure analysis. It was observed that the crystal structure of LaNiO3 was distorted after
+
2
+
Sr -substitution and formed tetragonal lanthanum-strontium nickelates (LSN) for x ≤ 0.8, and rhombohedral strontium nickelate
for x = 1.0. For all the samples, secondary phases (NiO for 0.2 ≤ x ≤ 1.0 and SrCO3 for 0.8 ≤ x ≤ 1.0) were also observed. Rietveld
2
+
analysis suggests that Sr -substitution caused the cell volume to contract. The oxidation state of Ni in the samples were investigated
4
+
2+
by XPS for the elusive Ni . An increase in the mass specific activity for OER was observed as the degree of Sr -substitution
increase until x = 0.6, however, the activity decreased for higher values of x. The LSN samples were significantly more active than
that of LaNiO3, and the state-of-the-art electrocatalyst Ba0.5Sr0.5Co0.8Fe0.2O3−δ.
©
The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0301815jes]
Manuscript submitted August 1, 2018; revised manuscript received October 1, 2018. Published October 16, 2018. This paper is part
of the JES Focus Issue on Electrocatalysis — In Honor of Radoslav Adzic.
In the pursuit of “hydrogen economy”, hydrogen (H
2
) gas has
Dedicated effort has been made to tune the electrocatalytic proper-
ties of perovskite oxides by systematic substitution of lanthanides or
alkaline earth elements at A-site, and/or transition elements at B-site
been proposed as an energy carrier.1–4 The H
gas acts as the most
2
promising clean fuel owing to its high specific energy density and
zero-emission applications. Thus, there is a tremendous scope for
production of hydrogen gas using renewable sources and its storage
10,17–19
of the ABO
3
structure.
The enhancement in the activity for
OER is attributed to the increase in the oxidation state of a transition
1
8,20
as most of the H
2
gas produced presently (upto 96%) is from age
element present at the B-site of ABO
tion of molecular level oxygen vacancies,
3
perovskites,
the forma-
broadening of the angle
between B-site transition element (M) and lattice oxygen (O), i.e.,
21,22
old hydrocarbon sources, which is costly and also acts as a potential
1
threat to the environment. Water electrolysis is a seminal electro-
18,20,23
18
chemical way to produce high purity H
2
gas, in which water is split
M-O-M,
other more recent study, reported a correlation between OER activity
of perovskite-type electrocatalysts and e -electrons, i.e., perovskite
with e
and improvement in the electrical conductivity. An-
into molecular H and oxygen (O ) gases by applying electricity (may
2
2
be generated by renewable sources). The use of electricity generated
g
9
,24
by renewable energy sources for water electrolysis could also provide
g
≈ 1 in the B-site has higher activity.
2
a way to store the energy in the form of H
is represented as 2H + O
O → 2H
oxygen evolution reaction (OER), 2H
.229 V vs. SHE, a multi-electron catalytic reaction involving many
2
. The splitting of water
Metallic nickel (Ni) has been the most extensively investigated
active electrocatalyst for alkaline water electrolysis with its supe-
rior stability and electrochemical activity than any other transition
2
2
2
. However, in practice the anodic
+
−
0
2
O → O + 4H + 4e ; E =
2
25,26
1
elements.
than nickel metal itself. Among other oxides, lanthanum nickelate
(LaNiO ) belonging to the perovskite family has been of consider-
able interest due to its better corrosion resistance in alkaline solution,
However, oxides of nickel have shown better activity
25
intermediates, is kinetically limiting and higher potential (overpoten-
tial) is required for an appreciable rate of the above water splitting
3
5
,6
reaction. In the last four decades, considerable research has been
done to enhance the efficiency of OER by minimizing overpotential
primarily by developing better and more efficient electrocatalysts.7–11
Oxides of Ir and Ru, and Pt metal were utilized for minimizing the
19,27–29
economically feasible, and high electrical conductivity.
suit of enhancing the activity for OER of perovskites, the substitution
In pur-
2
+
of divalent strontium (Sr ) cation at A-site has been shown to en-
1
8,20,21
20
overpotential and have shown better OER performance in comparison
hance the OER activity for lanthanum cobaltates
and ferrites.
with Ir and Ru metal electrocatalysts.1
2–14
Despite their better OER
This has been attributed to the increase in the oxidation state of the
1
8,20
activity, scale-up of these precious metal electrocatalysts is obstructed
due to their scarcity.
In the above context, an opportunity exists for use of inexpensive
and abundant transition elements for the synthesis of cost effective
transition elements present at the B-site,
which increase with the
2
+
degree of Sr substitution. Further, the bond strength between Ni
and OH ion is weaker as compared to the other transition metals
(Co, Fe, Mn, and Cr), which can have a positive effect on activity for
OER. Thus, it is desirable to have Ni at B-site and some amount of
−
7
8
7,30
and efficient electrocatalysts for alkaline water electrolysis. The an-
odic reaction, OER in an alkaline water electrolysis is represented as
Sr-substitution at A-site of ABO
3
perovskite. In this paper, we syn-
−
−
0
2+
4
OH → O
2
+ 2H
2
O + 4e ; E = 0.401 V vs. SHE. Specifically,
thesized lanthanum-strontium nickelate samples by substituting Sr
3
+
the OER activities of transition metal oxides have shown promising
alternatives to the metal electrodes.8 This opens up a vast fam-
ily of electrocatalysts including simple oxides, perovskites, spinels,
for La at A-site of LaNiO to adopt the aforementioned characteris-
3
,11
tics of Sr and Ni. Material and electrochemical characterizations were
carried out to find the efficient electrocatalyst and compare its activity
for OER with the state-of-the-art electrocatalysts in alkaline solution.
1
5
garnets, etc. In particular, oxides of the perovskite family (ABO
3
)
emerged as promising electrocatalysts for water electrolysis due to
7
,9,10,16
their high electrocatalytic activity and better chemical stability.
Materials and Methods
Synthesis of strontium-substituted lanthanum nickelates.—
∗
Strontium substituted lanthanum nickelate (La1−x Sr
x
NiO
3
, x = 0.0
Electrochemical Society Student Member.
E-mail: a.sarkar@iitb.ac.in
z
to 1.0) samples were synthesized using a citrate-nitrate solution
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Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
J3237
combustion method. Initially, stock solutions of 1 M La3 and Ni
cations were prepared by dissolving La(NO .6H O (AR, SDFCL)
and Ni(NO .6H O (AR, LOBA Chemie), respectively in deionized
water. Whereas, the stock solution of 0.5 M Sr cation was prepared
by initially dissolving 37.6607 g of SrCO (LR, SDFCL) in 75 mL of
9% nitric acid (EMPLURA, Merck) solution, and then it was diluted
to 500 mL using deionized water. Stoichiometric amounts of metal
+
2+
a mixture of 4 mg of electrocatalyst and 1 mg of Vulcan carbon (XC-
72R). The basic Nafion solution was prepared by mixing deionized
water and 5 wt% Nafion solution (Sigma Aldrich, U.S.A.) in the vol-
ume ratio of 1:1, and then adding few drops of NaOH solution (5 M)
to raise its pH to 14. Here, the carbon was used as a conductive addi-
tive, as most commonly used for the metal oxide electrocatalysts.9
Experiments done in similar condition (see Oxygen evolution reac-
tion activity measurement section) suggest that the contribution of
carbon to the activity for OER of the metal oxide electrocatalysts
is minuscule (∼0.39–2.52%). This electrocatalyst-carbon suspension
was sonicated for around 30 min prior to drop-casting onto a glassy
carbon electrode (GCE, Pine Instruments) with a geometrical surface
3
)
3
2
3
)
2
2
2
+
3
,10
6
3
+
2+
nitrate solutions (0–10 mL of 1 M La , 0–20 mL of 0.5 M Sr ,
2
+
and 10 mL of 1 M Ni ) were mixed together and then 14.8655 g
of citric acid monohydrate (EMPARTA, Merck) was added to it. The
molar ratio of fuel/oxidant (F/O) was maintained at 0.3 and 1.31–6.53
mL of 4 M nitric acid solution was used to regulate this F/O molar
ratio. The total volume of the solution was adjusted to 30 mL using
deionized water. The final concentrations of precursor metals (La
and Sr ) in 30 mL of the mixture were between 0 and 0.3333 M;
whereas Ni was 0.3333 M. Then, the solutions were initially heated
at 300 C under constant stirring at 250 rpm on a hot plate with mag-
netic stirrer. After some time when the metal-fuel-nitrate solution
turned into a viscous gel, the temperature of the hot plate was de-
creased to 150 C for slow auto-ignition. Under continuous heating
2
area of A = 0.196 cm . After sonication, 20 μL of this ink was
disk
3
+
drop-casted immediately onto a GCE and was dried under an infrared
2
+
lamp for around 15 min. The loading density of the electrocatalysts
2
+
−2
on the GCE was 80.82 μg cm . Prior to drop-casting, the GCE was
disk
◦
polished successively with 0.3, 0.1, and 0.05 μm alumina suspen-
sion and rinsed thoroughly with deionized water while transferring
between each particle size of alumina. The polishing was continued
till a mirror-finished surface was obtained.
◦
and stirring, the gel was ignited, producing voluminous solid pow-
◦
dery products. These were initially dried at 80 C in a hot-air oven
Oxygen evolution reaction activity measurement.—The activity for
for around 12 h and then ground into fine powders using an agate
mortar and pestle. These powders were heated again at 400 C for 10
min on a hot plate for complete ignition of any unreacted precursors
in the powders. Subsequently, these powder samples were calcined at
00 C for 6 h using a muffle furnace in the air with a ramp-up rate of
00 C h . The same procedure was used to synthesize the state-of-
3−δ (BSCF) oxygen electrocatalyst, but it
was calcined at 1000 C for 3 h. Also commercially available noble
OER was measured using a thin-film rotating disk electrode (RDE)
◦
33
setup as described elsewhere, which was similar to the measurement
34
of activity for oxygen reduction reaction. In brief, electrochemi-
cal measurements were carried out with a three-electrode system in
.5 M NaOH electrolyte. A thin film electrocatalyst coated GCE as
◦
◦
8
3
0
−1
described in the previous Electrocatalyst ink and thin film prepa-
ration subsection, platinum mesh, and Hg/HgO (0.5 M KOH) were
used as working, counter, and reference electrodes, respectively. How-
ever, the potential recorded using a Hg/HgO reference electrode was
converted to the reversible hydrogen electrode (RHE) as described
the-art Ba0.5Sr0.5Co0.8Fe0.2
O
◦
metal oxygen electrocatalysts such as IrO
(
2
(Alfa Aesar) and Pt black
Sigma-Aldrich) were used. Henceforth, these samples were referred
as electrocatalysts and these were used for further study.
33
elsewhere and henceforth, the potential values reported in this paper
are versus (vs.) RHE unless otherwise stated. The electrochemical
measurements were carried out using a Gamry potentiostat (Interface
Material characterization.—The elemental composition of
strontium-substituted lanthanum nickelate samples were measured
by the energy dispersive spectroscopy (EDS) using a field emis-
sion gun scanning electron microscope (JEOL JSM-7600F, Japan).
The Brunauer-Emmett-Teller (BET) surface area was measured with
a 3Flex surface characterization analyzer (Micromeritics Instrument
Corporation, U.S.A.). The powder X-ray diffraction (XRD) spectrum
were recorded on an EMPYREAN, PANalytical diffractometer with
Cu Kα radiation. The tube current was 40 mA and a generator voltage
of 45 kV was applied. The XRD spectrum were done in the scan
1
000, U.S.A.) and controlled using Gamry Echem Analyst software.
Prior to the measurements of activities for OER, the electrocatalyst
surface was cleaned by performing cyclic voltammetry (CV) in argon
(
Ar, 99.99%, Mars Gas Company, Mumbai)-saturated and continu-
ously purged 0.5 M NaOH solution (50 mL) between the potential
−1
window of −0.30 to 1.65 V vs. RHE at a scan rate of 50 mV s
for five cycles. Thereafter, the cleaned electrocatalyst thin film coated
GCE was fixed into the RDE assembly, which was controlled by
MSR rotator (AFMSRCE, Pine Research Instrumentation, U.S.A.).
The RDE was immersed in a similar but separate three electrode con-
figuration cell having 0.5 M NaOH solution as electrolyte (500 mL),
◦
range of 2θ = 10–100 . The phase formation in the samples was
identified using X’pert HighScore Plus (version 2.1.0) software by
comparing them with the International Centre for Diffraction Data
but saturated and continuously purged with O
Gas Company, Mumbai). The open circuit voltage (OCV) of the elec-
trocatalyst in O -saturated electrolyte and the ohmic resistance for
2
-gas (99.99%, Mars
(
ICDD). Further, the FullProf Suite program (3.00)31 was used for the
Rietveld analysis, which permits the refinement of lattice parameters
and quantification of phases.
2
iR compensation were measured in the same setup at a rotation rate
of 1600 rpm prior to the measurement of activity for OER. Subse-
X-ray photoelectron spectroscopy (XPS) measurements were
recorded in a Kratos AXIS-supra analytical system. The XPS was
obtained with Al Kα monochromatic radiations. The pass energy and
resolution were kept at 160 and 2 eV, respectively for survey scans;
whereas a pass energy of 20 eV with 0.5 eV resolution were used for
the high resolution scans. Surface charge correction of the binding
energies was performed using the C 1s spectral line (C-C) of adventi-
quently, CVs were performed between the applied potential (Eapplied
)
−1
of 1.0 and 1.8 V at a scan rate of 10 mV s for 10 cycles. For iR com-
pensation (EiRcompensated = Eapplied − iR), an inbuilt script “Get Ru”
available in the Gamry Framework software was employed, which
utilizes electrochemical impedance spectroscopy to determine the un-
compensated resistance, R. The overpotentials (η) were calculated
using, η = EiRcompensated − EOCV, where EiRcompensated and EOCV are iR
compensated potential (V) and the open circuit voltage (V) at equi-
librium, respectively as discussed earlier.
32
tious carbon at the binding energy of 284.8 eV. The collected XPS
survey and high resolution scans of all samples were analyzed using
the ESCApe software (version-1.1). In addition, the XPS spectra of Ni
3
p and Ni 2p3/2 (for x = 1.0) were deconvoluted into separate peaks
by specifying spin-orbital splitting (SOS), full width at half-maximum
FWHM) and the ratio of peak areas for doublets after Shirley back-
(
Results and Discussion
ground subtraction and employing a mixed Gaussian-Lorentzian line
shape function with 30% mixing.
Analysis of elemental composition by EDS.—The EDS analysis
x 3
results for calcined La1−x Sr NiO electrocatalysts are presented in
Electrochemical characterization.—Electrocatalyst ink and thin
film preparation.—Electrocatalyst ink (80 wt%) was prepared by
adding 5 mL of deionized water and 50 μL of basic Nafion (pH 14) to
Table I. The atomic ratios of (La + Sr)/Ni were found to vary from
0.8 to 1.1, which are close to the nominal atomic ratio of 1.0. While,
the atomic composition of most of the samples as determined from
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J3238
Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
Table I. The elemental composition of perovskite samples La1−xSrxNiO3 (x = 0.0 to 1.0) analyzed by the energy dispersive X-ray spectroscopy
(EDS). The values represent the average of two spots on the same sample.
Nominal atomic %
Measured atomic % by EDS
x
Sr2
+
)
La
Sr
Ni
O
La
Sr
Ni
O
(
0
0
0
0
0
1
.0
.2
.4
.6
.8
.0
20
16
12
8
4
0
0
4
8
12
16
20
20
20
20
20
20
20
60
60
60
60
60
60
21.9
17.7
15.2
10.1
4.0
0.0
3.9
6.6
9.8
14.2
18.4
20.3
20.7
24.2
24.8
19.0
22.1
57.8
57.7
54.0
55.2
62.9
59.5
0.0
the EDS analysis were in good agreement with the nominal values, a
slight deviation was observed for the samples with x = 0.4 and 0.6.
Sr
3
O
9
3 6
and two layers of Sr NiO ad infinitum; and it has an average
43
oxidation state of 3.61 (calculated). This suggests the possibility
of Sr
Ni ). Phases identified for each value of x are presented in Table II
along with their crystallographic information.
A substitution of divalent cation Sr for the trivalent La cation
will increase the oxidation state of Ni from 3+ toward 4+ if no oxygen
vacancies are created. Although, Ni is the highest oxidation state
usually attainable, formation of Ni cannot be completely ruled out
and the presence of quadrivalent Ni (Ni ) has been reported in the
2+
3+
9
Ni6.64
O
21 having mixed oxidation states of Ni (Ni , Ni and
4+
43
Phase identification by X-ray diffraction.—The XRD patterns of
calcined La1−x Sr
x
NiO
3
(x = 0.0 to 1.0) and the BSCF electrocatalyst
2+
3+
samples are presented in Figure 1. For x = 0.0, a single phase LaNiO
3
¯
perovskite with a rhombohedral crystal system of R3c (167) space
3+
group was obtained, which is in agreement with the ICDD reference
4+
35
2+
3+
card number 01-088-0633. When Sr was substituted for La in
the A-site of ABO
perovskite with x = 0.2, 0.4 and 0.6, a secondary
phase of NiO was also observed along with the primary phase of
lanthanum-strontium nickelate (La1.6Sr0.4NiO ). This primary phase
for 0.2 ꢀ x ꢀ 0.6 belongs to the A BO -type tetragonal crystal struc-
ture (I4/mmm), which consists of p-type doped NiO
4+
3
48
49
x
past for similar perovskites and NiO . The limited ability of Ni to
4+
form Ni might explain the possible distortion in the crystal structure
for 0.2 ꢀ x ꢀ 0.8 (tetragonal lanthanum-strontium nickelates with
secondary NiO phases) instead of the Sr substituted rhombohedral
lanthanum nickelate. Summarily, the analysis of XRD patterns show
that the Sr -substitution for La in the ABO
has distorted it, leading to a A BO layered perovskite structure.
The XRD pattern of synthesized state-of-the-art electrocatalyst BSCF
4
2
4
36
37
2
layers. These
A
2
BO
4
mixed oxides are built up by alternate layers of La1−x Sr
x
NiO
3
8
2+
3+
3
3
perovskite structure
(
ABO
3
) perovskite type and LaO rock salt (AO) type structures.
2
4
When x is further increased to 0.8, along with the secondary phase
of NiO, SrCO was also observed as an impurity, while the primary
3
(
Figure 1) was compared with XRD patterns available in the literature
phase consisted of La1.67Sr0.33NiO3.8, retaining the same crystal struc-
ture and space group of lanthanum-strontium nickelate. We believe
16,47
and it is in good agreement with them.
3
that the SrCO is formed via the reaction of strontium oxide with
the carbon dioxide formed on combustion of citric acid. This is also
in accordance with the literature which suggests the formation of
Structure analysis and phase quantification by Rietveld
refinement.—In addition to the phase identification by XRD pat-
terns, Rietveld refinement was employed in this study for refinement
of lattice parameters (a, b and c), quantification of phase, and the
calculation of cell volume (V ). Rietveld refinement profiles are pre-
sented in Figure 2 and their corresponding parameters in Table III for
the calcined electrocatalysts. The refinement results suggest that the
weight fraction of lanthanum-strontium nickelate decreased with an
increase in the value of x from 0.2 to 0.8, which was compensated
by an increase in the amount of secondary phase (NiO). For x = 0.8,
SrCO
ovskites for higher values of Sr -substitution.
rhombohedral Sr 21 was formed instead of the SrNiO
with secondary phases of NiO and SrCO . Moreover, the intensity of
3
during the synthesis of strontium substituted lanthanum per-
2
+
18,39–42
When x = 1.0,
9
Ni6.64
O
3
along
3
peaks corresponding to the SrCO
3
phase increased substantially for
Ni6.64O21 consists of stacking one layer of
x = 1.0. The structure of Sr
9
3
SrCO in a substantial amount was also observed in addition to NiO.
Further increase in the value of x to 1.0, the weight fraction of SrCO
3
increased to 62.44% and became dominant while the weight fraction
of strontium nickelate and NiO were in almost equal amount. More-
over, with an increase in the value of x from 0.2 to 0.8, the cell volume
of the lanthanum-strontium nickelate phases decreased. This decrease
in the cell volume can be attributed to either the substitution of smaller
ionic radius cation at A-site and/or formation of smaller ionic radius
33
cation at B-site. However, in the present study ionic radius of sub-
2
+
3+
stituted Sr (1.26 Å) is larger than that of the La (1.16 Å), hence
this substitution is expected to increase the cell volume, which is con-
trary to the observations. Hence, we suspect the possible oxidation of
2+
3+
larger ionic radius Ni (0.63 Å) to smaller ionic radius Ni (0.56 Å)
3
+
4+
and/or Ni to the still smaller ionic radius Ni (0.48 Å) might be re-
sponsible for the decrease in the cell volume of lanthanum-strontium
nickelate samples for x = 0.2 to 0.8. The decrease in the lattice pa-
rameters particularly ‘c’ (Table III), also supports the contraction in
the lattice structure. It has been also reported that the presence of any
oxygen vacancies in the perovskite structure can also contribute to the
50
contraction in the cell volume. Thus, the net contraction in the cell
volume of the samples can be a combined result of an increase in the
oxidation state of Ni and creation of oxygen vacancies. The ionic radii
Figure 1. X-ray diffraction patterns of synthesized La1−x Srx NiO3 (x = 0.0
◦
to 1.0) electrocatalysts after calcination at 800 C and the state-of-the-art elec-
◦
51
trocatalyst Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), which was calcined at 1000 C.
referred in this paper are adopted from the work of Shannon.
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Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
J3239
Table II. Phases identified by the X-ray diffraction technique with their crystal structure information.
x
Sr
Phase
identified
Crystal
system
Space group
(number)
PDF
number
2
+
(
)
Reference
¯
0
.0
LaNiO3
La1.6Sr0.4NiO4
NiO
La1.67Sr0.33NiO3.8
NiO
SrCO3
Sr9Ni6.64O21
NiO
SrCO3
Ba0.5Sr0.5Co0.8Fe0.2O3−δ
Rhombohedral
Tetragonal
Cubic
Tetragonal
Cubic
Orthorhombic
Rhombohedral
Cubic
Orthorhombic
Orthorhombic
R3c (167)
01-088-0633
01-089-8310
01-089-5881
01-089-0425
01-089-5881
01-071-2393
01-086-1217
01-089-5881
01-071-2393
-
35
36
44
45
44
46
43
44
46
0
.2–0.6
I4/mmm (139)
¯
Fm3m (225)
0
.8
.0
I4/mmm (139)
¯
Fm3m (225)
Pmcn (62)
¯
1
R3c (167)
¯
Fm3m (225)
Pmcn (62)
Pnma (62)
BSCF
16, 47
X-ray photoelectron spectroscopy analysis.—Further, the XPS
surface technique was employed in this study to examine the change
in the oxidation state of Ni at B-site of oxide and a possible change in
the binding energies of other elements. The XPS survey scans for all
the samples (x = 0.0–1.0) are presented in Figure 3. Before identifying
the peaks, all the binding energy values were adjusted with reference
to the C 1s peak of adventitious carbon (C-C) at 284.8 eV (peak
4). A sharp peak (peak 5) at around 528.5 eV, which corresponds
to O 1s peak is due to the lattice oxygen in the perovskites. The
binding energy at 134.0 eV corresponds to the Sr 3d (peak 2) and
those at 269.0 and 278.5 eV correspond to the Sr 3p3/2 (peak 3(a))
and Sr 3p1/2 (peak 3(b)), respectively. The peaks 2 and 3 are absent
◦
Figure 2. Rietveld refinement plots of the X-ray diffraction profiles of La1−x Srx NiO3 (x = 0.0 to 1.0) electrocatalyst samples calcined at 800 C.
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J3240
Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
Table III. Rietveld refinement results of La1−xSrxNiO3 electrocatalysts. Here W f is the weight fraction (%), a, b and c are lattice parameters (Å),
and V is the cell volume (Å).
Lattice parameters (Å)
2
x
Phase
χ
W f
V
Sr2
+
)
identified
(%)
(%)
a
b
c
(Å)
(
0
0
.0
.2
LaNiO3
La1.6Sr0.4NiO4
NiO
La1.6Sr0.4NiO4
NiO
La1.6Sr0.4NiO4
NiO
La1.67Sr0.33NiO3.8
NiO
SrCO3
Sr9Ni6.64O21
NiO
SrCO3
3.77
2.83
100
5.4600
3.8253
8.3540
3.8185
8.3531
3.8256
8.3567
3.8231
8.3535
5.1048
9.4813
8.3611
5.1100
5.4600
3.8253
8.3540
3.8185
8.3531
3.8256
8.3567
3.8231
8.3535
8.4119
9.4813
8.3611
8.4202
13.1760
12.7171
8.3540
12.6158
8.3531
12.4205
8.3567
12.3509
8.3535
6.0315
36.0734
8.3611
6.0378
340.17
186.09
583.02
183.95
582.84
181.79
583.59
180.53
582.92
259.00
2808.34
584.51
259.79
85.25
14.75
75.12
24.88
72.08
27.92
47.82
22.89
29.29
18.08
19.48
62.44
0.4
0.6
0.8
5.71
3.79
2.38
1.0
2.65
3p3/2) and D (Ni 3p1/2), which are attributed to the Ni3 also moved
to higher binding energies with an increase in the value of x from 0.0
to 0.6. But further increase in the value of x to 0.8, a slight decrease
in the binding energy values for peaks C and D was observed which
may be due to decrease in the average oxidation state of Ni present
in the lanthanum-strontium nickelate. However, when x value was
further increased to 1.0, a large shift in the binding energy (1.51 eV)
to higher values are distinguishable for all the peaks. The atomic
+
for x = 0.0 and their intensities increased with an increase in the
value of x as seen in Figure 3. The binding energies at 849.7 and
8
53.4 eV correspond to La 3d5/2 (peak 6(a)) and La 3d3/2 (peak 6(b)),
respectively, which are absent for x = 1.0. The peaks 7(a) and 7(b)
at 853.4 and 871.0 eV binding energies correspond to the Ni 2p3/2
and Ni 2p1/2, respectively. However, Ni 2p3/2 (peak 7(a)) strongly
overlap with the La 3d3/2 (peak 6(b)) and its satellite peak, which
3
+
4+
complicates distinguishing the possible presence of Ni and/or Ni
3+
2+
species in the lanthanum nickelate samples. Hence, Ni 3p (peak 1) at
ratio of Ni /Ni also decreased with an increase in the value of x
from 0.0 to 0.4, which is likely due to the formation of secondary
phase containing Ni species. This also corresponds to the XRD
the binding energy of around 67.0 eV is considered for further detailed
2
+
3+
2+
analysis of Ni and Ni species as it is not obstructed by other peaks
even though its relative intensity is weak. A slight increase in binding
energy values for all the peaks are observed with an increase in the
value of x. These binding energies are compared with the NIST XPS
results discussed in the Structure analysis and phase quantification
by Rietveld refinement subsection where the formation of NiO phase
increased with an increase in the x value from 0.2 to 0.6 resulting in
32
3+
2+
3+
2+
database.
a decrease in the Ni /Ni ratio. Interestingly, the Ni /Ni atomic
As mentioned earlier, due to the overlapping La 3d3/2 (peak 6(b))
and Ni 2p3/2 (peak 7(a)) peaks at around 853.4 eV binding energy,
ratio increased as the value of x increased to 0.8 and 1.0. This increase
3+
2+
in the ratio of Ni /Ni and shift in the binding energy values to
2
+
3+
4+
it is less reliable for identifying the presence of Ni and Ni even
higher values suggest the possible presence of the elusive Ni . To the
52
though a method was developed to separate both the peaks. Hence,
best of our knowledge, literature binding energy value of Ni 3p for
4
+
XPS spectra of Ni 3p between binding energy of 80 and 62 eV is
Ni is unavailable. However, Gottschall and co-works assumed that
2
+
3+
52
4+
considered to distinguish Ni and Ni as reported by Qiao and Bi.
binding energy of Ni 2p for Ni will be ca. 4.5 eV more than that of
3
/2
2
+
53
The deconvoluted peaks (A, B, C, and D) of high resolution scans for
all the samples are presented in Figure 4 and their results are given
in Table IV. Binding energies of peaks (A and B) corresponding to
Ni in oxides. If similar assumption holds for Ni 3p3/2 line, peaks
C) between 71.18 and 74.16 probably correspond to Ni , therefore
4+
(
samples with the value of 0.2 ≤ x ≤ 1.0 may contain this elusive
2
+
4
+
.
Ni increased with an increase in the value of x from 0.0 to 1.0.
Ni
4+
However, values of binding energies corresponding to peaks C (Ni
Further, an effort was made to identify the elusive Ni using a
high resolution spectra of Ni 2p3/2 for x = 1.0 sample as this sample
did not have lanthanum, and its deconvoluted spectra is presented in
Figure 5. It can be seen from the deconvoluted spectra that the net in-
tensity can be decomposed into four separate peaks at binding energies
of 854.1, 857.9, and 859.9, and 864.2 eV. Evidently, each peak should
correspond to the different oxidation states of Ni, nickel hydroxide,
and/or satellite peak of Ni. The peak at binding energy of 854.1 eV
2
+ 32
can be assigned to the oxide having Ni
. Moreover, according to
the NIST database, the peaks associated with nickel hydroxide are
between binding energies of 855.3 and 856.6 eV, however, no such
peak was observed in this range of binding energy. Thus, the peak
present at a higher binding energy of 857.9 eV can be assigned to
3
+
49
the Ni of nickel oxide (Ni
2
O
3
), although it is slightly higher than
53
8
56.0 eV. There is an additional peak at more higher binding energy
3+
of ≈859.9 eV to the Ni peak, which is lower than that of the satellite
peak (864.2 eV). Interestingly, it has been reported that binding en-
53
49
4+
ergy of 859.0 eV or 861.2 eV corresponds to Ni . Thus, the peak
4
+
at binding energy of ≈ 859.9 eV may be assigned to the Ni . How-
ever, further characterization such as X-ray absorption spectroscopy
Figure 3. XPS survey scans of La1−x Srx NiO3 (x = 0.0 to 1.0) electrocata-
lysts. Here numbers 1 to 7 represent the elements Ni 3p, Sr 3d, Sr 3p, C 1s,
O 1s, La 3d, and Ni 2p, respectively, and (a) and (b) are spin-orbital-splits of
corresponding elements.
54–56
(
XAS) or X-ray absorption near edge structure (XANES)
be required to confirm the presence of Ni ions in the lanthanum-
strontium nickelate samples.
may
4+
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Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
J3241
Figure 4. High resolution and deconvoluted XPS spectra of Ni 3p for La1−x Srx NiO3 (x = 0.0 to 1.0) electrocatalyst samples.
Oxygen evolution reaction activity.—The current density vs. iR
NiO samples
are presented in Figure 6a along with the state-of-the-art electrocata-
x = 0.8 and strontium nickelate for x = 1.0, and/or (iii) the presence
of impurity phase SrCO . To evaluate the effects of surface area and
weight fraction, the surface area specific activities, joxide (mA cm
compensated potential curves for OER on the La1−x Sr
x
3
3
−
2
)
oxide
−
1
lyst BSCF, and the most commonly used precious metal electrocata-
and weight fraction specific activities, jW f (A g ) were calculated
W
f
th
lysts IrO
2
and Pt black. It may be mentioned here that the 10 cycle
by normalizing with the BET surface area of electrocatalysts and the
weight fraction of lanthanum and strontium nickelates calculated by
Rietveld analysis (Table III), respectively, as presented in Figure 6b.
The measured BET surface areas of electrocatalysts were found to
−1
of the CVs performed at 10 mV s in O
2
-saturated 0.5 M NaOH
electrolyte on a RDE setup with 1600 rpm was extracted and the ob-
tained current-potential data were corrected for the ohmic resistance
before analysis. The corresponding specific activities normalized by
2
−1
be 4.08, 5.95, 4.28, 2.59, 1.56, and 0.47 m g for x = 0.0, 0.2,
0.4, 0.6, 0.8, and 1.0, respectively. After normalization of the current
density with respect to the BET surface area and the weight fraction,
no change is expected upon increasing the Sr content in the samples
for all values of x if the changes in specific activities are due to (i)
the BET surface area and (ii) weight fraction. However, from Figure
−
2
the geometric area of the GCE (disk) or jdisk (mA cm ) for OER
disk
at iR compensated potential of 1.70 V are also presented in the inset
of Figure 6a. It is evident that the current densities ( jdisk) correspond-
ing to OER activities for La1−x Sr NiO increased as the value of x
x 3
increased from 0.0 to 0.6. However, any further increase in the value
of x (0.8 and 1.0) decreased the current densities. The decrease in
the specific current density ( jdisk) of samples for x = 0.8 and 1.0 is
likely due to (i) the possible change in the surface area of samples, (ii)
decrease in the weight fraction of lanthanum-strontium nickelate for
6
b, it can be deduced that the specific activities ( joxide and jW f ) in-
2+
crease with increase in the degree of Sr -substitution. Thus, it clearly
demonstrates that the change in the activity for OER is intrinsic and
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J3242
Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
Table IV. X-ray photoelectron spectroscopy deconvolution results of peaks (A, B, C and D), binding energy (B.E.), relative area (R.A.), and full
width at half-maximum (FWHM) from Ni 3p spectra for strontium-substituted lanthanum nickelate (La1−xSrxNiO3; x = 0.0 to 1.0) samples.
Peak identification
Ni3 /Ni
+
2+
x
Sr2
B.E.
(eV)
R.A.
(%)
FWHM
(eV)
+
(
)
Peak
Species
Line
ratio
0
0
0
0
0
1
.0
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
66.68
68.88
70.46
72.66
66.96
69.16
71.62
73.82
67.13
69.33
71.94
74.14
67.24
69.44
72.21
74.41
67.28
69.48
71.92
74.12
69.66
71.86
73.48
75.64
53.73
26.87
12.94
6.47
56.37
28.17
10.30
5.16
2.59
2.59
2.82
2.82
3.00
3.00
2.65
2.65
2.69
2.69
2.49
2.49
2.95
2.95
2.15
2.15
2.94
2.94
4.64
4.64
3.52
3.52
3.58
3.58
NiO
NiO
Ni2O3
Ni2O3
NiO
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
3p3/2
3p1/2
0.241
0.183
0.158
0.208
0.378
.2
NiO
Ni2O3
Ni2O3
NiO
.4
.6
.8
.0
57.58
28.79
9.09
NiO
Ni2O3
Ni2O3
NiO
4.54
55.18
27.59
11.48
5.74
48.39
24.19
18.28
9.14
4.36
2.18
62.31
31.15
NiO
Ni2O3
Ni2O3
NiO
NiO
Ni2O3
Ni2O3
NiO
NiO
Ni2O3
Ni2O3
14.291
2
+
not due to the changes in the surface area or the weight fraction of
lanthanum-strontium nickelates. Juxtaposing the Rietveld refinement
results (Table III) with the OER activities (Figure 6a) also indicate
that the activity for OER ( jdisk) increased even though there was a
decrease in the amount of lanthanum-strontium nickelate formation
for an increase in the value of x from 0.2 to 0.6. It is also noteworthy to
highlight here that OER activity of the sample with x = 1.0 is higher
than that of x = 0.0 (Figure 6a) even though sample x = 1.0 has
only around 18 wt% of strontium nickelate (Table III). It has been re-
tained when Sr (x = 0.3) was substituted in La Sr NiO -layered
2
−x
x
4
38
perovskites. These results clearly demonstrate that the substitution
2
+
3+
of Sr for La in perovskite family oxides produced more efficient
OER electrocatalysts.
In addition to evaluating and comparing activities among the syn-
thesized electrocatalysts, an effort has been made to compare these
La1 Sr NiO electrocatalysts with laboratory synthesized the state-
of-the-art and various benchmark electrocatalysts from the literature
(Table V). The mass based specific activities, jmass (A g ) were con-
sidered for the comparison at an iR compensated potential of 1.70 V.
−x
x
3
−
1
ported that the impurity phase SrCO
synthesis was present on surface of electrocatalysts. This SrCO
impurity is inactive for the reactions and blocks the active site of elec-
3
formed during electrocatalysts
57
2+
3
It is observed that substitution of Sr , x = 0.6 achieved the high-
−
1
est mass based specific activity (816 A g ) among La Sr NiO
3
1
−x
x
57,58
trocatalysts, which negatively impact the electrocatalytic activity.
Presently, the presence of any inactive SrCO impurity does not seem
electrocatalysts and also the lanthanum-strontium nickelate electro-
catalysts surpassed the state-of-the-art and precious metal electrocat-
alysts. The electrocatalyst with x = 0.6 has around four times higher
3
to affect the electrocatalytic activity. Similar results were also ob-
2
mass specific activity than that of BSCF and IrO , and six times
that of Pt black at 1.70 V. Amongst the tabulated values of mass
specific activity for OER of perovskite-type electrocatalysts in Table
V, La0.4Sr0.6NiO
3
, i.e., for the value of x = 0.6 in La1−x Sr
shows higher activity than that of the state-of-the-art electrocatalyst
BSCF but lower than that of hybrid NiO-(La0.613Ca0.387
x 3
NiO
2
) NiO3.562
and precious metal electrocatalyst IrO
literature.
2
reported in the
It has been reported that oxides having mixed oxidation state and/or
2
0,33,48,60,62
higher oxidation state of transition metal present at B-site,
59
and hybrid NiO-(La1−x Ca
x
)
2
NiO4−δ have shown better electrocat-
alytic activity for OER. In the present case, both these effects might be
operative. While the Sr -substitution induces mixed oxidation states
2
+
2
+
3+
4+
of Ni (Ni , Ni , and possibly Ni ), the secondary phase of NiO
along with lanthanum-strontium nickelate possibly provides a syner-
gistic effect. Although, the contribution of the synergistic effect to the
overall OER activity might be small considering the weight fraction
of NiO in the samples. In essence, the higher activity for OER for
the synthesized lanthanum-strontium nickelates may be attributed to
the combined effect of the existence of mixed oxidation state of Ni
and presence of hybrid NiO-lanthanum-strontium nickelate electro-
catalyst.
Figure 5. High resolution and deconvoluted XPS spectra of Ni 2p3/2 for x =
1.0 i.e., SrNiO3 electrocatalyst sample.
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Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
J3243
Table V. Specific activities, jmass (A g 1) for oxygen evolution reaction of synthesized electrocatalysts and various benchmark electrocatalysts at
iR compensated potential of 1.70 V vs. RHE. The values reported here from the literature are extracted from the i − E polarization curves and
the mass activities in the present study were using the electrocatalyst loading of 15.84 μg, which included secondary phases NiO and SrCO3.
−
Electrocatalyst
Mass specific activity
jmass (A g 1) at 1.70 V
−
Reference
Type
Oxide
Electrolyte
ABO3
LaNiO3
LaNiO3
LaNiO3
SrFeO3
0.1 M KOH
0.1 M KOH
0.5 M NaOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.5 M NaOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.5 M NaOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.1 M KOH
0.5 M NaOH
0.5 M NaOH
≈ 3
559
278
8
45
45
563
21
816
89
753
27
15
38
59
This work
60
60
60
21
CaFeO3
CaCuFe4O12
SrCoO3−δ
La0.6Sr0.4CoO3
La0.4Sr0.6NiO3
BaNiO3
BSCF
BSCF
BSCF (950 C)
BSCF (1100 C)
18
This work
61
9
59
60
◦
◦
8
60
BSCF
La2NiO4
La1.7Sr0.3NiO4
193
≈ 2
≈ 5
764
8174
7569
563
170
125
This work
A2BO4
Noble
38
38
59
59
9
(
La0.6Ca0.4)2NiO4−δ
NiO-(La0.613Ca0.387)2NiO3.562
IrO2
IrO2
IrO2
Pt black
21
This work
This work
Tafel analysis.—Tafel analysis was done for the OER data as
shown in Figure 7 according to the Tafel equation, η = b log j +
Conclusions
x 3
Strontium-substituted lanthanum nickelate (La1−x Sr NiO )
−1
a, where b and j are the Tafel slope (V dec ) and current density
−
2
perovskite-type oxides were synthesized by the nitrate solution com-
bustion method. The values of the elemental composition of electro-
catalysts measured by the EDS are in good agreement with nominal
values. Further, structural characterization done with the help of XRD
(
A cm ), respectively. The transfer coefficient, α was calculated
disk
using α = 2.303RT/bF, where b is the Tafel slope. The kinetic pa-
rameters obtained by the Tafel analysis by plotting η vs. log j for OER
are presented in Table VI. The Tafel slope decreased with an increase
in the value of x till 0.6 and then it increased for x = 0.8, which follows
the trend of measured activities for OER of samples with 0.0 ≤ x ≤
2
+
3+
showed that the substitution of Sr for La at A-site distorted the
original crystal structure of lanthanum nickelate to form a tetragonal
lanthanum-strontium nickelates with smaller cell volume and also pro-
0
.8. However, the Tafel slope decreased again with further increase in
3
duced a secondary phase of NiO, and impurity SrCO for higher values
the value of x to 1.0. It has been reported that the Tafel slope for OER
of x. Rietveld analysis suggests that contraction in the lattice structure
was due to the formation of smaller ionic radii Ni ions at B-site of
oxides and/or due to creation of oxygen vacancies. It is also suspected
that a still higher oxidation state of Ni than Ni is present, however,
corroboration with XPS was found to be weak. The substitution of
−1
on pure nickelates were much smaller (2RT/3F ≈ 40 mV dec ) than
−
1 30
that of multiphase samples of nickel oxide (RT/F ≈ 60 mV dec ).
The Tafel slopes obtained for the La1−x Sr NiO samples in this study
are in good agreement with the literature value of 65 mV dec on
x
3
3+
−
1
7
,30
−1
LaNiO
3
and found to vary between 61.9 and 71.2 mV dec , which
2
+
Sr in La1−x Sr
x
NiO
3
enhanced the activity for OER until x = 0.6 and
are close to the Tafel slope of ≈ RT/F. The Tafel slope of 60 mV
decrease in the OER activities were observed with a further increase
in the value of x. Irrespective of the decrease in the OER activity at
higher values of x (0.8 and 1.0), all synthesized lanthanum-strontium
nickel oxide electrocatalysts have higher OER activities than the most
−
1
dec for the OER is indicative of the rate determining step involves a
chemical reaction, in which electrochemically oxygenated transition
30
2
metal surface (M-O) rearranges to produce O , which is commonly
15,63
observed on the perovskite oxides.
2
commonly used electrocatalysts (IrO and Pt) and the state-of-the-art
electrocatalyst (BSCF). Therefore, present results suggest that use of
single phase strontium substituted lanthanum nickel oxide and/or sin-
gle phase strontium nickel oxide can be a promising anode oxygen
electrocatalyst for the alkaline water electrolysis.
Table VI. Kinetic parameters of OER on La1−xSrxNiO3
electrocatalysts in 0.5 M NaOH electrolyte. The open circuit voltage
(OCV, V vs. RHE) of the electrocatalysts was measured in O2
saturated 0.5 M NaOH solution prior to the OER measurements.
−
1
Here b, α, and j0 represent Tafel slope (mV dec ), transfer
−2
coefficient, and exchange current density (mA cm ), respectively.
disk
Acknowledgments
Authors thank the Bhabha Atomic Research Centre (BARC),
Mumbai, India for funding this work (Sanction No. 36(1)/14/50/2016-
BRNS/36123). Authors also acknowledge the Department of Metal-
lurgical Engineering and Materials Science, IRCC-Central facility,
and Sophisticated Analytical Instrument Facility (SAIF) of IIT Bom-
bay for performing XRD, XPS, and EDS characterizations, respec-
tively. A. Sarkar greatly acknowledges the Department of Science and
Technology (DST), India with the Ramanujan Fellowship for financial
support.
x
Sr
OCV
(V)
b
j0
2
+
R2
−1
−2
(
)
(mV dec
)
α
(mA cm
)
disk
−
−
−
−
−
−
7
0
0
0
0
0
1
.0
.2
.4
.6
.8
.0
1.15
1.19
1.07
1.13
1.18
1.16
0.9991
0.9991
0.9976
0.9983
0.9987
0.9984
71.2
71.0
67.0
65.3
68.3
61.9
0.83
0.83
0.88
0.91
0.87
0.96
7.6 × 10
6.8 × 10
1.3 × 10
5.7 × 10
4.2 × 10
2.4 × 10
6
7
7
6
7
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J3244
Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
ORCID
Ravi Sankannavar https://orcid.org/0000-0002-1590-5609
A. Sarkar https://orcid.org/0000-0002-3873-4531
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Journal of The Electrochemical Society, 165 (15) J3236-J3245 (2018)
J3245
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