Structure-Activity Relationship in Manganese Perovskite Oxide Nanocrystals from Molten Salts for Efficient Oxygen Reduction Reaction Electrocatalysis

Article abstract of DOI:10.1021/acs.chemmater.0c00681

We report a synthesis pathway in molten salts toward ligand-free nanoparticles of the layered perovskite La0.5Sr1.5MnO4 (l-LSMO) and of the pseudocubic perovskite La0.7Sr0.3MnO3 (pc-LSMO). These particles are readily implemented as oxygen reduction reaction (ORR) electrocatalysts in alkaline conditions. They show high ORR selectivity for the 4-electron reduction of O2 in water. Among these two materials, pc-LSMO nanocrystals of 20 nm diameter exhibit high mass-normalized ORR activity for a perovskite material (21.4 A g-1oxide at 0.8 V/RHE) thanks to their relatively large surface area, high crystallinity, and electron mobility. These features provide pc-LSMO nanocrystals with remarkable stability compared to state-of-the-art perovskites, for instance, only a 5percent increase of the overpotential at -0.05 mA cm-2oxide over 40 h. Thus, pc-LSMO nanocrystals are the most stable perovskite ORR electrocatalyst reported to date. This performance combined with high activity, high selectivity, and the absence of precious metals make La0.7Sr0.3MnO3 nanocrystals one of the best compromises for alkaline oxygen reduction reaction.

Full text of DOI:10.1021/acs.chemmater.0c00681

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Article
Structure-activity relationship in manganese perovskite oxide nanocrystals
from molten salts for efficient oxygen reduction reaction electrocatalysis
Francisco Gonell, Carlos M. Sanchez-Sanchez, Vincent Vivier,
Christophe Méthivier, Christel Laberty-Robert, and David Portehault
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.0c00681 • Publication Date (Web): 01 Apr 2020
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Chemistry of Materials
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Structure-activity relationship in manganese perovskite oxide
nanocrystals from molten salts for efficient oxygen reduction
reaction electrocatalysis
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Francisco Gonell,a Carlos M. Sánchez-Sánchez,b Vincent Vivier,b Christophe Méthivier,c Christel
Laberty-Robert,a David Portehault*a
a Sorbonne Université, CNRS, Collège de France, Laboratoire de Chimie de la Matière Condensée de Paris (CMCP),
4 place Jussieu, F-75005, Paris, France
b Sorbonne Université, CNRS, Laboratoire Interfaces et Systèmes Electrochimiques (LISE), 4 place Jussieu, F-75005,
Paris, France
c Sorbonne Université, CNRS, Laboratoire de Réactivité de la Surface (LRS), 4 place Jussieu, F-75005, Paris, France
ABSTRACT: We report a synthesis pathway in molten salts toward ligand-free nanoparticles of the layered perovskite
La0.5Sr1.5MnO4 (l-LSMO) and of the pseudo-cubic perovskite La0.7Sr0.3MnO3 (pc-LSMO). These particles are readily
implemented as oxygen reduction reaction (ORR) electrocatalysts in alkaline conditions. They show high ORR selectivity
for the 4-electrons reduction of O2 in water. Among these two materials, pc-LSMO nanocrystals of 20 nm diameter
exhibit high mass-normalized ORR activity for a perovskite material (21.4 A g-1oxide at 0.8 V/RHE) thanks to their relatively
large surface area, high crystallinity and electron mobility. These features provide pc-LSMO nanocrystals with remarkable
stability compared to state-of-the-art perovskites, for instance only a 5% increase of the overpotential at -0.05 mA cm-2
oxide
over 40 hours. pc-LSMO nanocrystals are then the most stable perovskite ORR electrocatalyst reported up to now. These
performances, combined with high activity, high selectivity and absence of precious metals, make La0.7Sr0.3MnO3
nanocrystals one of the best compromises for alkaline oxygen reduction reaction.
Manganese-based oxides are promising precious metal-
free electrocatalysts of the oxygen reduction reaction
(ORR) for alkaline fuel cells.1,2 Both Mn octahedral
coordination and mixed valence Mn3+ / Mn4+ are critical
parameters to provide high ORR electrocatalytic activity.3–
in order to crystallize the perovskite structure. This
treatment delivers manganites with relatively low surface
area16,
highly
aggregated
particles,
ill-defined
morphology, and large density of crystal defects,18 which
hinder charge transfer, ORR electrocatalysis and overall
control of catalytic properties.3 The low surface area
bottleneck has been recently overcome by using
nanoparticles as reagents during sol-gel-based syntheses,
yielding high mass-normalized activity.19 Designing
manganese perovskites nanomaterials with small particle
size but also high crystallinity should further enhance
stability during electrocatalysis, but this level of materials
design could not been reached through sol-gel-based
processes.
5
These requirements are met in pseudo-cubic perovskite
manganites with general formula AMnO3 and layered
perovskite
Ruddlesden-Popper
(RP)
manganites
An+1MnnO3n+1 (or (AO)(AMnO3)n n = 1, 2…) that withstand
numerous cationic substitutions, mixed Mn valences and
lattice distortions.3,6–11 RP manganites contain layers of n
AMnO3 perovskite units alternating with AO rock-salt
layers, which provide additional anionic conductivity.9,12,13
This feature should increase electrocatalytic activity.14
Strategies explored to enhance the ORR electrocatalytic
activity of manganese-based oxides focus on increasing
the surface area of the materials in order to provide more
active sites per mass unit compared to the large crystals
typically obtained with solid-state synthesis. This goal has
been pursued either by decreasing particle size15 or by
designing macroporous perovskites16 by using sol-gel-
based synthesis related to the Pechini process.17 These
approaches require a final treatment at high temperature
In the present work we aim at synthesizing, studying
and comparing the ORR electrocatalytic properties of
nanoscaled pseudo-cubic and layered perovskite
manganites in order to (i) report the ORR electrocatalytic
properties of RP manganites at room temperature in
alkaline aqueous medium , (ii) study the role of the
dimensionality of perovskite-related crystal structures in
ORR electrocatalysis; (iii) highlight pseudo-cubic
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perovskite manganite nanocrystals as high performance
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show that the l-LSMO sample is made of cube-like
particles with a broad size distribution between 50-200
nm in agreement with the crystallite size. Selected area
electron diffraction (SAED, Figure 1e inset) and the Fast
Fourier Transform-analyzed HRTEM (Figure 1h) show
spot-like patterns indicating well-oriented crystallites
within the same particle. Nitrogen sorption yields a BET
surface area of 8 m2 g-1, smaller than the 36 m2 g-1
measured for pc-LSMO, in agreement with the larger
particle size of l-LSMO.
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ORR electrocatalysts.
We draw on a one-step colloidal synthesis in inorganic
molten salts. These liquids are stable at high
temperatures and are then compatible with direct
crystallization of the perovskites. In liquids, the
nucleation rate is higher than in the above-mentioned
methods, resulting in a decrease of the particle size. The
manganite perovskite particles we report exhibit small
sizes and organic ligand-free surfaces that are beneficial
for charge transfer at the electrocatalyst interfaces, hence
for electron percolation in the electrode, and for
adsorption of dioxygen reagent. They also exhibit low
amounts of crystal defects, again beneficial for charge
transport20,21 compared to sol-gel derived materials.
Hence, the approach delivers highly active and stable
manganite perovskite ORR electrocatalysts. In addition to
the pseudo-cubic manganite La0.7Sr0.3MnO3,20,21 we
synthesize the first layered manganite La0.5Sr1.5MnO4 RP
nanoparticles by tuning the oxo-basicity of the molten
medium. We report for the first time the mechanisms of
nanoparticles formation in high temperature molten salts
and highlight the key experimental parameters enabling
structural control. We show high mass-normalized ORR
activity for perovskites. Highly crystalline La0.7Sr0.3MnO3
nanocrystals also show remarkable stability of the ORR
performances over 40 hours of operation, thus out-
performing previously reported perovskites. These
precious metal-free nanomaterials are a step forward for
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pc-LSMO (Figure 1i) is a reaction intermediate at
600 °C that evolves to the layered phase at 800 °C. This
was confirmed by redispersing into NaNO2 pre-
synthesized pc-LSMO nanoparticles (Figure 1a-b and
low-cost
alternatives
to
precious
metal-based
electrocatalysts.
RESULTS AND DISCUSSION
Pseudo-cubic La0.7Sr0.3MnO3 (pc-LSMO) nanocubes of
20 nm (Figure 1a-b and Figure S1) were produced
according to a protocol we previously reported, using
molten KNO3 as a liquid medium at 600 °C where metal
nitrates are decomposed.21 When the reagents ratio was
adjusted to target the layered perovskite La0.5Sr1.5MnO4 (l-
LSMO) in the same conditions, the reaction yielded only
the pseudo-cubic structure (Figure S2). Increasing the
Sr/Mn ratio and the temperature up to 800 °C (Table S1)
triggered crystallization of the layered structure
(Figure S2), but the cationic content evaluated by
wavelength dispersive X-ray fluorescence (WDXRF, Table
S1) showed that all La3+ and Sr2+ cations were not
incorporated in the solid, as suggested by La(OH)3 and
SrCO3 impurities identified by X-ray diffraction (XRD,
Figure S2). To trigger full incorporation of the remaining
cations, the oxo-basicity of the solvent was increased by
using molten NaNO2 instead of KNO3.22,23 Although the
pseudo-cubic structure was again obtained at 600 °C
(Figure 1i and Figure S3), the reaction was completed at
800 °C and yielded l-LSMO as the only crystalline phase
with composition La0.5Sr1.5MnO4 according to WDXRF
(Table S1). The l-LSMO Scherrer crystal size of 67 nm is
smaller than reported values for RP manganite
particles.24–28 SEM and TEM (Figure 1c-g, respectively)
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specific current density of ca. -1 mA cm-2oxide, twice larger
than pc-LSMO. At a lower specific current density of -
40 µA cm-2oxide, pc-LSMO and l-LSMO operate at 0.86 and
0.83 V/RHE, respectively (Figure 3b inset), thus
indicating that the cubic phase is the most active at low
overpotentials. This is corroborated by Tafel slopes
(Figure S7) of 56 and 99 mV dec-1 for pc-LSMO and l-
LSMO, respectively, in good agreement with previous
works.3,5 Both materials exhibit surface-normalized
electrocatalytic activities (0.85 V vs RHE for 40 µA cm-
Figure 1. (a) TEM and (b) HRTEM images of pc-LSMO
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nanocubes. (c) SEM and (d-g) TEM images of l-LSMO
nanoparticles (SAED pattern as inset in (e) indexed along the
[331] zone axis). (h) HRTEM image of a l-LSMO nanoparticle
and corresponding Fast Fourier Transform. (i) XRD patterns
(Cu Kα) of the powders obtained by targeting l-LSMO in
molten NaNO2 at several reaction temperatures for 15
minutes.
Figure S1) with additional amounts of Sr(NO3)2 meeting
the stoichiometry of the layered phase. The XRD patterns
of the powders recovered after heating (Figure S4) show
a phase evolution towards l-LSMO with a temperature
increase, similar to the direct syntheses from the salts
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oxide) matching those of most active
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described above.
A blank treatment of pc-LSMO
nanocubes into molten NaNO2 at 800 °C without
additional Sr2+ source yielded crystalline La(OH)3
(Figure S5) and amorphous matter after water washing.
This suggests that before exposure to water, the reaction
medium actually contained dissolved La3+ ions. Thus, pc-
LSMO was solubilized in molten NaNO2. The dissolution
of pc-LSMO in this blank treatment indicates that the l-
LSMO on-pot synthesis proceeds through pc-LSMO
dissolution followed by l-LSMO recrystallization
(Figure 2).
The ORR activity in alkaline solution of l-LSMO and pc-
LSMO
nanomaterials
was
evaluated
on
catalyst/Nafion/carbon black composite electrode films
deposited on a glassy carbon rotating disk substrate. The
cyclic voltammograms (CVs) in 0.1 m KOH were first
normalized to the area of the glassy carbon collector,
corresponding to a constant catalyst mass loading (0.136
mg cm-2, Figure 3a). The cathodic current density at
Figure 2. Reaction pathway yielding layered perovskite (l-
LSMO) nanoparticles from pc-LSMO into molten NaNO2.
0.4 V/RHE for pc-LSMO is almost twice that of l-LSMO.
pc-LSMO nanocrystals show high mass-normalized ORR
activity for oxide perovskites (21.4 A g-1
at 0.8 V/RHE,
oxide
Table S2)4,5,14–16,29,30 thanks to the combination of their
high crystallinity and relatively large surface area. The
disk surface-normalized activity of pc-LSMO nanocrystals
is close to the benchmark precious metal catalyst (Pt/C)
(Figure S6). For comparison of the specific activities, the
current density was normalized versus BET surface areas
(Figure 3b). At 0.4 V/RHE, l-LSMO exhibits a cathodic
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Figure 3. ORR electrocatalytic activity of pseudo-cubic pc-
within a 1 nm-thick layer at the surface of pc-LSMO
nanocubes because of oxygen vacancies,21,33 yielding a
surface Mn oxidation state close to 3.3.33 Likewise, X-ray
photoelectron spectroscopy (XPS, Figure S16) yields a
surface Mn oxidation state of 3.2 for l-LSMO. Given the
precision of XPS (±0.1 on the oxidation state), pc-LSMO
and l-LSMO possess identical surface oxidation states.
Hence, both the pseudo-cubic and the layered perovskite
samples exhibit at their surface the Mn oxidation state
optimal for ORR, which rules out surface oxidation states
as a possible origin for the higher activity of pc-LSMO.
Therefore, the difference in ORR activity between both
materials presumably relates to charge-transfer kinetics
within the whole materials (Figure S9 right). At room
temperature with the measured average AOS, the pseudo-
cubic phase is metallic whereas the layered phase is semi-
conducting.34 Then, electronic conduction is larger in pc-
LSMO and enhances electrocatalytic activity at low
overpotential.3 On the opposite, at high overpotential,
below 0.6 V/RHE, we observe a higher ORR current for l-
LSMO compared to pc-LSMO. In these conditions, the
conduction band of l-LSMO becomes populated and
electronic conductivity is not limiting anymore the
electrocatalytic process. The higher activity of l-LSMO
may be related to the high ionic conductivity of the
layered phase.
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LSMO (blue curves) and layered l-LSMO (green curves) Mn
perovskites (O2-saturated 0.1 m KOH electrolyte, 10 mV s-1
and 1600 rpm). (a) Electrode area-normalized and (b) BET
surface-normalized
cyclic
voltammograms.
(c)
Chronopotentiometry curves of pc-LSMO at -0.05 mA cm-
2
(blue) and of pc-LSMO (black) and l-LSMO (green) at
oxide
several current densities.
manganese perovskite epitaxial thin films grown by
pulsed laser deposition.3 This demonstrates the high
crystalline quality of the nanocrystals produced in molten
salts, which provides high electronic conduction. The
synthesis in molten salts differs from conventional
colloidal synthesis in organic solvents, as it delivers
materials with surfaces free of any insulating organic
ligands. These ligand-free surfaces also contribute to
enhanced charge percolation and electrocatalytic activity.
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The number of electrons transferred during ORR is
between 3.9 and 4.0 according to a Koutecky-Levich
analysis of the diffusion current at 0.4 V/RHE
(Figure S8), in very good agreement with the theoretical
4-electrons value for the overall reduction reaction of O2
into H2O (Figure S9 left).4,5 This was also confirmed by
scanning electrochemical microscopy experiments
-
(SECM) that could not detect any corrosive HO2
byproduct during ORR for both electrocatalysts in the
whole potential range (Figure S10). In addition, the
acetylene black (AB) carbon additive shows a negligible
activity above 0.6 V vs RHE and a low activity at lower
potential (Figure S6). This further confirms that the
perovskites are the only active materials above 0.6 V vs
Finally, we have addressed the long-term stability of pc-
LSMO and l-LSMO during ORR operation conditions by
using chronopotentiometry (Figure 3c). pc-LSMO
exhibits remarkable stability at - 0.05 mA cm-2
with
oxide
only a slight decrease of 5 %in the voltage (from 0.77 to
0.73 V/RHE) over 40 hours. pc-LSMO appears as the most
stable ORR perovskite electrocatalyst reported up to now
(Table S2). By successively increasing the cathodic
-
RHE. The absence of any HO2 byproduct detected in the
vicinity of the electrode by SECM also enables to rule out
possible activity of the acetylene black additive through
formation of an H2O2 intermediate: the reaction
proceeds only through a 4-electrons reduction pathway at
the surface of the perovskite catalysts. This makes these
manganese-based perovskites ideal for implementation
into fuel cells. The reaction mechanism was further
confirmed by probing the effect of surface hydroxylation
and hydrophobicity on the overall ORR disk area-
normalized current density (see details in Figure S11-S15
and Table S3), as previously described for OER
electrocatalysts.31,32
current every
5
hours, pc-LSMO shows stable
where the
performances up to -0.30 mA cm-2
oxide
electrocatalytic activity drastically decreases. The
electrocatalytic activity of l-LSMO is less stable and drops
at -0.20 mA cm - 2oxide. These results are confirmed by the
CVs measured before and after each current step applied
in chronopotentiometry where a decrease of the cathodic
current is observed (Figure S17).
In order to gain insights into the deactivation process,
the materials were recovered after chonopotentiometry
and studied by HRTEM and STEM-EDS. After operation
during 40 h at -0.05 mA cm-2oxide, pc-LSMO exhibits the
same well faceted cubic morphology as the fresh material,
with particles still crystallized into the pc-LSMO structure
(Figure 4a-c). The same material observed after
successively increasing the cathodic current up to -0.35
mA cm-2oxide and 35 h still shows well faceted particles, but
others have lost their morphology and got amorphized
(Figure 4d-f). l-LSMO shows more severe damage, as up
to a current density of -0.20 mA cm-2oxide and after 20 h of
operation, a 3-4 nm-thick amorphous layer appeared at
the sur face of the particles (Figure 3g-i). EDS analysis of
electrode material performed in a scanning electron
microscope (SEM) indicates that the cationic composition
did not change significantly (Table S4). STEM-EDS
The ratio between Mn3+ and Mn4+ plays a crucial role in
the ORR kinetics. According to a model describing
localized orbitals on surface Mn cations,3,5 the optimum
AOS for ORR is expected at 3.3 for pseudo-cubic
manganite perovskites, where the best compromise is
-
reached between O2 /OH- exchange at the surface of the
catalyst, and the regeneration of OH-. In the present case,
chemical titration yields average Mn oxidation states
(AOS) of 3.6 and 3.3 in pc-LSMO and l-LSMO,
respectively. Hence, l-LSMO would be expected as the
most active electrocatalyst, which disagrees with our
observations at low overpotential. This apparent
discrepancy may originate first from a change in the
oxidation state of Mn at the surface of the particles.
Indeed, we have previously shown that Mn is reduced
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mapping of the post-mortem electrode also shows
and then extended reaction with the electrolyte. On the
contrary, pc-LSMO shows very high stability compared to
other Mn-based materials already reported (Table S2).
ORR has been previously evaluated only on Mn
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homogeneous element distribution (Figure S18-S20).
Hence, the deactivation during ORR is related to surface
amorphization. This phenomenon is more extended for
the layered perovskite, which may be ascribed to its
layered structure that provides larger ion conduction13
perovskite
nanoparticles
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Figure 4. TEM and HRTEM micrographs of pc-LSMO after chronopotentiometry in 0.1 m KOH at (a-c) -0.05 mA cm-2oxide during
40 hours and (d-f) after increasing current densities (from -0.05 to -0.35 mA cm-2oxide, see Figure 2c). (g-i) TEM and HRTEM
micrographs of l-LSMO after increasing current densities (from -0.05 to -0.20 mA cm-2oxide, see Figure 2c). The Fast Fourier
Transforms in (i) show that the material is crystallized in the particle core but amorphous in a 3-4 nm-thick layer.
produced by sol-gel-derived methods, which yield large
amounts of crystal defects.15,16 Herein we demonstrate that
the higher crystallinity of the molten salts-derived
nanocrystals ensures higher long-term stability under
ORR operation conditions of those materials and of their
electrochemical activity
pseudo-cubic phase. However, the layered structure is
poorly stable in operation conditions, probably due to its
high ionic conductivity. On the other hand, the pseudo-
cubic perovskite exhibits small particle size with low
amounts of crystal defects and no insulating organic
ligands. These features ensure high electronic
conductivity within the electrode, high activity and
remarkable stability under ORR operation for
La0.7Sr0.3MnO3 nanocrystals, making pseudo-cubic
manganite nanocrystals a promising precious metal-free
electrocatalyst for alkaline fuel cells.
CONCLUSIONS
Herein we have reported the synthesis of the layered
La0.5Sr1.5MnO4 perovskite (l-LMSO) in the nanoscale range
for the first time. Accurate control of the solvent
composition and thermal treatment allows obtaining pure
pc-LSMO and l-LSMO perovskites by deciphering the l-
LSMO crystallization process through the dissolution of a
pseudo-cubic perovskite followed by reprecipitation of
the layered phase in the molten salts. The pseudo-cubic
La0.7Sr0.3MnO3 (pc-LSMO) and layered La0.5Sr1.5MnO4
perovskites exhibit high electrocatalytic activity for the
ORR with high selectivity for the four-electrons O2
reduction thanks to an optimized Mn oxidation state in
the first surface atomic layers. l-LSMO shows high
intrinsic (surface-normalized) electrocatalytic activity,
despite its limited electronic conductivity versus the
EXPERIMENTAL PROCEDURES
Synthesis
All reagents used in the synthesis were purchased from
Aldrich, otherwise said, and used as received without
further purification. Water used for washing is MilliQ®
water.
Pseudo-cubic La0.7Sr0.3MnO3 (pc-LSMO) synthesis: pc-
LSMO was synthesized according to a previous study.21
Briefly, 6.5 mmol of Mn(NO3)2·4H2O, 3.9 mmol of
La(NO3)3·6H2O, 2.0 mmol of Sr(NO3)2 and 65 mmol of
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KNO3 were mixed with a few drops of ethanol and milled
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perovskite catalyst surface area was calculated from the
adsorption branch of the N2 sorption isotherm according
to the BET method at 77 K using an ASAP 2010
equipment. Prior to sorption measurements, the powders
were degassed at 300 °C for 4 h. The Mn Average
Oxidation State (AOS) was determined following a
previously reported protocol.35 Briefly, the powders were
dissolved in acidic media and the manganese species were
reduced by Fe2+ in excess. The excess Fe2+ ions were
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in a Retsch MM400 ball miller at 20 Hz for 2 min. The
mixture was then dried under vacuum at 40 °C overnight.
2.5 g were introduced in an alumina crucible and treated
at 600 oC for two hours in a preheated muffle oven in air
atmosphere. Then, the reaction was quenched at room
temperature by removing the crucible from the hot oven.
The material was washed by centrifugation with
deionized water several times until the conductance of
the supernatant reached the one of the water used for
washing. The powder was dried under vacuum at room
temperature overnight.
2-
titrated with Cr2O7 . The total amount of Mn was
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determined by UV-visible spectrophotometry by
oxidizing it into permanganate. With these two
independent experiments, the determination of MnIII and
MnIV proportions was made possible.
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Layered La0.5Sr1.5MnO4 (l-LSMO) synthesis: 6.5 mmol of
Mn(NO3)2·4H2O, 3.1 mmol of La(NO3)3·6H2O, 8.0 mmol of
Sr(NO3)2 and 65 mmol of KNO3 or 95 mmol of NaNO2
were mixed with a few drops of ethanol and milled in a
Retsch MM400 ball miller. The mixture was then dried
under vacuum at 40 °C overnight. 2.5 g were introduced in
Electrochemical characterization. The electrocatalytic
properties of the materials were studied through a three-
electrode setup in an electrochemical cell using a Pt wire
as a counter electrode, a saturated Ag/AgCl reference
electrode and a glassy carbon (GC) rotating disk electrode
(RDE) as a working electrode, connected to a VPS
Biologic potentiostat. All potentials are referred in the
o
an alumina crucible and treated at 600-800 C for 15 min
in air atmosphere. Then, the reaction was quenched at
room temperature by removing the crucible from the
oven. The material was washed by centrifugation with
deionized water several times until the conductance of
the washing waters reached the same value as deionized
water. The powder was dried under vacuum at room
temperature overnight.
text to the reversible hydrogen electrode (RHE, ERHE
=
EAg/AgCl(sat) + 0.197 + 0.059pH). The GC RDE (geometrical
disk area (0.07 cm²), Radiometer Analytical) was polished
with diamond paste (1 µm, BAS inc.) followed by diamond
paste (0.05 µm, BAS inc.) to reach mirror grade before
each experiment. Then, the GC disk was coated with a
conductive ink that contains the different perovskite
catalysts. This ink was composed by the perovskite
catalyst powder, hydrophilized Acetylene Black carbon
(AB, Alfa Aesar) (99.9+ %, 75 m² g-1) and Nafion-117
(5 wt. % solution in aliphatic alcohols, Sigma-Aldrich). AB
was hydrophilized by treating 0.5 g of AB in 50 mL of
Reactivity test of pc-LSMO into molten NaNO2: 0.25 g
(1.1 mmol) of pc-LSMO was mixed with 2.5 g of NaNO2
and milled in an agate mortar, then dried overnight at
40 °C under vacuum. This mixture was introduced in an
alumina crucible and treated at 800 °C for 15 min under
air atmosphere. The reaction was then quenched at room
temperature, the powder was washed with deionized
water and dried as explained above.
o
HNO3 20% at 80 C under stirring overnight, followed by
centrifugation, washing with water and vacuum drying. 10
mg of the perovskite catalyst powder, 10 mg of the
modified AB and 10 mL of absolute ethanol were mixed
and sonicated for 2 hours. Then, 435 µL of ion-exchanged
Nafion solution (5 wt. % in aliphatic alcohols, Aldrich)
was added and the suspension was sonicated for 5 min.
Reactivity test of pc-LSMO into molten NaNO2 in the
presence of Sr2+ ions: 0.25 g (1.1 mmol) of pc-LSMO,
1.48 mmol of Sr(NO3)2 and 2.5 g of NaNO2 were mixed
and milled in an agate mortar, then dried overnight at
40 °C under vacuum. This mixture was introduced in an
alumina crucible and treated at 700-800 °C for several
durations under air atmosphere. The reaction was then
quenched at room temperature, the powder was washed
with deionized water and dried as explained above.
10 µL (for cyclic voltammetry (CV)) or 2 µL (for the
chronopotentiometry) of the ink (catalyst:acetylene black
(AB) = 1:1) were deposited on the polished GC electrode
and dried during 1 hour. Lower amounts of the ink were
used for chronopotentiometry in order to facilitate the
Characterization
Materials characterization. Powder X-ray Diffraction
detection of catalysts deactivation.
A
20%Pt/C
(XRD) was performed on
a Bruker D8 advance
commercial sample (Alfa Aesar) was used for
benchmarking. The ink was prepared using 12.5 mg of the
material and the same amounts of EtOH and Nafion-117 as
described above. 1.5 µL of this ink was deposited on the
GC electrode for performing the CV. Note that higher
catalyst:AB ratio (5:1, 50 mg catalyst and 10 mg AB and
deposition of 6 µL on GC)4 resulted in high polarization
(Figure S6) corresponding to slow kinetics of electron
transfer. This is probably due to poor electron transport
in the electrode and between the electrode material and
the current collector. Indeed, the high amounts of
interfaces due to the nanoscale of the catalyst particles
diffractometer equipped with a Cu Kα source. The
crystalline phases were identified according to the
reference PDF cards 04-014-7255 for pseudo-cubic
La0.7Sr0.3MnO3, 01-083-2034 for La(OH)3, 01-084-1778 for
SrCO3 and 04-013-5773 for La0.5Sr1.5MnO4. Transmission
electron microscopy (TEM) was performed on a JEOL
JEM2100Plus LaB6 working at an accelerating voltage of
200 kV. Field-emission scanning electron microscopy (FE-
SEM) was carried out on a Hitachi SU-70 microscope.
Atomic composition given by wavelength dispersive X-ray
fluorescence (WDXRF) was measured on a Bruker S8
Tiger spectrometer after pelletizing the powder. The
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Chemistry of Materials
Type Oxide Solid Solution and Its Application to the Solid
Electrolyte Fuel Cell. Energy Convers. 1971, 11, 105–111.
https://doi.org/10.1016/0013-7480(71)90121-5.
Miyazaki, K.; Sugimura, N.; Matsuoka, K.; Iriyama, Y.; Abe,
T.; Matsuoka, M.; Ogumi, Z. Perovskite-Type Oxides
La1−xSrxMnO3 for Cathode Catalysts in Direct Ethylene
Glycol Alkaline Fuel Cells. J. Power Sources 2008, 178, 683–
686. https://doi.org/10.1016/j.jpowsour.2007.08.007.
Stoerzinger, K. A.; Lü, W.; Li, C.; Ariando; Venkatesan, T.;
Shao-Horn, Y. Highly Active Epitaxial La(1–x)SrxMnO3
Surfaces for the Oxygen Reduction Reaction: Role of
Charge Transfer. J. Phys. Chem. Lett. 2015, 6, 1435–1440.
https://doi.org/10.1021/acs.jpclett.5b00439.
Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.;
Goodenough, J. B.; Shao-Horn, Y. Design Principles for
Oxygen-Reduction Activity on Perovskite Oxide Catalysts
for Fuel Cells and Metal–Air Batteries. Nat. Chem. 2011, 3,
546–550. https://doi.org/10.1038/nchem.1069.
decreases electron percolation. The catalyst:AB ratio 1:1
was chosen to bypass this issue.
The films were hydrated with 0.1 M KOH
1
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electrolyte during 30 min before the experiments. All
cyclic voltammograms and chronopotentiometry curves
were performed in O2 saturated 0.1 M KOH solution
(pH = 13.3) at a scan rate of 10 mV s-1 and rotation speed of
1600 rpm. Each experiment was repeated 3 times to test
reproducibility. The pH-dependent study in KOH and
trimethylammonium hydroxide (TMAOH) was performed
in 0.1, 0.03, 0.005 and 0.001 M electrolytes (pH 13.3, 12.8,
12.0 and 11.2 respectively). The ionic strength was
corrected with KNO3 or trimethylammonium nitrate
(TMANO3) to a 0.1 M total concentration of K+ or TMA+.
The ORR activity of perovskite catalysts was evaluated by
comparing the specific activity current density (I (mA cm-
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55
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57
58
59
60
(5)
(6)
Stoerzinger, K. A.; Risch, M.; Han, B.; Shao-Horn, Y. Recent
Insights into Manganese Oxides in Catalyzing Oxygen
Reduction Kinetics. ACS Catal. 2015, 5, 6021–6031.
https://doi.org/10.1021/acscatal.5b01444.
Ansari, M. I. H.; Qurashi, A.; Nazeeruddin, M. K. Frontiers,
Opportunities, and Challenges in Perovskite Solar Cells: A
Critical Review. J. Photochem. Photobiol. C Photochem. Rev.
2
oxide)), being the current per perovskite oxide surface
measured by BET, and the mass activity current density (I
(A g-1oxide)), being the current per perovskite oxide mass.
The ORR selectivity was evaluated by the Koutecky-
Levich analysis from RDE measurements and by scanning
2018,
https://doi.org/10.1016/j.jphotochemrev.2017.11.002.
Hashim, S. S.; Somalu, M. R.; Loh, K. S.; Liu, S.; Zhou, W.;
Sunarso, J. Perovskite-Based Proton Conducting
Membranes for Hydrogen Separation: A Review. Int. J.
Hydrogen Energy 2018, 43, 15281–15305.
https://doi.org/10.1016/j.ijhydene.2018.06.045.
35,
1–24.
electrochemical
microscopy
(SECM).
SECM
measurements were carried out at room temperature
using the substrate generation-tip collection (SG/TC)
mode36–39 in a home-made SECM microscope. A four-
electrode configuration was employed, with the following
electrodes: the catalyst ink deposited on a glassy carbon
disk substrate (5 mm in diameter) as the substrate
generator electrode, a platinum ultramicroelectrode (25
µm in diameter) as the tip collector, a Pt wire as the
counter electrode and a Saturated Calomel Electrode
(SCE) as the reference electrode. A cyclic voltammogram
at the substrate electrode was recorded in a still air-
saturated 0.1 M KOH solution at 10 mV s-1. Meanwhile the
platinum tip collector biased at 0.4 V/SCE was positioned
at 25 µm from the substrate inside its diffusion layer in
(7)
(8)
(9)
(10)
(11)
(12)
Sunarso, J.; Hashim, S. S.; Zhu, N.; Zhou, W. Perovskite
Oxides Applications in High Temperature Oxygen
Separation, Solid Oxide Fuel Cell and Membrane Reactor: A
Review. Prog. Energy Combust. Sci. 2017, 61, 57–77.
https://doi.org/10.1016/j.pecs.2017.03.003.
Ebrahimizadeh Abrishami, M.; Risch, M.; Scholz, J.;
Roddatis, V.; Osterthun, N.; Jooss, C. Oxygen Evolution at
Manganite Perovskite Ruddlesden-Popper Type Particles:
Trends of Activity on Structure, Valence and Covalence.
Materials 2016, 9, 921. https://doi.org/10.3390/ma9110921.
Battle, P. D.; Green, M. A.; Laskey, N. S.; Millburn, J. E.;
Murphy, L.; Rosseinsky, M. J.; Sullivan, S. P.; Vente, J. F.
Layered Ruddlesden−Popper Manganese Oxides: Synthesis
and Cation Ordering. Chem. Mater. 1997, 9, 552–559.
https://doi.org/10.1021/cm960398r.
Sengodan, S.; Choi, S.; Jun, A.; Shin, T. H.; Ju, Y.-W.; Jeong,
H. Y.; Shin, J.; Irvine, J. T. S.; Kim, G. Layered Oxygen-
Deficient Double Perovskite as an Efficient and Stable
Anode for Direct Hydrocarbon Solid Oxide Fuel Cells. Nat.
Mater. 2015, 14, 205–209. https://doi.org/10.1038/nmat4166.
Tarancón, A.; Burriel, M.; Santiso, J.; Skinner, S. J.; Kilner, J.
A. Advances in Layered Oxide Cathodes for Intermediate
Temperature Solid Oxide Fuel Cells. J. Mater. Chem. 2010,
20, 3799. https://doi.org/10.1039/b922430k.
Lee, D.; Lee, H. Controlling Oxygen Mobility in
Ruddlesden–Popper Oxides. Materials. 2017, 10, 368.
https://doi.org/10.3390/ma10040368.
Chen, D.; Wang, J.; Zhang, Z.; Shao, Z.; Ciucci, F. Boosting
Oxygen Reduction/Evolution Reaction Activities with
Layered Perovskite Catalysts. Chem. Commun. 2016, 52,
10739–10742. https://doi.org/10.1039/C6CC04895A.
Du, J.; Zhang, T.; Cheng, F.; Chu, W.; Wu, Z.; Chen, J.
Nonstoichiometric Perovskite CaMnO3−δ for Oxygen
Electrocatalysis with High Activity. Inorg. Chem. 2014, 53,
9106–9114. https://doi.org/10.1021/ic501631h.
Lin, H.; Liu, P.; Wang, S.; Zhang, Z.; Dai, Z.; Tan, S.; Chen,
D. A Highly Efficient Electrocatalyst for Oxygen Reduction
Reaction: Three-Dimensionally Ordered Macroporous
Perovskite LaMnO3. J. Power Sources 2019, 412, 701–709.
-
order to detect the electroactive species (HO2 ) generated
during the ORR.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information ia availabre free of charge via
the Internet at http://pubs.acs.org.” at DOI:xxxxxxxxxxxxxxx
Characterization of the materials and electrocatalytic results
not shown in the manuscript.
AUTHOR INFORMATION
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Corresponding Author
*E-mail: david.portehault@sorbonne-universite.fr
ACKNOWLEDGMENT
This work has been supported by the Region Ile-de-France in
the framework of the Domaine d’Intérêt Majeur DIM Nano-K
and by the French national agency for research (ANR) under
the project SALTYSPIN ANR-17-CE09-0005. DP and FG also
thank the Fondation Collège de France for support.
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(16)
REFERENCES
(1)
Takahashi, T.; Iwahara, H. Ionic Conduction in Perovskite-
ACS Paragon Plus Environment

Chemistry of Materials
Page 8 of 9
https://doi.org/10.1016/j.jpowsour.2018.12.005.
Interplay between A-Site Segregation and B-Site Valency.
Catal. Sci. Technol. 2016, 6, 7231–7238.
https://doi.org/10.1039/c6cy01105e.
Poux, T.; Napolskiy, F. S.; Dintzer, T.; Kéranguéven, G.;
Istomin, S. Y.; Tsirlina, G. A.; Antipov, E. V.; Savinova, E. R.
Dual Role of Carbon in the Catalytic Layers of
Perovskite/Carbon Composites for the Electrocatalytic
Oxygen Reduction Reaction. Catal. Today 2012, 189, 83–92.
https://doi.org/10.1016/j.cattod.2012.04.046.
(17)
(18)
M.P. Pechini. Method of Preparing Lead and Alkaline Earth
Titanates and Niobates and Coating Method Using the
Same to Form a Capacitor. No. 3,330,697, 1967.
Epherre, R.; Duguet, E.; Mornet, S.; Pollert, E.; Louguet, S.;
Lecommandoux, S.; Schatz, C.; Goglio, G. Manganite
Perovskite Nanoparticles for Self-Controlled Magnetic Fluid
Hyperthermia: About the Suitability of an Aqueous
Combustion Synthesis Route. J. Mater. Chem. 2011, 21, 4393.
https://doi.org/10.1039/c0jm03963b.
Cai, B.; Akkiraju, K.; Mounfield, W. P.; Wang, Z.; Li, X.;
Huang, B.; Yuan, S.; Su, D.; Román-Leshkov, Y.; Shao-Horn,
Y. Solid-State Gelation for Nanostructured Perovskite
Oxide Aerogels. Chem. Mater. 2019, 31, 9422–9429.
10.1021/acs.chemmater.9b03182.
1
2
3
4
(30)
5
6
7
(31)
Yang, C.; Fontaine, O.; Tarascon, J.-M.; Grimaud, A.
Chemical Recognition of Active Oxygen Species on the
Surface of Oxygen Evolution Reaction Electrocatalysts.
8
(19)
9
Angew. Chem. Int. Ed.
2017,
56,
8652–8656.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
https://doi.org/10.1002/anie.201701984.
(32)
Yang, C.; Laberty-Robert, C.; Batuk, D.; Cibin, G.;
Chadwick, A. V.; Pimenta, V.; Yin, W.; Zhang, L.; Tarascon,
J.-M.; Grimaud, A. Phosphate Ion Functionalization of
Perovskite Surfaces for Enhanced Oxygen Evolution
Reaction. J. Phys. Chem. Lett. 2017, 8, 3466–3472.
https://doi.org/10.1021/acs.jpclett.7b01504.
Gonell, F.; Alem, N.; Dunne, P.; Crochet, G.; Beaunier, P.;
Méthivier, C.; Montero, D.; Laberty-Robert, C.; Doudin, B.;
Portehault, D. Versatile Molten Salt Synthesis of Manganite
Perovskite Oxide Nanocrystals and Their Magnetic
https://doi.org/10.1021/acs.chemmater.9b03182.
(20)
(21)
Portehault, D.; Delacroix, S.; Gouget, G.; Grosjean, R.;
Chan-Chang, T.-H.-C. Beyond the Compositional Threshold
of Nanoparticle-Based Materials. Acc. Chem. Res. 2018, 51,
930–939. https://doi.org/10.1021/acs.accounts.7b00429.
Thi N’Goc, H. Le; Mouafo, L. D. N.; Etrillard, C.; Torres-
Pardo, A.; Dayen, J.-F.; Rano, S.; Rousse, G.; Laberty-Robert,
C.; Calbet, J. G.; Drillon, M.; et al. Surface-Driven
Magnetotransport in Perovskite Nanocrystals. Adv. Mater.
2017, 29, 1604745. https://doi.org/10.1002/adma.201604745.
Al Raihani, H.; Durand, B.; Chassagneux, F.; Kerridge, D.
H.; Inman, D. Zirconia Formation by Reaction of Zirconium
Sulfate in Molten Alkali-Metal Nitrates or Nitrites. J. Mater.
Chem. 1994, 4, 1331. https://doi.org/10.1039/jm9940401331.
Houscroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd
Ed.; Pearson Education Limited: Harlow (England), 2005.
Ehsani, M. H.; Mehrabad, M. J.; Kameli, P.; Ghazi, M. E.;
Razavi, F. S. Low-Temperature Electrical Resistivity of
Bilayered LaSr2Mn2O7 Manganite. J. Low Temp. Phys. 2016,
183, 359–370. https://doi.org/10.1007/s10909-016-1520-1.
Ehsani, M. H.; Ghazi, M. E.; Kameli, P.; Razavi, F. S. DC
Magnetization Studies of Nano- and Micro-Particles of
Bilayered Manganite LaSr2Mn2O7. J. Alloys Compd. 2014,
586, 261–266. https://doi.org/10.1016/j.jallcom.2013.10.002.
Ehsani, M. H.; Ghazi, M. E.; Kameli, P. Effects of PH and
Sintering Temperature on the Synthesis and Electrical
Properties of the Bilayered LaSr2Mn2O7 Manganite Prepared
by the Sol–Gel Process. J. Mater. Sci. 2012, 47, 5815–5822.
https://doi.org/10.1007/s10853-012-6481-4.
(33)
Properties.
https://doi.org/10.1002/cnma.201800632.
Coey, J. M. D.; Viret, M.; von Molnár, S. Mixed-Valence
Manganites. Adv. Phys. 1999, 48, 167–293.
https://doi.org/10.1080/000187399243455.
ChemNanoMat
2019,
5,
358–363.
(22)
(34)
(35)
(23)
(24)
Portehault, D.; Cassaignon, S.; Baudrin, E.; Jolivet, J.-P.
Morphology Control of Cryptomelane Type MnO2
Nanowires by Soft Chemistry. Growth Mechanisms in
Aqueous Medium. Chem. Mater. 2007, 19, 5410–5417.
https://doi.org/10.1021/cm071654a.
Sánchez-Sánchez, C. M.; Bard, A. J. Hydrogen Peroxide
Production in the Oxygen Reduction Reaction at Different
Electrocatalysts as Quantified by Scanning Electrochemical
Microscopy. Anal. Chem. 2009, 81, 8094–8100.
https://doi.org/10.1021/ac901291v.
Villanueva-Rodríguez, M.; Sánchez-Sánchez, C. M.;
Montiel, V.; Brillas, E.; Peralta-Hernández, J. M.;
Hernández-Ramírez, A. Characterization of Ferrate Ion
Electrogeneration in Acidic Media by Voltammetry and
Scanning Electrochemical Microscopy. Assessment of Its
Reactivity on 2,4-Dichlorophenoxyacetic Acid Degradation.
(36)
(37)
(25)
(26)
(27)
Hue, D. T. M.; Manh, T. V; Anh, L. H.; Hong, L. V; Phan, M.
H.; Huyen, P. T.; Chinh, H. D. Sol–Gel Synthesis,
Characterization, and Magnetic Properties of Double-
Layered Perovskite Manganite La1.25Sr1.75Mn2O7. IEEE Trans.
Magn. 2014, 50, 2503204–2503204.
Ehsani, M. H.; Kameli, P.; Ghazi, M. E. Influence of Grain
Size on the Electrical Properties of the Double-Layered
LaSr2Mn2O7 Manganite. J. Phys. Chem. Solids 2012, 73, 744–
750. https://doi.org/10.1016/j.jpcs.2012.01.020.
Electrochim.
Acta
2012,
64,
196–204.
https://doi.org/10.1016/j.electacta.2012.01.021.
(38)
(39)
Hsu, H.-Y.; Ji, L.; Du, M.; Zhao, J.; Yu, E. T.; Bard, A. J.
Optimization of PbI2/MAPbI3 Perovskite Composites by
Scanning Electrochemical Microscopy. J. Phys. Chem. C
(28)
(29)
2016,
120,
19890–19895.
https://doi.org/10.1021/acs.jpcc.6b07850.
Bard, A. J.; Mirkin, M. V. Scanning Electrochemical
Microscopy; Marcel Dekker: New York, 2001.
Celorrio, V.; Calvillo, L.; Dann, E.; Granozzi, G.; Aguadero,
A.; Kramer, D.; Russell, A. E.; Fermín, D. J. Oxygen
Reduction Reaction at LaxCa1-xMnO3 Nanostructures:
ACS Paragon Plus Environment

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