Transforming bulk alloys into nanoporous lanthanum-based perovskite oxides with high specific surface areas and enhanced electrocatalytic activities

Article abstract of DOI:10.1039/c8ta07182a

The limited surface area of a perovskite oxide obtained through a conventional synthesis route has often limited its catalytic performance. Creating nanoporous perovskite oxides with abundant amounts of mesopores is one of the key challenges to realize perovskite potential for energy conversion and storage applications. Here, we report a novel, facile synthesis route that exploits the de-alloying process to transform bulk alloys into nanoporous lanthanum (La)-based perovskite oxides. The synthesized La-based perovskites display ultrahigh specific surface areas in the range of 8.10-20.13 m² g^-1 and a unique ligament-like porous framework. Our systematic ORR and OER tests revealed their high electrocatalytic activity and durability in an alkaline solution, which are consistent with the density functional theory (DFT) calculations. The synthesis route reported here opens up a new avenue to create a new generation of functional perovskite oxides that are applicable to other catalytic and electrochemical applications.

Full text of DOI:10.1039/c8ta07182a

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Materials Chemistry A
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COMMUNICATION
Transforming bulk alloys into nanoporous
lanthanum-based perovskite oxides with high
specific surface areas and enhanced
electrocatalytic activities†
Cite this: J. Mater. Chem. A, 2018, 6,
9979
1
Received 25th July 2018
Accepted 2nd October 2018
Conghui Si, Chi Zhang, Jaka Sunarso* and Zhonghua Zhang *a
ad
b
c
DOI: 10.1039/c8ta07182a
rsc.li/materials-a
The limited surface area of a perovskite oxide obtained through amounts of metal cation elements with different oxidation
a conventional synthesis route has often limited its catalytic perfor- states and electronegativities can be incorporated into the A-
mance. Creating nanoporous perovskite oxides with abundant amounts and/or B-sites of a perovskite to obtain desirable physi-
9
,10
of mesopores is one of the key challenges to realize perovskite potential ochemical and electrocatalytic properties.
For the oxygen
for energy conversion and storage applications. Here, we report a novel, reduction reaction (ORR) and oxygen evolution reaction (OER)
facile synthesis route that exploits the de-alloying process to transform applications in particular, the B-site cation constituents
bulk alloys into nanoporous lanthanum (La)-based perovskite oxides. usually determine the catalytic performance due to their
The synthesized La-based perovskites display ultrahigh specific surface correlation with the e
g
electron (s* orbital occupation) and
2
ꢀ1
areas in the range of 8.10–20.13 m
g
and a unique ligament-like the B–O bond strength, which affect the chemisorption energy
1
1
porous framework. Our systematic ORR and OER tests revealed their of the oxygenated species. Perovskites have conventionally
high electrocatalytic activity and durability in an alkaline solution, which been synthesized via an energy intensive route, i.e., solid state
1
2,13
are consistent with the density functional theory (DFT) calculations. The reaction
of metal oxide precursors that relies upon solid-
synthesis route reported here opens up a new avenue to create a new state diffusion. Several more energy efficient alternatives such
1
4,15
16
17
generation of functional perovskite oxides that are applicable to other as sol–gel,
catalytic and electrochemical applications.
freeze-drying, multi-step microemulsion, and
18
co-precipitation have recently been developed. High reaction
temperature above 1000 C nonetheless is still required in these
ꢁ
processes, which leads to the formation of micro-sized perovskite
Introduction
2
ꢀ1
particles with low specic surface areas (ꢂ1–2 m
g ) and
porosity. Such low specic surface areas and porosity ultimately
limit their electrocatalytic performance given the limited rates of
reaction kinetics, the mass transfer of the reactants and the
products from and to the reaction sites. Recently, a way of
producing nano-sized perovskites with high specic surface areas
Perovskite oxides have rapidly emerged as functional mate-
1
,2
3
rials in various applications such as solar cells, magnetism,
4
5,6
electronics, and catalysis given their high ionic and elec-
tronic conductivities, robust structure, and exible redox
behavior. A perovskite oxide has a compositional formula of
19,20
has been introduced using the ame-spray synthesis method.
ABO , where the A-site cation generally comes from alkaline
3
However, developing a new route that enables the synthesis of
perovskite products with high specic surface areas is currently
still one of the main challenges in the solid-state chemistry eld.
It has been demonstrated that nanomaterials with different
earth or rare earth metal elements while the B-site cation is
normally occupied by transition metal elements. Different
7
,8
a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials
(
Ministry of Education), School of Materials Science and Engineering, Shandong morphologies and crystallographic orientations such as nano-
21
22
23
University, Jinan 250061, China. E-mail: zh_zhang@sdu.edu.cn
wires, nanotubes, and nanosheets generally have superior
functional properties relative to their microparticle analogs.
Among them, light nanoporous materials with large surface
area to volume ratio and high electrical conductivity are of
b
School of Applied Physics and Materials, Wuyi University, Jiangmen 529020,
Guangdong, China
c
Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and
Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350, Kuching,
24
25
26
signicant interest for chemistry, physics, nanotechnology,
Sarawak, Malaysia. E-mail: jsunarso@swinburne.edu.my
27
d
and catalysis applications. Such materials can be prepared via
Key Laboratory of Processing and Testing Technology of Glass and Functional
28
29
30
Ceramics of Shandong Province, School of Material Science and Engineering, Qilu templating, sol–gel, bubble method, and vapor deposi-
University of Technology (Shandong Academy of Sciences), Jinan 250353, China
31
tion. De-alloying, i.e., the selective dissolution of the most
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta07182a
electrochemically active element(s) serves as one of the other
1
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attractive routes to create a bi-continuous open nanoporous oxides (refer to the Experimental section for the details, ESI†). X-
3
2
33
structure in metals. Numerous nanoporous metals, e.g., Au,
ray diffraction (XRD) patterns reveal that Al La M (M ¼ Co,
8
8
6
6
34
35
36
37
Pt, and Ag and multicomponent alloys, e.g., AuAg, PdNi,
and PtCo have been successfully created using de-alloying. Al (La,M), while the rapidly solidied Al La Ni precursor was
Mn, or Cr) precursors were primarily made of two phases: Al and
38
4
88
6
6
However, until now, no research studies have reported the composed of an amorphous phase (Fig. S1†). Compared to the
synthesis of nanoporous perovskite oxides from bulk alloys via standard prole of Al La (JCPDS 65-2679), the diffraction peaks
such a de-alloying route. of Al
(La,M) shi positively owing to the substitution of M (M ¼
4
4
Here, for the rst time, we reported a new design concept and Co, Mn or Cr) for La (Fig. S2†). Since the atomic radius of Co
synthesis protocol for direct transformation of bulk alloys into (0.126 nm), Mn (0.132 nm), and Cr (0.127 nm) is smaller than
nanoporous lanthanum (La)-based perovskite oxides via a facile that of La (0.187 nm), lattice contraction might occur in
de-alloying strategy. We started with ternary alloys consisting of Al (La,M). Aer de-alloying in 2 M sodium hydroxide (NaOH)
4
an active metal (Al) and non-active metals (La and M, M ¼ Co, aqueous solution at room temperature, the remaining phases in
Mn, Ni, or Cr) and adjusted the proportion of the metal elements the four samples took the form of a bimetallic hydroxide, i.e.,
3
to an appropriate content. The active alloy component, i.e., Al was (La,M) (OH) (Fig. S3†). The simultaneous presence of La and M
then dissolved in an alkaline solution to carry out the de-alloying in the cation site of the four samples is conrmed by the energy
process, resulting in the formation of a unique three-dimensional dispersive X-ray (EDX) spectra (Fig. S4†). Scanning electron
nanoporous hydroxide ((La,M)(OH)
Scheme 1. Subsequently, annealing (heat treatment) was per-
3
) as illustrated in the microscopy (SEM) image (Fig. 1a) shows the formation of
typical nanoporous ligament–channel structure of
a
formed to obtain La-based perovskite oxides that inherited the (La,Co)(OH) with a ligament diameter of ꢂ20 nm. The subse-
3
ꢁ
nanoporous structure of (La,M)(OH) . The nal nanoporous quent annealing process in air at 800 C led to the formation of
3
LaMO
tionally high ORR and OER activities, methanol resistance, and identied from their XRD patterns as LaCoO
high operational durability in an alkaline solution. The ORR and LaCrO (Fig. 1b). More importantly, the original nano-
proceeds via an apparently close to four- porous ligament–channel structure was retained in these
samples despite the signicant coarsening and sintering
3
products display ultrahigh specic surface areas, excep- a perovskite crystal structure in the four samples, which is
3
, LaMnO , LaNiO ,
3
3
3
catalyzed by LaMO
electron (4e ) pathway. Density functional theory (DFT) calcula- LaMO
3
ꢀ
3
tions were used to provide insights into the underlying electro- of the channels that occurred during the annealing process
catalytic mechanism.
(Fig. 1c–f for LaCoO , LaMnO , LaNiO , and LaCrO , respec-
3 3 3 3
tively, and Fig. S5†). The average ligament sizes of LaCoO3,
LaMnO , LaNiO , and LaCrO are 65, 30, 60, and 162 nm,
respectively (Fig. S6†). The particle sizes are about 2–5 mm for
the four catalyst samples based on the low magnication SEM
images. EDX spectra of these four samples show a close to 1 : 1
3
3
3
Results and discussion
Synthesis and characterization of nanoporous perovskite
oxides
As displayed in the Scheme 1, a novel synthesis route was proportion for La to M molar ratio, consistent with the
developed here to synthesize nanoporous La-based perovskite
Scheme 1 Schematic illustration of the systematic engineering of LaMO
nominal composition, at%) were chosen as the precursors. (b) The Al88La
the active metal Al element and then acquire bimetallic hydroxide (La,M)(OH)
annealed under certain conditions to transform into LaMO perovskite oxides.
3
(M ¼ Co, Mn, Ni or Cr) perovskite oxides. (a) Ternary Al88La
alloy foils were dealloyed in a NaOH alkaline solution to dissolve
intermediates. (c) The as-dealloyed (La,M)(OH) samples were
6 6
M alloys
(
6 6
M
3
3
3
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Fig. 1 (a) SEM image of the as-dealloyed (La,Co)(OH)
3
sample, inset: an enlarged structure. (b) XRD patterns of the synthesized LaCoO
, (d) LaMnO , (e) LaNiO and (f) LaCrO samples. (g) SEM-EDX
sample.
3 3
, LaMnO ,
LaNiO
3
and LaCrO
3
samples. (c–f) SEM images of the synthesized (c) LaCoO
sample. (h) EDX mapping images of the LaCoO
3
3
3
3
spectrum of the LaCoO
3
3
(La,M)(OH)3 formula (Fig. 1g and S7†). The homogeneous
Transmission electron microscopy (TEM) images display
distribution of all constituents, i.e., La, M, and O on the surfaces more clearly the nanoporous structures of LaCoO (Fig. 2a) and
3
of the four samples is furthermore veried using EDX mapping LaMnO3 (Fig. 2c, S8a and S8b†), which feature a signicant
(Fig. 1h, which shows typical EDX mapping proles for amount of mesopores. Selected area electron diffraction (SAED)
LaCoO ). patterns of LaCoO (inset of Fig. 2a and S9†) and LaMnO (inset
3
3
3
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˚
˚
of Fig. 2c and S8c†) exhibit a diffuse ring pattern, which is fringes with spacings of ꢂ2.724 A and ꢂ2.741 A, respectively,
a characteristic of polycrystalline materials. These rings can be which match their calculated (110) lattice spaces from XRD
indexed according to the (012), (110), (202), (024), and (214) patterns (Fig. 1b).
planes of LaCoO and LaMnO . High resolution transmission
N adsorption–desorption isotherms were utilized to determine
2
3
3
electron microscopy (HRTEM) images of LaCoO
3
(Fig. 2b) and the specic surface areas of the four samples. LaCoO
3 3
, LaMnO ,
LaMnO (Fig. 2d and S8d†) further reveal the presence of lattice LaNiO
3
3
, and LaCrO curves fall into type IV isotherms for
3
Fig. 2 (a) TEM image of the LaCoO
sample, inset: the SAED pattern. (d) HRTEM image of the LaMnO
and LaCrO samples. (f) La 3d XPS spectrum, (g) Co 2p XPS spectrum and (h) O 1s XPS spectrum of the LaCoO
3
sample, inset: the SAED pattern. (b) HRTEM image of the LaCoO
sample. (e) The BET surface areas of the synthesized LaCoO
sample.
3
sample. (c) TEM image of the LaMnO
3
3
3
3
, LaMnO , LaNiO
3
3
3
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mesoporous materials, which feature a hysteresis loop (Fig. S10†). conditions. The ORR onset potentials of LaCoO
3 3
, LaMnO ,
The Brunauer–Emmett–Teller (BET) specic surface areas for LaNiO , and LaCrO catalysts are approximately 0.96, 1.00,
3
3
LaCoO , LaMnO , LaNiO , and LaCrO are 9.25, 20.13, 11.75, and 0.86, and 0.84 V vs. the reversible hydrogen electrode (RHE),
3
3
3
3
2
ꢀ1
8
.10 m g , respectively (Fig. 2e). These specic surface areas are respectively (Fig. 3a). Table S2† lists the onset potentials of
signicantly larger than the typical values for perovskite oxides these four catalysts including the other parameters of rele-
2
ꢀ1
synthesized via conventional routes (ꢂ1–2 m g ).
vance to ORR characterization. These values are more positive
X-ray photoelectron spectroscopy (XPS) characterization was than several other previously reported perovskite-based cata-
1
1,48–56
performed to analyze the surface compositions and oxidation lysts
states of LaCoO (Fig. 2f–h and S11†), LaMnO (Fig. S12†), b and Table S3†). LaMnO
LaNiO3 (Fig. S13†) and LaCrO3 (Fig. S14†). The La 3d XPS onset potential that closely approaches that of Pt/C. The
and are quite close to those of Pt/C (Fig. 3a and
3
3
3
in particular has the most positive
spectra of LaCoO (Fig. 2f), LaMnO (Fig. S12b†), and LaCrO
3
limiting current densities of all four samples approach ꢀ6 mA
3
3
3
+
ꢀ2
(
Fig. S14b†) reveal the consistency of the peak positions for La
cm , which is clearly the limiting current density for Pt/C,
39
with those reported in the literature. La 3d5/2 and La 3d3/2 indicating that the ORR catalyzed by them proceeds through
ꢀ
peaks are split because of the charge transfer from the bonded a close to 4e pathway (Fig. 3a and c and Table S3†). Potential
3
+
ꢀ2
oxygen to the empty La 4f state. Co exists in a high-spin state at ꢀ3 mA cm
current density (E@J¼ꢀ3 mA cmꢀ2), current
in LaCoO as reected by the locations of the peaks of Co 2p3/2 density at 0.75 V vs. RHE (J@E¼0.75 V), and mass activity are
3
and Co 2p1/2 at ꢂ780.03 and ꢂ795.27 eV, respectively, and the other important ORR performance parameters. These three
presence of a shake-up satellite peak on the Co 2p XPS spectrum parameters clearly increase in the order of LaCrO , LaNiO ,
3
3
40
(
Fig. 2g). The Mn 2p XPS spectrum, on the other hand, shows LaCoO , and LaMnO (Fig. S15 and Table S2†). Kinetic data in
3 3
two different deconvoluted peak components, which come from the form of Tafel plots are shown next. The Tafel slopes are 87,
3
+
ꢀ1
Mn 2p3/2 and Mn 2p1/2 and indicate the presence of Mn and 73, 65, and 47 mV dec for LaCrO
3
, LaNiO
, respectively, and the enhancement order is consis-
sample may produce a small amount of oxygen vacancies tent with the other previous ORR parameters (Fig. S16 and
3 3
, LaCoO , and
2
+
41
slight Mn (Fig. S12c†). As reported in other literature, the LaMnO
3
ꢀ
1
during the preparation process of LaMnO
3
, which can be Table S2†). The smaller Tafel slope of 47 mV dec
written as LaMnO3ꢀd. Nominally such a lattice defect would LaMnO
for
ꢀ
1
3
relative to 60 mV dec
for Pt/C highlights its
3
+
2+
result in a mixed Mn /Mn state and electron doping. Many faster kinetics since lower slope allows achieving a high
studies indicate that the existence of cation deciencies and current at lower overpotentials. Electrochemical impedance
additional oxygen vacancies has signicant effects on both the spectra obtained at 0.65 V vs. RHE again reproduces the
physical–chemical properties and the electrochemical character- same trend, i.e., lowering resistance from faster charge
istics of perovskite oxides. The La 3d and Ni 2p XPS spectrum of transfer in the order of LaCrO
LaNiO is shown in Fig. S13b.† It is known that the La 3d3/2 peak (Fig. S17†).
overlaps with the Ni 2p3/2 peak. Thus the Ni 2p3/2 spectrum The electrochemical double layer capacitance (Cdl) of LaMO
42
3
, LaNiO
3
, LaCoO
3
, and LaMnO
3
3
3
cannot be analyzed separately without the analysis of the La 3d3/2 (M ¼ Co, Mn, Ni, or Cr) catalysts was quantied by obtaining
4
3
spectrum. The Cr 2p XPS spectrum splits into two peaks of Cr cyclic voltammograms (CVs) in an O -free 1 M KOH solution at
2
2
ꢀ
1
p3/2 and Cr 2p1/2, and the binding energies of Cr 2p3/2 and Cr various scan rates from 20 to 140 mV s between 1.21 and
are 575.95 and 579.29 eV, which indicates the existence of 1.26 V vs. RHE. The absence of O translates to the disappear-
Cr and slight Cr (Fig. S14c†). The O 1s peaks for LaCoO
2
p
1
/2
2
3
+
6+
44
3
ance of faradaic currents (Fig. S18a–S18d†). To determine the
effective surface area of the samples, one-half of the difference
(
3 3 3
Fig. 2h), LaMnO (Fig. S12d†), LaNiO (Fig. S13c†) and LaCrO
(Fig. S14d†) can be resolved into three peaks, which represent between the positive and negative current densities (DJgeo/2) at
the surface adsorbed water molecules or the hydroxyl species 1.235 V vs. RHE is plotted as a function of the scan rate. The
ꢀ
(
(
H
2
O/OH ), the hydroxyl groups or the surface-adsorbed oxygen regression line slope provides the electrochemical double layer
2ꢀ
–OH/O ), and the lattice oxygen species (O ), respectively, from capacitance value (Fig. S18e–S18h†). The linear relationship
high to low binding energy values. The –OH/O content appears between DJgeo/2 and the scan rate conrms the absence of
2
45
2
to be the highest relative to the other two species in LaCoO and faradaic processes. The capacitance increases in the order of
3
3 2 3 3 3 3
LaMnO (Table S1†). The higher content of –OH/O has previ- LaNiO , LaCrO , LaCoO , and LaMnO , which reects the
ously been associated with the higher ORR performance due to increasing electrochemically active surface area in this order, in
the presence of defect sites with lower oxygen coordination that accord with the ORR performance trend. The capacitance of
46,47
enhances the O
2
adsorption capacity.
LaNiO
error.
3 3
is identical to that of LaCrO within the experimental
Rotating ring-disk electrode (RRDE) experiments were also
performed to cross-check the ORR products (Fig. 3d), i.e.,
Electrochemical performance tests
To determine their ORR performances, linear sweep voltam- whether the products are water (H
metry (LSV) was performed on LaMO
(M ¼ Co, Mn, Ni, or Cr) (H ). The hydrogen peroxide yields from the ORR catalyzed by
catalyst samples deposited onto a glassy carbon rotating disk LaMO
(M ¼ Co, Mn, Ni, or Cr) are lower than 9%, i.e.,
electrode (RDE) in an O -saturated 0.1 M KOH solution at approximately 3.5%, 4.9%, 5.8%, and 8.4% for LaCoO
O) or hydrogen peroxide
2
3
2 2
O
3
3
,
2
ꢀ
1
1
600 rpm and 10 mV s . A commercial Pt/C catalyst was used LaMnO
3 3 3
, LaNiO , and LaCrO , respectively (Fig. 3e). Such minor
as the control reference under identical experimental hydrogen peroxide formation translates to an apparently close
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Fig. 3 (a) Polarization curves for the ORR on the LaCoO
3
, LaMnO
3
, LaNiO
3
, LaCrO
3
and commercial Pt/C catalysts at a rotation rate of 1600 rpm
ꢀ1
in the O
density of the LaCoO
tammograms of the LaCoO
transfer number (n) and peroxide yield of the LaCoO
RRDE data. (f) Comparison of polarization curves for the ORR on the LaCoO
600 rpm in the O -saturated 0.1 M KOH electrolyte containing 0.5 M methanol at a scan rate of 10 mV s . (g) ORR polarization curves for the
catalyst after 1 and 10 K potential cycles in the 0.1 M KOH solution. (h) Anodic linear scanning voltammograms for the OER on the
2
-saturated 0.1 M KOH electrolyte at a scan rate of 10 mV s . (b and c) Comparisons of (b) the onset potential and (c) the limiting current
11,48–56
3
, LaMnO
3
, LaNiO
3
and LaCrO
and LaCrO
, LaMnO
3
catalysts with the active perovskite electrocatalysts in the literature.
(d) RRDE vol-
-saturated 0.1 M KOH at 1600 rpm and 10 mV s . (e) The electron
and LaCrO catalysts at various potentials based on the corresponding
, LaMnO , LaNiO , LaCrO and Pt/C catalysts at a rotation rate of
ꢀ1
3
, LaMnO
3
, LaNiO
3
3
catalysts in the O
, LaNiO
2
3
3
3
3
3
3
3
3
ꢀ1
1
2
LaCoO
LaCoO
3
ꢀ1
3 3 3 3 2 2
, LaMnO , LaNiO , LaCrO and IrO catalysts at a rotation rate of 1600 rpm in the O -saturated 1 M KOH at a scan rate of 10 mV s .
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ꢀ
to 4e ORR pathway for these four catalysts, i.e., 3.93–3.99, 3.90– elements in LaMO
3
. Fig. 4a displays the electronic density of
3
.95, 3.87–3.98, and 3.83–3.92 for LaCoO , LaMnO , LaNiO , states (DOS) of the bulk LaMO (M ¼ Co, Mn, Ni, or Cr). The d-
3
3
3
3
and LaCrO , respectively (Fig. 3e and Table S2†). This is in band center lies at ꢀ1.52, ꢀ1.47, ꢀ1.97, and ꢀ1.30 eV relative to
3
perfect agreement with their limiting current densities, which the Fermi level for LaCoO , LaMnO , LaNiO , and LaCrO ,
3
3
3
3
ꢀ
2
are close to ꢀ6 mA cm (Fig. 3a) and the electron transfer respectively (Fig. 4a and Table S5†). According to Sabatier's
numbers obtained from the Koutecky–Levich equation, which principle, the most active catalyst does not bind adsorbed
8,62,63
require LSV proles at different rotation rates from 400 rpm to species too strongly or too weakly.
500 rpm (Fig. S19 and Table S2†). ported a volcano-type relationship between the ORR catalytic
Methanol resistance is important for application in a direct activity and the d-band center energy. The d-band centers of
methanol fuel cell (DMFC). The ORR current prole of Pt/C LaCoO and LaMnO in particular occupy the near to center
Shao-Horn's group re-
2
11
3
3
experienced an abrupt change to more negative potential position, which indicates their balanced O2 adsorption and
value aer the introduction of 0.1 M and 0.5 M methanol, which oxygenated intermediate desorption strengths. Fig. S23a–S23c†
indicates the occurrence of methanol oxidation. The original display the crystal lattice of LaMO
ORR current proles of LaMO
(M ¼ Co, Mn, Ni, or Cr) none- lattice plane, and side view onto its (001) lattice plane, respec-
theless were retained (Fig. 3f and S20†) aer such introduction, tively. DFT calculations to determine the adsorption energies of
which indicate their excellent methanol tolerance. This is on the (001) plane of LaMO
(M ¼ Co, Mn, Ni, or Cr) reveal
benecial in the case of methanol cross-over from the anode that these energies are moderate, i.e., vary from 1.59 to 3.09 eV
side to the cathode side during the DMFC operation. (Table S5†). The optimized structural congurations for adsor-
3
, the top view onto its (001)
3
O
2
3
Durability is another main indicator of a practical catalyst. bed oxygen on the (001) surface of LaMO3 are depicted in
LaCoO was subjected to LSV tests in an O -saturated 0.1 M Fig. 4b, S24a, S25a and S26a.†
3
2
ꢀ1
KOH solution at 1600 rpm and 10 mV s from 1.1 V to 0.3 V vs.
RHE aer 1 and 10 000 potential cycles (Fig. 3g). At the 10 000
DFT calculations were subsequently performed to evaluate
the methanol resistance of LaMO . The optimized structural
th
3
LSV scan, LaCoO
3
retains 95% of its original limiting current congurations for adsorbed methanol on the (001) plane of
density of the 1 LSV scan. The half-wave potential (E1/2) at the LaMO are illustrated in Fig. 4c, S24b, S25b and S26b.† The
calculated energies of the adsorption of methanol on the (001)
st
3
th
st
1
0 000 LSV scan also appears to be identical to that at the 1
LSV scan. This reects the high durability of the LaCoO catalyst plane of LaMO (M ¼ Co, Mn, Ni, or Cr) vary from 0.15 to 0.26 eV
3
3
for long term ORR operation.
(Table S5†). These values are much lower than the respective
64
The applicability of LaMO (M ¼ Co, Mn, Ni, or Cr) as OER value for Pt (0.44 eV), which is consistent with the signicantly
3
catalysts was additionally probed by performing LSV in an O
2
-
negligible methanol oxidation on these LaMO
3
catalysts
ꢀ
1
saturated 1 M KOH solution at 1600 rpm and 10 mV s from (Fig. S20f† and 3f). The change in the O–O bond length of the
1
.1 V to 1.9 V vs. RHE (positive direction). The OER onset adsorbed HOOH on the (001) plane of LaMO
3
further explains
ꢀ
potential increases in the order of LaNiO
LaCoO (1.60 V), LaCrO (1.61 V), and LaMnO
A lower OER onset potential generally indicates higher OER (Fig. 4d, S24c, S25c and S26c†), which elongates to 2.232, 2.421,
3
(1.45 V vs. RHE), their four-electron (4e ) ORR pathway. The O–O bond length in
(1.62 V) (Fig. 3h). an isolated hydrogen peroxide (HOOH) molecule is 1.465 A˚
3
3
3
ꢀ
2
˚
activity. The potential required to obtain a 10 mA cm current 2.087, and 2.200 A aer being adsorbed onto the (001) planes of
density (E
J¼10 mA cmꢀ2
) has been regarded as a metric of rele- LaCoO , LaMnO , LaNiO , and LaCrO , respectively (Fig. 4e,
3 3 3 3
vance to solar fuel synthesis. The most active OER catalyst, i.e., S24d, S25d and S26d†). This indicates the O–O breakage on
LaNiO required a potential of 1.60 V vs. RHE, which is the upon being adsorbed onto the surface of these catalysts.
Recent work from Shao-Horn's group indicates that the O p-
band of perovskite oxides relative to the Fermi level can be used
3
2 2
H O
lowest potential among the four catalysts. Notably, the EJ¼10 mA
ꢀ
2
of 1.60 vs. RHE for LaNiO is quite close to the E
J¼10 mA cmꢀ2
cm
3
of 1.57 vs. RHE for the OER benchmark IrO
2
catalyst, and much as the activity descriptor of the OER performance, identical to
65
higher than the other reported perovskite catalysts (Table the d-band center relationship for precious metal catalysts.
A
55–61
S4†).
The Tafel plots of the OER catalyzed by the four cata- large number of simulation experiments examined the trends
lysts reveal a decreasing trend in the order of LaMnO (103 mV between the O p-band center relative to the Fermi level and the
3
ꢀ
1
ꢀ1
ꢀ1
dec ), LaCrO
LaNiO (83 mV dec ). The Tafel slope of LaNiO
comparable with that of IrO
superior OER kinetics (Fig. S21†). Practical application of water energies on AO and BO
splitting requires the OER electrocatalysts to have excellent be attributed to the upshi and downshi of the electron energy
3
(92 mV dec ), LaCoO
3
(84 mV dec ), and adsorption energies. The results revealed that an excellent
in particular is correlation exists between the sub-surface (2nd surface layer)
(81 mV dec ), which highlights its layer O p-band center and the computed HO* adsorption
bare surfaces of ABO (001), which can
ꢀ1
3
3
ꢀ
1
2
2
3
ꢀ
2
stability. At a constant current density of 10 mA cm , the level near the surface due to compensating surface charge from
overpotential of the GC-loaded LaNiO catalyst shows a slight bulk polarity. The HO* adsorption energies result in the
3
change during the testing duration of 10 000 s (Fig. S22†).
observed binding energy difference between the two counter
surface terminations. On the other hand, the O p-band center
relative to the Fermi level of perovskites is closely associated
with the energy of oxygen vacancy formation and the oxygen
DFT calculations
3
DFT calculations were performed to provide insights into the surface exchange kinetics. The O p-band centers of LaCoO ,
observed ORR/OER performance as a function of different M LaMnO
3
, LaNiO
3
, and LaCrO
3
lie at ꢀ2.45, ꢀ3.25, ꢀ2.58, and
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Fig. 4 (a) The DOS of the bulk LaCoO
3
, LaMnO
3
, LaNiO
3
and LaCrO
3
. (b) The status of the O
2
3
molecule adsorbed on the LaCoO (001). (c) The
status of the CH
3
OH molecule adsorbed on the LaCoO
3
(001). (d and e) The status of HOOH molecules adsorbed on the LaCoO
3
(001) (d) before
and (e) after optimization.
ꢀ
4.12 eV relative to the Fermi level, respectively (Fig. S27†). The moderate O p-band center relative to the Fermi level can
metal–oxygen bonds of perovskite oxides have a covalent char- increase the OER activity and stability. LaNiO provides the
3
acter that correlates with the O p-band center. Increasing the most optimum O p-band center among the four catalysts,
covalency enhances the OER activity and stability in an alkaline exhibiting the highest OER activity and stability. Due to the
solution. However, excess covalency gives rise to non-stable unique ligament–channel structure, the electrocatalytic prop-
performance under OER conditions. In this context, LaNiO3 erties of LaMO are superior to those of the perovskite catalysts
3
55,56,66
provides the most optimum covalency (neither too close nor too with similar or even higher surface areas.
far from the Fermi level) among the four catalysts as reected by although the BET surface area and O p-band center of LaCoO
its O-p band center location, exhibiting the highest OER activity and LaNiO are quite close, many other factors such as the
origin of the overpotential for the oxygen evolution will also
are closely inuence the OER activity.
The intrinsic electrocatalytic activities of LaMO
Furthermore,
3
3
and stability.
The excellent ORR/OER activities of LaMO
3
related to the unique ligament–channel structure. The large
specic surface areas and the presence of mesopores are Mn, Ni, or Cr) are associated with the conguration of the d-
3
(M ¼ Co,
anticipated to provide high amount of catalytically active sites orbital electron of the transition metal M element, which
and facilitate the fast diffusion of oxygen and reactant/product bonds to six oxygen anions in an octahedral MO coordination.
6
species required for the enhanced ORR and OER performances. Shao-Horn's group showed that the electrocatalytic activities of
The BET surface area of LaMnO
3
seems extraordinarily high perovskite oxides can be scaled using the molecular orbital
compared to that of other three LaMO
samples, exhibiting the best ORR catalytic activity among the the e orbital leads to a high activity for the ORR and OER.
g
3
(M ¼ Co, Ni, or Cr) (MO) concept, where the near unity occupancy of an electron in
8
four catalysts. Moreover, the O 1s XPS spectrum of LaMnO
3
and Their recent work reveals that the ORR and OER activities of
LaCoO3 reveals that the –OH/O2 content appears to be the perovskite oxides exhibit a volcano-type relationship as a func-
11
highest relative to the other two species, indicating higher ORR tion of e lling of transition metal ions. In their work, the eg
g
performance for LaMnO and LaCoO . The computed O p-band occupancy decreases in the order of LaCoO (e z 1) z LaMnO
3
3
3
3
g
center relative to the Fermi level can be used as a descriptor to (e
g
¼ 1) z LaNiO
3
(e
g
z 1) > LaCrO
3
(e
g
¼ 0). Such an order
screen the OER activities and stabilities of LaMO
3
. The represents the increasing strength of the bond between M
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Journal of Materials Chemistry A
cation and O anion, where low e
bond such as in the case of LaCrO , while high eg lling
provides a weaker bond and consequently, a sluggish reaction
g
lling provides a stronger
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There are no conicts to declare.
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The authors gratefully acknowledge nancial support by the
National Natural Science Foundation of China (51671115 and
51871133), the support of the Department of Science and
Technology of Shandong Province for the Young Tip-top Talent
Support Project, and the Young Tip-top Talent Support Project
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