Lanthanum-Based Compounds: Electronic Band-Gap-Dependent Electrocatalytic Materials for Oxygen Reduction Reaction

Article abstract of DOI:10.1002/chem.201701136

The electronic energy level of lanthanum compounds plays an important role in the oxygen reduction reaction (ORR) electrocatalytic process. In this work, three lanthanum compounds, LaOHCO₃, La₂O₂CO₃, and La

Full text of DOI:10.1002/chem.201701136

A Journal of
Accepted Article
Title: Lanthanum-Based Compounds: Electronic Bandgap-Dependent
Electro-Catalytic Materials toward Oxygen Reduction Reaction
Authors: Weiwei Gu, Ye Song, Jingjun Liu, and Feng Wang
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Lanthanum-Based Compounds: Electronic Bandgap-Dependent
Electro-Catalytic Materials toward Oxygen Reduction Reaction
Weiwei Gu, Ye Song, Jingjun Liu,* and Feng Wang*
Abstract: The electronic energy level of lanthanum compounds
plays an important role in oxygen reduction reaction (ORR) electro-
catalytic process. In this work, three lanthanum compounds,
Secondly, the energy levels of La 4f orbitals locate below the
Fermi level about 5 eV, which leads to the 4f orbitals are partly
delocalized and couple with the conduction band (CB) of the
[
10-11]
LaOHCO
3
, La
2
O
2
CO
3
2 3
and La O , were synthesized via an in-situ
compounds.
Therefore, some electrons within 4f orbitals will
urea hydrolysis method, followed by annealing at different
temperatures. Among these lanthanum compounds, the layer-
be able to transfer into the conduction band through the strong
interaction between the localized 4f orbitals and the delocalized
conduction band, which can further alter the electronic
2 2 3
structured La O CO has the minimum bandgap and a moderate
[
12]
value of conduction band (CB) and valance band (VB).
Electrochemical measurements in 0.1 M KOH solution show that
configuration of these lanthanum compounds.
Such unique
electronic configurations for lanthanum-based compounds may
lead to their highly electro-catalytic activity toward ORR.
compared with the other catalysts, La CO
2
O
2
3
exhibits the best
electro-catalytic activity with lowest H O production and highest
2
2
durability for ORR, which proves the close correlation between
electronic energy level and electro-catalytic ORR activity. During the
To date, some lanthanum-based compounds, such as LaMnO
La CO and La with highly ORR electro-catalytic activity in
alkaline media have been widely investigated.
it has been reported that La O nanoparticles covered on carbon
3
,
2 2 3
ORR process over La O CO , some covalent electrons from VB are
2
O
2
3
2 3
O
[
13-15]
first excited into CB and then transfer into the unoccupied π*2p
orbitals of active oxygen molecular, leading to a strengthen oxygen
adsorption and promoting the reduction of oxygen. Moreover,
For example,
2
3
support exhibit low hydrogen peroxide production during ORR
process compared with other metal oxides. In generally, metal
La
2 2 3
O CO has an ability of chemical disproportionation for hydrogen
-
-
-
peroxide (HO
2
), and the produced HO
2
at the energy level of
oxide catalysts have relative higher HO2 production ratio than
-
O
O
2
/HO level can undergo prompt chemical disproportionation into
and OH species. The generated O
2
precious metal catalysts (Pt/C), which should be ascribed to the
-
2
2
at this stage is adsorbed on
complicated oxygen reduction process, where
O
2
is more
. In the ORR process, the oxygen
reduction reaction undergoes two/four electron transfer process
-
the catalyst surface, which can be utilized for further oxygen
reduction again.
feasible to be reduced to HO
2
-
-
2
with following equations, with the production of OH or HO .
-
-
0
O
O
2
+ 2H
2
O +4e = 4OH ;
E = 0.401 V vs SHE
Introduction
-
-
-
0
2
+ H O + 2e = HO
2
2
+ OH ;
E = -0.076 V vs SHE
Lanthanum-based compounds, such as LaOHCO
La CO , have been widely applied in photo-catalytic and
electro-catalytic fields, including methanol oxidation reaction
3 2 3
, La O and
For lanthanum-based compound, it has the ability to chemical
2
O
2
3
-
2
disproportionation for hydrogen peroxide (HO ), which can lead
-
to the produced HO
disproportionation into O
2
species undergo prompt chemical
and OH species, leading to lower
[
1-3]
(
MOR) and oxygen reduction reaction (ORR).
Based on their
-
2
unique electronic structures, the reasons for lanthanum-based
compounds as cheap and efficient catalysts can be ascribed to
two aspects. Firstly, lanthanum ions have unfilled 4f and 5d
orbitals that have characteristic f-f and f-d electron
configurations to form unique bonding characteristics because
hydrogen peroxide production during ORR process. Besides,
lanthanum-based compounds also show excellent activity in
catalyzing ORR. The reason for such superior ORR electro-
catalytic activity may be attributed to that La
2
16]
O
3
owns a
In the ORR
3
O as a Lewis base tends to provide
[
moderate band gap (between 2.8 and 5.4 eV).
catalyzing process, La
[4-6]
lanthanum ions are located in f-block.
As a result, the
2
fundamental bandgap of lanthanum compounds is dependent on
both 4f and 5d orbitals of lanthanum ions through intra-atomic
hybridization between the 4f orbitals and 5d orbitals via the
electrons, which can facilitate the sluggish ORR kinetics. In
addition, some investigators reported that the high ORR activity
may be correlated to the formation of La–O and C–O bonds at
[
7-9]
electron transfer within lanthanum-based compounds.
2 3
the interfaces between carbon and La O phase, which can
promote electrons transfer between the ligands and lanthanum
ions. As a result, some electrons will migrate to the unoccupied
W. Gu, Y. Song, J. Liu, F. Wang
e
g
2 3
orbitals splitted by La 5d orbitals of La O through the electron
State Key Laboratory of Chemical Resource Engineering
transfer, which contributes to the oxygen adsorption that is the
[17]
rate-determining step of ORR. Moreover, since La
moderate bandgap, which results in the ability to react with
intermediate CO to form La -CO bonds, and then the
generated La -CO bonds could react with oxygen containing
species such as OH from water to form CO
2 3
O owns a
Beijing Key Laboratory of Electrochemical Process and Technology for
Materials
2
O
3
Beijing University of Chemical Technology, Beijing 100029 (P.R. China)
E-mail: liujingjun@mail.buct.edu.cn,
2
O
3
-
2
, which further
[18-19]
wangf@mail.buct.edu.cn
relieves the poison of CO during catalysis.
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Therefore, the electronic bandgap may be a key factor to govern
corresponds to (121) plane of LaOHCO
(130) plane of La CO monoclinic phase and (101) plane of
La hexagonal phase, respectively. To further study the lattice
structure of these compounds, XRD measurements were carried
out and the obtained results are shown in Figure 1(h). The
peaks located at 15.7°, 20.6°, 23.7°, 24°, 29.9°, 31.8°, 35.6°,
38.0°,43.3°,44.7° are attributed to the (011), (020), (111), (021),
(121), (022), (200), (131), (221), and (212) planes of
3
orthorhombic phase,
catalytic activities toward ORR over La
2
O
3
and other lanthanum-
2
O
2
3
based compounds, because a suitable bandgap can not only
promote the active oxygen adsorption but also desorb the
adsorbed hydroxide into solution, and eventually lead to
improved ORR catalytic activity. Furthermore, for lanthanum-
based compounds like LaOHCO
bandgap can be tuned by their coordination environments
2 3
O
3 2 2 3
or La O CO , their electronic
[
20-22]
composed of various oxygen-containing functional groups.
orthorhombic phase of LaOHCO
La CO , the recorded pattern peaks located at 13.0°, 25.5°,
30.7°, 40.9°, 44.3°, 53.8° are (020), (120), (101), (141), (200),
(231) planes of monoclinic phase of La CO [PDF card No.48-
should also exhibit good catalytic activity toward ORR, 1113], while peaks at 26.6°, 29.9°, 39.5°, 46.0°, 55.9°
because they have similar electronic structures to La while correspond to the (100), (101), (102), (110), (112), (201) planes
of hexagonal phase of La [PDF card No.05-0602]. These
outcomes further confirm the formation of three lanthanum
compounds with different phases (orthorhombic LaOHCO
monoclinic La CO and hexagonal phase La ), via a simple
3
[PDF card No.49-0981]. For
Such variable bandgap would contribute to improve the catalytic
activity by reducing the activation energy of catalyzing ORR over
2
O
2
3
lanthanum-based compounds. In this way, La
LaOHCO
2
O
2
CO
3
or
2
O
2
3
3
2
O
3
the lanthanum ions in them are coordinated by different anions
with different electron affinity, which results in varying bandgap.
It has been verified that the reduced bandgap of some transition
metal oxides plays an important role in determining the catalytic
2 3
O
3
,
2
O
2
3
2 3
O
[
23]
properties toward ORR.
Unfortunately, La
2
O
2
CO
3
or
in-situ urea hydrolysis method, followed by annealing at different
temperatures shown in Figure 1(a).
LaOHCO as electrocatalysts for ORR have received little
3
attention by now, which needs further investigation.
The typical crystal structures of three lanthanum compounds are
In this work, we prepared three lanthanum compounds
shown in Figure 1(i). The orthorhombic LaOHCO
3
has
a
2
-
(
La
2
O
2
CO
3
,
La
2
O ,
3
LaOHCO
3
)
via
a
simple in-situ urea
separated layer structure, where the CO
3
layer is parallel with
2
+
2-
hydrolysis method, followed by annealing at different
temperatures. And then, the energy level and bandgap of these
samples have been determined by UV-vis and XPS
(LaOH) layer and oxygen atoms from CO
3
layer are located
-
2+
[24]
between the OH groups of two (LaOH) layers. Similar with
LaOHCO , the La CO also has a layer-like structure, while
the carbonate is separated with the (La
because the hydroxyl of LaOHCO is an electron donating group
while carbonate is an electron withdrawing group, leading to the
band strength of LaOHCO should be stronger than La CO
Such layer structure featured by LaOHCO CO is
different from that of La in which the LaO species of La
, the
or LaOHCO
) /(LaOH) and carbonate weakens the
band strength, which may lead to special electronic structure.
3
2
O
2
3
2
+
measurements, and the obtained results show that La
2
O
2
CO
3
2
O
2
)
groups. However,
owns the minimum bandgap with moderate energy level position
among all these compounds. We also investigated the
correlation between the bandgap and electro-catalytic activity of
these compounds for ORR in alkaline solution. It is noted that
3
3
2
O
2
3
.
3
2
or La
2
O
2
3
the reduced bandgap of La
2
O
2
CO
3
, derived by different oxygen
2
O
3
2 3
O
[
25]
functional groups, may be responsible for its improved ORR
activity here. Understanding the origin of catalytic activity may
be very important to develop cheap and stable lanthanum-based
compound materials as next-generation electrocatalysts for
ORR.
are directly stacked on each other. Compared with La
2 3
O
typical separated layer structure of La CO
composed by (La O
2 2
2
O
2
3
3
2
+
2+
Results and Discussion
Characterization of lanthanum-based compounds
Figure 1(a) shows the schematic diagram of synthesis process
for three lanthanum compounds, LaOHCO
La . First, the added urea hydrolyzes into NH
at a given temperature of 85 °C. Then, the produced CO
3
, La
2
O
2
CO
and OH
and
on the carbon support.
Finally, the lanthanum compound LaOHCO decomposes into
La CO at 550 °C and La at 800 °C under Ar atmosphere,
3
, and
-
2
O
3
3
, CO
2
2
-
3+
OH react with La to form LaOHCO
3
3
2
O
2
3
2 3
O
respectively. Figure 1(b)-(d) show the morphologies of three as-
prepared lanthanum compounds. It can be observed that all
these lanthanum compounds are mixed homogeneously with the
carbon without obvious aggregation, as evidenced by the TEM
images and BSE (Back Scattered Electron) images shown in
Figure S1 and Figure S2 in the supporting information. The
insets of Figure 1(b)-(d) show the particle sizes of the
synthesized LaOHCO
8, 16 nm respectively. Moreover, Figure 1(e)-(g) show the
parallel fringe of three lanthanum-based compounds, which
3 2 2 3 2 3
, La O CO , and La O , which is about 20,
1
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Figure 1. (a) Schematic diagram of the synthesis process for three lanthanum
compounds. (b)(c)(d) SEM and TEM (inset) images for LaOHCO
and La on carbon. (e)(f)(g) HR-TEM images for these above lanthanum
compounds nanoparticles. (h) XRD patterns for these above lanthanum
3 2 2 3
, La O CO
2 3
O
Figure 2. (a) CV curves for LaOHCO
solution. The dashed line is tested in N
tested in O -saturated solution. The scan rates is 20 mV/s. (b) LSV curves
corrected by the N -saturated solution results for these above lanthanum
compounds in O -saturated 0.1 M KOH solution. The scanning rate is 5 mV/s.
The current density is normalized by the work electrode area (0.247 cm ). (c)
The value of j for three lanthanum compounds and the potential is 0.65 V vs
RHE. (d) Specific mass activity for three lanthanum compounds and the
potential is set as 0.65 V vs RHE. (e) Specific surface area activity. (f) The
electron transfer number for three lanthanum compounds.
3
, La
2 2 3 2 3
O CO and La O in 0.1 M KOH
2
-saturated solution and solid line is
compounds and (i) schematic diagram of LaOHCO
crystal structure.
3 2 2 3 2 3
, La O CO and La O
2
2
2
2
Electro-catalytic ORR activity of three lanthanum
compounds
The electro-catalytic activity for ORR over each of the
K
synthesized LaOHCO
was first investigated by cyclic voltammetry (CV) in N
and O -saturated 0.1 M KOH solutions respectively, and the
obtained results are shown in Figure 2(a). In the case of N
saturated CV curve, there is no redox feature observed and the
value of current density is very small for these lanthanum-based
3
, La
2
O
2
CO
3
, and La
2
O
3
nanostructures
2
-saturated
2
Moreover,
the
rotating
ring-disk
electrode
(RRDE)
2
-
measurements were carried out to further investigate the
catalyzing ORR activities of these lanthanum-based compounds.
As shown in Figure 2(b), all the lanthanum compounds exhibit
highly ORR activity in alkaline solution, where La O CO exhibits
2 2 3
the most positive onset potential (0.8 V vs RHE) and the largest
compounds. However, for O
apparent reduction peak with larger current and the onset
potential of LaOHCO , La CO , and La are 0.77, 0.81 and
.79 V vs RHE, respectively. It indicates that the superior
intrinsic ORR catalyzing performance of lanthanum compounds
and La CO catalyst shows the best ORR activity compared
with other lanthanum compounds.
2
-saturated CV curve, there is an
O
2
3
2
O
3
-2
3
2
limited diffusion current density (~ 4 mA m ) among all the
0
tested samples. The slightly difference in limiting currents during
ORR over these catalysts may be attributed to the different
electrical conductivity for three lanthanum compounds. To
further study the ORR kinetics over these catalysts, the kinetic
2
O
2
3
current (I
K
) values are obtained by the formula:
/I=1/I +1/I
is the kinetic-limiting current, I is the current obtained at
.65 V vs RHE, I is the limiting current. To obtain the kinetic
current density (j ), the corresponding measured current (I ) was
1
L
K
where I
K
0
L
K
k
normalized by the surface area of each sample that is
determined by Brunauer Emmett and Teller (BET)
measurements shown in Figure S3. The obtained surface area
2
of LaOHCO
3
, La
2
O
2
CO
3
and La
2
O
3
is 34.04, 61.53 and 47.69 m
results are shown in Figure
-1
g
2
respectively. Therefore, the j
K
(c). As is observed, the kinetic current density (j
CO , and La is 20.78, 54.97 and 25.43 μ A cm ,
respectively, further indicating the highest catalytic activity of
La CO . Expect for the kinetic current density results, specific
K 3
) of LaOHCO ,
-
2
La
2
O
2
3
2 3
O
2
O
2
3
mass activities were also extracted for these catalysts to
investigate their intrinsic catalytic activities and the results are
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Chemistry - A European Journal
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FULL PAPER
shown in Figure 2(d). At the typical voltage of 0.65 V vs RHE,
the very cheap La
2
O
2
CO
3
as next-generation catalyst for the
the measured specific mass activity for LaOHCO
3
, La
2
O
2
CO
3
,
ORR in alkaline solution, alternative to expensive Pt catalyst.
-1
-1
-1
and La O is 7.07 A g , 33.82 A g and 11.7 A g , respectively.
2 3
Figure 2(e) shows the specific surface area activities of three
lanthanum compound catalysts at different potentials, which
further confirms that the La O CO has surprisingly high catalytic
2 2 3
activity toward ORR, even under different potential conditions.
As evidenced by Tafel plots in Figure S4, the ORR rate
catalyzed by La
The over-potential of LaOHCO
determined by the difference value between the onset potential
2
O
2
CO
3
is faster than that of La
2
O
3
or LaOHCO
3
.
3
,
La CO
2
O
2
3
and La is
2 3
O
[
26]
and oxygen reduction reaction potential (0.401 V).
derived over-potential of LaOHCO , La CO and La
3
.337, 0.318 V, where La CO exhibits the minimum over-
The
3
2
O
2
3
2
O
3
is 0.34,
0
2
O
2
Figure 3. (a) Durability measurements for LaOHCO
Pt/C for 10000 s, the potential is set as 0.7 V vs RHE. (b) Methanol crossover
tests for LaOHCO , La CO , La and Pt/C by injecting 1.5 ml methanol
into the electrolyte (60 ml) at 1000 s. The chronoamperometric measurements
were performed at the potential of 0.7 V vs RHE with a rotation rate of 900 rpm
3 2 2 3 2 3
, La O CO , La O and
potential in three lanthanum compounds.
In addition, the Koutecky-Levich analysis were carried out to
investigate the pathway of oxygen reduction, and the calculated
average transfer electron number (n) is shown in Figure 2(f). As
3
O
2 2
3
2 3
O
2
in 0.1 M O -saturated KOH solution.
is seen, the average transfer electron number (n) of La
calculated to be 3.7 from 0.2 to 0.6 V vs RHE, and 3.2 for La
and 3.0 for LaOHCO . The high electron transfer number for
La CO demonstrates nearly four-electron transferred
process for oxygen reduction. However, the electrocatalytic
activity of the lanthanum-based catalyst (La CO ) is slightly
2 2 3
O CO is
2
O
3
3
Determination bandgap values of three lanthanum
compounds
The bandgap values of LaOHCO , La O CO , and La O were
3 2 2 3 2 3
determined by UV-vis spectroscopy, and the results are shown
in Figure 4. In Figure 4(a), the UV/vis spectroscopy absorbance
spectra of the above lanthanum-based compounds show
apparent band edge absorption under 250 nm. It is noted that
the absorption coefficient of semiconductor has a characteristic
2
O
2
3
a
2
O
2
3
lower than that of Pt/C, as shown in Figure S5. It may be
attributed to its poor electrical conductivity of the catalyst,
compared with that of Pt/C. To improve conductivity of the
catalyst, by mixing with other high conductive carbon materials,
it is possible to further improve the activity.
Besides the catalytic activity, durability is another key parameter
evaluating the performance of catalysts. Therefore, the stabilities
3 2 2 3 2 3
for LaOHCO , La O CO , and La O as well as Pt/C catalyst
were measured via the chronoamperometric response method
at 0.7 V (vs RHE) in 0.1 M KOH solution for 10000 s and the
results are shown in Figure 3(a). As expected, all the lanthanum
compounds exhibit better stability than Pt/C catalyst. Moreover,
relation:
n
(
αhv) =B(hv-E
g
)
Where
B
is the semiconductor characteristic constant,
n
depends on the type of transition, which is 1/2 in this work. E
g
is
the bandgap that is the different value of conduction band and
valence band, hν is photon energy and α refers to the absorption
[28]
1/2
coefficient. Based on the deduced (αhν) - hν plots shown in
Figure 4(a) inset, the calculated E values are shown in Figure
2 3
(b). The obtained E value for LaOHCO , La CO , and La O
g
after 10000 s, there is still 93% current remaining for La
whereas only 90% for La and 88% for LaOHCO , indicating
La CO is more stable than La or LaOHCO during ORR
2 2 3
O CO ,
4
g
3
2
O
2
3
2
O
3
3
is 3.88, 3.37 and 3.61 eV, respectively. However, the measured
value of each compound is smaller than the derived value
according to the first principles pseudo-potential calculation that
is 4.51, 3.65, 3.8 eV for LaOHCO , La O CO , and La O bulk
3 2 2 3 2 3
materials, which should be ascribed to the quantum size effect
2
O
2
3
2
O
3
3
process. The resistance for methanol poisoning is another
important factor for the ORR catalysts. Accordingly,
chronoamperometric measurements were performed at the
2
potential of 0.7 V with a rotating rate of 900 rpm in 0.1 M O -
for these lanthanum compound nanoparticles, leading
saturated KOH solution to investigate the methanol crossover
effects for three lanthanum compounds as well as commercial
Pt/C catalyst and the results are shown in Figure 3(b). For Pt/C
[20-21]
g
decreasing value of E with decreasing particle size.
The different bandgap values for lanthanum-based compounds
can be explained by their coordination environments, which
2
catalyst, the injected methanol into O -saturated KOH electrolyte
mainly depends on the oxygen-containing functional groups and
at 1000 s leads to the ORR current sharply increase from
cathodic current to anodic current because methanol oxidation
[25]
layer structures.
As shown in Figure 1(i), among three
lanthanum compounds, La
structure that is composed by (La
2
O
2
CO
3
has
a
separated layer
[
27]
alters oxygen reduction reaction, whereas the current density
for three lanthanum compounds remains unaltered, indicating
high immunity towards methanol and selectivity to ORR than
Pt/C catalysts. The reason for such excellent durability and
resistance to methanol poisoning is that the lanthanum
compounds can react with methanol oxidation intermediate CO
to form La-CO bonds, and then the generated La-CO bonds
2+
2
O
2
)
and carbonate, which
weakens the band strength and leads to relatively small
bandgap, as shown in Figure 4(b). La is composed by LaO
layers that are directly stacked on each other, which leads to the
stronger band strength and larger bandgap of La than
La CO . For LaOHCO , its hydroxyl is an electron donating
2
O
3
2
2
O
3
2
O
2
3
3
group that is easily to lose electron, while carbonate is an
electron withdrawing group which is apt to obtain electron. As a
result, LaOHCO should have the largest E value.
3 g
The reduced band gap over lanthanum-based compounds would
also contribute to improve the catalytic activity by reducing the
activation energy of catalyzing ORR. This hypothesis has been
-
could react with the oxygen containing species such as OH from
water to form CO
catalysis.
2
, which further decay the poison of CO during
The good stability and high resistance to
methanol poisoning should contribute the practical application of
[
18-19]
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Chemistry - A European Journal
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FULL PAPER
verified by the results from Figure 4(c)-(d). Figure 4(c)-(d) has
given the correlation of bandgap and electro-catalytic activities
of three lanthanum compounds. As observed, with the decrease
La
and the reduction potential of O
VB and CB of La CO
covalent electrons under excitation from VB to CB to participate
the oxygen reduction. Moreover, compared with LaOHCO
La CO has a more negative VB level and more positive CB
level, indicating that covalent electrons in VB level owns higher
energy to transfer into CB level under excitation, and then easily
transfer from CB to adsorbed oxygen molecules. As a result,
2
O
3
, La
2
O
2
CO
3
and LaOHCO
3
have a more positive VB level,
-
2
/OH (0.401 V) is between the
2
O
2
3 3
and LaOHCO , which contributes the
of bandgap value, the onset potential, j
activity are totally increased. La CO
K
and specific mass
with the minimum
2
O
2
3
3
,
bandgap shows the maximum onset potential, kinetic current
density (j ) and specific mass activity shown in Figure 4(d),
2
O
2
3
K
further illustrating the bandgap may be a key factor to control the
ORR activity over these lanthanum compounds. On the one
hand, the decreased bandgap will lead to an increase of
electrical conductivity of lanthanum compounds, which is very
important to improve the nonconductive oxide catalytic activity.
On the other hand, the decreased bandgap can promote the
oxygen reduction kinetics by facilitating oxygen absorption and
electron transfer from catalyst bulk to the surface adsorbing
oxygen species, which can reduce the activation energy of
catalyzing ORR. This conclusion has been proved by transition
2 2 3
based on the energy level structure, La O CO with the smallest
bandgap and moderate energy level structure of VB and CB
should have the highest ORR activity among all the compounds.
Figure 5(c) shows the electron transfer process in the catalyzing
ORR of La
2
O
2
CO
3
. During the ORR over La
2 2 3
O CO , covalent
electron of La
2
O
2
CO
3
is first excited from VB level under the
condition of impressed current, then the excited electron
transfers from VB into CB due to the moderate bandgap value.
The excited electron has sufficient potential to transfer into the
unoccupied π*2p orbitals of oxygen, leading to a strengthen
[
29]
3 4
metal oxide like Co O as catalysts for the ORR.
[
31-33]
oxygen adsorption, which promotes the reduction of oxygen.
The high ORR activity of La CO has been verified by the
results from Figure 2. The detailed ORR process over La CO
3
O
2 2
3
2
O
2
has been shown in Figure 5(d). For lanthanum-based
compounds, their bandgaps and energy level structure of VB
and CB can impact the ORR activity of them together. This
conclusion can be confirmed by incomplete linear relationship of
the activity and bandgap as an activity description, as shown in
Figure 4(c) and (d).
Figure 4. (a) Absorption spectra for LaOHCO
3 2 2 3 2 3
, La O CO and La O
nanoparticles by UV-vis and the inset is apparent energy gap of LaOHCO
3
,
,
La
La
2
O
O
2
CO
CO
3
, La
2 3 3
O nanoparticles. (b) The calculated bandgap for LaOHCO
O nanoparticles. (c) Relationship between onset potential and
3
2
2
3
, La
2
bandgap and (d) relationship between the
bandgap for three lanthanum compounds.
K
j , specific mass activity and
The origin of catalytic activity for lanthanum compounds.
We investigate the origin of ORR catalytic activity for lanthanum-
based compounds based on their intrinsic electronic bandgaps.
XPS measurements were employed to locate the valance band
level, and the results are shown in Figure 5(a). Their valance
band maximum (VBM) positions are determined by the leading
edges linear extrapolation of valance band (VB) spectra to the
3 2 2 3 2 3
Figure 5. (a) Valence band maximum for LaOHCO , La O CO and La O
nanoparticles determined by XPS valance band spectra via linear
extrapolation of the leading edges and the base lines. (b) Energy level
diagram for the LaOHCO
Schematic diagram for electron transfer process of La
electro-catalytic process.
3
, La
2
O
2
CO
3
and La
2
O
3
compounds. (c) and (d)
CO in the ORR
2
O
2
3
base line. As is shown, the calculated VBM for LaOHCO
La CO , and La is 1.89, 1.66, -0.45 eV respectively, which
corresponds to the potential (vs NHE) is 1.48, 1.26, -0.85 V.
Therefore, the value of conduction band (CB) for LaOHCO
La CO and La is -2.39, -4.46, -2.11 vs NHE,
3
,
2
O
2
3
2 3
O
[
30]
3
,
Concerning that the energy level of lanthanum-based
compounds plays an important role in catalyzing ORR, we also
2
O
2
3
,
2
O
3
V
respectively. The energy level diagram for three lanthanum
compounds is shown in Figure 5(b). According to the diagram,
study the influence of La
pathway apart from the bandgap. Figure 6(a) gives the possible
schematic diagram of electron transfer process of La CO
during ORR, based on the energy level of both VB and CB,
2 2 3
O CO energy level on the ORR
both the VB and CB levels of La
2
O
3
are more negative than the
2
O
2
3
-
reduction potential of O
2
/OH (0.401 V), which makes it difficult
-
-
for the electrons under excitation to participate in the
electrochemical reaction of oxygen reduction. Different from
relative to the redox potentials of O
2 2 2
/HO and O /OH pairs as
references. We can observe that the reduction potential of
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Chemistry - A European Journal
10.1002/chem.201701136
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-
-
O
2
/HO
2
or O
2
/OH is in the middle position between the VB and
and La
stack layer structure. The particle sizes of the synthesized
LaOHCO , La CO , and La are about 20, 18, 16 nm
respectively. Compared with the other two compounds, the
La CO exhibits the best electro-catalytic activity, the lowest
2 2
H O production and superior durability for the ORR in 0.1 M
2 2 3 2 3
O CO own layer structure while La O has a directed
CB levels. As a result, the excited electron occupied in CB
-
originated from VB of La
2
O
2
CO
through a 2-electron pathway; at the
same time, these excited electrons can also transfer into the
3
can transfer into the O
2
/HO
2
3
2
O
2
3
2 3
O
-
level to generate HO
2
2
O
2
3
-
-
O
2
/OH level to produce OH via a 4-electron pathway during, as
-
2
shown in Figure 6(a). Since the reduction potential of O
2
/HO
KOH solution. The improved ORR activity of the lanthanum
compound can be attributed to its minimum bandgap and
moderate value of conduction band (CB) and valance band (VB)
in electronic structure, which facilitates the excited electrons
-
pair is more negative than O
transferred electrons will occupy the O
in a lot of H produced as ORR intermediate product.
However, as shown in Figure 6(b), the H production (26%)
on the La CO catalyst is much lower than the OH (74%), as
2
/OH pair, it is possible that many
-
2
2
/HO level, which results
2
O
2
-
-
-
-
2
O
2
2 2 2 2
transferring into O /HO or O /OH levels to produce HO or OH
-
2
O
2
3
species via a 2/4-electron pathway during the ORR process.
Considering that the lanthanum compound has the ability of
-
shown in Figure 6(c). The high OH production over this catalyst
can be explained by two aspects as following. On the one hand,
-
chemical disproportionation for hydrogen peroxide (HO
2
), it can
-
for the La
chemical interaction between La
2
O
2
CO
3
/C hybrid catalyst, there remains a strong
CO and carbon support in
lead to HO
disproportionation into O
this stage adsorbs on the La
oxygen storage due to its unique layered structure. It can
facilitate the reuse of O from hydrogen peroxide decomposition
2
species undergoing
a
prompt chemical
and OH species. The generated O in
CO surface that is beneficial for
-
2
O
2
3
2
2
their hybrid, which can facilitate the ORR undergoing through
the 4-electron pathway to generate dominated OH rather than
2
O
2
3
-
[
13]
H
2
O
2
, as confirmed by recently published report. This leads to
2
-
the low production of H
catalyst, although the energy level diagram in Figure 6(a) is
beneficial to the formation of On the other hand,
considering that the lanthanum compound has an ability to
2
O
2
with respect to OH produced by the
process, which further contributes the ORR kinetics. Therefore,
understanding the origin of the catalytic activity may be very
important to develop cheap and stable lanthanum-based
compound materials as next-generation electrocatalysts for the
ORR.
2 2
H O .
-
chemical disproportionation for hydrogen peroxide (HO
2
), which
species produced at the energy level of
2
level undergo prompt chemical disproportionation into
-
can lead to that the HO
2
-
O
O
2
/HO
and OH species.
-
[34-35]
2
The generated O
2
in this stage Experimental Section
adsorbs on the catalyst surface, which can be utilized for further
oxygen reduction once again. Moreover, it has been reported
that existence of (CO
Synthesis of materials
2
-
3 2 2 3
) in La O CO leads to a larger radius,
which are not only beneficial for isolating the active sites but also
Prior to the preparation of preferred La O CO and La O , LaOHCO was
2
2
3
2
3
3
[
36]
for store oxygen.
facilitate the reuse of O
Thus, the ability of oxygen storage can
from hydrogen peroxide decomposition
initially prepared via a simple in-situ urea hydrolysis method. The typical
preparing procedure of LaOHCO is shown as follows. First, 70.1 mg of
La(NO •6H O and 780 mg urea were dissolved in 65 ml deionized
water (18.2 MΩ) to form a clear solution. After that, 100 mg carbon black
Vulcan XC-72, Cabot) was added into the solution as disperser to
3
2
3
)
3
2
process, leading to an enhanced ORR activity. The above two
aspects should be responsible for the low hydro-peroxide yield
on this catalyst shown in Figure 6(c) and the highly ORR activity
(
control lanthanum compounds particle size and increase the electronic
conductivity of lanthanum-based catalyst. The mixed solution was then
heated at 85 °C for 2 h with vigorous stirring and the resulting mixture
was washed with deionized water and ethanol for several times and then
(
Figure 2(a) and (b)). Therefore, we conclude that the ORR
process catalyzed by lanthanum-based compound via a nearly
-electron pathway (apparent 4-electron mechanism). This
4
conclusion is in agreement with the results shown in Figure 2(f).
dried for five hours at 80 °C, which obtained the LaOHCO
the as-synthesized LaOHCO were used as precursor to prepare
La CO and La samples by calcining at 550 °C and 800 °C for 2 h
3
sample. Then,
3
O
2 2
3
2 3
O
under Ar atmosphere, respectively. The mass ratio of these lanthanum
compounds to their carbon supports is fixed at 10:1 by varying the added
amount of carbon black, as shown in Figure S6.
Characterization of materials
The morphologies of these lanthanum compounds were observed via the
transmission electron microscopy (TEM) (JEOL TEM 2010 microscope)
and scanning electron microscopy (SEM) (FE-SEM, JEOL, JSM-6701F),
respectively. The crystal structure of each lanthanum compound was
determined by X-ray diffraction patterns (XRD) from Rigaku RINT 2200
V/PC at 30 kV and 40 mA, where the scan rate was 4 °/min from 10° to
-
Figure 6. (a) Schematic diagram for HO
RRDE plots of La CO
ratio and OH ratio of La
2
production in the ORR process. (b)
6
0°, and the schematic diagram for lanthanum compounds crystal
structure was drawn by Diamond 3.1. X-ray photoelectron spectroscopy
XPS, Thermo ESCALAB 250) was applied for the valence band spectra
O
2 2
3
in 0.1 M KOH solution with the rotating rate at 1600
-
2
-
rpm. (c) HO
2
O
2
CO
3
at the potential of 0.7 V vs RHE.
(
of three lanthanum compounds and the XPS spectra were calibrated by
C 1s at 285.0 eV. UV/vis spectroscopy on a UV2450 was used to
Conclusions
characterize the optical absorbance of the pure LaOHCO
3 2 2 3
, La O CO ,
and La nanoparticles which were dissolved in deionized water, and
2 3
O
Three lanthanum compounds (LaOHCO
were synthesized via an in-situ urea hydrolysis method, followed
by annealing at different temperatures. The obtained LaOHCO
3
, La
2
O
2
CO
3
, and La
2
O
3
)
the slit width was set at 2.0. The surface area of lanthanum compounds
was measured by BET (Quadrasorb SI-MP). The amount of lanthanum
compound relative to carbon support was determined by thermo-
3
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Chemistry - A European Journal
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FULL PAPER
gravimetric and differential thermo-gravimetric analysis (TG-DTA,
Thermoplus TG8120, Rigaku)
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bandgap• hydro-peroxide decomposition • electronic energy
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Energy level diagram of La
2
O
2
CO
production in
the ORR electro-catalytic process
3
Weiwei Gu, Jingjun Liu,* Ye Song
-
compounds with HO
2
Page No. – Page No.
Lanthanum-Based Compounds:
Electronic Bandgap-Dependent
Electro-Catalytic Materials toward
Oxygen Reduction Reaction
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