Phase controllable synthesis of three-dimensional star-like MnO2 hierarchical architectures as highly efficient and stable oxygen reduction electrocatalysts

Article abstract of DOI:10.1039/c6ta04530h

To achieve high-performance fuel cells and metal-air batteries, inexpensive and earth-abundant catalysts with enhanced activity and durability for the oxygen reduction reaction (ORR) are currently sought after. Herein, three-dimensional (3D) α-MnO₂ and ?-MnO₂ hierarchical star-like architectures with tunable crystal phases and desirable ORR activity were readily prepared by a facile hydrothermal method with no surfactants or templates. The effects of reaction temperature, anion type, and dwell time on the morphologies of the MnO₂ products were studied in detail, and the possible formation mechanism of the 3D MnO₂ hierarchical stars was proposed. Due to the improved electrical conductivity and O₂ adsorption ability, the resulting α-MnO₂ catalyst showed substantially enhanced ORR activity, compared to the ?-MnO₂ and bulk MnO₂ catalysts, with a more positive onset potential, a larger limiting current density, and better durability. Our results provide a facile chemical route towards the phase-controlled synthesis of 3D MnO₂ architectures, which can serve as efficient catalysts for ORR-based applications.

Full text of DOI:10.1039/c6ta04530h

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Phase Controllable Synthesis of Three-Dimensional Star-like MnO2
Hierarchical Architectures as Highly Efficient and Stable Oxygen
Reduction Electrocatalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Gao Cheng,a Shilei Xie,b Bang Lan,c Xiaoying Zheng,a Fei Ye,a Ming Sun,a Xihong Lu,b* and Lin Yua*
DOI: 10.1039/x0xx00000x
www.rsc.org/
To achieve high performance of fuel cells and metal-air batteries, searching the inexpensive and earth-abundant catalysts
with enhanced activity and durability for oxygen reduction reaction (ORR) are currently pursued. Herein, three-
dimensional (3D) α-MnO2 and ε-MnO2 hierarchical star-like architectures with tunable crystal phase and desirable ORR
activity were readily prepared by a facile hydrothermal method without any surfactants or templates. The effects of
reaction temperature, anion type and dwell time on the morphologies of the MnO2 products were studied in detail, and
the possible formation mechanism of the 3D MnO2 hierarchical stars was proposed. Due to the improved electrical
conductivity and O2 adsorption ability, the resulting α-MnO2 catalyst showed substantially enhanced ORR activity
compared to the ε-MnO2 and bulk MnO2 catalysts, with a more positive onset potential, larger limiting current density and
better durability. Our results provide a facile chemical route towards phase-controlled synthesis of the 3D MnO2
architectures, which can be served as the efficient catalysts for ORR-based applications.
the most current MnO2 electrocatalysts are still insufficient and
needed to further boost.
Introduction
The ever-growing demands for energy and environment
protection have stimulated significant interests in the
development of sustainable energy conversion and storage
devices. Fuel cells and metal-air batteries are recognizing as the
most promising energy conversion and storage devices for their
theoretically high energy density and wide applications.1-5 The
essential to gain their optimal performance is to develop
efficient electrocatalysts for the oxygen reduction reaction
(ORR). Noble metal and its alloys are the most active ORR
electrocatalysts,6,7 but they are highly expensive and scarce.
Alternatively, substantial efforts have been devoted to
exploring the non-precious ORR electrocatalysts. Up to now,
multifarious electrocatalysts such as transition-metal oxides,8-10
doped carbons11,12 and metal-N4 chelates macrocycles13 have
been reported. Among them, manganese dioxide (MnO2) holds
great promise as alternative and cost-effective electrocatalyst
for ORR due to its low cost, environmental friendliness,
structural diversity (e. g., α-, β-, γ-, δ-, and ε- type, etc.) and
acceptable catalytic activity.14-19 However, owing to the limited
electrical conductivity (10-5-10-6 S cm-1), the ORR activities of
To improve the ORR performance of MnO2, many strategies
including nanostructuring,20 integrating with highly conductive
materials,21,22 doping with cations,23,24 coating with metal
nanoparticles,25,26 introducing native oxygen defects27 and
hydrogenation,28 have been proposed. Among these strategies,
designing MnO2 nanostructures with rational architecture and
desired phase have been emerging as one of the most effective
approaches to boost their electrocatalytic ORR activity. For
instance, free-standing and ultrathin α-MnO2 nanoflakes
synthesized by a cationic surfactant assistant method showed a
good ORR mass activity of 21 ± 1.2 mA mg-1 at 0.75 V vs.
RHE.20 The oxygen-deficient β-MnO2 nanorods exhibited
approximately 50 mV (vs. RHE) higher potential than the
pristine β-MnO2 nanorods.27 Overall, these synthetic methods
often require relatively complicated processes and the catalytic
durability of these MnO2-based catalysts is not yet satisfactory.
More importantly, the effect of the phase variation on the ORR
activity of MnO2 is still not clear since it is difficult to obtain
similar MnO2 morphology with different crystal phases through
the current synthetic approaches. Therefore, the development of
a facile and effective method to prepare nanostructured MnO2
electrocatalysts with controllable crystal phase and morphology
as well as high ORR activity is of great interest and greatly
challenging.
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Three-dimensional
(3D)
hierarchical
architectures, electron microscopy (FESEM) images of the α-MnO2 and ε-
assembling from one-dimensional (1D) or two-dimensional (2D) MnO2 samples. Both the MnO2 samples exhibit a homogenous
micro/nanoscale building blocks, not only can provide high 3D star-like hierarchical architecture that consists of six
surface area and fast pathway for ion diffusion and charge,29,30 branches. The α-MnO2 hierarchical stars have a typical size of
but also prevent ostwald ripening or agglomeration of the about 40-50 μm (Fig. 2a), while the ε-MnO2 hierarchical stars
catalyst.31 Such these features make them highly promising as have a typical size of 10-25 μm (Fig. 2c). Further observations
electrocatalysts for ORR.32 Herein, we demonstrated for the reveal that every branch of both the MnO2 samples is composed
first time that the facile synthesis of 3D star-like MnO2 of parallel arrays of MnO2 nanowires, and the angle between
hierarchical architectures with tunable crystal phase based on a every pair of adjacent branches is nearly 60° (Fig. 2b and d).
simple redox reaction and their implementation as advanced The surfaces of the α-MnO2 and ε-MnO2 hierarchical stars are
electrocatalysts for ORR. The novel 3D α-MnO2 and ε-MnO2 relatively smooth (Fig. S2). Additionally, the big difference
hierarchical stars were readily achieved by a direct redox between these two MnO2 samples is that the length of the α-
reaction using KClO3/NaClO3 and Mn(CH3COO)2 as MnO2 branched nanowires (~ 25 μm) is substantially longer
precursors under mild conditions. Significantly, the resulting than that of the ε-MnO2 branched nanowires (~ 13 μm). All
MnO2 hierarchical stars presented a close relationship between these results fully confirmed that similar 3D MnO2 hierarchical
the phase and ORR activity. The α-MnO2 hierarchical stars stars with controllable crystal phase were successfully prepared
exhibited substantially superior ORR performance than the ε- via this simple approach.
MnO2 hierarchical stars in alkaline medium, with a more
To acquire more detailed structural information of α-MnO2
positive onset potential of 0.84 V and higher diffusion-limiting and ε-MnO2 hierarchical stars, transmission electron
current of 5.67 mA cm-2. Furthermore, the α-MnO2 stars also microscopy (TEM) and selected area electron diffraction
had remarkably pronounced durability with more than 93.3 % (SAED) analyses were carried out. Fig. 3a and 3c show the
retention after 10 h.
typical TEM images of an individual six-branched α-MnO2 and
Results
Structures and morphologies
As illustrated in Fig. S1, the α-MnO2 and ε-MnO2 star-like
hierarchical architectures were prepared via a direct redox
reaction by using KClO3/NaClO3 and Mn(CH3COO)2 as
precursors under acidic conditions, respectively. X-ray
diffraction (XRD) studies were firstly conducted to investigate
the phase of the products (Fig. 1a and c). Interestingly, all the
reflections for the sample obtained by using KClO3 as the
oxidant can be well indexed to a tetragonal α-MnO2 (JCPDS
44-0141), which possesses a 2 × 2 tunnel structure in Fig. 1b.
When using NaClO3 as the oxidant, a hexagonal ε-MnO2
(JCPDS 12-141) with an intergrowth of 1 × 1 tunnels and 1 × 2
tunnels (Fig. 1d) was obtained. This indicates that the crystal
phase of the MnO2 can be readily tuned by elaborately choosing
the oxidant. Fig. 2 shows the typical field emission scanning
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ε-MnO2 hierarchical star, respectively. The HRTEM image of a the uniform 3D MnO2 hierarchical stars can be formed at
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DOI: 10.1039/C6TA04530H
single nanowire on α-MnO2 branches shows two sets of lattice 160 °C. However, when the reaction temperature further
spacings of 4.89 Å and 2.74 Å with an angle of 73° that increased to 180 °C, only rod-like MnO2 nanostructures are
correspond to the (200) and (101) planes (Fig. 3b), suggesting observed for the products prepared by using the two different
that the nanowire grows along the [001] direction. The three oxidants (Fig. S3e and S3f). This is because higher temperature
bright spots from the circle could be well indexed to (-101), (180 °C) leads to
a faster nucleation and growth of
(101), and (400) planes (inset of Fig. 3b). The zone axis is nanostructures, which can destroy the star-like shapes during
[010]. The SAED pattern also confirms that the nanowire is a the violently reaction.36 Similar results were obtained for α-
single crystal of α-MnO2. Fig. 3d shows the HRTEM image of MnO2 flower-like particles and CoOOH flower-like
a single ε-MnO2 nanowire. The lattice spacings of 2.42 Å and nanostructures synthesized at moderate temperature.36,37
2.42 Å with an angle of 60° are in good agreement with the Moreover, we also investigated the effect of anion on the
(100) and (010) planes, indicating the nanowire has a growth morphology of the MnO2 products. With other conditions kept
direction along of [001]. The three bright spots in the SAED the same, the 3D MnO2 hierarchical stars with six branches of
pattern (inset of Fig. 3d) could be corresponded to the (-110), MnO2 nanowires can’t be observed when the other kinds of
-
(010), and (100) planes. Additionally, the ε-MnO2 nanowire is anions (SO42-, Cl- and NO3 ) were used in our system (Fig. S4),
also single crystalline, which is confirmed by the well-arranged indicating the CHCOO- play a key role in the formation of the
spots in the SAED pattern.
MnO2 star-like shapes. Under the hydrothermal system, the
Precise control the reaction parameters such as the reaction carbonyl groups from CH3COO- could form hydrogen bond
temperature, dwell time and anion type are the key factors for with the surface OH groups of the MnO2 nanostructures.38 Thus
the formation of uniform hierarchical nanostructures, since the lateral growth of MnO2 can be restricted through the steric
these parameters significantly affect their nucleation and effects, resulting in the MnO2 nanowires with high aspect ratios
growth process. As shown in Fig. S3, various reaction of the MnO2 hierarchical stars.39
temperatures (120, 140 and 180 °C) were applied to study the
In order to understand the formation mechanism of these
effect of temperature on the morphology of the MnO2 products MnO2 hierarchical stars, time-dependent studies were
with other parameters unchanged. At lower reaction performed and the results are shown in Fig. S5 and Fig. S6.
temperatures (120 and 140 °C), the products prepared by using Based on the FESEM results of the 3D MnO2 star-like
KClO3 as the oxidant are both the urchin-like MnO2 hierarchical architectures after various reaction times, we
nanostructures (Fig. S3a and S3b), while the products obtained proposed these MnO2 hierarchical stars may grow according to
by using NaClO3 as the oxidant are both made up of the urchin- a “nucleation-growth” process. In the early stage, primary
like and star-like MnO2 nanostructures (Fig. S3d and S3e). In nucleation occurs in solution, followed by producing lots of
general, increasing the reaction temperature will facilitate the MnO2 units spontaneously. Afterward, aggregation of the
homogeneous nucleation and growth of nanostructures,35 thus MnO2 units will then occur to generate relatively small 3D
MnO2 nanostructures. Since the difference in the redox
-
potentials between ClO3 /Cl and MnO2/Mn2+ is small (~ 0.227
eV), the reaction may proceed very mildly, allowing the MnO2
intermediates to grow slowly and homogeneously. Then, the
nanorods growing from the core start to convert to the
nanowires with high aspect ratios, and the 3D MnO2
nanostructures evolve into the novel star-like hierarchical
nanostructures. Finally, prolonged reaction time results in the
formation of the MnO2 hierarchical stars. A similar process has
been observed in the formation of 3D MnO2 nanostructures
under the hydrothermal method.34,40 In our experiments, the
reaction conditions remained the same except for the type of
oxidizing reagent. Generally, the size of the inorganic cations is
believed to have a critical impact on the formation of different
MnO2 crystal phases. The cations with large sizes (such as K+,
Rb+ and Ba2+) can serve as a template to stabilize the 2 × 2
tunnel structure, while the Na+ ions are beneficial for the
synthesis of the MnO2 structure with an intergrowth of 1 × 1
and 1 × 2 tunnels.40 Therefore, the α-MnO2 and ε-MnO2
hierarchical stars with tunable crystal phase can be readily
obtained under our synthesis conditions.
Electrocatalytic results
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α-MnO2 and ε-MnO2 samples. The corresponding slopes are
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DOI: 10.1039/C6TA04530H
very similar for the Pt/C, implying an apparent quasi-four-
electron mechanism for these two MnO2 samples.
The electron transfer number (n) and peroxide yields (yperoxide
are two indicators to evaluate the ORR catalytic performance.
As shown in Fig. 4d, the determined yield of peroxide species
and n vary with disk potential. From 0.4 to 0.8 V, n is well
approaches 3.81 for the α-MnO2 sample and 3.77 for the ε-
MnO2 sample, again confirming the ORR with both of the two
MnO2 samples is an apparent quasi-four-electron process.
Moreover, the yperoxide for α-MnO2 sample is substantially lower
than those of ε-MnO2 sample and bulk MnO2, further indicating
the superior ORR activity of the α-MnO2 hierarchical stars. Fig.
S9 shows the Tafel plots of the α-MnO2 and ε-MnO2 samples.
Obviously, two Tafel slopes can be observed for both MnO2
samples at high and low overpotentials, respectively. The small
Tafel slope is advantageous for practical catalytic applications,
since it leads to a rapid increase of ORR rate with
overpotential.41 The calculated Tafel slopes for the α-MnO2
sample are about 78 mV/dec in low overpotential range and 138
mV/dec in high overpotential range, which are distinctly
smaller than those of the ε-MnO2 sample (95 mV/dec in low
overpotential range and 141 mV/dec in high overpotential
range). This indicates the faster ORR reaction kinetics of the α-
MnO2 sample.
Besides the excellent catalytic activity, the α-MnO2
hierarchical stars also show favourable catalytic stability in
alkaline solution (Fig. 5). In a continuous polarization period of
10 h, more than ~ 93.3 % of the initial current is retained for the
α-MnO2 catalyst, while only ~ 78.7 % retention for the ε-MnO2
catalyst and ~ 64.5 % for the bulk MnO2. This present stability
also outperforms the value of the Pt/C catalyst for ORR (~ 87.9 %
retention after 10 h). The high durability of the α-MnO2
hierarchical stars could be attributed to their unique 3D star-like
morphology constructed by nanowires, providing some
advantage against particulate agglomeration.42 However, the
durability of the ε-MnO2 hierarchical stars is not ideal, which
)
Fig. 4 (a) CV curves of the MnO2 catalysts in both N2-saturated and O2-saturated
0.1 M KOH solution; (b) Polarization curves of bulk MnO2, ε-MnO2, α-MnO2 and
Pt/C at 1600 rpm; (c) K-L plots of the different samples at 0.45 V. (d) Electron
transfer number and H2O2 production yield determined from RRDE data.
The electrocatalytic activity of the MnO2 samples for the ORR
was tested in 0.1 M KOH solution saturated with high-purity O2.
Fig. 4a compares the typical cyclic voltammograms (CVs) of
the α-MnO2 hierarchical stars, ε-MnO2 hierarchical stars and
bulk MnO2 (Fig. S7) in N2- and O2- saturated solution at a scan
rate of 50 mV s-1. The CV curves of the three MnO2 samples in
N2-saturated solution exhibit no obvious peaks, while the CV
curves in the O2-saturated have a distinct cathodic peak. Note
that the cathodic peak for α-MnO2 sample is located at 0.72 V,
which is more positive than those of ε-MnO2 sample (0.69 V)
and bulk MnO2 (0.64 V), indicating the superior electrocatalytic
activity of the α-MnO2 sample. The ORR activity of the as-
prepared α-MnO2 and ε-MnO2 catalysts were further
characterized using rotational ring-disk-electrode (RRDE) at
different rotating speeds. These results were also compared
with those of bulk MnO2 and Pt/C (20 wt%). As shown in Fig.
S8, all these catalysts exhibit substantial rise in limiting current
densities as the rotation speeds continually enhance due to the
increased mass transport at the electrode surface. Fig. 4b
compares the typical LSV curves of the different catalysts
recorded at 1600 rpm in 0.1 M KOH solution saturated with O2.
Significantly, the α-MnO2 hierarchical stars yielded a more
positive onset potential (Eonset) and larger limiting current
density (around 0.84 V and 5.67 mA cm-2, respectively) than
those of the ε-MnO2 hierarchical stars (around 0.80 V and 5.28
mA cm-2, respectively) and bulk MnO2 (around 0.75 V and 5.10
mA cm-2, respectively). In addition, the half-wave potential
(E1/2) for α-MnO2 sample is ~ 0.72 V, which is the highest of
the three MnO2 samples. The electrocatalytic performances
(Eonset and E1/2) of the α-MnO2 hierarchical stars are comparable
to those of recently reported non-precious ORR catalysts,
which are summarized in Table S1. However, compared with
the Pt/C catalyst, the ORR activity of the α-MnO2 hierarchical
stars is not ideal and still needed to improve. Fig. 4c depicts the
Koutecky-Levich (K-L) curves derived from rotation-rate-
dependent currents at 0.45 V, showing a good linearity for both
may be ascribed to its unstable chemical stability causing by a
43,44
trace amount of dissolved MnO2
.
The gradual decay of
reduction current may be due to the chemical stability of the
catalysts during the ORR test. Moreover, the α-MnO2
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hierarchical stars were further examined by a CV test from 0.2 be 14.9 and 17.8 m2 g-1 for α-MnO2 and ε-MnO2,VrieewspAreticclteivOenllinye,
to -0.7 V for 2000 cycles at a scan rate of 100 mV s-1. As and the average pore diameter calculateDdObI:y10B.10a3rr9e/Cte6-TJAo0y4n5e3r0eH-
shown in Fig. S10, there is a negative shift of
∼
29 mV in the Halenda (BJH) method for α-MnO2 and ε-MnO2 is about 12.10
E1/2 value at 1600 rpm after the CV test.
and 13.55 nm (Fig. S12). Generally, the electrocatalyst with
In alkaline solutions, the ORR contains a series of complex larger surface area may have more active sites on its surface for
electrocatalytic reactions, which is involved in several electron the contact with the reactants, resulting in the higher ORR
transfer steps. On the surface of metal catalysts, the ORR activity.26 However, our results reveal that the α-MnO2 and ε-
mechanism is proposed to follow by a four-electron (4e-) MnO2 samples have close surface area and pore size, which
pathway or a two-electron (2e-) pathway.23,43 As we known, suggests the difference in ORR activities between the two
-
MnO2 exhibits outstanding activity toward the HO2 MnO2 samples may be due to the other factors such as the O2
decomposition and/or reduction, thus the ORR pathway on the adsorption affinity and electrical conductivity.46
surface of MnO2 could be recognized as an apparent 4e-
reduction mechanism involving the following steps:45,46
X-ray photoelectron spectroscopy (XPS) was employed to
determine the chemical state of the surface Mn and O elements.
Fig. S13 shows the core level Mn 2p XPS spectra of the α-
MnO2 and ε-MnO2 samples. Two peaks, corresponding to the
binding energies (BEs) of Mn 2p3/2 and Mn 2p1/2, are observed
in the Mn 2p spectra. The BEs values of Mn 2p3/2 for both two
MnO2 samples are larger than 642.0 eV, indicating their
Or
Accordingly, we can speculate that the ORR pathway on the oxidation states are close to +4.48,49 The typical Mn 3s XPS
surface of the 3D MnO2 hierarchical stars also follows the spectra of the α-MnO2 and ε-MnO2 samples are presented in Fig
apparent 4e- reduction mechanism because of their RRDE 6a. The energy separation (
△
E) of α-MnO2 sample (4.61 eV) is
-
results (high values of n and low yields of HO2 ). The adsorbed larger than that of ε-MnO2 sample (4.50 eV), revealing the Mn
O2 to replace adsorbed OH- is considered to be the rate-limiting oxidation state in α-MnO2 sample is lower than the ε-MnO2
step of this mechanism.45
The ORR performance is largely determined by the surface splitting energy (
sample.50 Detailed studies have shown that the multiplet
E) of the Mn 3s decreases monotonically
△
area, morphology, size, composition and crystal structure of the with the increase of the average oxidation state (AOS) of Mn
MnO2 catalysts. To gain the insights into the enhanced ORR element.51 The AOS for the α-MnO2 and ε-MnO2 samples can
performance of the α-MnO2 hierarchical stars, various be calculated to be 3.74 and 3.87, respectively.52,53 Thus, the α-
physicochemical characterization techniques were performed. MnO2 may have more surface oxygen vacancies than ε-MnO2
Fig. S11 displays the N2 adsorption-desorption isotherms of the according to the principle of electroneutrality, indicating more
two MnO2 samples. It can be seen that both of the two MnO2 multivalent Mn cations in α-MnO2 hierarchical stars.42,54 On the
samples show a characteristic of type II isotherm pattern with a other hand, the O1s spectra can be fitted into three peaks (Fig.
hysteresis loop of type H3, suggesting the existence of slit- 6b). The peaks at 529.7 eV, 531.1 eV and 533.4 eV are
shaped capillaries with parallel plates.47 Additionally, the corresponding to the lattice oxygen (Olatt), surface absorbed
Brunauer-Emmett-Teller (BET) surface areas were measured to oxygen (Oads) and adsorbed H2O (H2Oads), respectively.28 After
making quantitative analysis, the ratio of Oads/Olatt for α-MnO2
(0.38) is bigger than that of the ε-MnO2 (0.32), implying a
stronger interaction between the α-MnO2 hierarchical stars and
adsorbed oxygen species since the surface oxygen vacancies
can be the active centers in adsorbing the oxygen molecules.
The O1s XPS spectra results indicate that the α-MnO2 sample
possesses higher O2 adsorption affinity than the ε-MnO2 sample.
As indicated in the proposed ORR pathway, the strong
adsorption of O2 on the active centers can be expected to
promote the rate-limiting step, leading to the faster kinetics of
the α-MnO2 hierarchical stars.24
The O2 temperature-programmed desorption (O2-TPD)
spectra of the α-MnO2 and ε-MnO2 hierarchical stars were
collected in Fig. 6c to further understand their O2 adsorption
affinities. On the whole, the oxygen desorption peaks below
400 °C are attributed to the release of Oads, while the desorption
55,56
peaks above 400 °C are caused by the release of Olatt
.
Accordingly, the α-MnO2 sample displays much stronger peak
from 150 to 400 °C corresponding to the Oads species than that
of the ε-MnO2 sample, indicating that O2 is preferentially
absorbed on α-MnO2 hierarchical stars. This improved O2
adsorption capability of the α-MnO2 hierarchical stars again
Fig. 6 XPS spectra of (a) Mn 3s and (b) O 1s for α-MnO2 and ε-MnO2 samples;
(c) O2-TPD profiles of the α-MnO2 and ε-MnO2 samples; (d) Comparison of the
I-V curves of the two MnO2 samples.
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H. Wang, Y. Yang, Y. Liang, G. Zheng, Y. Li, Y. Cui and H.
4
5
confirms the above XPS results, which is highly favorable for
promoting the rate of ORR.57 Moreover, we measured the
electrical conductivity of the as-prepared MnO2 samples by
two-point current-potential (I-V) curves in a typical two
electrode system. As illustrated in Fig. 6d, the current density
of the α-MnO2 sample is substantially higher than that of the ε-
MnO2 sample at the same potential, demonstrating the α-MnO2
hierarchical stars possess superior conductivity. The enhanced
electrical conductivity of the α-MnO2 hierarchical stars can be
ascribed to its higher concentration of surface oxygen vacancies,
leading to the fast electron transfer and reduce electrode
polarization during the ORR process.50
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Conclusions
In summary, 3D α-MnO2 and ε-MnO2 hierarchical stars with
similar morphology and desirable phase have been successfully
synthesized via a facile redox reaction, and their possible
formation mechanism were seriously investigated. It was found
that the reaction temperature, anion type and dwell time played
a significant role in controlling the morphologies of the MnO2
products. These 3D hierarchical stars offer abundant active sites
on the surface of the MnO2 and prevent their aggregation as
well as facilitate the O2/electrolyte diffusion during the oxygen
reduction process. Benefiting from the improved O2 adsorption
capability and conductivity, the α-MnO2 hierarchical stars
presented remarkably enhanced ORR activity compared to the
ε-MnO2 hierarchical stars and the bulk MnO2. It achieved a
more positive ORR onset potential of 0.84 V and higher
diffusion-limited current of 5.67 mA cm-2 with excellent long-
term durability. Our present findings not only afford a reliable
synthetic approach to precisely prepare the 3D MnO2
hierarchical architectures with different crystallographic forms
for studying the phase-ORR activity of the electrocatalysts, but
also provide new insights into design high-efficient MnO2
catalysts for ORR.
5
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DOI: 10.1039/C6TA04530H
Graphical Abstract
Phase Controllable Synthesis of Three-Dimensional Star-like MnO2
Hierarchical Architectures as Highly Efficient and Stable Oxygen
Reduction Electrocatalysts
Gao Cheng,a Shilei Xie,b Bang Lan,c Xiaoying Zheng,a Fei Ye,a Ming Sun,a Xihong Lub* and Lin
Yua*
aKey Laboratory of Clean Chemistry Technology of Guangdong Regular Higher
Education Institutions, School of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006, P. R. China.
Eꢀmail: gych@gdut.edu.cn (Lin Yu).
bMOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and
Chemical Engineering, Sun YatꢀSen University, Guangzhou 510275, P. R. China.
Eꢀmail: luxh6@mail.sysu.edu.cn (Xihong Lu).
cGuangDong MeiZhou Quality & Metrology Supervision and Testing Institution, Meizhou 514072,
P. R. China.
Threeꢀdimensional (3D) αꢀMnO2 and εꢀMnO2 hierarchical starꢀlike architectures were readily
prepared by a facile hydrothermal method without any surfactants or templates. The asꢀobtained
αꢀMnO2 catalyst showed substantially enhanced ORR activity compared to the εꢀMnO2 catalyst,
which can be served as efficient catalysts for ORRꢀbased applications.