Efficient synthesis of 2-arylquinazolin-4-amines via a copper-catalyzed diazidation and ring expansion cascade of 2-arylindoles

Article abstract of DOI:10.1039/C8CC07721E

Copper-catalyzed synthesis of 2-arylquinazolin-4-amines from readily available 2-arylindoles and TMSN₃ has been developed. The mechanism study shows that the domino reaction may involve a free radical diazidation, denitrogenation, intramolecular cyclization and ring expansion sequence.

Full text of DOI:10.1039/C8CC07721E

Journal of
Materials Chemistry A
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PAPER
Constructing bundle-like Co-Mn oxides and Co-Mn
selenides for efficient overall water splitting†
Cite this: J. Mater. Chem. A, 2018, 6,
22697
Hui Xu,‡a Jingjing Wei,‡a Ke Zhang,ab Min Zhang,a Chaofan Liu,a Jun Guoc
ab
*
and Yukou Du
Making easy-to-make, cost-efficient, and durable electrocatalysts for overall water splitting to produce
oxygen and hydrogen is of paramount significance for future renewable energy systems but is still
a challenge. Herein, mesoporous CoMn and CoMnSe nanobundles are successfully constructed via
a facile one-pot hydrothermal approach. It is discovered that the Mn introduction in Co oxides can
simultaneously tune their electronic structure and modulate the nanobundle morphology. As a result,
benefitting from the 3D open nanobundle structure, the self-supported Co1Mn1 oxide exhibits
unprecedented oxygen evolution reaction (OER) activity with an ultralow overpotential of 221 mV at 10
mA cmꢀ2. Moreover, such a nanobundle-like structure also enables the Co1Mn1Se catalyst to exhibit
outstanding hydrogen evolution reaction (HER) activity with a relatively low overpotential of 87.3 mV at
10 mA cmꢀ2, surpassing those of previously reported non-precious metal catalysts. More importantly,
taking advantage of their excellent OER and HER activity, an advanced water electrolyzer through
exploiting Co1Mn1 oxide and Co1Mn1Se nanobundles as the anode and cathode is fabricated, which gives
an impressive water-splitting current density of 10 mA cmꢀ2 in 1.0 M KOH solution at 1.60 V with
remarkable stability over 36 h, holding great potential for efficient overall water splitting electrocatalysis.
Received 1st August 2018
Accepted 8th October 2018
DOI: 10.1039/c8ta07449f
rsc.li/materials-a
earth-abundant electrocatalysts for electrochemical water
splitting are highly imperative.8–10
1. Introduction
Thanks to their earth-abundant nature and theoretically
excellent electrocatalytic performance, 3d transition-metal (3d
TM) (e.g., Fe, Co, Ni, and Mn)-based electrocatalysts have
attracted extensive attention and been generally considered as
ideal substitutes for noble-metal based materials for the HER
and OER,11,12 such as metal chalcogenides, metal alloys, and
metal phosphates for the HER, metal hydroxides and metal
oxides for the OER.13–22 However, 3d TMs in traditional forms
such as aggregated particles or bulk counterparts generally
display no competitive advantages in electrocatalysis, particu-
larly for overall water splitting, due to their limited active
surface areas and few catalytically active sites.23,24 Moreover, the
catalytic activities of bare 3d TMs in high-concentration alkaline
solutions and high overpotential are also unstable; all of these
shortcomings have also limited their practical application to
some extent. To overcome these disadvantages and obtain high-
activity and stable electrocatalysts, various strategies have been
well developed. And one of the most efficient approaches is to
rationally design the morphology of 3d TMs since the structures
of materials deeply affect their electrocatalytic properties. For
the sake of increasing the utilization efficiency of catalytically
active sites, precisely controlling the morphology and structure
of electrocatalysts is of paramount importance for promoting
electrocatalytic overall water splitting performance to a higher
level.25–27 Another choice to greatly improve electrocatalytic
Regarding the rapid consumption of fossil fuels and increasing
environmental pollution problems, it is highly expected to
exploit clean and sustainable energy sources.1,2 Electrochemical
water splitting is an essential and convenient technology for the
production of sustainable energy sources due to its environ-
mental benignity and high efficiency, which includes supplying
molecular hydrogen via the cathodic HER and generating
oxygen molecules through the anodic OER.3–5 Regardless of
these advantages, the large overpotential for OER and HER
severely restricted the practical application of electrochemical
water splitting. Accordingly, searching for highly efficient elec-
trocatalysts is necessary. Currently, state-of-the-art electro-
catalysts for the HER and OER are Pt-group materials and Ru/Ir-
based oxides, respectively.6,7 However, drawbacks such as
exorbitant price, limited natural reserves, and unsatisfactory
performances greatly impede their practical applications. In
this regard, the design and development of cost-effective and
aCollege of Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou 215123, P. R. China. E-mail: duyk@suda.edu.cn
bSchool of Environmental Science and Engineering, Shaanxi University of Science and
Technology, Xian 710021, China
cTesting & Analysis Center, Soochow University, Jiangsu 215123, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta07449f
‡ These authors contributed equally to this work.
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performances is to form bimetal or bimetallic oxides, which can ethanol and acetone. The CoMn NBs were synthesized through
modulate their adsorption energies and electronic struc- the same approach just without the addition of 7.9 mg Se
tures.28–32 Besides, doping phosphorus, sulfur, or selenium into powder. And the Co(OH)2 nanobelts (NBs) and Mn2O3 were also
bimetallic oxides has also been generally considered as an prepared under the same conditions without the addition of Se
efficient avenue for the construction of high-performance powder, Mn(NO3)2$4H2O, and Co(NO3)2$6H2O, respectively. For
electrocatalysts for overall water splitting.33–37 To this end, the comparison, the Co1Mn0.8 NBs, Co1Mn1.2 NBs, Co1Mn0.8Se NBs,
design and development of both efficient and stable electro- and Co1Mn1.2Se NBs were also synthesized under the same
catalysts for overall water splitting by integrating the above conditions while tuning the amount of Mn with the designed
strategies are highly desirable, but are still a grand challenge.
Motivated by the above considerations, we herein estab-
lished a facile one-pot hydrothermal method for the successful
construction of a novel class of nanobundle-like CoMn oxide
and CoMn selenide (denoted as CoMn NBs and CoMnSe NBs)
electrocatalysts, which can act as anodic and cathodic electro-
catalysts for the OER and HER, respectively. Owing to the large
surface area, accelerated electron mobility and mass transfer,
invulnerable dissolution, and the strong synergistic coupling
between Co and Mn, the resultant Co1Mn1 NBs can display
superior OER activity with the overpotentials of only 221 and
330 mV to reach the current density of 10 and 100 mA cmꢀ2 in
1.0 M KOH solution. Notably, the electrocatalysts doped with Se
were fabricated for the HER, of which the optimized Co1Mn1Se
NBs could also exhibit excellent electrocatalytic activity with an
ultralow overpotential of 87.3 mV to achieve the HER current
density of 10 mA cmꢀ2, which was much superior to that of
most of the other electrocatalysts. More signicantly, the opti-
mized Co1Mn1 NBs//Co1Mn1Se NB couple could also exhibit
high activity for overall water splitting with a cell voltage of only
1.60 V to afford the current density of 10 mA cmꢀ2 in 1.0 M KOH
concentrations.
2.3 Characterization
Scanning electron microscopy (SEM) and energy dispersive X-
ray (EDX) were carried out on an XL30 ESEM FEG scanning
electron microscope at a voltage of 20 kV. The transmission
electron microscopy (TEM) images were acquired using
a HITACHI HT7700 TEM at an accelerating voltage of 120 kV.
High-resolution transmission electron microscopy (HRTEM)
images, selected area electron diffraction (SAED) patterns, and
high-angle annular dark-eld scanning transmission electron
microscopy (HAADF-STEM) images were obtained by using an
FEI Tecnai F20 transmission electron microscope. Powder X-ray
diffraction (PXRD) patterns were collected on an X'Pert-Pro
MPD diffractometer (Netherlands PANalytical) with a Cu Ka X-
˚
ray source (l ¼ 1.540598 A). X-ray photoelectron spectroscopy
spectra (XPS) were recorded on a Thermo Scientic ESCALAB
250 XI X-ray photoelectron spectrometer.
2.4 Electrochemical measurements
solution, which was much superior to that of the Ir/C//Pt/C The electrochemical measurements were conducted with a CHI
couple, demonstrating the potential of novel CoMn NB cata- 760e electrochemical workstation, employing CoMn NBs and
lysts for application in practical water electrolysis.
CoMnSe NBs as working electrodes, a Ag/AgCl electrode as the
reference electrode, and a carbon rod as the counter electrode,
respectively. To prepare the working electrodes, 2 mg samples
were rst dissolved in a solution (1 : 9, v/v) containing Naon
(5%) and water (2 mL) by sonication. Then, 20 mL freshly
prepared suspension was dropped on the surface of a pre-
polished glassy carbon electrode (GCE, 3 mm in diameter)
and dried at room temperature before measurements. Before
the HER and OER activity tests, cyclic voltammetry (CV) for
these electrocatalysts was performed in 1.0 M KOH solution at
a scan rate of 50 mV sꢀ1 to activate the electrocatalysts. The
polarization curves were acquired using linear sweep voltam-
metry (LSV) for the HER and OER in 1.0 M KOH solution at
a sweep rate of 5 mV sꢀ1. iR compensation of 95% was applied
via electrochemical soware. The Tafel plots were derived from
the OER and HER polarization curves (1 mV sꢀ1) and con-
structed using the Tafel equation. The chronopotentiometric
(CP) and chronoamperometric (CA) measurements, as well as
2. Experimental section
2.1 Chemicals and materials
Potassium hydroxide (KOH, analytical reagent), cobaltous
nitrate (Co(NO3)2$6H2O, analytical reagent), manganese nitrate
(Mn(NO3)2$4H2O, analytical reagent), and ethylene glycol
(CH2OH)2 were all purchased from Sinopharm Chemical
Reagent Co. Ltd. (Shanghai, China). Se powder and urea
(CO(NH2)2, 99%) were purchased from Sigma-Aldrich. All the
chemicals were used as received without further purication.
Water (18 MU cmꢀ1) used in all experiments was prepared by
passing through an ultra-pure purication system (Aqua
Solutions).
2.2 Synthesis of CoMnSe and CoMn NBs
The mesoporous CoMn NBs are synthesized by a facile one-pot LSV aer 3000 continuous CV cycles were also conducted to
hydrothermal method. In a standard synthesis, 29.1 mg evaluate their durability. The electrocatalytic performances
Co(NO3)2$6H2O, 25.1 mg Mn(NO3)2$4H2O, 7.9 mg Se powder, toward overall water splitting were measured by employing
and 60 mg urea were added to a solution containing 9 mL Co1Mn1 NBs and Co1Mn1Se NBs as working electrodes for the
ethylene glycol and 1 mL deionized water under ultrasound OER and HER, respectively; the sweep rate was 5 mV sꢀ1. For
irradiation for 30 min. The mixture was then transferred into comparison, an Ir/C (+)//Pt/C (ꢀ) couple was also used as the
a reactor and heated at 180 ꢁC for 10 h. The products were then baseline catalyst. It was worth noting that all the potentials
collected via centrifugation and washed several times with measured were calibrated to a reversible hydrogen electrode
22698 | J. Mater. Chem. A, 2018, 6, 22697–22704
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(RHE), and state-of-the-art Ir/C and Pt/C catalysts (20 wt%, 2–
5 nm Pt nanoparticles) purchased from Johnson Matthey (JM)
Corporation were also employed as baseline catalysts for all the
electrochemical measurements.
3. Results and discussion
The syntheses of CoMn NBs and CoMnSe NBs were carried out
through a facile one-step method as illustrated in Scheme 1.
The morphologies and microstructures of the as-prepared
Co1Mn1 NBs are characterized by TEM and SEM. The unique
mesoporous nanobundles assembled with abundant ultrathin
nanosheets can be clearly observed in the HAADF-STEM images
(Fig. 1a and b), which can efficiently provide ion-diffusion
routes that enable mass transfer and electron mobility. A clear
view also indicates that the mesoporous Co1Mn1 NBs are
composed of parallelly interconnected nanosheets (Fig. 1c, S1a
and S1b†). Also, the two ends of the nanobundle comprise three
to ve forks generated by a plurality of nanosheets (Fig. 1d and
S2†) and form a mesoporous nanobundle-like arrangement,
which is also favorable for the mass transfer and ion
diffusion.36,37
The HRTEM image was recorded by focusing individual
nanobundles to investigate their crystal structure. As seen, the
lattice spacing of ca. 0.284 and 0.301 nm was matched well with
the d-spacing of the (200) and (112) planes of CoMn NBs
(Fig. 1e), respectively, and no obvious continuous lattice fringes
can be observed, corroborating the low crystallinity of CoMn
NBs.38–40 The crystal structures of mesoporous Co1Mn1 NBs were
further conrmed by employing the PXRD technique. As shown
in Fig. 1f, the mesoporous Co1Mn1 NBs displayed the typical
diffraction peaks of CoMn oxides (JCPDS: # 77-0471), while
some weak peaks could be indexed to Mn2O3.41,42 The BET
technique has also been employed to deeply investigate the
structural features of this nanobundle catalyst. As seen in
Fig. S3,† the BET surface area of Co1Mn1 NBs is 208.3 m2 gꢀ1
,
and the average pore size is about 5.1 nm, suggesting the high
surface area and mesoporous properties, which is in accordance
with the TEM observations.
For comparison, the TEM images of Co1Mn0.8 NBs, Co1Mn1.2
NBs, and Co(OH)2 were also acquired. As seen in Fig. S4,† both
Fig. 1 (a and b) HAADF-STEM images, (c and d) representative TEM
images, (e) HRTEM image and SAED pattern, and (f) PXRD pattern and
(g) EDX spectrum of Co1Mn1 NBs.
Co1Mn0.8 and Co1Mn1.2 NBs also displayed the typical nano-
bundle structure similar to Co1Mn1 NBs. However, the thick-
nesses of individual Co1Mn0.8 NBs and Co1Mn1.2 NBs were
found to be smaller than that of Co1Mn1 NBs, indicating that
the feed ratio of precursors plays a signicant role in deter-
mining the nal shapes of the products.43 Apart from these, we
have also studied the specic morphology of Co(OH)2. Differ-
ently, Fig. S5† shows that the unique Co(OH)2 NBs are
comprised of many nanosheets that distributed uniformly, and
the thickness of the nanobelts is around 30 nm, which is much
thinner than that of CoMn NBs. For gaining insight into the
composition information of these products, the EDX tests have
also been well conducted. As seen in Fig. 1g, the atomic ratio of
Co/Mn for Co1Mn1 NBs was 47.9 : 52.1, being consistent with
Scheme 1 Schematic illustration of the synthetic route for CoMn and
CoMnSe nanobundles.
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the feed amount. And the atomic ratios of Co/Mn for Co1Mn0.8 has no effects on the ultimate morphology and structure of the
NBs and Co1Mn1.2 NBs were also calculated to be 55.9 : 44.1 and CoMn NBs.
45.4 : 54.6, respectively (Fig. S4c and S4f†), both of which were
also greatly in agreement with their feed amounts.
XPS was applied to understand the surface chemical
compositions and valences of the resultant Co1Mn1 NBs and
In order to produce effective HER catalysts, we have also Co1Mn1Se NBs. Fig. 3a shows the survey spectra of Co1Mn1 NBs
successfully constructed the CoMnSe catalysts through the and Co1Mn1Se NBs. It is clearly displayed that the signals of
same method. As displayed in Fig. 2a–d, the Co1Mn1Se also both Co and Mn are clearly observed for both Co1Mn1 NBs and
showed a similar nanobundle morphology assembled by ultra- Co1Mn1Se NBs. However, for the Co1Mn1Se NBs, the Mn, Se,
thin nanosheets to Co1Mn1 NBs. To corroborate the phase and O signals are remarkably enhanced, conrming the
composition of the as-obtained CoMnSe catalysts, the PXRD successful incorporation of Mn and Se in Co oxides. Fig. 3b
patterns have also been obtained. As displayed in Fig. S6,† the exhibits the Co 2p XPS spectrum of Co1Mn1Se NBs; as seen, the
PXRD patterns of the Co1Mn1Se NBs showed a slight difference obvious peaks at 798.3 and 782.9 eV are corresponded to Co2+
in comparison with Co1Mn1, caused by the dopant Se, demon- 2p1/2 and Co2+ 2p3/2, and the peak at 784.5 eV is ascribed to Co3+
strating the successful Se doping. Moreover, we have also 2p1/2, while the other peaks are typical satellite peaks. More
prepared the other two types of CoMnSe NBs for comparison. As importantly, the peak area of Co2+ is obviously higher than that
seen in Fig. S7,† both Co1Mn0.8Se NBs and Co1Mn1.2Se NBs also of Co3+, indicating the presence of oxygen vacancies, which may
possessed a similar morphology to Co1Mn0.8 NBs and Co1Mn1.2 be key for promoting the overall water splitting performances.44
NBs. And the compositions of these three types of electro- With regard to Mn 2p, as shown in Fig. 3c, the peaks at 642.3
catalysts have also been measured; all of them well correspond and 653.9 eV can be assigned to Mn(II), while another peak at
to the theoretical ratios (Fig. 2e and S7c and S7f†). From these 654.7 eV is characteristic of Mn(III) cation.45,46 As for Se 3d, the
results, we can come to a conclusion that the Se introduction XPS spectrum (Fig. 3d) can be well deconvoluted into two peaks
at 54.4 and 55.8 eV, being consistent with the Se2ꢀ of Se 3d5/2
and Se 3d3/2, which may be ascribed to the surface oxidation of
the selenide.47,48 These results have indicated the presence of
the mixed valence of metal ions (Mn2+/Mn3+ and Co2+/Co3+) in
Co1Mn1Se NBs, which is highly expected to play a signicant
role in the enhancement of electrocatalytic activity.
In consideration of the unique nanobundle structure, the
newly generated CoMn NBs are thus highly desirable to obtain
excellent electrochemical properties. In this regard, we here
conducted the electrochemical measurements to evaluate their
OER catalytic properties by selecting state-of-the-art Ir/C and
Co(OH)2 NB catalysts for comparison. The OER activity was rst
assessed using LSV at a scan rate of 5 mV sꢀ1 in 1.0 M KOH
solution. Fig. 4a showed the iR-compensated LSV curves of
these ve electrocatalysts. As seen, the Co1Mn1 NB electrode
exhibits the most excellent activity in comparison with the other
four electrocatalysts. From Fig. 4b, we can nd that the optimal
Co1Mn1 NB electrode requires an overpotential of only 221 mV
to achieve the current density of 10 mA cmꢀ2, which is 34, 54,
84, 118, and 195 mV lower than those of Co1Mn1.2 NBs,
Fig. 2 (a and b) HAADF-STEM, (c and d) representative TEM, (e) SEM- Fig. 3 XPS spectra of (a) the survey scan of Co1Mn1 NBs and Co1Mn1Se
EDX spectrum of Co1Mn1Se NBs.
NBs, (b) Co 2p, (c) Mn 2p, and (d) Se 3d.
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signicant for practical applications. The electrochemically
active surface area (ECSA) has been generally regarded as
a crucial factor affecting the electrocatalytic performances of
catalysts, which is assumed by capacitance measurements via
CV in the double layer region at different scan rates
(Fig. S10†).50,51 From Fig. S11,† it was found that the ECSA of
Co1Mn1 NBs was 40 cmꢀ2, which was 1.14, 1.23, and 1.60 times
higher than those of Co1Mn1.2 NBs (35 cmꢀ2), Co1Mn0.8 NBs
(32.5 cmꢀ2), and Co(OH)2 NBs (25 cmꢀ2), respectively. The
superior OER activity of Co1Mn1 NBs can mainly be associated
with the high ECSA.52
The electrocatalytic activity of CoMnSe NBs for hydrogen
evolution was also investigated in depth. For a good compar-
ison, the HER activities of Co(OH)2 NBs and commercial Pt/C
catalysts were also tested. An inspection of the polarization
curves indicates that the electrocatalytic activity of these cata-
lysts obey the following sequence: Pt/C > Co1Mn1Se NBs >
Co1Mn1.2Se NBs > Co1Mn0.8Se NBs > Co(OH)2 NBs (Fig. 5a). For
instance, to achieve the current density of 10 mA cmꢀ2, an
overpotential of 87.3 mV is required for Co1Mn1Se NBs, which is
34.9, 74.9, and 102.6 mV lower than those of Co1Mn1.2Se NBs
(122.2 mV), Co1Mn0.8Se NBs (162.2 mV), and Co(OH)2 NBs
(189.9 mV) (Fig. 5b), respectively. Moreover, the electrocatalytic
activity of Co1Mn1Se NBs also compares favorably to that of
many previously reported 3d TM catalysts (Table S2†). More
importantly, the current densities of all the electrocatalysts
Fig. 4 (a) Polarization curves, (b) overpotentials at the current densi-
ties of 10 and 100 mA cmꢀ2, and (c) Tafel slopes of Co1Mn1 NBs,
Co1Mn1.2 NBs, Co1Mn0.8 NBs, Ir/C, and Co(OH)2 NBs. (d) Polarization
curves of Co1Mn1 NBs obtained before and after 3000 potential cycles.
(e) Prolonged CP of Co1Mn1 NBs for 36 h.
Co1Mn0.8 NBs, benchmark Ir/C, Co(OH)2 NBs, and Mn2O3
(Fig. S8†), respectively. What's more, the OER electrocatalytic
activity of Co1Mn1 NBs is also superior to most of the recently
reported binary or ternary electrocatalysts (Table S1†).
Furthermore, the optimized Co1Mn1 NBs also need only
330 mV to reach a current density of 100 mA cmꢀ2, which is
even 89 mV lower than that of the Ir/C catalyst, further con-
rming the remarkable OER catalytic activity.
Meanwhile, for assessing the reaction kinetics, the Tafel
slopes of these electrocatalysts have also been calculated. As
shown in Fig. 4c, the Tafel slope of Co1Mn1 NBs was 39.8 mV
decꢀ1, which was considerably lower than those of Co1Mn1.2
NBs (62.2 mV decꢀ1), Co1Mn0.8 NBs (65.5 mV decꢀ1), Ir/C
(67.8 mV decꢀ1), and Co(OH)2 NBs (109.2 mV decꢀ1), indi-
cating more benecial OER kinetics for Co1Mn1 NBs.49 Besides
the OER activity and kinetics, the durability is also another
crucial parameter for evaluating their electrochemical proper-
ties. Accordingly, LSV aer 3000 continuous CV cycles and
prolonged CP test were also conducted to assess their dura-
bility. As shown in Fig. 4d, the polarization curve of Co1Mn1 NBs
aer CV of 3000 cycles is almost overlapped with the original
one. Moreover, the overpotential only increases by 32 mV for
continuous 18 h CP test (Fig. 4e). And the current densities of
Co1Mn1 NBs in CA measurements at the overpotentials of 400
and 300 mV only decayed slightly aer successive electrolysis
for more than 50 000 s (Fig. S9†). These results have demon-
Fig. 5 (a) Polarization curves, (b) overpotentials at a current density of
10 mA cmꢀ2, and (c) Tafel slopes of Co1Mn1Se NBs, Co1Mn1.2Se NBs,
Co1Mn0.8Se NBs, Pt/C, and Co(OH)2 NBs. (d) Polarization curves of
Co1Mn1Se NBs obtained before and after 3000 potential cycles. (e)
strated the outstanding electrocatalytic OER durability, which is Prolonged CP of Co1Mn1Se NBs for 18 h.
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based on the real surface area are much higher than those
based on the projected area (Fig. S12†). Remarkably, the
Co1Mn1 NBs and Co1Mn1Se NBs require the overpotentials of
210 and 75.6 mV to achieve the current density of 10 mA cmꢀ2
for the OER and HER, respectively, showing great potential for
practical water splitting. Fig. 5c presents the Tafel slopes of
these ve catalyst-modied electrodes. As seen, the measured
Tafel slope of Co1Mn1Se NBs was about 71.2 mV decꢀ1, which
was obviously smaller than those of Co1Mn1.2 Se NBs (93.5 mV
decꢀ1), Co1Mn0.8Se NBs (132.5 mV decꢀ1), and Co(OH)2 NBs
(192.3 mV decꢀ1). Additionally, the Tafel slope of Co1Mn1 Se
NBs falls within the range of 40 to 120 mV decꢀ1, suggesting
that the HER is probably taking place on the surface of Co1-
Mn1Se NBs and proceeding through the Volmer–Heyrovsky
mechanism.53
The Co1Mn1Se NBs also displayed excellent HER durability,
in which negligible electrocatalytic activity degradation was
observed aer being subjected to CV of 3000 cycles in 1.0 M
KOH solution (Fig. 5d). The prolonged CP results also indicated
that the overpotential of Co1Mn1Se NBs exhibited the negligible
increase aer continuous HER testing for 36 h (Fig. 5f). More-
over, the CA results also verify that the Co1Mn1Se NBs show
a very slow decay of current density and maintain 72% and 83%
of the initial current at the overpotentials of ꢀ200 and ꢀ100 mV
(Fig. S13†). These results have directly conrmed the
outstanding long-term stability of Co1Mn1Se NBs for the HER.54
Encouraged by the outstanding OER performance of Co1Mn1
NBs, excellent HER performance of Co1Mn1Se NBs in 1.0 M
KOH solution, and their potential for practical applications, we
here employed Co1Mn1 NBs as the anode and Co1Mn1Se NBs as
the cathode to assemble an alkaline electrolyzer for overall
water splitting (Fig. 6a).55 For comparison, the Ir/C (+)//Pt/C (ꢀ)
was also constructed and measured in a two-electrode cong-
uration. The Co1Mn1 NBs (+)//Co1Mn1Se NB (ꢀ) couple can
deliver a water splitting current density of 10 mA cmꢀ2 by only
employing a voltage of 1.60 V (Fig. 6b), outperforming the Ir/C
(+)//Pt/C (ꢀ) couple (1.62 V) and many previous catalysts
(Table S3†), which indicates the outstanding electrochemical
water splitting activity. Signicantly, the Co1Mn1 NBs (+)//Co1-
Mn1Se NB (ꢀ) couple can also deliver a larger current density
than the Ir/C (+)//Pt/C (ꢀ) couple at high potential (Fig. 6c). In
detail, the current density of the Co1Mn1 NBs (+)//Co1Mn1Se NB
(ꢀ) couple was 189.3 mA cmꢀ2 at a potential of 1.8 V, which was
much higher than that of the Ir/C (+)//Pt/C (ꢀ) couple (65.4 mA
cmꢀ2), further conrming the excellent electrocatalytic activity.
To study the oxygen and hydrogen conversion rates, the faradaic
efficiency measurements have also been performed. As seen in
Fig. S14a,† the end values of dissolved oxygen are 2.65 and 2.52
for red and black lines, respectively. Therefore, the faradaic
Fig. 6 (a) Illustration of overall water electrolysis. (b) Polarization
curves of the Co1Mn1 NBs (+)//Co1Mn1Se NB (ꢀ) couple and Ir/C
(+)//Pt/C (ꢀ) couple in 1.0 M KOH solution with a scan rate of 5 mV sꢀ1
,
and (c) current densities of the Co1Mn1 NBs (+)//Co1Mn1Se NB (ꢀ)
couple and Ir/C (+)//Pt/C (ꢀ) couple at different voltages. (d) Pro-
longed CP of the Co1Mn1 NBs (+)//Co1Mn1Se NB (ꢀ) couple for 36 h.
suggesting the remarkable durability of electrocatalytic activity
toward overall water splitting.
In addition, the morphology and composition of the Co1-
Mn1Se NBs aer long-term CP test are also studied attentively
(Fig. S15†). Remarkably, the Co1Mn1Se NBs could also maintain
their unique nanobundle structure and composition with
negligible change aer the long-term stability test, which was
also another piece of evidence conrming their excellent elec-
trocatalytic durability, thus holding great potential for practical
applications.58 To further study the surface compositions, we
have conducted the XPS tests for the Co1Mn1 and Co1Mn1Se NB
catalysts aer long-term CP testing. As shown in Fig. S16,† both
Co1Mn1 and Co1Mn1Se NBs aer long-term CP testing
possessed the mixed metal valences (Co2+, Co3+, Mn2+, and
Mn3+) similar to what was observed in the XPS spectra before CP
tests. However, aer a detailed observation, we could also nd
that the XPS spectra of Co 2p showed a peak area increase for
Co3+, which corresponded to the oxidized Co species during the
overall water splitting.59,60 Such a transformation explains the
excellent overall water splitting activity of the Co1Mn1 NBs
(+)//Co1Mn1Se NB (ꢀ) couple and is in accordance with the
above results.
4. Conclusions
efficiency is 95.1%, indicating that the detected oxidation In summary, an advanced class of nanobundle-like CoMn
current can be ascribed to the OER process catalyzed by Co1Mn1 oxides and CoMn selenides with a controlled shape and struc-
NB catalysts. From Fig. S14b,† the faradaic efficiency for ture have been successfully constructed through a facile and
hydrogen conversion is 92.3%.56,57 Furthermore, the Co1Mn1 one-pot hydrothermal method for the rst time. The systematic
NBs (+)//Co1Mn1Se NB (ꢀ) couple also displays superior long- experiments indicated that the Mn incorporation could
term stability, where its cell potential has an unobvious varia- favourably and signicantly modify the electronic structure,
tion at the current density of 10 mA cmꢀ2 for 36 h (Fig. 6d), shape, and ECSA of Co oxides. Impressively, the CoMn and
CoMnSe NBs can exhibit excellent OER and HER activity, and
22702 | J. Mater. Chem. A, 2018, 6, 22697–22704
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Journal of Materials Chemistry A
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Conflicts of interest
There are no conicts to declare.
19 X. Ma, J. Wen, S. Zhang, H. Yuan, K. Li, F. Yan, X. Zhang and
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Acknowledgements
This work was supported by the National Natural Science 21 J. X. Feng, L. X. Ding, S. H. Ye, X. J. He, H. Xu, Y. X. Tong and
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